<- RFC Index (3701..3800)
RFC 3775
Obsoleted by RFC 6275
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.
Johnson, et al. Standard Track [Page 1]
RFC 3775 Mobility Support in IPv6 June 2004
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
Johnson, et al. Standard Track [Page 2]
RFC 3775 Mobility Support in IPv6 June 2004
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
Johnson, et al. Standard Track [Page 3]
RFC 3775 Mobility Support in IPv6 June 2004
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
Johnson, et al. Standard Track [Page 4]
RFC 3775 Mobility Support in IPv6 June 2004
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
Johnson, et al. Standard Track [Page 5]
RFC 3775 Mobility Support in IPv6 June 2004
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
Johnson, et al. Standard Track [Page 6]
RFC 3775 Mobility Support in IPv6 June 2004
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].
Johnson, et al. Standard Track [Page 7]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 8]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 9]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 10]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 11]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 12]
RFC 3775 Mobility Support in IPv6 June 2004
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
Johnson, et al. Standard Track [Page 13]
RFC 3775 Mobility Support in IPv6 June 2004
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
Johnson, et al. Standard Track [Page 14]
RFC 3775 Mobility Support in IPv6 June 2004
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:
Johnson, et al. Standard Track [Page 15]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 16]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 17]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 18]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 19]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 20]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 21]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 22]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 23]
RFC 3775 Mobility Support in IPv6 June 2004
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
Johnson, et al. Standard Track [Page 24]
RFC 3775 Mobility Support in IPv6 June 2004
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
Johnson, et al. Standard Track [Page 25]
RFC 3775 Mobility Support in IPv6 June 2004
* 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.
Johnson, et al. Standard Track [Page 26]
RFC 3775 Mobility Support in IPv6 June 2004
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)))
Johnson, et al. Standard Track [Page 27]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 28]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 29]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Johnson, et al. Standard Track [Page 30]
RFC 3775 Mobility Support in IPv6 June 2004
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.
6. New IPv6 Protocol, Message Types, and Destination Option
6.1. Mobility Header
The Mobility Header is an extension header used by mobile nodes,
correspondent nodes, and home agents in all messaging related to the
creation and management of bindings. The subsections within this
section describe the message types that may be sent using the
Mobility Header.
Mobility Header messages MUST NOT be sent with a type 2 routing
header, except as described in Section 9.5.4 for Binding
Acknowledgement. Mobility Header messages also MUST NOT be used with
a Home Address destination option, except as described in Section
11.7.1 and Section 11.7.2 for Binding Update. Binding Update List or
Binding Cache information (when present) for the destination MUST NOT
be used in sending Mobility Header messages. That is, Mobility
Header messages bypass both the Binding Cache check described in
Section 9.3.2 and the Binding Update List check described in Section
Johnson, et al. Standard Track [Page 31]
RFC 3775 Mobility Support in IPv6 June 2004
11.3.1 which are normally performed for all packets. This applies
even to messages sent to or from a correspondent node which is itself
a mobile node.
6.1.1. Format
The Mobility Header is identified by a Next Header value of 135 in
the immediately preceding header, and has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Proto | Header Len | MH Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. Message Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Payload Proto
8-bit selector. Identifies the type of header immediately
following the Mobility Header. Uses the same values as the IPv6
Next Header field [11].
This field is intended to be used by a future extension (see
Appendix B.1).
Implementations conforming to this specification SHOULD set the
payload protocol type to IPPROTO_NONE (59 decimal).
Header Len
8-bit unsigned integer, representing the length of the Mobility
Header in units of 8 octets, excluding the first 8 octets.
The length of the Mobility Header MUST be a multiple of 8 octets.
MH Type
8-bit selector. Identifies the particular mobility message in
question. Current values are specified in Section 6.1.2 and
onward. An unrecognized MH Type field causes an error indication
to be sent.
Johnson, et al. Standard Track [Page 32]
RFC 3775 Mobility Support in IPv6 June 2004
Reserved
8-bit field reserved for future use. The value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.
Checksum
16-bit unsigned integer. This field contains the checksum of the
Mobility Header. The checksum is calculated from the octet string
consisting of a "pseudo-header" followed by the entire Mobility
Header starting with the Payload Proto field. The checksum is the
16-bit one's complement of the one's complement sum of this
string.
The pseudo-header contains IPv6 header fields, as specified in
Section 8.1 of RFC 2460 [11]. The Next Header value used in the
pseudo-header is 2. The addresses used in the pseudo-header are
the addresses that appear in the Source and Destination Address
fields in the IPv6 packet carrying the Mobility Header.
Note that the procedures of calculating upper layer checksums
while away from home described in Section 11.3.1 apply even for
the Mobility Header. If a mobility message has a Home Address
destination option, then the checksum calculation uses the home
address in this option as the value of the IPv6 Source Address
field. The type 2 routing header is treated as explained in [11].
The Mobility Header is considered as the upper layer protocol for
the purposes of calculating the pseudo-header. The Upper-Layer
Packet Length field in the pseudo-header MUST be set to the total
length of the Mobility Header.
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 value is greater than the
length required for the message specified here, the remaining octets
are interpreted as mobility options. These options include padding
options that can be used to ensure that other options are aligned
Johnson, et al. Standard Track [Page 33]
RFC 3775 Mobility Support in IPv6 June 2004
properly, and that the total length of the message is divisible by 8.
The encoding and format of defined options are described in Section
6.2.
Alignment requirements for the Mobility Header are the same as for
any IPv6 protocol Header. That is, they MUST be aligned on an 8-
octet boundary.
6.1.2. Binding Refresh Request Message
The Binding Refresh Request (BRR) message requests a mobile node to
update its mobility binding. This message is sent by correspondent
nodes according to the rules in Section 9.5.5. When a mobile node
receives a packet containing a Binding Refresh Request message it
processes the message according to the rules in Section 11.7.4.
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. This field
contains zero 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.
Johnson, et al. Standard Track [Page 34]
RFC 3775 Mobility Support in IPv6 June 2004
There MAY be additional information, associated with this Binding
Refresh Request message that need not be present in all Binding
Refresh Request messages sent. Mobility options allow future
extensions to the format of the Binding Refresh Request message to
be defined. This specification does not define any options valid
for the Binding Refresh Request message.
If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 0.
6.1.3. Home Test Init Message
A mobile node uses the Home Test Init (HoTI) message to initiate the
return routability procedure and request a home keygen token from a
correspondent node (see Section 11.6.1). The Home Test Init 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Init 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.
Home Init Cookie
64-bit field which contains a random value, the home init cookie.
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. This field
contains zero or more TLV-encoded mobility options. The receiver
Johnson, et al. Standard Track [Page 35]
RFC 3775 Mobility Support in IPv6 June 2004
MUST ignore and skip any options which it does not understand.
This specification does not define any options valid for the Home
Test Init message.
If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 1.
This message 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
indicated by the IPsec security policy database. The protection of
Home Test Init messages is unrelated to the requirement to protect
regular payload traffic, which MAY use such tunnels as well.
6.1.4. Care-of Test Init Message
A mobile node uses the Care-of Test Init (CoTI) message to initiate
the return routability procedure and request a care-of keygen token
from a correspondent node (see Section 11.6.1). The Care-of Test
Init 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Init 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.
Care-of Init Cookie
64-bit field which contains a random value, the care-of init
cookie.
Johnson, et al. Standard Track [Page 36]
RFC 3775 Mobility Support in IPv6 June 2004
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. This field
contains zero or more TLV-encoded mobility options. The receiver
MUST ignore and skip any options which it does not understand.
This specification does not define any options valid for the
Care-of Test Init message.
If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 1.
6.1.5. Home Test Message
The Home Test (HoT) message is a response to the Home Test Init
message, and is sent from the correspondent node to the mobile node
(see Section 5.2.5). The Home Test 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:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Keygen Token +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Nonce Index
This field will be echoed back by the mobile node to the
correspondent node in a subsequent Binding Update.
Home Init Cookie
64-bit field which contains the home init cookie.
Johnson, et al. Standard Track [Page 37]
RFC 3775 Mobility Support in IPv6 June 2004
Home Keygen Token
This field contains the 64 bit home keygen token used in the
return routability procedure.
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. This field
contains zero or more TLV-encoded mobility options. The receiver
MUST ignore and skip any options which it does not understand.
This specification does not define any options valid for the Home
Test message.
If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 2.
6.1.6. Care-of Test Message
The Care-of Test (CoT) message is a response to the Care-of Test Init
message, and is sent from the correspondent node to the mobile node
(see Section 11.6.2). The Care-of Test 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:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Keygen Token +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Care-of Nonce Index
This value will be echoed back by the mobile node to the
correspondent node in a subsequent Binding Update.
Johnson, et al. Standard Track [Page 38]
RFC 3775 Mobility Support in IPv6 June 2004
Care-of Init Cookie
64-bit field which contains the care-of init cookie.
Care-of Keygen Token
This field contains the 64 bit care-of keygen token used in the
return routability procedure.
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. This field
contains zero or more TLV-encoded mobility options. The receiver
MUST ignore and skip any options which it does not understand.
This specification does not define any options valid for the
Care-of Test message.
If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 2.
6.1.7. Binding Update Message
The Binding Update (BU) message is used by a mobile node to notify
other nodes of a new care-of address for itself. Binding Updates are
sent as described in Section 11.7.1 and Section 11.7.2.
The Binding Update 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:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|L|K| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. 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.
Johnson, et al. Standard Track [Page 39]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Link-Local Address Compatibility (L)
The Link-Local Address Compatibility (L) bit is set when the home
address reported by the mobile node has the same interface
identifier as the mobile node's link-local address.
Key Management Mobility Capability (K)
If this bit is cleared, the protocol used for establishing the
IPsec security associations between the mobile node and the home
agent does not survive movements. It may then have to be rerun.
(Note that the IPsec security associations themselves are expected
to survive movements.) If manual IPsec configuration is used, the
bit MUST be cleared.
This bit is valid only in Binding Updates sent to the home agent,
and MUST be cleared in other Binding Updates. Correspondent nodes
MUST ignore this bit.
Reserved
These fields are unused. They MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Sequence #
A 16-bit unsigned integer used by the receiving node to sequence
Binding Updates and by the sending node to match a returned
Binding Acknowledgement with this Binding Update.
Lifetime
16-bit unsigned integer. The number of time units remaining
before the binding MUST be considered expired. A value of zero
indicates that the Binding Cache entry for the mobile node MUST be
deleted. (In this case the specified care-of address MUST also be
set equal to the home address.) One time unit is 4 seconds.
Johnson, et al. Standard Track [Page 40]
RFC 3775 Mobility Support in IPv6 June 2004
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. This field
contains zero 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.
The following options are valid in a Binding Update:
* Binding Authorization Data option (this option is mandatory in
Binding Updates sent to a correspondent node)
* Nonce Indices option.
* Alternate Care-of Address option
If no options are present in this message, 4 octets of padding are
necessary and the Header Len field will be set to 1.
The care-of address is specified either by the Source Address field
in the IPv6 header or by the Alternate Care-of Address option, if
present. The care-of address MUST be a unicast routable address.
IPv6 Source Address MUST be a topologically correct source address.
Binding Updates for a care-of address which is not a unicast routable
address MUST be silently discarded. Similarly, the Binding Update
MUST be silently discarded if the care-of address appears as a home
address in an existing Binding Cache entry, with its current location
creating a circular reference back to the home address specified in
the Binding Update (possibly through additional entries).
The deletion of a binding can be indicated by setting the Lifetime
field to 0 and by setting the care-of address equal to the home
address. In deletion, the generation of the binding management key
depends exclusively on the home keygen token, as explained in Section
5.2.5. (Note that while the senders are required to set both the
Lifetime field to 0 and the care-of address equal to the home
address, Section 9.5.1 rules for receivers are more liberal, and
interpret either condition as a deletion.)
Correspondent nodes SHOULD NOT delete the Binding Cache entry before
the lifetime expires, if any application hosted by the correspondent
node is still likely to require communication with the mobile node.
A Binding Cache entry that is de-allocated prematurely might cause
subsequent packets to be dropped from the mobile node, if they
contain the Home Address destination option. This situation is
recoverable, since a Binding Error message is sent to the mobile node
Johnson, et al. Standard Track [Page 41]
RFC 3775 Mobility Support in IPv6 June 2004
(see Section 6.1.9); however, it causes unnecessary delay in the
communications.
6.1.8. Binding Acknowledgement Message
The Binding Acknowledgement is used to acknowledge receipt of a
Binding Update (Section 6.1.7). This packet is sent as described in
Section 9.5.4 and Section 10.3.1.
The Binding Acknowledgement 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 |K| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Key Management Mobility Capability (K)
If this bit is cleared, the protocol used by the home agent for
establishing the IPsec security associations between the mobile
node and the home agent does not survive movements. It may then
have to be rerun. (Note that the IPsec security associations
themselves are expected to survive movements.)
Correspondent nodes MUST set the K bit to 0.
Reserved
These fields are unused. They MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Johnson, et al. Standard Track [Page 42]
RFC 3775 Mobility Support in IPv6 June 2004
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. Values
greater than or equal to 128 indicate that the Binding Update was
rejected by the receiving node. The following Status values are
currently defined:
0 Binding Update accepted
1 Accepted but prefix discovery necessary
128 Reason unspecified
129 Administratively prohibited
130 Insufficient resources
131 Home registration not supported
132 Not home subnet
133 Not home agent for this mobile node
134 Duplicate Address Detection failed
135 Sequence number out of window
136 Expired home nonce index
137 Expired care-of nonce index
138 Expired nonces
139 Registration type change disallowed
Up-to-date values of the Status field are to be specified in the IANA
registry of assigned numbers [19].
Sequence #
The Sequence Number in the Binding Acknowledgement is copied from
the Sequence Number field in the Binding Update. It is used by
the mobile node in matching this Binding Acknowledgement with an
outstanding Binding Update.
Johnson, et al. Standard Track [Page 43]
RFC 3775 Mobility Support in IPv6 June 2004
Lifetime
The granted lifetime, in time units of 4 seconds, for which this
node SHOULD retain the entry for this mobile node in its Binding
Cache.
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. This field
contains zero 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 that need not be present in all Binding
Acknowledgements sent. Mobility options allow future extensions
to the format of the Binding Acknowledgement to be defined. The
following options are valid for the Binding Acknowledgement:
* Binding Authorization Data option (this option is mandatory in
Binding Acknowledgements sent by a correspondent node, except
where otherwise noted in Section 9.5.4)
* Binding Refresh Advice option
If no options are present in this message, 4 octets of padding are
necessary and the Header Len field will be set to 1.
6.1.9. Binding Error 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; see Section 9.3.3 for details.
Johnson, et al. Standard Track [Page 44]
RFC 3775 Mobility Support in IPv6 June 2004
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 values are currently defined:
1 Unknown binding for Home Address destination option
2 Unrecognized MH Type value
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.
Johnson, et al. Standard Track [Page 45]
RFC 3775 Mobility Support in IPv6 June 2004
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. This field
contains zero 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. Mobility options allow future extensions to the
format of 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.
If no actual options are present in this message, no padding is
necessary and the Header Len field will be set to 2.
6.2. Mobility Options
Mobility messages can include zero or more mobility options. This
allows optional fields that may not be needed in every use of a
particular Mobility Header, as well as future extensions to the
format of the messages. Such options are included in the Message
Data field of the message itself, after the fixed portion of the
message data specified in the message subsections of Section 6.1.
The presence of such options will be indicated by the Header Len of
the Mobility Header. If included, the Binding Authorization Data
option (Section 6.2.7) MUST be the last option and MUST NOT have
trailing padding. Otherwise, options can be placed in any order.
6.2.1. Format
Mobility options are encoded within the remaining space of the
Message Data field of a mobility 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 Length | Option Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Johnson, et al. Standard Track [Page 46]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Option Length
8-bit unsigned integer, representing the length in octets of the
mobility option, not including the Option Type and Option Length
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.
Mobility options may have alignment requirements. Following the
convention in IPv6, these options are 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) [11].
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
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
NOTE! the format of the Pad1 option is a special case - it has
neither Option Length nor Option Data fields.
Johnson, et al. Standard Track [Page 47]
RFC 3775 Mobility Support in IPv6 June 2004
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 1 | Option Length | Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
The PadN option is used to insert two or more octets of padding in
the Mobility Options area of a mobility message. For N octets of
padding, the Option Length field contains the value N-2, and the
Option Data consists of N-2 zero-valued octets. PadN Option data
MUST be ignored by the receiver.
6.2.4. Binding Refresh Advice
The Binding Refresh Advice 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Refresh Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Binding Refresh Advice option is only valid in the Binding
Acknowledgement, and only on Binding Acknowledgements sent from the
mobile node's home agent in reply to a home registration. The
Refresh Interval is measured in units of four seconds, and indicates
remaining time until the mobile node SHOULD send a new home
registration to the home agent. The Refresh Interval MUST be set to
indicate a smaller time interval than the Lifetime value of the
Binding Acknowledgement.
Johnson, et al. Standard Track [Page 48]
RFC 3775 Mobility Support in IPv6 June 2004
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Alternate Care-of Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Normally, a Binding Update specifies the desired care-of address in
the Source Address field of the IPv6 header. However, this is not
possible in some cases, such as when the mobile node wishes to
indicate a care-of address which it cannot use as a topologically
correct source address (Section 6.1.7 and Section 11.7.2) or when the
used security mechanism does not protect the IPv6 header (Section
11.7.1).
The Alternate Care-of Address option is provided for these
situations. This option is valid only in Binding Update. 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.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Nonce Index | Care-of Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Johnson, et al. Standard Track [Page 49]
RFC 3775 Mobility Support in IPv6 June 2004
The Nonce Indices option is valid only in the Binding Update message
sent to a correspondent node, and only when present together with a
Binding Authorization Data option. When the correspondent node
authorizes the Binding Update, it needs to produce home and care-of
keygen tokens from its stored random nonce values.
The Home Nonce Index field tells the correspondent node which nonce
value to use when producing the home keygen token.
The Care-of Nonce Index field is ignored in requests to delete a
binding. Otherwise, it tells the correspondent node which nonce
value to use when producing the care-of keygen token.
6.2.7. Binding Authorization Data
The Binding Authorization Data option does not have alignment
requirements as such. However, since this option must be the last
mobility option, an implicit alignment requirement is 8n + 2. The
format of this 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 = 5 | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Authenticator |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Binding Authorization Data option is valid in the Binding Update
and Binding Acknowledgement.
The Option Length field contains 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
authority. Rules for calculating this value depends on the used
authorization procedure.
Johnson, et al. Standard Track [Page 50]
RFC 3775 Mobility Support in IPv6 June 2004
For the return routability procedure, this option can appear in the
Binding Update and Binding Acknowledgements. Rules for calculating
the Authenticator value are the following:
Mobility Data = care-of address | correspondent | MH Data
Authenticator = First (96, HMAC_SHA1 (Kbm, Mobility Data))
Where | denotes concatenation. "Care-of address" is the care-of
address which will be registered for the mobile node if the Binding
Update succeeds, or the home address of the mobile node if this
option is used in de-registration. Note also that this address might
be different from the source address of the Binding Update message,
if the Alternative Care-of Address mobility option is used, or when
the lifetime of the binding is set to zero.
The "correspondent" is the IPv6 address of the correspondent node.
Note that, if the message is sent to a destination which is itself
mobile, the "correspondent" address may not be the address found in
the Destination Address field of the IPv6 header; instead the home
address from the type 2 Routing header should be used.
"MH Data" is the content of the Mobility Header, excluding the
Authenticator field itself. The Authenticator value is calculated as
if the Checksum field in the Mobility Header was zero. The Checksum
in the transmitted packet is still calculated in the usual manner,
with the calculated Authenticator being a part of the packet
protected by the Checksum. Kbm is the binding management key, which
is typically created using nonces provided by the correspondent node
(see Section 9.4). Note that while the contents of a potential Home
Address destination option are not covered in this formula, the rules
for the calculation of the Kbm do take the home address in account.
This ensures that the MAC will be different for different home
addresses.
The first 96 bits from the MAC result are used as the Authenticator
field.
6.3. Home Address Option
The Home Address option is carried by the Destination Option
extension header (Next Header value = 60). It is used in a packet
sent by a mobile node while away from home, to inform the recipient
of the mobile node's home address.
Johnson, et al. Standard Track [Page 51]
RFC 3775 Mobility Support in IPv6 June 2004
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
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. This
address MUST be a unicast routable address.
The alignment requirement [11] for the Home Address option is 8n+6.
The three highest-order bits of the Option Type field are encoded to
indicate specific processing of the option [11]; for the Home Address
option, these three bits are set to 110. This indicates the
following processing requirements:
o Any IPv6 node that does not recognize the Option Type must discard
the packet, and 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. The Pointer field in the
ICMP message SHOULD point at the Option Type field. Otherwise,
for multicast addresses, the ICMP message MUST NOT be sent.
o The data within the option cannot change en route to the packet's
final destination.
Johnson, et al. Standard Track [Page 52]
RFC 3775 Mobility Support in IPv6 June 2004
The Home Address option MUST be placed as follows:
o After the routing header, if that header is present
o Before the Fragment Header, if that header is present
o Before the AH Header or ESP Header, if either one of those headers
are present
For each IPv6 packet header, the Home Address Option MUST NOT appear
more than once. However, an encapsulated packet [15] MAY contain a
separate Home Address option associated with each encapsulating IP
header.
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.
6.4. Type 2 Routing Header
Mobile IPv6 defines a new routing header variant, the type 2 routing
header, to allow the packet to be routed directly from a
correspondent to the mobile node's care-of address. The mobile
node's care-of address is inserted into the IPv6 Destination Address
field. Once the packet arrives at the care-of address, the mobile
node retrieves its home address from the routing header, and this is
used as the final destination address for the packet.
The new routing header uses a different type than defined for
"regular" IPv6 source routing, enabling firewalls to apply different
rules to source routed packets than to Mobile IPv6. 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. The IP
address contained in the routing header, since it is the mobile
node's home address, MUST be a unicast routable address.
Furthermore, if the scope of the home address is smaller than the
scope of the care-of address, the mobile node MUST discard the packet
(see Section 4.6).
Johnson, et al. Standard Track [Page 53]
RFC 3775 Mobility Support in IPv6 June 2004
6.4.1. 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 IPv6
Next Header field [11].
Hdr Ext Len
2 (8-bit unsigned integer); length of the routing header in 8-
octet units, not including the first 8 octets.
Routing Type
2 (8-bit unsigned integer).
Segments Left
1 (8-bit unsigned integer).
Reserved
32-bit reserved field. The value MUST be initialized to zero by
the sender, and MUST be ignored by the receiver.
Home Address
The Home Address of the destination Mobile Node.
Johnson, et al. Standard Track [Page 54]
RFC 3775 Mobility Support in IPv6 June 2004
For a type 2 routing header, the Hdr Ext Len MUST be 2. The Segments
Left value describes the number of route segments remaining; i.e.,
number of explicitly listed intermediate nodes still to be visited
before reaching the final destination. Segments Left MUST be 1. The
ordering rules for extension headers in an IPv6 packet are described
in Section 4.1 of RFC 2460 [11]. The type 2 routing header defined
for Mobile IPv6 follows the same ordering as other routing headers.
If both a type 0 and a type 2 routing header are present, the type 2
routing header should follow the other routing header. A packet
containing such nested encapsulation should be created as if the
inner (type 2) routing header was constructed first and then treated
as an original packet by the outer (type 0) routing header
construction process.
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 Section 11.4.1. The mobile node sends the
Home Agent Address Discovery Request message to the Mobile IPv6
Home-Agents anycast address [16] for its own home subnet prefix.
(Note that the currently defined anycast addresses may not work with
all prefix lengths other than those defined in RFC 2373 [3, 35].)
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
144
Code
0
Johnson, et al. Standard Track [Page 55]
RFC 3775 Mobility Support in IPv6 June 2004
Checksum
The ICMP checksum [14].
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 is typically one of the mobile node's current care-of
addresses. At the time of performing this dynamic home agent address
discovery procedure, it is likely that the mobile node is not
registered with any home agent. Therefore, neither the nature of the
address nor the identity of the mobile node can be established at
this time. The home agent MUST then return the Home Agent Address
Discovery Reply message directly to the Source Address chosen by the
mobile node.
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 that uses the dynamic home agent
address discovery mechanism, as described in Section 10.5.
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 .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Johnson, et al. Standard Track [Page 56]
RFC 3775 Mobility Support in IPv6 June 2004
Type
145
Code
0
Checksum
The ICMP checksum [14].
Identifier
The identifier from the invoking 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.
Home Agent Addresses
A list of addresses of home agents on the home link for the mobile
node. The number of addresses presented in the list is indicated
by the remaining length of the IPv6 packet carrying the Home Agent
Address Discovery Reply message.
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
and update home address(es) according to changes in prefix
information supplied by the home agent.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Johnson, et al. Standard Track [Page 57]
RFC 3775 Mobility Support in IPv6 June 2004
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 that the mobile node wishes to learn prefix
information about.
Hop Limit
Set to an initial hop limit value, similarly to any other unicast
packet sent by the mobile node.
Destination Option:
A Home Address destination option MUST be included.
ESP header:
IPsec headers MUST be supported and SHOULD be used as described in
Section 5.4.
ICMP Fields:
Type
146
Code
0
Checksum
The ICMP checksum [14].
Identifier
An identifier to aid in matching a future Mobile Prefix
Advertisement to this Mobile Prefix Solicitation.
Johnson, et al. Standard Track [Page 58]
RFC 3775 Mobility Support in IPv6 June 2004
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
The Mobile Prefix Solicitation messages may have options. These
options MUST use the option format defined in RFC 2461 [12]. This
document does not define any option types for the Mobile Prefix
Solicitation message, but future documents may define new options.
Home agents MUST silently ignore any options they do not recognize
and continue processing the message.
6.8. ICMP Mobile Prefix Advertisement Message Format
A home agent will send a Mobile Prefix Advertisement 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
10.6.
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 |M|O| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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,
this field contains the Source Address field from that packet.
For unsolicited messages, the mobile node's care-of address SHOULD
be used. Note that unsolicited messages can only be sent if the
mobile node is currently registered with the home agent.
Johnson, et al. Standard Track [Page 59]
RFC 3775 Mobility Support in IPv6 June 2004
Routing header:
A type 2 routing header MUST be included.
ESP header:
IPsec headers MUST be supported and SHOULD be used as described in
Section 5.4.
ICMP Fields:
Type
147
Code
0
Checksum
The ICMP checksum [14].
Identifier
An identifier to aid in matching this Mobile Prefix Advertisement
to a previous Mobile Prefix Solicitation.
M
1-bit Managed Address Configuration flag. When set, hosts use the
administered (stateful) protocol for address autoconfiguration in
addition to any addresses autoconfigured using stateless address
autoconfiguration. The use of this flag is described in [12, 13].
O
1-bit Other Stateful Configuration flag. When set, hosts use the
administered (stateful) protocol for autoconfiguration of other
(non-address) information. The use of this flag is described in
[12, 13].
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Johnson, et al. Standard Track [Page 60]
RFC 3775 Mobility Support in IPv6 June 2004
The Mobile Prefix Advertisement messages may have options. These
options MUST use the option format defined in RFC 2461 [12]. This
document defines one option which may be carried in a Mobile Prefix
Advertisement message, but future documents may define new options.
Mobile nodes MUST silently ignore any options they do not recognize
and continue processing the message.
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 10.6 describes which
prefixes should be advertised to the mobile node.
The Prefix Information option is defined in Section 4.6.2 of RFC
2461 [12], with modifications defined in Section 7.2 of this
specification. The home agent MUST use this modified Prefix
Information option to send home network prefixes as defined in
Section 10.6.1.
If the Advertisement is sent in response to a Mobile Prefix
Solicitation, the home agent MUST copy the Identifier value from that
message into the Identifier field of the Advertisement.
The home agent MUST NOT send more than one Mobile Prefix
Advertisement message per second to any mobile node.
The M and O bits MUST be cleared if the Home Agent DHCPv6 support is
not provided. If such support is provided then they are set in
concert with the home network's administrative settings.
7. Modifications to IPv6 Neighbor Discovery
7.1. Modified Router Advertisement Message Format
Mobile IPv6 modifies the format of the Router Advertisement message
[12] 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:
Johnson, et al. Standard Track [Page 61]
RFC 3775 Mobility Support in IPv6 June 2004
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 [12]:
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 IPv6 home agent on this link.
Reserved
Reduced from a 6-bit field to a 5-bit field to account for the
addition of the above bit.
7.2. Modified Prefix Information Option Format
Mobile IPv6 requires knowledge of a router's global address in
building a Home Agents List as part of the dynamic home agent address
discovery mechanism.
However, Neighbor Discovery [12] 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 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:
Johnson, et al. Standard Track [Page 62]
RFC 3775 Mobility Support in IPv6 June 2004
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 [12]:
Router Address (R)
1-bit router address flag. When set, indicates that the Prefix
field contains a complete IP address assigned to the sending
router. The indicated prefix is the first Prefix Length bits of
the Prefix field. The 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 bits. 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 above bit.
In a Router Advertisement, a home agent MUST, and all other routers
MAY, 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 [12]. Also, when
sending unsolicited multicast Router Advertisements more frequently
Johnson, et al. Standard Track [Page 63]
RFC 3775 Mobility Support in IPv6 June 2004
than the limit specified in RFC 2461 [12], the sending router need
not include all options in each of these Advertisements. However, in
both of these cases the router SHOULD include at least one Prefix
Information option with the Router Address (R) bit set in each such
advertisement, if this bit is set in some advertisement sent by the
router.
In addition, the following requirement can assist mobile nodes in
movement detection. Barring changes in the prefixes for the link,
routers that send multiple Router Advertisements with the Router
Address (R) bit set in some of the included Prefix Information
options SHOULD provide at least one option and router address which
stays the same in all of the Advertisements.
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) is 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.
Johnson, et al. Standard Track [Page 64]
RFC 3775 Mobility Support in IPv6 June 2004
Advertisement Interval
32-bit unsigned integer. The maximum time, in milliseconds,
between successive unsolicited Router Advertisement messages sent
by this router on this network interface. Using the conceptual
router configuration variables defined by Neighbor Discovery [12],
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 11.5.1.
This option MUST be silently ignored for other Neighbor Discovery
messages.
7.4. New Home Agent Information Option Format
Mobile IPv6 defines a new Home Agent Information option, used in
Router Advertisements 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.
Johnson, et al. Standard Track [Page 65]
RFC 3775 Mobility Support in IPv6 June 2004
Home Agent Preference
16-bit unsigned 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 MUST be considered to be 0.
Greater values indicate a more preferable home agent than lower
values.
The manual configuration of the Home Agent Preference value is
described in Section 8.4. 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, 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. 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.
Johnson, et al. Standard Track [Page 66]
RFC 3775 Mobility Support in IPv6 June 2004
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.
7.5. Changes to Sending Router Advertisements
The Neighbor Discovery protocol specification [12] 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.
One method which can provide for faster movement detection, is to
increase the rate at which unsolicited Router Advertisements are
sent. Mobile IPv6 relaxes this limit such that routers MAY send
unsolicited multicast Router Advertisements more frequently. This
method can be applied 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).
Routers supporting mobility SHOULD be able to be configured with a
smaller MinRtrAdvInterval value and MaxRtrAdvInterval value to allow
sending of unsolicited multicast Router Advertisements more often.
The minimum allowed values are:
Johnson, et al. Standard Track [Page 67]
RFC 3775 Mobility Support in IPv6 June 2004
o MinRtrAdvInterval 0.03 seconds
o MaxRtrAdvInterval 0.07 seconds
In the case where the minimum intervals and delays are used, the mean
time between unsolicited multicast router advertisements is 50 ms.
Use of these modified limits MUST be configurable (see also the
configuration variable MinDelayBetweenRas in Section 13 which may
also have to be modified accordingly). Systems where these values
are available MUST NOT default to them, and SHOULD default to values
specified in RFC 2461. Knowledge of the type of network interface
and operating environment SHOULD be taken into account in configuring
these limits for each network interface. This is important with some
wireless links, where increasing the frequency of multicast beacons
can cause considerable overhead. Routers SHOULD adhere to the
intervals specified in RFC 2461 [12], if this overhead is likely to
cause service degradation.
Additionally, the possible low values of MaxRtrAdvInterval may cause
some problems with movement detection in some mobile nodes. To
ensure that this is not a problem, Routers SHOULD add 20 ms to any
Advertisement Intervals sent in RAs, which are below 200 ms, in order
to account for scheduling granularities on both the MN and the
Router.
Note that multicast Router Advertisements are not always required in
certain wireless networks that have limited bandwidth. Mobility
detection or link changes in such networks may be done at lower
layers. Router advertisements in such networks SHOULD be sent only
when solicited. In such networks it SHOULD be possible to disable
unsolicited multicast Router Advertisements on specific interfaces.
The MinRtrAdvInterval and MaxRtrAdvInterval in such a case can be set
to some high values.
Home agents MUST include the Source Link-Layer Address option in all
Router Advertisements they send. This simplifies the process of
returning home, as discussed in Section 11.5.4.
Note that according to RFC 2461 [12], AdvDefaultLifetime is by
default based on the value of MaxRtrAdvInterval. AdvDefaultLifetime
is used in the Router Lifetime field of Router Advertisements. Given
that this field is expressed in seconds, a small MaxRtrAdvInterval
value can result in a zero value for this field. To prevent this,
routers SHOULD keep AdvDefaultLifetime in at least one second, even
if the use of MaxRtrAdvInterval would result in a smaller value.
Johnson, et al. Standard Track [Page 68]
RFC 3775 Mobility Support in IPv6 June 2004
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.
The requirements are set for the following groups of nodes:
o All IPv6 nodes.
o All IPv6 nodes with support for route optimization.
o All IPv6 routers.
o All Mobile IPv6 home agents.
o All Mobile IPv6 mobile nodes.
It is outside the scope of this specification to specify which of
these groups are mandatory in IPv6. We only describe what is
mandatory for a node that supports, for instance, route optimization.
Other specifications are expected to define the extent of IPv6.
8.1. All IPv6 Nodes
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. There are no Mobile IPv6 specific MUST
requirements for such nodes, and basic IPv6 techniques are
sufficient. If a mobile node attempts to set up route optimization
with a node with only basic IPv6 support, an ICMP error will signal
that the node does not support such optimizations (Section 11.3.5),
and communications will flow through the home agent.
An IPv6 node MUST NOT support the Home Address destination option,
type 2 routing header, or the Mobility Header unless it fully
supports the requirements listed in the next sections for either
route optimization, mobile node, or home agent functionality.
8.2. IPv6 Nodes with Support for Route Optimization
Nodes that implement route optimization are a subset of all IPv6
nodes on the Internet. The ability of a correspondent node to
participate in route optimization is essential for the efficient
operation of the IPv6 Internet, for the following reasons:
Johnson, et al. Standard Track [Page 69]
RFC 3775 Mobility Support in IPv6 June 2004
o Avoidance of congestion in the home network, and enabling the use
of lower-performance home agent equipment even for supporting
thousands of mobile nodes.
o Reduced network load across the entire Internet, as mobile devices
begin to predominate.
o Reduction of jitter and latency for the communications.
o Greater likelihood of success for QoS signaling as tunneling is
avoided and, again, fewer sources of congestion.
o Improved robustness against network partitions, congestion, and
other problems, since fewer routing path segments are traversed.
These effects combine to enable much better performance and
robustness for communications between mobile nodes and IPv6
correspondent nodes. Route optimization introduces a small amount of
additional state for the peers, some additional messaging, and up to
1.5 roundtrip delays before it can be turned on. However, it is
believed that the benefits far outweigh the costs in most cases.
Section 11.3.1 discusses how mobile nodes may avoid route
optimization for some of the remaining cases, such as very short-term
communications.
The following requirements apply to all correspondent nodes that
support route optimization:
o The node MUST be able to validate a Home Address option using an
existing Binding Cache entry, as described in Section 9.3.1.
o The node MUST be able to insert a type 2 routing header into
packets to be sent to a mobile node, as described in Section
9.3.2.
o Unless the correspondent node is also acting as a mobile node, it
MUST ignore type 2 routing headers and silently discard all
packets that it has received with such headers.
o The node SHOULD be able to interpret ICMP messages as described in
Section 9.3.4.
o The node MUST be able to send Binding Error messages as described
in Section 9.3.3.
o The node MUST be able to process Mobility Headers as described in
Section 9.2.
Johnson, et al. Standard Track [Page 70]
RFC 3775 Mobility Support in IPv6 June 2004
o The node MUST be able to participate in a return routability
procedure (Section 9.4).
o The node MUST be able to process Binding Update messages (Section
9.5).
o The node MUST be able to return a Binding Acknowledgement (Section
9.5.4).
o The node MUST be able to maintain a Binding Cache of the bindings
received in accepted Binding Updates, as described in Section 9.1
and Section 9.6.
o The node SHOULD allow route optimization to be administratively
enabled or disabled. The default SHOULD be enabled.
8.3. All IPv6 Routers
All IPv6 routers, even those not serving as a home agent for Mobile
IPv6, have an effect on how well mobile nodes can communicate:
o Every IPv6 router SHOULD be able to send an Advertisement Interval
option (Section 7.3) in each of its Router Advertisements [12], to
aid movement detection by mobile nodes (as in Section 11.5.1).
The use of this option in Router Advertisements SHOULD be
configurable.
o Every IPv6 router SHOULD be able to support sending unsolicited
multicast Router Advertisements at the faster rate described in
Section 7.5. If the router supports a faster rate, the used rate
MUST be configurable.
o Each router SHOULD include at least one prefix with the Router
Address (R) bit set and with its full IP address in its Router
Advertisements (as described in Section 7.2).
o Routers supporting filtering packets with routing headers SHOULD
support different rules for type 0 and type 2 routing headers (see
Section 6.4) so that filtering of source routed packets (type 0)
will not necessarily limit Mobile IPv6 traffic which is delivered
via type 2 routing headers.
8.4. 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 that serve as a home agent:
Johnson, et al. Standard Track [Page 71]
RFC 3775 Mobility Support in IPv6 June 2004
o 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 (Section 10.1 and Section 10.3.1).
o Every home agent MUST be able to intercept packets (using proxy
Neighbor Discovery [12]) 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 (Section 10.4.1).
o Every home agent MUST be able to encapsulate [15] 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 (Section 10.4.2).
o Every home agent MUST support decapsulating [15] 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 (Section 10.4.5).
o The node MUST be able to process Mobility Headers as described in
Section 10.2.
o Every home agent MUST be able to return a Binding Acknowledgement
in response to a Binding Update (Section 10.3.1).
o 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 10.1 and Section 10.5.1.
o Every home agent MUST be able to accept packets addressed to the
Mobile IPv6 Home-Agents anycast address [16] for the subnet on
which it is serving as a home agent, and MUST be able to
participate in dynamic home agent address discovery (Section
10.5).
o 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
(Section 7.4).
o Every home agent SHOULD support sending ICMP Mobile Prefix
Advertisements (Section 6.8), and SHOULD respond to Mobile Prefix
Solicitations (Section 6.7). If supported, this behavior MUST be
configurable, so that home agents can be configured to avoid
sending such Prefix Advertisements according to the needs of the
network administration in the home domain.
Johnson, et al. Standard Track [Page 72]
RFC 3775 Mobility Support in IPv6 June 2004
o Every home agent MUST support IPsec ESP for protection of packets
belonging to the return routability procedure (Section 10.4.6).
o Every home agent SHOULD support the multicast group membership
control protocols as described in Section 10.4.3. If this support
is provided, the home agent MUST be capable of using it to
determine which multicast data packets to forward via the tunnel
to the mobile node.
o Home agents MAY support stateful address autoconfiguration for
mobile nodes as described in Section 10.4.4.
8.5. IPv6 Mobile Nodes
Finally, the following requirements apply to all IPv6 nodes capable
of functioning as mobile nodes:
o The node MUST maintain a Binding Update List (Section 11.1).
o The node MUST support sending packets containing a Home Address
option (Section 11.3.1), and follow the required IPsec interaction
(Section 11.3.2).
o The node MUST be able to perform IPv6 encapsulation and
decapsulation [15].
o The node MUST be able to process type 2 routing header as defined
in Section 6.4 and Section 11.3.3.
o The node MUST support receiving a Binding Error message (Section
11.3.6).
o The node MUST support receiving ICMP errors (Section 11.3.5).
o The node MUST support movement detection, care-of address
formation, and returning home (Section 11.5).
o The node MUST be able to process Mobility Headers as described in
Section 11.2.
o The node MUST support the return routability procedure (Section
11.6).
o The node MUST be able to send Binding Updates, as specified in
Section 11.7.1 and Section 11.7.2.
o The node MUST be able to receive and process Binding
Acknowledgements, as specified in Section 11.7.3.
Johnson, et al. Standard Track [Page 73]
RFC 3775 Mobility Support in IPv6 June 2004
o The node MUST support receiving a Binding Refresh Request (Section
6.1.2), by responding with a Binding Update.
o The node MUST support receiving Mobile Prefix Advertisements
(Section 11.4.3) and reconfiguring its home address based on the
prefix information contained therein.
o The node SHOULD support use of the dynamic home agent address
discovery mechanism, as described in Section 11.4.1.
o The node MUST allow route optimization to be administratively
enabled or disabled. The default SHOULD be enabled.
o The node MAY support the multicast address listener part of a
multicast group membership protocol as described in Section
11.3.4. If this support is provided, the mobile node MUST be able
to receive tunneled multicast packets from the home agent.
o The node MAY support stateful address autoconfiguration mechanisms
such as DHCPv6 [29] on the interface represented by the tunnel to
the home agent.
9. Correspondent Node Operation
9.1. Conceptual Data Structures
IPv6 nodes with route optimization support maintain a Binding Cache
of bindings for other nodes. A separate Binding Cache SHOULD be
maintained by each IPv6 node for each of its unicast routable
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 [12]. When sending a packet, the
Binding Cache is searched before the Neighbor Discovery conceptual
Destination Cache [12].
Each Binding Cache entry conceptually contains the following fields:
o 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.
o The care-of address for the mobile node indicated by the home
address field in this Binding Cache entry.
Johnson, et al. Standard Track [Page 74]
RFC 3775 Mobility Support in IPv6 June 2004
o 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.
o A flag indicating whether or not this Binding Cache entry is a
home registration entry (applicable only on nodes which support
home agent functionality).
o The maximum value of the Sequence Number field received in
previous Binding Updates for this home address. The Sequence
Number field is 16 bits long. Sequence Number values MUST be
compared modulo 2**16 as explained in Section 9.5.1.
o Usage information for this Binding Cache entry. This is needed to
implement the cache replacement policy in use in the Binding
Cache. Recent use of a cache entry also serves as an indication
that 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.
9.2. Processing Mobility Headers
Mobility Header processing MUST observe the following rules:
o The checksum must be verified as per Section 6.1. Otherwise, the
node MUST silently discard the message.
o The MH Type field MUST have a known value (Section 6.1.1).
Otherwise, the node MUST discard the message and issue a Binding
Error message as described in Section 9.3.3, with Status field set
to 2 (unrecognized MH Type value).
o The Payload Proto field MUST be IPPROTO_NONE (59 decimal).
Otherwise, the node MUST discard the message and SHOULD send ICMP
Parameter Problem, Code 0, directly to the Source Address of the
packet as specified in RFC 2463 [14]. Thus no Binding Cache
information is used in sending the ICMP message. The Pointer
field in the ICMP message SHOULD point at the Payload Proto field.
Johnson, et al. Standard Track [Page 75]
RFC 3775 Mobility Support in IPv6 June 2004
o The Header Len field in the Mobility Header MUST NOT be less than
the length specified for this particular type of message in
Section 6.1. Otherwise, the node MUST discard the message and
SHOULD send ICMP Parameter Problem, Code 0, directly to the Source
Address of the packet as specified in RFC 2463 [14]. (The Binding
Cache information is again not used.) The Pointer field in the
ICMP message SHOULD point at the Header Len field.
Subsequent checks depend on the particular Mobility Header.
9.3. Packet Processing
This section describes how the correspondent node sends packets to
the mobile node, and receives packets from it.
9.3.1. Receiving Packets with Home Address Option
Packets containing a Home Address option MUST be dropped if the given
home address is not a unicast routable address.
Mobile nodes can include a Home Address destination option in a
packet if they believe the correspondent node has a Binding Cache
entry for the home address of a mobile node. Packets containing a
Home Address option MUST be dropped if there is no corresponding
Binding Cache entry. A corresponding Binding Cache entry MUST have
the same home address as appears in the Home Address destination
option, and the currently registered care-of address MUST be equal to
the source address of the packet. These tests MUST NOT be done for
packets that contain a Home Address option and a Binding Update.
If the packet is dropped due the above tests, the correspondent node
MUST send the Binding Error message as described in Section 9.3.3.
The Status field in this message should be set to 1 (unknown binding
for Home Address destination option).
The correspondent node MUST process the option in a manner consistent
with exchanging the Home Address field from the Home Address option
into the IPv6 header and replacing the original value of the Source
Address field there. After all IPv6 options have been processed, it
MUST be possible for upper layers to process the packet without the
knowledge that it came originally from a care-of address or that a
Home Address option was used.
The use of IPsec Authentication Header (AH) for the Home Address
option is not required, except that if the IPv6 header of a packet is
covered by AH, then the 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
Johnson, et al. Standard Track [Page 76]
RFC 3775 Mobility Support in IPv6 June 2004
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 AH 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.
When attempting to verify AH authentication data in a packet that
contains a Home Address option, the receiving node MUST calculate the
AH authentication data as if the following were true: The Home
Address option contains the care-of address, and the source IPv6
address field of the IPv6 header contains the home address. This
conforms with the calculation specified in Section 11.3.2.
9.3.2. 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 type 2 routing header
to route the packet to this mobile node (the destination node) by way
of its care-of address. However, the sending node MUST not do this
in the following cases:
o When sending an IPv6 Neighbor Discovery [12] packet.
o Where otherwise noted in Section 6.1.
When calculating authentication data in a packet that contains a type
2 routing header, the correspondent node MUST calculate the AH
authentication data as if the following were true: The routing header
contains the care-of address, the destination IPv6 address field of
the IPv6 header contains the home address, and the Segments Left
field is zero. The IPsec Security Policy Database lookup MUST based
on the mobile node's home address.
For instance, assuming there are no additional routing headers in
this packet beyond those needed by Mobile IPv6, the correspondent
node could set the fields in the packet's IPv6 header and routing
header as follows:
o The Destination Address in the packet's IPv6 header is set to the
mobile node's home address (the original destination address to
which the packet was being sent).
Johnson, et al. Standard Track [Page 77]
RFC 3775 Mobility Support in IPv6 June 2004
o The routing header is initialized to contain a single route
segment, containing the mobile node's care-of address copied from
the Binding Cache entry. The Segments Left field is, however,
temporarily set to zero.
The IP layer will insert the routing header before performing any
necessary IPsec processing. Once all IPsec processing has been
performed, the node swaps the IPv6 destination field with the Home
Address field in the routing header, sets the Segments Left field to
one, and sends the packet. This ensures the AH calculation is done
on the packet in the form it will have on the receiver after
advancing the routing header.
Following the definition of a type 2 routing header in Section 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 to the
mobile node's current primary care-of address.
9.3.3. Sending Binding Error Messages
Section 9.2 and Section 9.3.1 describe error conditions that lead to
a need to send a Binding Error message.
A Binding Error message is sent directly to the address that appeared
in the IPv6 Source Address field of the offending packet. If the
Source Address field does not contain a unicast address, the Binding
Error message MUST NOT be sent.
The Home Address field in the Binding Error message MUST be copied
from the Home Address field in the Home Address destination option of
the offending packet, or set to the unspecified address if no such
option appeared in the packet.
Johnson, et al. Standard Track [Page 78]
RFC 3775 Mobility Support in IPv6 June 2004
Note that the IPv6 Source Address and Home Address field values
discussed above are the values from the wire, i.e., before any
modifications possibly performed as specified in Section 9.3.1.
Binding Error messages SHOULD be subject to rate limiting in the same
manner as is done for ICMPv6 messages [14].
9.3.4. Receiving ICMP Error Messages
When the correspondent node has a Binding Cache entry for a mobile
node, all traffic destined to the mobile node goes directly to the
current care-of address of the mobile node using a routing header.
Any ICMP error message caused by packets on their way to the care-of
address will be returned in the normal manner 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 through the
mobile node's home link. 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. By the definition of
IPv6 encapsulation [15], the home agent MUST relay certain ICMP error
messages back to the original sender of the packet, which in this
case is the correspondent 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. Note that
if the mobile node continues to send packets with the Home Address
destination option to this correspondent node, they will be dropped
due to the lack of a binding. For this reason it is important that
only persistent ICMP messages lead to the deletion of the Binding
Cache entry.
9.4. Return Routability Procedure
This subsection specifies actions taken by a correspondent node
during the return routability procedure.
Johnson, et al. Standard Track [Page 79]
RFC 3775 Mobility Support in IPv6 June 2004
9.4.1. Receiving Home Test Init Messages
Upon receiving a Home Test Init message, the correspondent node
verifies the following:
o The packet MUST NOT include a Home Address destination option.
Any packet carrying a Home Test Init message which fails to satisfy
all of these tests MUST be silently ignored.
Otherwise, in preparation for sending the corresponding Home Test
Message, the correspondent node checks that it has the necessary
material to engage in a return routability procedure, as specified in
Section 5.2. The correspondent node MUST have a secret Kcn and a
nonce. If it does not have this material yet, it MUST produce it
before continuing with the return routability procedure.
Section 9.4.3 specifies further processing.
9.4.2. Receiving Care-of Test Init Messages
Upon receiving a Care-of Test Init message, the correspondent node
verifies the following:
o The packet MUST NOT include a Home Address destination option.
Any packet carrying a Care-of Test Init message which fails to
satisfy all of these tests MUST be silently ignored.
Otherwise, in preparation for sending the corresponding Care-of Test
Message, the correspondent node checks that it has the necessary
material to engage in a return routability procedure in the manner
described in Section 9.4.1.
Section 9.4.4 specifies further processing.
9.4.3. Sending Home Test Messages
The correspondent node creates a home keygen token and uses the
current nonce index as the Home Nonce Index. It then creates a Home
Test message (Section 6.1.5) and sends it to the mobile node at the
latter's home address.
Johnson, et al. Standard Track [Page 80]
RFC 3775 Mobility Support in IPv6 June 2004
9.4.4. Sending Care-of Test Messages
The correspondent node creates a care-of keygen token and uses the
current nonce index as the Care-of Nonce Index. It then creates a
Care-of Test message (Section 6.1.6) and sends it to the mobile node
at the latter's care-of address.
9.5. Processing Bindings
This section explains how the correspondent node processes messages
related to bindings. These messages are:
o Binding Update
o Binding Refresh Request
o Binding Acknowledgement
o Binding Error
9.5.1. Receiving Binding Updates
Before accepting a Binding Update, the receiving node MUST validate
the Binding Update according to the following tests:
o The packet MUST contain a unicast routable home address, either in
the Home Address option or in the Source Address, if the Home
Address option is not present.
o The Sequence Number field in the Binding Update is greater than
the Sequence Number received in the previous valid Binding Update
for this home address, if any.
If the receiving node has no Binding Cache entry for the indicated
home address, it MUST accept any Sequence Number value in a received
Binding Update from this mobile node.
This Sequence Number comparison MUST be performed modulo 2**16, i.e.,
the number is a free running counter represented modulo 65536. A
Sequence Number in a received Binding Update is considered less than
or equal to the last received number if its value lies in the range
of the last received number and the preceding 32768 values,
inclusive. For example, if the last received sequence number was 15,
then messages with sequence numbers 0 through 15, as well as 32783
through 65535, would be considered less than or equal.
Johnson, et al. Standard Track [Page 81]
RFC 3775 Mobility Support in IPv6 June 2004
When the Home Registration (H) bit is not set, the following are also
required:
o A Nonce Indices mobility option MUST be present, and the Home and
Care-of Nonce Index values in this option MUST be recent enough to
be recognized by the correspondent node. (Care-of Nonce Index
values are not inspected for requests to delete a binding.)
o The correspondent node MUST re-generate the home keygen token and
the care-of keygen token from the information contained in the
packet. It then generates the binding management key Kbm and uses
it to verify the authenticator field in the Binding Update as
specified in Section 6.1.7.
o The Binding Authorization Data mobility option MUST be present,
and its contents MUST satisfy rules presented in Section 5.2.6.
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. When such a 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.
o The Binding Authorization Data mobility option MUST be the last
option and MUST NOT have trailing padding.
If the Home Registration (H) bit is set, the Nonce Indices mobility
option MUST NOT be present.
If the mobile node sends a sequence number which is not greater than
the sequence number from the last valid Binding Update for this home
address, then the receiving node MUST send back a Binding
Acknowledgement with status code 135, and the last accepted sequence
number in the Sequence Number field of the Binding Acknowledgement.
If a binding already exists for the given home address and the home
registration flag has a different value than the Home Registration
(H) bit in the Binding Update, then the receiving node MUST send back
a Binding Acknowledgement with status code 139 (registration type
change disallowed). The home registration flag stored in the Binding
Cache entry MUST NOT be changed.
If the receiving node no longer recognizes the Home Nonce Index
value, Care-of Nonce Index value, or both values from the Binding
Update, then the receiving node MUST send back a Binding
Acknowledgement with status code 136, 137, or 138, respectively.
Johnson, et al. Standard Track [Page 82]
RFC 3775 Mobility Support in IPv6 June 2004
Packets carrying Binding Updates that fail to satisfy all of these
tests for any reason other than insufficiency of the Sequence Number,
registration type change, or expired nonce index values, MUST be
silently discarded.
If the Binding Update is valid according to the tests above, then the
Binding Update is processed further as follows:
o The Sequence Number value received from a mobile node in a Binding
Update is stored by the receiving node in its Binding Cache entry
for the given home address.
o 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 home
address. If the Home Registration (H) bit is set in the Binding
Update, the Binding Update is processed according to the procedure
specified in Section 10.3.1; otherwise, it is processed according
to the procedure specified in Section 9.5.2.
o 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 cached binding for
the home address. In this case, the Binding Update MUST include a
valid home nonce index, and the care-of nonce index MUST be
ignored by the correspondent node. The generation of the binding
management key depends then exclusively on the home keygen token
(Section 5.2.5). If the Home Registration (H) bit is set in the
Binding Update, the Binding Update is processed according to the
procedure specified in Section 10.3.2; otherwise, it is processed
according to the procedure specified in Section 9.5.3.
The specified care-of address MUST be determined as follows:
o If the Alternate Care-of Address option is present, the care-of
address is the address in that option.
o Otherwise, the care-of address is the Source Address field in the
packet's IPv6 header.
The home address for the binding MUST be determined as follows:
o If the Home Address destination option is present, the home
address is the address in that option.
o Otherwise, the home address is the Source Address field in the
packet's IPv6 header.
Johnson, et al. Standard Track [Page 83]
RFC 3775 Mobility Support in IPv6 June 2004
9.5.2. Requests to Cache a Binding
This section describes the processing of a valid Binding Update that
requests a node to cache a 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 home address, or update its existing Binding
Cache entry for this home address, if such an entry already exists.
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 of its lifetime.
Note that if the mobile node did not request a Binding
Acknowledgement, then it is not aware of the selected shorter
lifetime. The mobile node may thus use route optimization and send
packets with the Home Address destination option. As discussed in
Section 9.3.1, such packets will be dropped if there is no binding.
This situation is recoverable, but can cause temporary packet loss.
The correspondent node MAY refuse to accept a new Binding Cache entry
if it does not have sufficient resources. A new entry MAY also be
refused if the correspondent node believes its resources are utilized
more efficiently in some other purpose, such as serving another
mobile node with higher amount of traffic. In both cases the
correspondent node SHOULD return a Binding Acknowledgement with
status value 130.
9.5.3 Requests to Delete a Binding
This section describes the processing of a valid Binding Update that
requests a node to delete a binding when the Home Registration (H)
bit is not set in the Binding Update.
Any existing binding for the given home address MUST be deleted. A
Binding Cache entry for the home address MUST NOT be created in
response to receiving the Binding Update.
If the Binding Cache entry was created by use of return routability
nonces, the correspondent node MUST ensure that the same nonces are
not used again with the particular home and care-of address. If both
nonces are still valid, the correspondent node has to remember the
particular combination of nonce indexes, addresses, and sequence
number as illegal until at least one of the nonces has become too
old.
Johnson, et al. Standard Track [Page 84]
RFC 3775 Mobility Support in IPv6 June 2004
9.5.4. Sending Binding Acknowledgements
A Binding Acknowledgement may be sent to indicate receipt of a
Binding Update as follows:
o If the Binding Update was discarded as described in Section 9.2 or
Section 9.5.1, a Binding Acknowledgement MUST NOT be sent.
Otherwise the treatment depends on the following rules.
o If the Acknowledge (A) bit set is set in the Binding Update, a
Binding Acknowledgement MUST be sent. Otherwise, the treatment
depends on the below rule.
o If the node rejects the Binding Update due to an expired nonce
index, sequence number being out of window (Section 9.5.1), or
insufficiency of resources (Section 9.5.2), a Binding
Acknowledgement MUST be sent. If the node accepts the Binding
Update, the Binding Acknowledgement SHOULD NOT be sent.
If the node accepts the Binding Update and creates or updates an
entry for this binding, the Status field in the Binding
Acknowledgement MUST be set to a value less than 128. Otherwise, the
Status field MUST be set to a value greater than or equal to 128.
Values for the Status field are described in Section 6.1.8 and in the
IANA registry of assigned numbers [19].
If the Status field in the Binding Acknowledgement contains the value
136 (expired home nonce index), 137 (expired care-of nonce index), or
138 (expired nonces) then the message MUST NOT include the Binding
Authorization Data mobility option. Otherwise, the Binding
Authorization Data mobility option MUST be included, and MUST meet
the specific authentication requirements for Binding Acknowledgements
as defined in Section 5.2.
If the Source Address field of the IPv6 header that carried the
Binding Update does not contain a unicast address, the Binding
Acknowledgement MUST NOT be sent and the Binding Update packet MUST
be silently discarded. Otherwise, the acknowledgement MUST be sent
to the Source Address. Unlike the treatment of regular packets, this
addressing procedure does not use information from the Binding Cache.
However, a routing header is needed in some cases. If the Source
Address is the home address of the mobile node, i.e., the Binding
Update did not contain a Home Address destination option, then the
Binding Acknowledgement MUST be sent to that address and the routing
header MUST NOT be used. Otherwise, the Binding Acknowledgement MUST
be sent using a type 2 routing header which contains the mobile
node's home address.
Johnson, et al. Standard Track [Page 85]
RFC 3775 Mobility Support in IPv6 June 2004
9.5.5. Sending Binding Refresh Requests
If a Binding Cache entry being deleted is still in active use when
sending packets to a mobile node, then the next packet sent to the
mobile node will be routed normally to the mobile node's home link.
Communication with the mobile node continues, but the tunneling from
the home network creates additional overhead and latency in
delivering packets to the mobile node.
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. This message is always sent to
the home address of the mobile node.
The correspondent node MAY retransmit Binding Refresh Request
messages as long as the rate limitation is applied. The
correspondent node MUST stop retransmitting when it receives a
Binding Update.
9.6. 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.
A node MAY choose to drop any entry already in its Binding Cache in
order to make space for a new entry. For example, a "least-recently
used" (LRU) strategy for cache entry replacement among entries should
work well, unless the size of the Binding Cache is substantially
insufficient. When entries are deleted, the correspondent node MUST
follow the rules in Section 5.2.8 in order to guard the return
routability procedure against replay attacks.
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 through
the mobile node's home link. The mobile node can detect this and
establish a new binding if necessary.
Johnson, et al. Standard Track [Page 86]
RFC 3775 Mobility Support in IPv6 June 2004
However, if the mobile node believes that the binding still exists,
it may use route optimization and send packets with the Home Address
destination option. This can create temporary packet loss, as
discussed earlier, in the context of binding lifetime reductions
performed by the correspondent node (Section 9.5.2).
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 the same for home
agents and correspondent nodes and have already been described in
Section 9.1.
The Home Agents List is maintained by each home agent, recording
information about each router on the same link that is acting as a
home agent. This list is used by the dynamic home agent address
discovery mechanism. A router is known to be acting as a home agent,
if it sends a Router Advertisement in which the Home Agent (H) bit is
set. When the lifetime for a list entry (defined below) expires,
that entry is removed from the Home Agents List. The Home Agents
List is similar to the Default Router List conceptual data structure
maintained by each host for Neighbor Discovery [12]. 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. A new entry is created or an
existing entry is updated in response to receipt of a valid Router
Advertisement in which the Home Agent (H) bit is set. Each Home
Agents List entry conceptually contains the following fields:
o The link-local IP address of a home agent on the link. This
address is learned through the Source Address of the Router
Advertisements [12] received from the router.
o One or more global IP addresses for this home agent. Global
addresses are learned through Prefix Information options with the
Router Address (R) bit set and received in Router Advertisements
from this link-local address. Global addresses for the router in
a Home Agents List entry MUST be deleted once the prefix
associated with that address is no longer valid [12].
Johnson, et al. Standard Track [Page 87]
RFC 3775 Mobility Support in IPv6 June 2004
o 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 (if present); otherwise, the
lifetime is initialized from the Router Lifetime field in the
received Router Advertisement. If Home Agents List entry lifetime
reaches zero, the entry MUST be deleted from the Home Agents List.
o 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 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 when
it sends an ICMP Home Agent Address Discovery message.
10.2. Processing Mobility Headers
All IPv6 home agents MUST observe the rules described in Section 9.2
when processing Mobility Headers.
10.3. Processing Bindings
10.3.1. 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 9.5.1. Furthermore, it MUST authenticate the Binding
Update as described in Section 5.1. An authorization step specific
for the home agent is also needed to ensure that only the right node
can control a particular home address. This is provided through the
home address unequivocally identifying the security association that
must be used.
This section describes the processing of a valid and authorized
Binding Update when it requests the registration of the mobile node's
primary care-of address.
To begin processing the Binding Update, the home agent MUST perform
the following sequence of tests:
o If the node implements only correspondent node functionality, or
has not been configured to act as a home agent, then the node MUST
reject the Binding Update. The node MUST also return a Binding
Acknowledgement to the mobile node, in which the Status field is
set to 131 (home registration not supported).
Johnson, et al. Standard Track [Page 88]
RFC 3775 Mobility Support in IPv6 June 2004
o Else, if the home address for the binding (the Home Address field
in the packet's Home Address option) is not an on-link IPv6
address with respect to the home agent's current Prefix List, then
the home agent MUST reject the Binding Update and SHOULD return a
Binding Acknowledgement to the mobile node, in which the Status
field is set to 132 (not home subnet).
o 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.
o A Home Address destination option MUST be present in the message.
It MUST be validated as described in Section 9.3.1 with the
following additional rule. The Binding Cache entry existence test
MUST NOT be done for IPsec packets when the Home Address option
contains an address for which the receiving node could act as a
home agent.
If home agent accepts the Binding Update, it 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 field as received in the Home Address option provides
the home address of the mobile node.
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 9.6) and MUST NOT be removed from the
Binding Cache until the expiration of the Lifetime period.
Unless this home agent already has a binding for the given home
address, the home agent MUST perform Duplicate Address Detection [13]
on the mobile node's home link before returning the Binding
Acknowledgement. This ensures that no other node on the home link
was using the mobile node's home address when the Binding Update
arrived. If this Duplicate Address Detection fails for the given
home address or an associated link local address, then the home agent
MUST reject the complete Binding Update and MUST return a Binding
Acknowledgement to the mobile node, in which the Status field is set
to 134 (Duplicate Address Detection failed). When the home agent
sends a successful Binding Acknowledgement to the mobile node, the
home agent assures to the mobile node that its address(es) will be
kept unique by the home agent for as long as the lifetime was granted
for the binding.
Johnson, et al. Standard Track [Page 89]
RFC 3775 Mobility Support in IPv6 June 2004
The specific addresses, which are to be tested before accepting the
Binding Update and later to be defended by performing Duplicate
Address Detection, depend on the setting of the Link-Local Address
Compatibility (L) bit, as follows:
o L=0: Defend only the given address. Do not derive a link-local
address.
o L=1: Defend both the given non link-local unicast (home) address
and the derived link-local. The link-local address is derived by
replacing the subnet prefix in the mobile node's home address with
the link-local prefix.
The lifetime of the Binding Cache entry depends on a number of
factors:
o The lifetime for the Binding Cache entry MUST NOT be greater than
the Lifetime value specified in the Binding Update.
o 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. The
remaining valid lifetime for this prefix is determined by the home
agent based on its own Prefix List entry [12].
The remaining preferred lifetime SHOULD NOT have any impact on the
lifetime for the binding cache entry.
The home agent MUST remove a binding when the valid lifetime of
the prefix associated with it expires.
o 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 Acknowledge (A) bit in the Binding
Update, the home agent MUST return a Binding Acknowledgement to the
mobile node constructed as follows:
o The Status field MUST be set to a value indicating success. The
value 1 (accepted but prefix discovery necessary) MUST be used if
the subnet prefix of the specified home address is deprecated, or
becomes deprecated during the lifetime of the binding, or becomes
invalid at the end of the lifetime. The value 0 MUST be used
Johnson, et al. Standard Track [Page 90]
RFC 3775 Mobility Support in IPv6 June 2004
otherwise. For the purposes of comparing the binding and prefix
lifetimes, the prefix lifetimes are first converted into units of
four seconds by ignoring the two least significant bits.
o The Key Management Mobility Capability (K) bit is set if the
following conditions are all fulfilled, and cleared otherwise:
* The Key Management Mobility Capability (K) bit was set in the
Binding Update.
* The IPsec security associations between the mobile node and the
home agent have been established dynamically.
* The home agent has the capability to update its endpoint in the
used key management protocol to the new care-of address every
time it moves.
Depending on the final value of the bit in the Binding
Acknowledgement, the home agent SHOULD perform the following
actions:
K = 0
Discard key management connections, if any, to the old care-of
address. If the mobile node did not have a binding before
sending this Binding Update, discard the connections to the
home address.
K = 1
Move the peer endpoint of the key management protocol
connection, if any, to the new care-of address. For an IKE
phase 1 connection, this means that any IKE packets sent to the
peer are sent to this address, and packets from this address
with the original ISAKMP cookies are accepted.
Note that RFC 2408 [8] Section 2.5.3 gives specific rules that
ISAKMP cookies must satisfy: they must depend on specific
parties and can only be generated by the entity itself. Then
it recommends a particular way to do this, namely a hash of IP
addresses. With the K bit set to 1, the recommended
implementation technique does not work directly. To satisfy
the two rules, the specific parties must be treated as the
original IP addresses, not the ones in use at the specific
moment.
o The Sequence Number field MUST be copied from the Sequence Number
given in the Binding Update.
Johnson, et al. Standard Track [Page 91]
RFC 3775 Mobility Support in IPv6 June 2004
o 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.
o If the home agent stores the Binding Cache entry in nonvolatile
storage, then the Binding Refresh Advice mobility option MUST be
omitted. Otherwise, the home agent MAY include this option to
suggest that the mobile node refreshes its binding before the
actual lifetime of the binding ends.
If the Binding Refresh Advice mobility option is present, the
Refresh Interval field in the option MUST be set to a value less
than the Lifetime value being returned in the Binding
Acknowledgement. This indicates that the mobile node SHOULD
attempt to refresh its home registration at the indicated shorter
interval. The home agent MUST still retain the registration for
the Lifetime period, even if the mobile node does not refresh its
registration within the Refresh period.
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 9.5.4.
In addition, the home agent MUST follow the procedure defined in
Section 10.4.1 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 10.4.5. Finally,
the home agent MUST also propagate new home network prefixes, as
described in Section 10.6.
10.3.2. Primary Care-of Address De-Registration
A binding may need to be de-registered when the mobile node returns
home or when the mobile node knows that it will not have any care-of
addresses in the visited network.
A Binding Update is validated and authorized in the manner described
in the previous section; note that when the mobile node de-registers
when it is at home, it may not include the Home Address destination
option, in which case the mobile node's home address is the source IP
address of the de-registration Binding Update. 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.
Johnson, et al. Standard Track [Page 92]
RFC 3775 Mobility Support in IPv6 June 2004
To begin processing the Binding Update, the home agent MUST perform
the following test:
o 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 133 (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. Then, the home agent MUST return a Binding
Acknowledgement to the mobile node, constructed as follows:
o The Status field MUST be set to a value 0, indicating success.
o The Key Management Mobility Capability (K) bit is set or cleared
and actions based on its value are performed as described in the
previous section. The mobile node's home address is used as its
new care-of address for the purposes of moving the key management
connection to a new endpoint.
o The Sequence Number field MUST be copied from the Sequence Number
given in the Binding Update.
o The Lifetime field MUST be set to zero.
o The Binding Refresh Advice mobility option MUST be omitted.
In addition, the home agent MUST stop intercepting packets on the
mobile node's home link that are addressed to the mobile node
(Section 10.4.1).
The rules for selecting the Destination IP address (and, if required,
routing header construction) for the Binding Acknowledgement to the
mobile node are the same as in the previous section. When the Status
field in the Binding Acknowledgement is greater than or equal to 128
and the Source Address of the Binding Update is on the home link, the
home agent MUST send it to the mobile node's link layer address
(retrieved either from the Binding Update or through Neighbor
Solicitation).
Johnson, et al. Standard Track [Page 93]
RFC 3775 Mobility Support in IPv6 June 2004
10.4. Packet Processing
10.4.1. Intercepting Packets for a Mobile Node
While a node is serving as the home agent for mobile node it MUST
attempt to intercept packets on the mobile node's home link that are
addressed to the mobile node.
In order to do this, when a node begins serving as the home agent it
MUST multicast onto the home link a Neighbor Advertisement message
[12] on behalf of the mobile node. For the home address specified in
the Binding Update, the home agent sends a Neighbor Advertisement
message [12] 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. If the Link-Layer Address
Compatibility (L) flag has been specified in the Binding Update, the
home agent MUST do the same for the link-local address of the mobile
node.
All fields in each Neighbor Advertisement message SHOULD be set in
the same way they would be set by the mobile node if it was sending
this Neighbor Advertisement [12] while at home, with the following
exceptions:
o The Target Address in the Neighbor Advertisement MUST be set to
the specific IP address for the mobile node.
o The Advertisement MUST include a Target Link-layer Address option
specifying the home agent's link-layer address.
o The Router (R) bit in the Advertisement MUST be set to zero.
o The Solicited Flag (S) in the Advertisement MUST NOT be set, since
it was not solicited by any Neighbor Solicitation.
o 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.
o The Source Address in the IPv6 header MUST be set to the home
agent's IP address on the interface used to send the
advertisement.
Any node on the home link that receives one of the Neighbor
Advertisement messages (described above) will 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
normally destined to the mobile node to the mobile node's home agent.
Johnson, et al. Standard Track [Page 94]
RFC 3775 Mobility Support in IPv6 June 2004
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_NEIGHBOR_ADVERTISEMENT (see
[12]) times to increase its reliability. It is still possible that
some nodes on the home link will not receive any of the Neighbor
Advertisements, but these nodes will eventually be able to detect the
link-layer address change for the mobile node's address through use
of Neighbor Unreachability Detection [12].
While a node is serving as a home agent for some mobile node, the
home agent uses IPv6 Neighbor Discovery [12] to intercept unicast
packets on the home link addressed to the mobile node. 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 Solicitations for
it. When a home agent receives a Neighbor Solicitation, it MUST
check if the Target Address specified in the message matches the
address of any mobile node for which it has a Binding Cache entry
marked as a home registration.
If such an entry exists in the home agent's Binding Cache, the home
agent MUST reply to the Neighbor Solicitation with a Neighbor
Advertisement 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 set to zero. Acting
as a proxy in this way allows other nodes on the mobile node's home
link to resolve the mobile node's address and for the home agent to
defend these addresses on the home link for Duplicate Address
Detection [12].
10.4.2. Processing Intercepted Packets
For any packet sent to a mobile node from the mobile node's home
agent (in 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 9.3.2
apply. The home agent then uses a routing header to route the packet
to the mobile node by way of the primary care-of address in the home
agent's Binding Cache.
While the mobile node is away from home, the home agent intercepts
any packets on the home link addressed to the mobile node's home
address, as described in Section 10.4.1. 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 [15]. 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
Johnson, et al. Standard Track [Page 95]
RFC 3775 Mobility Support in IPv6 June 2004
address. When received by the mobile node, normal processing of the
tunnel header [15] will result in decapsulation and processing of the
original packet by the mobile node.
However, packets addressed to the mobile node's link-local address
MUST NOT be tunneled to the mobile node. Instead, these packets 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 NOT be tunneled to the
mobile node by default.
Interception and tunneling of the following multicast addressed
packets on the home network are only done if the home agent supports
multicast group membership control messages from the mobile node as
described in the next section. 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 [3], to which the mobile node is subscribed,
MUST NOT be tunneled to the mobile node. These packets SHOULD be
silently discarded (after delivering to other local multicast
recipients). Multicast packets addressed to a multicast address with
a scope larger than link-local, but smaller than global (e.g., site-
local and organization-local [3], to which the mobile node is
subscribed, SHOULD NOT be tunneled to the mobile node. Multicast
packets addressed with a global scope, to which the mobile node has
successfully subscribed, MUST be tunneled to the mobile node.
Before tunneling a packet to the mobile node, the home agent MUST
perform any IPsec processing as indicated by the security policy data
base.
10.4.3. Multicast Membership Control
This section is a prerequisite for the multicast data packet
forwarding, described in the previous section. If this support is
not provided, multicast group membership control messages are
silently ignored.
In order to forward multicast data packets from the home network to
all the proper mobile nodes, the home agent SHOULD be capable of
receiving tunneled multicast group membership control information
from the mobile node in order to determine which groups the mobile
node has subscribed to. These multicast group membership messages
are Listener Report messages specified in MLD [17] or in other
protocols such as [37].
Johnson, et al. Standard Track [Page 96]
RFC 3775 Mobility Support in IPv6 June 2004
The messages are issued by the mobile node, but sent through the
reverse tunnel to the home agent. These messages are issued whenever
the mobile node decides to enable reception of packets for a
multicast group or in response to an MLD Query from the home agent.
The mobile node will also issue multicast group control messages to
disable reception of multicast packets when it is no longer
interested in receiving multicasts for a particular group.
To obtain the mobile node's current multicast group membership the
home agent must periodically transmit MLD Query messages through the
tunnel to the mobile node. These MLD periodic transmissions will
ensure the home agent has an accurate record of the groups in which
the mobile node is interested despite packet losses of the mobile
node's MLD group membership messages.
All MLD packets are sent directly between the mobile node and the
home agent. Since all of these packets are destined to a link-scope
multicast address and have a hop limit of 1, there is no direct
forwarding of such packets between the home network and the mobile
node. The MLD packets between the mobile node and the home agent are
encapsulated within the same tunnel header used for other packet
flows between the mobile node and home agent.
Note that at this time, even though a link-local source is used on
MLD packets, no functionality depends on these addresses being
unique, nor do they elicit direct responses. All MLD messages are
sent to multicast destinations. To avoid ambiguity on the home
agent, due to mobile nodes which may choose identical link-local
source addresses for their MLD function, it is necessary for the home
agent to identify which mobile node was actually the issuer of a
particular MLD message. This may be accomplished by noting which
tunnel such an MLD arrived by, which IPsec SA was used, or by other
distinguishing means.
This specification puts no requirement on how the functions in this
section and the multicast forwarding in Section 10.4.2 are to be
achieved. At the time of this writing it was thought that a full
IPv6 multicast router function would be necessary on the home agent,
but it may be possible to achieve the same effects through a "proxy
MLD" application coupled with kernel multicast forwarding. This may
be the subject of future specifications.
Johnson, et al. Standard Track [Page 97]
RFC 3775 Mobility Support in IPv6 June 2004
10.4.4. Stateful Address Autoconfiguration
This section describes how home agents support the use of stateful
address autoconfiguration mechanisms such as DHCPv6 [29] from the
mobile nodes. If this support is not provided, then the M and O bits
must remain cleared on the Mobile Prefix Advertisement Messages. Any
mobile node which sends DHCPv6 messages to the home agent without
this support will not receive a response.
If DHCPv6 is used, packets are sent with link-local source addresses
either to a link-scope multicast address or a link-local address.
Mobile nodes desiring to locate a DHCPv6 service may reverse tunnel
standard DHCPv6 packets to the home agent. Since these link-scope
packets cannot be forwarded onto the home network, it is necessary
for the home agent to either implement a DHCPv6 relay agent or a
DHCPv6 server function itself. The arriving tunnel or IPsec SA of
DHCPv6 link-scope messages from the mobile node must be noted so that
DHCPv6 responses may be sent back to the appropriate mobile node.
DHCPv6 messages sent to the mobile node with a link-local destination
must be tunneled within the same tunnel header used for other packet
flows.
10.4.5. Handling Reverse Tunneled Packets
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. Home agents MUST support reverse
tunneling as follows:
o The tunneled traffic arrives to the home agent's address using
IPv6 encapsulation [15].
o Depending on the security policies used by the home agent, reverse
tunneled packets MAY be discarded unless accompanied by a valid
ESP header. 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.
o Otherwise, when a home agent decapsulates a tunneled packet from
the mobile node, the home agent MUST verify that the Source
Address in the tunnel IP header is the mobile node's primary
care-of address. Otherwise, any node in the Internet could send
traffic through the home agent and escape ingress filtering
limitations. This simple check forces the attacker to know the
current location of the real mobile node and be able to defeat
ingress filtering. This check is not necessary if the reverse-
tunneled packet is protected by ESP in tunnel mode.
Johnson, et al. Standard Track [Page 98]
RFC 3775 Mobility Support in IPv6 June 2004
10.4.6. Protecting Return Routability Packets
The return routability procedure, described in Section 5.2.5, assumes
that the confidentiality of the Home Test Init and Home Test messages
is protected as they are tunneled between the home agent and the
mobile node. Therefore, the home agent MUST support tunnel mode
IPsec ESP for the protection of packets belonging to the return
routability procedure. Support for a non-null encryption transform
and authentication algorithm MUST be available. It is not necessary
to distinguish between different kinds of packets during the return
routability procedure.
Security associations are needed to provide this protection. When
the care-of address for the mobile node changes as a result of an
accepted Binding Update, special treatment is needed for the next
packets sent using these security associations. The home agent MUST
set the new care-of address as the destination address of these
packets, as if the outer header destination address in the security
association had changed [21].
The above protection SHOULD be used with all mobile nodes. The use
is 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. The Mobility Header protocol carries both these messages as
well as the return routability messages. From the point of view of
the security policy database these messages are indistinguishable.
When IPsec is used to protect return routability signaling or payload
packets, this protection MUST only be applied to the return
routability packets entering the IPv6 encapsulated tunnel interface
between the mobile node and the home agent. This can be achieved,
for instance, by defining the security policy database entries
specifically for the tunnel interface. That is, the policy entries
are not generally applied on all traffic on the physical interface(s)
of the nodes, but rather only on traffic that enters the tunnel.
This makes use of per-interface security policy database entries [4]
specific to the tunnel interface (the node's attachment to the tunnel
[11]).
10.5. Dynamic Home Agent Address Discovery
This section describes how a home agent can help mobile nodes to
discover the addresses of the home agents. The home agent keeps
track of the other home agents on the same link and responds to
queries sent by the mobile node.
Johnson, et al. Standard Track [Page 99]
RFC 3775 Mobility Support in IPv6 June 2004
10.5.1. 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 10.5.
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 [12]. 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 [12], the home
agent performs the following steps in addition to any steps already
required of it by Neighbor Discovery:
o If the Home Agent (H) bit in the Router Advertisement is not set,
delete the sending node's entry in the current Home Agents List
(if one exists). Skip all the following steps.
o 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 [12].
o Determine 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.
o Determine 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.
o 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.
o 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.
Johnson, et al. Standard Track [Page 100]
RFC 3775 Mobility Support in IPv6 June 2004
o If the link-local address of the home agent sending this
Advertisement is not already present in the Home Agents List
maintained by the receiving home agent, and the lifetime for the
sending home agent is non-zero, create a new entry in the list,
and initialize its lifetime and preference to the values
determined above.
o 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). Add all
such global addresses to the list of global addresses in this Home
Agents List entry.
A home agent SHOULD maintain an entry in its Home Agents List for
each valid home agent address until that entry's lifetime expires,
after which time the entry MUST be deleted.
As described in Section 11.4.1, 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 [16] for its home IP subnet prefix. A home agent receiving a
Home Agent Address Discovery Request message that serves this subnet
SHOULD return an ICMP Home Agent Address Discovery Reply message to
the mobile node 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:
o The Home Agent Addresses field SHOULD contain all global IP
addresses for each home agent currently listed in this home
agent's own Home Agents List (Section 10.1).
o The IP addresses in the Home Agent Addresses field SHOULD be
listed in order of decreasing preference values, 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).
o 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 every time
a Home Agent Address Discovery Reply message is returned by this
home agent.
o If more than one global IP address is associated with a home
agent, these addresses SHOULD be listed in a randomized order.
Johnson, et al. Standard Track [Page 101]
RFC 3775 Mobility Support in IPv6 June 2004
o The home agent SHOULD reduce the number of home agent IP addresses
so that the packet fits within the minimum IPv6 MTU [11]. The
home agent addresses selected for inclusion in the packet SHOULD
be those from the complete list with the highest preference. This
limitation avoids the danger of the Reply message packet being
fragmented (or rejected by an intermediate router with an ICMP
Packet Too Big message [14]).
10.6. Sending Prefix Information to the Mobile Node
10.6.1. List of Home Network Prefixes
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. These messages include Prefix Information
Options for the prefixes configured on the home subnet interface(s)
of the home agent.
If there are multiple home agents, differences in the advertisements
sent by different home agents can lead to an inability to use a
particular home address when changing to another home agent. In
order to ensure that the mobile nodes get the same information from
different home agents, it is preferred that all of the home agents on
the same link be configured in the same manner.
To support this, the home agent monitors prefixes advertised by
itself and other home agents on the home link. In RFC 2461 [12] it
is acceptable for two routers to advertise different sets of prefixes
on the same link. For home agents, the differences should be
detected for a given home address because the mobile node
communicates only with one home agent at a time and the mobile node
needs to know the full set of prefixes assigned to the home link.
All other comparisons of Router Advertisements are as specified in
Section 6.2.7 of RFC 2461.
10.6.2. Scheduling Prefix Deliveries
A home agent serving a mobile node will schedule the delivery of the
new prefix information to that mobile node when any of the following
conditions occur:
MUST:
o The state of the flags changes for the prefix of the mobile node's
registered home address.
Johnson, et al. Standard Track [Page 102]
RFC 3775 Mobility Support in IPv6 June 2004
o The valid or preferred lifetime is reconfigured or changes for any
reason other than advancing real time.
o The mobile node requests the information with a Mobile Prefix
Solicitation (see Section 11.4.2).
SHOULD:
o A new prefix is added to the home subnet interface(s) of the home
agent.
MAY:
o 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.
o If a mobile node sends a solicitation, answer right away.
o If no Mobile Prefix Advertisement has been sent to the mobile node
in the last MaxMobPfxAdvInterval seconds (see Section 13), then
ensure that a transmission is scheduled. The actual transmission
time is randomized as described below.
o If a prefix matching the mobile node's home registration is added
on the home subnet interface or if its information changes in any
way that does not deprecate the mobile node's address, ensure that
a transmission is scheduled. The actual transmission time is
randomized as described below.
o If a home registration expires, cancel any scheduled
advertisements to the mobile node.
The list of prefixes is sent in its entirety in all cases.
If the home agent has already scheduled the transmission of a Mobile
Prefix Advertisement to the mobile node, then the home agent will
replace the advertisement with a new one to be sent at the scheduled
time.
Otherwise, the home agent computes a fresh value for RAND_ADV_DELAY
which offsets from the current time for the scheduled transmission.
First calculate the maximum delay for the scheduled Advertisement:
Johnson, et al. Standard Track [Page 103]
RFC 3775 Mobility Support in IPv6 June 2004
MaxScheduleDelay = min (MaxMobPfxAdvInterval, Preferred Lifetime),
where MaxMobPfxAdvInterval is as defined in Section 12. Then compute
the final delay for the advertisement:
RAND_ADV_DELAY = MinMobPfxAdvInterval +
(rand() % abs(MaxScheduleDelay - MinMobPfxAdvInterval))
Here rand() returns a random integer value in the range of 0 to the
maximum possible integer value. This 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, when
necessary to avoid overwhelming local network resources. The home
agent SHOULD periodically continue to retransmit an unsolicited
Advertisement to the mobile node, until it is acknowledged by the
receipt of a Mobile Prefix Solicitation from the mobile node.
The home agent MUST wait PREFIX_ADV_TIMEOUT (see Section 12) before
the first retransmission and double the retransmission wait time for
every succeeding retransmission until a maximum number of
PREFIX_ADV_RETRIES attempts (see Section 12) has been tried. 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. In the meantime, if the mobile node sends another
Binding Update without returning home, then the home agent SHOULD
begin transmitting the unsolicited Advertisement again.
If some condition, as described above, occurs on the home link and
causes another Prefix Advertisement to be sent to the mobile node,
before the mobile node acknowledges a previous transmission, the home
agent SHOULD combine any Prefix Information options in the
unacknowledged Mobile Prefix Advertisement into a new Advertisement.
The home agent then discards the old Advertisement.
10.6.3. Sending Advertisements
When sending a Mobile Prefix Advertisement to the mobile node, the
home agent MUST construct the packet as follows:
o 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.
Johnson, et al. Standard Track [Page 104]
RFC 3775 Mobility Support in IPv6 June 2004
o If the advertisement was solicited, it MUST be destined 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.
o A type 2 routing header MUST be included with the mobile node's
home address.
o IPsec headers MUST be supported and SHOULD be used.
o The home agent MUST send the packet as it would any other unicast
IPv6 packet that it originates.
o Set the Managed Address Configuration (M) flag if the
corresponding flag has been set in any of the Router
Advertisements from which the prefix information has been learned
(including the ones sent by this home agent).
o Set the Other Stateful Configuration (O) flag if the corresponding
flag has been set in any of the Router Advertisements from which
the prefix information has been learned (including the ones sent
by this home agent).
10.6.4. Lifetimes for Changed Prefixes
As described in Section 10.3.1, the lifetime returned by the home
agent in a Binding Acknowledgement MUST not be 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.
The Binding Update List records information for each Binding Update
sent by this mobile node, in which the lifetime of the binding has
not yet expired. The Binding Update List includes all bindings sent
by the mobile node either to its home agent or correspondent nodes.
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
Johnson, et al. Standard Track [Page 105]
RFC 3775 Mobility Support in IPv6 June 2004
any manner consistent with the external behavior described in this
document.
Each Binding Update List entry conceptually contains the following
fields:
o The IP address of the node to which a Binding Update was sent.
o The home address for which that Binding Update was sent.
o 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 while giving its new care-of address to this
destination after changing its care-of address.
o The initial value of the Lifetime field sent in that Binding
Update.
o 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.
o 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 (see Section 9.5.1).
o 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.
o The state of any retransmissions needed for 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.
o A flag specifying whether or not future Binding Updates should 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 11.3.5.
The Binding Update List is used to determine whether a particular
packet is sent directly to the correspondent node or tunneled via the
home agent (see Section 11.3.1).
Johnson, et al. Standard Track [Page 106]
RFC 3775 Mobility Support in IPv6 June 2004
The Binding Update list also conceptually contains the following data
related to running the return routability procedure. This data is
relevant only for Binding Updates sent to correspondent nodes.
o 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.
o 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.
o Cookie values used in the Home Test Init and Care-of Test Init
messages.
o Home and care-of keygen tokens received from the correspondent
node.
o Home and care-of nonce indices received from the correspondent
node.
o The time at which each of the tokens and nonces were received from
the correspondent node, as needed to implement reuse while moving.
11.2. Processing Mobility Headers
All IPv6 mobile nodes MUST observe the rules described in Section 9.2
when processing Mobility Headers.
11.3. Packet Processing
11.3.1. 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:
o Protocols layered over IP will generally treat the mobile node's
home address as its IP address for most packets. 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. If a binding exists, the mobile node SHOULD
Johnson, et al. Standard Track [Page 107]
RFC 3775 Mobility Support in IPv6 June 2004
send the packets directly to the correspondent node. Otherwise,
if a binding does not exist, the mobile node MUST use reverse
tunneling.
o The mobile node MAY choose to directly use one of its care-of
addresses as the source of the packet, not requiring the use of a
Home Address option in the packet. This is particularly useful
for short-term communication that may easily be retried if it
fails. 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. Such packets can be routed
normally, directly between their source and destination without
relying on Mobile IPv6. If application running on 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.
The choice of the most efficient communications method is
application specific, and outside the scope of this specification.
The APIs necessary for controlling the choice are also out of
scope.
o While not at its home link, the mobile node MUST NOT use the Home
Address destination option when communicating with link-local or
site-local peers, if the scope of the home address is larger than
the scope of the peer's address.
Similarly, the mobile node MUST NOT use the Home Address
destination option for IPv6 Neighbor Discovery [12] packets.
Detailed operation of these cases is described later in this section
and also discussed in [31].
For packets sent by a mobile node while it is at home, no special
Mobile IPv6 processing is required. Likewise, if the mobile node
uses any address other than one of its home addresses as the source
of a packet sent while away from home, no special Mobile IPv6
processing is required. In either case, the packet is simply
addressed and transmitted in the same way as any normal IPv6 packet.
For packets sent by the mobile node sent while away from home using
the mobile node's home address as the source, special Mobile IPv6
processing of the packet is required. This can be done in the
following two ways:
Johnson, et al. Standard Track [Page 108]
RFC 3775 Mobility Support in IPv6 June 2004
Route Optimization
This manner of delivering packets does not require going through the
home network, and typically will enable faster and more reliable
transmission.
The mobile node needs to ensure that a Binding Cache entry exists for
its home address so that the correspondent node can process the
packet (Section 9.3.1 specifies the rules for Home Address
Destination Option Processing at a correspondent node). The mobile
node SHOULD examine its Binding Update List for an entry which
fulfills the following conditions:
* The Source Address field of the packet being sent is equal to the
home address in the entry.
* The Destination Address field of the packet being sent is equal to
the address of the correspondent node in the entry.
* One of the current care-of addresses of the mobile node appears as
the care-of address in the entry.
* The entry indicates that a binding has been successfully created.
* The remaining lifetime of the binding is greater than zero.
When these conditions are met, the mobile node knows that the
correspondent node has a suitable Binding Cache entry.
A mobile node SHOULD arrange to supply the home address in a Home
Address option, and MUST set the IPv6 header's Source Address field
to the care-of address which the mobile node has registered to be
used with this correspondent node. 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 includes the calculation of upper layer
checksums using the home address as the value of the source.
* 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.
Johnson, et al. Standard Track [Page 109]
RFC 3775 Mobility Support in IPv6 June 2004
* 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 an
address assigned to the interface on the link being used.
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 [26].
Reverse Tunneling
This is the mechanism which tunnels the packets via the home
agent. It is not as efficient as the above mechanism, but is
needed if there is no binding yet with the correspondent node.
This mechanism is used for packets that have the mobile node's
home address as the Source Address in the IPv6 header, or with
multicast control protocol packets as described in Section 11.3.4.
Specifically:
* The packet is sent to the home agent using IPv6 encapsulation
[15].
* 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.
Then, the home agent will pass the encapsulated packet to the
correspondent node.
11.3.2. Interaction with Outbound IPsec Processing
This section sketches the interaction between outbound Mobile IPv6
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 [4] 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 11.3.1):
Johnson, et al. Standard Track [Page 110]
RFC 3775 Mobility Support in IPv6 June 2004
o The packet is created by higher layer protocols and applications
(e.g., by TCP) as if the mobile node were at home and Mobile IPv6
were not being used.
o Determine the outgoing interface for the packet. (Note that the
selection between reverse tunneling and route optimization may
imply different interfaces, particularly if tunnels are considered
interfaces as well.)
o 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 [4].
o 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.
o 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 the care-of address used
with this correspondent node, as described in Section 11.3.1. 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 IPsec (AH [5] or ESP [6])
header, so that the Home Address destination option is processed
by the destination node before the IPsec 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 IPsec header.
RFC 2402 treatment of destination options is extended as follows.
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,
Johnson, et al. Standard Track [Page 111]
RFC 3775 Mobility Support in IPv6 June 2004
* the Home Address field of the Home Address destination option
(Section 6.3) contains the new care-of address.
o 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 to reach the above situation,
simplifying processing for all subsequent packet headers.
However, such an exchange is not required, as long as the result
of the authentication calculation remains the same.
When an automated key management protocol is used to create new
security associations for a peer, it is important to ensure that the
peer can send the key management protocol packets to the mobile node.
This may not be possible if the peer is the home agent of the mobile
node and the purpose of the security associations would be to send a
Binding Update to the home agent. Packets addressed to the home
address of the mobile node cannot be used before the Binding Update
has been processed. For the default case of using IKE [9] as the
automated key management protocol, such problems can be avoided by
the following requirements when communicating with its home agent:
o When the mobile node is away from home, it 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 IPv6 for these
packets, as suggested in Section 11.3.1).
o In addition, for all security associations bound to the mobile
node's home address established by IKE, the mobile node MUST
include an ISAKMP Identification Payload [8] in the IKE phase 2
exchange, giving the mobile node's home address as the initiator
of the Security Association [7].
The Key Management Mobility Capability (K) bit in Binding Updates and
Acknowledgements can be used to avoid the need to rerun IKE upon
movements.
11.3.3. Receiving Packets While Away from Home
While away from home, a mobile node will receive packets addressed to
its home address, by one of two methods:
o Packets sent by a correspondent node, that does not have a Binding
Cache entry for the mobile node, will be sent to the home address,
captured by the home agent and tunneled to the mobile node.
o 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
Johnson, et al. Standard Track [Page 112]
RFC 3775 Mobility Support in IPv6 June 2004
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.
For packets received by the first method, the mobile node MUST check
that the IPv6 source address of the tunneled packet is the IP address
of its home agent. In this method, the mobile node may also send a
Binding Update to the original sender of the packet as described in
Section 11.7.2 and subject to the rate limiting defined in Section
11.8. The mobile node MUST also process the received packet in the
manner defined for IPv6 encapsulation [15], 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, 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.
o The length field in the routing header is exactly 2.
o The segments left field in the routing header is 1 on the wire.
(But implementations may process the routing header so that the
value may become 0 after the routing header has been processed,
but before the rest of the packet is processed.)
o The Home Address field in the routing header is one of the node's
home addresses, if the segments left field was 1. Thus, in
particular the address field is required to be a unicast routable
address.
Once the above checks have been performed, the node swaps the IPv6
destination field with the Home Address field in the routing header,
decrements segments left by one from the value it had on the wire,
and resubmits the packet to IP for processing the next header.
Johnson, et al. Standard Track [Page 113]
RFC 3775 Mobility Support in IPv6 June 2004
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 the same way 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.
11.3.4. 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. Therefore, this section 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, in which a mobile
node MAY join the group, is via a (local) multicast router on the
foreign link being visited. In this case, the mobile node MUST use
its care-of address and MUST NOT use the Home Address destination
option when sending MLD packets [17].
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 (such as those defined in
[17] or in [37]) to its home agent, and the home agent forwards
multicast packets down the tunnel to the mobile node. A mobile node
MUST NOT tunnel multicast group membership control packets until (1)
the mobile node has a binding in place at the home agent, and (2) the
latter sends at least one multicast group membership control packet
via the tunnel. Once this condition is true, the mobile node SHOULD
assume it does not change as long as the binding does not expire.
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.
The application is aware of the care-of address and uses it as a
source address for multicast traffic, just like it would use a
stationary address. The mobile node MUST NOT use Home Address
destination option in such traffic.
Johnson, et al. Standard Track [Page 114]
RFC 3775 Mobility Support in IPv6 June 2004
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.
Note that direct sending from the foreign link is only applicable
while the mobile node is at that foreign link. This is because the
associated multicast tree is specific to that source location and any
change of location and source address will invalidate the source
specific tree or branch and the application context of the other
multicast group members.
This specification does not provide mechanisms to enable such local
multicast session to survive hand-off and to seamlessly continue from
a new care-of address on each new foreign link. Any such mechanism,
developed as an extension to this specification, needs to take into
account the impact of fast moving mobile nodes on the Internet
multicast routing protocols and their ability to maintain the
integrity of source specific multicast trees and branches.
While the use of bidirectional tunneling can ensure that multicast
trees are independent of the mobile nodes movement, in some case such
tunneling can have adverse affects. The latency of specific types of
multicast applications (such as multicast based discovery protocols)
will be affected when the round-trip time between the foreign subnet
and the home agent is significant compared to that of the topology to
be discovered. In addition, the delivery tree from the home agent in
such circumstances relies on unicast encapsulation from the agent to
the mobile node. Therefore, bandwidth usage is inefficient compared
to the native multicast forwarding in the foreign multicast system.
11.3.5. Receiving ICMP Error Messages
Any node that does not recognize the Mobility header will return an
ICMP Parameter Problem, Code 1, message to the sender of the packet.
If the mobile node receives such an ICMP error message in response to
a return routability procedure or Binding Update, it SHOULD record in
its Binding Update List that future Binding Updates SHOULD NOT be
sent to this destination. Such Binding Update List entries SHOULD be
removed after a period of time in order to allow for retrying route
optimization.
New Binding Update List entries MUST NOT be created as a result of
receiving ICMP error messages.
Johnson, et al. Standard Track [Page 115]
RFC 3775 Mobility Support in IPv6 June 2004
Correspondent nodes that have participated in the return routability
procedure MUST implement the ability to correctly process received
packets containing a Home Address destination 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 that it
does not support the Home Address option, the mobile node SHOULD log
the error and then discard the ICMP message.
11.3.6. 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 source of the Binding Error message. If
the mobile node does not have such an 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 (unknown binding for
Home Address destination option), the mobile node should perform one
of the following two actions:
o If the mobile node has recent upper layer progress information,
which indicates that 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.
o If the mobile node has 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 5.2).
If the message Status field was 2 (unrecognized MH Type value), the
mobile node should perform one of the following two actions:
o If the mobile node is not expecting an acknowledgement or response
from the correspondent node, the mobile node SHOULD ignore this
message.
o 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.
Johnson, et al. Standard Track [Page 116]
RFC 3775 Mobility Support in IPv6 June 2004
11.4. Home Agent and Prefix Management
11.4.1. 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 11.7.1, 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 [16] for its home subnet
prefix. As described in Section 10.5, the home agent on its home
link that receives this Request message will return an ICMP Home
Agent Address Discovery Reply message. This message gives the
addresses for the home agents 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
any of the unicast IP addresses listed in the Home Agent Addresses
field in the Reply. For example, the mobile node MAY attempt its
home registration to each of these addresses, in turn, until its
registration is accepted. The mobile node sends a Binding Update to
an address and waits for the matching Binding Acknowledgement, moving
on to the next address if there is no response. The mobile node
MUST, however, wait at least InitialBindackTimeoutFirstReg seconds
(see Section 13) before sending a Binding Update to the next home
agent. In trying each of the returned home agent addresses, the
mobile node SHOULD try each of them in the order they appear in the
Home Agent Addresses field in the received Home Agent Address
Discovery Reply message.
If the mobile node has a current registration with some home agent
(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., timed out or
rejected), the mobile node SHOULD then reattempt this registration
with another home agent. If the mobile node knows of no other
suitable home agent, then it MAY attempt the dynamic home agent
address discovery mechanism described above.
Johnson, et al. Standard Track [Page 117]
RFC 3775 Mobility Support in IPv6 June 2004
If, after a mobile node transmits a Home Agent Address Discovery
Request message to the Home Agents Anycast address, it does not
receive a corresponding Home Agent Address Discovery Reply message
within INITIAL_DHAAD_TIMEOUT (see Section 12) 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 (see Section 12) attempts. Each retransmission MUST be
delayed by twice the time interval of the previous retransmission.
11.4.2. Sending Mobile Prefix Solicitations
When a mobile node has a home address that is about to become
invalid, it SHOULD send 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 10.6.
The mobile node MUST use the Home Address destination option to carry
its home address. The mobile node MUST support and SHOULD use IPsec
to protect the solicitation. The mobile node MUST set the Identifier
field in the ICMP header to a random value.
As described in Section 11.7.2, 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 valid lifetime (see
Section 10.6.2) 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.
11.4.3. Receiving Mobile Prefix Advertisements
Section 10.6 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 Advertisements 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.
Johnson, et al. Standard Track [Page 118]
RFC 3775 Mobility Support in IPv6 June 2004
The Mobile Prefix Solicitation is similar to the Router Solicitation
used in Neighbor Discovery [12], 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.
When a mobile node receives a Mobile Prefix Advertisement, it MUST
validate it according to the following test:
o 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.
o The packet MUST have a type 2 routing header and SHOULD be
protected by an IPsec header as described in Section 5.4 and
Section 6.8.
o If the ICMP Identifier value matches the ICMP Identifier value of
the most recently sent Mobile Prefix Solicitation and no other
advertisement has yet been received for this value, then the
advertisement is considered to be solicited and will be processed
further.
Otherwise, the advertisement is unsolicited, and MUST be
discarded. In this case the mobile node SHOULD send a Mobile
Prefix Solicitation.
Any received Mobile Prefix Advertisement not meeting these tests MUST
be silently discarded.
For an accepted Mobile Prefix Advertisement, the mobile node MUST
process Managed Address Configuration (M), Other Stateful
Configuration (O), and the Prefix Information Options as if they
arrived in a Router Advertisement [12] on the mobile node's home
link. (This specification does not, however, describe how to acquire
home addresses through stateful protocols.) 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 communications by the mobile node, in
the same way as for nodes that are at home.
This specification assumes that any security associations and
security policy entries that may be needed for new prefixes have been
pre-configured in the mobile node. Note that while dynamic key
Johnson, et al. Standard Track [Page 119]
RFC 3775 Mobility Support in IPv6 June 2004
management avoids the need to create new security associations, it is
still necessary to add policy entries to protect the communications
involving the home address(es). Mechanisms for automatic set-up of
these entries are outside the scope of this specification.
11.5. Movement
11.5.1. Movement Detection
The primary goal of movement detection is to detect L3 handovers.
This section does not attempt to specify a fast movement detection
algorithm which will function optimally for all types of
applications, link-layers and deployment scenarios; instead, it
describes a generic method that uses the facilities of IPv6 Neighbor
Discovery, including Router Discovery and Neighbor Unreachability
Detection. At the time of this writing, this method is considered
well enough understood to recommend for standardization, however it
is expected that future versions of this specification or other
specifications may contain updated versions of the movement detection
algorithm that have better performance.
Generic movement detection uses Neighbor Unreachability Detection to
detect when the default router is no longer bi-directionally
reachable, in which case the mobile node must discover a new default
router (usually on a new link). However, this detection only occurs
when the mobile node has packets to send, and in the absence of
frequent Router Advertisements or indications from the link-layer,
the mobile node might become unaware of an L3 handover that occurred.
Therefore, the mobile node should supplement this method with other
information whenever it is available to the mobile node (e.g., from
lower protocol layers).
When the mobile node detects an L3 handover, it performs Duplicate
Address Detection [13] on its link-local address, selects a new
default router as a consequence of Router Discovery, and then
performs Prefix Discovery with that new router to form new care-of
address(es) as described in Section 11.5.2. It then registers its
new primary care-of address with its home agent as described in
Section 11.7.1. After updating its home registration, the mobile
node then updates associated mobility bindings in correspondent nodes
that it is performing route optimization with as specified in Section
11.7.2.
Due to the temporary packet flow disruption and signaling overhead
involved in updating mobility bindings, the mobile node should avoid
performing an L3 handover until it is strictly necessary.
Specifically, when the mobile node receives a Router Advertisement
from a new router that contains a different set of on-link prefixes,
Johnson, et al. Standard Track [Page 120]
RFC 3775 Mobility Support in IPv6 June 2004
if the mobile node detects that the currently selected default router
on the old link is still bi-directionally reachable, it should
generally continue to use the old router on the old link rather than
switch away from it to use a new default router.
Mobile nodes can use the information in received Router
Advertisements to detect L3 handovers. In doing so the mobile node
needs to consider the following issues:
o There might be multiple routers on the same link, thus hearing a
new router does not necessarily constitute an L3 handover.
o When there are multiple routers on the same link they might
advertise different prefixes. Thus even hearing a new router with
a new prefix might not be a reliable indication of an L3 handover.
o The link-local addresses of routers are not globally unique, hence
after completing an L3 handover the mobile node might continue to
receive Router Advertisements with the same link-local source
address. This might be common if routers use the same link-local
address on multiple interfaces. This issue can be avoided when
routers use the Router Address (R) bit, since that provides a
global address of the router.
In addition, the mobile node should consider the following events as
indications that an L3 handover may have occurred. Upon receiving
such indications, the mobile node needs to perform Router Discovery
to discover routers and prefixes on the new link, as described in
Section 6.3.7 of RFC 2461 [12].
o 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. The
mobile node can then implement its own policy to determine how
many lost Advertisements from its current default router
constitute an L3 handover indication.
o Neighbor Unreachability Detection determines that the default
router is no longer reachable.
Johnson, et al. Standard Track [Page 121]
RFC 3775 Mobility Support in IPv6 June 2004
o With some types of networks, notification that an L2 handover has
occurred might be obtained from lower layer protocols or device
driver software within the mobile node. While further details
around handling L2 indications as movement hints is an item for
further study, at the time of writing this specification the
following is considered reasonable:
An L2 handover indication may or may not imply L2 movement and L2
movement may or may not imply L3 movement; the correlations might
be a function of the type of L2 but might also be a function of
actual deployment of the wireless topology.
Unless it is well-known that an L2 handover indication is likely
to imply L3 movement, instead of immediately multicasting a router
solicitation it may be better to attempt to verify whether the
default router is still bi-directionally reachable. This can be
accomplished by sending a unicast Neighbor Solicitation and
waiting for a Neighbor Advertisement with the solicited flag set.
Note that this is similar to Neighbor Unreachability detection but
it does not have the same state machine, such as the STALE state.
If the default router does not respond to the Neighbor
Solicitation it makes sense to proceed to multicasting a Router
Solicitation.
11.5.2. Forming New Care-of Addresses
After detecting that it has moved a mobile node SHOULD generate a new
primary care-of address using normal IPv6 mechanisms. This SHOULD
also be done when the current primary care-of address becomes
deprecated. A mobile node MAY form a new primary care-of address at
any time, but a mobile node MUST NOT send a Binding Update about a
new care-of address to its home agent more than MAX_UPDATE_RATE times
within a second.
In addition, a mobile node MAY form new non-primary care-of addresses
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 11.5.3.
Johnson, et al. Standard Track [Page 122]
RFC 3775 Mobility Support in IPv6 June 2004
As described in Section 4, in order to form a new care-of address, a
mobile node MAY use either stateless [13] or stateful (e.g., DHCPv6
[29]) Address Autoconfiguration. If a mobile node needs to use a
source address (other than the unspecified address) in packets sent
as a part of address autoconfiguration, it MUST use an IPv6 link-
local address rather than its own IPv6 home address.
RFC 2462 [13] specifies that in normal processing for Duplicate
Address Detection, the node SHOULD delay sending the initial Neighbor
Solicitation message by a random delay between 0 and
MAX_RTR_SOLICITATION_DELAY. Since delaying DAD can result in
significant delays in configuring a new care-of address when the
Mobile Node moves to a new link, the Mobile Node preferably SHOULD
NOT delay DAD when configuring a new care-of address. The Mobile
Node SHOULD delay according to the mechanisms specified in RFC 2462
unless the implementation has a behavior that desynchronizes the
steps that happen before the DAD in the case that multiple nodes
experience handover at the same time. Such desynchronizing behaviors
might be due to random delays in the L2 protocols or device drivers,
or due to the movement detection mechanism that is used.
11.5.3. Using Multiple Care-of Addresses
As described in Section 11.5.2, 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. The
mobile node MUST ensure that its primary care-of address always has a
prefix that is advertised by its current default router. After
selecting a new primary care-of address, the mobile node MUST send a
Binding Update containing that care-of address to its home agent.
The Binding Update MUST have the Home Registration (H) and
Acknowledge (A) bits set its home agent, as described on Section
11.7.1.
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 [29], the mobile
node may not wish to release the address immediately upon switching
to a new primary care-of address.
Johnson, et al. Standard Track [Page 123]
RFC 3775 Mobility Support in IPv6 June 2004
Whenever a mobile node determines that it is no longer reachable
through a given link, it SHOULD invalidate all care-of addresses
associated with address prefixes that it discovered from routers on
the unreachable link which are not in the current set of address
prefixes advertised by the (possibly new) current default router.
11.5.4. Returning Home
A mobile node detects that it has returned to its home link through
the movement detection algorithm in use (Section 11.5.1), 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 home registration, the mobile node MUST set the
Acknowledge (A) and Home Registration (H) bits, set the Lifetime
field to zero, and set the care-of address for the binding to the
mobile node's own home address. The mobile node MUST use its home
address as the source address in the Binding Update.
When sending this Binding Update to its home agent, the mobile node
must be careful in how it uses Neighbor Solicitation [12] (if needed)
to learn the home agent's link-layer address, since the home agent
will be currently configured to intercept packets to the mobile
node's home address using Duplicate Address Detection (DAD). In
particular, the mobile node is unable to use its home address as the
Source Address in the Neighbor Solicitation until the home agent
stops defending the home address.
Neighbor Solicitation by the mobile node for the home agent's address
will normally not be necessary, since the mobile node has already
learned the home agent's link-layer address from a Source Link-Layer
Address option in a Router Advertisement. However, if there are
multiple home agents it may still be necessary to send a
solicitation. In this special case of the mobile node returning
home, the mobile node MUST multicast 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). The target of the Neighbor Solicitation
MUST be set to the mobile node's home address. The destination IP
address MUST be set to the Solicited-Node multicast address [3]. The
home agent will send a multicast Neighbor Advertisement back to the
mobile node with the Solicited flag (S) set to zero. In any case,
the mobile node SHOULD record the information from the Source Link-
Layer Address option or from the advertisement, and set the state of
the Neighbor Cache entry for the home agent to REACHABLE.
The mobile node then sends its Binding Update to 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
Johnson, et al. Standard Track [Page 124]
RFC 3775 Mobility Support in IPv6 June 2004
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. This rule also applies to the
derived link-local address of the mobile node, if the Link Local
Address Compatibility (L) bit was set when the binding was created.
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, it MUST be prepared
to reply to Neighbor Solicitations for its home address. Such
replies MUST be sent using a unicast Neighbor Advertisement to the
sender's link-layer address. It is necessary to reply, since sending
the Binding Acknowledgement from the home agent may require
performing Neighbor Discovery, and the mobile node may not be able to
distinguish Neighbor Solicitations coming from the home agent from
other Neighbor Solicitations. Note that a race condition exists
where both the mobile node and the home agent respond to the same
solicitations sent by other nodes; this will be only temporary,
however, until the Binding Update is accepted.
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 [12], to
advertise the mobile node's own link-layer address for its own home
address. The Target Address in this Neighbor Advertisement 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 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. 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 Advertisements [12] up to MAX_NEIGHBOR_ADVERTISEMENT times
to increase their reliability. It is still possible that some nodes
Johnson, et al. Standard Track [Page 125]
RFC 3775 Mobility Support in IPv6 June 2004
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 [12].
Note that the tunnel via the home agent typically stops operating at
the same time that the home registration is deleted.
11.6. Return Routability Procedure
This section defines the rules that the mobile node must follow when
performing the return routability procedure. Section 11.7.2
describes the rules when the return routability procedure needs to be
initiated.
11.6.1. Sending 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. However, if the mobile node has recently received (see
Section 5.2.7) one or both home or care-of keygen tokens, and
associated nonce indices for the desired addresses, it MAY reuse
them. Therefore, the return routability procedure may in some cases
be completed with only one message pair. It may even be completed
without any messages at all, if the mobile node has a recent home
keygen token and has previously visited the same care-of address so
that it also has a recent care-of keygen token. If the mobile node
intends to send a Binding Update with the Lifetime set to zero and
the care-of address equal to its home address - such as when
returning home - sending a Home Test Init message is sufficient. In
this case, generation of the binding management key depends
exclusively on the home keygen token (Section 5.2.5).
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:
o The IP address of the node to which the message was sent.
o The home address of the mobile node. This value will appear in
the Source Address field of the Home Test Init message. When
sending the Care-of Test Init message, this address does not
appear in the message, but represents the home address for which
the binding is desired.
Johnson, et al. Standard Track [Page 126]
RFC 3775 Mobility Support in IPv6 June 2004
o The time at which each of these messages was sent.
o The cookies used in the messages.
Note that a single Care-of Test Init message may be sufficient even
when there are multiple home addresses. In this case the mobile node
MAY record the same information in multiple Binding Update List
entries.
11.6.2. Receiving Test Messages
Upon receiving a packet carrying a Home Test message, a mobile node
MUST validate the packet according to the following tests:
o 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.
Note that there may be multiple such entries.
o The Binding Update List indicates that no home keygen token has
been received yet.
o 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.
o The Home Init 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 keygen token in the Binding Update List. If the
Binding Update List entry does not have a care-of keygen token, the
mobile node SHOULD continue waiting for the Care-of Test message.
Upon receiving a packet carrying a Care-of Test message, a mobile
node MUST validate the packet according to the following tests:
o 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.
Note that there may be multiple such entries.
o The Binding Update List indicates that no care-of keygen token has
been received yet.
o The Destination Address of the packet is the current care-of
address of the mobile node.
Johnson, et al. Standard Track [Page 127]
RFC 3775 Mobility Support in IPv6 June 2004
o The Care-of Init 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 keygen token in the Binding Update List. If
the Binding Update List entry does not have a home keygen token, the
mobile node SHOULD continue waiting for the Home Test message.
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 keygen tokens, the return routability
procedure is complete. The mobile node SHOULD then proceed with
sending a Binding Update as described in Section 11.7.2.
Correspondent nodes from the time before this specification was
published may 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 cannot provide
route optimization, and revert back to the use of bidirectional
tunneling.
11.6.3. Protecting Return Routability Packets
The mobile node MUST support the protection of Home Test and Home
Test Init messages as described in Section 10.4.6.
When IPsec is used to protect return routability signaling or payload
packets, the mobile node MUST set the source address it uses for the
outgoing tunnel packets to the current primary care-of address. The
mobile node starts to use a new primary care-of address immediately
after sending a Binding Update to the home agent to register this new
address.
11.7. Processing Bindings
11.7.1. Sending Binding Updates to the Home Agent
After deciding to change its primary care-of address as described in
Section 11.5.1 and Section 11.5.2, a mobile node MUST register this
care-of address with its home agent in order to make this its primary
care-of address.
Also, if the mobile node wants the services of the home agent beyond
the current registration period, the mobile node should send a new
Binding Update to it well before the expiration of this period, even
if it is not changing its primary care-of address. However, if the
Johnson, et al. Standard Track [Page 128]
RFC 3775 Mobility Support in IPv6 June 2004
home agent returned a Binding Acknowledgement for the current
registration with Status field set to 1 (accepted but prefix
discovery necessary), the mobile node should not try to register
again before it has learned the validity of its home prefixes through
mobile prefix discovery. This is typically necessary every time this
Status value is received, because information learned earlier may
have changed.
To register a care-of address or to extend the lifetime of an
existing registration, the mobile node sends a packet to its home
agent containing a Binding Update, with the packet constructed as
follows:
o The Home Registration (H) bit MUST be set in the Binding Update.
o The Acknowledge (A) bit MUST be set in the Binding Update.
o The packet MUST contain a Home Address destination option, giving
the mobile node's home address for the binding.
o 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. This
option MUST be included in all home registrations, as the ESP
protocol will not be able to protect care-of addresses in the IPv6
header. (Mobile IPv6 implementations that know they are using
IPsec AH to protect a particular message might avoid this option.
For brevity the usage of AH is not discussed in this document.)
o If the mobile node's link-local address has the same interface
identifier as the home address for which it is supplying a new
care-of address, then the mobile node SHOULD set the Link-Local
Address Compatibility (L) bit.
o If the home address was generated using RFC 3041 [18], then the
link local address is unlikely to have a compatible interface
identifier. In this case, the mobile node MUST clear the Link-
Local Address Compatibility (L) bit.
o If the IPsec security associations between the mobile node and the
home agent have been established dynamically, and the mobile node
has the capability to update its endpoint in the used key
management protocol to the new care-of address every time it
moves, the mobile node SHOULD set the Key Management Mobility
Capability (K) bit in the Binding Update. Otherwise, the mobile
node MUST clear the bit.
Johnson, et al. Standard Track [Page 129]
RFC 3775 Mobility Support in IPv6 June 2004
o The value specified in the Lifetime field MUST be non-zero and
SHOULD be less than or equal to the remaining valid lifetime of
the home address and the care-of address specified for the
binding.
Mobile nodes that use dynamic home agent address discovery should
be careful with long lifetimes. If the mobile node loses the
knowledge of its binding with a specific home agent, registering a
new binding with another home agent may be impossible as the
previous home agent is still defending the existing binding.
Therefore, to ensure that mobile nodes using home agent address
discovery do not lose information about their binding, they SHOULD
de-register before losing this information, or use small
lifetimes.
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 returned during dynamic home agent address discovery
(see Section 11.4.1). If there was only one home agent, 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 11.8
for information about retransmitting Binding Updates.
With the Binding Update, the mobile node requests the home agent to
serve as the home agent for the given home address. Until the
lifetime of this registration expires, the home agent considers
itself the home agent for this home address.
Each Binding Update MUST be authenticated as coming from the right
mobile node, as defined in Section 5.1. The mobile node MUST use its
home address - either in the Home Address destination option or in
the Source Address field of the IPv6 header - in Binding Updates sent
to the home agent. This is necessary in order to allow the IPsec
policies to be matched with the correct home address.
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 11.7.2.
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 correct Sequence Number value, it may start at
Johnson, et al. Standard Track [Page 130]
RFC 3775 Mobility Support in IPv6 June 2004
any value. If the home agent rejects the value, it sends back a
Binding Acknowledgement with a status code 135, 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, 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.
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 the 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 134 (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 the
care-of addresses for its other home addresses, if any. (Mechanisms
outlined in Appendix B.5 may in the future allow mobile nodes to
acquire new home addresses to replace the one for which Status 134
was received.)
11.7.2. Correspondent Registration
When the mobile node is assured that its home address is valid, it
can initiate a correspondent registration with the purpose of
allowing the correspondent node to cache the mobile node's current
care-of address. This procedure consists of the return routability
procedure followed by a registration.
This section defines when the correspondent registration is to be
initiated and the rules to follow while it is being performed.
After the mobile node has sent a Binding Update to its home agent,
registering a new primary care-of address (as described in Section
11.7.1), the mobile node SHOULD initiate a correspondent registration
for each node that already appears in the mobile node's Binding
Update List. The initiated procedures can be used to either update
or delete binding information in the correspondent node.
Johnson, et al. Standard Track [Page 131]
RFC 3775 Mobility Support in IPv6 June 2004
For nodes that do not appear in the mobile node's Binding Update
List, the mobile node MAY initiate a correspondent registration at
any time after sending the Binding Update to its home agent.
Considerations regarding when (and if) to initiate the procedure
depend on the specific movement and traffic patterns of the mobile
node and are outside the scope of this document.
In addition, the mobile node MAY initiate the correspondent
registration in response to receiving a packet that meets all of the
following tests:
o The packet was tunneled using IPv6 encapsulation.
o The Destination Address in the tunnel (outer) IPv6 header is equal
to any of the mobile node's care-of addresses.
o The Destination Address in the original (inner) IPv6 header is
equal to one of the mobile node's home addresses.
o The Source Address in the tunnel (outer) IPv6 header differs from
the Source Address in the original (inner) IPv6 header.
o The packet does not contain a Home Test, Home Test Init, Care-of
Test, or Care-of Test Init message.
If a mobile node has multiple home addresses, it becomes important to
select the right home address to use in the correspondent
registration. The used home address MUST be the Destination Address
of the original (inner) packet.
The peer address used in the procedure MUST be determined as follows:
o If a Home Address destination option is present in the original
(inner) packet, the address from this option is used.
o Otherwise, the Source Address in the original (inner) IPv6 header
of the packet is used.
Note that the validity of the original packet is checked before
attempting to initiate a correspondent registration. For instance,
if a Home Address destination option appeared in the original packet,
then rules in Section 9.3.1 are followed.
A mobile node MAY also choose to keep its topological location
private from certain correspondent nodes, and thus need not initiate
the correspondent registration.
Johnson, et al. Standard Track [Page 132]
RFC 3775 Mobility Support in IPv6 June 2004
Upon successfully completing the return routability procedure, and
after receiving a successful Binding Acknowledgement from the Home
Agent, a Binding Update MAY be sent to the correspondent node.
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 also send a Binding Update to such a correspondent
node, instructing it to delete any existing binding for the mobile
node from its Binding Cache, as described in Section 6.1.7. Even in
this case a successful completion of the return routability procedure
is required first.
If the care-of address is not set to the mobile node's home address,
the Binding Update requests that the correspondent node create or
update an entry for the mobile node in the correspondent node's
Binding Cache. This is done in order to record a 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
registration and the care-of address specified for the binding. The
care-of address given in the Binding Update MAY differ from the
mobile node's primary care-of address.
If the Binding Update is sent to the correspondent node, requesting
the deletion of any existing Binding Cache entry it has for the
mobile node, the care-of address is set to the mobile node's home
address and the Lifetime field set to zero. In this case, generation
of the binding management key depends exclusively on the home keygen
token (Section 5.2.5). The care-of nonce index SHOULD be set to zero
in this case. In keeping with the Binding Update creation rules
below, the care-of address MUST be set to the home address if the
mobile node is at home, or to the current care-of address if it is
away from home.
If the mobile node wants to ensure that its new care-of address has
been entered into a correspondent node's Binding Cache, the mobile
node needs to request an acknowledgement by setting the Acknowledge
(A) bit in the Binding Update.
Johnson, et al. Standard Track [Page 133]
RFC 3775 Mobility Support in IPv6 June 2004
A Binding Update is created as follows:
o The current care-of address of the mobile node MUST be sent either
in the Source Address of the IPv6 header, or in the Alternate
Care-of Address mobility option.
o The Destination Address of the IPv6 header MUST contain the
address of the correspondent node.
o The Mobility Header is constructed according to rules in Section
6.1.7 and Section 5.2.6, including the Binding Authorization Data
(calculated as defined in Section 6.2.7) and possibly the Nonce
Indices mobility options.
o The home address of the mobile node MUST be added to the packet in
a Home Address destination option, unless the Source Address is
the home address.
Each Binding Update MUST have a Sequence Number greater than the
Sequence Number value sent in the previous Binding Update to the same
destination address (if any). The sequence numbers are compared
modulo 2**16, as described in Section 9.5.1. 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. 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.
The mobile node is responsible for the completion of the
correspondent registration, as well as any retransmissions that may
be needed (subject to the rate limitation defined in Section 11.8).
11.7.3. Receiving Binding Acknowledgements
Upon receiving a packet carrying a Binding Acknowledgement, a mobile
node MUST validate the packet according to the following tests:
o The packet meets the authentication requirements for Binding
Acknowledgements defined in Section 6.1.8 and Section 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 Binding Authorization Data mobility option
MUST be present and have a valid value.
Johnson, et al. Standard Track [Page 134]
RFC 3775 Mobility Support in IPv6 June 2004
o The Binding Authorization Data mobility option, if present, MUST
be the last option and MUST not have trailing padding.
o 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:
o 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, 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.
Mobile nodes SHOULD send a new Binding Update well before the
expiration of this period in order to extend the lifetime. This
helps to avoid disruptions in communications which might otherwise
be caused by network delays or clock drift.
Johnson, et al. Standard Track [Page 135]
RFC 3775 Mobility Support in IPv6 June 2004
o Additionally, if the Status field value is 1 (accepted but prefix
discovery necessary), the mobile node SHOULD send a Mobile Prefix
Solicitation message to update its information about the available
prefixes.
o 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 can 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
11.8. If this is not done or it fails, then the mobile node
SHOULD record in its Binding Update List that future Binding
Updates SHOULD NOT be sent to this destination.
The treatment of a Binding Refresh Advice mobility option within the
Binding Acknowledgement depends on where the acknowledgement came
from. This option MUST be ignored if the acknowledgement came from a
correspondent node. If it came from the home agent, the mobile node
uses the Refresh Interval field in the option as a suggestion that it
SHOULD attempt to refresh its home registration at the indicated
shorter interval.
If the acknowledgement came from the home agent, the mobile node
examines the value of the Key Management Mobility Capability (K) bit.
If this bit is not set, the mobile node SHOULD discard key management
protocol connections, if any, to the home agent. The mobile node MAY
also initiate a new key management connection.
If this bit is set, the mobile node SHOULD move its own endpoint in
the key management protocol connections to the home agent, if any.
The mobile node's new endpoint should be the new care-of address.
For an IKE phase 1 connection, this means that packets sent to this
address with the original ISAKMP cookies are accepted.
11.7.4. Receiving Binding Refresh Requests
When a mobile node receives a packet containing a Binding Refresh
Request message, the mobile node has a Binding Update List entry for
the source of the Binding Refresh Request, and the mobile node wants
to retain its binding cache entry at the correspondent node, then the
mobile node should start a return routability procedure. If the
mobile node wants to have its binding cache entry removed, it can
either ignore the Binding Refresh Request and wait for the binding to
time out, or at any time, it can delete its binding from a
correspondent node with an explicit binding update with a zero
lifetime and the care-of address set to the home address. If the
Johnson, et al. Standard Track [Page 136]
RFC 3775 Mobility Support in IPv6 June 2004
mobile node does not know if it needs the binding cache entry, it can
make the decision in an implementation dependent manner, such as
based on available resources.
Note that the mobile node should be careful to not respond to Binding
Refresh Requests for addresses not in the Binding Update List to
avoid being subjected to a denial of service attack.
If the return routability procedure completes successfully, a Binding
Update message SHOULD be sent, as described in Section 11.7.2. 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 the 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.
11.8. Retransmissions and Rate Limiting
The mobile node is responsible for retransmissions and rate limiting
in the return routability procedure, registrations, and in solicited
prefix discovery.
When the mobile node sends a Mobile Prefix Solicitation, Home Test
Init, Care-of Test Init or Binding Update for which it expects a
response, the mobile node has to determine a value for the initial
retransmission timer:
o If the mobile node is sending a Mobile Prefix Solicitation, it
SHOULD use an initial retransmission interval of
INITIAL_SOLICIT_TIMER (see Section 12).
o If the mobile node is sending a Binding Update and does not have
an existing binding at the home agent, it SHOULD use
InitialBindackTimeoutFirstReg (see Section 13) as a value for the
initial retransmission timer. This long retransmission interval
will allow the home agent to complete the Duplicate Address
Detection procedure mandated in this case, as detailed in Section
11.7.1.
o Otherwise, the mobile node should use the specified value of
INITIAL_BINDACK_TIMEOUT for the initial retransmission timer.
Johnson, et al. Standard Track [Page 137]
RFC 3775 Mobility Support in IPv6 June 2004
If the mobile node fails to receive a valid matching response within
the selected initial retransmission interval, the mobile node SHOULD
retransmit the message until a response 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 response or the
timeout period reaches the value MAX_BINDACK_TIMEOUT. The mobile
node MAY continue to send these messages at this slower rate
indefinitely.
The mobile node SHOULD start a separate back-off process for
different message types, different home addresses and different
care-of addresses. However, in addition an overall rate limitation
applies for messages sent to a particular correspondent node. This
ensures that the correspondent node has a sufficient amount of time
to respond when bindings for multiple home addresses are registered,
for instance. The mobile node MUST NOT send Mobility Header messages
of a particular type to a particular correspondent node more than
MAX_UPDATE_RATE times within a second.
Retransmitted Binding Updates MUST use a Sequence Number value
greater than that used for the previous transmission of this Binding
Update. Retransmitted Home Test Init and Care-of Test Init messages
MUST use new cookie values.
12. Protocol Constants
DHAAD_RETRIES 4 retransmissions
INITIAL_BINDACK_TIMEOUT 1 second
INITIAL_DHAAD_TIMEOUT 3 seconds
INITIAL_SOLICIT_TIMER 3 seconds
MAX_BINDACK_TIMEOUT 32 seconds
MAX_NONCE_LIFETIME 240 seconds
MAX_TOKEN_LIFETIME 210 seconds
MAX_RR_BINDING_LIFETIME 420 seconds
MAX_UPDATE_RATE 3 times
PREFIX_ADV_RETRIES 3 retransmissions
PREFIX_ADV_TIMEOUT 3 seconds
13. Protocol Configuration Variables
MaxMobPfxAdvInterval Default: 86,400 seconds
MinDelayBetweenRAs Default: 3 seconds,
Min: 0.03 seconds
MinMobPfxAdvInterval Default: 600 seconds
InitialBindackTimeoutFirstReg Default: 1.5 seconds
Johnson, et al. Standard Track [Page 138]
RFC 3775 Mobility Support in IPv6 June 2004
Home agents MUST allow the first three variables to be configured by
system management, and mobile nodes MUST allow the last variable to
be configured by system management.
The default value for InitialBindackTimeoutFirstReg has been
calculated as 1.5 times the default value of RetransTimer [12] times
the default value of DupAddrDetectTransmits [13].
The value MinDelayBetweenRAs overrides the value of the protocol
constant MIN_DELAY_BETWEEN_RAS, as specified in RFC 2461 [12]. This
variable SHOULD be set to MinRtrAdvInterval, if MinRtrAdvInterval is
less than 3 seconds.
14. IANA Considerations
This document defines a new IPv6 protocol, the Mobility Header,
described in Section 6.1. This protocol has been assigned protocol
number 135.
This document also creates a new name space "Mobility Header Type",
for the MH Type field in the Mobility Header. The current message
types are described starting from Section 6.1.2, and are 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
or IESG Approval [10].
Furthermore, each mobility message may contain mobility options as
described in Section 6.2. This document defines a new name space
"Mobility Option" to identify these options. The current mobility
options are defined starting from Section 6.2.2 and are the
following:
Johnson, et al. Standard Track [Page 139]
RFC 3775 Mobility Support in IPv6 June 2004
0 Pad1
1 PadN
2 Binding Refresh Advice
3 Alternate Care-of Address
4 Nonce Indices
5 Authorization Data
Future values of the Option Type can be allocated using Standards
Action or IESG Approval [10].
Finally, this document creates a third new name space "Status Code"
for the Status field in the Binding Acknowledgement message. The
current values are described in Section 6.1.8, and are the following:
0 Binding Update accepted
1 Accepted but prefix discovery necessary
128 Reason unspecified
129 Administratively prohibited
130 Insufficient resources
131 Home registration not supported
132 Not home subnet
133 Not home agent for this mobile node
134 Duplicate Address Detection failed
135 Sequence number out of window
136 Expired home nonce index
137 Expired care-of nonce index
138 Expired nonces
139 Registration type change disallowed
Johnson, et al. Standard Track [Page 140]
RFC 3775 Mobility Support in IPv6 June 2004
Future values of the Status field can be allocated using Standards
Action or IESG Approval [10].
All fields labeled "Reserved" are only to be assigned through
Standards Action or IESG Approval.
This document also defines a new IPv6 destination option, the Home
Address option, described in Section 6.3. This option has been
assigned the Option Type value 0xC9.
This document also defines a new IPv6 type 2 routing header,
described in Section 6.4. The value 2 has been allocated by IANA.
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. These messages have been
assigned ICMPv6 type numbers from the informational message range:
o The Home Agent Address Discovery Request message, described in
Section 6.5;
o The Home Agent Address Discovery Reply message, described in
Section 6.6;
o The Mobile Prefix Solicitation, described in Section 6.7; and
o The Mobile Prefix Advertisement, described in Section 6.8.
This document also defines two new Neighbor Discovery [12] options,
which have been assigned Option Type values within the option
numbering space for Neighbor Discovery messages:
o The Advertisement Interval option, described in Section 7.3; and
o The Home Agent Information option, described in Section 7.4.
Johnson, et al. Standard Track [Page 141]
RFC 3775 Mobility Support in IPv6 June 2004
15. Security Considerations
15.1. Threats
Any mobility solution must protect itself against misuses of the
mobility features and mechanisms. In Mobile IPv6, most of the
potential threats are concerned with false Bindings, usually
resulting in Denial-of-Service attacks. Some of the threats also
pose potential for Man-in-the-Middle, Hijacking, Confidentiality, and
Impersonation attacks. The main threats this protocol protects
against are the following:
o Threats involving 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 a home agent accepts such spoofed information sent
to it, the mobile node might not get traffic destined to it.
Similarly, a malicious (mobile) node might use the home address of
a victim node in a forged Binding Update sent to a correspondent
node.
These pose threats against confidentiality, integrity, and
availability. That is, an attacker might learn the contents of
packets destined to another node by redirecting the traffic to
itself. Furthermore, an attacker might use the redirected packets
in an attempt to set itself as a Man-in-the-Middle between a
mobile and a correspondent node. This would allow the attacker to
impersonate the mobile node, leading to integrity and availability
problems.
A malicious (mobile) node might also send Binding Updates in which
the care-of address is set to the address of a victim node. If
such Binding Updates were accepted, the malicious node could lure
the correspondent node into sending potentially large amounts of
data to the victim; 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. For example, the correspondent node might be a
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 does not help, because the
attacker can receive the first few segments (including the ISN) at
its own address, and only then redirect the stream to the victim's
address. These types of attacks may also be directed to networks
instead of nodes. Further variations of this threat are described
elsewhere [27, 34].
Johnson, et al. Standard Track [Page 142]
RFC 3775 Mobility Support in IPv6 June 2004
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 as
opposed to its current location.
In conclusion, there are Denial-of-Service, Man-in-the-Middle,
Confidentiality, and Impersonation threats against the parties
involved in sending legitimate Binding Updates, and Denial-of-
Service threats against any other party.
o Threats associated with payload packets: Payload packets exchanged
with mobile nodes are exposed to similar threats as that of
regular IPv6 traffic. However, Mobile IPv6 introduces the Home
Address destination option, a new routing header type (type 2),
and uses tunneling headers in the payload packets. The protocol
must protect against potential new threats involving the use of
these mechanisms.
Third parties become exposed to a reflection threat via the Home
Address destination option, unless appropriate security
precautions are followed. The Home Address destination option
could be used to direct response traffic toward a node whose IP
address appears in the option. In this case, ingress filtering
would not catch the forged "return address" [36, 32].
A similar threat exists with the tunnels between the mobile node
and the home agent. An attacker might forge tunnel 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. Note
that an attacker who is able to forge tunnel packets would
typically also be able to forge packets that appear to come
directly from the mobile node. This is not a new threat as such.
However, it may make it easier for attackers to escape detection
by avoiding ingress filtering and packet tracing mechanisms.
Furthermore, spoofed tunnel packets might be used to gain access
to the home network.
Finally, a routing header could also be used in reflection
attacks, and in attacks designed to bypass firewalls. The
generality of the regular routing header would allow circumvention
of IP-address based rules in firewalls. It would also allow
reflection of traffic to other nodes. These threats exist with
routing headers in general, even if the usage that Mobile IPv6
requires is safe.
o Threats associated with dynamic home agent and mobile prefix
discovery.
Johnson, et al. Standard Track [Page 143]
RFC 3775 Mobility Support in IPv6 June 2004
o Threats against the Mobile IPv6 security mechanisms themselves: An
attacker might, for instance, lure the participants into executing
expensive cryptographic operations or allocating memory for the
purpose of keeping state. The victim node would have no resources
left to handle other tasks.
As a fundamental service in an IPv6 stack, Mobile IPv6 is expected to
be deployed in most nodes of the IPv6 Internet. The above threats
should therefore be considered as being applicable to the whole
Internet.
It should also be noted that some additional threats result from
movements as such, even without the involvement of mobility
protocols. Mobile nodes must be capable to defend themselves in the
networks that they visit, as typical perimeter defenses applied in
the home network no longer protect them.
15.2. Features
This specification provides a series of features designed to mitigate
the risk introduced by the threats listed above. The main security
features are the following:
o Reverse Tunneling as a mandatory feature.
o Protection of Binding Updates sent to home agents.
o Protection of Binding Updates sent to correspondent nodes.
o Protection against reflection attacks that use the Home Address
destination option.
o Protection of tunnels between the mobile node and the home agent.
o Closing routing header vulnerabilities.
o Mitigating Denial-of-Service threats to the Mobile IPv6 security
mechanisms themselves.
The support for encrypted reverse tunneling (see Section 11.3.1)
allows mobile nodes to defeat certain kinds of traffic analysis.
Protecting those Binding Updates that are sent to home agents and
those that are sent to arbitrary correspondent nodes requires very
different security solutions due to the different situations. Mobile
nodes and home agents are naturally expected to be subject to the
network administration of the home domain.
Johnson, et al. Standard Track [Page 144]
RFC 3775 Mobility Support in IPv6 June 2004
Thus, they can and are supposed to have a security association that
can be used to reliably authenticate the exchanged messages. See
Section 5.1 for the description of the protocol mechanisms, and
Section 15.3 below for a discussion of the resulting level of
security.
It is expected that Mobile IPv6 route optimization will be used on a
global basis between nodes belonging to different administrative
domains. It would be a very demanding task to build an
authentication infrastructure on this scale. Furthermore, a
traditional authentication infrastructure cannot be easily used to
authenticate IP addresses because IP addresses can change often. It
is not sufficient to just authenticate the mobile nodes;
Authorization to claim the right to use an address is needed as well.
Thus, an "infrastructureless" approach is necessary. The chosen
infrastructureless method is described in Section 5.2, and Section
15.4 discusses the resulting security level and the design rationale
of this approach.
Specific rules guide the use of the Home Address destination option,
the routing header, and the tunneling headers in the payload packets.
These rules are necessary to remove the vulnerabilities associated
with their unrestricted use. The effect of the rules is discussed in
Section 15.7, Section 15.8, and Section 15.9.
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 15.4.5.
15.3. Binding Updates to Home Agent
Signaling between the mobile node and the home agent requires message
integrity. This is necessary to assure the home agent that a Binding
Update is from a legitimate mobile node. In addition, correct
ordering and anti-replay protection are optionally needed.
IPsec ESP protects the integrity of the Binding Updates and Binding
Acknowledgements by securing mobility messages between the mobile
node and the home agent.
IPsec can provide anti-replay protection only if dynamic keying is
used (which may not always be the case). IPsec does not guarantee
correct ordering of packets, only that they have not been replayed.
Because of this, sequence numbers within the Mobile IPv6 messages are
used to ensure correct ordering (see Section 5.1). However, if the
16 bit Mobile IPv6 sequence number space is cycled through, or the
home agent reboots and loses its state regarding the sequence
Johnson, et al. Standard Track [Page 145]
RFC 3775 Mobility Support in IPv6 June 2004
numbers, replay and reordering attacks become possible. The use of
dynamic keying, IPsec anti-replay protection, and the Mobile IPv6
sequence numbers can together prevent such attacks. It is also
recommended that use of non-volatile storage be considered for home
agents, to avoid losing their state.
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.
The above mechanisms do not show that the care-of address given in
the Binding Update is correct. This opens the possibility for
Denial-of-Service attacks against third parties. However, since the
mobile node and home agent have a security association, the home
agent can always identify an ill-behaving mobile node. This allows
the home agent operator to discontinue the mobile node's service, and
possibly take further actions based on the business relationship with
the mobile node's owner.
Note that the use of a single pair of manually keyed security
associations conflicts with the generation of a new home address [18]
for the mobile node, or with the adoption of a new home subnet
prefix. This is because IPsec security associations are bound to the
used addresses. While certificate-based automatic keying alleviates
this problem to an extent, it is still necessary to ensure that a
given mobile node cannot send Binding Updates for the address of
another mobile node. In general, this leads to the inclusion of home
addresses in certificates in the Subject AltName field. This again
limits the introduction of new addresses without either manual or
automatic procedures to establish new certificates. Therefore, this
specification restricts the generation of new home addresses (for any
reason) to those situations where a security association or
certificate for the new address already exists. (Appendix B.4 lists
the improvement of security for new addresses as one of the future
developments for Mobile IPv6.)
Support for IKE has been specified as optional. The following should
be observed about the use of manual keying:
o As discussed above, with manually keyed IPsec, only a limited form
of protection exists against replay and reordering attacks. A
vulnerability exists if either the sequence number space is cycled
through, or if the home agent reboots and forgets its sequence
numbers (and uses volatile memory to store the sequence numbers).
Assuming the mobile node moves continuously every 10 minutes, it
Johnson, et al. Standard Track [Page 146]
RFC 3775 Mobility Support in IPv6 June 2004
takes roughly 455 days before the sequence number space has been
cycled through. Typical movement patterns rarely reach this high
frequency today.
o A mobile node and its home agent belong to the same domain. If
this were not the case, manual keying would not be possible [28],
but in Mobile IPv6 only these two parties need to know the
manually configured keys. Similarly, we note that Mobile IPv6
employs standard block ciphers in IPsec, and is not vulnerable to
problems associated with stream ciphers and manual keying.
o It is expected that the owner of the mobile node and the
administrator of the home agent agree on the used keys and other
parameters with some off-line mechanism.
The use of IKEv1 with Mobile IPv6 is documented in more detail in
[21]. The following should be observed from the use of IKEv1:
o It is necessary to prevent a mobile node from claiming another
mobile node's home address. The home agent must verify that the
mobile node trying to negotiate the SA for a particular home
address is authorized for that home address. This implies that
even with the use of IKE, a policy entry needs to be configured
for each home address served by the home agent.
It may be possible to include home addresses in the Subject
AltName field of certificate to avoid this. However,
implementations are not guaranteed to support the use of a
particular IP address (care-of address) while another address
(home address) appears in the certificate. In any case, even this
approach would require user-specific tasks in the certificate
authority.
o If preshared secret authentication is used, IKEv1 main mode cannot
be used. Aggressive mode or group preshared secrets need to be
used with corresponding security implications instead.
Note that, like many other issues, this is a general IKEv1 issue
related to the ability to use different IP addresses, and not
specifically related to Mobile IPv6. For further information, see
Section 4.4 in [21].
o Due to the problems outlined in Section 11.3.2, IKE phase 1
between the mobile node and its home agent is established using
the mobile node's current care-of address. This implies that when
the mobile node moves to a new location, it may have to re-
establish phase 1. A Key Management Mobility Capability (K) flag
Johnson, et al. Standard Track [Page 147]
RFC 3775 Mobility Support in IPv6 June 2004
is provided for implementations that can update the IKE phase 1
endpoints without re-establishing phase 1, but the support for
this behavior is optional.
o When certificates are used, IKE fragmentation can occur as
discussed in Section 7 in [21].
o Nevertheless, even if per-mobile node configuration is required
with IKE, an important benefit of IKE is that it automates the
negotiation of cryptographic parameters, including the SPIs,
cryptographic algorithms, and so on. Thus, less configuration
information is needed.
o The frequency of movements in some link layers or deployment
scenarios may be high enough to make replay and reordering attacks
possible, if only manual keying is used. IKE SHOULD be used in
such cases. Potentially vulnerable scenarios involve continuous
movement through small cells, or uncontrolled alternation between
available network attachment points.
o Similarly, in some deployment scenarios the number of mobile nodes
may be very large. In these cases, it can be necessary to use
automatic mechanisms to reduce the management effort in the
administration of cryptographic parameters, even if some per-
mobile node configuration is always needed. IKE SHOULD also be
used in such cases.
o Other automatic key management mechanisms exist beyond IKEv1, but
this document does not address the issues related to them. We
note, however, that most of the above discussion applies to IKEv2
[30] as well, at least as it is currently specified.
15.4. Binding Updates to Correspondent Nodes
The motivation for designing the return routability procedure was to
have sufficient support for Mobile IPv6, without creating significant
new security problems. The goal for this procedure was not to
protect against attacks that were already possible before the
introduction of Mobile IPv6.
The next sections will describe the security properties of the used
method, both from the point of view of possible on-path attackers who
can see those cryptographic values that have been sent in the clear
(Section 15.4.2 and Section 15.4.3) and from the point of view of
other attackers (Section 15.4.6).
Johnson, et al. Standard Track [Page 148]
RFC 3775 Mobility Support in IPv6 June 2004
15.4.1. Overview
The chosen infrastructureless method verifies that the mobile node is
"live" (that is, it responds to probes) at its home and care-of
addresses. Section 5.2 describes the return routability procedure in
detail. The procedure uses the following principles:
o A message exchange verifies that the mobile node is reachable at
its addresses, i.e., is at least able to transmit and receive
traffic at both the home and care-of addresses.
o The eventual Binding Update is cryptographically bound to the
tokens supplied in the exchanged messages.
o Symmetric exchanges are employed to avoid the use of this protocol
in reflection attacks. In a symmetric exchange, the responses are
always sent to the same address the request was sent from.
o The correspondent node operates in a stateless manner until it
receives a fully authorized Binding Update.
o Some additional protection is provided by encrypting the tunnels
between the mobile node and home agent with IPsec ESP. As the
tunnel also transports the nonce exchanges, the ability of
attackers to see these nonces is limited. For instance, this
prevents attacks from being launched from the mobile node's
current foreign link, even when no link-layer confidentiality is
available.
The resulting level of security is in theory the same even without
this additional protection: the return routability tokens are
still exposed only to one path within the whole Internet.
However, the mobile nodes are often found on an insecure link,
such as a public access Wireless LAN. Thus, in many cases, this
addition makes a practical difference.
For further information about the design rationale of the return
routability procedure, see [27, 34, 33, 32]. The mechanisms used
have been adopted from these documents.
15.4.2. Achieved Security Properties
The return routability procedure protects Binding Updates against all
attackers who are unable to monitor the path between the home agent
and the correspondent node. The procedure does not defend against
attackers who can monitor this path. Note that such attackers are in
any case able to mount an active attack against the mobile node when
Johnson, et al. Standard Track [Page 149]
RFC 3775 Mobility Support in IPv6 June 2004
it is at its home location. The possibility of such attacks is not
an impediment to the deployment of Mobile IPv6 because these attacks
are possible regardless of whether or not Mobile IPv6 is in use.
This procedure also protects against Denial-of-Service attacks in
which the attacker pretends to be mobile, but uses the victim's
address as the care-of address. This would cause the correspondent
node to send the victim some unexpected traffic. This procedure
defends against these attacks by requiring at least the passive
presence of the attacker at the care-of address or on the path from
the correspondent to the care-of address. Normally, this will be the
mobile node.
15.4.3. Comparison to Regular IPv6 Communications
This section discusses the protection offered by the return
routability method by comparing it to the security of regular IPv6
communications. We will divide vulnerabilities into three classes:
(1) those related to attackers on the local network of the mobile
node, home agent, or the correspondent node, (2) those related to
attackers on the path between the home network and the correspondent
node, and (3) off-path attackers, i.e., the rest of the Internet.
We will now discuss the vulnerabilities of regular IPv6
communications. The on-link vulnerabilities of IPv6 communications
include Denial-of-Service, Masquerading, Man-in-the-Middle,
Eavesdropping, and other attacks. These attacks can be launched
through spoofing Router Discovery, Neighbor Discovery and other IPv6
mechanisms. Some of these attacks can be prevented with the use of
cryptographic protection in the packets.
A similar situation exists with on-path attackers. That is, without
cryptographic protection, the traffic is completely vulnerable.
Assuming that attackers have not penetrated the security of the
Internet routing protocols, attacks are much harder to launch from
off-path locations. Attacks that can be launched from these
locations are mainly Denial-of-Service attacks, such as flooding and/
or reflection attacks. It is not possible for an off-path attacker
to become a Man-in-the-Middle.
Next, we will consider the vulnerabilities that exist when IPv6 is
used together with Mobile IPv6 and the return routability procedure.
On the local link, the vulnerabilities are the same as those in IPv6,
but Masquerade and Man-in-the-Middle attacks can now also be launched
against future communications, and not just against current
communications. If a Binding Update was sent while the attacker was
present on the link, its effects remain for the lifetime of the
Johnson, et al. Standard Track [Page 150]
RFC 3775 Mobility Support in IPv6 June 2004
binding. This happens even if the attacker moves away from the link.
In contrast, an attacker who uses only plain IPv6 generally has to
stay on the link in order to continue the attack. Note that in order
to launch these new attacks, the IP address of the victim must be
known. This makes this attack feasible, mainly in the context of
well-known interface IDs, such as those already appearing in the
traffic on the link or registered in the DNS.
On-path attackers can exploit similar vulnerabilities as in regular
IPv6. There are some minor differences, however. Masquerade, Man-
in-the-Middle, and Denial-of-Service attacks can be launched with
just the interception of a few packets, whereas in regular IPv6 it is
necessary to intercept every packet. The effect of the attacks is
the same regardless of the method, however. In any case, the most
difficult task an attacker faces in these attacks is getting on the
right path.
The vulnerabilities for off-path attackers are the same as in regular
IPv6. Those nodes that are not on the path between the home agent
and the correspondent node will not be able to receive the home
address probe messages.
In conclusion, we can state the following main results from this
comparison:
o Return routability prevents any off-path attacks beyond those that
are already possible in regular IPv6. This is the most important
result, preventing attackers on the Internet from exploiting any
vulnerabilities.
o Vulnerabilities to attackers on the home agent link, the
correspondent node link, and the path between them are roughly the
same as in regular IPv6.
o However, one difference is that in basic IPv6 an on-path attacker
must be constantly present on the link or the path, whereas with
Mobile IPv6, an attacker can leave a binding behind after moving
away.
For this reason, this specification limits the creation of
bindings to at most MAX_TOKEN_LIFETIME seconds after the last
routability check has been performed, and limits the duration of a
binding to at most MAX_RR_BINDING_LIFETIME seconds. With these
limitations, attackers cannot take any practical advantages of
this vulnerability.
Johnson, et al. Standard Track [Page 151]
RFC 3775 Mobility Support in IPv6 June 2004
o There are some other minor differences, such as an effect to the
Denial-of-Service vulnerabilities. These can be considered to be
insignificant.
o 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. The security on layer 2 of the links plays
then a major role in the resulting overall network security.
Similarly, security of IPv6 Neighbor and Router Discovery on these
links has a large impact. If these were secured using some new
technology in the future, this could change the situation
regarding the easiest point of attack.
For a more in-depth discussion of these issues, see [32].
15.4.4. Replay Attacks
The return routability procedure also protects the participants
against replayed Binding Updates. The attacker is unable replay the
same message due to the sequence number which is a part of the
Binding Update. It is also unable to modify the Binding Update since
the MAC verification would fail after such a modification.
Care must be taken when removing bindings at the correspondent node,
however. If a binding is removed while the nonce used in its
creation is still valid, an attacker could replay the old Binding
Update. Rules outlined in Section 5.2.8 ensure that this cannot
happen.
15.4.5. Denial-of-Service Attacks
The return routability procedure has protection against resource
exhaustion Denial-of-Service attacks. The correspondent nodes do not
retain any state about individual mobile nodes until an authentic
Binding Update arrives. This is achieved through the construct of
keygen tokens from the nonces and node keys that are not specific to
individual mobile nodes. The keygen tokens can be reconstructed by
the correspondent node, 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
symmetric cryptography, the correspondent nodes are relatively safe
against CPU resource exhaustion attacks as well.
Johnson, et al. Standard Track [Page 152]
RFC 3775 Mobility Support in IPv6 June 2004
Nevertheless, as [27] 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.
Even if route optimization is a very important optimization, it is
still only an optimization. A mobile node can communicate with a
correspondent node even if the correspondent refuses to accept any
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 these resource exhaustion
attacks as follows. If the correspondent node is flooded with a
large number of Binding Updates that fail the cryptographic integrity
checks, it can stop processing Binding Updates. 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 silently discard some or all Binding
Updates without performing any cryptographic operations.
Layers above IP can usually provide additional information to help
decide if there is a need to establish a binding with a specific
peer. For example, TCP knows if the node has a queue of data that it
is trying to send to a peer. An implementation of this specification
is not required to make use of information from higher protocol
layers, but some implementations are likely to be able to manage
resources more effectively by making use of such information.
We also require that all implementations be capable of
administratively disabling route optimization.
15.4.6. Key Lengths
Attackers can try to break the return routability procedure in many
ways. Section 15.4.2 discusses the situation where the attacker can
see the cryptographic values sent in the clear, and Section 15.4.3
discusses the impact this has on IPv6 communications. This section
discusses whether attackers can guess the correct values without
seeing them.
While the return routability procedure is in progress, 64 bit cookies
are used to protect spoofed responses. This is believed to be
sufficient, given that to blindly spoof a response a very large
number of messages would have to be sent before success would be
probable.
Johnson, et al. Standard Track [Page 153]
RFC 3775 Mobility Support in IPv6 June 2004
The tokens used in the return routability procedure provide together
128 bits of information. This information is used internally as
input to a hash function to produce a 160 bit quantity suitable for
producing the keyed hash in the Binding Update using the HMAC_SHA1
algorithm. The final keyed hash length is 96 bits. The limiting
factors in this case are the input token lengths and the final keyed
hash length. The internal hash function application does not reduce
the entropy.
The 96 bit final keyed hash is of typical size and is believed to be
secure. The 128 bit input from the tokens is broken in two pieces,
the home keygen token and the care-of keygen token. An attacker can
try to guess the correct cookie value, but again this would require a
large number of messages (an the average 2**63 messages for one or
2**127 for two). Furthermore, given that the cookies are valid only
for a short period of time, the attack has to keep a high constant
message rate to achieve a lasting effect. This does not appear
practical.
When the mobile node is returning home, it is allowed to use just the
home keygen token of 64 bits. This is less than 128 bits, but
attacking it blindly would still require a large number of messages
to be sent. If the attacker is on the path and capable of seeing the
Binding Update, it could conceivably break the keyed hash with brute
force. However, in this case the attacker has to be on the path,
which appears to offer easier ways for denial-of-service than
preventing route optimization.
15.5. Dynamic Home Agent Address Discovery
The dynamic home agent address discovery function could be used to
learn the addresses of home agents in the home network.
The ability to learn addresses of nodes may be useful to attackers
because brute-force scanning of the address space is not practical
with IPv6. Thus, they could benefit from any means which make
mapping the networks easier. For example, if a security threat
targeted at routers or even home agents is discovered, having a
simple ICMP mechanism to easily find out possible targets may prove
to be an additional (though minor) security risk.
Apart from discovering the address(es) of home agents, attackers will
not be able to learn much from this information, and mobile nodes
cannot be tricked into using wrong home agents, as all other
communication with the home agents is secure.
Johnson, et al. Standard Track [Page 154]
RFC 3775 Mobility Support in IPv6 June 2004
15.6. Mobile Prefix Discovery
The mobile prefix discovery function may leak interesting information
about network topology and prefix lifetimes to eavesdroppers; for
this reason, requests for this information has to be authenticated.
Responses and unsolicited prefix information needs to be
authenticated to prevent the mobile nodes from being tricked into
believing false information about the prefixes and possibly
preventing communications with the existing addresses. Optionally,
encryption may be applied to prevent leakage of the prefix
information.
15.7. Tunneling via the Home Agent
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
5.5.
Binding Updates to the home agents are secure. When receiving
tunneled traffic, the home agent verifies that the outer IP address
corresponds to the current location of the mobile node. This acts as
a weak form of protection against spoofing packets that appear to
come from the mobile node. This is particularly useful, if no end-
to-end security is being applied between the mobile and correspondent
nodes. The outer IP address check prevents attacks where the
attacker is controlled by ingress filtering. It also prevents
attacks when 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 send
packets that appear to come from the mobile node without attacking
the tunnel; the attacker could simply send packets with the source
address set to the mobile node's home address. However, this attack
does not work if the final destination of the packet is in the home
network, and some form of perimeter defense is being applied for
packets sent to those destinations. In such cases it is recommended
that either end-to-end security or additional tunnel protection be
applied, as is usual in remote access situations.
Home agents and mobile nodes may use IPsec ESP to protect payload
packets tunneled between themselves. This is useful for protecting
communications against attackers on the path of the tunnel.
When site local home addresses 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
Johnson, et al. Standard Track [Page 155]
RFC 3775 Mobility Support in IPv6 June 2004
addresses. In particular, the outer IP address check described above
is not sufficient against all attackers. The use of encrypted
tunnels is particularly useful for these kinds of home addresses.
15.8. Home Address Option
When the mobile node sends packets directly to the correspondent
node, the Source Address field of the packet's IPv6 header is the
care-of address. Therefore, ingress filtering [26] works in the
usual manner even for mobile nodes, as the Source Address is
topologically correct. The Home Address option is used to inform the
correspondent node of the mobile node's home address.
However, the care-of address in the Source Address field does not
survive in replies sent by the correspondent node unless it has a
binding for this mobile node. Also, not all attacker tracing
mechanisms work when packets are being reflected through
correspondent nodes using the Home Address option. For these
reasons, this specification restricts the use of the Home Address
option. It may only be used when a binding has already been
established with the participation of the node at the home address,
as described in Section 5.5 and Section 6.3. This prevents
reflection attacks through the use of the Home Address option. It
also ensures that the correspondent nodes reply to the same address
that the mobile node sends traffic from.
No special authentication of the Home Address option is required
beyond the above, but note that if the IPv6 header of a packet is
covered by IPsec Authentication Header, then that authentication
covers the Home Address option as well. 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, any
field in the IPv6 header, including the Source Address field or any
other part of the packet and 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.
15.9. Type 2 Routing Header
The definition of the type 2 routing header is described in Section
6.4. This definition and the associated processing rules have been
chosen so that the header cannot be used for what is traditionally
viewed as source routing. In particular, the Home Address in the
routing header will always have to be assigned to the home address of
the receiving node; otherwise the packet will be dropped.
Johnson, et al. Standard Track [Page 156]
RFC 3775 Mobility Support in IPv6 June 2004
Generally, source routing has a number of security concerns. These
include the automatic reversal of unauthenticated source routes
(which is an issue for IPv4, but not for IPv6). Another concern is
the ability to use source routing to "jump" between nodes inside, as
well as outside a firewall. These security concerns are not issues
in Mobile IPv6, due to the rules mentioned above.
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.
This implies that a device which implements the filtering of packets
should be able to distinguish between a type 2 routing header and
other routing headers, as required in Section 8.3. This is necessary
in order to allow Mobile IPv6 traffic while still having the option
of filtering out other uses of routing headers.
16. Contributors
Tuomas Aura, Mike Roe, Greg O'Shea, Pekka Nikander, Erik Nordmark,
and Michael Thomas worked on the return routability protocols
eventually led to the procedures used in this protocol. The
procedures described in [34] were adopted in the protocol.
Significant contributions were made by members of the Mobile IPv6
Security Design Team, including (in alphabetical order) Gabriel
Montenegro, Erik Nordmark and Pekka Nikander.
17. 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, Josh
Broch, Samita Chakrabarti, Robert Chalmers, Noel Chiappa, Greg Daley,
Vijay Devarapalli, Rich Draves, Francis Dupont, Thomas Eklund, Jun-
Ichiro Itojun Hagino, Brian Haley, Marc Hasson, John Ioannidis, James
Kempf, Rajeev Koodli, Krishna Kumar, T.J. Kniveton, Joe Lau, Jiwoong
Lee, Aime Le Rouzic, Vesa-Matti Mantyla, Kevin Miles, Glenn Morrow,
Thomas Narten, Karen Nielsen, Simon Nybroe, David Oran, Brett
Pentland, Lars Henrik Petander, Basavaraj Patil, Mohan Parthasarathy,
Alexandru Petrescu, Mattias Petterson, Ken Powell, Phil Roberts, Ed
Remmell, Patrice Romand, Luis A. Sanchez, Jeff Schiller, Pekka
Savola, Arvind Sevalkar, Keiichi Shima, Tom Soderlund, Hesham
Soliman, Jim Solomon, Tapio Suihko, Dave Thaler, Benny Van Houdt,
Jon-Olov Vatn, Carl E. Williams, Vladislav Yasevich, Alper Yegin, and
Johnson, et al. Standard Track [Page 157]
RFC 3775 Mobility Support in IPv6 June 2004
Xinhua Zhao, 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 of the Mobile IPv6
testing event (1999), implementors who participated in Mobile IPv6
interoperability testing at Connectathons (2000, 2001, 2002, and
2003), and the participants at the ETSI interoperability testing
(2000, 2002). Finally, we would like to thank the TAHI project who
has provided test suites for Mobile IPv6.
18. References
18.1. Normative References
[1] Eastlake 3rd., D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[4] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[5] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[6] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[7] Piper, D., "The Internet IP Security Domain of Interpretation
for ISAKMP", RFC 2407, November 1998.
[8] Maughan, D., Schertler, M., Schneider, M. and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[9] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[10] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
Johnson, et al. Standard Track [Page 158]
RFC 3775 Mobility Support in IPv6 June 2004
[11] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[12] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[13] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[14] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
[15] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6
Specification", RFC 2473, December 1998.
[16] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
Addresses", RFC 2526, March 1999.
[17] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[18] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[19] Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced by
an On-line Database", RFC 3232, January 2002.
[20] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS PUB 180-1, April 1995, <http://
www.itl.nist.gov/fipspubs/fip180-1.htm>.
[21] Arkko, J., Devarapalli, V. and F. Dupont, "Using IPsec to
Protect Mobile IPv6 Signaling Between Mobile Nodes and Home
Agents", RFC 3776, June 2004.
18.2. Informative References
[22] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
August 2002.
[23] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
1996.
[24] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
Johnson, et al. Standard Track [Page 159]
RFC 3775 Mobility Support in IPv6 June 2004
[25] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[26] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[27] Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses", Work in
Progress, March 2002.
[28] Bellovin, S., "Guidelines for Mandating Automated Key
Management", Work in Progress, August 2003.
[29] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C. and
M. Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[30] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", Work in
Progress, April 2003.
[31] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[32] Nikander, P., Aura, T., Arkko, J., Montenegro, G. and E.
Nordmark, "Mobile IP version 6 Route Optimization Security
Design Background", Work in Progress, April 2003.
[33] Nordmark, E., "Securing MIPv6 BUs using return routability
(BU3WAY)", Work in Progress, November 2001.
[34] Roe, M., Aura, T., O'Shea, G. and J. Arkko, "Authentication of
Mobile IPv6 Binding Updates and Acknowledgments", Work in
Progress, March 2002.
[35] Savola, P., "Use of /127 Prefix Length Between Routers
Considered Harmful", RFC 3627, September 2003.
[36] Savola, P., "Security of IPv6 Routing Header and Home Address
Options", Work in Progress, December 2002.
[37] Vida, R. and L. Costa, Eds., "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
Johnson, et al. Standard Track [Page 160]
RFC 3775 Mobility Support in IPv6 June 2004
Appendix A. Future Extensions
A.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 issues such as the
interaction between piggybacking and IPsec are fully resolved (see
also Appendix A.3). The return routability messages can indicate
support for piggybacking with a new mobility option.
A.2. Triangular Routing
Due to the concerns about opening reflection attacks with the Home
Address destination option, this specification requires that this
option 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.
A.3. New Authorization Methods
While the return routability procedure provides a good level of
security, there exist methods that have even higher levels of
security. Secondly, as discussed in Section 15.4, future
enhancements of IPv6 security may cause a need to also improve the
security of the return routability procedure. 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.
A.4. Dynamically Generated Home Addresses
A future version of this specification may include functionality that
allows the generation of new home addresses without requiring pre-
arranged security associations or certificates even for the new
addresses.
Johnson, et al. Standard Track [Page 161]
RFC 3775 Mobility Support in IPv6 June 2004
A.5. Remote Home Address Configuration
The method for initializing a mobile node's home address upon 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 while the
mobile node is disconnected, a robust mobile node would not rely
solely on storing these addresses locally.
Such a mobile node could be initialized by using the following
procedure:
1. Generate a care-of address.
2. Query DNS for an anycast address associated with the FQDN of the
home agent(s).
3. Perform home agent address discovery, and select a home agent.
4. Configure one home address based on the selected home agent's
subnet prefix and the interface identifier of the mobile node.
5. Create security associations and security policy database entries
for protecting the traffic between the selected home address and
home agent.
6. Perform a home registration on the selected home agent.
7. Perform mobile prefix discovery.
8. Make a decision if further home addresses need to be configured.
This procedure is restricted to those situations where the home
prefix is 64 bits and the mobile node knows its own interface
identifier, which is also 64 bits.
Johnson, et al. Standard Track [Page 162]
RFC 3775 Mobility Support in IPv6 June 2004
A.6. Neighbor Discovery Extensions
Future specifications may improve the efficiency of Neighbor
Discovery tasks, which could be helpful for fast movements. One
factor is currently being looked at: the delays caused by the
Duplicate Address Detection mechanism. Currently, Duplicate Address
Detection needs to be performed for every new care-of address as the
mobile node moves, and for the mobile node's link-local address on
every new link. In particular, the need and the trade-offs of re-
performing Duplicate Address Detection for the link-local address
every time the mobile node moves on to new links will need to be
examined. Improvements in this area are, however, generally
applicable and progress independently from the Mobile IPv6
specification.
Future functional improvements may also be relevant for Mobile IPv6
and other applications. For instance, mechanisms that would allow
recovery from a Duplicate Address Detection collision would be useful
for link-local, care-of, and home addresses.
Johnson, et al. Standard Track [Page 163]
RFC 3775 Mobility Support in IPv6 June 2004
Authors' Addresses
David B. Johnson
Rice University
Dept. of Computer Science, MS 132
6100 Main Street
Houston TX 77005-1892
USA
EMail: dbj@cs.rice.edu
Charles E. Perkins
Nokia Research Center
313 Fairchild Drive
Mountain View CA 94043
USA
EMail: charliep@iprg.nokia.com
Jari Arkko
Ericsson
02420 Jorvas
Finland
EMail: jari.arkko@ericsson.com
Johnson, et al. Standard Track [Page 164]
RFC 3775 Mobility Support in IPv6 June 2004
Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Johnson, et al. Standard Track [Page 165]