IETF Mobile IP Working Group David B. Johnson
INTERNET-DRAFT Rice University
Charles Perkins
Nokia Research Center
Jari Arkko
Ericsson
1 May 2002
Mobility Support in IPv6
<draft-ietf-mobileip-ipv6-17.txt>
Status of This Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note
that other groups may also distribute working documents as
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This document specifies the operation of mobile computers using IPv6.
Each mobile node is always identified by its home address, regardless
of its current point of attachment to the Internet. While situated
away from its home, a mobile node is also associated with a care-of
address, which provides information about the mobile node's current
location. IPv6 packets addressed to a mobile node's home address are
transparently routed to its care-of address. The protocol enables
IPv6 nodes to cache the binding of a mobile node's home address
with its care-of address, and to then send any packets destined for
the mobile node directly to it at this care-of address. To support
this operation, Mobile IPv6 defines a new IPv6 protocol and a new
destination option. All IPv6 nodes, whether mobile or stationary,
MUST support communications with mobile nodes.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Comparison with Mobile IP for IPv4 2
3. Terminology 4
3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 6
4. Overview of Mobile IPv6 9
4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 9
4.2. New IPv6 Protocols . . . . . . . . . . . . . . . . . . . 12
4.3. New IPv6 Destination Options . . . . . . . . . . . . . . 13
4.4. New IPv6 ICMP Messages . . . . . . . . . . . . . . . . . 14
4.5. Conceptual Data Structures . . . . . . . . . . . . . . . 15
4.6. Binding Management . . . . . . . . . . . . . . . . . . . 16
5. Overview of Mobile IPv6 Security 17
5.1. Threats . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2. Features . . . . . . . . . . . . . . . . . . . . . . . . 18
5.3. Tunnels to and from the Home Agents . . . . . . . . . . . 20
5.4. Binding Updates to Home Agents . . . . . . . . . . . . . 20
5.5. Binding Updates to Correspondent Nodes . . . . . . . . . 21
5.5.1. Node Keys . . . . . . . . . . . . . . . . . . . . 22
5.5.2. Nonces . . . . . . . . . . . . . . . . . . . . . 23
5.5.3. Cookies . . . . . . . . . . . . . . . . . . . . . 23
5.5.4. Cryptographic Functions . . . . . . . . . . . . . 24
5.5.5. Return Routability Procedure . . . . . . . . . . 24
5.5.6. Applying Return Routability for Correspondent
Bindings . . . . . . . . . . . . . . . . . 28
5.5.7. Updating Node Keys and Nonces . . . . . . . . . . 29
5.5.8. Preventing Replay Attacks . . . . . . . . . . . . 30
5.5.9. Preventing Denial-of-Service Attacks . . . . . . 30
5.5.10. Correspondent Binding Procedure Extensibility . . 31
6. New IPv6 Protocols, Message Types, and Destination Option 31
6.1. Mobility Header . . . . . . . . . . . . . . . . . . . . . 31
6.1.1. Format . . . . . . . . . . . . . . . . . . . . . 32
6.1.2. Binding Refresh Request (BRR) Message . . . . . . 33
6.1.3. Home Test Init (HoTI) Message . . . . . . . . . . 34
6.1.4. Care-of Test Init (CoTI) Message . . . . . . . . 36
6.1.5. Home Test (HoT) Message . . . . . . . . . . . . . 37
6.1.6. Care-of Test (CoT) Message . . . . . . . . . . . 39
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6.1.7. Binding Update (BU) Message . . . . . . . . . . . 41
6.1.8. Binding Acknowledgement (BA) Message . . . . . . 45
6.1.9. Binding Error (BE) Message . . . . . . . . . . . 49
6.2. Mobility Options . . . . . . . . . . . . . . . . . . . . 51
6.2.1. Format . . . . . . . . . . . . . . . . . . . . . 51
6.2.2. Pad1 . . . . . . . . . . . . . . . . . . . . . . 52
6.2.3. PadN . . . . . . . . . . . . . . . . . . . . . . 52
6.2.4. Unique Identifier . . . . . . . . . . . . . . . . 53
6.2.5. Alternate Care-of Address . . . . . . . . . . . . 53
6.2.6. Nonce Indices . . . . . . . . . . . . . . . . . . 54
6.2.7. Binding Authorization Data . . . . . . . . . . . 54
6.3. Home Address Destination Option . . . . . . . . . . . . . 55
6.4. Routing Header type 2 . . . . . . . . . . . . . . . . . . 58
6.4.1. Routing Header Packet format . . . . . . . . . . 58
6.5. ICMP Home Agent Address Discovery Request Message . . . . 59
6.6. ICMP Home Agent Address Discovery Reply Message . . . . . 61
6.7. ICMP Mobile Prefix Solicitation Message Format . . . . . 63
6.8. ICMP Mobile Prefix Advertisement Message Format . . . . . 65
7. Modifications to IPv6 Neighbor Discovery 67
7.1. Modified Router Advertisement Message Format . . . . . . 67
7.2. Modified Prefix Information Option Format . . . . . . . . 68
7.3. New Advertisement Interval Option Format . . . . . . . . 70
7.4. New Home Agent Information Option Format . . . . . . . . 71
7.5. Changes to Sending Router Advertisements . . . . . . . . 73
7.6. Changes to Sending Router Solicitations . . . . . . . . . 74
8. Requirements for Types of IPv6 Nodes 75
8.1. Requirements for All IPv6 Hosts and Routers . . . . . . . 75
8.2. Requirements for All IPv6 Routers . . . . . . . . . . . . 75
8.3. Requirements for IPv6 Home Agents . . . . . . . . . . . . 76
8.4. Requirements for IPv6 Mobile Nodes . . . . . . . . . . . 77
9. Correspondent Node Operation 78
9.1. Conceptual Data Structures . . . . . . . . . . . . . . . 78
9.2. Receiving Packets from a Mobile Node . . . . . . . . . . 79
9.2.1. Processing Mobility Header (MH) Messages . . . . 79
9.2.2. Receiving Packets with Home Address Destination
Option . . . . . . . . . . . . . . . . . . 80
9.3. Return Routability Procedure . . . . . . . . . . . . . . 80
9.3.1. Receiving HoTI Messages . . . . . . . . . . . . . 81
9.3.2. Receiving CoTI Messages . . . . . . . . . . . . . 81
9.3.3. Sending HoT Messages . . . . . . . . . . . . . . 82
9.3.4. Sending CoT Messages . . . . . . . . . . . . . . 82
9.4. Processing Bindings . . . . . . . . . . . . . . . . . . . 82
9.4.1. Receiving Binding Updates . . . . . . . . . . . . 82
9.4.2. Requests to Cache a Binding . . . . . . . . . . . 84
9.4.3. Requests to Delete a Binding . . . . . . . . . . 84
9.4.4. Sending Binding Acknowledgements . . . . . . . . 85
9.4.5. Sending Binding Refresh Requests . . . . . . . . 86
9.4.6. Sending Binding Error Messages . . . . . . . . . 87
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9.5. Cache Replacement Policy . . . . . . . . . . . . . . . . 87
9.6. Sending Packets to a Mobile Node . . . . . . . . . . . . 88
9.7. Receiving ICMP Error Messages . . . . . . . . . . . . . . 89
10. Home Agent Operation 90
10.1. Conceptual Data Structures . . . . . . . . . . . . . . . 90
10.2. Primary Care-of Address Registration . . . . . . . . . . 91
10.3. Primary Care-of Address De-Registration . . . . . . . . . 94
10.4. Intercepting Packets for a Mobile Node . . . . . . . . . 95
10.5. Tunneling Intercepted Packets to a Mobile Node . . . . . 97
10.6. Handling Reverse Tunneled Packets from a Mobile Node . . 98
10.7. Protecting Return Routability Packets . . . . . . . . . . 99
10.8. Receiving Router Advertisement Messages . . . . . . . . . 99
10.9. Dynamic Home Agent Address Discovery . . . . . . . . . . 101
10.9.1. Aggregate List of Home Network Prefixes . . . . . 102
10.9.2. Scheduling Prefix Deliveries to the Mobile Node . 104
10.9.3. Sending Advertisements to the Mobile Node . . . . 106
10.9.4. Lifetimes for Changed Prefixes . . . . . . . . . 107
11. Mobile Node Operation 107
11.1. Conceptual Data Structures . . . . . . . . . . . . . . . 107
11.2. Packet Processing . . . . . . . . . . . . . . . . . . . . 110
11.2.1. Sending Packets While Away from Home . . . . . . 110
11.2.2. Interaction with Outbound IPsec Processing . . . 112
11.2.3. Receiving Packets While Away from Home . . . . . 114
11.2.4. Routing Multicast Packets . . . . . . . . . . . . 116
11.3. Home Agent and Prefix Management . . . . . . . . . . . . 116
11.3.1. Receiving Local Router Advertisement Messages . . 116
11.3.2. Dynamic Home Agent Address Discovery . . . . . . 118
11.3.3. Sending Mobile Prefix Solicitations . . . . . . . 119
11.3.4. Receiving Mobile Prefix Advertisements . . . . . 120
11.4. Movement . . . . . . . . . . . . . . . . . . . . . . . . 121
11.4.1. Movement Detection . . . . . . . . . . . . . . . 121
11.4.2. Forming New Care-of Addresses . . . . . . . . . . 124
11.4.3. Using Multiple Care-of Addresses . . . . . . . . 125
11.5. Return Routability Procedure . . . . . . . . . . . . . . 126
11.5.1. Sending Home and Care-of Test Init Messages . . . 126
11.5.2. Receiving Return Routability Messages . . . . . . 126
11.5.3. Retransmitting in the Return Routability Procedure 128
11.5.4. Rate Limiting for Return Routability Procedure . 128
11.6. Processing Bindings . . . . . . . . . . . . . . . . . . . 128
11.6.1. Sending Binding Updates to the Home Agent . . . . 128
11.6.2. Correspondent Binding Procedure . . . . . . . . . 130
11.6.3. Receiving Binding Acknowledgements . . . . . . . 133
11.6.4. Receiving Binding Refresh Requests . . . . . . . 134
11.6.5. Receiving Binding Error Messages . . . . . . . . 135
11.6.6. Forwarding from a Previous Care-of Address . . . 136
11.6.7. Returning Home . . . . . . . . . . . . . . . . . 137
11.6.8. Retransmitting Binding Updates . . . . . . . . . 139
11.6.9. Rate Limiting Binding Updates . . . . . . . . . . 140
11.7. Receiving ICMP Error Messages . . . . . . . . . . . . . . 140
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12. Protocol Constants 141
13. IANA Considerations 142
14. Security Considerations 143
14.1. Security for the Tunneling to and from the Home Agent . . 143
14.2. Security for the Binding Updates to the Home Agent . . . 144
14.3. Security for the Binding Updates to the Correspondent
Nodes . . . . . . . . . . . . . . . . . . . . . . . . 144
14.4. Security for the Home Address Destination Option . . . . 145
14.5. Firewall considerations . . . . . . . . . . . . . . . . . 145
Acknowledgements 146
References 147
A. State Machine for the Correspondent Binding Procedure 150
B. Changes from Previous Version of the Draft 159
B.1. Changes from Draft Version 16 . . . . . . . . . . . . . . 159
B.2. Changes from Draft Version 15 . . . . . . . . . . . . . . 161
B.3. Changes from Earlier Versions of the Draft . . . . . . . 162
C. Remote Home Address Configuration 164
D. Future Extensions 165
D.1. Piggybacking . . . . . . . . . . . . . . . . . . . . . . 165
D.2. Triangular Routing and Unverified Home Addresses . . . . 166
D.3. New Authorization Methods beyond Return Routability . . . 166
Chairs' Addresses 167
Authors' Addresses 167
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1. Introduction
This document specifies the operation of mobile computers using
Internet Protocol Version 6 (IPv6) [6]. Without specific support
for mobility in IPv6, packets destined to a mobile node (host or
router) would not be able to reach it while the mobile node is away
from its home link (the link on which its home IPv6 subnet prefix is
in use), since routing is based on the subnet prefix in a packet's
destination IP address. In order to continue communication in spite
of its movement, a mobile node could change its IP address each time
it moves to a new link, but the mobile node would then not be able
to maintain transport and higher-layer connections when it changes
location. Mobility support in IPv6 is particularly important, as
mobile computers are likely to account for a majority or at least a
substantial fraction of the population of the Internet during the
lifetime of IPv6.
The protocol defined in this document, known as Mobile IPv6, allows
a mobile node to move from one link to another without changing the
mobile node's IP address. A mobile node is always addressable by
its "home address", an IP address assigned to the mobile node within
its home subnet prefix on its home link. Packets may be routed to
the mobile node using this address regardless of the mobile node's
current point of attachment to the Internet, and the mobile node may
continue to communicate with other nodes (stationary or mobile) after
moving to a new link. The movement of a mobile node away from its
home link is thus transparent to transport and higher-layer protocols
and applications.
The Mobile IPv6 protocol is just as suitable for mobility across
homogeneous media as for mobility across heterogeneous media. For
example, Mobile IPv6 facilitates node movement from one Ethernet
segment to another as well as it facilitates node movement from an
Ethernet segment to a wireless LAN cell, with the mobile node's IP
address remaining unchanged in spite of such movement.
One can think of the Mobile IPv6 protocol as solving the
network-layer mobility management problem. Some mobility management
applications -- for example, handover among wireless transceivers,
each of which covers only a very small geographic area -- have been
solved using link-layer techniques. For example, in many current
wireless LAN products, link-layer mobility mechanisms allow a
"handover" of a mobile node from one cell to another, reestablishing
link-layer connectivity to the node in each new location. Within
the natural limitations imposed by link-management solutions, and as
long as such handover occurs only within cells of the mobile node's
home link, such link-layer mobility mechanisms MAY offer faster
convergence and lower overhead than Mobile IPv6. Extensions to the
Mobile IPv6 protocol have been proposed to support a more local,
hierarchical form of mobility management, but such extensions are
beyond the scope of this document.
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The protocol specified in this document solves the problem of
transparently routing packets to and from mobile nodes while away
from home. However, it does not attempt to solve all general
problems related to the use of mobile computers or wireless networks.
In particular, this protocol does not attempt to solve:
- Handling links with partial reachability, or unidirectional
connectivity, such as are often found in wireless networks (but
see Section 11.4.1).
- Access control on a link being visited by a mobile node.
- Assistance for adaptive applications
- Mobile routers
- Service Discovery
- Distinguishing between packets lost due to bit errors vs.
network congestion
2. Comparison with Mobile IP for IPv4
The design of Mobile IP support in IPv6 (Mobile IPv6) represents a
natural combination of the experiences gained from the development
of Mobile IP support in IPv4 (Mobile IPv4) [25, 24, 26], together
with the opportunities provided by the design and deployment of a new
version of IP itself (IPv6) and the new protocol features offered
by IPv6. Mobile IPv6 thus shares many features with Mobile IPv4,
but the protocol is now fully integrated into IP and provides many
improvements over Mobile IPv4. This section summarizes the major
differences between Mobile IPv4 and Mobile IPv6:
- Support for what is known in Mobile IPv4 as "Route
Optimization" [27] is now built in as a fundamental part
of the protocol, rather than being added on as an optional
set of extensions that may not be supported by all nodes
as in Mobile IPv4. This integration of Route Optimization
functionality allows direct routing from any correspondent
node to any mobile node, without needing to pass through
the mobile node's home network and be forwarded by its home
agent, and thus eliminates the problem of "triangle routing"
present in the base Mobile IPv4 protocol [25]. The Mobile IPv4
"registration" functionality and the Mobile IPv4 Route
Optimization functionality are performed by a single protocol
rather than two separate (and different) protocols.
- Support is also integrated into Mobile IPv6 -- and into IPv6
itself -- for allowing Route Optimization to coexist efficiently
with routers that perform "ingress filtering" [7]. A mobile
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node uses its care-of address as the Source Address in the
IP header of packets it sends, allowing the packets to pass
normally through ingress filtering routers. The home address
of the mobile node is carried in the packet in a Home Address
destination option, allowing the use of the care-of address in
the packet to be transparent above the IP layer. The ability to
correctly process a Home Address option in a received packet is
required in all IPv6 nodes, whether mobile or stationary, whether
host or router.
- The use of the care-of address as the Source Address in each
packet's IP header also simplifies routing of multicast packets
sent by a mobile node. With Mobile IPv4, the mobile node
had to tunnel multicast packets to its home agent in order to
transparently use its home address as the source of the multicast
packets. With Mobile IPv6, the use of the Home Address option
allows the home address to be used but still be compatible with
multicast routing that is based in part on the packet's Source
Address.
- There is no longer any need to deploy special routers as
"foreign agents" as are used in Mobile IPv4. In Mobile IPv6,
mobile nodes make use of IPv6 features, such as Neighbor
Discovery [20] and Address Autoconfiguration [33], to operate in
any location away from home without any special support required
from the local router.
- The movement detection mechanism in Mobile IPv6 provides
bidirectional confirmation of a mobile node's ability to
communicate with its default router in its current location
(packets that the router sends are reaching the mobile node, and
packets that the mobile node sends are reaching the router).
This confirmation provides a detection of the "black hole"
situation that may exist in some wireless environments where the
link to the router does not work equally well in both directions,
such as when the mobile node has moved out of good wireless
transmission range from the router. The mobile node may then
attempt to find a new router and begin using a new care-of
address if its link to its current router is not working well.
In contrast, in Mobile IPv4, only the forward direction (packets
from the router are reaching the mobile node) is confirmed,
allowing the black hole condition to persist.
- Most packets sent to a mobile node while away from home in
Mobile IPv6 are sent using an IPv6 Routing header rather than IP
encapsulation, whereas Mobile IPv4 must use encapsulation for all
packets. The use of a Routing header requires less additional
header bytes to be added to the packet, reducing the overhead
of Mobile IP packet delivery. To avoid modifying the packet in
flight, however, packets intercepted and tunneled by a mobile
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node's home agent in Mobile IPv6 must still use encapsulation for
delivery to the mobile node.
- While a mobile node is away from home, its home agent intercepts
any packets for the mobile node that arrive at the home network,
using IPv6 Neighbor Discovery [20] rather than ARP [29] as is
used in Mobile IPv4. The use of Neighbor Discovery improves
the robustness of the protocol (e.g., due to the Neighbor
Advertisement "override" bit) and decouples Mobile IP from any
particular link layer, unlike in ARP.
- The use of IPv6 encapsulation (and the Routing header) removes
the need in Mobile IPv6 to manage "tunnel soft state", which was
required in Mobile IPv4 due to limitations in ICMP for IPv4. Due
to the definition of ICMP for IPv6, the use of tunnel soft state
is no longer required in IPv6 for correctly relaying ICMP error
messages from within the tunnel back to the original sender of
the packet.
- The dynamic home agent address discovery mechanism in Mobile IPv6
uses IPv6 anycast [11] and returns a single reply to the mobile
node, rather than the corresponding Mobile IPv4 mechanism that
uses IPv4 directed broadcast and returns a separate reply from
each home agent on the mobile node's home link. The Mobile IPv6
mechanism is more efficient and more reliable, since only one
packet has to be sent back to the mobile node.
- Mobile IPv6 defines an Advertisement Interval option for
Router Advertisements (equivalent to Agent Advertisements in
Mobile IPv4), allowing a mobile node to decide for itself how
many Router Advertisements (Agent Advertisements) it is willing
to miss before declaring its current router unreachable.
- The return routability procedure (see section 5.5) provides a
way to verify the that a mobile node is reachable at its claimed
home address and at its claimed care-of address. This allows
correspondent nodes to verify the authority of the Binding
Updates sent to it. Given that the return routability procedure
is light-weight and does not require participation in a security
infrastructure, it is expected that Route Optimization can
be deployed on a global scale between all mobile nodes and
correspondent nodes.
3. Terminology
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
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3.1. General Terms
IP
Internet Protocol Version 6 (IPv6).
node
A device that implements IP.
router
A node that forwards IP packets not explicitly addressed to
itself.
host
Any node that is not a router.
link
A communication facility or medium over which nodes can
communicate at the link layer, such as an Ethernet (simple or
bridged). A link is the layer immediately below IP.
interface
A node's attachment to a link.
subnet prefix
A bit string that consists of some number of initial bits of an
IP address.
interface identifier
A number used to identify a node's interface on a link. The
interface identifier is the remaining low-order bits in the
node's IP address after the subnet prefix.
link-layer address
A link-layer identifier for an interface, such as IEEE 802
addresses on Ethernet links.
packet
An IP header plus payload.
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security association
A security object shared between two nodes which includes the
data mutually agreed on for operation of some cryptographic
algorithm (typically including a key).
security policy database
A database of rules that describe what security associations
should be applied for different kinds of packets.
destination option
Destination options are carried by the IPv6 Destination Options
extension header. Mobile IPv6 defines one new destination
option, the Home Address destination option.
3.2. Mobile IPv6 Terms
home address
An IP address assigned to a mobile node, used as the permanent
address of the mobile node. This address is within the mobile
node's home link. Standard IP routing mechanisms will deliver
packets destined for a mobile node's home address to its home
link.
home subnet prefix
The IP subnet prefix corresponding to a mobile node's home
address.
home link
The link on which a mobile node's home subnet prefix is
defined.
mobile node
A node that can change its point of attachment from one link to
another, while still being reachable via its home address.
movement
A change in a mobile node's point of attachment to the Internet
such that it is no longer connected to the same link as it was
previously. If a mobile node is not currently attached to its
home link, the mobile node is said to be "away from home".
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correspondent node
A peer node with which a mobile node is communicating. The
correspondent node may be either mobile or stationary.
foreign subnet prefix
Any IP subnet prefix other than the mobile node's home subnet
prefix.
foreign link
Any link other than the mobile node's home link.
care-of address
An IP address associated with a mobile node while visiting a
foreign link; the subnet prefix of this IP address is a foreign
subnet prefix. Among the multiple care-of addresses that a
mobile node may have at any given time (e.g., with different
subnet prefixes), the one registered with the mobile node's
home agent is called its "primary" care-of address.
home agent
A router on a mobile node's home link with which the mobile
node has registered its current care-of address. While the
mobile node is away from home, the home agent intercepts
packets on the home link destined to the mobile node's home
address, encapsulates them, and tunnels them to the mobile
node's registered 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.
binding procedure
A binding procedure is initiated by the mobile node to inform
either a correspondent node or the mobile node's home agent of
the current binding of the mobile node.
binding authorization
Binding procedure needs to be authorized to allow the recipient
to believe that the sender has the right to specify a new
binding.
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return routability procedure
The return routability procedure authorizes binding procedures
by the use of a cryptographic cookie exchange.
correspondent binding procedure
A return routability procedure followed by a binding procedure,
run between the mobile node and a correspondent node.
home binding procedure
A binding procedure between the mobile node and its home agent,
authorized by the use of IPsec.
nonce
Nonces are random numbers used internally by the correspondent
node in the creation of cookies related to the return
routability procedure. The nonces are not specific to a mobile
node, and are kept secret within the correspondent node, only
used as one input in the creation of the cookies.
cookie
Cookies are numbers that are used by mobile nodes in the return
routability procedure.
care-of cookie
A cookie sent directly to the mobile node's claimed care-of
address from the correspondent node.
home cookie
A cookie sent to the mobile node's claimed home address from
the correspondent node.
mobile cookie
A cookie sent to the correspondent node from the mobile node,
and later returned to the mobile node. Mobile cookies are
produced randomly.
nonce index
The mobile node uses a particular set of cookies in the return
routability procedure. The cookies have been produced using a
particular set of nonces. A nonce index is used to indicate
which nonces have been used, without revealing the nonces
themselves.
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binding key
a key used for authenticating binding cache management
messages.
binding security association
a security association established specifically for the purpose
of producing and verifying authentication data passed with a
Binding Authorization Data option.
4. Overview of Mobile IPv6
4.1. Basic Operation
A mobile node is always addressable at its home address, whether it
is currently attached to its home link or is away from home. While
a mobile node is at home, packets addressed to its home address are
routed to it using conventional Internet routing mechanisms in the
same way as if the node were stationary. Since the subnet prefix of
a mobile node's home address is one of the subnet prefixes of the
mobile node's home link, packets addressed to the mobile node will be
routed to its home link.
While a mobile node is attached to some foreign link away from home,
it is also addressable at one or more care-of addresses, in addition
to its home address. A care-of address is an IP address associated
with a mobile node while visiting a particular foreign link. The
subnet prefix of a mobile node's care-of address is one of the subnet
prefixes on the foreign link being visited by the mobile node; if
the mobile node is connected to this foreign link while using that
care-of address, packets addressed to this care-of address will be
routed to the mobile node in its location away from home.
The association between a mobile node's home address and care-of
address is known as a "binding" for the mobile node. A mobile node
typically acquires its care-of address through stateless [33] or
stateful (e.g., DHCPv6 [2]) Address Autoconfiguration, according
to the methods of IPv6 Neighbor Discovery [20]. Other methods
of acquiring a care-of address are also possible, such as static
pre-assignment by the owner or manager of a particular foreign
link, but details of such other methods are beyond the scope of
this document. The operation of the mobile node is specified in
Section 11.
While away from home, a mobile node registers one of its care-of
addresses with a router on its home link, requesting this router to
function as the "home agent" for the mobile node. The mobile node
performs this binding registration by sending a "Binding Update"
message to the home agent; the home agent then replies to the mobile
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node by returning a "Binding Acknowledgement" message. The care-of
address associated with this binding registration is known as the
mobile node's "primary care-of address". The mobile node's home
agent thereafter uses proxy Neighbor Discovery to intercept any
IPv6 packets addressed to the mobile node's home address (or home
addresses) on the home link, and tunnels each intercepted packet
to the mobile node's primary care-of address. To tunnel each
intercepted packet, the home agent encapsulates the packet using IPv6
encapsulation [4], with the outer IPv6 header addressed to the mobile
node's primary care-of address. The operation of the home agent is
specified in Section 10.
The Binding Update and Binding Acknowledgement messages, together
with a "Binding Refresh Request" message, are also used to allow IPv6
nodes communicating with a mobile node are capable of dynamically
learning and caching the mobile node's binding. This happens
through the correspondent binding procedure which involves a return
routability test in order to authorize the establishment of the
binding, as specified in Sections 5.5.5 and 5.5.6. 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 a new
type of IPv6 Routing header [6] (see section 6.4) to route the packet
to the mobile node by way of the care-of address indicated in this
binding. If, instead, the sending node has no cached binding for
this destination address, the node sends the packet normally (with
no Routing header), and the packet is subsequently intercepted and
tunneled by the mobile node's home agent as described above. Any
node communicating with a mobile node is referred to in this document
as a "correspondent node" of the mobile node, and may itself be
either a stationary node or a mobile node. The operation of the
correspondent node is specified in Section 9.
Mobile IPv6 also defines one additional IPv6 destination option.
When a mobile node sends a packet while away from home, it could
generally use a tunnel via the home agent to send this packet.
However, if the correspondent node in question has a binding for this
mobile node it can use deliver packets more directly. In this case
the mobile node can the Source Address in the packet's IPv6 header to
one of its current care-of addresses, and include a "Home Address"
destination option in the packet, giving the mobile node's home
address. Many routers implement security policies such as "ingress
filtering" [7] that do not allow forwarding of packets having a
Source Address that appears topologically incorrect. By using the
care-of address as the IPv6 header Source Address, the packet will
be able to pass normally through such routers, and ingress filtering
rules will still be able to locate the true topological source of
the packet in the same way as packets from non-mobile nodes. By
also including the Home Address destination option in each packet,
the sending mobile node can communicate its home address to the
correspondent node receiving this packet, allowing the use of the
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care-of address to be transparent above the Mobile IPv6 support level
(e.g., at the transport layer). The inclusion of a Home Address
destination option in a packet affects only the correspondent node's
receipt of this single packet; no state is created or modified in
the correspondent node as a result of receiving a Home Address
destination option in a packet.
It is possible that while a mobile node is away from home, some nodes
on its home link may be reconfigured, such that the router that was
operating as the mobile node's home agent is replaced by a different
router serving this role. In this case, the mobile node may not
know the IP address of its own home agent. Mobile IPv6 provides a
mechanism, known as "dynamic home agent address discovery", that
allows a mobile node to dynamically discover the IP address of a
home agent on its home link with which it may register its (primary)
care-of address while away from home. The mobile node sends an ICMP
"Home Agent Address Discovery Request" message to the "Mobile IPv6
Home-Agents" anycast address for its own home subnet prefix [11] and
thus reaches one of the routers on its home link currently operating
as a home agent. This home agent then returns an ICMP "Home Agent
Address Discovery Reply" message to the mobile node, including a list
of home agents on the home link. This procedure is specified in
Sections 10.9 and 11.3.2.
When a mobile node moves from one care-of address to a new care-of
address on a new link, it is desirable for packets arriving at
the previous care-of address to be tunneled to the mobile node's
new care-of address. Since the purpose of a Binding Update is
to establish exactly this kind of tunneling, it can be used (at
least temporarily) for tunnels originating at the mobile node's
previous care-of address, in exactly the same way that it is used
for establishing tunnels from the mobile node's home address to the
mobile node's current care-of address. Section 11.6.6 describes the
use of the Binding Update for this purpose.
Section 11.4.3 discusses the reasons why it may be desirable for a
mobile node to use more than one care-of address at the same time.
However, a mobile node's primary care-of address is distinct among
these in that the home agent maintains only a single care-of address
registered for each home address belonging to a mobile node, and
always tunnels packets sent to a mobile node's home address and
intercepted from its home link to this mobile node's registered
primary care-of address. The home agent thus need not implement any
policy to determine the particular care-of address to which it will
tunnel each intercepted packet. The mobile node alone controls the
policy by which it selects the care-of addresses to register with its
home agent.
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4.2. New IPv6 Protocols
Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header
(see Section 6.1). This Header is used to carry the following
messages:
Home Test Init
The Home Test Init message is used to initiate the return
routability procedure from the mobile node to a correspondent
node. This procedure ensures that subsequent Binding Updates
are properly authorized to redirect the traffic of a particular
home address. The Home Test Init message is described in
detail in Section 6.1.3.
Care-of Test Init
The Care-of Test Init message is used to initiate the
correspondent routability procedure, for a particular care-of
address. The Care-of Test Init message is described in detail
in Section 6.1.4.
Home Test
The Home Test message carries a cookie which the mobile node
needs before it can properly authorize itself for sending a
Binding Update. This message is sent in reply to the Home Test
Init message, and is described in detail in Section 6.1.5.
Care-of Test
The Care-of Test message carries another cookie which the
mobile node needs before it can properly authorize itself for
sending a Binding Update. This message is sent in reply to
the Care-of Test Init message, and is described in detail in
Section 6.1.6.
Binding Update
A Binding Update message is used by a mobile node to notify
a correspondent node or the mobile node's home agent of its
current binding. The Binding Update sent to the mobile node's
home agent to register its primary care-of address is marked
as a "home registration". The Binding Update message and its
specific authentication requirements are described in detail in
Section 6.1.7.
Binding Acknowledgement
A Binding Acknowledgement message is used to acknowledge
receipt of a Binding Update, if an acknowledgement was
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requested in the Binding Update. The Binding Acknowledgement
message and its specific authentication requirements are
described in detail in Section 6.1.8.
Binding Refresh Request
A Binding Refresh Request message is used to request that
a mobile node send to the requesting node a Binding Update
containing the mobile node's current binding. This message
is typically used by a correspondent node to refresh a cached
binding for a mobile node, when the cached binding is in active
use but the binding's lifetime is close to expiration. The
Binding Refresh Request message is described in detail in
Section 6.1.2.
No authentication is required for the Binding Refresh Request
message.
Binding Error
The Binding Error message is used by the correspondent node to
signal an error related to mobility, such as an inappropriate
attempt to use the Home Address destination option without
an existing binding. This message is described in detail in
Section 6.1.9.
4.3. New IPv6 Destination Options
Mobile IPv6 defines a new IPv6 destination option, the Home Address
destination option. This option is used in a packet sent by a mobile
node to inform the recipient of that packet of the mobile node's home
address. For packets sent by a mobile node while away from home,
the mobile node generally uses one of its care-of addresses as the
Source Address in the packet's IPv6 header. By including a Home
Address option in the packet, the correspondent node receiving the
packet is able to substitute the mobile node's home address for this
care-of address when processing the packet, thus making the use of
the care-of address transparent to the correspondent node above the
Mobile IPv6 support level. If the IP header of a packet carrying
a Home Address option is covered by authentication, then the Home
Address option MUST also be covered by this authentication, but no
other authentication is required for the Home Address option. See
Sections 6.3 and 11.2.2 for additional details about requirements
for the calculation and verification of the authentication data.
The Home Address destination option is described in detail in
Section 6.3.
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4.4. New IPv6 ICMP Messages
Mobile IPv6 also introduces four new ICMP message types, two for use
in the dynamic home agent address discovery mechanism, and two for
renumbering and mobile configuration mechanisms. As discussed in
general in Section 4.1, the following two new ICMP message types are
used for home agent address discovery:
Home Agent Address Discovery Request
The ICMP Home Agent Address Discovery Request message is used
by a mobile node to initiate the dynamic home agent address
discovery mechanism. When attempting a home registration, the
mobile node may use this mechanism to discover the address of
one or more routers currently operating as home agents on its
home link, with which it may register while away from home.
The Home Agent Address Discovery Request message is described
in detail in Section 6.5.
Home Agent Address Discovery Reply
The ICMP Home Agent Address Discovery Reply message is used by
a home agent to respond to a mobile node using the dynamic home
agent address discovery mechanism. When a home agent receives
a Home Agent Address Discovery Request message, it replies with
a Home Agent Address Discovery Reply message, giving a list
of the routers on the mobile node's home link serving as home
agents. The Home Agent Address Discovery Reply message is
described in detail in Section 6.6.
The next two message types are used for network renumbering
and address configuration on the mobile node, as described in
Section 10.9.1:
Mobile Prefix Solicitation
The ICMP Mobile Prefix Solicitation message is used by a mobile
node to request prefix information about the home subnet, in
order to retrieve prefixes that are served by home agents and
can be used to configure one or more home addresses, or to
refresh home addresses before the expiration of their validity.
This message is specified in Section 6.7.
Mobile Prefix Advertisement
The ICMP Mobile Prefix Advertisement is used by a home agent
to distribute information to a mobile node about prefixes on
the home link which are available for use by the mobile node
while away from home. This message may be sent as a response
to a Mobile Prefix Solicitation, or due to network renumbering
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or other prefix changes. This message is specified in Section
Section 10.9.3.
4.5. Conceptual Data Structures
This document describes the Mobile IPv6 protocol in terms of the
following three conceptual data structures:
Binding Cache
A cache, maintained by each IPv6 node, of bindings for other
nodes. A separate Binding Cache is maintained by each IPv6
node for each of its IPv6 addresses. When sending a packet,
the Binding Cache is searched before the Neighbor Discovery
conceptual Destination Cache [20].
The Binding Cache for any one of a node's IPv6 addresses may
contain at most one entry for each mobile node home address.
The contents of all of a node's Binding Cache entries are
cleared when it reboots.
Binding Cache entries are marked either as "home registration"
entries or "correspondent registration" entries. Home
registration entries are deleted when its binding lifetime
expires, while other entries may be replaced at any time
through a local cache replacement policy.
Binding Update List
A list, maintained by each mobile node, recording information
for each Binding Update sent by this mobile node, for which the
Lifetime sent in that Binding Update has not yet expired. The
Binding Update List includes all bindings sent by the mobile
node: those to correspondent nodes, those to the mobile node's
home agent, and those to a home agent on the link on which the
mobile node's previous care-of address is located.
Home Agents List
A list, maintained by each home agent and each mobile node,
recording information about each home agent from which this
node has received recent a Router Advertisement in which the
Home Agent (H) bit is set. The home agents list is thus
similar to the Default Router List conceptual data structure
maintained by each host for Neighbor Discovery [20].
Each home agent maintains a separate Home Agents List for each
link on which it is serving as a home agent; this list is used
by a home agent in the dynamic home agent address discovery
mechanism. Each mobile node, while away from home, also
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maintains a Home Agents List, to enable it to notify a home
agent on its previous link when it moves to a new link.
4.6. Binding Management
When a mobile node configures a new care-of address and decides to
use this new address as its primary care-of address, the mobile
node registers this new binding with its home agent by sending
the home agent a Binding Update. The mobile node indicates
that an acknowledgement is needed for this Binding Update and
continues to periodically retransmit it until acknowledged. The
home agent acknowledges the Binding Update by returning a Binding
Acknowledgement to the mobile node.
When a mobile node receives a packet tunneled to it from its home
agent, the mobile node uses that as an indication 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 SHOULD then start a correspondent binding procedure in
order to establish a binding. This would allow the correspondent
node to cache the mobile node's binding for routing future packets to
it.
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 Refresh 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 Refresh Request, it MAY reply
by initiating a correspondent binding procedure.
A mobile node may use more than one care-of address at the same
time. Use of more than one care-of address by a mobile node may be
useful, for example, to improve smooth handover when the mobile node
moves from one wireless link to another. If each of these wireless
links is connected to the Internet through a separate base station,
such that the wireless transmission range from the two base stations
overlap, the mobile node may be able to remain connected to both
links while in the area of overlap. In this case, the mobile node
could acquire a new care-of address on the new link before moving
out of transmission range and disconnecting from the old link. The
mobile node may thus still accept packets at its old care-of address
while it works to update its home agent and correspondent nodes,
notifying them of its new care-of address on the new link.
Since correspondent nodes cache bindings, it is expected that
correspondent nodes usually will route packets directly to the mobile
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node's care-of address, so that the home agent is rarely involved
with packet transmission to the mobile node. This is important for
scalability and reliability, and for minimizing overall network load.
By caching the care-of address of a mobile node, direct delivery of
packets can be achieved from the correspondent node to the mobile
node. Routing packets directly to the mobile node's care-of address
also eliminates congestion at the mobile node's home agent and home
link. In addition, the impact of any possible failure of the home
agent, the home link, or intervening networks leading to or from the
home link is reduced, since these nodes and links are not involved in
the delivery of most packets to the mobile node.
5. Overview of Mobile IPv6 Security
5.1. Threats
Any mobility solution must protect itself against misuses of the
mobility features. In Mobile IPv6, most of the potential threats
are concerned with denial of service. Some of the threats also
include potential for man-in-the-middle, hijacking, and impersonation
attacks. The main threats this protocol protects against are as
follows:
1. Threats against Binding Updates sent to home agents and
correspondent nodes. For instance, an attacker might claim that
a certain mobile node is currently at a different location than
it really is. If the home agent accepts the information sent to
it as is, the mobile node might not get traffic destined to it,
and other nodes might get traffic they did not want.
Similarly, a malicious mobile node might use the home address of
a victim node in a forged Binding Update to a correspondent node.
If such Binding Updates were accepted, the communications between
the correspondent node and the victim would be then be disrupted,
because packets that the correspondent node intended to send to
the victim would be sent to the wrong care-of address. This is
a threat to confidentiality as well as availability, because an
attacker might redirect packets meant for another node to itself
in order to learn the content of those packets.
A malicious mobile node might also send Binding Updates in
which the care-of address is set to the address of a victim
node or an address within a victim network. If such Binding
Updates were accepted, the malicious mobile node could force the
correspondent node into sending data to the victim node or the
victim network; the correspondent node's replies to messages sent
by the malicious mobile node will be sent to the victim host
or network. This could be used to cause a distributed denial
of service attack. Variations of this threat are described
elsewhere [1][31].
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A malicious node might also send a large number of invalid
Binding Updates to a victim node. If each Binding Update takes a
significant amount of resources (such as CPU) to process before
it can be recognized either as valid or as invalid, then a denial
of service attack can be caused by sending the correspondent node
so many invalid Binding Updates that it has no resources left for
other tasks.
An attacker might also attempt to disrupt a mobile node's
communications by replaying a Binding Update that the node had
sent earlier. If the old Binding Update was accepted, packets
destined for the mobile node would be sent to its old location
and not its current location.
2. Reflection attack threats against third partied with the help
of Mobile IPv6 correspondent nodes that do not use appropriate
security precautions. The Home Address destination option can be
used to direct response traffic toward a node whose IP address
appears in the option, without allowing ingress filtering to
catch the forged "return address" [32] [23].
3. Threats where an attacker forges tunneled packets between the
mobile node and the home agent, making it appear that the traffic
is coming from the mobile node when it is not.
4. Threats against IPv6 functionality used by Mobile IPv6, such as
the Routing header. The generality of the regular Routing Header
would allow circumvention of IP-address based rules in firewalls
or reflection of traffic to other nodes, even if the usage that
Mobile IPv6 requires is safe.
5. The security mechanisms of Mobile IPv6 may also be attacked
themselves, e.g. in order to force the participants to execute
expensive cryptographic operations or allocate memory for the
purpose of keeping state.
5.2. Features
This specification provides a number of security features. The main
features are:
- Protection of Binding Updates to home agents.
- Protection of Binding Updates to correspondent nodes.
- Protection against reflection attacks through the Home Address
destination option.
- Protection of tunnels between the mobile node and the home agent.
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- Preventing Routing Header vulnerabilities.
- Preventing Denial-of-Service attacks to the Mobile IPv6 security
mechanisms themselves.
Protecting the Binding Updates to home agents and to arbitrary
correspondent nodes require very different security solutions due
to the different situations. Mobile nodes and home agents are
expected to be naturally subject to the network administration of
the home domain, and thus to have a strong security association to
reliably authenticate the exchanged messages. With such a security
arrangement, IPsec Encapsulating Security Payload (ESP) can be used
to implement the necessary security features. See Section 5.4.
It is expected that Mobile IPv6 will be used on a global basis
between nodes belonging to different administrative domains.
Building an authentication infrastructure to authenticate mobile
nodes and correspondent nodes would be a very demanding task in this
scale. Furthermore, traditional authentication infrastructure keep
track of correct IP addresses for all hosts is either impossible or
at least very hard. That is, it isn't sufficient to authenticate
mobile nodes, authorization to claim right to use an address is
needed. Thus, an "infrastructureless" approach is necessary.
The chosen infrastructureless method verifies that the mobile
node is "live" (that is, it responds to probes) at its home and
care-of addresses by performing a cookie exchange with the nodes
in question, and by requiring that the eventual Binding Update is
cryptographically bound to the exchanged cookies. Some additional
protection is provided by requiring the cookies be protected by
ESP when exchanged between the mobile node and the correspondent
node via the home agent. This method limits the vulnerabilities to
those attackers who are on the path between the home agent and the
correspondent node. As adversaries on this path would be able to
cause also other types of attacks, this is seen as sufficient base
security between mobile and correspondent nodes.
Vulnerabilities relating to the use of correspondent nodes as
reflectors via the Home Address destination option can be solved as
follows: We ensure that the mobile node is authorized to use a given
home address before this option can be used. Such authorization is
already performed in the context of Route Optimization, and therefore
this specification limits the use of the Home Address option to the
situation where the correspondent node already has a binding cache
entry for the given home address.
Tunnels between the mobile node and the home agent can be
protected by ensuring proper use of source addresses, and optional
cryptographic protection. These procedures are discussed in
Section 5.3.
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Potential abuses of the Routing Header can be prevented by using a
Mobile IPv6 specific type of a Routing Header. This type provides
the necessary functionality but does not open vulnerabilities.
Denial-of-Service threats against Mobile IPv6 security mechanisms
themselves concern mainly the Binding Update procedures with
correspondent nodes. The protocol has been designed to limit the
effects of such attacks, as will be described in Section 5.5.9.
5.3. Tunnels to and from the Home Agents
Mobile IPv6 tunneling -- as tunneling in general -- needs protection
so that it isn't possible, e.g., for anyone to pose as the home agent
and send traffic to the mobile node. To protect the tunnels to the
mobile node, the mobile node verifies that the outer IP address
corresponds to its home agent, to prevent attacks against the tunnel
from other IP addresses.
Tunnels from the mobile node to the home agent need protection
so that it isn't possible for anyone to send traffic through the
home agent, pose as the mobile node, and escape detection through
traditional tracing mechanisms.
Binding Updates sent to the home agents are secure. The home
agent verifies that the outer IP address corresponds to the current
location of the mobile node, to prevent attacks against the tunnel
from other IP addresses.
For tunneled traffic to and from the mobile node, encapsulating the
traffic inside IPsec ESP offers an optional mechanism to protect
the confidentiality and integrity of the traffic against on-path
attackers.
5.4. Binding Updates to Home Agents
Signaling between the mobile node and the home agent requires message
integrity, correct ordering and replay protection.
In order to have this protection, the mobile node and the home agent
must have a security association. IPsec Encapsulating Security
Payload (ESP) can be used for integrity protection when a non-null
authentication algorithm is applied.
However, IPsec can easily provide replay protection only if dynamic
security association establishment is used. This may not always be
possible, and manual keying would be preferred in some cases. IPsec
also does not guarantee correct ordering of packets, only that they
have not been replayed. Because of this, Mobile IPv6 provides its
own mechanism inside the Binding Update and Acknowledgement messages.
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A sequence number field is used to ensure correct ordering. If the
mobile node reboots and forgets its current sequence number, the home
agent uses the status value 141 (Sequence number out of window, see
Section 6.1.8) to inform the mobile node of the use of an improper
sequence number.
Note that the the sequence number mechanism provides also a weak form
of replay protection. However, if a home agent reboots and loses its
state regarding the sequence numbers, replay attacks become possible.
If the home agent is vulnerable to this, the use of a key management
mechanism together with IPsec can be used to prevent replay attacks.
A sliding window scheme is used for the sequence numbers. The
protection against replays and reordering attacks without a key
management mechanism works when the attacker remembers up to a
maximum of 2**15 Binding Updates.
In order to protect messages exchanged between the mobile node and
the home agent with IPsec, appropriate security policy database
entries must be created. We need to avoid the possibility that a
mobile node could use its security association to send a Binding
Update on behalf of another mobile node using the same home agent.
In order to do this, the security policy database entries MUST
unequivocally identify a single SA for any given home address and
home agent. In order for the home address of the mobile node to be
visible when the policy check is made, the mobile node MUST use the
Home Address destination option in Binding Updates sent to the home
agent. The home address in the Home Address destination option and
the Binding Update message MUST be equal and MUST be checked by the
home agent.
5.5. Binding Updates to Correspondent Nodes
Binding Updates to correspondent nodes are protected using the return
routability procedure. The motivation for designing the return
routability procedure was to have sufficient support for Mobile IP,
without creating major new security problems. It was not our goal
to protect against attacks that were already possible before the
introduction of Mobile IP. This protocol does not defend against
an attacker who can monitor the home agent to correspondent node
path, as such attackers would in any case be able to mount an active
attack against the mobile node when it is at its home location. The
possibility of such attacks is not an impediment to the deployment of
Mobile IP, because these attacks are possible regardless of whether
Mobile IP is in use.
This protocol also protects against denial of service attacks in
which the attacker pretends to be a mobile, but uses the victim's
address as the care of address, and so causes the correspondent node
to send the victim traffic that it does not expect. For example,
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suppose that the correspondent node is a news site that will send a
high-bandwidth stream of video to anyone who asks for it. Note that
the use of flow-control protocols such as TCP does not necessarily
defend against this type of attack, because the attacker can fake the
acknowledgements. Even keeping TCP initial sequence numbers secret
doesn't help, because the attacker can receive the first few segments
(including the ISN) at its own address, and then redirect the stream
to the victim's address. This protocol defends against these attacks
by only completing if packets sent by the correspondent node to the
care of address are received and processed by an entity that is
willing to participate in the protocol. Normally, this will be the
mobile node.
For further information about the design rationale of the return
routability procedure, see [1] [31] [22] [23].
The return routability procedure method uses the following
principles:
- A cookie exchange verifies that the mobile node is reachable at
its addresses i.e. is at least able to transmit and receive
traffic at its addresses.
- The eventual Binding Update is protected cryptographically using
the cookies.
- Requiring that the cookies be protected by ESP when forwarded by
the home agent to the mobile node.
- The use of symmetric exchanges where responses are sent to the
same address as the request was sent from, to avoid the use of
this protocol in reflection attacks.
- Correspondent nodes operate in a stateless manner until they
receive a Binding Update that can be authorized.
The return routability procedure can be broken by an attacker on the
route between the home agent and the correspondent node, but not by
attackers on the network the mobile node is currently at and not from
elsewhere on the Internet.
5.5.1. Node Keys
Each correspondent node has a secret key, Kcn. This key is used by
the correspondent node to accept only the use of cookies which it has
created itself. This key does not need to be shared with any other
entity, so no key distribution mechanism is needed for it.
A correspondent node can generate a fresh Kcn each time that it boots
to avoid the need for secure persistent storage for Kcn. Kcn can be
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either a fixed value or regularly updated. Procedures for updating
Kcn are discussed later in Section 5.5.7.
Kcn consists of 20 octets.
5.5.2. Nonces
Each correspondent node also generates a nonce at regular intervals,
for example every few minutes. A correspondent node uses the same
Kcn and nonce with all the mobiles it is in communication with, so
that it does not need to generate and store a new nonce when a new
mobile contacts it. Each nonce is identified by a nonce index.
Nonce indices are 16-bit values that are e.g. incremented each time
a new nonce is created. The index value is communicated in the
protocol, so that if a nonce is replaced by new nonce during the run
of a protocol, the correspondent node can distinguish messages that
should be checked against the old nonce from messages that should be
checked against the new nonce. Correspondent nodes keep both the
current nonce and a small set of old nonces. Older values can be
discarded, and messages using them will be rejected as replays.
The specific nonce index values can not be used by mobile nodes to
determine the validity of the nonce. Expected validity times for
the nonces values and the procedures for updating them are discussed
later in Section 5.5.7.
Nonce is an octet string of any length. The recommended length is 16
octets.
5.5.3. Cookies
Three different types of cookies are used in the protocol:
- Mobile cookie is sent to the correspondent node from the mobile
node, and later returned to the mobile node. Mobile cookies are
produced randomly, and used to verify that the response matches
the request, and to ensure that parties who have not seen the
request can not spoof responses.
- A home cookie sent to the mobile node from the correspondent node
via the home agent. Home cookies are produced cryptographically
from nonces.
- A care-of cookie sent directly to the mobile node from the
correspondent node. Home cookies are produced cryptographically
from nonces.
Mobile cookies are typically newly generated random values for each
new request that needs them. They could also be changed periodically
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only. The policy to use new or old mobile cookies is purely a local
matter for the mobile node.
Home and care-of cookies are produced by the correspondent node, and
they are based on the currently active secret keys and nonces of the
correspondent node as well as the home or care-of address. Such a
cookie is valid as long as both the secret key and the nonce used to
create it are valid.
5.5.4. Cryptographic Functions
MAC_K(m) denotes a Message Authentication Code computed on message
m with key K. In this specification, HMAC SHA1 function [15][21] is
used to compute these codes.
H(m) denotes a hash of message m. In this specification, SHA1
function [21] is used to compute the hash.
5.5.5. Return Routability Procedure
The return routability signaling happens as follows:
Mobile node Home agent Correspondent node
| |
| Home Test Init(HoTI) |
| Src = home address, |
| Dst = correspondent | |
| Parameters: | |
| - mobile cookie 1 | |
|------------------------->|------------------------->|
| | |
| |
| Care-of Test Init(CoTI) |
| Src = care-of address |
| Dst = correspondent |
| Parameters: |
| - mobile cookie 2 |
|---------------------------------------------------->|
| |
| Home Test (HoT) |
| Src = correspondent, |
| Dst = home address |
| Parameters: |
| - mobile cookie 1 |
| | - home cookie |
| | - home nonce index |
|<-------------------------|<-------------------------|
| | |
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| |
| Care-of Test(CoT) |
| Src = correspondent, |
| Dst = care-of address |
| Parameters: |
| - mobile cookie 2 |
| - care-of cookie |
| - care-of nonce index |
|<----------------------------------------------------|
| |
The HoTI and CoTI messages are sent at the same time. The
correspondent node returns the HoT and CoT messages as quickly as
possible, and perhaps nearly simultaneously, requiring very little
processing. The four messages form the return routability procedure.
(After the return routability procedure, a binding will be created
with a single request with an optional response.) Due to the
simultaneous sending of messages, the return routability procedure
completes in 1 roundtrip (and the whole process completes in 1.5
roundtrips excluding the acknowledgement message).
The four messages (HoTI, CoTI, HoT, and CoT) belonging to the return
routability procedure are described in more detail below. The use of
the results of the return routability procedure for authenticating a
correspondent binding procedure is described in Section 5.5.6.
HoTI
Home Test Init Message:
When a mobile nodes wants to perform route optimization it
sends a HoTI message to the correspondent node in order to
initiate the return routability verification for the Home
Address.
Src = home address
Dst = correspondent
Parameters:
- mobile cookie 1
This message conveys the mobile node's home address to the
correspondent node. The mobile node also sends along mobile
cookie C0 that the correspondent node must return later,
along with its own cookie that it generates based on the home
address. The HoTI message is reverse tunneled through the home
agent.
CoTI
Care-of Test Init Message:
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When a mobile nodes wants to perform route optimization it
sends a CoTI message to the correspondent node in order to
initiate the return routability verification for the care-of
Address.
Src = care-of address
Dst = correspondent
Parameters:
- mobile cookie 2
The second message is sent in parallel with the first one. It
conveys the mobile node's care-of address to the correspondent
node. The mobile node also sends along mobile cookie C1 that
the correspondent node must return later, along with its own
cookie that it generates based on the care-of address. The
CoTI message is sent directly to the correspondent node.
HoT
Home Test Message:
This message is sent in response to a HoTI message.
Src = correspondent
Dst = home address
Parameters:
- mobile cookie 1
- home cookie
- home nonce index
When the correspondent node receives the HoTI message, it
generates a 16 octet home cookie as follows:
home cookie = MAC_Kcn(home address | nonce)
The cookie is sent in the message to the mobile node via the
Home Agent; it is an assumption of the protocol that the home
agent - mobile node route is secure. Home cookie also acts as
a challenge to test that the mobile can receive messages sent
to its home address. Kcn is used in the production of home
cookie in order to allow the correspondent node to verify that
the cookies used later really came from itself, without forcing
the correspondent node to remember a list of all cookies it has
handed out.
Mobile cookie 1 from the mobile node is returned as well in the
HoT message, to ensure that the message comes from someone on
the path to the correspondent node.
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The home nonce index is carried along in the protocol to allow
the correspondent node to later efficiently find the nonce
value Ni that it used in creating this cookie.
CoT
Care-of Test Message:
This message is sent in response to a CoTI message.
Src = correspondent
Dst = care-of address
Parameters:
- mobile cookie 2
- care-of cookie
- care-of nonce index
The correspondent node also sends a challenge to the mobile's
care-of address. When the correspondent node receives the CoTI
message, it generates a 16 octet care-of cookie as follows:
care-of cookie = MAC_Kcn(care-of address | nonce)
The cookie is sent directly to the mobile node at its care-of
address. Mobile cookie 2 from the mobile node is returned as
well, to ensure that the message comes from someone on the path
to the correspondent node.
Again, an index is sent along the cookie in order to identify
the used nonce. Note that home and care-of nonce indices are
likely to be the same in HoT and CoT messages, except when
the correspondent node changed its nonce value between the
reception of HoTI and the CoTI messages.
When the mobile node has received both the HoT and CoT messages, the
return routability procedure is complete. As a result, the mobile
node has the means to prove its authority to send a Binding Update
to the correspondent node. The mobile node hashes together the
challenges to form a 20 octet session key (Kbu):
Kbu = H(home cookie | care-of cookie)
Note that the correspondent node has not created any state at this
point. It is unaware of the session key Kbu, though it can recreate
Kbu if it is presented the right addresses and nonce indices.
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5.5.6. Applying Return Routability for Correspondent Bindings
After the return routability procedure, the mobile node can proceed
to perform a binding procedure with the correspondent node. An
overview of the binding procedure is shown below.
Mobile Node Correspondent node
| |
| 1. Binding Update |
| Src = care-of address, Dst = correspondent |
| Parameters: |
| - home address |
| - a MAC |
| - home nonce index |
| - care-of nonce index |
| - sequence number |
| - ... |
|---------------------------------------------------->|
| |
| 2. Binding Acknowledgement |
| (if requested) |
| Src = correspondent, |
| Dst = care-of address |
| Parameters: |
| - sequence number |
| - ... |
|<----------------------------------------------------|
| |
Message 1 actually creates a binding, and message 2 is optional. The
correspondent binding procedure consists of the return routability
procedure followed by the messages 1 and 2.
1.
Binding Update (BU) Message:
The mobile node uses the created session key Kbu to authorize
the Binding Update.
Src = care-of address
Dst = correspondent
Parameters:
- home address
- MAC_Kbu(care-of address | correspondent node address | BU)
- home nonce index
- care-of nonce index
- sequence number
- ...
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The message contains home and care-of nonce indices, so that
the correspondent node knows which nonces to use to recompute
the session key. "BU" is the content of the Binding Update
message, excluding (1) the IP header, (2) any extension
headers between the IP header the Mobility Header, and (3) the
Authenticator field inside the Binding Update. The result of
the MAC_Kbu function is used as the Authenticator field in
the Binding Update. A sequence number will be used to match
an eventual acknowledgement with this message. The sequence
numbers start from a random value, which offers a weak form
of authentication also to the acknowledgement messages. The
three dots represent all the remaining (not security related)
information in the message.
Once the correspondent node has verified the MAC, it can create
a binding cache entry for the mobile.
2.
Binding Acknowledgement (BA) Message:
The Binding Update is optionally acknowledged by the
correspondent node.
Src = correspondent
Dst = care-of address
Parameters:
- sequence number
- ...
The Binding Acknowledgement is not authenticated in other ways
than including the right sequence number in the reply. The
three dots represent all the remaining (not security related)
information in the message.
5.5.7. Updating Node Keys and Nonces
An update of Kcn can be done at the same time as an update of Ni, so
that i identifies both the nonce and the key. Old Kcn values have to
be therefore remembered as long as old nonce values.
Before sending a Binding Update in Step 3, the mobile node has
to wait for both the Home and Care-of Cookies to arrive. Due
to resource limitations, rapid deletion of bindings, or reboots
it can not be guaranteed that the cookies are still fresh and
acceptable when the correspondent node uses them in the processing
of the Binding Update. If the cookies have become too old, the
correspondent node replies with an an error code in the Binding
Acknowledgement. The mobile node can then retry the return
routability procedure. However, it is recommended that correspondent
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nodes try to keep these cookies acceptable as long as possible and
SHOULD NOT accept them beyond MAX_COOKIE_LIFE seconds.
Given that the cookies are normally expected to be usable for
some time, the mobile node MAY use them beyond a single run of the
return routability procedure. A fast moving mobile node may reuse
a recent Home Cookie from a correspondent node when moving to a new
location, and just acquire a new Care-of Cookie to show routability
in the new location. While this does not save roundtrips due to the
parallel nature of the home and care-of return routability tests, the
roundtrip through the home agent may be longer, and consequently this
optimization is often useful. A mobile node that has multiple home
addresses, may also use the same Care-of Cookie for Binding Updates
concerning all of these addresses.
5.5.8. Preventing Replay Attacks
The return routability procedure also protects the participants
against replayed Binding Updates. The attacker can't replay the
same message due to the sequence number which is a part of the
Binding Update, and the attacker can't modify the Binding Update
since the MAC would not verify after that. Care must be taken when
removing bindings at the correspondent node, however. If a binding
is removed either due to garbage collection, request, or expiration
and the nonce used in its creation is still valid, an attacker can
replay the old Binding Update. This can be prevented by having the
correspondent node change the nonce often enough to ensure that the
nonces used when removed entries were created are no longer valid.
If many such deletions occur the correspondent node can batch them
together to avoid having to increment the nonce index too often.
5.5.9. Preventing Denial-of-Service Attacks
The return routability procedure has been designed with protection
against resource exhaustion Denial-of-Service attacks. In these
attacks the victim has only a limited amount of some resource (such
as network bandwidth or CPU cycles), and the attack consumes some of
this resource. This leaves the victim without enough resources to
carry out other work.
The correspondent nodes do not have to retain any state about
individual mobile nodes until an authentic Binding Update arrives.
This is achieved through the use of the nonces and Kcn that are not
specific to individual mobile nodes. The cookies are specific, but
they can be reconstructed based on the home and care-of address
information that arrives with the Binding Update. This means that
the correspondent nodes are safe against memory exhaustion attacks
except where on-path attackers are concerned. Due to the use of
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symmetric cryptography, the correspondent nodes are relatively safe
against CPU resource exhaustion attacks as well.
Nevertheless, as [1] describes, there are situations in which it is
impossible for the mobile and correspondent nodes to determine if
they actually need a binding or whether they just have been fooled
into believing so by an attacker. Therefore, it is necessary to
consider situations where such attacks are being made.
The binding updates that are used in Mobile IPv6 are only an
optimization, albeit a very important optimization. A mobile node
can communicate with a correspondent node even if the correspondent
refuses to accept any of its binding updates. However, performance
will suffer because packets from the correspondent node to the mobile
node will be routed via the mobile's home agent rather than a more
direct route. A correspondent node can protect itself against some
of the resource exhaustion attacks by not processing binding updates
when it is flooded with a large number of binding updates that fail
the cryptographic integrity checks. If a correspondent node finds
that it is spending more resources on checking bogus binding updates
than it is likely to save by accepting genuine binding updates, then
it MAY reject some or all Binding Updates without performing any
cryptographic operations.
Additional information needed to make this decision about responding
to requests will usually originate in layers above IP. For example,
TCP knows if the node has a queue of data that it is trying to send
to a peer. A conformant implementation of the protocols in this
specification is not required to make use of information from higher
protocol layers, but implementations are likely to be able to manage
resources more effectively by making use of such information.
5.5.10. Correspondent Binding Procedure Extensibility
As discussed in Appendix D.3, in the future there may be other
mechanisms beyond the return routability procedure for authorizing
mobile nodes to correspondent nodes. The nodes can use other methods
based on future definition of flag values in the Reserved fields of
HoTI, HoT, CoTI, CoT, and BU messages. Nodes need assurance against
bidding down attacks in this selection by following the procedure
described in Section 14.3.
6. New IPv6 Protocols, Message Types, and Destination Option
6.1. Mobility Header
The Mobility Header is used by mobile nodes, correspondent nodes, and
home agents in all messaging related to the creation and management
of bindings. The Mobility Header is an IPv6 protocol. Rules
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regarding how it is sent and what addresses are used in the IPv6
header are given separately in Sections 6.1.2 through 6.1.9, which
describe the message types used in this protocol.
6.1.1. Format
The Mobility Header is identified by a Next Header value of 62 (XXX)
in the immediately preceding header, and has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Payload Proto | Header Len | MH Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. Message Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Payload Proto
8-bit selector. Identifies the type of header immediately
following the Mobility Header. Uses the same values as the
IPv4 Protocol field [10].
This field is intended to be used by a future specification
of piggybacking binding messages on payload packets (see
Section D.1).
Implementations conforming to this specification SHOULD set the
payload protocol type to NO_NXTHDR (59 decimal).
Header Len
8-bit unsigned integer. Length of the Mobility Header in units
of 8 octets, including the the Payload Proto, MH Type, Header
Len, Checksum, and Message Data fields.
MH Type
16-bit selector. Identifies the particular mobility message
in question. Current values are specified in Sections 6.1.2
to 6.1.9. An unrecognized MH Type field causes an error to be
sent to the source.
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Checksum
16-bit unsigned integer. This field contains the checksum
of the Mobility Header. The checksum is the 16-bit one's
complement of the one's complement sum of an octet string
consisting of a "pseudo-header" followed by the entire
Mobility Header starting with the Payload Proto field. The
pseudo-header contains IPv6 header fields, as specified
in Section 8.1 of [6]. The Next Header value used in the
pseudo-header is 62 (XXX). For computing the checksum, the
checksum field is set to zero.
Message Data
A variable length field containing the data specific to the
indicated Mobility Header type.
Mobile IPv6 also defines a number of "mobility options" for use
within these messages; if included, any options MUST appear after the
fixed portion of the message data specified in this document. The
presence of such options will be indicated by the Header Len field
within the message. When the Header Len is greater than the length
required for the message specified here, the remaining octets are
interpreted as mobility options options. The encoding and format of
defined options are described in Section 6.2.
Alignment requirements for the Mobility Header are same as for any
IPv6 protocol Header. That is, they MUST be aligned on an 8-octet
boundary. We also require that the Mobility Header length is a
multiple of 8 octets.
6.1.2. Binding Refresh Request (BRR) Message
The Binding Refresh Request (BRR) message is used to request a
mobile node's binding from the mobile node. A packet containing
a Binding Refresh Request message is sent in the same way as any
packet to a mobile node (Section 9.6). When a mobile node receives
a packet containing a Binding Refresh Request message and there
already exists a Binding Update List entry for the source of the
Binding Refresh Request, it MAY start a return routability procedure
(see Section 5.5) if it believes the amount of traffic with the
correspondent justifies the use of Route Optimization. Note that
the mobile node SHOULD NOT respond to Binding Refresh Requests from
previously unknown correspondent nodes due to Denial-of-Service
concerns.
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The Binding Refresh Request message uses the MH Type value 0. When
this value is indicated in the MH Type field, the format of the
Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
16-bit field reserved for future use. The value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.
Mobility options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The encoding and format
of defined options are described in Section 6.2. The receiver
MUST ignore and skip any options which it does not understand.
There MAY be additional information, associated with this
Binding Refresh Request message, that need not be present in
all Binding Requests sent. This use of mobility options also
allows for future extensions to the format of the Binding
Refresh Request message to be defined. The following options
are valid in a Binding Refresh Request message:
- Unique Identifier Option
- Binding Authorization option
The Header Length field in the Mobility Header for this message
MUST be set to 1 (since unit is 8 octets) plus the total length of
all mobility options present (also in 8 octet units). If no actual
options are present in this message, no padding is necessary.
6.1.3. Home Test Init (HoTI) Message
The Home Test Init (HoTI) message is used to initiate the return
routability procedure from the mobile node to a correspondent node
(see Section 11.6.2). The purpose of this message is to test the
reachability of the home address. This message is always sent with
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the Source Address set to the home address of the mobile node,
Destination Address set to the correspondent node's address, and is
tunneled through the home agent when the mobile node is away from
home. Such tunneling SHOULD employ IPsec ESP in tunnel mode between
the home agent and the mobile node. This protection is guided by the
IPsec Policy Data Base. (Note the protection of HoTI messages is
different from the requirement to protect regular payload traffic,
which MAY use such tunnels as well.)
The HoTI message uses the MH Type value 1. When this value is
indicated in the MH Type field, the format of the Message Data field
in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
16-bit field reserved for future use. This value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.
Mobile cookie
32-bit field which contains a random value, mobile cookie 1,
selected by the mobile node.
Mobility options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The receiver MUST ignore
and skip any options which it does not understand.
There MAY be additional information, associated with this
message that need not be present in all HoTI messages. This
use of mobility options also allows for future extensions to
the format of the HoTI message to be defined. The encoding and
format of defined options are described in Section 6.2. The
following options are valid in a HoTI message:
- Unique Identifier Option
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The Header Length field in the Mobility Header for this message
MUST be set to 2 (since unit is 8 octets) plus the total length of
all mobility options present (also in 8 octet units). If no actual
options are present in this message, 4 bytes of padding is necessary.
A packet that includes a HoTI message MUST NOT include a Home Address
destination option.
6.1.4. Care-of Test Init (CoTI) Message
The Care-of Test Init (CoTI) message is used to initiate the return
routability procedure from the mobile node to a correspondent node
(see Section 11.6.2). The purpose of this message is to test the
reachability of the care-of address. This message is always sent
with the Source Address set to the care-of address of the mobile
node, and is sent directly to the correspondent node.
The CoTI message uses the MH Type value 2. When this value is
indicated in the MH Type field, the format of the Message Data field
in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
16-bit field reserved for future use. The value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.
Mobile cookie
32-bit field which contains a random value, mobile cookie 2,
selected by the mobile node.
Mobility options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The receiver MUST ignore
and skip any options which it does not understand.
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There MAY be additional information, associated with this
message that need not be present in all CoTI messages. This
use of mobility options also allows for future extensions to
the format of the CoTI message to be defined. The encoding and
format of defined options are described in Section 6.2. The
following options are valid in a CoTI message:
- Unique Identifier Option
The Header Length field in the Mobility Header for this message
MUST be set to 2 (since unit is 8 octets) plus the total length of
all mobility options present (also in 8 octet units). If no actual
options are present in this message, 4 bytes of padding is necessary.
A packet that includes a CoTI message MUST NOT include a Home Address
destination option.
6.1.5. Home Test (HoT) Message
The Home Test (HoT) message is a response to the HoTI message, and
is sent from the correspondent node to the mobile node (see Section
8.2). This message is always sent with the Destination Address set
to the home address of the mobile node, Source Address set to the
address of the correspondent node, and is tunneled through the home
agent when the mobile node is away from home. Such tunneling SHOULD
employ IPsec ESP in tunnel mode between the home agent and the mobile
node. This protection is guided by the IPsec Policy Data Base.
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The HoT message uses the MH Type value 3. When this value is
indicated in the MH Type field, the format of the Message Data field
in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Nonce Index | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Home Cookie (128 bits) |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
The two 16-bit fields are reserved for future use. These
values MUST be initialized to zero by the sender, and MUST be
ignored by the receiver.
Home Nonce Index
This field will be echoed back by the mobile node to the
correspondent node in a subsequent binding update. Strictly
speaking, this value is not necessary in the authentication,
but allows the correspondent node to efficiently find the nonce
value Ni that it used in creating the Home Cookie. Without
this field, the correspondent node would have to search through
all currently acceptable nonce values when testing for the
correctness of the authenticator sent in a Binding Update.
Mobile cookie
32-bit field which contains mobile cookie 1, returned by the
correspondent node.
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Home Cookie
This field contains the home cookie in the return routability
procedure; it is the first of two cookies which are to be
processed to form a key which is then used to authenticate a
binding update.
Mobility options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The receiver MUST ignore
and skip any options which it does not understand.
There MAY be additional information, associated with this
message that need not be present in all HoT messages. Mobility
options are used to carry that information. The encoding and
format of defined options are described in Section 6.2. This
use of mobility options also allows for future extensions
to the format of the HoT message to be defined. This
specification does not define any options valid for the HoT
message.
The Header Length field in the Mobility Header for this message
MUST be set to 4 (since unit is 8 octets) plus the total length of
all mobility options present (also in 8 octet units). If no actual
options are present in this message, no padding is necessary.
6.1.6. Care-of Test (CoT) Message
The Care-of Test (CoT) message is a response to the CoTI message, and
is sent from the correspondent node to the mobile node (see Section
8.2). This message is always sent with the Source Address set to the
address of the correspondent node, the Destination Address set to
the care-of address of the mobile node, and is sent directly to the
mobile node.
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The CoT message uses the MH Type value 4. When this value is
indicated in the MH Type field, the format of the Message Data field
in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Nonce Index | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Care-of Cookie (128 bits) |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
The two 16-bit fields and the one 32-bit field are reserved for
future use. These values MUST be initialized to zero by the
sender, and MUST be ignored by the receiver.
Care-of Nonce Index
This field will be echoed back by the mobile node to the
correspondent node in a subsequent binding update. It
will allow the correspondent node to select the appropriate
challenge values to authenticate the binding update.
Mobile cookie
32-bit field which contains the mobile cookie 2, returned by
the correspondent node.
Care-of Cookie
This field contains the care-of cookie in the return
routability procedure; it is the second of two cookies which
are to be processed to form a key which is then used to
authenticate a binding update.
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Mobility options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The receiver MUST ignore
and skip any options which it does not understand.
There MAY be additional information, associated with this
message that need not be present in all CoT messages. Mobility
options are used to carry that information. The encoding and
format of defined options are described in Section 6.2. This
use of mobility options also allows for future extensions
to the format of the CoT message to be defined. This
specification does not define any options valid for the CoT
message.
The Header Length field in the Mobility Header for this message
MUST be set to 4 (since unit is 8 octets) plus the total length of
all mobility options present (also in 8 octet units). If no actual
options are present in this message, no padding is necessary.
6.1.7. Binding Update (BU) Message
The Binding Update (BU) message is used by a mobile node to notify
other nodes of a new care-of address for itself. A packet containing
a Binding Update message is sent with the Source Address set to the
care-of address of the mobile node and the Destination Address set to
the correspondent node's address.
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The Binding Update message uses the MH Type value 5. When this value
is indicated in the MH Type field, the format of the Message Data
field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|S|D| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Acknowledge (A)
The Acknowledge (A) bit is set by the sending mobile node to
request a Binding Acknowledgement (Section 6.1.8) be returned
upon receipt of the Binding Update.
Home Registration (H)
The Home Registration (H) bit is set by the sending mobile
node to request that the receiving node should act as this
node's home agent. The destination of the packet carrying this
message MUST be that of a router sharing the same subnet prefix
as the home address of the mobile node in the binding.
Single Address Only (S)
If the `S' bit is set, the mobile node requests that the home
agent make no changes to any other Binding Cache entry except
for the particular one containing the home address specified
in the Home Address destination option. This disables home
agent processing for other related addresses, as is described
in Section 10.2.
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Duplicate Address Detection (D)
The Duplicate Address Detection (D) bit is set by the sending
mobile node to request that the receiving node (the mobile
node's home agent) perform Duplicate Address Detection [33]
on the mobile node's home link for the home address in this
binding. This bit is only valid when the Home Registration (H)
and Acknowledge (A) bits are also set, and MUST NOT be set
otherwise. If the Duplicate Address Detection performed by
the home agent fails, the Status field in the returned Binding
Acknowledgement will be set to 138 (Duplicate Address Detection
failed).
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Sequence #
A 16-bit number used by the receiving node to sequence Binding
Updates and by the sending node to match a returned Binding
Acknowledgement with this Binding Update. Each Binding Update
sent by a mobile node MUST use a Sequence Number greater than
the Sequence Number value sent in the previous Binding Update
(if any) to the same destination address (modulo 2**16, as
defined in Section 4.5). There is no requirement, however,
that the Sequence Number value strictly increase by 1 with each
new Binding Update sent or received, as long as the value stays
within the window. A Binding Acknowledgement with Status field
set to 141 (Sequence number out of window) will be returned
if the value is outside the window. Both home agents and
correspondent nodes use the sequence number also to prevent
replay attacks.
Lifetime
32-bit unsigned integer. The number of seconds remaining
before the binding MUST be considered expired. A value of all
one bits (0xffffffff) indicates infinity. A value of zero
indicates that the Binding Cache entry for the mobile node MUST
be deleted.
Bindings established with correspondent nodes using the return
routability procedure MUST NOT exceed MAX_RR_BINDING_LIFE
seconds.
Home Address
The home address of the mobile node associated with this
Binding Update.
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Mobility options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The encoding and format
of defined options are described in Section 6.2. The receiver
MUST ignore and skip any options which it does not understand.
A Binding Update sent to a correspondent node MUST include the
following options when the return routability procedure is used
as the authorization method:
- Nonce Indices option. This option contains information the
correspondent node needs in order to find the challenge
values Ni and Nj.
- Binding Authorization Data option. This option contains
a cryptographic hash value which is used to ensure that
it has been sent by the same party who received the HoT
and CoT messages. The authenticator covering a Binding
Update MUST be 96 bits and computed over a string of octets
containing the following fields of the IPv6 header and the
Mobility Header, in order:
* Care-of Address, in the Source Address field of the
IPv6 header
* The address of the correspondent node, in the
Destination Address field of the IPv6 header.
* The contents of the Mobility Header, excluding the
Authenticator field (within the Binding Authorization
Data mobility option) which is not included for the
purposes of calculating the Authenticator. Options of
the Mobility Header are included in the calculation.
The actual authenticator calculation over a sequence of
bits is described in Section 5.5.
There MAY be additional information, associated with this
Binding Update message, that need not be present in all Binding
Updates sent. This use of mobility options also allows for
future extensions to the format of the Binding Update message
to be defined. The following options are valid in a Binding
Update message:
- Unique Identifier option
- Binding Authorization Data option
- Alternate Care-of Address option
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The Header Length field in the Mobility Header for this message
MUST be set to 4 (since unit is 8 octets) plus the total length of
all mobility options present (also in 8 octet units). If no actual
options are present in this message, no padding is necessary.
A Binding Update to the home agent MUST include the Home Address
destination option in order to allow for the use of manually keyed
IPsec in the protection of these messages. Note also that as
described in Section 6.3, the Home Address destination option is not
accepted by correspondent nodes that do not have an existing binding
with the sender.
When a packet contains both a Home Address destination option and a
Binding Update message, the sender MUST use the same address in both.
The receiver MUST check for equal values and MUST silently discard a
packet that does not pass this test.
The care-of address for the binding given in the Binding Update
message is normally that which was received as the value in the
Source Address field in the IPv6 header of the packet carrying the
Binding Update message. However, a care-of address different from
the Source Address MAY be specified by including an Alternate Care-of
Address mobility option in the Binding Update message. When such
message is sent to the correspondent node and the return routability
procedure is used as the authorization method, the Care-of Test Init
and Care-of Test messages MUST have been performed for the address in
the Alternate Care-of Address option (not the Source Address). The
contents of the Nonce Indices and the Authenticator mobility options
MUST be based on information gained in this test.
In any case, the care-of address MUST NOT be any IPv6 address
which is prohibited for use within a Routing Header; thus multicast
addresses, the unspecified address, loop-back address, and link-local
addresses are excluded. Binding Updates indicating any such excluded
care-of address MUST be silently discarded.
The deletion of a binding can be indicated by setting the Lifetime
field to 0 or by setting the care-of address as equal to the home
address (the care-of address can be specified either in an Alternate
Care-of Address mobility option in the Binding Update message, if
present, or in the Source Address field in the packet's IPv6 header).
6.1.8. Binding Acknowledgement (BA) Message
The Binding Acknowledgement message is used to acknowledge receipt
of a Binding Update message (Section 6.1.7). When a node receives
a packet containing a Binding Update message, with this node being
the destination of the packet, this node MUST return a Binding
Acknowledgement to the mobile node, if the Acknowledge (A) bit
is set in the the Binding Update. The Binding Acknowledgement
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message is sent to the Source Address of the Binding Update message
which is being acknowledged. The Source Address of the Binding
Acknowledgement is the Destination Address from the Binding Update.
The Binding Acknowledgement message has the MH Type value 6. When
this value is indicated in the MH Type field, the format of the
Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Refresh |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
These fields are unused. They MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
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
130
Administratively prohibited
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131
Insufficient resources
132
Home registration not supported
133
Not home subnet
137
Not home agent for this mobile node
138
Duplicate Address Detection failed
141
Sequence number out of window
142
Route optimization unnecessary due to low traffic
143
Invalid authenticator
144
Expired Home Nonce Index
145
Expired Care-of Nonce Index
Up-to-date values of the Status field are to be specified in
the most recent "Assigned Numbers" [30].
Sequence #
The Sequence Number in the Binding Acknowledgement is copied
from the Sequence Number field in the Binding Update being
acknowledged, for use by the mobile node in matching this
Acknowledgement with an outstanding Binding Update.
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Lifetime
The granted lifetime, in seconds, for which this node SHOULD
retain the entry for this mobile node in its Binding Cache.
Correspondent nodes should make an effort to honor the
lifetimes, since an entry that was garbage collected too early
might cause subsequent packets from the mobile node to be
dropped, if they contained the Home Address destination option.
While this situation is recoverable since an error message is
sent to the mobile node, it causes an unnecessary break in the
communications.
Mobile nodes SHOULD send a new Binding Update well before the
expiration of this period in order to extend the lifetime and
not cause a disruption in communications. This is particularly
necessary in order to prevent packets from being dropped due
to the use of the Home Address destination option without an
existing Binding Cache Entry, and the possibility of clock
drift.
If the node sending the Binding Acknowledgement is serving
as the mobile node's home agent, the Lifetime period also
indicates the period for which this node will continue this
service; if the mobile node requires home agent service from
this node beyond this period, the mobile node MUST send a new
Binding Update to it before the expiration of this period (even
if it is not changing its primary care-of address), in order
to extend the lifetime. The value of this field is undefined
if the Status field indicates that the Binding Update was
rejected.
Refresh
The recommended interval, in seconds, at which the mobile
node SHOULD send a new Binding Update to this node in order
to "refresh" the mobile node's binding in this node's Binding
Cache. This refreshing of the binding is useful in case the
node fails and loses its cache state. The Refresh period is
determined by the node sending the Binding Acknowledgement (the
node caching the binding). If this node is serving as the
mobile node's home agent, the Refresh value may be set, for
example, based on whether the node stores its Binding Cache in
volatile storage or in nonvolatile storage.
If the node sending the Binding Acknowledgement is not
serving as the mobile node's home agent, the Refresh period
SHOULD be set equal to the Lifetime period in the Binding
Acknowledgement; even if this node loses this cache entry due
to a failure of the node, packets from it can still reach the
mobile node through the mobile node's home agent, causing a new
Binding Update to this node to allow it to recreate this cache
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entry. The value of this field is undefined if the Status
field indicates that the Binding Update was rejected.
Mobility options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The encoding and format
of defined options are described in Section 6.2. The receiver
MUST ignore and skip any options which it does not understand.
There MAY be additional information, associated with this
Binding Acknowledgement message, that need not be present
in all Binding Acknowledgements sent. This use of mobility
options also allows for future extensions to the format of the
Binding Acknowledgement message to be defined. The following
options are valid for the Binding Acknowledgement message:
- Binding Authorization Data option
The Header Length field in the Mobility Header for this message
MUST be set to 3 (since unit is 8 octets) plus the total length of
all mobility options present (also in 8 octet units). If no actual
options are present in this message, 4 bytes of Pad1 or PadN mobility
options are needed to make the length of the message a multiple of 8.
The Header Length field does include this padding.
The Binding Acknowledgement is sent to the source address of the
Binding Update message, regardless of whether the Binding Update
succeeded or failed. No Routing Headers are added to the message.
If the mobile node sends a sequence number which is not within the
window of acceptable sequence numbers, then the home agent MUST send
back a Binding Acknowledgement with status code 141, and the last
accepted sequence number in the Sequence Number field of the Binding
Acknowledgement message.
6.1.9. Binding Error (BE) Message
The Binding Error (BE) message is used by the correspondent node to
signal an error related to mobility, such as an inappropriate attempt
to use the Home Address destination option without an existing
binding. A packet containing a Binding Error message is sent to the
source address of the offending packet. For instance, in the case
of the Home Address destination option error, the packet is the one
that contained the Home Address destination option and therefore
the Binding Error message is sent to the care-of address of the
mobile node. The source address of the Binding Error message is the
correspondent node's address.
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The Binding Error message uses the MH Type value 7. When this value
is indicated in the MH Type field, the format of the Message Data
field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Status
8-bit unsigned integer indicating the reason for this message.
The following such Status values are currently defined:
1
Home Address destination option used without a binding
2
Received message had an unknown value for the MH Type field
Reserved
A 8-bit field reserved for future use. The value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.
Home Address
The home address that was contained in the Home Address
destination option. The mobile node uses this information to
determine which binding does not exist, in cases where the
mobile node has several home addresses.
Mobility options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
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or more TLV-encoded mobility options. The receiver MUST ignore
and skip any options which it does not understand.
There MAY be additional information, associated with this
Binding Error message, that need not be present in all Binding
Error messages sent. This use of mobility options also allows
for future extensions to the format of the Binding Error
message to be defined. The encoding and format of defined
options are described in Section 6.2. This specification does
not define any options valid for the Binding Error message.
The Header Length field in the Mobility Header for this message
MUST be set to 3 (since unit is 8 octets) plus the total length of
all mobility options present (also in 8 octet units). If no actual
options are present in this message, no padding is necessary.
6.2. Mobility Options
6.2.1. Format
In order to allow optional fields that may not be needed in every use
of any given Mobility Header, and to allow future extensions to the
format of these messages to be defined, any of the Mobility Header
messages defined in this document MAY include one or more mobility
options.
Such options are included in the data portion of the message itself,
after the fixed portion of the message data specified in section 6.1.
The presence of such options will be indicated by the Header Len of
the Mobility Header.
These options are encoded within the remaining space of the message
data for that message, using a type-length-value (TLV) format as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Option Type | Option Len | Option Data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
8-bit identifier of the type of mobility option. When
processing a Mobility Header containing an option for which
the Option Type value is not recognized by the receiver,
the receiver MUST quietly ignore and skip over the option,
correctly handling any remaining options in the message.
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Option Length
8-bit unsigned integer. Length of this mobility option, in
octets. The Option Len does not include the length of the
Option Type and Option Len fields.
Option Data
A variable length field that contains data specific to the
option.
The following subsections specify the Option types which are
currently defined for use in the Mobility Header.
Implementations MUST silently ignore any mobility options that they
do not understand.
6.2.2. Pad1
The Pad1 option does not have any alignment requirements. Its format
is as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
NOTE! the format of the Pad1 option is a special case -- it has
neither Option Len nor Option Data fields.
The Pad1 option is used to insert one octet of padding in the
Mobility Options area of a Mobility Header. If more than one octet
of padding is required, the PadN option, described next, should be
used rather than multiple Pad1 options.
6.2.3. PadN
The PadN option does not have any alignment requirements. Its format
is as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| 1 | Option Len | Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
The PadN option is used to insert two or more octets of padding in
the Mobility Options area of a Mobility Header message. For N octets
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of padding, the Option Len field contains the value N, and the Option
Data consists of N-2 zero-valued octets. Option data MUST be ignored
by the receiver.
6.2.4. Unique Identifier
The Unique Identifier option has the alignment requirement of 2n.
Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 4 | Unique Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Unique Identifier option is valid only in Binding Refresh
Request, HoTI, CoTI, and Binding Update messages. The Unique
Identifier field contains a 16-bit value that serves to uniquely
identify a Binding Request among those sent by this Source Address,
and to allow the HoTI, CoTI, and Binding Update to identify the
specific Binding Refresh Request to which it responds. This matching
of Binding Updates to Binding Refresh Requests is required in the
procedure for renumbering the home subnet while a mobile node is away
from home (Section 10.9.1).
6.2.5. Alternate Care-of Address
The Alternate Care-of Address option has an alignment requirement of
8n+6. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Alternate Care-of Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Alternate Care-of Address option is valid only in Binding Update
message. The Alternate Care-of Address field contains an address to
use as the care-of address for the binding, rather than using the
Source Address of the packet as the care-of address.
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6.2.6. Nonce Indices
The Nonce Indices option has an alignment requirement of 2n. Its
format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Nonce Index | Care-of Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Nonce Indices option is valid only in the Binding Update message,
and only when present together with an Binding Authorization Data
option.
The Home Nonce Index field tells the correspondent node that receives
the message which of the challenge values (Ni) are to be used to
authenticate the Binding Update.
The Care-of Nonce Index field tells the correspondent node that
receives the message which of the challenge values (Nj) are to be
used to authenticate the Binding Update.
6.2.7. Binding Authorization Data
The Binding Authorization Data option has an alignment requirement of
4n+2. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 2 + Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Authenticator |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Binding Authorization Data option is valid only in the Binding
Refresh Request, Binding Update, and Binding Acknowledgment messages.
The Option Len field contains the value 2 + Len, where Len is the
length of the authenticator in octets.
The Authenticator field contains a cryptographic value which can be
used to determine that the message in question comes from the right
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authority. Rules for calculating this value depend on the used
authorization procedure. This specification gives the rules only for
the return routability procedure. For this procedure, this option
can only appear in a Binding Update message and rules for calculating
the Authenticator value are described in Section 6.1.7.
6.3. Home Address Destination Option
The Home Address destination option is used in a packet sent by a
mobile node while away from home, to inform the recipient of that
packet of the mobile node's home address. For packets sent by a
mobile node while away from home, the mobile node generally uses one
of its care-of addresses as the Source Address in the packet's IPv6
header. By including a Home Address option in the IPv6 Destination
Options header of the packet, the correspondent node receiving the
packet is able to substitute the mobile node's home address for
this care-of address when processing the packet. This makes the
use of the care-of address transparent to the correspondent node
above the Mobile IPv6 support level. Note that multicast addresses,
link-local addresses, loopback addresses, IPv4 mapped addresses,
and the unspecified address, MUST NOT be used within a Home Address
option. The Home Address Option MUST not appear more than once in
any given packet, except inside the payload part of the packet if
tunneling is involved.
The Home Address option is encoded in type-length-value (TLV) format
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
201 = 0xC9
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Option Length
8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. This field
MUST be set to 16.
Home Address
The home address of the mobile node sending the packet.
IPv6 requires that options appearing in a Hop-by-Hop Options
header or Destination Options header be aligned in a packet so that
multi-octet values within the Option Data field of each option fall
on natural boundaries (i.e., fields of width n octets are placed at
an integer multiple of n octets from the start of the header, for
n = 1, 2, 4, or 8) [6]. The alignment requirement [6] for the Home
Address option is 8n+6.
The three highest-order bits of the Option Type are encoded to
indicate specific processing of the option [6]. For the Home Address
option, these three bits are set to 110, indicating that any IPv6
node processing this option that does not recognize the Option Type
must discard the packet and, only if the packet's Destination Address
was not a multicast address, return an ICMP Parameter Problem,
Code 2, message to the packet's Source Address; and that the data
within the option cannot change en-route to the packet's final
destination.
A packet MUST NOT contain more than one Home Address option, except
that an encapsulated packet [4] MAY contain a separate Home Address
option associated with each encapsulating IP header.
The Home Address option MUST be placed as follows:
- After the Routing Header, if that header is present
- Before the Fragment Header, if that header is present
- Before the AH Header or ESP Header, if either one of those
headers is present
Due to the threat of reflection attacks through the use of this
option, this specification requires that packets containing Home
Address option MUST be dropped if there is no corresponding Binding
Cache Entry for that home address with the currently registered
care-of address matching the source address of the packet. If the
packet is dropped, the correspondent nodes SHOULD send the Binding
Error message to the source address of the packet that contained the
Home Address option (see Section 6.1.9). The Status field in this
message should be set to 1. These messages SHOULD be rate-limited.
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No additional authentication of the Home Address option is
required, except that if the IPv6 header of a packet is covered
by authentication, then that authentication MUST also cover the
Home Address option; this coverage is achieved automatically by the
definition of the Option Type code for the Home Address option, since
it indicates that the data within the option cannot change en-route
to the packet's final destination, and thus the option is included in
the authentication computation. By requiring that any authentication
of the IPv6 header also cover the Home Address option, the security
of the Source Address field in the IPv6 header is not compromised by
the presence of a Home Address option. Security issues related to
the Home Address option are discussed further in Section 5. When
attempting to verify authentication data in a packet that contains
a Home Address option, the receiving node MUST make the calculation
as if the care-of address were present in the Home Address option,
and the home address were present in the source IPv6 address field
of the IPv6 header. This conforms with the calculation specified in
section 11.2.2.
The inclusion of a Home Address destination option in a packet
affects the receiving node's processing of only this single packet;
no state is created or modified in the receiving node as a result
of receiving a Home Address option in a packet. In particular, the
presence of a Home Address option in a received packet MUST NOT alter
the contents of the receiver's Binding Cache and MUST NOT cause any
changes in the routing of subsequent packets sent by this receiving
node.
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6.4. Routing Header type 2
Mobile IPv6 uses a Routing header to carry the Home Address for
packets sent from a correspondent node to a mobile node. The Care of
Address of the mobile node is carried in the IPv6 destination field.
This uses a different Routing header type than defined for "regular"
IPv6 source routing, enabling firewalls to apply different rules
to source routed packets than to MIPv6. This Routing header type
(Type 2) is restricted to carry only one IPv6 address. All IPv6
nodes which process this Routing header MUST verify that the address
contained within is the node's own home address in order to prevent
packets from being forwarded outside the node.
6.4.1. Routing Header Packet format
The Type 2 Routing header has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=2 | Routing Type=2|Segments Left=1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header
8-bit selector. Identifies the type of header immediately
following the Routing header. Uses the same values as the IPv4
Protocol field [10].
Hdr Ext Len
8-bit unsigned integer. Length of the Routing header in
8-octet units, not including the first 8 octets. For the Type
2 Routing header, Hdr Ext Len is always 2.
Routing Type
8-bit unsigned integer that contains the value 2.
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Segments Left
8-bit unsigned integer. Number of route segments remaining;
i.e., number of explicitly listed intermediate nodes still to
be visited before reaching the final destination. Packets
transmitted through an interface have Segments left is always 1
in this type of Routing header.
Reserved
32-bit reserved field. Initialized to zero for transmission,
and ignored on reception.
Home Address
The Home Address of the destination Mobile Node.
The ordering rules for extension headers in an IPv6 packet are
described in Section 4.1 of [6]. The new Routing header (Type 2)
defined for Mobile IPv6 follows the same ordering as other routing
headers. If more than one Routing header (e.g., both a Type 0 and a
Type 2 Routing header are present), the Type 2 Routing header should
follow all other Routing headers. Otherwise the order of routing
headers is independent of their type and follows [6].
In addition, the general procedures defined by IPv6 for Routing
headers suggest that a received Routing header MAY be automatically
"reversed" to construct a Routing header for use in any response
packets sent by upper-layer protocols, if the received packet is
authenticated [6]. This MUST NOT be done automatically for Type 2
Routing headers.
6.5. ICMP Home Agent Address Discovery Request Message
The ICMP Home Agent Address Discovery Request message is used by a
mobile node to initiate the dynamic home agent address discovery
mechanism, as described in Sections 10.9 and 11.3.2. The mobile
node sends a Home Agent Address Discovery Request message to the
"Mobile IPv6 Home-Agents" anycast address for its own home subnet
prefix [11], and one of the home agents responds to the mobile node
with a Home Agent Address Discovery Reply message, providing a list
of the routers on the mobile node's home link serving as home agents.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type
150 <To Be Assigned by IANA>
Code
0
Checksum
The ICMP checksum [5].
Identifier
An identifier to aid in matching Home Agent Address Discovery
Reply messages to this Home Agent Address Discovery Request
message.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
The Source Address of the Home Agent Address Discovery Request
message packet MUST be one of the mobile node's current care-of
addresses. The home agent MUST then return the Home Agent Address
Discovery Reply message directly to the Source Address chosen by the
mobile node. Note that, at the time of performing this dynamic home
agent address discovery, it is likely that the mobile node is not
registered with any home agent within the specified anycast group.
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6.6. ICMP Home Agent Address Discovery Reply Message
The ICMP Home Agent Address Discovery Reply message is used by
a home agent to respond to a mobile node using the dynamic home
agent address discovery mechanism, as described in Sections 10.9
and 11.3.2. The mobile node sends a Home Agent Address Discovery
Request message to the "Mobile IPv6 Home-Agents" anycast address
for its own home subnet prefix [11], and one of the home agents
responds to the mobile node with a Home Agent Address Discovery Reply
message, providing a list of the routers on the mobile node's home
link serving as home agents.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ Reserved +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Home Agent Addresses .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
151 <To Be Assigned by IANA>
Code
0
Checksum
The ICMP checksum [5].
Identifier
The identifier from the invoking Home Agent Address Discovery
Request message.
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Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Home Agent Addresses
A list of addresses of home agents on the home link for the
mobile node. The number of addresses present in the list is
indicated by the remaining length of the IPv6 packet carrying
the Home Agent Address Discovery Reply message.
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6.7. ICMP Mobile Prefix Solicitation Message Format
The ICMP Mobile Prefix Solicitation Message is sent by a mobile node
to its home agent while it is away from home. The purpose of the
message is to solicit a Mobile Prefix Advertisement from the home
agent, which will allow the mobile node to gather prefix information
about its home network. This information can be used to configure
home address(es) by stateless address autoconfiguration [33],
or update address(es) according to changes in prefix information
supplied by the home agent.
The Mobile Prefix Solicitation is similar to the Router Solicitation
used in Neighbor Discovery [20], except it is routed from the mobile
node on the visited network to the home agent on the home network by
usual unicast routing rules.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
Source Address
The mobile node's care-of address.
Destination Address
The address of the mobile node's home agent. This home agent
must be on the link which the mobile node wishes to learn
prefix information about.
Hop Limit
Set to an initial hop limit value, and this message is routed
according to the rules of a typical unicast packet. A hop
limit of 64 is currently suggested [30].
Authentication Header
If a Security Association for the IP Authentication Header
exists between the sender and the destination address, then the
sender SHOULD include this header. [subject to change]
ICMP Fields:
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Type
152 <To Be Assigned by IANA>
Code
0
Checksum
The ICMP checksum [5].
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
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6.8. ICMP Mobile Prefix Advertisement Message Format
A home agent will send a Mobile Prefix Advertisement message to a
mobile node to distribute prefix information about the home link
while the mobile node is traveling away from the home network. This
will occur in response to a Mobile Prefix Solicitation with an
Advertisement, or by an unsolicited Advertisement sent according to
the rules in Section 10.9.1.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
The home agent's address as the mobile node would expect to see
it (i.e., same network prefix)
Destination Address
If this message is a response to a Mobile Prefix Solicitation,
the Source Address field from that packet. For unsolicited
messages, the mobile node's care-of address SHOULD be used, if
it is currently registered with the home agent. Otherwise, the
mobile node's home address SHOULD be used.
Authentication Header
An AH header MUST be included unless the mobile node has yet to
configure a home address.
ICMP Fields:
Type
153 <To Be Assigned by IANA>
Code
0
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Checksum
The ICMP checksum [5].
Options:
Prefix Information
Each message contains one or more Prefix Information options.
Each option carries the prefix(es) that the mobile node
should use to configure its home address(es). Section 10.9.1
describes which prefixes should be advertised to the mobile
node.
The Prefix Information option is defined in Section 4.6.2
of [20], with modifications defined in Section 7.2 of this
specification. The home agent MUST use this modified Prefix
Information option to send the aggregate list of home network
prefixes as defined in Section 10.9.1.
The Mobile Prefix Advertisement sent by the home agent MAY include
the Source Link-layer Address option defined in RFC 2461 [20], or the
Advertisement Interval option specified in Section 7.3.
Future versions of this protocol may define new option types. Mobile
nodes MUST silently ignore any options they do not recognize and
continue processing the message.
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7. Modifications to IPv6 Neighbor Discovery
7.1. Modified Router Advertisement Message Format
Mobile IPv6 modifies the format of the Router Advertisement
message [20] by the addition of a single flag bit to indicate that
the router sending the Advertisement message is serving as a home
agent on this link. The format of the Router Advertisement message
is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cur Hop Limit |M|O|H| Reserved| Router Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reachable Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Retrans Timer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
This format represents the following changes over that originally
specified for Neighbor Discovery [20]:
Home Agent (H)
The Home Agent (H) bit is set in a Router Advertisement to
indicate that the router sending this Router Advertisement is
also functioning as a Mobile IP home agent on this link.
Reserved
Reduced from a 6-bit field to a 5-bit field to account for the
addition of the Home Agent (H) bit.
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7.2. Modified Prefix Information Option Format
Mobile IPv6 requires knowledge of a router's global address for two
reasons:
- To allow a home agent (a router) to learn the address of all
other home agents on the link for which it is providing home
agent service, for use in building its Home Agents List as
part of the dynamic home agent address discovery mechanism
(Sections 10.9 and 11.3.2).
- To allow a mobile node to send a Binding Update to a router on
the link on which its previous care-of address is located, for
purposes of establishing forwarding from this previous care-of
address to its new care-of address (Section 11.6.6).
However, Neighbor Discovery [20] only advertises a router's
link-local address, by requiring this address to be used as the IP
Source Address of each Router Advertisement.
Mobile IPv6 extends Neighbor Discovery to allow a router to easily
and efficiently advertise its global address, by the addition of a
single flag bit in the format of a Prefix Information option for
use in Router Advertisement messages. The format of the Prefix
Information option is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Prefix Length |L|A|R|Reserved1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preferred Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This format represents the following changes over that originally
specified for Neighbor Discovery [20]:
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Router Address (R)
1-bit router address flag. When set, indicates that the
Prefix field, in addition to advertising the indicated prefix,
contains a complete IP address assigned to the sending router.
This router IP address has the same scope and conforms to the
same lifetime values as the advertised prefix. This use of
the Prefix field is compatible with its use in advertising
the prefix itself, since prefix advertisement uses only the
leading number Prefix bits specified by the Prefix Length
field. Interpretation of this flag bit is thus independent
of the processing required for the On-Link (L) and Autonomous
Address-Configuration (A) flag bits.
Reserved1
Reduced from a 6-bit field to a 5-bit field to account for the
addition of the Router Address (R) bit.
In a solicited Router Advertisement, a home agent MUST, and all other
routers SHOULD, include at least one Prefix Information option with
the Router Address (R) bit set. Neighbor Discovery specifies that,
if including all options in a Router Advertisement causes the size of
the Advertisement to exceed the link MTU, multiple Advertisements can
be sent, each containing a subset of the options [20]. In this case,
at least one of these multiple Advertisements being sent instead
of a single larger solicited Advertisement, MUST include a Prefix
Information option with the Router Address (R) bit set.
All routers SHOULD include at least one Prefix Information option
with the Router Address (R) bit set, in each unsolicited multicast
Router Advertisement that they send. If multiple Advertisements
are being sent instead of a single larger unsolicited multicast
Advertisement, at least one of these multiple Advertisements SHOULD
include a Prefix Information option with the Router Address (R) bit
set.
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7.3. New Advertisement Interval Option Format
Mobile IPv6 defines a new Advertisement Interval option, used in
Router Advertisement messages to advertise the interval at which the
sending router sends unsolicited multicast Router Advertisements.
The format of the Advertisement Interval option is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertisement Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
7
Length
8-bit unsigned integer. The length of the option (including
the type and length fields) in units of 8 octets. The value of
this field MUST be 1.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Advertisement Interval
32-bit unsigned integer. The maximum time, in milliseconds,
between successive unsolicited router Router Advertisement
messages sent by this router on this network interface. Using
the conceptual router configuration variables defined by
Neighbor Discovery [20], this field MUST be equal to the value
MaxRtrAdvInterval, expressed in milliseconds.
Routers MAY include this option in their Router Advertisements. A
mobile node receiving a Router Advertisement containing this option
SHOULD utilize the specified Advertisement Interval for that router
in its movement detection algorithm, as described in Section 11.4.1.
This option MUST be silently ignored for other Neighbor Discovery
messages.
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7.4. New Home Agent Information Option Format
Mobile IPv6 defines a new Home Agent Information option, used in
Router Advertisement messages sent by a home agent to advertise
information specific to this router's functionality as a home agent.
The format of the Home Agent Information option is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent Preference | Home Agent Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
8
Length
8-bit unsigned integer. The length of the option (including
the type and length fields) in units of 8 octets. The value of
this field MUST be 1.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Home Agent Preference
16-bit signed, twos-complement integer. The preference for
the home agent sending this Router Advertisement, for use in
ordering the addresses returned to a mobile node in the Home
Agent Addresses field of a Home Agent Address Discovery Reply
message. Higher values mean more preferable. If this option
is not included in a Router Advertisement in which the Home
Agent (H) bit is set, the preference value for this home agent
SHOULD be considered to be 0. Values greater than 0 indicate a
home agent more preferable than this default value, and values
less than 0 indicate a less preferable home agent.
The manual configuration of the Home Agent Preference value
is described in Section 8.3. In addition, the sending home
agent MAY dynamically set the Home Agent Preference value, for
example basing it on the number of mobile nodes it is currently
serving or on its remaining resources for serving additional
mobile nodes; such dynamic settings are beyond the scope of
this document. Any such dynamic setting of the Home Agent
Preference, however, MUST set the preference appropriately,
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relative to the default Home Agent Preference value of 0 that
may be in use by some home agents on this link (i.e., a home
agent not including a Home Agent Information option in its
Router Advertisements will be considered to have a Home Agent
Preference value of 0).
Home Agent Lifetime
16-bit unsigned integer. The lifetime associated with the
home agent in units of seconds. The default value is the same
as the Router Lifetime, as specified in the main body of the
Router Advertisement message. The maximum value corresponds
to 18.2 hours. A value of 0 MUST NOT be used. The Home Agent
Lifetime applies only to this router's usefulness as a home
agent; it does not apply to information contained in other
message fields or options.
Home agents MAY include this option in their Router Advertisements.
This option MUST NOT be included in a Router Advertisement in which
the Home Agent (H) bit (see Section 7.1) is not set. If this option
is not included in a Router Advertisement in which the Home Agent (H)
bit is set, the lifetime for this home agent MUST be considered to
be the same as the Router Lifetime in the Router Advertisement.
If multiple Advertisements are being sent instead of a single
larger unsolicited multicast Advertisement, all of the multiple
Advertisements with the Router Address (R) bit set MUST include this
option with the same contents, otherwise this option MUST be omitted
from all Advertisements.
This option MUST be silently ignored for other Neighbor Discovery
messages.
If both the Home Agent Preference and Home Agent Lifetime are set
to their default values specified above, this option SHOULD NOT be
included in the Router Advertisement messages sent by this home
agent.
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7.5. Changes to Sending Router Advertisements
The Neighbor Discovery protocol specification [20] limits routers to
a minimum interval of 3 seconds between sending unsolicited multicast
Router Advertisement messages from any given network interface
(limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that:
"Routers generate Router Advertisements frequently enough
that hosts will learn of their presence within a few
minutes, but not frequently enough to rely on an absence
of advertisements to detect router failure; a separate
Neighbor Unreachability Detection algorithm provides failure
detection."
This limitation, however, is not suitable to providing timely
movement detection for mobile nodes. Mobile nodes detect their
own movement by learning the presence of new routers as the mobile
node moves into wireless transmission range of them (or physically
connects to a new wired network), and by learning that previous
routers are no longer reachable. Mobile nodes MUST be able to
quickly detect when they move to a link served by a new router, so
that they can acquire a new care-of address and send Binding Updates
to register this care-of address with their home agent and to notify
correspondent nodes as needed.
Thus, to provide good support for mobile nodes, Mobile IPv6 relaxes
this limit such that routers MAY send unsolicited multicast Router
Advertisements more frequently. In particular, on network interfaces
where the router is expecting to provide service to visiting mobile
nodes (e.g., wireless network interfaces), or on which it is serving
as a home agent to one or more mobile nodes (who may return home and
need to hear its Advertisements), the router SHOULD be configured
with a smaller MinRtrAdvInterval value and MaxRtrAdvInterval value,
to allow sending of unsolicited multicast Router Advertisements more
often. Recommended values for these limits are:
- MinRtrAdvInterval 0.05 seconds
- MaxRtrAdvInterval 1.5 seconds
Use of these modified limits MUST be configurable, and specific
knowledge of the type of network interface in use SHOULD be taken
into account in configuring these limits for each network interface.
When sending unsolicited multicast Router Advertisements more
frequently than the standard limit on unsolicited multicast
Advertisement frequency, the sending router need not include all
options in each of these Advertisements, but it SHOULD include at
least one Prefix Information option with the Router Address (R) bit
set (Section 7.2) in each.
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7.6. Changes to Sending Router Solicitations
In addition to the limit on routers sending unsolicited multicast
Router Advertisement messages (Section 7.5), Neighbor Discovery
defines limits on nodes sending Router Solicitation messages, such
that a node SHOULD send no more than 3 Router Solicitations, and that
these 3 transmissions SHOULD be spaced at least 4 seconds apart.
However, these limits prevent a mobile node from finding a new
default router (and thus a new care-of address) quickly as it moves
about.
Mobile IPv6 relaxes this limit such that, while a mobile node is away
from home, it MAY send Router Solicitations more frequently. The
following limits for sending Router Solicitations are recommended for
mobile nodes while away from home:
- A mobile node that is not configured with any current care-of
address (e.g., the mobile node has moved since its previous
care-of address was configured), MAY send more than the defined
Neighbor Discovery limit of MAX_RTR_SOLICITATIONS Router
Solicitations.
- The rate at which a mobile node sends Router Solicitations MUST
be limited, although a mobile node MAY send Router Solicitations
more frequently than the defined Neighbor Discovery limit of
RTR_SOLICITATION_INTERVAL seconds. The minimum interval MUST
be configurable, and specific knowledge of the type of network
interface in use SHOULD be taken into account in configuring this
limit for each network interface. A recommended minimum interval
is 1 second.
- After sending at most MAX_RTR_SOLICITATIONS Router Solicitations,
a mobile node MUST reduce the rate at which it sends subsequent
Router Solicitations. Subsequent Router Solicitations SHOULD
be sent using a binary exponential backoff mechanism, doubling
the interval between consecutive Router Solicitations, up to a
maximum interval. The maximum interval MUST be configurable and
SHOULD be chosen appropriately based on the characteristics of
the type of network interface in use.
- While still searching for a new default router and care-of
address, a mobile node MUST NOT increase the rate at which it
sends Router Solicitations unless it has received a positive
indication (such as from lower network layers) that it has moved
to a new link. After successfully acquiring a new care-of
address, the mobile node SHOULD also increase the rate at which
it will send Router Solicitations when it next begins searching
for a new default router and care-of address.
- A mobile node that is currently configured with a care-of address
SHOULD NOT send Router Solicitations to the default router
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on it current link, until its movement detection algorithm
(Section 11.4.1) determines that it has moved and that its
current care-of address might no longer be valid.
8. Requirements for Types of IPv6 Nodes
Mobile IPv6 places some special requirements on the functions
provided by different types of IPv6 nodes. This section summarizes
those requirements, identifying the functionality each requirement
is intended to support. Further details on this functionality is
provided in the following sections.
8.1. Requirements for All IPv6 Hosts and Routers
Since any IPv6 node may at any time be a correspondent node of a
mobile node, either sending a packet to a mobile node or receiving a
packet from a mobile node, the following requirements apply to ALL
IPv6 nodes (whether host or router, whether mobile or stationary):
- Every IPv6 node MUST be able to process a Home Address option
received in any IPv6 packet.
- Every IPv6 node SHOULD be able to participate in a return
routability procedure, process Binding Update messages, and to
return a Binding Acknowledgement option if the Acknowledge (A)
bit is set in the received Binding Update.
- Every IPv6 node SHOULD be able to maintain a Binding Cache of the
bindings received in accepted Binding Updates.
8.2. Requirements for All IPv6 Routers
The following requirements apply to all IPv6 routers, even those not
serving as a home agent for Mobile IPv6:
- Every IPv6 router SHOULD be able to send an Advertisement
Interval option in each of its Router Advertisements, to aid
movement detection by mobile nodes. The use of this option in
Router Advertisements MUST be configurable.
- Every IPv6 router SHOULD be able to support sending unsolicited
multicast Router Advertisements at the faster rate described in
Section 7.5. The use of this faster rate MUST be configurable.
- Each router SHOULD include at least one prefix with the 'R' bit
set and with its full IP address in its router advertisements.
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- Filtering routers SHOULD support different rules for Type 0 and
Type 2 Routing headers so that filtering of source routed packets
(Type 0) will not necessarily limit MIPv6 traffic via Type 2
Routing headers.
8.3. Requirements for IPv6 Home Agents
In order for a mobile node to operate correctly while away from home,
at least one IPv6 router on the mobile node's home link must function
as a home agent for the mobile node. The following additional
requirements apply to all IPv6 routers capable of serving as a home
agent:
- Every home agent MUST be able to maintain an entry in its Binding
Cache for each mobile node for which it is serving as the home
agent. Each such Binding Cache entry records the mobile node's
binding with its primary care-of address and is marked as a "home
registration".
- Every home agent MUST be able to intercept packets (using proxy
Neighbor Discovery) addressed to a mobile node for which it is
currently serving as the home agent, on that mobile node's home
link, while the mobile node is away from home.
- Every home agent MUST be able to encapsulate such intercepted
packets in order to tunnel them to the primary care-of address
for the mobile node indicated in its binding in the home agent's
Binding Cache.
- Every home agent MUST support decapsulating reverse tunneled
packets sent to it from a mobile node's home address. Every home
agent MUST also check that the source address in the tunneled
packets corresponds to the currently registered location of the
mobile node.
- Every home agent MUST be able to return a Binding Acknowledgement
message in response to a Binding Update option received with the
Acknowledge (A) bit set.
- Every home agent MUST maintain a separate Home Agents List for
each link on which it is serving as a home agent, as described in
Section 4.5.
- Every home agent MUST be able to accept packets addressed to
the "Mobile IPv6 Home-Agents" anycast address for the subnet
on which it is serving as a home agent [11], and MUST be
able to participate in dynamic home agent address discovery
(Section 10.9).
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- Every home agent SHOULD support a configuration mechanism to
allow a system administrator to manually set the value to be sent
by this home agent in the Home Agent Preference field of the Home
Agent Information Option in Router Advertisements that it sends.
- Every home agent SHOULD support sending ICMP Mobile
Prefix Advertisements, and SHOULD respond to Mobile Prefix
Solicitations.
8.4. Requirements for IPv6 Mobile Nodes
Finally, the following requirements apply to all IPv6 nodes capable
of functioning as mobile nodes:
- Every IPv6 mobile node MUST be able to perform IPv6 encapsulation
and decapsulation [4].
- Every IPv6 mobile node MUST support the return routability
procedure and sending Binding Update messages, as specified in
Sections 11.6.1, 11.6.2, and 11.6.6; and MUST be able to receive
and process Binding Acknowledgement messages, as specified in
Section 11.6.3.
- Every IPv6 mobile node MUST support use of the dynamic home agent
address discovery mechanism, as described in Section 11.3.2.
- 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.
- Every IPv6 mobile node MUST support receiving a Binding Refresh
Request, by responding with a Binding Update message.
- Every IPv6 mobile node MUST support sending packets containing a
Home Address option. This option MUST be included in all packets
sent to a correspondent node when the following three conditions
apply: The correspondent node has a binding with this mobile
node. The mobile node is away from home. The packet would
otherwise have been sent with the mobile node's home address as
the IP Source Address.
- Every IPv6 mobile node MUST maintain a Home Agents List, as
described in Section 4.5.
- Every mobile node MUST support receiving Mobile Prefix
Advertisements and reconfiguring its home address based on the
prefix information contained therein.
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9. Correspondent Node Operation
This section explains the special processing required for the return
routability and binding procedures, as well as to manage the binding
cache, handle ICMP messages and send packets to a mobile node.
9.1. Conceptual Data Structures
Each IPv6 node maintains a Binding Cache of bindings for other nodes.
A separate Binding Cache SHOULD be maintained by each IPv6 node for
each of its IPv6 addresses. The Binding Cache MAY be implemented in
any manner consistent with the external behavior described in this
document, for example by being combined with the node's Destination
Cache as maintained by Neighbor Discovery [20]. When sending a
packet, the Binding Cache is searched before the Neighbor Discovery
conceptual Destination Cache [20] (i.e., any Binding Cache entry for
this destination SHOULD take precedence over any Destination Cache
entry for the same destination).
Each Binding Cache entry conceptually contains the following fields:
- The home address of the mobile node for which this is the Binding
Cache entry. This field is used as the key for searching the
Binding Cache for the destination address of a packet being sent.
If the destination address of the packet matches the home address
in the Binding Cache entry, this entry SHOULD be used in routing
that packet.
- The care-of address for the mobile node indicated by the home
address field in this Binding Cache entry. If the destination
address of a packet being routed by a node matches the home
address in this entry, the packet SHOULD be routed to this
care-of address, as described in Section 9.6, for packets
originated by this node, or in Section 10.5, if this node is the
mobile node's home agent and the packet was intercepted by it on
the home link.
- A lifetime value, indicating the remaining lifetime for this
Binding Cache entry. The lifetime value is initialized from
the Lifetime field in the Binding Update that created or last
modified this Binding Cache entry. Once the lifetime of this
entry expires, the entry MUST be deleted from the Binding Cache.
- A flag indicating whether or not this Binding Cache entry is a
"home registration" entry.
- A flag indicating whether or not this Binding Cache entry
represents a mobile node that should be advertised as a router in
proxy Neighbor Advertisements sent by this node on its behalf.
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This flag is only valid if the Binding Cache entry indicates that
this is a "home registration" entry.
- The length of the routing prefix for the home address. This
field is only valid if the "home registration" flag is set on
this Binding Cache entry.
- The maximum value of the Sequence Number field received in
previous Binding Updates for this mobile node home address.
The Sequence Number field is 16 bits long, and all comparisons
between Sequence Number values MUST be performed modulo 2**16.
For example, using an implementation in the C programming
language, a Sequence Number value A is greater than another
Sequence Number value B if ((short)((a) - (b)) > 0), if the
"short" data type is a 16-bit signed integer.
- Recent usage information for this Binding Cache entry, as needed
to implement the cache replacement policy in use in the Binding
Cache and to assist in determining whether a Binding Refresh
Request should be sent when the lifetime of this entry nears
expiration.
Binding Cache entries not marked as "home registrations" MAY be
replaced at any time by any reasonable local cache replacement policy
but SHOULD NOT be unnecessarily deleted. The Binding Cache for any
one of a node's IPv6 addresses may contain at most one entry for
each mobile node home address. The contents of a node's Binding
Cache MUST NOT be changed in response to a Home Address option in
a received packet. The contents of all of a node's Binding Cache
entries, for each of its IPv6 addresses, MUST be cleared when the
node reboots.
9.2. Receiving Packets from a Mobile Node
Packets sent by a mobile node with either a Home Address destination
option or a Mobility Header (or both) require special processing at
the correspondent node as explained below.
9.2.1. Processing Mobility Header (MH) Messages
All IPv6 correspondent nodes MUST observe the following rules when
processing Mobility Header messages:
1. If an MH message of unknown type is received (Section 6.1, the
correspondent node SHOULD issue a Binding Error message to the
packet's Source Address with Status field set to 2. Finally, the
correspondent node MUST discard the packet.
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2. If the "Next Header" field is not NO_NXTHDR (59 decimal), the
packet MUST be silently discarded.
3. The checksum must be verified as per Section 6.1.
Subsequent checks depend on the particular Mobility Header message.
There are two types of Mobility Header messages. The return
routability procedure (Section 9.3) is used to verify liveness of the
mobile node at both its home address as well as its care-of address.
These liveness probes are used to secure binding updates.
The other type of Mobility Header messages are directly concerned
with managing bindings (Section 9.4).
9.2.2. Receiving Packets with Home Address Destination Option
Packets sent by a mobile node while away from home MAY include a Home
Address destination option, if the correspondent node has a Binding
Cache Entry for that home address. It MUST process the option in a
manner consistent with exchanging the Home Address field from the
Home Address option into the IPv6 header, replacing the original
value of the Source Address field there. However, any actual
modifications to the Source Address field in the packet's IPv6 header
MUST be carried out in such a fashion that further processing of such
a packet after all IPv6 options processing (e.g., at the transport
layer) does not depend on that information to know that the original
Source Address was a care-of address, or that the Home Address option
was used in the packet.
Since the sending mobile node uses its home address at the transport
layer when sending such a packet, the use of the care-of address
and Home Address option is transparent to both the mobile node and
the correspondent node above the level of the Home Address option
generation and processing.
Packets containing Home Address Option MUST be dropped if there is
no corresponding Binding Cache Entry for that home address. In this
case, the correspondent nodes SHOULD send the Binding Error message
to the source address of the packet that contained the Home Address
Option (see Section 6.1.9).
9.3. Return Routability Procedure
A correspondent node engages in the return routability procedure in
order to secure a subsequent Binding Update. This is a requirement
in order to authorize the creation of new bindings as well as to
refresh existing ones. In particular, these messages are used to
establish the mobile node's liveness (responsiveness to packets) at
both its care-of address as well as its home address.
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9.3.1. Receiving HoTI Messages
The HoTI message initiates the return routability procedure from the
mobile node's home address to the correspondent node.
The correspondent node verifies the following:
- MH Type field for this message is 1.
- The Header Extension Length field MUST be greater than or equal
to the length specified in Section 6.1.3.
- The packet MUST NOT include a Home Address destination option.
In preparation for sending the corresponding HoT Message, the
correspondent node checks that it has the necessary material
to engage in a return routability procedure, as specified in
Section 5.5. For that procedure, the correspondent node MUST have a
secret Kcn and a nonce Nj. If it does not have this material yet,
it MUST produce it before continuing with the return routability
procedure.
Section 9.3.3 specifies further processing.
9.3.2. Receiving CoTI Messages
The CoTI message initiates the return routability procedure from the
mobile node's care-of address location to the correspondent node.
The correspondent node verifies the following:
- MH Type field for this message is 2.
- The Header Extension Length field MUST be greater than or equal
to the length specified in Section 6.1.4.
- The packet MUST NOT include a Home Address destination option.
In preparation for sending the corresponding CoT Message, the
correspondent node checks that it has the necessary material
to engage in a return routability procedure, as specified in
Section 5.5. For that procedure, the correspondent node MUST have a
secret Kcn and a nonce Nl. If it does not have this material yet,
it MUST produce it before continuing with the return routability
procedure.
Section 9.3.4 specifies further processing.
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9.3.3. Sending HoT Messages
Unless already created, the correspondent node creates a "Home
Cookie" and an associated "Home Nonce Index". It then creates a
HoT message (Section 6.1.5) and sends it to the mobile node at the
latter's home address.
9.3.4. Sending CoT Messages
Unless already created, the correspondent node creates a "Care-of
Cookie" and an associated "Care-of Nonce Index". It then creates a
CoT message (Section 6.1.6) and sends it to the mobile node at the
latter's care-of address.
9.4. Processing Bindings
This section explains how the correspondent node processes the
binding cache messages. These messages are:
- Binding Update
- Binding Refresh Request
- Binding Acknowledgement
- Binding Error
9.4.1. Receiving Binding Updates
Before accepting a Binding Update message, the receiving node MUST
validate the Binding Update according to the following tests:
- The packet MUST NOT contain a Home Address option.
- The Header Len field in the Binding Update option is greater than
or equal to the length specified in Section 6.1.7.
- The Sequence Number field in the Binding Update message is
greater than the Sequence Number received in the previous Binding
Update for this home address, if any. As noted in Section 5.5,
this Sequence Number comparison MUST be performed modulo 2**16.
- The packet meets the specific authentication requirements for
Binding Updates, defined in Section 5.5.
When the return routability procedure is used as an authorization
method, the following are also required:
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- The correspondent node MUST re-generate the Home Cookie and the
Care-of Cookie from the information contained in the packet.
It then generates the session key Kbu and uses it to verify
the authenticator field in the Binding Update as specified in
Section 6.1.7. Note that a care-of address different from the
Source Address MAY have been specified by including an Alternate
Care-of Address mobility option in the Binding Update message.
When such message is received and the return routability
procedure is used as an authorization method, the correspondent
node MUST verify the authenticator by using the address within
the Alternate Care-of Address in the calculations.
- The Home and Care-of Nonce Index values in the Nonce Indices
mobility option are recognized by the correspondent node. As
described in Section 5.5, the correspondent node discards Nonce
values that are too old.
If the mobile node sends a sequence number which is not greater than
the sequence number from the last successful Binding Update, then the
receiving node MUST send back a Binding Acknowledgement with status
code 141, and the last accepted sequence number in the Sequence
Number field of the Binding Acknowledgement.
If the mobile node sends a Home or Care-of Nonce Index value which is
no longer recognized by the correspondent node, then the receiving
node MUST send back a Binding Acknowledgement with status code 144 or
145, respectively.
Any Binding Update which fails to satisfy all of these tests for
any reason other than insufficiency of the Sequence Number or Nonce
Indices MUST be silently ignored, and the packet carrying the Binding
Update MUST be discarded.
In this section, the care-of address refers to the IPv6 address,
which was originally located in the IPv6 header when the packet was
transmitted by the mobile node.
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 home address
for the binding, then this is a request to cache a binding for
the mobile node. If the Home Registration (H) bit is set in the
Binding Update, the Binding Update is processed according to the
procedure specified in Section 10.2; otherwise, it is processed
according to the procedure specified in Section 9.4.2.
- If the Lifetime specified in the Binding Update is zero or the
specified Care-of Address matches the home address for the
binding, then this is a request to delete the mobile node's
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cached binding. If the Home Registration (H) bit is set in the
Binding Update, the Binding Update is processed according to the
procedure specified in Section 10.3; otherwise, it is processed
according to the procedure specified in Section 9.4.3.
9.4.2. Requests to Cache a Binding
When a node receives a Binding Update, it MUST validate it and
determine the type of Binding Update according to the steps described
in Section 9.4.1. This section describes the processing of a valid
Binding Update that requests a node to cache a mobile node's binding,
for which the Home Registration (H) bit is not set in the Binding
Update.
In this case, the receiving node SHOULD create a new entry in its
Binding Cache for this mobile node, or update its existing Binding
Cache entry for this mobile node, if such an entry already exists.
The Binding Cache entry records the association between this home
address and the care-of address for the binding. The lifetime for
the Binding Cache entry is initialized from the Lifetime field
specified in the Binding Update, although this lifetime MAY be
reduced by the node caching the binding; the lifetime for the Binding
Cache entry MUST NOT be greater than the Lifetime value specified in
the Binding Update. Any Binding Cache entry MUST be deleted after
the expiration its lifetime.
The Sequence Number value received from a mobile node in a Binding
Update is stored by a correspondent node in its Binding Cache entry
for that mobile node. If the receiving correspondent node has no
Binding Cache entry for the sending mobile node, it MUST accept any
Sequence Number value in a received Binding Update from this mobile
node.
9.4.3. Requests to Delete a Binding
When a node receives a Binding Update, it MUST validate it and
determine the type of Binding Update according to the steps described
in Section 9.4.1. This section describes the processing of a valid
Binding Update that requests a node to delete a mobile node's binding
from its Binding Cache, for which the Home Registration (H) bit is
not set in the Binding Update.
Any existing binding for the mobile node MUST be deleted. A Binding
Cache entry for the mobile node MUST NOT be created in response to
receiving the Binding Update.
In order to prevent replayed binding updates after a binding cache
entry has been deleted the correspondent node needs to make sure that
the nonce indices used to create the binding are no longer valid.
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This applies whether the binding is deleted due to it timing out
(lifetime expiry) or being deleted explicitly by the mobile node.
If a binding cache entry is logically deleted and either the home
nonce index or the care-of nonce index used to create (or last
update) the binding are still valid, the correspondent node must
behave as if it retains the state about the binding (including the
sequence number) until at least one of the cookies has become too
old.
A possible way to implement this is to mark the binding cache entry
so that it does not effect sending and receiving of packets, but
so that it is found when a binding update is received. Another
way is to mark the used nonces immediately too old. However, this
method may cause some unnecessary failures and retries with ongoing
return routability procedures with other mobile nodes. Furthermore,
unless the mobile node has requested a Binding Acknowledgement,
it is possible that this method may even cause an error in the
return routability procedure procedure to go unnoticed, and data
packets to be dropped through the use of the Home Address destination
option without an existing binding. The effect is similar to packet
loss during the return routability procedure, but may in certain
circumstances significantly increase the problems.
9.4.4. Sending Binding Acknowledgements
When any node receives a packet containing a Binding Update message
in which the Acknowledge (A) bit is set, it MUST return a Binding
Acknowledgement message acknowledging receipt of the Binding Update.
If the node accepts the Binding Update and creates or updates an
entry in its Binding Cache for this binding, the Status field in the
Binding Acknowledgement MUST be set to a value less than 128; if, on
the other hand the Binding Update is accepted and the `A' bit is not
set, the node SHOULD NOT send a Binding Acknowledgement. If the node
rejects the Binding Update and does not create or update an entry for
this binding, a Binding Acknowledgement MUST be sent even if the `A'
bit was not set, and the Status field in the Binding Acknowledgement
MUST be set to a value greater than or equal to 128. Specific values
for the Status field are described in Section 6.1.8 and in the most
recent "Assigned Numbers" [10].
The packet in which the Binding Acknowledgement is returned
MUST meet the specific authentication requirements for Binding
Acknowledgements, defined in Section 5.5. Furthermore, if the packet
is to be sent to the mobile node at any address other than the mobile
node's home address, it MUST be sent using a Routing header (even if
the binding was rejected). The intermediate IP address, to which
the packet will be delivered immediately before the home address, is
determined as follows:
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- Whenever the Binding Update is accepted with a nonzero lifetime,
the routing header will be constructed using the care-of address
as described in Section 9.6.
- Otherwise, if the Source IP Address of the packet containing
the Binding Update, is legal for inclusion in a Routing Header,
the routing header will be constructed using that IP address.
Note that multicast addresses, link-local addresses, loopback
addresses, IPv4 mapped addresses, and the unspecified address,
MUST NOT be used within a Routing Header for the Binding
Acknowledgement.
Otherwise, if the Binding Update has a zero lifetime but the Source
IP address is not allowable for use within the Routing Header,
the Binding Acknowledgment MUST be sent to the mobile node's home
address.
9.4.5. Sending Binding Refresh Requests
Entries in a node's Binding Cache MUST be deleted when their lifetime
expires. If such an entry is still in active use in sending packets
to a mobile node, the next packet sent to the mobile node will be
routed normally to the mobile node's home link, where it will be
intercepted and tunneled to the mobile node. The mobile node will
then return a Binding Update to the sender, allowing it to create
a new Binding Cache entry for sending future packets to the mobile
node. Communication with the mobile node continues uninterrupted,
but the forwarding of this packet through the mobile node's home
agent creates additional overhead and latency in delivering packets
to the mobile node. Such routing paths could, for instance,
temporarily or permanently disrupt any negotiated Quality of Service
reservations which had been made by the mobile node on its home
network.
If the sender knows that the Binding Cache entry is still in active
use, it MAY send a Binding Refresh Request message to the mobile node
in an attempt to avoid this overhead and latency due to deleting and
recreating the Binding Cache entry. When the mobile node receives a
packet from some sender containing a Binding Refresh Request option,
it MAY start a return routability procedure, if necessary, before
sending its current binding and a new lifetime in a new Binding
Update.
The correspondent node MAY retransmit Binding Refresh Request
messages provided that rate limitation is applied. The correspondent
node SHOULD stop retransmitting when it receive a Home Test Init
message, as the mobile node is responsible for retransmissions during
the return routability procedure.
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9.4.6. Sending Binding Error Messages
If the correspondent node receives a packet with a Home Address
destination option it MUST verify that it has a binding for that
mobile node. Specifically, it MUST have a binding entry for the
mobile node's home address (as obtained from the Home Address option)
at the mobile node's care-of address (from the IP source address of
the packet). If the correspondent node does not find such a binding
entry, it MUST discard the packet and return a Binding Error message
(Section 6.1.9).
9.5. Cache Replacement Policy
Conceptually, a node maintains a separate timer for each entry in its
Binding Cache. When creating or updating a Binding Cache entry in
response to a received and accepted Binding Update, the node sets the
timer for this entry to the specified Lifetime period. Any entry in
a node's Binding Cache MUST be deleted after the expiration of the
Lifetime specified in the Binding Update from which the entry was
created or last updated.
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 10.2) MUST NOT be deleted from the cache until
the expiration of its lifetime period. When such "home registration"
entries are deleted, the home agent MUST also cease intercepting
packets on the mobile node's home link addressed to the mobile node
(Section 10.4), just as if the mobile node had de-registered its
primary care-of address (see Section 10.3).
When attempting to add a new "home registration" entry in response
to a Binding Update with the Home Registration (H) bit set, if no
sufficient space can be found, the node MUST reject the Binding
Update and MUST return a Binding Acknowledgement to the sending
mobile node, in which the Status field is set to 131 (insufficient
resources). When otherwise attempting to add a new entry to its
Binding Cache, a node MAY, if needed, choose to drop any entry
already in its Binding Cache, other than "home registration"
entries, in order to make space for the new entry. For example, a
"least-recently used" (LRU) strategy for cache entry replacement
among entries not marked as "home registrations" is likely to
work well unless the size of the Binding Cache is substantially
insufficient.
Any binding dropped from a node's Binding Cache due to lack of cache
space will be rediscovered and a new cache entry created, if the
binding is still in active use by the node for sending packets. If
the node sends a packet to a destination for which it has dropped the
entry from its Binding Cache, the packet will be routed normally,
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leading to the mobile node's home link. There, the packet will be
intercepted by the mobile node's home agent and tunneled to the
mobile node's current primary care-of address. This indirect routing
to the mobile node through its home agent will result in the mobile
node sending a Binding Update to this sending node when it receives
the tunneled packet, allowing it to again add an entry for this
destination mobile node to its Binding Cache.
9.6. Sending Packets to a Mobile Node
Before sending any packet, the sending node SHOULD examine its
Binding Cache for an entry for the destination address to which the
packet is being sent. If the sending node has a Binding Cache entry
for this address, the sending node SHOULD use a Routing header to
route the packet to this mobile node (the destination node) by way
of the care-of address in the binding recorded in that Binding Cache
entry. For example, assuming use of a Type 2 Routing header (see
Section 6.4), if no other use of a Routing header is involved in
the routing of this packet, the mobile node sets the fields in the
packet's IPv6 header and Routing header as follows:
- The Destination Address in the packet's IPv6 header is set to
the mobile node's care-of address copied from the Binding Cache
entry.
- The Routing header is initialized to contain a single route
segment, with an Address of the mobile node's home address (the
original destination address to which the packet was being sent).
Following the definition of a Type 2 Routing header 6.4, this packet
will be routed to the mobile node's care-of address, where it will
be delivered to the mobile node (the mobile node has associated the
care-of address with its network interface).
Note that following the above conceptual model in an implementation
creates some additional requirements for path MTU discovery since the
layer that decides the packet size (e.g., TCP and applications using
UDP) needs to be aware of the size of the headers added by the IP
layer on the sending node.
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 10.5. The mobile node MAY then send
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a Binding Update to the sending node, as described in Section 11.6.2,
allowing the sending node to create a Binding Cache entry for its use
in sending subsequent packets to this mobile node.
9.7. Receiving ICMP Error Messages
When a correspondent node sends a packet to a mobile node, if the
correspondent node has a Binding Cache entry for the destination
address of the packet, then the correspondent node uses a Routing
header to deliver the packet to the mobile node through the care-of
address in the binding recorded in the Binding Cache entry. Any ICMP
error message caused by the packet on its way to the mobile node will
be returned normally to the correspondent node.
On the other hand, if the correspondent node has no Binding Cache
entry for the mobile node, the packet will be routed to the mobile
node's home link. There, it will be intercepted by the mobile node's
home agent, encapsulated, and tunneled to the mobile node's primary
care-of address. Any ICMP error message caused by the packet on
its way to the mobile node while in the tunnel, will be transmitted
to the mobile node's home agent (the source of the tunnel). By
the definition of IPv6 encapsulation [4], the home agent (as the
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 link and is intercepted there by a home agent for the mobile
node's previous care-of address as described in Section 11.6.6 (e.g.,
the mobile node moved after the packet was sent), that home agent
will encapsulate and tunnel the packet to the mobile node's new
care-of address. As above, any ICMP error message caused by the
packet while in this tunnel will be returned to that home agent (the
source of the tunnel), which MUST relay certain ICMP error messages
back to the correspondent node [4]. The relayed packet MUST NOT
contain a routing header entry with the care-of address of the mobile
node.
Thus, in all cases, any meaningful ICMP error messages caused
by packets from a correspondent node to a mobile node will be
returned to the correspondent node. If the correspondent node
receives persistent ICMP Destination Unreachable messages after
sending packets to a mobile node based on an entry in its Binding
Cache, the correspondent node SHOULD delete this Binding Cache
entry. If the correspondent node subsequently transmits another
packet to the mobile node, the packet will be routed to the mobile
node's home link, intercepted by the mobile node's home agent, and
tunneled to the mobile node's primary care-of address using IPv6
encapsulation. The mobile node will then return a Binding Update to
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the correspondent node, allowing it to recreate a (correct) Binding
Cache entry for the mobile node.
10. Home Agent Operation
10.1. Conceptual Data Structures
Each home agent MUST maintain a Binding Cache and Home Agents List.
The rules for maintaining a Binding Cache are same for home
agents and correspondent nodes, and have already been described in
Section 9.1. In addition, if 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, it SHOULD NOT be deleted by the home agent until
the expiration of its binding lifetime.
The Home Agents List is maintained by each home agent (as well as
each mobile node), recording information about each home agent from
which this node has received a Router Advertisement in which the Home
Agent (H) bit is set, for which the remaining lifetime for this list
entry (defined below) has not yet expired. The home agents list is
thus similar to the Default Router List conceptual data structure
maintained by each host for Neighbor Discovery [20], although the
Home Agents List MAY be implemented in any manner consistent with the
external behavior described in this document.
Each home agent maintains a separate Home Agents List for each link
on which it is serving as a home agent; this list is used by a home
agent in the dynamic home agent address discovery mechanism. Each
mobile node, while away from home, also maintains a Home Agents
List, to enable it to notify a home agent on its previous link when
it moves to a new link; a mobile node MAY maintain a separate Home
Agents List for each link to which it is (or has recently) connected,
or it MAY maintain a single list for all links. Each Home Agents
List entry conceptually contains the following fields:
- The link-local IP address of a router on the link, that this
node currently believes is operating as a home agent for that
link. A new entry is created or an existing entry is updated
in the Home Agents List in response to receipt of a valid
Router Advertisement in which the Home Agent (H) bit is set.
The link-local address of the home agent is learned through
the Source Address of the Router Advertisements received from
it [20].
- One or more global IP addresses for this home agent, learned
through Prefix Information options with the Router Address (R)
bit set, received in Router Advertisements from this link-local
address. Global addresses for the router in a Home Agents List
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entry MUST be deleted once the prefix associated with that
address is no longer valid [20].
Are there interactions with the new Router Advertisement
stuff?
- The remaining lifetime of this Home Agents List entry. If a Home
Agent Information Option is present in a Router Advertisement
received from a home agent, the lifetime of the Home Agents List
entry representing that home agent is initialized from the Home
Agent Lifetime field in the option; otherwise, the lifetime
is initialized from the Router Lifetime field in the received
Router Advertisement. The Home Agents List entry lifetime is
decremented until it reaches zero, at which time this entry MUST
be deleted from the Home Agents List.
- The preference for this home agent; higher values indicate a more
preferable home agent. The preference value is taken from the
Home Agent Preference field (a signed, twos-complement integer)
in the received Router Advertisement, if the Router Advertisement
contains a Home Agent Information Option, and is otherwise set
to the default value of 0. A home agent uses this preference
in ordering the Home Agents List returned in an ICMP Home
Agent Address Discovery message in response to a mobile node's
initiation of dynamic home agent address discovery. A mobile
node uses this preference in determining which of the home agents
on its previous link to notify when it moves to a new link.
Can we delete the preference stuff? Is anyone using it?
10.2. Primary Care-of Address Registration
When a node receives a Binding Update, it MUST validate it and
determine the type of Binding Update according to the steps described
in Section 9.4.1. This section describes the processing of a valid
Binding Update that requests the receiving node to serve as its home
agent, registering its primary care-of address.
To begin processing the Binding Update, the home agent MUST perform
the following sequence of tests:
- If the node is not a router that implements home agent
functionality, then the node MUST reject the Binding Update
and MUST return a Binding Acknowledgement to the mobile node,
in which the Status field is set to 132 (home registration not
supported).
- Else, if the home address for the binding (the Home Address field
in the packet's Home Address option) is not an on-link IPv6
address with respect to the home agent's current Prefix List,
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then the home agent MUST reject the Binding Update and SHOULD
return a Binding Acknowledgement to the mobile node, in which the
Status field is set to 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 to the mobile node, in which the Status
field is set to an appropriate value to indicate the reason for
the rejection.
- A Home Address destination option MUST be present in the message,
and the value of the Home Address field in this option MUST
correspond to the Home Address field in the Binding Update.
- Finally, if the Duplicate Address Detection (D) bit is set in
the Binding Update, this home agent MUST perform Duplicate
Address Detection [33] on the mobile node's home link for the
link-local address associated with the home address in this
binding, before returning the Binding Acknowledgement. This
ensures that no other node on the home link can possibly use
the mobile node's home address. The address used for Duplicate
Address Detection SHOULD be the mobile node's link-local address.
Normal processing for Duplicate Address Detection specifies that,
in certain cases, the node SHOULD delay sending the initial
Neighbor Solicitation message of Duplicate Address Detection by a
random delay between 0 and MAX_RTR_SOLICITATION_DELAY [20, 33];
however, in this case, the home agent SHOULD NOT perform such a
delay. If this Duplicate Address Detection fails, then the home
agent MUST reject the Binding Update and MUST return a Binding
Acknowledgement to the mobile node, in which the Status field is
set to 138 (Duplicate Address Detection failed). When the home
agent sends a successful Binding Acknowledgement to the mobile
node, in response to a Binding Update with the `D' bit set, the
home agent assures to the mobile node that its home address will
continue to be kept unique by the home agent at least as long
as the mobile node transmits Binding Updates with new care-of
addresses for that home address.
If the home agent does not reject the Binding Update, then it becomes
or remains the home agent for the mobile node. The home agent MUST
then create a new entry in its Binding Cache for this mobile node,
or update its existing Binding Cache entry, if such an entry already
exists. The home address of the mobile node is taken to be the
value which, when the packet was originally received, was located
in the Home Address field in the packet's Home Address option. The
care-of address for this Binding Cache entry is taken to be the value
which, when the packet was originally received, was located either in
the Alternate Care-of Address option in the Binding Update option,
if present, or from the Source Address field in the packet's IPv6
header, otherwise.
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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 9.5) and MUST NOT be
removed from the Binding Cache until the expiration of the Lifetime
period.
If the `S' bit field in the Binding Update is zero, The home agent
creates or updates Binding Cache entries for each of possibly
several home addresses. The set of such home addresses is formed
by replacing the routing prefix for the given home address with
all other routing prefixes that are supported by the home agent
processing the Binding Update. The home agent creates such a
separate primary care-of address registration for each such home
address. Note that the same considerations for Duplicate Address
Detection apply for each affected home address.
The lifetime of the Binding Cache entry depends on a number of
factors:
- The lifetime for the Binding Cache entry MUST NOT be greater
than the remaining valid lifetime for the subnet prefix in the
mobile node's home address specified with the Binding Update,
and MUST NOT be greater than the Lifetime value specified in the
Binding Update. The remaining valid lifetime for this prefix is
determined by the home agent based on its own Prefix List entry
for this prefix [20].
- , However, if the `S' bit field in the Binding Update is zero,
the lifetime for the each Binding Cache entry MUST NOT be greater
than the minimum remaining valid lifetime for all subnet prefixes
on the mobile node's home link. If the value of the Lifetime
field specified by the mobile node in its Binding Update is
greater than this prefix lifetime, the home agent MUST decrease
the binding lifetime to less than or equal to the prefix valid
lifetime.
- The home agent MAY further decrease the specified lifetime for
the binding, for example based on a local policy. The resulting
lifetime is stored by the home agent in the Binding Cache entry,
and this Binding Cache entry MUST be deleted by the home agent
after the expiration of this lifetime.
Regardless of the setting of the 'A' bit in the Binding Update, the
home agent MUST return a Binding Acknowledgement to the mobile node,
constructed as follows:
- The Status field MUST be set to a value 0, indicating success.
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- The Sequence Number field MUST be copied from the Sequence Number
given in the Binding Update.
- The Lifetime field MUST be set to the remaining lifetime for
the binding as set by the home agent in its "home registration"
Binding Cache entry for the mobile node, as described above.
- The Refresh field MUST be set to a value less than or equal to
the Lifetime value being returned in the Binding Update. If the
home agent stores the Binding Cache entry in nonvolatile storage
(that survives the crash or other failure of the home agent),
then the Refresh field SHOULD be set to the same value as the
Lifetime field; otherwise, the home agent MAY set the Refresh
field to a value less than the Lifetime field, to indicate that
the mobile node SHOULD attempt to refresh its home registration
at the indicated shorter interval (although the home agent will
still retain the registration for the Lifetime period, even if
the mobile node does not refresh its registration within the
Refresh period).
In addition, the home agent MUST follow the procedure defined in
Section 10.4 to intercept packets on the mobile node's home link
addressed to the mobile node, while the home agent is serving as
the home agent for this mobile node. The home agent MUST also be
prepared to accept reverse tunneled packets from the new care-of
address of the mobile node, as described in Section 10.6. Finally,
the home agent MUST also propagate new home network prefixes, as
described in Section 10.9.1.
10.3. Primary Care-of Address De-Registration
When a node receives a Binding Update, it MUST validate it and
determine the type of Binding Update according to the steps described
in Section 9.4.1. This section describes the processing of a valid
Binding Update that requests the receiving node to no longer serve as
its home agent, de-registering its primary care-of address.
To begin processing the Binding Update, the home agent MUST perform
the following test:
- If the receiving node has no entry marked as a "home
registration" in its Binding Cache for this mobile node, then
this node MUST reject the Binding Update and SHOULD return a
Binding Acknowledgement to the mobile node, in which the Status
field is set to 137 (not home agent for this mobile node).
If the home agent does not reject the Binding Update as described
above, then it MUST delete any existing entry in its Binding Cache
for this mobile node, and proceed as follows.
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The home agent MUST return a Binding Acknowledgement to the mobile
node, constructed as follows:
- The Status field MUST be set to a value 0, indicating success.
- The Sequence Number field MUST be copied from the Sequence Number
given in the Binding Update.
- The Lifetime field MUST be set to zero.
- The Refresh field MUST be set to zero.
In addition, the home agent MUST stop intercepting packets on
the mobile node's home link that are addressed to the mobile node
(Section 10.4).
The rules for selecting the Destination IP address (and possibly
Routing Header construction) for the Binding Acknowledgement to the
mobile node are the same as in section 9.4.4.
10.4. Intercepting Packets for a Mobile Node
While a node is serving as the home agent for mobile node (while the
node has an entry in its Binding Cache for this mobile node that is
marked as a "home registration"), this node MUST attempt to intercept
packets on the mobile node's home link that are addressed to the
mobile node, and MUST tunnel each intercepted packet to the mobile
node using IPv6 encapsulation [4].
In order to intercept such packets on the home link, when a node
begins serving as the home agent for some mobile node (it did not
already have a Binding Cache entry for this mobile node marked as a
"home registration"), then the home agent MUST multicast onto the
home link a "gratuitous" Neighbor Advertisement message [20] on
behalf of the mobile node. Specifically, the home agent performs the
following steps:
- The home agent examines the value of the `S' bit in the new "home
registration" Binding Cache entry. If this bit is nonzero,
the following step is carried out only for the individual home
address specified for this binding. If, instead, this bit is
zero, then the following step is carried out for each address
for the mobile node formed from the interface identifier in
the mobile node's home address in this binding (the remaining
low-order bits in the address after the configured subnet
prefix), together with each one of the subnet prefixes currently
considered by the home agent to be on-link (including both the
link-local and site-local prefix).
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- For each specific IP address for the mobile node determined
in the first step above, the home agent sends a Neighbor
Advertisement message [20] to the all-nodes multicast address
on the home link, to advertise the home agent's own link-layer
address for this IP address on behalf of the mobile node.
All fields in each such Neighbor Advertisement message SHOULD be
set in the same way they would be set by the mobile node itself
if sending this Neighbor Advertisement while at home [20], with
the following exceptions:
* The Target Address in the Neighbor Advertisement message MUST
be set to the specific IP address for the mobile node.
* The Advertisement MUST include a Target Link-layer Address
option specifying the home agent's link-layer address.
* The Router (R) bit in the Advertisement MUST be set to zero.
* The Solicited Flag (S) in the Advertisement MUST NOT be set,
since it was not solicited by any Neighbor Solicitation
message.
* The Override Flag (O) in the Advertisement MUST be set,
indicating that the Advertisement SHOULD override any
existing Neighbor Cache entry at any node receiving it.
Any node on the home link receiving one of the Neighbor Advertisement
messages described above will thus update its Neighbor Cache to
associate the mobile node's address with the home agent's link
layer address, causing it to transmit any future packets for the
mobile node normally destined to this address instead to the mobile
node's home agent. Since multicasting on the local link (such as
Ethernet) is typically not guaranteed to be reliable, the home
agent MAY retransmit this Neighbor Advertisement message up to
MAX_ADVERT_REXMIT times to increase its reliability. It is still
possible that some nodes on the home link will not receive any of
these Neighbor Advertisements, but these nodes will eventually be
able to detect the link-layer address change for the mobile node's
home address, through use of Neighbor Unreachability Detection [20].
While a node is serving as a home agent for some mobile node (it
still has a "home registration" entry for this mobile node in its
Binding Cache), the home agent uses IPv6 Neighbor Discovery [20] to
intercept unicast packets on the home link addressed to the mobile
node's home address. In order to intercept packets in this way, the
home agent MUST act as a proxy for this mobile node, and 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
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registration". (Note that Binding Update messages with the `S' bit
set to zero will result in multiple Binding Cache entries, so checks
on all these entries necessarily include all possible home addresses
for the mobile node).
If such an entry exists in the home agent's Binding Cache, the home
agent MUST reply to the Neighbor Solicitation message with a Neighbor
Advertisement message, giving the home agent's own link-layer address
as the link-layer address for the specified Target Address. In
addition, the Router (R) bit in the Advertisement MUST be copied from
the corresponding bit in the home agent's Binding Cache entry for the
mobile node. Acting as a proxy in this way allows other nodes on
the mobile node's home link to resolve the mobile node's IPv6 home
address, and allows the home agent to defend these addresses on the
home link for Duplicate Address Detection [20].
10.5. 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 9.6 apply. The home agent (as a correspondent node) uses a
Routing header to route the packet to the mobile node by way of the
care-of address in the home agent's Binding Cache (the mobile node's
primary care-of address, in this case).
While the mobile node is away from home and this node is acting
as the mobile node's home agent, the home agent intercepts any
packets on the home link addressed to the mobile node's home address
(including addresses formed from other on-link prefixes, if the
Prefix Length field was nonzero in the Binding Update), as described
in Section 10.4. The home agent cannot use a Routing header to
forward these intercepted packets to the mobile node, since it cannot
modify the packet in flight without invalidating any existing IPv6
AH [12] or ESP [13] header present in the packet.
In order to forward each intercepted packet to the mobile node, the
home agent MUST tunnel the packet to the mobile node using IPv6
encapsulation [4]; the tunnel entry point node is the home agent,
and the tunnel exit point node is the primary care-of address as
registered with the home agent. When a home agent encapsulates
an intercepted packet for forwarding to the mobile node, the home
agent sets the Source Address in the new tunnel IP header to the
home agent's own IP address, and sets the Destination Address
in the tunnel IP header to the mobile node's primary care-of
address. When received by the mobile node (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.
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However, packets addressed to the mobile node's link-local address
MUST NOT be tunneled to the mobile node. Instead, such a packet MUST
be discarded, and the home agent SHOULD return an ICMP Destination
Unreachable, Code 3, message to the packet's Source Address (unless
this Source Address is a multicast address). Packets addressed to
the mobile node's site-local address SHOULD be tunneled to the mobile
node by default, but this behavior MUST be configurable to disable
it; currently, the exact definition and semantics of a "site" and a
site-local address are incompletely defined in IPv6, and this default
behavior might change at some point in the future.
Tunneling of multicast packets to a mobile node follows similar
limitations to those defined above for unicast packets addressed to
the mobile node's link-local and site-local addresses. Multicast
packets addressed to a multicast address with link-local scope [9],
to which the mobile node is subscribed, MUST NOT be tunneled
to the mobile node; such packets SHOULD be silently discarded
(after delivering to other local multicast recipients). Multicast
packets addressed to a multicast address with scope larger
than link-local but smaller than global (e.g., site-local and
organization-local) [9], to which the mobile node is subscribed,
SHOULD be tunneled to the mobile node by default, but this behavior
MUST be configurable to disable it; this default behavior might
change at some point in the future as the definition of these scopes
become more completely defined in IPv6.
10.6. Handling Reverse Tunneled Packets from a Mobile Node
Unless a binding has been established between the mobile node and a
correspondent node, traffic from the mobile node to the correspondent
node goes through a reverse tunnel. This tunnel extends between the
mobile node and the home agent. Home agents MUST support reverse
tunneling as follows:
- The tunneled traffic arrives to the home agent using IPv6
encapsulation [4].
- The tunnel entry point is the primary care-of address as
registered with the home agent and the tunnel exit point is the
home agent.
- When a home agent decapsulates a tunneled packet from the mobile
node, the home agent verifies that the Source Address in the
tunnel IP header is the mobile node's primary care-of address.
Reverse tunneled packets MAY be discarded unless accompanied by a
valid AH or ESP header, depending on the security policies used by
the home agent. In any case, the home agent MUST check that the
source address in the tunneled packets corresponds to the currently
registered location of the mobile node, as otherwise any node in the
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Internet could send traffic through the home agent and escape ingress
filtering limitations.
The support for authenticated reverse tunneling allows the home agent
to protect the home network and correspondent nodes from malicious
nodes masquerading as a mobile node, even if they know the current
location of the real mobile node.
10.7. Protecting Return Routability Packets
The return routability procedure described in Section 5 assumes that
the confidentiality of the HoTI and HoT messages is protected as
it is tunneled from the home agent to the mobile node. Therefore,
the home agent MUST support IPsec ESP for the protection of packets
belonging to the return routability procedure. Support for a
non-null encryption transform MUST be available. In this case it
isn't necessary to distinguish between different kinds of packets
within the return routability procedure.
The use of ESP for protection of the return routability procedure is
optional and controlled by configuration of the IPsec security policy
database both at the mobile node and at the home agent.
As described earlier, the Binding Update and Binding Acknowledgement
messages require protection between the home agent and the mobile
node. These messages and the return routability messages employ
the same protocol from the point of view of the security policy
database, the Mobility Header. One way to set up the security policy
database is to have one rule for the Mobility Header traffic between
the mobile node and the home agent addresses, and an optional rule
following it for Mobility Header traffic between the mobile node and
any other address.
10.8. Receiving Router Advertisement Messages
For each link on which a router provides service as a home agent,
the router maintains a Home Agents List recording information
about all other home agents on that link. This list is used in
the dynamic home agent address discovery mechanism, described in
Section 10.9. The information for the list is learned through
receipt of the periodic unsolicited multicast Router Advertisements,
in a manner similar to the Default Router List conceptual data
structure maintained by each host for Neighbor Discovery [20]. In
the construction of the Home Agents List, the Router Advertisements
are from each other home agent on the link, and the Home Agent (H)
bit is set in them.
On receipt of a valid Router Advertisement, as defined in the
processing algorithm specified for Neighbor Discovery [20], the home
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agent performs the following steps, in addition to any steps already
required of it by Neighbor Discovery:
- If the Home Agent (H) bit in the Router Advertisement is not set,
check to see if the sending node has an entry in the current Home
Agents List. If it does, delete the corresponding entry. In any
case all of the following steps are skipped.
- Otherwise, extract the Source Address from the IP header of the
Router Advertisement. This is the link-local IP address on this
link of the home agent sending this Advertisement [20].
- Determine from the Router Advertisement the preference for this
home agent. If the Router Advertisement contains a Home Agent
Information Option, then the preference is taken from the Home
Agent Preference field in the option; otherwise, the default
preference of 0 MUST be used.
- Determine from the Router Advertisement the lifetime for
this home agent. If the Router Advertisement contains a Home
Agent Information Option, then the lifetime is taken from
the Home Agent Lifetime field in the option; otherwise, the
lifetime specified by the Router Lifetime field in the Router
Advertisement SHOULD be used.
- If the link-local address of the home agent sending this
Advertisement is already present in this home agent's Home
Agents List and the received home agent lifetime value is zero,
immediately delete this entry in the Home Agents List.
- Otherwise, if the link-local address of the home agent sending
this Advertisement is already present in the receiving home
agent's Home Agents List, reset its lifetime and preference to
the values determined above.
- If the link-local address of the home agent sending this
Advertisement, as determined above, is not already present in
the Home Agents List maintained by the receiving home agent, and
the lifetime for the sending home agent, as determined above,
is non-zero, create a new entry in the list, and initialize its
lifetime and preference to the values determined above.
- If the Home Agents List entry for the link-local address of
the home agent sending this Advertisement was not deleted as
described above, determine any global address(es) of the home
agent based on each Prefix Information option received in
this Advertisement in which the Router Address (R) bit is set
(Section 7.2). For each such global address determined from this
Advertisement, add this global address to the list of global
addresses for this home agent in this Home Agents List entry.
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A home agent SHOULD maintain an entry in its Home Agents List for
each such valid home agent address until that entry's lifetime
expires, after which time the entry MUST be deleted.
10.9. Dynamic Home Agent Address Discovery
A mobile node, while away from home, MAY use the dynamic home agent
address discovery mechanism in section 11.3.2 to attempt to discover
the address of one or more routers serving as home agents on its home
link. This discovery might become necessary, for example, if some
nodes on its home link have been reconfigured while the mobile node
has been away from home, such that the router that was operating as
the mobile node's home agent has been replaced by a different router
serving this role.
As described in Section 11.3.2, a mobile node attempts dynamic
home agent address discovery by sending an ICMP Home Agent Address
Discovery Request message to the "Mobile IPv6 Home-Agents" anycast
address [11] for its home IP subnet prefix, using its care-of address
as the Source Address of the packet. A home agent receiving such a
Home Agent Address Discovery Request message that is serving this
subnet (the home agent is configured with this anycast address on one
of its network interfaces) SHOULD return an ICMP Home Agent Address
Discovery Reply message to the mobile node (at its care-of address
that was used as the Source Address of the Request message), with the
Source Address of the Reply packet set to one of the global unicast
addresses of the home agent. The Home Agent Addresses field in the
Reply message is constructed as follows:
- The Home Agent Addresses field SHOULD contain one global IP
address for each home agent currently listed in this home
agent's own Home Agents List (Section 4.5). However, if this
home agent's own global IP address would be placed in the list
(as described below) as the first entry in the list, then this
home agent SHOULD NOT include its own address in the Home Agent
Addresses field in the Reply message. Not placing this home
agent's own IP address in the list will cause the receiving
mobile node to consider this home agent as the most preferred
home agent; otherwise, this home agent will be considered to be
preferred in its order given by its place in the list returned.
- The IP addresses in the Home Agent Addresses field SHOULD be
listed in order of decreasing preference value, based either
on the respective advertised preference from a Home Agent
Information option or on the default preference of 0 if no
preference is advertised (or on the configured home agent
preference for this home agent itself). The home agent with
the highest preference SHOULD be listed first in the Home Agent
Addresses field, and the home agent with the lowest preference
SHOULD be listed last.
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- Among home agents with equal preference, their IP addresses
in the Home Agent Addresses field SHOULD be listed in an
order randomized with respect to other home agents with equal
preference, each time a Home Agent Address Discovery Reply
message is returned by this home agent.
- For each entry in this home agent's Home Agents List, if more
than one global IP address is associated with this list entry,
then one of these global IP addresses SHOULD be selected
to include in the Home Agent Addresses field in the Reply
message. As described in Section 4.5, one Home Agents List
entry, identified by the home agent's link-local address,
exists for each home agent on the link; associated with that
list entry is one or more global IP addresses for this home
agent, learned through Prefix Information options with the
Router Address (R) bit is set, received in Router Advertisements
from this link-local address.
The selected global IP address for each home agent to include in
forming the Home Agent Addresses field in the Reply message MUST
be the global IP address of the respective home agent sharing a
prefix with the Destination IP address of the Request message;
if no such global IP address is known for some home agent, an
entry for that home agent MUST NOT be included in the Home Agent
Addresses field in the Reply message.
- In order to avoid the possibility of the Reply message packet
being fragmented (or rejected by an intermediate router with an
ICMP Packet Too Big message [5]), if the resulting total packet
size containing the complete list of home agents in the Home
Agent Addresses field would exceed the minimum IPv6 MTU [6], the
home agent SHOULD reduce the number of home agent IP addresses
returned in the packet to the number of addresses that will fit
without exceeding this limit. The home agent addresses returned
in the packet SHOULD be those from the complete list with the
highest preference.
10.9.1. Aggregate List of Home Network Prefixes
IPv6 provides mechanisms for node configuration when it turns on,
and in renumbering a subnet, such as when a site switches to a new
network service provider. These mechanisms are a part of Neighbor
Discovery [20] and Address Autoconfiguration [33].
In renumbering, new prefixes and addresses can be introduced for the
subnet and old ones can be deprecated and removed. These mechanisms
are defined to work while all nodes using the old prefixes are at
home, connected to the link using these prefixes. Mobile IPv6
extends these mechanisms to work also with mobile nodes that are away
from home when the renumbering takes place.
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Mobile IPv6 arranges to propagate relevant prefix information to the
mobile node when it is away from home, so that it may be used in
mobile node home address configuration, and in network renumbering.
In this mechanism, mobile nodes away from home receive Mobile Prefix
Advertisements messages with Prefix Information Options, which give
the valid lifetime and preferred lifetime for available prefixes on
the home link.
To avoid possible security attacks from forged Mobile Prefix
Advertisements all such Advertisements must be authenticated to the
mobile node by its home agent using IPsec [14, 12, 13] if a security
associate exists (i.e. unless the mobile node does not yet have a
home address configured).
A mobile node on a remote network SHOULD autoconfigure all of the
global IP addresses, which it would autoconfigure if it were attached
to its home network, from network prefixes representing network
addresses that are served by home agents. Site-local addresses MAY
be autoconfigured if the mobile node is roaming in a network on the
same site as its home addresses. Site-local addresses and addresses
not served by a home agent MUST NOT be autoconfigured, since they are
unusable in the remote network.
To support this, the home agent monitors prefixes advertised by
itself and other home agents routers on the home link, and passes
this aggregated list of relevant subnet prefixes on to the mobile
node in Mobile Prefix Advertisements.
The home agent SHOULD construct the aggregate list of home subnet
prefixes as follows:
- Copy prefix information defined in the home agent's AdvPrefixList
on the home subnet's interfaces to the aggregate list. Also
apply any changes made to the AdvPrefixList on the home agent to
the aggregate list.
- Check valid prefixes received in Router Advertisements
from the home network for consistency with the home agent's
AdvPrefixList, as specified in section 6.2.7 of RFC 2461
(Neighbor Discovery [20]). Do not update the aggregate list with
any information from received prefixes that fail this check.
- Check Router Advertisements which contain an `H' bit (from other
home agents) for valid prefixes that are not yet in the aggregate
list, and if they are usable for autoconfiguration (`A' bit set,
and prefix length is valid for address autoconfiguration on the
home subnet) add them and preserve the `L' flag value. Clear the
`R' flag and zero the interface-id portion of the prefix field
to prevent mobile nodes from treating another router's interface
address as belonging to the home agent. Treat the lifetimes
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of these prefixes as decrementing in real time, as defined in
section 6.2.7 of RFC 2461 [20].
- Do not perform consistency checks on valid prefixes received in
Router Advertisements on the home network that do not exist in
the home agent's AdvPrefixList. Instead, if the prefixes already
exist in the aggregate list, update the prefix lifetime fields in
the aggregate list according to the rules specified for hosts in
section 6.3.4 of RFC 2461 (Neighbor Discovery [20]) and section
5.5.3 of RFC 2462 (Stateless Address Autoconfiguration [33]).
- If the L flag is set on valid prefixes received in a Router
Advertisement, and that prefix already exists in the aggregate
list, set the flag in the aggregate list. Ignore the flag if it
is clear.
- Delete prefixes from the aggregate list when their valid
lifetimes expire.
The home agent uses the information in the aggregate list to
construct Mobile Prefix Advertisements. It may be possible to
construct an aggregate list by combining information contained in the
home agent's AdvPrefixList and its Home Agents List used for Dynamic
Home Agent Address Discovery (Section 11.3.2).
10.9.2. Scheduling Prefix Deliveries to the Mobile Node
A home agent serving a mobile node will schedule the delivery of new
prefix information to that mobile node when any of the following
conditions occur:
MUST:
- The valid or preferred lifetime or the state of the flags changes
for the prefix of the mobile node's registered home address.
- The mobile node requests the information with a Mobile Prefix
Solicitation (see section 11.3.3).
MAY:
- A new prefix is added to the aggregate list.
- The valid or preferred lifetime or the state of the flags changes
for a prefix which is not used in any binding cache entry for
this mobile node.
The home agent uses the following algorithm to determine when to send
prefix information to the mobile node.
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- If the mobile node has not received the prefix information within
the last HomeRtrAdvInterval seconds, then transmit the prefix
information. This MAY be done according to a periodically
scheduled transmission.
- If a mobile node sends a solicitation, answer right away.
- If a prefix in the aggregate list that matches the mobile node's
home registration is added, or if its information changes in
any way that does not cause the mobile node's address to go
deprecated, ensure that a transmission is scheduled (as described
below), and calculate RAND_ADV_DELAY in order to randomize the
time at which the transmission is scheduled.
- If a home registration expires, cancel any scheduled
advertisements to the mobile node.
Assume that the home agent already has scheduled the transmission of
a Router Advertisement to the mobile node. New information should
be added to the existing scheduled transmission, if the freshly
calculated RAND_ADV_DELAY would cause another transmission before
the expiration of the Preferred Lifetime of the mobile node's home
address derived from the prefix whose advertisement information has
changed. In this case, the home agent does not perform the following
algorithm to schedule an advertisement to the mobile node.
Otherwise, the home agent uses the following algorithm to compute
a fresh value for RAND_ADV_DELAY, the offset from the current time
for the scheduled transmission. If there is already a scheduled
transmission, add the data from the existing scheduled transmission
to the newly scheduled transmission, deleting the previously
scheduled transmission event.
RAND_ADV_DELAY is the offset from the current time to be used
to schedule the necessary advertisement to the mobile node. The
computation is expected to alleviate bursts of advertisements when
prefix information changes. In addition, a home agent MAY further
reduce the rate of packet transmission by further delaying individual
advertisements, if needed to avoid overwhelming local network
resources.
Calculate the newly advertised Preferred Lifetime as follows.
MAX_SCHEDULE_DELAY == min (MAX_PFX_ADV_DELAY, Preferred Lifetime)
Then compute RAND_ADV_DELAY ==
MinRtrAdvInt + rand()*(MAX_SCHEDULE_DELAY - MinRtrAdvInt)
The home agent SHOULD periodically continue to retransmit an
unsolicited Advertisement to the mobile node, until it is
acknowledged by the receipt from the mobile node of a Binding Update
matching the Binding Refresh Request in the packet (i.e., with
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matching Unique Identifier mobility option). The home agent MUST
wait PREFIX_ADV_TIMEOUT before the first retransmission, and double
the retransmission wait time for every succeeding retransmission, up
until a maximum of PREFIX_ADV_RETRIES attempts. If the mobile node's
bindings expire before the matching Binding Update has been received,
then the home agent MUST NOT attempt any more retransmissions, even
if not all PREFIX_ADV_RETRIES have been retransmitted. After another
Binding Update is received from the mobile node, and if the mobile
node has not returned to the home network in the meantime, the home
agent SHOULD begin the process again of transmitting the unsolicited
Advertisement.
A Binding Update matches a Binding Refresh Request if it specifies
a binding for the mobile node to which the Binding Refresh Request
was sent and contains a Unique Identifier mobility option matching
the unique identifier sent in the Unique Identifier option in the
Binding Refresh Request. In the solicited case, the mobile node will
retransmit solicitations until one is received; thus, the home agent
SHOULD NOT retransmit the responding advertisement.
If while the home agent is still retransmitting a Mobile Prefix
Advertisement to the mobile node, another condition as described
above occurs on the home link causing another Prefix Advertisement to
be sent to the mobile node, the home agent SHOULD combine any Prefix
Information options in the unacknowledged Mobile Prefix Advertisement
into the new Advertisement, discard the old Advertisement, and then
begin retransmitting the new one. according to the algorithm in
section 10.9.2. The home agent MUST generate a new unique identifier
for use in the Unique Identifier Option in the Binding Refresh
Request tunneled with the new Mobile Prefix Advertisement.
10.9.3. Sending Advertisements to the Mobile Node
When sending a Mobile Prefix Advertisement to the mobile node, the
home agent MUST construct the packet as follows:
- The Source Address in the packet's IPv6 header MUST be set to
the home agent's IP address to which the mobile node addressed
its current home registration, or its default global home agent
address if no binding exists.
- If a security association exists with the mobile node's address,
the packet MUST be protected by IPsec [14, 12, 13] to guard
against malicious Mobile Prefix Advertisements. The IPsec
protection MUST provide sender authentication, data integrity
protection, and replay protection, covering the Mobile Prefix
Advertisement.
- A separate Binding Refresh Request message MUST be sent in
addition to the advertisement, if this is the first advertisement
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for a home registration, or if there was a change in prefix
information since the last acknowledged advertisement was sent to
the mobile node for the home registration. The Binding Refresh
Request message MUST include a Unique Identifier mobility option
(Section 6.2.4), with the unique identifier in the option data
set to a value different than that in any other Binding Refresh
Request sent recently by this home agent. It is assumed that
this requirement can be met by maintaining a simple 16-bit
"wrap-around" counter to generate unique identifiers for Binding
Refresh Requests that contain a Unique Identifier option,
incremented each time a Binding Refresh Request containing a
Unique Identifier option is sent.
- If the advertisement was solicited, it should be destined
(and authenticated, if possible) to the source address of
the solicitation. If it was triggered by prefix changes or
renumbering, the advertisement's destination will be the mobile
node's home address in the binding which triggered the rule.
- The packet MUST be sent as any other unicast IPv6 packet. If a
care-of address is used, the packet will be delivered directly.
If a binding exists, the home agent will send the packet with
a routing header containing the care-of address, as any other
packet sent to the mobile node originated by the home agent
(rather than using IPv6 encapsulation, as would be used by the
home agent for intercepted packets).
10.9.4. Lifetimes for Changed Prefixes
As described in Section 10.2, the lifetime returned by the home agent
in a Binding Acknowledgement MUST be no greater than the remaining
valid lifetime for the subnet prefix in the mobile node's home
address. This limit on the binding lifetime serves to prohibit use
of a mobile node's home address after it becomes invalid.
11. Mobile Node Operation
11.1. Conceptual Data Structures
Each mobile node MUST maintain a Binding Update List and Home Agents
List.
The rules for maintaining a Home Agents List are same for home agents
and correspondent nodes, and have been described in Section 10.1.
The Binding Update List records information for each Binding Update
sent by this mobile node, for which the Lifetime sent in that Binding
Update has not yet expired. The Binding Update List includes all
bindings sent by the mobile node: those to correspondent nodes,
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those to the mobile node's home agent, and those to a home agent
on the link on which the mobile node's previous care-of address is
located. It also contains Binding Updates which are waiting for
the completion of the return routability procedure before they can
be sent. However, for multiple Binding Updates sent to the same
destination address, the Binding Update List contains only the most
recent Binding Update (i.e., with the greatest Sequence Number value)
sent to that destination. The Binding Update List MAY be implemented
in any manner consistent with the external behavior described in this
document.
Each Binding Update List entry conceptually contains the following
fields:
- The IP address of the node to which a Binding Update was sent.
If the Binding Update was successfully received by that node
(e.g., not lost by the network), a Binding Cache entry may have
been created or updated based on this Binding Update. The
Binding Cache entry may still exist, if that node has not deleted
the entry before its expiration (e.g., to reclaim space in its
Binding Cache for other entries).
- The home address for which that Binding Update was sent. This
will be one of the following:
* one the mobile node's home addresses for typical Binding
Updates (Sections 11.6.1 and 11.6.2), or
* the mobile node's previous care-of address for Binding
Updates sent to establish forwarding from the mobile node's
previous location (Section 11.6.6).
- The care-of address sent in that Binding Update. This value
is necessary for the mobile node to determine if it has sent a
Binding Update giving its new care-of address to this destination
after changing its care-of address.
- The initial value of the Lifetime field sent in that Binding
Update.
- The remaining lifetime of that binding. This lifetime is
initialized from the Lifetime value sent in the Binding Update
and is decremented until it reaches zero, at which time this
entry MUST be deleted from the Binding Update List.
- The maximum value of the Sequence Number field sent in previous
Binding Updates to this destination. The Sequence Number field
is 16 bits long, and all comparisons between Sequence Number
values MUST be performed modulo 2**16. For example, using an
implementation in the C programming language, a Sequence Number
value A is greater than another Sequence Number value B if
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((short)((a) - (b)) > 0), if the "short" data type is a 16-bit
signed integer.
- The time at which a Binding Update was last sent to this
destination, as needed to implement the rate limiting restriction
for sending Binding Updates.
- The state of any retransmissions needed for this Binding Update,
if the Acknowledge (A) bit was set in this Binding Update. This
state includes the time remaining until the next retransmission
attempt for the Binding Update, and the current state of the
exponential back-off mechanism for retransmissions.
- A flag that, when set, indicates that future Binding Updates
should not be sent to this destination. The mobile node sets
this flag in the Binding Update List entry when it receives an
ICMP Parameter Problem, Code 1, error message in response to
a return routability message or Binding Update sent to that
destination, as described in Section 11.7.
The Binding Update list also conceptually contains data related to
running the return routability procedure. This data is relevant only
for Binding Updates sent to correspondent nodes.
- The time at which a Home Test Init or Care-of Test Init message
was last sent to this destination, as needed to implement the
rate limiting restriction for the return routability procedure.
- The state of any retransmissions needed for this return
routability procedure. This state includes the time remaining
until the next retransmission attempt and the current state of
the exponential back-off mechanism for retransmissions.
- Mobile cookie values used the Home Test Init and Care-of Test
Init messages.
- Home and care-of cookies received from the correspondent node.
- Home and care-of nonce indices received from the correspondent
node.
- The time at which each of the cookies was received from this
correspondent node, as needed to implement cookie reuse while
moving.
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11.2. Packet Processing
11.2.1. Sending Packets While Away from Home
While a mobile node is away from home, it continues to use its home
address, as well as also using one or more care-of addresses. When
sending a packet while away from home, a mobile node MAY choose among
these in selecting the address that it will use as the source of the
packet, as follows:
- From the point of view of protocol layers and applications
above Mobile IP (e.g., transport protocols), the mobile node
will generally use its home address as the source of the packet
for most packets, even while away from home, since Mobile IP
is designed to make mobility transparent to such software.
For packets sent that are part of transport-level connections
established while the mobile node was at home, the mobile node
MUST use its home address. Likewise, for packets sent that are
part of transport-level connections that the mobile node may
still be using after moving to a new location, the mobile node
SHOULD use its home address in this way. When sending such
packets, the delivery method depends on whether a binding exists
with the correspondent node. If a binding exists, the mobile
node SHOULD send the packets directly to the correspondent node.
Otherwise, if a binding does not exist, the mobile node MUST use
reverse tunneling. Detailed operation for both of these cases is
described later in this section.
- For short-term communication, particularly for communication that
may easily be retried if it fails, the mobile node MAY choose
to directly use one of its care-of addresses as the source of
the packet, thus not requiring the use of a Home Address option
in the packet. An example of this type of communication might
be DNS queries sent by the mobile node [17, 18]. Using the
mobile node's care-of address as the source for such queries will
generally have a lower overhead than using the mobile node's
home address, since no extra options need be used in either the
query or its reply, and all packets can be routed normally,
directly between their source and destination without relying
on Mobile IP. If the mobile node has no particular knowledge
that the communication being sent fits within this general type
of communication, however, the mobile node SHOULD NOT use its
care-of address as the source of the packet in this way.
For packets sent by a mobile node while it is at home, no special
Mobile IP processing is required for sending this packet. Likewise,
if the mobile node uses any address other than its home address as
the source of a packet sent while away from home (from the point of
view of higher protocol layers or applications, as described above),
no special Mobile IP processing is required for sending that packet.
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In each case, the packet is simply addressed and transmitted in the
same way as any normal IPv6 packet.
For each other packet sent by the mobile node (i.e., packets
sent while away from home, using the mobile node's home address
as the source, from the point of view of higher protocol layers
and applications), special Mobile IP processing of the packet is
required. This can be done in two ways, as described above. These
ways are:
direct delivery
This is manner of delivering packets does not require going
through the home network, and typically will enable faster and
more reliable transmission. A mobile node SHOULD arrange to
supply the home address in a Home Address option, and allowing
the IPv6 header's Source Address field to be set to one of the
mobile node's care-of addresses; the correspondent node will
then use the address supplied in the Home Address option to
serve the function traditionally done by the Source IP address
in the IPv6 header. the mobile node's home address is then
supplied to higher protocol layers and applications.
Specifically:
- Construct the packet using the mobile node's home address
as the packet's Source Address, in the same way as if the
mobile node were at home. This preserves the transparency
of Mobile IP to higher protocol layers (e.g., TCP).
- Insert a Home Address option into the packet, with the Home
Address field copied from the original value of the Source
Address field in the packet.
- Change the Source Address field in the packet's IPv6 header
to one of the mobile node's care-of addresses. This will
typically be the mobile node's current primary care-of
address, but MUST be a care-of address with a subnet prefix
that is on-link on the network interface on which the
mobile node will transmit the packet.
By using the care-of address as the Source Address in the IPv6
header, with the mobile node's home address instead in the Home
Address option, the packet will be able to safely pass through
any router implementing ingress filtering [7].
reverse tunneling
This is the mechanism which tunnels the packets via the home
agent. It isn't as efficient as the above mechanism, but is
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needed if there is no binding yet with the correspondent node.
Specifically:
- The packet is sent to the home agent using IPv6
encapsulation [4].
- The Source Address in the tunnel packet is the primary
care-of address as registered with the home agent.
- The Destination Address in the tunnel packet is the home
agent's address.
Reverse tunneled packets MAY be protected using a AH or ESP
header, depending on the security policies used by the home
agent. The support for encrypted reverse tunneling allows
mobile nodes to defeat certain kinds of traffic analysis, and
provides a mechanism by which routers on the home network can
distinguish authorized traffic from other possibly malicious
traffic.
11.2.2. Interaction with Outbound IPsec Processing
This section sketches the interaction between outbound Mobile
IP processing and outbound IP Security (IPsec) processing for
packets sent by a mobile node while away from home. Any specific
implementation MAY use algorithms and data structures other than
those suggested here, but its processing MUST be consistent with the
effect of the operation described here and with the relevant IPsec
specifications. In the steps described below, it is assumed that
IPsec is being used in transport mode [14] and that the mobile node
is using its home address as the source for the packet (from the
point of view of higher protocol layers or applications, as described
in Section 11.2.1):
- The packet is created by higher layer protocols and applications
(e.g., by TCP) as if the mobile node were at home and Mobile IP
were not being used. Mobile IP is transparent to such higher
layers.
- As part of outbound packet processing in IP, the packet is
compared against the IPsec security policy database to determine
what processing is required for the packet [14].
- If IPsec processing is required, the packet is either mapped to
an existing Security Association (or SA bundle), or a new SA (or
SA bundle) is created for the packet, according to the procedures
defined for IPsec.
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- Since the mobile node is away from home, the mobile is either
using reverse tunneling or route optimization to reach the
correspondent node.
If reverse tunneling is used, the packet is constructed in the
normal manner and then tunneled through the home agent.
If route optimization is in use, the mobile node inserts a Home
Address destination option into the packet, replacing the Source
Address in the packet's IP header with a care-of address suitable
for the link on which the packet is being sent, as described in
Section 11.2.1. The Destination Options header in which the Home
Address destination option is inserted MUST appear in the packet
after the Routing Header, if present, and before the AH [12] (or
ESP [13]) header, so that the Home Address destination option is
processed by the destination node before the AH or ESP header is
processed.
Finally, once the packet is fully assembled, the necessary IPsec
authentication (and encryption, if required) processing is
performed on the packet, initializing the Authentication Data
in the AH or ESP header. The AH authentication data MUST be
calculated as if the following were true:
* the IPv6 source address in the IPv6 header contains the
mobile node's home address,
* the Home Address field of the Home Address destination option
(section 6.3) contains the new care-of address.
- This allows, but does not require, the receiver of the packet
containing a Home Address destination option to exchange the
two fields of the incoming packet, simplifying processing for
all subsequent packet headers. The mechanics of implementation
do not absolutely require such an exchange to occur; other
implementation strategies may be more appropriate, as long as the
result of the authentication calculation remains the same.
In addition, when using any automated key management protocol [14]
(such as IKE [8]) to create a new SA (or SA bundle) while away from
home, a mobile node MUST take special care in its processing of the
key management protocol. Otherwise, other nodes with which the
mobile node must communicate as part of the automated key management
protocol processing may be unable to correctly deliver packets to
the mobile node if they and/or the mobile node's home agent do
not then have a current Binding Cache entry for the mobile node.
For the default case of using IKE as the automated key management
protocol [8][14], such problems can be avoided by the following
requirements on the use of IKE by a mobile node while away from home:
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- The mobile node MUST use its care-of address as the Source
Address of all packets it sends as part of the key management
protocol (without use of Mobile IP for these packets, as
suggested in Section 11.2.1).
- In addition, for all security associations bound to the mobile
node's home address established by way of IKE, the mobile node
MUST include an ISAKMP Identification Payload [16] in the IKE
exchange, giving the mobile node's home address as the initiator
of the Security Association [28].
11.2.3. Receiving Packets While Away from Home
While away from home, a mobile node will receive packets addressed to
its home address, by one of three methods:
- Packets sent by a correspondent node that does not have a
Binding Cache entry for the mobile node, will be sent by the
correspondent node in the same way as any normal IP packet. Such
packets will then be intercepted by the mobile node's home agent,
encapsulated using IPv6 encapsulation [4], and tunneled to the
mobile node's primary care-of address.
- Packets sent by a correspondent node that has a Binding Cache
entry for the mobile node that contains the mobile node's current
care-of address, will be sent by the correspondent node using
a type 2 Routing header. The packet will be addressed to the
mobile node's care-of address, with the final hop in the Routing
header directing the packet to the mobile node's home address;
the processing of this last hop of the Routing header is entirely
internal to the mobile node, since the care-of address and home
address are both addresses within the mobile node.
- Packets sent by a correspondent node that has a Binding
Cache entry for the mobile node that contains an out-of-date
care-of address for the mobile node, will also be sent by the
correspondent node using a type 2 Routing header, as described
above. If the mobile node sent a Binding Update to a home agent
on the link on which its previous care-of address is located
(Section 11.6.6), and if this home agent is still serving as
a home agent for the mobile node's previous care-of address,
then such a packet will be intercepted by this home agent,
encapsulated using IPv6 encapsulation [4], and tunneled to the
mobile node's new care-of address (registered with this home
agent).
For packets received by the first of these methods, the mobile node
MUST check that the IPv6 source address of the tunnel packet is the
IP address of its home agent.
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For packets received by either the first or last of these three
methods, the mobile node SHOULD send a Binding Update to the original
sender of the packet, as described in Section 11.6.2, subject to
the rate limiting defined in Section 11.6.9. The mobile node MUST
also process the received packet in the manner defined for IPv6
encapsulation [4], which will result in the encapsulated (inner)
packet being processed normally by upper-layer protocols within the
mobile node, as if it had been addressed (only) to the mobile node's
home address.
For packets received by the second method above (using a Type 2
Routing header), the following rules will result in the packet being
processed normally by upper-layer protocols within the mobile node,
as if it had been addressed to the mobile node's home address.
A node receiving a packet addressed to itself (i.e., one of the
node's addresses is in the IPv6 destination field) follows the next
header chain of headers and processes them. When it encounters
a Type 2 Routing header during this processing it performs the
following checks. If any of these checks fail the node MUST silently
discard the packet.
- The length field in the RH is exactly 2.
- The segments left field in the RH is either 0 or 1.
- The Home Address field in the RH is one of the node's home
addresses, if the segments left field was 1.
Once the above checks have been performed, the node swaps the IPv6
destination field with the Home Address field in the RH, decrements
segments left, and resubmits the packet to IP for processing the
next header. Conceptually this follows the same model as in RFC
2460. However, in the case of Type 2 Routing header this can be
simplified since it is known that the packet will not be forwarded to
a different node.
The definition of AH requires the sender to calculate the AH
integrity check value of a routing header in a way as it appears in
the receiver after it has processed the header. Since IPsec headers
follow the Routing Header, any IPsec processing will operate on
the packet with the home address in the IP destination field and
segments left being zero. Thus, the AH calculations at the sender
and receiver will have an identical view of the packet.
/
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11.2.4. Routing Multicast Packets
A mobile node that is connected to its home link functions in the
same way as any other (stationary) node. Thus, when it is at home,
a mobile node functions identically to other multicast senders and
receivers. This section therefore describes the behavior of a mobile
node that is not on its home link.
In order to receive packets sent to some multicast group, a mobile
node must join that multicast group. One method by which a mobile
node MAY join the group is via a (local) multicast router on the
foreign link being visited. The mobile node SHOULD use one of its
care-of addresses that shares a subnet prefix with the multicast
router, as the source IPv6 address of its multicast group membership
control messages. If the multicast applications depend on the
address of the joining node, the mobile node MAY establish a binding
with the router and use the Home Address destination option in the
sent control messages.
Alternatively, a mobile node MAY join multicast groups via a
bi-directional tunnel to its home agent. The mobile node tunnels its
multicast group membership control packets to its home agent, and the
home agent forwards multicast packets down the tunnel to the mobile
node.
A mobile node that wishes to send packets to a multicast group
also has two options: (1) send directly on the foreign link being
visited; or (2) send via a tunnel to its home agent. Because
multicast routing in general depends upon the Source Address used in
the IPv6 header of the multicast packet, a mobile node that tunnels a
multicast packet to its home agent MUST use its home address as the
IPv6 Source Address of the inner multicast packet.
11.3. Home Agent and Prefix Management
11.3.1. Receiving Local Router Advertisement Messages
Each mobile node maintains a Home Agents List recording information
about all home agents from which it receives a Router Advertisement,
for which the home agent lifetime indicated in that Router
Advertisement has not yet expired. This list is used by the mobile
node to enable it to send a Binding Update to the global unicast
address of a home agent on its previous link when it moves to a new
link, as described in Section 11.6.6. On receipt of a valid Router
Advertisement, as defined in the processing algorithm specified for
Neighbor Discovery [20], the mobile node performs the following
steps, in addition to any steps already required of it by Neighbor
Discovery.
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- If the Home Agent (H) bit in the Router Advertisement is not set,
and the sending node currently has an entry in the node's Home
Agents List, delete the corresponding entry. Subsequently, skip
all of the following steps.
- Otherwise, extract the Source Address from the IP header of the
Router Advertisement. This is the link-local IP address on this
link of the home agent sending this Advertisement [20].
- Determine from the Router Advertisement the preference for this
home agent. If the Router Advertisement contains a Home Agent
Information Option, then the preference is taken from the Home
Agent Preference field in the option; otherwise, the default
preference of 0 MUST be used.
- Determine from the Router Advertisement the lifetime for
this home agent. If the Router Advertisement contains a Home
Agent Information Option, then the lifetime is taken from
the Home Agent Lifetime field in the option; otherwise, the
lifetime specified by the Router Lifetime field in the Router
Advertisement SHOULD be used.
- If the link-local address of the home agent sending this
Advertisement is already present in this mobile node's Home
Agents List and the received home agent lifetime value is zero,
immediately delete this entry in the Home Agents List.
- Otherwise, if the link-local address of the home agent sending
this Advertisement is already present in the receiving mobile
node's Home Agents List, reset its lifetime and preference to the
values determined above.
- If the link-local address of the home agent sending this
Advertisement, as determined above, is not already present in the
Home Agents List maintained by the receiving mobile node, and
the lifetime for the sending home agent, as determined above,
is non-zero, create a new entry in the list, and initialize its
lifetime and preference to the values determined above.
- If the Home Agents List entry for the link-local address of
the home agent sending this Advertisement was not deleted as
described above, determine any global address(es) of the home
agent based on each Prefix Information option received in
this Advertisement in which the Router Address (R) bit is set
(Section 7.2). For each such global address determined from this
Advertisement, add this global address to the list of global
addresses for this home agent in this Home Agents List entry.
A mobile node SHOULD maintain an entry in its Home Agents List for
each such valid home agent address until that entry's lifetime
expires, after which time the entry MUST be deleted.
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11.3.2. Dynamic Home Agent Address Discovery
Sometimes, when the mobile node needs to send a Binding Update to its
home agent to register its new primary care-of address, as described
in Section 11.6.1, the mobile node may not know the address of any
router on its home link that can serve as a home agent for it. For
example, some nodes on its home link may have been reconfigured while
the mobile node has been away from home, such that the router that
was operating as the mobile node's home agent has been replaced by a
different router serving this role.
In this case, the mobile node MAY attempt to discover the address of
a suitable home agent on its home link. To do so, the mobile node
sends an ICMP Home Agent Address Discovery Request message to the
"Mobile IPv6 Home-Agents" anycast address [11] for its home subnet
prefix. As described in Section 10.9, the home agent on its home
link that receives this Request message will return an ICMP Home
Agent Address Discovery Reply message, giving this home agent's own
global unicast IP address along with a list of the global unicast IP
address of each other home agent operating on the home link.
The mobile node, upon receiving this Home Agent Address Discovery
Reply message, MAY then send its home registration Binding Update to
the home agent address given as the IP Source Address of the packet
carrying the Reply message or to any of the unicast IP addresses
listed in the Home Agent Addresses field in the Reply. For example,
if necessary, the mobile node MAY attempt its home registration
with each of these home agents, in turn, by sending each a Binding
Update and waiting for the matching Binding Acknowledgement, until
its registration is accepted by one of these home agents. In trying
each of the returned home agent addresses, the mobile node SHOULD try
each in the order listed in the Home Agent Addresses field in the
received Home Agent Address Discovery Reply message. If the home
agent identified by the Source Address field in the IP header of the
packet carrying the Home Agent Address Discovery Reply message is
not listed in the Home Agent Addresses field in the Reply, it SHOULD
be tried before the first address given in the list; otherwise, it
SHOULD be tried in its listed order.
If the mobile node has a current registration with some home agent
on its home link (the Lifetime for that registration has not yet
expired), then the mobile node MUST attempt any new registration
first with that home agent. If that registration attempt fails
(e.g., times out or is rejected), the mobile node SHOULD then
reattempt this registration with another home agent on its home link.
If the mobile node knows of no other suitable home agent, then it MAY
attempt the dynamic home agent address discovery mechanism described
above.
If, after a mobile node transmits a Home Agent Address Discovery
Request message to the Home Agents Anycast address, it does not
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receive a corresponding Home Agent Address Discovery Reply message
within INITIAL_DHAAD_TIMEOUT seconds, the mobile node MAY retransmit
the same Request message to the same anycast address. This
retransmission MAY be repeated up to a maximum of DHAAD_RETRIES
attempts. Each retransmission MUST be delayed by twice the time
interval of the previous retransmission.
11.3.3. Sending Mobile Prefix Solicitations
When a mobile node has a home address that is about to become
invalid, it sends a Mobile Prefix Solicitation to its home agent
in an attempt to acquire fresh routing prefix information. The
new information also enables the mobile node to participate in
renumbering operations affecting the home network, as described in
section 10.9.1.
The mobile node SHOULD send a Solicitation to the home agent when
its home address will become invalid within MaxRtrAdvInterval
seconds, where this value is acquired in a previous Mobile Prefix
Advertisement from the home agent. If no such value is known, the
value MAX_PFX_ADV_DELAY seconds is used instead (see section 12).
If the mobile node does not have a valid home address available for
use as the IP source address, it MAY use its care-of address, but
there will not be a security association between the home agent
and the care-of address for the corresponding Advertisement to be
authenticated.
This solicitation follows the same retransmission rules specified for
Router Solicitations [20], except that the initial retransmission
interval is specified to be INITIAL_SOLICIT_TIMER (see section 12).
As described in Section 11.6.2, Binding Updates sent by the mobile
node to other nodes MUST use a lifetime no greater than the remaining
lifetime of its home registration of its primary care-of address.
The mobile node SHOULD further limit the lifetimes that it sends on
any Binding Updates to be within the remaining preferred lifetime
(see Section 10.9.2) for the prefix in its home address.
When the lifetime for a changed prefix decreases, and the change
would cause cached bindings at correspondent nodes in the Binding
Update List to be stored past the newly shortened lifetime, the
mobile node MUST issue a Binding Update to all such correspondent
nodes.
These limits on the binding lifetime serve to prohibit use of a
mobile node's home address after it becomes invalid.
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11.3.4. Receiving Mobile Prefix Advertisements
Section 10.9.1 describes the operation of a home agent to support
boot time configuration and renumbering a mobile node's home subnet
while the mobile node is away from home. The home agent sends Mobile
Prefix Advertisement messages to the mobile node while away from
home, giving "important" Prefix Information options that describe
changes in the prefixes in use on the mobile node's home link.
When a mobile node receives a Mobile Prefix Advertisement, it MUST
validate it according to the following tests:
- The Source Address of the IP packet carrying the Mobile Prefix
Advertisement is the same as the home agent address to which the
mobile node last sent an accepted "home registration" Binding
Update to register its primary care-of address. Otherwise, if
no such registrations have been made, it SHOULD be the mobile
node's stored home agent address, if one exists. Otherwise, if
the mobile node has not yet discovered its home agent's address,
it MUST NOT accept Mobile Prefix Advertisements.
- The packet MUST be protected by IPsec [14, 12, 13] to guard
against malicious prefix advertisements, if a security
association exists (i.e. unless the mobile node does not yet
have a home address configured). The IPsec protection MUST
provide sender authentication, data integrity protection, and
replay protection, covering the advertisement.
Any received Mobile Prefix Advertisement not meeting all of these
tests MUST be silently discarded.
If a received Mobile Prefix Advertisement is not discarded according
to the tests listed above, the mobile node MUST process the Prefix
Information Options as if they arrived in a Router Advertisement
on the mobile node's home link [20]. Such processing may result
in the mobile node configuring a new home address, although due
to separation between preferred lifetime and valid lifetime, such
changes should not affect most communication by the mobile node, in
the same way as for nodes that are at home.
If the advertisement contains a Binding Refresh Request option, the
mobile node SHOULD return a Binding Update, which will be viewed by
the home agent as an acknowledgement of the corresponding Mobile
Prefix Advertisement, which it can cease transmitting.
In addition, if processing of this Advertisement resulted in the
mobile node configuring a new home address, and if the method used
for this new home address configuration would require the mobile node
to perform Duplicate Address Detection [33] for the new address if
the mobile node were located at home, then the mobile node MUST set
the Duplicate Address Detection (D) bit in this Binding Update to
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its home agent, to request the home agent to perform this Duplicate
Address Detection on behalf of the mobile node.
11.4. Movement
11.4.1. Movement Detection
A mobile node MAY use any combination of mechanisms available to it
to detect when it has moved from one link to another. The primary
movement detection mechanism for Mobile IPv6 defined here uses the
facilities of IPv6 Neighbor Discovery, including Router Discovery and
Neighbor Unreachability Detection, although the mobile node SHOULD
supplement this mechanism with other information whenever it is
available to the mobile node (e.g., from lower protocol layers). The
description here is based on the conceptual model of the organization
and data structures defined by Neighbor Discovery [20].
Mobile nodes SHOULD use Router Discovery to discover new routers and
on-link subnet prefixes; a mobile node MAY send Router Solicitation
messages, or MAY wait for unsolicited (periodic) multicast Router
Advertisement messages, as specified for Router Discovery [20].
Based on received Router Advertisement messages, a mobile node (in
the same way as any other node) maintains an entry in its Default
Router List for each router, and an entry in its Prefix List for each
subnet prefix, that it currently considers to be on-link. Each entry
in these lists has an associated invalidation timer value (extracted
from the Router Advertisement and Prefix Information options) used to
expire the entry when it becomes invalid.
While away from home, a mobile node typically selects one router
from its Default Router List to use as its default router, and one
subnet prefix advertised by that router from its Prefix List to use
as the subnet prefix in its primary care-of address. A mobile node
MAY also have associated additional care-of addresses, using other
subnet prefixes from its Prefix List. The method by which a mobile
node selects and forms a care-of address from the available subnet
prefixes is described in Section 11.4.2. The mobile node registers
its primary care-of address with its home agent, as described in
Section 11.6.1.
While a mobile node is away from home and using some router as its
default router, it is important for the mobile node to be able to
quickly detect when that router becomes unreachable, so that it
can switch to a new default router and (if needed, according to
prefix advertisement) 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 for packets sent from its default
router, so that the mobile node can take steps to ensure that any
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correspondent nodes attempting to communicate with it can still reach
it through some other route.
To detect when its default router becomes unreachable, a mobile
node SHOULD use Neighbor Unreachability Detection. As specified in
Neighbor Discovery [20], while the mobile node is actively sending
packets to (or through) its default router, the mobile node can
detect that the router (as its neighbor) is still reachable either
through indications from upper layer protocols on the mobile node
that a connection is making "forward progress" (e.g., receipt of TCP
acknowledgements for new data transmitted), or through receipt of a
Neighbor Advertisement message from its default router in response
to an explicit Neighbor Solicitation messages to it. Note that
although this mechanism detects that the mobile node's default router
has become unreachable to the mobile node only while the mobile node
is actively sending packets to it, this is the only time that this
direction of reachability confirmation is needed. Confirmation
that the mobile node is still reachable from the router is handled
separately, as described below.
For a mobile node to detect when it has become unreachable from its
default router, the mobile node cannot efficiently rely on Neighbor
Unreachability Detection alone, since the network overhead would be
prohibitively high in many cases for a mobile node to continually
probe its default router with Neighbor Solicitation messages even
when it is not otherwise actively sending packets to it. Instead,
when a mobile node receives any IPv6 packets from its current default
router at all, irrespective of the source IPv6 address, it SHOULD use
that as an indication that it is still reachable from the router.
Since the router SHOULD be sending periodic unsolicited multicast
Router Advertisement messages, the mobile node will have frequent
opportunity to check if it is still reachable from its default
router, even in the absence of other packets to it from the router.
If Router Advertisements that the mobile node receives include
an Advertisement Interval option, the mobile node MAY use its
Advertisement Interval field as an indication of the frequency with
which it SHOULD expect to continue to receive future Advertisements
from that router. This field specifies the minimum rate (the maximum
amount of time between successive Advertisements) that the mobile
node SHOULD expect. If this amount of time elapses without the
mobile node receiving any Advertisement from this router, the mobile
node can be sure that at least one Advertisement sent by the router
has been lost. It is thus possible for the mobile node to implement
its own policy for determining the number of Advertisements from
its current default router it is willing to tolerate losing before
deciding to switch to a different router from which it may currently
be correctly receiving Advertisements.
On some types of network interfaces, the mobile node MAY also
supplement this monitoring of Router Advertisements, by setting its
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network interface into "promiscuous" receive mode, so that it is able
to receive all packets on the link, including those not addressed to
it at the link layer (i.e., disabling link-level address filtering).
The mobile node will then be able to detect any packets sent by the
router, in order to detect reachability from the router. This use of
promiscuous mode may be useful on very low bandwidth (e.g., wireless)
links, but its use MUST be configurable on the mobile node since it
is likely to consume additional energy resources.
If the above means do not provide indication that the mobile node
is still reachable from its current default router (for instance,
the mobile node receives no packets from the router for a period
of time), then the mobile node SHOULD attempt to actively probe
the router with Neighbor Solicitation messages, even if it is not
otherwise actively sending packets to the router. If it receives a
solicited Neighbor Advertisement message in response from the router,
then the mobile node can deduce that it is still reachable. It is
expected that the mobile node will in most cases be able to determine
its reachability from the router by listening for packets from the
router as described above, and thus, such extra Neighbor Solicitation
probes should rarely be necessary.
With some types of networks, indications about link-layer mobility
might be obtained from lower-layer protocol or device driver software
within the mobile node. However, all link-layer mobility indications
from lower layers do not necessarily indicate a movement of the
mobile node to a new link, such that the mobile node would need to
switch to a new default router and primary care-of address. For
example, movement of a mobile node from one cell to another in many
wireless LANs can be made transparent to the IP level through use of
a link-layer "roaming" protocol, as long as the different wireless
LAN cells all operate as part of the same IP link with the same
subnet prefix. Upon lower-layer indication of link-layer mobility,
the mobile node MAY send Router Solicitation messages to determine if
additional on-link subnet prefixes are available 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 subnet prefixes
currently available through different routers from which the mobile
node is reachable. For example, a mobile node MAY use signal
strength or signal quality information (with suitable hysteresis) for
its link with the available routers to decide when to switch to a new
primary care-of address using that router rather than its current
default router (and current primary care-of address). Even though
the mobile node's current default router may still be reachable in
terms of Neighbor Unreachability Detection, the mobile node MAY use
such lower-layer information to determine that switching to a new
default router would provide a better connection.
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11.4.2. Forming New Care-of Addresses
After detecting that it has moved from one link to another (i.e., its
current default router has become unreachable and it has discovered
a new default router), a mobile node SHOULD form a new primary
care-of address using one of the on-link subnet prefixes advertised
by the new router. A mobile node MAY form a new primary care-of
address at any time, except that it MUST NOT do so too frequently.
Specifically, a mobile node MUST NOT send a Binding Update about a
new care-of address to its home agent (which is required to register
the new address as its primary care-of address) more often than once
per MAX_UPDATE_RATE seconds.
In addition, after discovering a new on-link subnet prefix, a mobile
node MAY form a new (non-primary) care-of address using that subnet
prefix, even when it has not switched to a new default router. A
mobile node can have only one primary care-of address at a time
(which is registered with its home agent), but it MAY have an
additional care-of address for any or all of the prefixes on its
current link. Furthermore, since a wireless network interface may
actually allow a mobile node to be reachable on more than one link at
a time (i.e., within wireless transmitter range of routers on more
than one separate link), a mobile node MAY have care-of addresses
on more than one link at a time. The use of more than one care-of
address at a time is described in Section 11.4.3.
As described in Section 4, in order to form a new care-of address,
a mobile node MAY use either stateless [33] or stateful (e.g.,
DHCPv6 [2]) Address Autoconfiguration. If a mobile node needs to
send packets as part of the method of address autoconfiguration,
it MUST use an IPv6 link-local address rather than its own IPv6
home address as the Source Address in the IPv6 header of each such
autoconfiguration packet.
In some cases, a mobile node may already know a (constant) IPv6
address that has been assigned to it for its use only while
visiting a specific foreign link. For example, a mobile node may be
statically configured with an IPv6 address assigned by the system
administrator of some foreign link, for its use while visiting that
link. If so, rather than using Address Autoconfiguration to form a
new care-of address using this subnet prefix, the mobile node MAY use
its own pre-assigned address as its care-of address on this link.
After forming a new care-of address, a mobile node MAY perform
Duplicate Address Detection [33] on that new address to confirm its
uniqueness. However, doing so represents a trade-off between safety
(ensuring that the new address is not used if it is a duplicate
address) and overhead (performing Duplicate Address Detection
requires the sending of one or more additional packets over what
may be, for example, a slow wireless link through which the mobile
node is connected). Performing Duplicate Address Detection also in
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general would cause a delay before the mobile node could use the
new care-of address, possibly causing the mobile node to be unable
to continue communication with correspondent nodes for some period
of time. For these reasons, a mobile node, after forming a new
care-of address, MAY begin using the new care-of address without
performing Duplicate Address Detection. Furthermore, the mobile node
MAY continue using the address without performing Duplicate Address
Detection, although it SHOULD in most cases (e.g., unless network
bandwidth or battery consumption for communication is of primary
concern) begin Duplicate Address Detection asynchronously when it
begins use of the address, allowing the Duplicate Address Detection
procedure to complete in parallel with normal communication using the
address.
In addition, normal processing for Duplicate Address Detection
specifies that, in certain cases, the node SHOULD delay sending the
initial Neighbor Solicitation message of Duplicate Address Detection
by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY [20, 33];
however, in this case, the mobile node SHOULD NOT perform such a
delay in its use of Duplicate Address Detection, unless the mobile
node is initializing after rebooting.
11.4.3. Using Multiple Care-of Addresses
As described in Section 11.4.2, a mobile node MAY use more than one
care-of address at a time. Particularly in the case of many wireless
networks, a mobile node effectively might be reachable through
multiple links at the same time (e.g., with overlapping wireless
cells), on which different on-link subnet prefixes may exist. A
mobile node SHOULD select a primary care-of address from among those
care-of addresses it has formed using any of these subnet prefixes,
based on the movement detection mechanism in use, as described in
Section 11.4.1. When the mobile node selects a new primary care-of
address, it MUST register it with its home agent by sending it a
Binding Update with the Home Registration (H) and Acknowledge (A)
bits set, as described in Section 11.6.1.
To assist with smooth handovers, a mobile node SHOULD retain
its previous primary care-of address as a (non-primary) care-of
address, and SHOULD still accept packets at this address, even after
registering its new primary care-of address with its home agent.
This is reasonable, since the mobile node could only receive packets
at its previous primary care-of address if it were indeed still
connected to that link. If the previous primary care-of address was
allocated using stateful Address Autoconfiguration [2], the mobile
node may not wish to release the address immediately upon switching
to a new primary care-of address.
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11.5. Return Routability Procedure
This section defines the rules that the mobile node must follow
when performing the return routability procedure. Appendix A
specifies also a (non-normative) state-machine that describes the
same procedure. Section 11.6.2 describes the rules when the return
routability procedure needs to be initiated.
11.5.1. Sending Home and Care-of Test Init Messages
A mobile node that initiates a return routability procedure MUST
send (in parallel) a Home Test Init message and a Care-of Test Init
messages. A Home Test Init message MUST be created as described
in Section 6.1.3. A Care-of Test Init message MUST be created as
described in Section 6.1.4.
When sending a Home Test Init or Care-of Test Init message the mobile
node MUST record in its Binding Update List the following fields from
the messages:
- The IP address of the node to which the message was sent.
- The home address for which the binding is desired. This value
will appear in the Source Address field of the Home Test Init
message.
- The time at which each of these messages was sent.
- The mobile cookie used in the messages.
11.5.2. Receiving Return Routability Messages
Upon receiving a packet carrying a Home Test message, a mobile node
MUST validate the packet according to the following tests:
- The Header Len field in the Mobility Header is greater than or
equal to the length specified in Section 6.1.5.
- The Source Address of the packet belongs to a correspondent
node for which the mobile node has a Binding Update List entry
with a state indicating that return routability procedure is in
progress.
- The Binding Update List indicates that no home cookie has been
received yet.
- The Destination Address of the packet has the home address of the
mobile node, and the packet has been received in a tunnel from
the home agent.
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- The Mobile Cookie field in the message matches the value stored
in the Binding Update List.
Any Home Test message not satisfying all of these tests MUST be
silently ignored. Otherwise, the mobile node MUST record the Home
Nonce Index and Home Cookie in the Binding Update List. If the
Binding Update List entry does not have a Care-of Cookie, the mobile
node SHOULD continue waiting for additional messages.
Upon receiving a packet carrying a Care-of Test message, a mobile
node MUST validate the packet according to the following tests:
- The Header Len field in the Mobility Header is greater than or
equal to the length specified in Section 6.1.6.
- The Source Address of the packet belongs to a correspondent
node for which the mobile node has a Binding Update List entry
with a state indicating that return routability procedure is in
progress.
- The Binding Update List indicates that no care-of cookie has been
received yet.
- The Destination Address of the packet is the current care-of
address of the mobile node.
- The Mobile Cookie field in the message matches the value stored
in the Binding Update List.
Any Care-of Test message not satisfying all of these tests MUST be
silently ignored. Otherwise, the mobile node MUST record the Care-of
Nonce Index and Care-of Cookie in the Binding Update List. If the
Binding Update List entry does not have a Home Cookie, the mobile
node SHOULD continue waiting for additional messages.
If after receiving either the Home Test or the Care-of Test message
and performing the above actions, the Binding Update List entry
has both the Home and the Care-of Cookies, the return routability
procedure is complete. The mobile node SHOULD then proceed with
sending a Binding Update message as described in Section 11.6.2.
Correspondent nodes from the time before this specification was
published may not not support the Mobility Header protocol. These
nodes will respond to Home Test Init and Care-of Test Init messages
with an ICMP Parameter Problem code 1. The mobile node SHOULD
take such messages as an indication that the correspondent node
can not provide route optimization, and revert back to the use of
bidirectional routing.
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11.5.3. Retransmitting in the Return Routability Procedure
The mobile node is responsible for retransmissions in the return
routability procedure.
When the mobile node sends a Home Test Init or Care-of Test Init
message, it has to determine a value for the initial retransmission
timer. It should use the specified value of INITIAL_BINDACK_TIMEOUT
for this initial retransmission timer.
If, after sending either a Home Test Init or Care-of Test Init
message and the mobile node fails to receive a valid, matching
Home Test or Care-of Test message within the selected initial
retransmission interval, the mobile node SHOULD retransmit
the original message, until a valid answer is received. The
retransmissions by the mobile node MUST use an exponential
back-off process, in which the timeout period is doubled upon each
retransmission until either the node receives a valid response or the
timeout period reaches the value MAX_BINDACK_TIMEOUT.
11.5.4. Rate Limiting for Return Routability Procedure
A mobile node MUST NOT send Home Test Init or Care-of Test
Init messages to any individual node more often than once per
MAX_UPDATE_RATE seconds. After sending MAX_FAST_UPDATES consecutive
messages to a particular node with the same care-of address, the
mobile node SHOULD reduce its rate of sending these messages to that
node, to the rate of SLOW_UPDATE_RATE per second. The mobile node
MAY continue to send these messages at this slower rate indefinitely,
in hopes that the node will eventually be able to complete the return
routability procedure.
11.6. Processing Bindings
11.6.1. Sending Binding Updates to the Home Agent
After deciding to change its primary care-of address as described in
Sections 11.4.1 and 11.4.2, 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 message, with the packet constructed as
follows:
- The Home Registration (H) bit MUST be set in the Binding Update.
- The Acknowledge (A) bit MUST be set in the Binding Update.
- The packet MUST contain a Home Address destination option, giving
the mobile node's home address for the binding.
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- The care-of address for the binding MUST be used as the Source
Address in the packet's IPv6 header, unless an Alternate Care-of
Address mobility option is included in the Binding Update
message.
- The `S' bit is set to the zero to request the mobile node's home
agent to serve as a home agent for all home addresses for the
mobile node based on all on-link subnet prefixes on the home
link; this is the default behavior. If the mobile node desires
that only a single home address should be affected by this
Binding Update, the `S' bit can be set to 1.
- The value specified in the Lifetime field SHOULD be less than
or equal to the remaining lifetime of the home address and the
care-of address specified for the binding.
The Acknowledge (A) bit in the Binding Update requests the home agent
to return a Binding Acknowledgement in response to this Binding
Update. As described in Section 6.1.8, the mobile node SHOULD
retransmit this Binding Update to its home agent until it receives
a matching Binding Acknowledgement. Once reaching a retransmission
timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart
the process of delivering the Binding Update, but trying instead the
next home agent from its Home Agents List (see Section 11.3.2). If
there is only one home agent in the Home Agents List, the mobile node
instead SHOULD continue to periodically retransmit the Binding Update
at this rate until acknowledged (or until it begins attempting to
register a different primary care-of address). See Section 11.6.8
for information about retransmitting Binding Updates.
Depending on the value of the Single Address Only (S) bit in the
Binding Update, the home agent is requested to serve either a single
home address or all home home addresses for the mobile node. Until
the lifetime of this registration expires, the home agent considers
itself the home agent for each such home address of the mobile node.
As the set of on-link subnet prefixes on the home link changes over
time, the home agent changes the set of home addresses for this
mobile node for which it is serving as the home agent.
Each Binding Update MUST be authenticated as coming from the right
mobile node, as defined in Section 5.4. The mobile node MUST use a
Home Address destination option in Binding Updates sent to the home
agent in order to allow the IPsec policies to be matched with the
right home address. The home address in the Home Address destination
option and the Binding Update message MUST be equal (and this will be
checked by the home agent).
When sending a Binding Update to its home agent, the mobile node MUST
also create or update the corresponding Binding Update List entry, as
specified in Section 11.6.2.
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The last Sequence Number value sent to the home agent in a Binding
Update is stored by the mobile node. If the sending mobile node has
no knowledge of the right Sequence Number value, it may start at any
value. If the home agent rejects the value, it sends back a Binding
Acknowledgement with status code 141, and the last accepted sequence
number in the Sequence Number field of the Binding Acknowledgement.
The mobile node MUST store this information and use the next Sequence
Number value for the next Binding Update it sends.
If the mobile node has additional home addresses using a different
interface identifier, then the mobile node SHOULD send an additional
packet containing a Binding Update to its home agent to register the
care-of address for each such other home address (or set of home
addresses sharing an interface identifier).
While the mobile node is away from home, it relies on the home agent
to participate in Duplicate Address Detection (DAD) to defend its
home address against stateless autoconfiguration performed by another
node. Therefore, the mobile node SHOULD set the Duplicate Address
Detection (D) bit based on any requirements for DAD that would apply
to the mobile node if it were at home [20][33]. If the mobile
node's recent Binding Update was accepted by the home agent, and the
lifetime for that Binding Update has not yet expired, the mobile node
SHOULD NOT set the `D' bit in the new Binding Update; the home agent
will already be defending the home address(es) of the mobile node and
does not need to perform DAD again.
The home agent will only perform DAD for the mobile node's home
address when the mobile node has supplied a valid binding between
its home address and a care-of address. If some time elapses during
which the mobile node has no binding at the home agent, it might
be possible for another node to autoconfigure the mobile node's
home address. Therefore, the mobile node MUST treat creation of
a new binding with the home agent using an existing home address
the same as creation of a new home address. In the unlikely event
that the mobile node's home address is autoconfigured as the IPv6
address of another network node on the home network, the home agent
will reply to the mobile node's subsequent Binding Update with a
Binding Acknowledgement containing a Status of 138, Duplicate Address
Detection failed. In this case, the mobile node MUST NOT attempt to
re-use the same home address. It SHOULD continue to register care-of
addresses for its other home addresses, if any. The mobile node MAY
also attempt to acquire a new home address to replace the one for
which Status 138 was received, for instance by using the techniques
described in Appendix B.
11.6.2. Correspondent Binding Procedure
When the mobile node is assured that its home address is valid, it
MAY at any time initiate a correspondent binding procedure with
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the purpose of allowing the correspondent node to cache the mobile
node's current care-of address. The mobile node is responsible for
the initiation and completion of this procedure, as well as any
retransmissions that may be needed (subject to the rate limiting
defined in Section 11.6.9).
This section defines the rules that the mobile node must follow
when performing the correspondent binding procedure. Appendix A
specifies also a (non-normative) state-machine that describes the
same procedure.
The mobile node can be assured that its home address is still valid,
for example, by the home agent's use the 'D' bit of Binding Updates
(see Section 10.2). In any Binding Update sent by a mobile node,
the care-of address (either the Source Address in the packet's IPv6
header or the Care-of Address in the Alternate Care-of Address
mobility option of the Binding Update) MUST be set to one of the
care-of addresses currently in use by the mobile node or to the
mobile node's home address. A mobile node MAY set the care-of
address differently for sending Binding Updates to different
correspondent nodes.
A mobile node MAY choose to keep its location private from
certain correspondent nodes, and thus need not initiate the
return routability procedure, or 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 6.1.7. However, all Binding Updates to the correspondent
node require the successful completion of the return routability
procedure first, as no other IPv6 nodes are authorized to send
Binding Updates on behalf of a mobile node.
If set to one of the mobile node's current care-of addresses (the
care-of address given MAY differ from the mobile node's primary
care-of address), the Binding Update requests the correspondent node
to create or update an entry for the mobile node in the correspondent
node's Binding Cache in order to record this care-of address for use
in sending future packets to the mobile node. In this case, the
value specified in the Lifetime field sent in the Binding Update
SHOULD be less than or equal to the remaining lifetime of the home
address and the care-of address specified for the binding.
If, instead, the care-of address is set to the mobile node's home
address, the Binding Update requests the correspondent node to delete
any existing Binding Cache entry that it has for the mobile node.
When a mobile node sends a Binding Update to its home agent
to register a new primary care-of address (as described in
Section 11.6.1), the mobile node SHOULD also start a return
routability procedure to each other node for which an entry exists
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in the mobile node's Binding Update List, as detailed below. Upon
successful return routability procedure, a Binding Update message is
sent. Thus, other relevant nodes are generally kept updated about
the mobile node's binding and can send packets directly to the mobile
node using the mobile node's current care-of address.
The mobile node, however, need not initiate these actions immediately
after configuring a new care-of address. For example, the mobile
node MAY delay initiating the return routability procedure to any
correspondent node for a short period of time, if it isn't certain
that there's traffic to the correspondent node. This is particularly
useful if the mobile node anticipates that it is not going to stay
long in this location.
In addition, when a mobile node receives a packet for which the
mobile node can deduce that the original sender of the packet either
has no Binding Cache entry for the mobile node, or a stale entry
for the mobile node in its Binding Cache, the mobile node SHOULD
initiate a return routability procedure with the sender, in order to
finally update the sender's Binding Cache with the current care-of
address (subject to the rate limiting defined in Section 11.6.9).
In particular, the mobile node SHOULD initiate a return routability
procedure in response to receiving a packet that meets all of the
following tests:
- The packet was tunneled using IPv6 encapsulation.
- The Destination Address in the tunnel (outer) IPv6 header is
equal to any of the mobile node's care-of addresses.
- The Destination Address in the original (inner) IPv6 header
is equal to one of the mobile node's home addresses; or this
Destination Address is equal to one of the mobile node's previous
care-of addresses for which the mobile node has an entry in its
Binding Update List, representing an unexpired Binding Update
sent to a home agent on the link on which its previous care-of
address is located (Section 11.6.6).
- The Source Address in the tunnel (outer) IPv6 header differs from
the Source Address in the original (inner) IPv6 header.
The destination address to which the procedure should be initiated to
in response to receiving a packet meeting all of the above tests is
the Source Address in the original (inner) IPv6 header of the packet.
The home address for which this Binding Update is sent should be the
Destination Address of the original (inner) packet.
Binding Updates sent to correspondent nodes are not generally
required to be acknowledged. However, if the mobile node wants
to be sure that its new care-of address has been entered into a
correspondent node's Binding Cache, the mobile node MAY request an
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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.
The mobile node SHOULD create a Binding Update message as follows:
- The Source Address of the IPv6 header MUST contain the current
care-of address of the mobile node.
- The Destination Address of the IPv6 header MUST contain the
address of the correspondent node.
- The Mobility Header is constructed according to rules in
Section 6.1.7, including the authenticator field which is
calculated based on the received Home and Care-of Cookies.
The last Sequence Number value sent to a destination in a Binding
Update is stored by the mobile node in its Binding Update List entry
for that destination. If the sending mobile node has no Binding
Update List entry, the Sequence Number SHOULD start at a random
value. The mobile node MUST NOT use the same Sequence Number in two
different Binding Updates to the same correspondent node, even if the
Binding Updates provide different care-of addresses.
11.6.3. 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 meets the authentication requirements for Binding
Acknowledgements, defined in Sections 6.1.8 and 5. That is,
if the Binding Update was sent to the home agent, underlying
IPsec protection is used. If the Binding Update was sent to the
correspondent node, the authenticator field MUST be present and
have a valid value.
- The Header Len field in the Binding Acknowledgement message is
greater than or equal to the length specified in Section 6.1.8.
- The Sequence Number field matches the Sequence Number sent by the
mobile node to this destination address in an outstanding Binding
Update.
Any Binding Acknowledgement not satisfying all of these tests MUST be
silently ignored.
When a mobile node receives a packet carrying a valid Binding
Acknowledgement, the mobile node MUST examine the Status field as
follows:
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- If the Status field indicates that the Binding Update was
accepted (the Status field is less than 128), then the mobile
node MUST update the corresponding entry in its Binding Update
List to indicate that the Binding Update has been acknowledged;
the mobile node MUST then stop retransmitting the Binding Update.
In addition, if the value specified in the Lifetime field in the
Binding Acknowledgement is less than the Lifetime value sent
in the Binding Update being acknowledged, then the mobile node
MUST subtract the difference between these two Lifetime values
from the remaining lifetime for the binding as maintained in the
corresponding Binding Update List entry (with a minimum value
for the Binding Update List entry lifetime of 0). That is, if
the Lifetime value sent in the Binding Update was L_update, the
Lifetime value received in the Binding Acknowledgement was L_ack,
and the current remaining lifetime of the Binding Update List
entry is L_remain, then the new value for the remaining lifetime
of the Binding Update List entry should be
max((L_remain - (L_update - L_ack)), 0)
where max(X, Y) is the maximum of X and Y. The effect of this
step is to correctly manage the mobile node's view of the
binding's remaining lifetime (as maintained in the corresponding
Binding Update List entry) so that it correctly counts down from
the Lifetime value given in the Binding Acknowledgement, but with
the timer countdown beginning at the time that the Binding Update
was sent.
- If the Status field indicates that the Binding Update was
rejected (the Status field is greater than or equal to 128), then
the mobile node MUST delete the corresponding Binding Update List
entry, and it MUST also stop retransmitting the Binding Update.
Optionally, the mobile node MAY then take steps to correct the
cause of the error and retransmit the Binding Update (with a new
Sequence Number value), subject to the rate limiting restriction
specified in Section 11.6.9.
11.6.4. Receiving Binding Refresh Requests
When a mobile node receives a packet containing a Binding Refresh
Request message and there already exists a Binding Update List
entry for the source of the Binding Refresh Request, it MAY start
a return routability procedure (see Section 5) if it believes
the amount of traffic with the correspondent justifies the use of
Route Optimization. Note that the mobile node SHOULD NOT respond
Binding Requests from previously unknown correspondent nodes due to
Denial-of-Service concerns.
If the return routability procedure completes successfully, a
Binding Update message SHOULD be sent as described in Section 11.6.2.
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The Lifetime field in this Binding Update SHOULD be set to a new
lifetime, extending any current lifetime remaining from a previous
Binding Update sent to this node (as indicated in any existing
Binding Update List entry for this node), and lifetime SHOULD
again be less than or equal to the remaining lifetime of the home
registration and the care-of address specified for the binding. When
sending this Binding Update, the mobile node MUST update its Binding
Update List in the same way as for any other Binding Update sent by
the mobile node.
Note, however, that the mobile node MAY choose to delete its binding
from the sender of the Binding Refresh Request. In this case, the
mobile node instead SHOULD return a Binding Update to the sender, in
which the Lifetime field is set to zero and the care-of address is
set to the mobile node's home address.
If the Binding Refresh Request for which the Binding Update is being
returned contains a Unique Identifier mobility option, the resulting
Home Test Init, Care-of Test Init, and Update messages MUST also
include a Unique Identifier mobility option. The unique identifier
in the Option Data field of the Unique Identifier mobility option
MUST be copied from the unique identifier carried in the Binding
Refresh Request.
11.6.5. Receiving Binding Error Messages
When a mobile node receives a packet containing a Binding Error
message, it should first check if the mobile node has a Binding
Update List entry for the the source of the Binding Error message.
If the mobile node does not have such entry, it MUST ignore the
message. This is necessary to prevent a waste of resources on e.g.
return routability procedure due to spoofed Binding Error messages.
Otherwise, if the message Status field was 1 (Home Address
destination option used without a binding), the mobile node should
perform one of the following two actions:
- If the mobile node does have a Binding Update List entry but
has recent upper layer progress information that indicates
communications with the correspondent node are progressing, it
MAY ignore the message. This can be done in order to limit the
damage that spoofed Binding Error messages can cause to ongoing
communications.
- If the mobile node does have a Binding Update List entry but
no upper layer progress information, it MUST remove the entry
and route further communications through the home agent. It
MAY also optionally start a return routability procedure (see
Section 5.5).
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If the message Status field was 2 (received message had an unknown
value for the MH Type field), the mobile node should perform one of
the following two actions:
- If the mobile node is not expecting an acknowledgement or
response from the correspondent node, the mobile node SHOULD
ignore this message.
- Otherwise, the mobile node SHOULD cease the use of any extensions
to this specification. If no extensions had been used, the
mobile node should cease the attempt to use Route Optimization.
11.6.6. Forwarding from a Previous Care-of Address
When a mobile node connects to a new link and forms a new care-of
address, it MAY establish forwarding of packets from a previous
care-of address to this new care-of address. To do so, the mobile
node sends a Binding Update to any home agent on the link on which
the previous care-of address is located, indicating this previous
care-of address as the home address for the binding, and giving its
new care-of address as the binding's care-of address. Such packet
forwarding allows packets destined to the mobile node from nodes that
have not yet learned the mobile node's new care-of address, to be
forwarded to the mobile node rather than being lost once the mobile
node is no longer reachable at this previous care-of address.
This Binding Update is sent to a home agent, albeit a temporary
one. Nevertheless, the authentication requirements for Binding
Updates from a mobile node to its home agent apply, as specified in
Section 11.6.1. This means that the mobile node MUST employ IPsec
ESP as specified further below.
In constructing this Binding Update, the mobile node utilizes the
following specific steps:
- The Home Address field in the Home Address destination option
in the packet carrying the Binding Update MUST be set to the
previous care-of address for which packet forwarding is being
established.
- The care-of address for the new binding MUST be set to the new
care-of address to which packets destined to the previous care-of
address are to be forwarded. Normally, this care-of address for
the binding is specified by setting the Source Address of the
packet carrying the Binding Update, to this address. However,
the mobile node MAY instead include an Alternate Care-of Address
mobility option in the Binding Update message, with its Alternate
Care-of Address field set to the care-of address for the binding.
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- The Home Registration (H) bit MUST also be set in this Binding
Update, to request this home agent to temporarily act as a home
agent for this previous care-of address.
This home agent will thus tunnel packets for the mobile node (packets
destined to its specified previous care-of address) to its new
care-of address. All of the procedures defined for home agent
operation MUST be followed by this home agent for this registration.
Note that this home agent does not necessarily know (and need not
know) the mobile node's (permanent) home address as part of this
registration.
The packet carrying the Binding Update MUST be addressed to
this home agent's global unicast address. Normally, this global
unicast address is learned by the mobile node based on the Router
Advertisements received by the mobile node (Section 7.2) while
attached to the link on which this previous care-of address and this
home agent are located; the mobile node obtains this home agent
address from its Home Agents List (Section 4.4). Alternatively,
the mobile node MAY use dynamic home agent address discovery
(Section 10.9) to discover the global unicast address of a home agent
on this previous link, but it SHOULD use an address from its Home
Agents List if available for the prefix it used to form this previous
care-of address.
As with any packet containing a Binding Update (see Section 6.1.7),
the Binding Update packet to this home agent MUST meet the
authentication requirements for Binding Updates, defined in
Section 5.4. Each Binding Update MUST be authenticated as coming
from the right mobile node. This means that the mobile node and the
home agent MUST have a security association that employs IPsec ESP
for protecting the Mobility Header with a non-null authentication
algorithm. The mobile node MUST use a Home Address destination
option in Binding Updates sent to the home agent in order to allow
the IPsec policies to be matched with the right home address. The
home address in the Home Address destination option and the Binding
Update message MUST be equal (and this will be checked by the home
agent), that is, it MUST be the mobile node's previous care-of
address for which forwarding is being established.
11.6.7. Returning Home
A mobile node detects that it has returned to its home link through
the movement detection algorithm in use (Section 11.4.1), when the
mobile node detects that its home subnet prefix is again on-link.
The mobile node SHOULD then send a Binding Update to its home agent,
to instruct its home agent to no longer intercept or tunnel packets
for it. In this Binding Update, the mobile node MUST set the care-of
address for the binding (the Source Address field in the packet's
IPv6 header) to the mobile node's own home address. As with other
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Binding Updates sent to register with its home agent, the mobile
node MUST set the Acknowledge (A) and Home Registration (H) bits,
and SHOULD retransmit the Binding Update until a matching Binding
Acknowledgement is received.
When sending this Binding Update to its home agent, the mobile
node must be careful in how it uses Neighbor Solicitation [20] (if
needed) to learn the home agent's link-layer address, since the home
agent will be currently configured to defend the mobile node's home
address for Duplicate Address Detection. In particular, a Neighbor
Solicitation from the mobile node using its home address as the
Source Address would be detected by the home agent as a duplicate
address. In many cases, Neighbor Solicitation by the mobile node
for the home agent's address will not be necessary, since the mobile
node may have already learned the home agent's link-layer address,
for example from a Source Link-Layer Address option in the Router
Advertisement from which it learned that its home address was on-link
and that the mobile node had thus returned home. If the mobile node
does Neighbor Solicitation to learn the home agent's link-layer
address, in this special case of the mobile node returning home, the
mobile node MUST unicast the packet, and in addition set the Source
Address of this Neighbor Solicitation to the unspecified address
(0:0:0:0:0:0:0:0). Since the solicitation is unicast, the home
agent will be able to distinguish from a similar packet that would
only be used for DAD. The home agent will send a multicast Neighbor
Advertisement back to the mobile node with the Solicited flag ('S')
set to zero. The mobile node SHOULD accept this advertisement, and
set the state of the Neighbor Cache entry for the home agent to
REACHABLE.
The mobile node then sends its Binding Update using the home agent's
link-layer address, instructing its home agent to no longer serve
as a home agent for it. By processing this Binding Update, the
home agent will cease defending the mobile node's home address for
Duplicate Address Detection and will no longer respond to Neighbor
Solicitations for the mobile node's home address. The mobile node
is then the only node on the link receiving packets at the mobile
node's home address. In addition, when returning home prior to the
expiration of a current binding for its home address, and configuring
its home address on its network interface on its home link, the
mobile node MUST NOT perform Duplicate Address Detection on its own
home address, in order to avoid confusion or conflict with its home
agent's use of the same address. If the mobile node returns home
after the bindings for all of its care-of addresses have expired,
then it SHOULD perform DAD.
After the Mobile Node sends the Binding Update, the Home Agent MUST
remove the Proxy Neighbor Cache entry for the Mobile Node and MAY
learn its link-layer address based on the link-layer packet or cached
information, or if that is not available, it SHOULD send a Neighbor
Solicitation with the target address equal to the Binding Update's
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source IP address. The Mobile Node MUST then reply with a unicast
Neighbor Advertisement to the Home Agent with its link-layer address.
While the Mobile Node is waiting for a Binding Acknowledgement, it
MUST NOT respond to any Neighbor Solicitations for its Home Address
other than those originating from the IP address to which it sent the
Binding Update.
After receiving the Binding Acknowledgement for its Binding Update
to its home agent, the mobile node MUST multicast onto the home
link (to the all-nodes multicast address) a Neighbor Advertisement
message [20], to advertise the mobile node's own link-layer address
for its own home address. The Target Address in this Neighbor
Advertisement message MUST be set to the mobile node's home address,
and the Advertisement MUST include a Target Link-layer Address option
specifying the mobile node's link-layer address. The mobile node
MUST multicast such a Neighbor Advertisement message for each of its
home addresses, as defined by the current on-link prefixes, including
its link-local address and site-local address. The Solicited
Flag (S) in these Advertisements MUST NOT be set, since they were
not solicited by any Neighbor Solicitation message. The Override
Flag (O) in these Advertisements MUST be set, indicating that the
Advertisements SHOULD override any existing Neighbor Cache entries at
any node receiving them.
Since multicasting on the local link (such as Ethernet) is typically
not guaranteed to be reliable, the mobile node MAY retransmit these
Neighbor Advertisement messages up to MAX_ADVERT_REXMIT times to
increase their reliability. It is still possible that some nodes on
the home link will not receive any of these Neighbor Advertisements,
but these nodes will eventually be able to recover through use of
Neighbor Unreachability Detection [20].
11.6.8. Retransmitting Binding Updates
The mobile node is responsible for retransmissions in the binding
procedure.
When the mobile node sends a Binding Update message, it has to
determine a value for the initial retransmission timer. If the
mobile node is changing or updating an existing binding at the home
agent, it should use the specified value of INITIAL_BINDACK_TIMEOUT
for this initial retransmission timer. If on the other hand the
mobile node does not have an existing binding at the home agent, it
SHOULD use a value for the initial retransmission timer that is at
least 1.5 times longer than (RetransTimer * DupAddrDetectTransmits).
This value is likely to be substantially longer than the otherwise
specified value of INITIAL_BINDACK_TIMEOUT that would be used by the
mobile node. This longer retransmission interval will allow the the
home agent to complete the DAD procedure which is mandated in this
case, as detailed in Section 11.6.1.
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If, after sending a Binding Update in which the care-of address has
changed and the Acknowledge (A) bit is set, a mobile node fails
to receive a valid, matching Binding Acknowledgement within the
selected initial retransmission interval, the mobile node SHOULD
retransmit the Binding Update, until a Binding Acknowledgement is
received. Such a retransmitted Binding Update MUST use a Sequence
Number value greater than that used for the previous transmission of
this Binding Update. The retransmissions by the mobile node MUST
use an exponential back-off process, in which the timeout period
is doubled upon each retransmission until either the node receives
a Binding Acknowledgement or the timeout period reaches the value
MAX_BINDACK_TIMEOUT.
11.6.9. Rate Limiting Binding Updates
A mobile node MUST NOT send Binding Update messages for the
same binding to any individual node more often than once per
MAX_UPDATE_RATE seconds. After sending MAX_FAST_UPDATES consecutive
messages to a particular node with the same care-of address, the
mobile node SHOULD reduce its rate of sending these messages to that
node, to the rate of SLOW_UPDATE_RATE per second. The mobile node
MAY continue to send these messages at this slower rate indefinitely,
in hopes that the node will eventually be able to process a Binding
Update, and begin to route its packets directly to the mobile node at
its new care-of address.
11.7. Receiving ICMP Error Messages
Any node receiving a Mobility header that does not recognize the
protocol SHOULD return an ICMP Parameter Problem, Code 1, message
to the sender of the packet. If a node performing the return
routability procedure or sending a Binding Update receives such an
ICMP error message in response, it SHOULD record in its Binding
Update List that future Binding Updates SHOULD NOT be sent to this
destination.
Correspondent nodes who have participated in the return routability
procedure MUST implement the ability to correctly process received
packets containing a Home Address option. Therefore, correctly
implemented correspondent nodes should always be able to recognize
Home Address options. If a mobile node receives an ICMP Parameter
Problem, Code 2, message from some node indicating the that the Home
Address option, the mobile node SHOULD log the error and then discard
the ICMP message.
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12. Protocol Constants
HomeRtrAdvInterval 3,600 seconds
DHAAD_RETRIES 3 retransmissions
INITIAL_BINDACK_TIMEOUT 1 second
INITIAL_DHAAD_TIMEOUT 2 seconds
INITIAL_SOLICIT_TIMER 2 seconds
MAX_ADVERT_REXMIT 3 transmissions
MAX_BINDACK_TIMEOUT 256 seconds
MAX_COOKIE_LIFE 240 seconds
MAX_FAST_UPDATES 5 transmissions
MAX_PFX_ADV_DELAY 1,000 seconds
MAX_RR_BINDING_LIFE 300 seconds
MAX_UPDATE_RATE once per second
PREFIX_ADV_RETRIES 3 retransmissions
PREFIX_ADV_TIMEOUT 5 seconds
SLOW_UPDATE_RATE once per 10 second interval
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13. IANA Considerations
This document defines a new IPv6 protocol, the Mobility Header,
described in Section 6.1. This protocol must be assigned a protocol
number. The MH Type field in the Mobility Header is used to indicate
a particular type of a message. The current message types are
described in Sections 6.1.2 through 6.1.9, and include the following:
0 Binding Refresh Request
1 Home Test Init
2 Care-of Test Init
3 Home Test
4 Care-of Test
5 Binding Update
6 Binding Acknowledgement
7 Binding Error
Future values of the MH Type can be allocated using standards
action [19].
Furthermore, each Mobility Header message may contain mobility
options as described in Section 6.2. The current mobility options
are defined in Sections 6.2.2 through 6.2.5, and include the
following:
0 Pad1
1 PadN
2 Unique Identifier
3 Alternate Care-of Address
4 Nonce Indices
5 Authorization Data
Future values of the Option Type can be allocated using standards
action [19].
This document also defines a new IPv6 destination option, the Home
Address option, described in Section 6.3. This option must be
assigned an Option Type value.
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This document also defines a new IPv6 Type 2 Routing Header,
described in Section 6.4. The value 2 must be allocated by IANA when
this specification becomes an RFC.
In addition, this document defines four ICMP message types, two used
as part of the dynamic home agent address discovery mechanism and
two used in lieu of router solicitations and advertisements when the
mobile node is away from the home link:
- The Home Agent Address Discovery Request message, described in
Section 6.5;
- The Home Agent Address Discovery Reply message, described in
Section 6.6;
- The Mobile Prefix Solicitation message, described in Section 6.7;
and
- The Mobile Prefix Advertisement message, described in
Section 6.8.
This document also defines two new Neighbor Discovery [20] options,
which must be assigned Option Type values within the option numbering
space for Neighbor Discovery messages:
- The Advertisement Interval option, described in Section 7.3; and
- The Home Agent Information option, described in Section 7.4.
14. Security Considerations
14.1. Security for the Tunneling to and from the Home Agent
Binding updates to the home agents are secure. When receiving
tunneled traffic the home agent verifies the outer IP address
corresponds to the current location of the mobile node. This
prevents attacks where the attacker is controlled by ingress
filtering, as well as attacks where the attacker does not know the
current care-of address of the mobile node. Attackers who know the
care-of address and are not controlled by ingress filtering could
still send traffic through the home agent. This includes attackers
on the same local link as the mobile node is currently on. But such
attackers could also send spoofed packets without using a tunnel.
It is possible to use IPsec ESP to protect payload packets tunneled
to the mobile node and back. While this specification does not
mandate the use of ESP, its use is recommended to protect the payload
communications against attackers on the path between the home agent
and the current location of the mobile node.
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When site local home address are used, reverse tunneling can be used
to send site local traffic from another location. Administrators
should be aware of this when allowing such home addresses. In
particular, the outer IP address check described above is not
sufficient against all attackers and the use of encrypted tunnels is
particularly useful for this kind of home addresses.
14.2. Security for the Binding Updates to the Home Agent
The use of IPsec ESP to protect Mobility Header messages between
the mobile node and the home agent protects the integrity of the
Binding Updates and Binding Acknowledgements. Sequence numbers with
the Mobile IPv6 messages ensure correct ordering (see Section 5.4).
However, if a home agent reboots and loses its state regarding the
sequence numbers, replay attacks become possible. If the home agent
is vulnerable to this, the use of a key management mechanism together
with IPsec can be used to prevent replay attacks.
14.3. Security for the Binding Updates to the Correspondent Nodes
The use of home address and care-of-address based return routability
tests prevents any off-path attacks beyond those that are already
possible in basic IPv6 [23].
Protection against attackers on the home agent link and the
correspondent node link, as well as on the path between, are
roughly similar to the situation in existing IPv6 as well. However,
one difference is that in basic IPv6 an on-path attacker must be
constantly present on the link or the path (e.g., in order to perform
a man-in-the-middle attack), whereas with Mobile IPv6 an attacker
can leave an existing binding behind, even after it is no longer on
the link or on the path [23]. For this reason, this specification
limits the validity of bindings authorized by return routability to
a maximum of MAX_COOKIE_LIFE + MAX_RR_BINDING_LIFE seconds after the
last routability check has been performed.
The path between the home agent and a correspondent node is typically
easiest to attack on the links at either end, in particular if these
links are publicly accessible wireless LANs. Attacks against the
routers or switches on the path are typically harder to accomplish.
Thus, the weakest points are typically on the links at either end,
and their mechanisms for layer 2 security or IPv6 Neighbour and
Router Discovery. If these were secured using some new technology in
the future, this could make the key establishment mechanism specified
in this document to be an easier route for attackers to use. For
this reason, this specification should have a protection mechanism
for selecting between return routability and potential other future
mechanisms.
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14.4. Security for the Home Address Destination Option
The use of the Home Address destination option allows packets sent by
a mobile node to pass normally through routers implementing ingress
filtering [7]. Since the care-of address used in the Source Address
field of the packet's IPv6 header is topologically correct for the
sending location of the mobile node, ingress filtering can trace the
location of the mobile node in the same way as can be done with any
sender when ingress filtering is in use. As this location does not
survive in replies sent by the correspondent node, this document
restricts the use of the Home Address option to those situations
where a binding has been established with the participation of the
node at the home address. This prevents reflection attacks through
the use of the Home Address option.
No special authentication of the Home Address option is required
beyond the above, except that if the IPv6 header of a packet is
covered by authentication, then that authentication MUST also cover
the Home Address option; this coverage is achieved automatically by
the definition of the Option Type code for the Home Address option
(Section 6.3), since it indicates that the option is included in the
authentication computation. Thus, even when authentication is used
in the IPv6 header, the security of the Source Address field in the
IPv6 header is not compromised by the presence of a Home Address
option. Without authentication of the packet, then any field in the
IPv6 header, including the Source Address field, and any other parts
of the packet, including the Home Address option, can be forged or
modified in transit. In this case, the contents of the Home Address
option is no more suspect than any other part of the packet.
14.5. Firewall considerations
The definition of Routing Header 2 in Section 6.4 and the associated
processing rules have been chosen so that the header can not be used
for what is traditionally viewed as source routing. In particular,
the IPv6 destination and the Home Address in the routing header will
always have to be assigned to the same node otherwise the packet will
be dropped.
This means that the typical security concerns for source routing
including the automatic reversal of unauthenticated source routes
(which is an issue for IPv4 but not for IPv6 source routing) and the
ability to use source routing to "jump" between nodes inside, as well
as outside a firewall, are not at play.
In essence the semantics of the type 2 routing header is the same as
a special form of IP-in-IP tunneling where the inner and outer source
addresses are the same.
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This implies that a device which implements filtering of packets
should be able to distinguish between a Type 2 Routing header and
other Routing headers, as required in section 8.2. This is necessary
in order to allow Mobile IPv6 traffic while still having the option
to filter out other uses of Routing headers.
Acknowledgements
We would like to thank the members of the Mobile IP and IPng Working
Groups for their comments and suggestions on this work. We would
particularly like to thank (in alphabetical order) Fred Baker
(Cisco), Josh Broch (Carnegie Mellon University), Robert Chalmers
(University of California, Santa Barbara), Noel Chiappa (MIT),
Vijay Devarapalli (Nokia Research Center), Rich Draves (Microsoft
Research), Francis Dupont (ENST Bretagne), Thomas Eklund (Xelerated),
Jun-Ichiro Itojun Hagino (IIJ Research Laboratory), Krishna Kumar
(IBM Research), T.J. Kniveton (Nokia Research), Jiwoong Lee (KTF),
Aime Lerouzic (Bull S.A.), Thomas Narten (IBM), Erik Nordmark (Sun
Microsystems), Simon Nybroe (Ericsson Telebit), David Oran (Cisco),
Lars Henrik Petander (HUT), Basavaraj Patil (Nokia), Ken Powell
(Compaq), Phil Roberts (Motorola), Patrice Romand (Bull S.A.),
Jeff Schiller (MIT) Tom Soderlund (Nokia Research), Hesham Soliman
(Ericsson), Jim Solomon (RedBack Networks), Tapio Suihko (Technical
Research Center of Finland), Benny Van Houdt (University of Antwerp),
Jon-Olov Vatn (KTH), Alper Yegin (Sun Microsystems), and Xinhua Zhao
(Stanford University) for their detailed reviews of earlier versions
of this document. Their suggestions have helped to improve both the
design and presentation of the protocol.
We would also like to thank the participants in the Mobile IPv6
testing event held at Nancy, France, September 15-17, 1999, for
their valuable feedback as a result of interoperability testing
of four Mobile IPv6 implementations coming from four different
organizations: Bull (AIX), Ericsson Telebit (FreeBSD), NEC
(FreeBSD), and INRIA (FreeBSD). Further, we would like to thank the
feedback from the implementors who participated in the Mobile IPv6
interoperability testing at Connectathons 2000, 2001, and 2002
in San Jose, California. Similarly, we would like to thank the
participants at the ETSI interoperability testing at ETSI, in Sophia
Antipolis, France, during October 2-6, 2000, including teams from
Compaq, Ericsson, INRIA, Nokia, and Technical University of Helsinki.
Lastly, we must express our appreciation for the significant
contributions made by members of the Mobile IPv6 Security Design
Team, including (in alphabetical order) Gabriel Montenegro, Erik
Nordmark, and Pekka Nikander, who have contributed volumes of text to
this specification.
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A. State Machine for the Correspondent Binding Procedure
Home agents and correspondent nodes are stateless until a binding is
actually established.
The mobile node, however, is responsible for initiating the
correspondent binding procedure, keeping track of its state, handle
retransmissions and failures, and completing the procedure.
Section 11.6.2 defines the normative rules that the mobile node
must follow when performing the correspondent procedure. This
appendix specifies an additional, non-normative, state-machine that
illustrates the behaviour of the mobile node.
The mobile node will keep the following states in its Binding List:
- Idle: This is an abstract state that refers to the situation
that the correspondent node in question does not appear in
the Binding List. In this state, all RR and binding related
messaging is silently ignored.
- WaitHC: In this state, the mobile node has sent the Home Test
Init and CoT Init messages, and is waiting for the Home Test and
CoT messages to come back. It will also be necessary to keep
state of retransmissions for both.
- WaitH: In this state, the mobile node has a recent Care-of Cookie
and is only waiting for the Home Test message to arrive.
- WaitC: In this state, the mobile node has a recent Home Cookie
and is only waiting for the CoT message to arrive.
- WaitA: In this state, the mobile node has sent a Binding Update,
and is only waiting for the Binding Acknowledgement message to
arrive.
- WaitD: In this state, the mobile node has sent a de-registration
Binding Update, and is only waiting for the Binding
Acknowledgement message to arrive.
- WaitDH: In this state, the mobile node intends to send a
de-registration Binding Update later but is first waiting for a
home cookie before this can be done. Note that if the mobile
node is at home, it can use a home cookie also as care-of cookie.
- Bound: In this state, the mobile node has established a binding
with the correspondent node.
The following events are possible:
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- Route Optimization desired. This is a decision taken by
the mobile node based on observing traffic to and from the
correspondent node.
- Route Optimization not needed. This is another decision taken by
the mobile node, perhaps due to running out of resources or lack
of sufficient traffic to justify route optimization with this
particular correspondent node. Another reason for not needing
Route Optimization any more is that the mobile node has returned
home.
- Movement.
- Valid BRR received. A valid Binding Refresh Request message has
been received.
- Valid HoT received. A valid Home Test message has been received.
- Valid CoT received. A valid Care-of Test message has been
received.
- Valid BA received. A valid Binding Acknowledgement message has
been received.
- Valid BE received. A valid Binding Error message has been
received.
- ICMP Parameter Problem Code 1 received. This can happen if the
peer does not support this specification.
- Invalid BRR received.
- Invalid HoT received.
- Invalid CoT received.
- Invalid BA received.
- Invalid BE received.
- Retransmission needed. A timer is set to expire when a
retransmission of a packet needs to be made.
- Retransmission failed. A timer is set to expire when all
retransmissions have failed.
The following additional conditions are also used:
- Acknowledgements are required. This is a local configuration on
the mobile node side, and indicates whether acknowledgements are
required to binding updates.
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- Home cookie too old. A cookie is too old if it has been received
MIN_COOKIE_LIFE or over seconds ago.
- Care-of cookie too old.
- Reason to believe forward progress is being made. Upper layer
protocols such as TCP may provide hints to the IP layer regarding
the successfullness of the recent communications.
- Tests of the Status values received in a BE or BA message.
- Binding lifetime left. The remaining lifetime field of a Binding
Update List entry tells whether the binding currently registered
at the correspondent node still has some lifetime left, even if
we are trying to create a new one. This has relevance when an
attempt at re-binding is aborted for some reason.
The state machine for the mobile node is as follows:
State Event Action New State
--------------------------------------------------------------
Idle Route Optimization desired Send HoTI, WaitHC
Send CoTI,
Start retrans-
mission and
failure timers
Idle Valid HoT received (None) Idle
Idle Valid CoT received (None) Idle
Idle Valid BA received (None) Idle
Idle Valid BRR received (None) Idle
Idle ICMP Parameter Problem Code 1 (None) Idle
received
Idle Valid BE received and (None) Idle
status = 1
Idle Valid BE received and (None) Idle
status = 2
Idle Movement (None) Idle
State Event Action New State
--------------------------------------------------------------
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WaitHC Valid HoT received Store cookie WaitC
and nonce
index
WaitHC Valid CoT received Store cookie WaitH
and nonce
index
WaitHC Valid BA received (None) WaitHC
WaitHC Valid BRR received (None) WaitHC
WaitHC Retransmission needed Send HoTI, WaitHC
Send CoTI,
Start timer
TRetr
WaitHC Valid BE received and (None) WaitHC
status = 1
WaitHC Valid BE received and Stop timers Idle
status = 2
WaitHC Movement Send CoTI, WaitHC
Restart
retransmission
and failure
timers
WaitHC Route Optimization not needed (None) WaitHC
WaitHC ICMP Parameter Problem Code 1 Stop timers Idle
received
State Event Action New State
--------------------------------------------------------------
WaitH Valid HoT received and Store cookie WaitA
acknowledgements required and nonce
index,
Send BU,
Start retrans-
mission timer
WaitH Valid HoT received and Store cookie Bound
acknowledgements not required and nonce
index,
Send BU,
Stop timers
WaitH Valid CoT received (None) WaitH
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WaitH Valid BA received (None) WaitH
WaitH Valid BRR received (None) WaitH
WaitH Retransmission needed Send HoTI, WaitH
Start retrans-
mission timer
WaitH Valid BE received and (None) WaitH
status = 1
WaitH Valid BE received and Stop timers Idle
status = 2
WaitH Movement Send CoTI, WaitH
Restart
retransmission
and failure
timers
WaitH Route Optimization not needed (None) WaitH
WaitH ICMP Parameter Problem Code 1 (None) WaitH
received
State Event Action New State
--------------------------------------------------------------
WaitC Valid CoT received and Store cookie WaitA
acknowledgements required and nonce
index,
Send BU,
Start retrans-
mission timers
WaitC Valid CoT received Store cookie Bound
and acknowledgements not and nonce
required index,
Send BU,
Stop timers
WaitC Valid HoT received (None) WaitC
WaitC Valid BA received (None) WaitC
WaitC Valid BRR received (None) WaitC
WaitC Valid BE received and (None) WaitC
status = 1
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WaitC Valid BE received and Stop timers Idle
status = 2
WaitC Retransmission needed Send CoTI, WaitC
Start retrans-
mission timer
WaitC Movement Send CoTI, WaitC
Restart
retransmission
and failure
timers
WaitC Route Optimization not needed (None) WaitC
WaitC ICMP Parameter Problem Code 1 (None) WaitC
received
State Event Action New State
--------------------------------------------------------------
WaitA Valid BA received and Stop timers Bound
status < 128
WaitA Valid BA received and Set sequence#, WaitA
status = 141 Send BU,
Restart
retransmission
and failure
timers
WaitA Valid BA received and Send HoTI, WaitHC
status = 144 or 145 Send CoTI,
Restart
retransmission
and failure
timers
WaitA Valid BA received and Stop timers Idle
status anything else
WaitA Valid HoT received (None) WaitA
WaitA Valid CoT received (None) WaitA
WaitA Valid BRR received (None) WaitA
WaitA Retransmission needed Send BU, WaitA
Start retrans-
mission timer
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WaitA Valid BE received and (None) WaitA
status = 1
WaitA Valid BE received and Stop timers Idle
status = 2
WaitA Movement Send CoTI, WaitC
Restart
retransmission
and failure
timers
WaitA Route Optimization not needed (None) WaitA
WaitA ICMP Parameter Problem Code 1 (None) WaitA
received
State Event Action New State
--------------------------------------------------------------
WaitD Valid BA received and Stop timers Idle
status < 128
WaitD Valid BA received and Set sequence#, WaitD
status = 141 Send BU,
Restart
retransmission
and failure
timers
WaitD Valid BA received and Send HoTI, WaitDH
status = 144 or 145 Restart
retransmission
and failure
timers
WaitD Valid BA received and Stop timers Idle
status anything else
WaitD Valid HoT received (None) WaitD
WaitD Valid CoT received (None) WaitD
WaitD Valid BRR received (None) WaitD
WaitD Retransmission needed Send BU, WaitD
Start retrans-
mission timer
WaitD Valid BE received Stop timers Idle
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WaitD Movement (None) WaitD
WaitD Route Optimization Desired Send HoTI, WaitHC
Send CoTI,
Restart
retransmission
and failure
timers
WaitD ICMP Parameter Problem Code 1 (None) WaitD
received
State Event Action New State
--------------------------------------------------------------
WaitDH Valid HoT received and Send BU, WaitD
acknowledgements required Restart
retransmission
and failure
timers
WaitDH Valid HoT received and Send BU, Idle
acknowledgements not Stop timers
required
WaitDH Valid CoT received (None) WaitDH
WaitDH Valid BA received (None) WaitDH
WaitDH Valid BRR received (None) WaitDH
WaitDH Retransmission needed Send HoTI, WaitDH
Start retrans-
mission timer
WaitDH Valid BE received Stop timers Idle
WaitDH Movement (none) WaitDH
WaitDH Route Optimization Desired Send HoTI, WaitHC
Send CoTI,
Restart
retransmission
and failure
timers
WaitDH ICMP Parameter Problem Code 1 (None) WaitDH
received
State Event Action New State
--------------------------------------------------------------
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Bound Valid BRR received Send HoTI, WaitHC
Send CoTI,
Start retrans-
mission timers
Bound Valid HoT received (None) Bound
Bound Valid CoT received (None) Bound
Bound Valid BA received (None) Bound
Bound Route Optimization not Send BU Idle
needed and home cookie not
too old and acknowledgements
not required
Bound Route Optimization not Send BU, WaitD
needed and home cookie not Start retrans-
too old and acknowledgements mission and
required failure timers
Bound Route Optimization not Send HoTI, WaitDH
needed and home cookie too Start retrans-
old mission and
failure timers
Bound ICMP Parameter Problem Code 1 (None) Bound
received
Bound Movement and home cookie Send CoTI, WaitC
not too old Start retrans-
mission and
failure timers
Bound Movement and home cookie Send HoTI, WaitHC
too old Send CoTI,
Start retrans-
mission and
failure timers
Bound Valid BE received and (None) Bound
status = 1 and reason to
believe forward progress
is being made
Bound Valid BE received and Send HoTI, WaitHC
status = 1 and no reason to Send CoTI,
believe forward progress Start retrans-
is being made mission and
failure timers
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Bound Valid BE received and (None) Bound
status = 2
Bound ICMP Parameter Problem Code 1 (None) Bound
received
State Event Action New State
--------------------------------------------------------------
(Any) Retransmission failed Stop retrans- Idle
mission timer
(Any) Invalid BRR received (No change)
(Any) Invalid HoT received (No change)
(Any) Invalid CoT received (No change)
(Any) Invalid BA received (No change)
(Any) Invalid BE received (No change)
(Any) Invalid MH Type received Send BE with (No change)
status 2
B. Changes from Previous Version of the Draft
This appendix briefly lists some of the major changes in this
draft relative to the previous version of this same draft,
draft-ietf-mobileip-ipv6-15.txt:
B.1. Changes from Draft Version 16
- The "rest" of the document has been updated to correspond to the
new packet formats and messages.
- Correspondent node operation has been updated to include the new
security mechanisms.
- Procedures for reverse tunneling have been described for both
home agents and mobile nodes, and these requirements have been
take in account in Section 8.
- Terminology has been aligned throughout the document. Parameters
are now mobility options. Binding Request is Binding Refresh
Request. Capitalization of the terms has been aligned throughout
the document.
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- Overview section is now shorter, security issues are discussed
elsewhere and data structures are fully described later.
- Parts of the mobile node requirements under Section 10.9 were
moved to Section 11.3.3.
- A mechanism for Binding Acknowledgement authorization has been
clarified.
- Alignment rules, minimum lengths, and packet formats of Mobility
Header message have been updated.
- Discussion on the use of Type 0 Routing header in addition to
Type 2 Routing header has been removed from the correspondent
node operation section, and we now rely only on the ordering
requirements specified by the Routing Header Type 2 description.
- Type 2 Routing header rules have been rewritten to allow for
Segments Left to be 0. Explanation on how AH works with Routing
header has been clarified. Much of the text has been moved
to the Mobile Node Operation and Correspondent Node Operation
sections.
- The concept of "persistent" ICMP messages is no longer referred
to by a MUST keyword in Section 9.7.
- References to the "Router (R)" bit have been changed to "Router
Address (R)" bit.
- The Home Agent Information option now has to appear on all Prefix
Advertisements, or on none of them.
- Sub-options have been removed.
- The Dynamic Home Agent Address Discovery procedures have been
updated to not use piggybacking. Binding Refresh Requests
are still sent during these procedures in certain cases,
however. the Unique Identifier mobility option has been used
to synchronize BRR and BU instead of the sequence number. The
scheduling of the prefix deliveries has been changed to send new
information even when the current binding is close to expiring.
- Section 11.7 now uses ICMP Parameter Problem Code 1 instead of 2.
- Sections 11.3.4 and 10.9 now agree that IPsec need not be used
for the first advertisement.
- The rules regarding addresses for receiving and sending multicast
traffic and control messages have been clarified for mobile
nodes.
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- The Binding Missing message has been renamed to Binding Error.
- Eliminated the use of symbols in the description of the return
routability procedure.
- Wrote a new description of the return routability procedure.
B.2. Changes from Draft Version 15
- A binding update authorization mechanism suitable for use
between previously unknown peers in the global Internet has been
incorporated to the specification. As a result, Sections 5, 6.1,
14 and others have been substantially revised.
- A new IPv6 protocol has replaced IPv6 Destination Options for
some of the MIPv6 signaling. This was done in order to enable
the use of standard IPsec for the protection of binding updates
between the mobile node and the home agent, the protection
of return routability packets as they are forwarded to the
mobile node from the home agent, and possibly in the future the
protection of binding updates themselves to the correspondent
nodes. This has resulted in substantial modifications in
Section 6.
- The use of the Home Address destination option has been
restricted to the situation where a binding already exists. This
has been done in order to limit distributed Denial-of-Service
attacks through reflections attacks that employ the Home Address
Option.
- A new Binding Missing message has been added to signal the mobile
node that it has used the Home Address destination option when
the correspondent node has no existing binding to the node.
- The Authorization Data mobility option has been made a part of
the Binding Update and Acknowledgement messages, and is now
calculated in the specific manner required by the authorization
mechanism (return routability).
- Sequence number length for Binding Update messages has been
increased to 32 bits to protect home registrations against replay
attacks.
- Mobile IPv6 uses now Routing Header type 2 instead of the
general type 0, in order to limit potential dangers that
general capabilities offers type 0 and to ensure that firewall
administrators want to allow the type of Routing Header that
Mobile IPv6 uses through.
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- Requirements for all IPv6 routers have also been updated in order
to describe the considerations relating to the new Routing Header
type.
- Processing rules for mobile nodes, correspondent nodes, and to
some extent home agents have been substantially modified in order
to explain the new authorization scheme.
- Piggybacking is no longer possible due to the use of a new IPv6
protocol and not a destination option. (However, a separate
extension to this specification will allow piggybacking and takes
in account the necessary IPsec policy considerations to avoid
problems.)
- The security considerations in Section 14 have been revised to
describe the threats that this specification protects against as
well as any residual threats.
B.3. Changes from Earlier Versions of the Draft
- Strengthened mandates for mobile nodes so that now a mobile node
MUST support decapsulation and processing for routing headers
(section 11.2.3).
- Enabled ESP to be a valid way to secure reverse tunneled packets
(section 10.6).
- Removed mandate that mobile node select a default router, and
instead described it as typical behavior (section 11.4.1).
Also made it clear that picking a new default router does not
automatically mean picking a new primary care-of address.
- Modified mandated behavior from Home Agent upon reception of a
`D' bit in a Binding Update. The home agent only has to make
sure that DAD has been run, and that no other node on the home
network could be using the mobile node's link-local address.
- Added provisional ICMP numbers for the new message types, which
may be reassigned by IANA, but which will be useful for testing
purposes.
- Removed the Mobile Router Prefix Length Sub-Option
- Removed the Prefix Length field from the Binding Update, and
references to error number 136.
- Added the `S' bit so that the home agent can be instructed to
*override* its default behavior. That is, with the `S' bit
set, the home agent will not attempt to be helpful by changing
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multiple Binding Cache entries, for multiple routing prefixes,
after receiving only one Binding Update.
- Reworded the specification so that the Home Agent now has to
perform Duplicate Address Detection for the mobile node's address
on all the prefixes for which the router is performing home agent
service.
- Removed the section about Mobile Routers
- Added the Authentication Data Sub-option; reorganized the section
about computing authentication data.
- Specified that the Home Agent lifetime is by default the same as
the Router lifetime, in a Router Advertisement.
- Specified that Binding Updates with zero lifetime and the 'A' bit
set should cause a Binding Acknowledgement to be sent back to the
Source IP address of the Binding Update.
- Qualified the allowable times when a mobile node can send a
Binding Update to a correspondent node
- Added text allowing the correspondent node to extend an existing
Routing Header by also including the care-of address as the entry
of a routing header to be visited immediately before the home
address. In this way, for instance, the mobile node can be an
intermediate node of a path along the way to some other node.
- Removed the Home Address field from the Home Agent Address
Discovery Request Message.
- Noted that ICMP Unreachable forms a potential mechanism by which
a malicious node can cause a correspondent node to delete a valid
entry from its Binding Cache.
- Specified that, when a router stops offering home agent services
by turning off the 'H' flag, the mobile node has to delete the
corresponding entry from its Home Agent list.
- Clarified language about how the aggregate list of prefixes is
built by the home agent, to include only prefixes with the 'H'
bit set.
- Specified a new error status (141) to handle cases for sequence
number mismatches (e.g., when a mobile node reboots).
- Moved this section to the appendix, and reorganized other
appendix sections.
- Reorganized some related sections to be adjacent to each other.
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- Changed the Prefix Length of the Binding Update to be 7-bit only,
in order to reserve more flag bits for the future.
- Changed the Sequence Number of the Binding Update and Binding
Acknowledgement to be 8-bit only.
- Inserted specification that, after returning home and sending a
Neighbor Solicitation to the home agent, a mobile node should
accept any Neighbor Advertisement from the home agent as an
indication that the home agent is REACHABLE.
- Inserted new terminology for binding key and binding security
association in anticipation of eliminating the use of AH
- Eliminated use of AH for authenticating Binding Update, and for
authenticating Binding Acknowledgement
- Specified that all correspondent nodes MUST implement a base
protocol for establishing a Binding Key; this has become the
return routability procedure in this document.
- Added the following protocol constants:
INITIAL_SOLICIT_TIMER: XXX
- Created new ICMP messages for Mobile Prefix Solicitations and
Advertisements (see sections 6.7 and 6.8).
- Changed Network Renumbering (Section 10.9.1) to encompass mobile
node configuration issues, remove unspecified address usage,
simplify rules for prefix maintenance and sending, and use new
ICMP message types noted above.
- Added a paragraph to Returning Home (section 11.6.7) to describe
how the Home Agent discovers the mobile node's link-layer address
- Reworded parts of Appendix C as needed.
- Added the Mobile Router Prefix Length Sub-Option along with text
describing what a Mobile Router should do with it.
C. Remote Home Address Configuration
The method for initializing a mobile node's home addresses on
power-up or after an extended period of being disconnected from
the network is beyond the scope of this specification. Whatever
procedure is used should result in the mobile node having the same
stateless or stateful (e.g., DHCPv6) home address autoconfiguration
information it would have if it were attached to the home network.
Due to the possibility that the home network could be renumbered
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while the mobile node is disconnected, a robust mobile node would not
rely solely on storing these addresses locally.
Such a mobile node could initialize by using the following procedure:
1. Generate a care-of address using stateless or stateful
autoconfiguration.
2. Query DNS for the home network's mobile agent anycast address.
3. Send a Home Agent Address Discovery Request message to the home
network.
4. Receive Home Agent Address Discovery Reply message.
5. Select the most preferred home agent and establish a security
association between the mobile node's current care-of address and
the home agent for temporary use during initialization only.
6. Send a Home Prefix Solicitation message with the Request All
Prefixes flag set to the home agent from the mobile node's
care-of address.
7. Receive a Home Prefix Advertisement message from the home agent,
follow stateless address autoconfiguration rules to configure
home addresses for prefixes received.
8. Create a security association between the mobile node's home
address and the home agent.
9. Send a binding update(s) to the home agent to register the mobile
node's home addresses.
10. Receive binding acknowledgement(s) then begin normal
communications.
D. Future Extensions
D.1. Piggybacking
This document does not specify how to piggyback payload packets on
the binding related messages. However, it is envisioned that this
can be specified in a separate document when currently discussed
issues such as the interaction between piggybacking and IPsec are
fully resolved (see also Section D.3).
The idea is to use the Flag field in the HoTI message so that the
mobile node can indicate that it supports the receipt of piggybacked
messages, use the Flag field in the HoT message for the correspondent
node to indicate that it can support the receipt of piggybacked
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messages, and then carry the piggybacked payload after the MH header
by specifying a payload protocol type other than NO_NXTHDR (59).
Until such a separate specification exists implementations conforming
to this specification MUST set the payload protocol type to NO_NXTHDR
(59 decimal).
D.2. Triangular Routing and Unverified Home Addresses
Due to the concerns about opening reflection attacks with the Home
Address destination option, this specification requires that this
option must be verified against the binding cache, i.e., there must
be a binding cache entry for the Home Address and Care-of Address.
Future extensions may be specified that allow the use of unverified
Home Address destination options in ways that do not introduce
security issues.
D.3. New Authorization Methods beyond Return Routability
While the return routability procedure provides a good level
of security, there exists methods that have even higher levels
of security. Secondly, as discussed in Section 14.3, future
enhancements of IPv6 security may cause a need to improve also the
security of the return routability procedure. The question is then
what is the method to securely agree on the use of another method,
while still allowing return routability procedure for some hosts
during a transition period. In some cases, a third party can help to
make this selection. But in general infrastructureless methods have
little information beyond the exchanged messages and their contents.
For these reasons, the final version of this specification requires
a protection mechanism for selecting between the return routability
procedure and potential other future mechanisms (see Section 14.3)
but this isn't ready yet.
Using IPsec as the sole method for authorizing Binding Updates
to correspondent nodes is also possible. The protection of the
Mobility Header for this purpose is easy, though one must ensure
that the IPsec SA was created with appropriate authorization to use
the home address referenced in the Binding Update. For instance,
a certificate used by IKE to create the security association might
contain the home address. A future specification may specify how
this is done.
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Chairs' Addresses
The Working Group can be contacted via its current chairs:
Basavaraj Patil Phil Roberts
Nokia Corporation Megisto Corp.
6000 Connection Drive Suite 120
M/S M8-540 20251 Century Blvd
Irving, TX 75039 Germantown MD 20874
USA USA
Phone: +1 972-894-6709 Phone: +1 847-202-9314
Fax : +1 972-894-5349 Email: PRoberts@MEGISTO.com
EMail: Raj.Patil@nokia.com
Authors' Addresses
Questions about this document can also be directed to the authors:
David B. Johnson Charles Perkins
Rice University Nokia Research Center
Dept. of Computer Science, MS 132
6100 Main Street 313 Fairchild Drive
Houston, TX 77005-1892 Mountain View, CA 94043
USA USA
Phone: +1 713 348-3063 Phone: +1 650 625-2986
Fax: +1 713 348-5930 Fax: +1 650 625-2502
E-mail: dbj@cs.rice.edu E-mail: charliep@iprg.nokia.com
Jari Arkko
Ericsson
Jorvas 02420
Finland
Phone: +358 40 5079256
E-mail: jari.arkko@ericsson.com
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