RFC 3775
Mobility Support in IPv6
Network Working Group D. Johnson Request for Comments: 3775 Rice University Category: Standards Track C. Perkins Nokia Research Center J. Arkko Ericsson June 2004 Mobility Support in IPv6 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2004).Abstract
This document specifies a protocol which allows nodes to remain reachable while moving around in the IPv6 Internet. 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, can communicate with mobile nodes.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 5 2. Comparison with Mobile IP for IPv4 . . . . . . . . . . . . 6 3. Terminology. . . . . . . . . . . . . . . . . . . . . . . . 7 3.1. General Terms . . . . . . . . . . . . . . . . . . . 8 3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . 10 4. Overview of Mobile IPv6. . . . . . . . . . . . . . . . . . 13 4.1. Basic Operation . . . . . . . . . . . . . . . . . . 13 4.2. New IPv6 Protocol . . . . . . . . . . . . . . . . . 15 4.3. New IPv6 Destination Option . . . . . . . . . . . . 17 4.4. New IPv6 ICMP Messages. . . . . . . . . . . . . . . 17 4.5. Conceptual Data Structure Terminology . . . . . . . 17 4.6. Site-Local Addressability . . . . . . . . . . . . . 18 5. Overview of Mobile IPv6 Security . . . . . . . . . . . . . 18 5.1. Binding Updates to Home Agents. . . . . . . . . . . 18 5.2. Binding Updates to Correspondent Nodes. . . . . . . 20 5.2.1. Node Keys . . . . . . . . . . . . . . . . . 20 5.2.2. Nonces. . . . . . . . . . . . . . . . . . . 20 5.2.3. Cookies and Tokens. . . . . . . . . . . . . 21 5.2.4. Cryptographic Functions . . . . . . . . . . 22 5.2.5. Return Routability Procedure. . . . . . . . 22 5.2.6. Authorizing Binding Management Messages . . 27 5.2.7. Updating Node Keys and Nonces . . . . . . . 29 5.2.8. Preventing Replay Attacks . . . . . . . . . 30 5.3. Dynamic Home Agent Address Discovery. . . . . . . . 30 5.4. Mobile Prefix Discovery . . . . . . . . . . . . . . 30 5.5. Payload Packets . . . . . . . . . . . . . . . . . . 30 6. New IPv6 Protocol, Message Types, and Destination Option . 31 6.1. Mobility Header . . . . . . . . . . . . . . . . . . 31 6.1.1. Format. . . . . . . . . . . . . . . . . . . 32 6.1.2. Binding Refresh Request Message . . . . . . 34 6.1.3. Home Test Init Message. . . . . . . . . . . 35 6.1.4. Care-of Test Init Message . . . . . . . . . 36 6.1.5. Home Test Message . . . . . . . . . . . . . 37 6.1.6. Care-of Test Message. . . . . . . . . . . . 38 6.1.7. Binding Update Message. . . . . . . . . . . 39 6.1.8. Binding Acknowledgement Message . . . . . . 42 6.1.9. Binding Error Message . . . . . . . . . . . 44 6.2. Mobility Options. . . . . . . . . . . . . . . . . . 46 6.2.1. Format. . . . . . . . . . . . . . . . . . . 46 6.2.2. Pad1. . . . . . . . . . . . . . . . . . . . 47 6.2.3. PadN. . . . . . . . . . . . . . . . . . . . 48 6.2.4. Binding Refresh Advice. . . . . . . . . . . 48 6.2.5. Alternate Care-of Address . . . . . . . . . 49 6.2.6. Nonce Indices . . . . . . . . . . . . . . . 49 6.2.7. Binding Authorization Data. . . . . . . . . 50 6.3. Home Address Option . . . . . . . . . . . . . . . . 51
6.4. Type 2 Routing Header . . . . . . . . . . . . . . . 53
6.4.1. Format. . . . . . . . . . . . . . . . . . . 54
6.5. ICMP Home Agent Address Discovery Request Message . 55
6.6. ICMP Home Agent Address Discovery Reply Message . . 56
6.7. ICMP Mobile Prefix Solicitation Message Format. . . 57
6.8. ICMP Mobile Prefix Advertisement Message Format . . 59
7. Modifications to IPv6 Neighbor Discovery . . . . . . . . . 61
7.1. Modified Router Advertisement Message Format. . . . 61
7.2. Modified Prefix Information Option Format . . . . . 62
7.3. New Advertisement Interval Option Format. . . . . . 64
7.4. New Home Agent Information Option Format. . . . . . 65
7.5. Changes to Sending Router Advertisements. . . . . . 67
8. Requirements for Types of IPv6 Nodes . . . . . . . . . . . 69
8.1. All IPv6 Nodes. . . . . . . . . . . . . . . . . . . 69
8.2. IPv6 Nodes with Support for Route Optimization. . . 69
8.3. All IPv6 Routers. . . . . . . . . . . . . . . . . . 71
8.4. IPv6 Home Agents. . . . . . . . . . . . . . . . . . 71
8.5. IPv6 Mobile Nodes . . . . . . . . . . . . . . . . . 73
9. Correspondent Node Operation . . . . . . . . . . . . . . . 74
9.1. Conceptual Data Structures. . . . . . . . . . . . . 74
9.2. Processing Mobility Headers . . . . . . . . . . . . 75
9.3. Packet Processing . . . . . . . . . . . . . . . . . 76
9.3.1. Receiving Packets with Home Address Option. 76
9.3.2. Sending Packets to a Mobile Node. . . . . . 77
9.3.3. Sending Binding Error Messages. . . . . . . 78
9.3.4. Receiving ICMP Error Messages . . . . . . . 79
9.4. Return Routability Procedure. . . . . . . . . . . . 79
9.4.1. Receiving Home Test Init Messages . . . . . 80
9.4.2. Receiving Care-of Test Init Messages. . . . 80
9.4.3. Sending Home Test Messages. . . . . . . . . 80
9.4.4. Sending Care-of Test Messages . . . . . . . 81
9.5. Processing Bindings . . . . . . . . . . . . . . . . 81
9.5.1. Receiving Binding Updates . . . . . . . . . 81
9.5.2. Requests to Cache a Binding . . . . . . . . 84
9.5.3. Requests to Delete a Binding. . . . . . . . 84
9.5.4. Sending Binding Acknowledgements. . . . . . 85
9.5.5. Sending Binding Refresh Requests. . . . . . 86
9.6. Cache Replacement Policy. . . . . . . . . . . . . . 86
10. Home Agent Operation . . . . . . . . . . . . . . . . . . . 87
10.1. Conceptual Data Structures. . . . . . . . . . . . . 87
10.2. Processing Mobility Headers . . . . . . . . . . . . 88
10.3. Processing Bindings . . . . . . . . . . . . . . . . 88
10.3.1. Primary Care-of Address Registration. . . . 88
10.3.2. Primary Care-of Address De-Registration . . 92
10.4. Packet Processing . . . . . . . . . . . . . . . . . 94
10.4.1. Intercepting Packets for a Mobile Node. . . 94
10.4.2. Processing Intercepted Packets. . . . . . . 95
10.4.3. Multicast Membership Control. . . . . . . . 96
10.4.4. Stateful Address Autoconfiguration. . . . . 98
10.4.5. Handling Reverse Tunneled Packets . . . . . 98
10.4.6. Protecting Return Routability Packets . . . 99
10.5. Dynamic Home Agent Address Discovery. . . . . . . . 99
10.5.1. Receiving Router Advertisement Messages . . 100
10.6. Sending Prefix Information to the Mobile Node . . . 102
10.6.1. List of Home Network Prefixes . . . . . . . 102
10.6.2. Scheduling Prefix Deliveries. . . . . . . . 102
10.6.3. Sending Advertisements. . . . . . . . . . . 104
10.6.4. Lifetimes for Changed Prefixes. . . . . . . 105
11. Mobile Node Operation. . . . . . . . . . . . . . . . . . . 105
11.1. Conceptual Data Structures. . . . . . . . . . . . . 105
11.2. Processing Mobility Headers . . . . . . . . . . . . 107
11.3. Packet Processing . . . . . . . . . . . . . . . . . 107
11.3.1. Sending Packets While Away from Home. . . . 107
11.3.2. Interaction with Outbound IPsec Processing. 110
11.3.3. Receiving Packets While Away from Home. . . 112
11.3.4. Routing Multicast Packets . . . . . . . . . 114
11.3.5. Receiving ICMP Error Messages . . . . . . . 115
11.3.6. Receiving Binding Error Messages. . . . . . 116
11.4. Home Agent and Prefix Management. . . . . . . . . . 117
11.4.1. Dynamic Home Agent Address Discovery. . . . 117
11.4.2. Sending Mobile Prefix Solicitations . . . . 118
11.4.3. Receiving Mobile Prefix Advertisements. . . 118
11.5. Movement. . . . . . . . . . . . . . . . . . . . . . 120
11.5.1. Movement Detection. . . . . . . . . . . . . 120
11.5.2. Forming New Care-of Addresses . . . . . . . 122
11.5.3. Using Multiple Care-of Addresses. . . . . . 123
11.5.4. Returning Home. . . . . . . . . . . . . . . 124
11.6. Return Routability Procedure. . . . . . . . . . . . 126
11.6.1. Sending Test Init Messages. . . . . . . . . 126
11.6.2. Receiving Test Messages . . . . . . . . . . 127
11.6.3. Protecting Return Routability Packets . . . 128
11.7. Processing Bindings . . . . . . . . . . . . . . . . 128
11.7.1. Sending Binding Updates to the Home Agent . 128
11.7.2. Correspondent Registration. . . . . . . . . 131
11.7.3. Receiving Binding Acknowledgements. . . . . 134
11.7.4. Receiving Binding Refresh Requests. . . . . 136
11.8. Retransmissions and Rate Limiting . . . . . . . . . 137
12. Protocol Constants . . . . . . . . . . . . . . . . . . . . 138
13. Protocol Configuration Variables . . . . . . . . . . . . . 138
14. IANA Considerations. . . . . . . . . . . . . . . . . . . . 139
15. Security Considerations. . . . . . . . . . . . . . . . . . 142
15.1. Threats . . . . . . . . . . . . . . . . . . . . . . 142
15.2. Features. . . . . . . . . . . . . . . . . . . . . . 144
15.3. Binding Updates to Home Agent . . . . . . . . . . . 145
15.4. Binding Updates to Correspondent Nodes. . . . . . . 148
15.4.1. Overview. . . . . . . . . . . . . . . . . . 149
15.4.2. Achieved Security Properties. . . . . . . . 149
15.4.3. Comparison to Regular IPv6 Communications . 150
15.4.4. Replay Attacks. . . . . . . . . . . . . . . 152
15.4.5. Denial-of-Service Attacks . . . . . . . . . 152
15.4.6. Key Lengths . . . . . . . . . . . . . . . . 153
15.5. Dynamic Home Agent Address Discovery. . . . . . . . 154
15.6. Mobile Prefix Discovery . . . . . . . . . . . . . . 155
15.7. Tunneling via the Home Agent. . . . . . . . . . . . 155
15.8. Home Address Option . . . . . . . . . . . . . . . . 156
15.9. Type 2 Routing Header . . . . . . . . . . . . . . . 156
16. Contributors . . . . . . . . . . . . . . . . . . . . . . . 157
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 157
18. References . . . . . . . . . . . . . . . . . . . . . . . . 158
18.1. Normative References. . . . . . . . . . . . . . . . 158
18.2. Informative References. . . . . . . . . . . . . . . 159
Appendix A. Future Extensions . . . . . . . . . . . . . . . . . 161
A.1. Piggybacking. . . . . . . . . . . . . . . . . . . . 161
A.2. Triangular Routing. . . . . . . . . . . . . . . . . 161
A.3. New Authorization Methods . . . . . . . . . . . . . 161
A.4. Dynamically Generated Home Addresses. . . . . . . . 161
A.5. Remote Home Address Configuration . . . . . . . . . 162
A.6. Neighbor Discovery Extensions . . . . . . . . . . . 163
Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . . 164
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 165
1. Introduction
This document specifies a protocol which allows nodes to remain
reachable while moving around in the IPv6 Internet. Without specific
support for mobility in IPv6 [11], packets destined to a mobile node
would not be able to reach it while the mobile node is away from its
home link. 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 "home address". Packets may be routed to the mobile
node using this address regardless of the mobile node's current point
of attachment to the Internet. The mobile node may also 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, re-establishing
link-layer connectivity to the node in each new location.
Mobile IPv6 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:
o Handling links with unidirectional connectivity or partial
reachability, such as the hidden terminal problem where a host is
hidden from only some of the routers on the link.
o Access control on a link being visited by a mobile node.
o Local or hierarchical forms of mobility management (similar to
many current link-layer mobility management solutions).
o Assistance for adaptive applications.
o Mobile routers.
o Service Discovery.
o 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) benefits both
from the experiences gained from the development of Mobile IP support
in IPv4 (Mobile IPv4) [22, 23, 24], and from the opportunities
provided by IPv6. Mobile IPv6 thus shares many features with Mobile
IPv4, but is integrated into IPv6 and offers many other improvements.
This section summarizes the major differences between Mobile IPv4 and
Mobile IPv6:
o There is no need to deploy special routers as "foreign agents", as
in Mobile IPv4. Mobile IPv6 operates in any location without any
special support required from the local router.
o Support for route optimization is a fundamental part of the
protocol, rather than a nonstandard set of extensions.
o Mobile IPv6 route optimization can operate securely even without
pre-arranged security associations. It is expected that route
optimization can be deployed on a global scale between all mobile
nodes and correspondent nodes.
o Support is also integrated into Mobile IPv6 for allowing route
optimization to coexist efficiently with routers that perform
"ingress filtering" [26].
o The IPv6 Neighbor Unreachability Detection assures symmetric
reachability between the mobile node and its default router in the
current location.
o 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, reducing the amount of resulting overhead compared
to Mobile IPv4.
o Mobile IPv6 is decoupled from any particular link layer, as it
uses IPv6 Neighbor Discovery [12] instead of ARP. This also
improves the robustness of the protocol.
o The use of IPv6 encapsulation (and the routing header) removes the
need in Mobile IPv6 to manage "tunnel soft state".
o The dynamic home agent address discovery mechanism in Mobile IPv6
returns a single reply to the mobile node. The directed broadcast
approach used in IPv4 returns separate replies from each home
agent.
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 BCP 14, RFC 2119 [2].
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. unicast routable address An identifier for a single interface such that a packet sent to it from another IPv6 subnet is delivered to the interface identified by that address. Accordingly, a unicast routable address must have either a global or site-local scope (but not link-local). 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.
security association
An IPsec security association is a cooperative relationship formed
by the sharing of cryptographic keying material and associated
context. Security associations are simplex. That is, two
security associations are needed to protect bidirectional traffic
between two nodes, one for each direction.
security policy database
A database that specifies what security services are to be offered
to IP packets and in what fashion.
destination option
Destination options are carried by the IPv6 Destination Options
extension header. Destination options include optional
information that need be examined only by the IPv6 node given as
the destination address in the IPv6 header, not by routers in
between. Mobile IPv6 defines one new destination option, the Home
Address destination option (see Section 6.3).
routing header
A routing header may be present as an IPv6 header extension, and
indicates that the payload has to be delivered to a destination
IPv6 address in some way that is different from what would be
carried out by standard Internet routing. In this document, use
of the term "routing header" typically refers to use of a type 2
routing header, as specified in Section 6.4.
"|" (concatenation)
Some formulas in this specification use the symbol "|" to indicate
bytewise concatenation, as in A | B. This concatenation requires
that all of the octets of the datum A appear first in the result,
followed by all of the octets of the datum B.
First (size, input)
Some formulas in this specification use a functional form "First
(size, input)" to indicate truncation of the "input" data so that
only the first "size" bits remain to be used.
3.2. Mobile IPv6 Terms
home address
A unicast routable 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. Mobile nodes can have multiple home addresses, for
instance when there are multiple home prefixes on the 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".
L2 handover
A process by which the mobile node changes from one link-layer
connection to another. For example, a change of wireless access
point is an L2 handover.
L3 handover
Subsequent to an L2 handover, a mobile node detects a change in an
on-link subnet prefix that would require a change in the primary
care-of address. For example, a change of access router
subsequent to a change of wireless access point typically results
in an L3 handover.
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
A unicast routable 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 for a given home address 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.
registration
The process during which a mobile node sends a Binding Update to
its home agent or a correspondent node, causing a binding for the
mobile node to be registered.
mobility message
A message containing a Mobility Header (see Section 6.1).
binding authorization
Correspondent registration needs to be authorized to allow the
recipient to believe that the sender has the right to specify a
new binding.
return routability procedure
The return routability procedure authorizes registrations by the
use of a cryptographic token exchange.
correspondent registration
A return routability procedure followed by a registration, run
between the mobile node and a correspondent node.
home registration
A registration 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 keygen tokens related to the return
routability procedure. The nonces are not specific to a mobile
node, and are kept secret within the correspondent node.
nonce index
A nonce index is used to indicate which nonces have been used when
creating keygen token values, without revealing the nonces
themselves.
cookie
A cookie is a random number used by a mobile node to prevent
spoofing by a bogus correspondent node in the return routability
procedure.
care-of init cookie
A cookie sent to the correspondent node in the Care-of Test Init
message, to be returned in the Care-of Test message.
home init cookie
A cookie sent to the correspondent node in the Home Test Init
message, to be returned in the Home Test message.
keygen token
A keygen token is a number supplied by a correspondent node in the
return routability procedure to enable the mobile node to compute
the necessary binding management key for authorizing a Binding
Update.
care-of keygen token
A keygen token sent by the correspondent node in the Care-of Test
message.
home keygen token
A keygen token sent by the correspondent node in the Home Test
message.
binding management key (Kbm)
A binding management key (Kbm) is a key used for authorizing a
binding cache management message (e.g., Binding Update or Binding
Acknowledgement). Return routability provides a way to create a
binding management key.
4. Overview of Mobile IPv6
4.1. Basic Operation
A mobile node is always expected to be addressable at its home
address, whether it is currently attached to its home link or is away
from home. The "home address" is an IP address assigned to the
mobile node within its home subnet prefix on its home link. While a
mobile node is at home, packets addressed to its home address are
routed to the mobile node's home link, using conventional Internet
routing mechanisms.
While a mobile node is attached to some foreign link away from home,
it is also addressable at one or more care-of addresses. A care-of
address is an IP address associated with a mobile node that has the
subnet prefix of a particular foreign link. The mobile node can
acquire its care-of address through conventional IPv6 mechanisms,
such as stateless or stateful auto-configuration. As long as the
mobile node stays in this location, packets addressed to this care-of
address will be routed to the mobile node. The mobile node may also
accept packets from several care-of addresses, such as when it is
moving but still reachable at the previous link.
The association between a mobile node's home address and care-of
address is known as a "binding" for the mobile node. While away from
home, a mobile node registers its primary care-of address 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 replies to the mobile node by returning a
"Binding Acknowledgement" message. The operation of the mobile node
is specified in Section 11, and the operation of the home agent is
specified in Section 10.
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. Mobile nodes can
provide information about their current location to correspondent
nodes. This happens through the correspondent registration. As a
part of this procedure, a return routability test is performed in
order to authorize the establishment of the binding. The operation
of the correspondent node is specified in Section 9.
There are two possible modes for communications between the mobile
node and a correspondent node. The first mode, bidirectional
tunneling, does not require Mobile IPv6 support from the
correspondent node and is available even if the mobile node has not
registered its current binding with the correspondent node. Packets
from the correspondent node are routed to the home agent and then
tunneled to the mobile node. Packets to the correspondent node are
tunneled from the mobile node to the home agent ("reverse tunneled")
and then routed normally from the home network to the correspondent
node. In this mode, the home agent uses proxy Neighbor Discovery to
intercept any IPv6 packets addressed to the mobile node's home
address (or home addresses) on the home link. Each intercepted packet is tunneled to the mobile node's primary care-of address. This tunneling is performed using IPv6 encapsulation [15]. The second mode, "route optimization", requires the mobile node to register its current binding at the correspondent node. Packets from the correspondent node can be routed directly to the care-of address of the mobile node. When sending a packet to any IPv6 destination, the correspondent 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 [11] (see Section 6.4) to route the packet to the mobile node by way of the care-of address indicated in this binding. Routing packets directly to the mobile node's care-of address allows the shortest communications path to be used. It 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 or networks on the path to or from it is reduced. When routing packets directly to the mobile node, the correspondent node sets the Destination Address in the IPv6 header to the care-of address of the mobile node. A new type of IPv6 routing header (see Section 6.4) is also added to the packet to carry the desired home address. Similarly, the mobile node sets the Source Address in the packet's IPv6 header to its current care-of addresses. The mobile node adds a new IPv6 "Home Address" destination option (see Section 6.3) to carry its home address. The inclusion of home addresses in these packets makes the use of the care-of address transparent above the network layer (e.g., at the transport layer). Mobile IPv6 also provides support for multiple home agents, and a limited support for the reconfiguration of the home network. In these cases, the mobile node may not know the IP address of its own home agent, and even the home subnet prefixes may change over time. A mechanism, known as "dynamic home agent address discovery" allows a mobile node to dynamically discover the IP address of a home agent on its home link, even when the mobile node is away from home. Mobile nodes can also learn new information about home subnet prefixes through the "mobile prefix discovery" mechanism. These mechanisms are described starting from Section 6.5.4.2. New IPv6 Protocol
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
Home Test
Care-of Test Init
Care-of Test
These four messages are used to perform the return routability
procedure from the mobile node to a correspondent node. This
ensures authorization of subsequent Binding Updates, as described
in Section 5.2.5.
Binding Update
A Binding Update 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".
Binding Acknowledgement
A Binding Acknowledgement is used to acknowledge receipt of a
Binding Update, if an acknowledgement was requested in the Binding
Update, the binding update was sent to a home agent, or an error
occurred.
Binding Refresh Request
A Binding Refresh Request is used by a correspondent node to
request a mobile node to re-establish its binding with the
correspondent node. This message is typically used when the
cached binding is in active use but the binding's lifetime is
close to expiration. The correspondent node may use, for
instance, recent traffic and open transport layer connections as
an indication of active use.
Binding Error
The Binding Error 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.
4.3. New IPv6 Destination Option
Mobile IPv6 defines a new IPv6 destination option, the Home Address destination option. This option is described in detail in Section 6.3.4.4. 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 described in Section 10.5 and Section 11.4.1, the following two new ICMP message types are used for home agent address discovery: o Home Agent Address Discovery Request, described in Section 6.5. o Home Agent Address Discovery Reply, described 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 10.6: o Mobile Prefix Solicitation, described in Section 6.7. o Mobile Prefix Advertisement, described in Section 6.8.4.5. Conceptual Data Structure Terminology
This document describes the Mobile IPv6 protocol in terms of the following conceptual data structures: Binding Cache A cache of bindings for other nodes. This cache is maintained by home agents and correspondent nodes. The cache contains both "correspondent registration" entries (see Section 9.1) and "home registration" entries (see Section 10.1). Binding Update List This list is maintained by each mobile node. The list has an item for every binding that the mobile node has or is trying to establish with a specific other node. Both correspondent and home registrations are included in this list. Entries from the list are deleted as the lifetime of the binding expires. See Section 11.1.
Home Agents List
Home agents need to know which other home agents are on the same
link. This information is stored in the Home Agents List, as
described in more detail in Section 10.1. The list is used for
informing mobile nodes during dynamic home agent address
discovery.
4.6. Site-Local Addressability
This specification requires that home and care-of addresses MUST be
unicast routable addresses. Site-local addresses may be usable on
networks that are not connected to the Internet, but this
specification does not define when such usage is safe and when it is
not. Mobile nodes may not be aware of which site they are currently
in, it is hard to prevent accidental attachment to other sites, and
ambiguity of site-local addresses can cause problems if the home and
visited networks use the same addresses. Therefore, site-local
addresses SHOULD NOT be used as home or care-of addresses.
5. Overview of Mobile IPv6 Security
This specification provides a number of security features. These
include the protection of Binding Updates both to home agents and
correspondent nodes, the protection of mobile prefix discovery, and
the protection of the mechanisms that Mobile IPv6 uses for
transporting data packets.
Binding Updates are protected by the use of IPsec extension headers,
or by the use of the Binding Authorization Data option. This option
employs a binding management key, Kbm, which can be established
through the return routability procedure. Mobile prefix discovery is
protected through the use of IPsec extension headers. Mechanisms
related to transporting payload packets - such as the Home Address
destination option and type 2 routing header - have been specified in
a manner which restricts their use in attacks.
5.1. Binding Updates to Home Agents
The mobile node and the home agent MUST use an IPsec security
association to protect the integrity and authenticity of the Binding
Updates and Acknowledgements. Both the mobile nodes and the home
agents MUST support and SHOULD use the Encapsulating Security Payload
(ESP) [6] header in transport mode and MUST use a non-NULL payload
authentication algorithm to provide data origin authentication,
connectionless integrity and optional anti-replay protection. Note
that Authentication Header (AH) [5] is also possible but for brevity
not discussed in this specification.
In order to protect messages exchanged between the mobile node and the home agent with IPsec, appropriate security policy database entries must be created. A mobile node must be prevented from using its security association to send a Binding Update on behalf of another mobile node using the same home agent. This MUST be achieved by having the home agent check that the given home address has been used with the right security association. Such a check is provided in the IPsec processing, by having the security policy database entries unequivocally identify a single security association for protecting Binding Updates between any given home address and home agent. In order to make this possible, it is necessary that the home address of the mobile node is visible in the Binding Updates and Acknowledgements. The home address is used in these packets as a source or destination, or in the Home Address Destination option or the type 2 routing header. As with all IPsec security associations in this specification, manual configuration of security associations MUST be supported. The used shared secrets MUST be random and unique for different mobile nodes, and MUST be distributed off-line to the mobile nodes. Automatic key management with IKE [9] MAY be supported. When IKE is used, either the security policy database entries or the Mobile IPv6 processing MUST unequivocally identify the IKE phase 1 credentials which can be used to authorize the creation of security associations for protecting Binding Updates for a particular home address. How these mappings are maintained is outside the scope of this specification, but they may be maintained, for instance, as a locally administered table in the home agent. If the phase 1 identity is a Fully Qualified Domain Name (FQDN), secure forms of DNS may also be used. Section 11.3.2 discusses how IKE connections to the home agent need a careful treatment of the addresses used for transporting IKE. This is necessary to ensure that a Binding Update is not needed before the IKE exchange which is needed for securing the Binding Update. When IKE version 1 is used with preshared secret authentication between the mobile node and the home agent, aggressive mode MUST be used. The ID_IPV6_ADDR Identity Payload MUST NOT be used in IKEv1 phase 1. Reference [21] contains a more detailed description and examples on using IPsec to protect the communications between the mobile node and the home agent.
5.2. Binding Updates to Correspondent Nodes
The protection of Binding Updates sent to correspondent nodes does not require the configuration of security associations or the existence of an authentication infrastructure between the mobile nodes and correspondent nodes. Instead, a method called the return routability procedure is used to assure that the right mobile node is sending the message. This method does not protect against attackers who are on the path between the home network and the correspondent node. However, attackers in such a location are capable of performing the same attacks even without Mobile IPv6. The main advantage of the return routability procedure is that it limits the potential attackers to those having an access to one specific path in the Internet, and avoids forged Binding Updates from anywhere else in the Internet. For a more in depth explanation of the security properties of the return routability procedure, see Section 15. The integrity and authenticity of the Binding Updates messages to correspondent nodes is protected by using a keyed-hash algorithm. The binding management key, Kbm, is used to key the hash algorithm for this purpose. Kbm is established using data exchanged during the return routability procedure. The data exchange is accomplished by use of node keys, nonces, cookies, tokens, and certain cryptographic functions. Section 5.2.5 outlines the basic return routability procedure. Section 5.2.6 shows how the results of this procedure are used to authorize a Binding Update to a correspondent node.5.2.1. Node Keys
Each correspondent node has a secret key, Kcn, called the "node key", which it uses to produce the keygen tokens sent to the mobile nodes. The node key MUST be a random number, 20 octets in length. The node key allows the correspondent node to verify that the keygen tokens used by the mobile node in authorizing a Binding Update are indeed its own. This key MUST NOT be shared with any other entity. A correspondent node MAY generate a fresh node key at any time; this avoids the need for secure persistent key storage. Procedures for optionally updating the node key are discussed later in Section 5.2.7.5.2.2. Nonces
Each correspondent node also generates nonces at regular intervals. The nonces should be generated by using a random number generator that is known to have good randomness properties [1]. A correspondent node may use the same Kcn and nonce with all the mobiles it is in communication with.
Each nonce is identified by a nonce index. When a new nonce is generated, it must be associated with a new nonce index; this may be done, for example, by incrementing the value of the previous nonce index, if the nonce index is used as an array pointer into a linear array of nonces. However, there is no requirement that nonces be stored that way, or that the values of subsequent nonce indices have any particular relationship to each other. 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. Strictly speaking, indices are not necessary in the authentication, but allow the correspondent node to efficiently find the nonce value that it used in creating a keygen token. Correspondent nodes keep both the current nonce and a small set of valid previous nonces whose lifetime has not yet expired. Expired values MUST be discarded, and messages using stale or unknown indices will be rejected. The specific nonce index values cannot 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 5.2.7. A nonce is an octet string of any length. The recommended length is 64 bits.5.2.3. Cookies and Tokens
The return routability address test procedure uses cookies and keygen tokens as opaque values within the test init and test messages, respectively. o The "home init cookie" and "care-of init cookie" are 64 bit values sent to the correspondent node from the mobile node, and later returned to the mobile node. The home init cookie is sent in the Home Test Init message, and returned in the Home Test message. The care-of init cookie is sent in the Care-of Test Init message, and returned in the Care-of Test message. o The "home keygen token" and "care-of keygen token" are 64-bit values sent by the correspondent node to the mobile node via the home agent (via the Home Test message) and the care-of address (by the Care-of Test message), respectively.
The mobile node should set the home init or care-of init cookie to a newly generated random number in every Home or Care-of Test Init message it sends. The cookies are used to verify that the Home Test or Care-of Test message matches the Home Test Init or Care-of Test Init message, respectively. These cookies also serve to ensure that parties who have not seen the request cannot spoof responses. Home and care-of keygen tokens are produced by the correspondent node based on its currently active secret key (Kcn) and nonces, as well as the home or care-of address (respectively). A keygen token is valid as long as both the secret key (Kcn) and the nonce used to create it are valid.5.2.4. Cryptographic Functions
In this specification, the function used to compute hash values is SHA1 [20]. Message Authentication Codes (MACs) are computed using HMAC_SHA1 [25, 20]. HMAC_SHA1(K,m) denotes such a MAC computed on message m with key K.5.2.5. Return Routability Procedure
The Return Routability Procedure enables the correspondent node to obtain some reasonable assurance that the mobile node is in fact addressable at its claimed care-of address as well as at its home address. Only with this assurance is the correspondent node able to accept Binding Updates from the mobile node which would then instruct the correspondent node to direct that mobile node's data traffic to its claimed care-of address. This is done by testing whether packets addressed to the two claimed addresses are routed to the mobile node. The mobile node can pass the test only if it is able to supply proof that it received certain data (the "keygen tokens") which the correspondent node sends to those addresses. These data are combined by the mobile node into a binding management key, denoted Kbm. The figure below shows the message flow for the return routability procedure.
Mobile node Home agent Correspondent node
| |
| Home Test Init (HoTI) | |
|------------------------->|------------------------->|
| | |
| Care-of Test Init (CoTI) |
|---------------------------------------------------->|
| |
| | Home Test (HoT) |
|<-------------------------|<-------------------------|
| | |
| Care-of Test (CoT) |
|<----------------------------------------------------|
| |
The Home and Care-of Test Init messages are sent at the same time.
The procedure requires very little processing at the correspondent
node, and the Home and Care-of Test messages can be returned quickly,
perhaps nearly simultaneously. These four messages form the return
routability procedure.
Home Test Init
A mobile node sends a Home Test Init message to the correspondent
node (via the home agent) to acquire the home keygen token. The
contents of the message can be summarized as follows:
* Source Address = home address
* Destination Address = correspondent
* Parameters:
+ home init cookie
The Home Test Init message conveys the mobile node's home address
to the correspondent node. The mobile node also sends along a
home init cookie that the correspondent node must return later.
The Home Test Init message is reverse tunneled through the home
agent. (The headers and addresses related to reverse tunneling
have been omitted from the above discussion of the message
contents.) The mobile node remembers these cookie values to
obtain some assurance that its protocol messages are being
processed by the desired correspondent node.
Care-of Test Init
The mobile node sends a Care-of Test Init message to the
correspondent node (directly, not via the home agent) to acquire
the care-of keygen token. The contents of this message can be
summarized as follows:
* Source Address = care-of address
* Destination Address = correspondent
* Parameters:
+ care-of init cookie
The Care-of Test Init message conveys the mobile node's care-of
address to the correspondent node. The mobile node also sends
along a care-of init cookie that the correspondent node must
return later. The Care-of Test Init message is sent directly to
the correspondent node.
Home Test
The Home Test message is sent in response to a Home Test Init
message. It is sent via the home agent. The contents of the
message are:
* Source Address = correspondent
* Destination Address = home address
* Parameters:
+ home init cookie
+ home keygen token
+ home nonce index
When the correspondent node receives the Home Test Init message,
it generates a home keygen token as follows:
home keygen token :=
First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0)))
where | denotes concatenation. The final "0" inside the HMAC_SHA1
function is a single zero octet, used to distinguish home and
care-of cookies from each other.
The home keygen token is formed from the first 64 bits of the MAC.
The home keygen token tests that the mobile node can receive were
messages sent to its home address. Kcn is used in the production
of home keygen token in order to allow the correspondent node to
verify that it generated the home and care-of nonces, without
forcing the correspondent node to remember a list of all tokens it
has handed out.
The Home Test message is sent to the mobile node via the home
network, where it is presumed that the home agent will tunnel the
message to the mobile node. This means that the mobile node needs
to already have sent a Binding Update to the home agent, so that
the home agent will have received and authorized the new care-of
address for the mobile node before the return routability
procedure. For improved security, the data passed between the
home agent and the mobile node is made immune to inspection and
passive attacks. Such protection is gained by encrypting the home
keygen token as it is tunneled from the home agent to the mobile
node as specified in Section 10.4.6. The security properties of
this additional security are discussed in Section 15.4.1.
The home init cookie from the mobile node is returned in the Home
Test message, to ensure that the message comes from a node on the
route between the home agent and the correspondent node.
The home nonce index is delivered to the mobile node to later
allow the correspondent node to efficiently find the nonce value
that it used in creating the home keygen token.
Care-of Test
This message is sent in response to a Care-of Test Init message.
This message is not sent via the home agent, it is sent directly
to the mobile node. The contents of the message are:
* Source Address = correspondent
* Destination Address = care-of address
* Parameters:
+ care-of init cookie
+ care-of keygen token
+ care-of nonce index
When the correspondent node receives the Care-of Test Init
message, it generates a care-of keygen token as follows:
care-of keygen token :=
First (64, HMAC_SHA1 (Kcn, (care-of address | nonce | 1)))
Here, the final "1" inside the HMAC_SHA1 function is a single
octet containing the hex value 0x01, and is used to distinguish
home and care-of cookies from each other. The keygen token is
formed from the first 64 bits of the MAC, and sent directly to the
mobile node at its care-of address. The care-of init cookie from
the Care-of Test Init message is returned to ensure that the
message comes from a node on the route to the correspondent node.
The care-of nonce index is provided to identify the nonce used for
the care-of keygen token. The home and care-of nonce indices MAY
be the same, or different, in the Home and Care-of Test messages.
When the mobile node has received both the Home and Care-of Test
messages, the return routability procedure is complete. As a result
of the procedure, the mobile node has the data it needs to send a
Binding Update to the correspondent node. The mobile node hashes the
tokens together to form a 20 octet binding key Kbm:
Kbm = SHA1 (home keygen token | care-of keygen token)
A Binding Update may also be used to delete a previously established
binding (Section 6.1.7). In this case, the care-of keygen token is
not used. Instead, the binding management key is generated as
follows:
Kbm = SHA1(home keygen token)
Note that the correspondent node does not create any state specific
to the mobile node, until it receives the Binding Update from that
mobile node. The correspondent node does not maintain the value for
the binding management key Kbm; it creates Kbm when given the nonce
indices and the mobile node's addresses.
5.2.6. Authorizing Binding Management Messages
After the mobile node has created the binding management key (Kbm), it can supply a verifiable Binding Update to the correspondent node. This section provides an overview of this registration. The below figure shows the message flow. Mobile node Correspondent node | | | Binding Update (BU) | |---------------------------------------------->| | (MAC, seq#, nonce indices, care-of address) | | | | | | Binding Acknowledgement (BA) (if sent) | |<----------------------------------------------| | (MAC, seq#, status) | Binding Update To authorize a Binding Update, the mobile node creates a binding management key Kbm from the keygen tokens as described in the previous section. The contents of the Binding Update include the following: * Source Address = care-of address * Destination Address = correspondent * Parameters: + home address (within the Home Address destination option if different from the Source Address) + sequence number (within the Binding Update message header) + home nonce index (within the Nonce Indices option) + care-of nonce index (within the Nonce Indices option) + First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent | BU)))
The Binding Update contains a Nonce Indices option, indicating to
the correspondent node which home and care-of nonces to use to
recompute Kbm, the binding management key. The MAC is computed as
described in Section 6.2.7, using the correspondent node's address
as the destination address and the Binding Update message itself
("BU" above) as the MH Data.
Once the correspondent node has verified the MAC, it can create a
Binding Cache entry for the mobile.
Binding Acknowledgement
The Binding Update is in some cases acknowledged by the
correspondent node. The contents of the message are as follows:
* Source Address = correspondent
* Destination Address = care-of address
* Parameters:
+ sequence number (within the Binding Update message header)
+ First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
| BA)))
The Binding Acknowledgement contains the same sequence number as
the Binding Update. The MAC is computed as described in Section
6.2.7, using the correspondent node's address as the destination
address and the message itself ("BA" above) as the MH Data.
Bindings established with correspondent nodes using keys created
by way of the return routability procedure MUST NOT exceed
MAX_RR_BINDING_LIFETIME seconds (see Section 12).
The value in the Source Address field in the IPv6 header carrying
the Binding Update is normally also the care-of address which is
used in the binding. However, a different care-of address MAY be
specified by including an Alternate Care-of Address mobility
option in the Binding Update (see Section 6.2.5). When such a
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 nonce indices and MAC value MUST be
based on information gained in this test.
Binding Updates may also be sent to delete a previously
established binding. In this case, generation of the binding
management key depends exclusively on the home keygen token and
the care-of nonce index is ignored.
5.2.7. Updating Node Keys and Nonces
Correspondent nodes generate nonces at regular intervals. It is
recommended to keep each nonce (identified by a nonce index)
acceptable for at least MAX_TOKEN_LIFETIME seconds (see Section 12)
after it has been first used in constructing a return routability
message response. However, the correspondent node MUST NOT accept
nonces beyond MAX_NONCE_LIFETIME seconds (see Section 12) after the
first use. As the difference between these two constants is 30
seconds, a convenient way to enforce the above lifetimes is to
generate a new nonce every 30 seconds. The node can then continue to
accept tokens that have been based on the last 8 (MAX_NONCE_LIFETIME
/ 30) nonces. This results in tokens being acceptable
MAX_TOKEN_LIFETIME to MAX_NONCE_LIFETIME seconds after they have been
sent to the mobile node, depending on whether the token was sent at
the beginning or end of the first 30 second period. Note that the
correspondent node may also attempt to generate new nonces on demand,
or only if the old nonces have been used. This is possible, as long
as the correspondent node keeps track of how long a time ago the
nonces were used for the first time, and does not generate new nonces
on every return routability request.
Due to resource limitations, rapid deletion of bindings, or reboots
the correspondent node may not in all cases recognize the nonces that
the tokens were based on. If a nonce index is unrecognized, the
correspondent node replies with an error code in the Binding
Acknowledgement (either 136, 137, or 138 as discussed in Section
6.1.8). The mobile node can then retry the return routability
procedure.
An update of Kcn SHOULD be done at the same time as an update of a
nonce, so that nonce indices can identify both the nonce and the key.
Old Kcn values have to be therefore remembered as long as old nonce
values.
Given that the tokens are normally expected to be usable for
MAX_TOKEN_LIFETIME seconds, the mobile node MAY use them beyond a
single run of the return routability procedure until
MAX_TOKEN_LIFETIME expires. After this the mobile node SHOULD NOT
use the tokens. A fast moving mobile node MAY reuse a recent home
keygen token from a correspondent node when moving to a new location,
and just acquire a new care-of keygen token to show routability in
the new location.
While this does not save the number of round-trips due to the simultaneous processing of home and care-of return routability tests, there are fewer messages being exchanged, and a potentially long round-trip through the home agent is avoided. Consequently, this optimization is often useful. A mobile node that has multiple home addresses, MAY also use the same care-of keygen token for Binding Updates concerning all of these addresses.5.2.8. Preventing Replay Attacks
The return routability procedure also protects the participants against replayed Binding Updates through the use of the sequence number and a MAC. Care must be taken when removing bindings at the correspondent node, however. Correspondent nodes must retain bindings and the associated sequence number information at least as long as the nonces used in the authorization of the binding are still valid. Alternatively, if memory is very constrained, the correspondent node MAY invalidate the nonces that were used for the binding being deleted (or some larger group of nonces that they belong to). This may, however, impact the ability to accept Binding Updates from mobile nodes that have recently received keygen tokens. This alternative is therefore recommended only as a last measure.5.3. Dynamic Home Agent Address Discovery
No security is required for dynamic home agent address discovery.5.4. Mobile Prefix Discovery
The mobile node and the home agent SHOULD use an IPsec security association to protect the integrity and authenticity of the Mobile Prefix Solicitations and Advertisements. Both the mobile nodes and the home agents MUST support and SHOULD use the Encapsulating Security Payload (ESP) header in transport mode with a non-NULL payload authentication algorithm to provide data origin authentication, connectionless integrity and optional anti-replay protection.5.5. Payload Packets
Payload packets exchanged with mobile nodes can be protected in the usual manner, in the same way as stationary hosts can protect them. However, Mobile IPv6 introduces the Home Address destination option, a routing header, and tunneling headers in the payload packets. In the following we define the security measures taken to protect these, and to prevent their use in attacks against other parties.
This specification limits the use of the Home Address destination option to the situation where the correspondent node already has a Binding Cache entry for the given home address. This avoids the use of the Home Address option in attacks described in Section 15.1. Mobile IPv6 uses a Mobile IPv6 specific type of a routing header. This type provides the necessary functionality but does not open vulnerabilities discussed in Section 15.1. Tunnels between the mobile node and the home agent are protected by ensuring proper use of source addresses, and optional cryptographic protection. The mobile node verifies that the outer IP address corresponds to its home agent. The home agent verifies that the outer IP address corresponds to the current location of the mobile node (Binding Updates sent to the home agents are secure). The home agent identifies the mobile node through the source address of the inner packet. (Typically, this is the home address of the mobile node, but it can also be a link-local address, as discussed in Section 10.4.2. To recognize the latter type of addresses, the home agent requires that the Link-Local Address Compatibility (L) was set in the Binding Update.) These measures protect the tunnels against vulnerabilities discussed in Section 15.1. For traffic tunneled via the home agent, additional IPsec ESP encapsulation MAY be supported and used. If multicast group membership control protocols or stateful address autoconfiguration protocols are supported, payload data protection MUST be supported.
(page 31 continued on part 2)
