IETF Mobile IP Working Group David B. Johnson
INTERNET-DRAFT Rice University
Charles E. Perkins
Nokia Research Center
Jari Arkko
Ericsson
29 Oct 2002
Mobility Support in IPv6
<draft-ietf-mobileip-ipv6-19.txt>
Status of This Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note
that other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents, valid for a maximum of six
months, and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This document specifies the operation of the IPv6 Internet with
mobile computers. 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, Perkins, Arkko Expires 29 April 2003 [Page i]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Comparison with Mobile IP for IPv4 2
3. Terminology 3
3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 5
4. Overview of Mobile IPv6 7
4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 7
4.2. New IPv6 Protocol . . . . . . . . . . . . . . . . . . . . 9
4.3. New IPv6 Destination Option . . . . . . . . . . . . . . . 10
4.4. New IPv6 ICMP Messages . . . . . . . . . . . . . . . . . 10
4.5. Conceptual Data Structure Terminology . . . . . . . . . . 11
4.6. Site-Local Addressability . . . . . . . . . . . . . . . . 11
5. Overview of Mobile IPv6 Security 12
5.1. Binding Updates to Home Agents . . . . . . . . . . . . . 12
5.2. Binding Updates to Correspondent Nodes . . . . . . . . . 13
5.2.1. Node Keys . . . . . . . . . . . . . . . . . . . . 13
5.2.2. Nonces . . . . . . . . . . . . . . . . . . . . . 13
5.2.3. Cookies and Tokens . . . . . . . . . . . . . . . 14
5.2.4. Cryptographic Functions . . . . . . . . . . . . . 15
5.2.5. Return Routability Procedure . . . . . . . . . . 15
5.2.6. Authorizing Binding Management Messages . . . . . 19
5.2.7. Updating Node Keys and Nonces . . . . . . . . . . 20
5.2.8. Preventing Replay Attacks . . . . . . . . . . . . 22
5.3. Dynamic Home Agent Address Discovery . . . . . . . . . . 22
5.4. Prefix Discovery . . . . . . . . . . . . . . . . . . . . 22
5.5. Payload Packets . . . . . . . . . . . . . . . . . . . . . 22
6. New IPv6 Protocol, Message Types, and Destination Option 23
6.1. Mobility Header . . . . . . . . . . . . . . . . . . . . . 23
6.1.1. Format . . . . . . . . . . . . . . . . . . . . . 23
6.1.2. Binding Refresh Request Message . . . . . . . . . 25
6.1.3. Home Test Init Message . . . . . . . . . . . . . 26
6.1.4. Care-of Test Init Message . . . . . . . . . . . . 27
6.1.5. Home Test Message . . . . . . . . . . . . . . . . 28
6.1.6. Care-of Test Message . . . . . . . . . . . . . . 29
6.1.7. Binding Update Message . . . . . . . . . . . . . 31
6.1.8. Binding Acknowledgement Message . . . . . . . . . 33
6.1.9. Binding Error Message . . . . . . . . . . . . . . 35
Johnson, Perkins, Arkko Expires 29 April 2003 [Page ii]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
6.2. Mobility Options . . . . . . . . . . . . . . . . . . . . 36
6.2.1. Format . . . . . . . . . . . . . . . . . . . . . 37
6.2.2. Pad1 . . . . . . . . . . . . . . . . . . . . . . 37
6.2.3. PadN . . . . . . . . . . . . . . . . . . . . . . 38
6.2.4. Alternate Care-of Address . . . . . . . . . . . . 38
6.2.5. Nonce Indices . . . . . . . . . . . . . . . . . . 39
6.2.6. Binding Authorization Data . . . . . . . . . . . 39
6.2.7. Binding Refresh Advice . . . . . . . . . . . . . 40
6.3. Home Address Option . . . . . . . . . . . . . . . . . . . 41
6.4. Type 2 Routing Header . . . . . . . . . . . . . . . . . . 43
6.4.1. Format . . . . . . . . . . . . . . . . . . . . . 43
6.5. ICMP Home Agent Address Discovery Request Message . . . . 44
6.6. ICMP Home Agent Address Discovery Reply Message . . . . . 46
6.7. ICMP Mobile Prefix Solicitation Message Format . . . . . 47
6.8. ICMP Mobile Prefix Advertisement Message Format . . . . . 49
7. Modifications to IPv6 Neighbor Discovery 51
7.1. Modified Router Advertisement Message Format . . . . . . 51
7.2. Modified Prefix Information Option Format . . . . . . . . 52
7.3. New Advertisement Interval Option Format . . . . . . . . 54
7.4. New Home Agent Information Option Format . . . . . . . . 55
7.5. Changes to Sending Router Advertisements . . . . . . . . 57
7.6. Changes to Sending Router Solicitations . . . . . . . . . 59
7.7. Changes to Duplicate Address Detection . . . . . . . . . 60
8. Requirements for Types of IPv6 Nodes 60
8.1. All IPv6 Nodes . . . . . . . . . . . . . . . . . . . . . 61
8.2. IPv6 Nodes with Support for Route Optimization . . . . . 61
8.3. All IPv6 Routers . . . . . . . . . . . . . . . . . . . . 62
8.4. IPv6 Home Agents . . . . . . . . . . . . . . . . . . . . 62
8.5. IPv6 Mobile Nodes . . . . . . . . . . . . . . . . . . . . 63
9. Correspondent Node Operation 65
9.1. Conceptual Data Structures . . . . . . . . . . . . . . . 65
9.2. Processing Mobility Headers . . . . . . . . . . . . . . . 66
9.3. Packet Processing . . . . . . . . . . . . . . . . . . . . 66
9.3.1. Receiving Packets with Home Address Destination
Option . . . . . . . . . . . . . . . . . . 66
9.3.2. Sending Packets to a Mobile Node . . . . . . . . 67
9.3.3. Sending Binding Error Messages . . . . . . . . . 68
9.3.4. Receiving ICMP Error Messages . . . . . . . . . . 69
9.4. Return Routability Procedure . . . . . . . . . . . . . . 69
9.4.1. Receiving Home Test Init Messages . . . . . . . . 69
9.4.2. Receiving Care-of Test Init Messages . . . . . . 70
9.4.3. Sending Home Test Messages . . . . . . . . . . . 70
9.4.4. Sending Care-of Test Messages . . . . . . . . . . 70
9.5. Processing Bindings . . . . . . . . . . . . . . . . . . . 70
9.5.1. Receiving Binding Updates . . . . . . . . . . . . 71
9.5.2. Requests to Cache a Binding . . . . . . . . . . . 73
9.5.3. Requests to Delete a Binding . . . . . . . . . . 74
9.5.4. Sending Binding Acknowledgements . . . . . . . . 74
Johnson, Perkins, Arkko Expires 29 April 2003 [Page iii]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
9.5.5. Sending Binding Refresh Requests . . . . . . . . 75
9.6. Cache Replacement Policy . . . . . . . . . . . . . . . . 75
10. Home Agent Operation 76
10.1. Conceptual Data Structures . . . . . . . . . . . . . . . 76
10.2. Processing Mobility Headers . . . . . . . . . . . . . . . 77
10.3. Processing Bindings . . . . . . . . . . . . . . . . . . . 77
10.3.1. Primary Care-of Address Registration . . . . . . 77
10.3.2. Primary Care-of Address De-Registration . . . . . 81
10.4. Packet Processing . . . . . . . . . . . . . . . . . . . . 82
10.4.1. Intercepting Packets for a Mobile Node . . . . . 82
10.4.2. Tunneling Intercepted Packets to a Mobile Node . 83
10.4.3. Handling Reverse Tunneled Packets from a Mobile
Node . . . . . . . . . . . . . . . . . . . 85
10.4.4. Protecting Return Routability Packets . . . . . . 85
10.5. Dynamic Home Agent Address Discovery . . . . . . . . . . 86
10.5.1. Receiving Router Advertisement Messages . . . . . 86
10.6. Sending Prefix Information to the Mobile Node . . . . . . 89
10.6.1. Aggregate List of Home Network Prefixes . . . . . 89
10.6.2. Scheduling Prefix Deliveries to the Mobile Node . 90
10.6.3. Sending Advertisements to the Mobile Node . . . . 92
10.6.4. Lifetimes for Changed Prefixes . . . . . . . . . 92
11. Mobile Node Operation 93
11.1. Conceptual Data Structures . . . . . . . . . . . . . . . 93
11.2. Processing Mobility Headers . . . . . . . . . . . . . . . 94
11.3. Packet Processing . . . . . . . . . . . . . . . . . . . . 95
11.3.1. Sending Packets While Away from Home . . . . . . 95
11.3.2. Interaction with Outbound IPsec Processing . . . 97
11.3.3. Receiving Packets While Away from Home . . . . . 99
11.3.4. Receiving ICMP Error Messages . . . . . . . . . . 100
11.3.5. Routing Multicast Packets . . . . . . . . . . . . 101
11.4. Home Agent and Prefix Management . . . . . . . . . . . . 102
11.4.1. Dynamic Home Agent Address Discovery . . . . . . 102
11.4.2. Sending Mobile Prefix Solicitations . . . . . . . 103
11.4.3. Receiving Mobile Prefix Advertisements . . . . . 104
11.5. Movement . . . . . . . . . . . . . . . . . . . . . . . . 105
11.5.1. Movement Detection . . . . . . . . . . . . . . . 105
11.5.2. Forming New Care-of Addresses . . . . . . . . . . 107
11.5.3. Using Multiple Care-of Addresses . . . . . . . . 109
11.5.4. Returning Home . . . . . . . . . . . . . . . . . 109
11.6. Return Routability Procedure . . . . . . . . . . . . . . 111
11.6.1. Sending Home and Care-of Test Init Messages . . . 111
11.6.2. Receiving Return Routability Messages . . . . . . 112
11.6.3. Protecting Return Routability Packets . . . . . . 113
11.7. Processing Bindings . . . . . . . . . . . . . . . . . . . 114
11.7.1. Sending Binding Updates to the Home Agent . . . . 114
11.7.2. Correspondent Binding Procedure . . . . . . . . . 116
11.7.3. Receiving Binding Acknowledgements . . . . . . . 119
11.7.4. Receiving Binding Refresh Requests . . . . . . . 121
11.7.5. Receiving Binding Error Messages . . . . . . . . 121
Johnson, Perkins, Arkko Expires 29 April 2003 [Page iv]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
11.8. Retransmissions and Rate Limiting . . . . . . . . . . . . 122
12. Protocol Constants 123
13. IANA Considerations 124
14. Security Considerations 125
14.1. Threats . . . . . . . . . . . . . . . . . . . . . . . . . 125
14.2. Features . . . . . . . . . . . . . . . . . . . . . . . . 127
14.3. Binding Updates to Home Agent . . . . . . . . . . . . . . 128
14.4. Binding Updates to Correspondent Nodes . . . . . . . . . 130
14.4.1. Overview . . . . . . . . . . . . . . . . . . . . 130
14.4.2. Offered Protection . . . . . . . . . . . . . . . 131
14.4.3. Comparison to Regular IPv6 Communications . . . . 131
14.4.4. Return Routability Replays . . . . . . . . . . . 133
14.4.5. Return Routability Denial-of-Service . . . . . . 133
14.5. Dynamic Home Agent Address Discovery . . . . . . . . . . 134
14.6. Prefix Discovery . . . . . . . . . . . . . . . . . . . . 135
14.7. Tunneling via the Home Agent . . . . . . . . . . . . . . 135
14.8. Home Address Option . . . . . . . . . . . . . . . . . . . 135
14.9. Type 2 Routing Header . . . . . . . . . . . . . . . . . . 136
Contributors 137
Acknowledgements 137
References 139
A. Changes from Previous Version of the Draft 142
A.1. Changes from Draft Version 18 . . . . . . . . . . . . . . 142
B. Future Extensions 146
B.1. Piggybacking . . . . . . . . . . . . . . . . . . . . . . 146
B.2. Triangular Routing and Unverified Home Addresses . . . . 146
B.3. New Authorization Methods beyond Return Routability . . . 146
B.4. Security and Dynamically Generated Home Addresses . . . . 147
B.5. Remote Home Address Configuration . . . . . . . . . . . . 147
Chairs' Addresses 149
Authors' Addresses 149
Johnson, Perkins, Arkko Expires 29 April 2003 [Page v]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
1. Introduction
This document specifies how the IPv6 Internet operates with mobile
computers. Without specific support for mobility in IPv6 [11],
packets destined to a mobile node would not be able to reach it while
the mobile node is away from its home link. In order to continue
communication in spite of its movement, a mobile node could change
its IP address each time it moves to a new link, but the mobile
node would then not be able to maintain transport and higher-layer
connections when it changes location. Mobility support in IPv6 is
particularly important, as mobile computers are likely to account for
a majority or at least a substantial fraction of the population of
the Internet during the lifetime of IPv6.
The protocol defined in this document, known as Mobile IPv6, allows
a mobile node to move from one link to another without changing the
mobile node's "home address". Packets may be routed to the mobile
node using this address regardless of the mobile node's current point
of attachment to the Internet. The mobile node may also continue
to communicate with other nodes (stationary or mobile) after moving
to a new link. The movement of a mobile node away from its home
link is thus transparent to transport and higher-layer protocols and
applications.
The Mobile IPv6 protocol is just as suitable for mobility across
homogeneous media as for mobility across heterogeneous media. For
example, Mobile IPv6 facilitates node movement from one Ethernet
segment to another as well as it facilitates node movement from an
Ethernet segment to a wireless LAN cell, with the mobile node's IP
address remaining unchanged in spite of such movement.
One can think of the Mobile IPv6 protocol as solving the
network-layer mobility management problem. Some mobility management
applications -- for example, handover among wireless transceivers,
each of which covers only a very small geographic area -- have been
solved using link-layer techniques. For example, in many current
wireless LAN products, link-layer mobility mechanisms allow a
"handover" of a mobile node from one cell to another, re-establishing
link-layer connectivity to the node in each new location.
Mobile IPv6 does not attempt to solve all general problems related
to the use of mobile computers or wireless networks. In particular,
this protocol does not attempt to solve:
- Handling links with partial reachability, or unidirectional
connectivity, such as are often found in wireless networks (but
see Section 11.5.1).
- Access control on a link being visited by a mobile node.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 1]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- Local or hierarchical forms of mobility management (similar to
many current link-layer mobility management solutions).
- Assistance for adaptive applications
- Mobile routers
- Service Discovery
- Distinguishing between packets lost due to bit errors vs.
network congestion
2. Comparison with Mobile IP for IPv4
The design of Mobile IP support in IPv6 (Mobile IPv6) benefits both
from the experiences gained from the development of Mobile IP support
in IPv4 (Mobile IPv4) [20, 21, 22], and from the opportunities
provided by IPv6. Mobile IPv6 thus shares many features with
Mobile IPv4, but is integrated into IPv6 and offers many other
improvements. This section summarizes the major differences between
Mobile IPv4 and Mobile IPv6:
- 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.
- Support for route optimization is a fundamental part of the
protocol, rather than a nonstandard set of extensions.
- 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.
- Support is also integrated into Mobile IPv6 for allowing route
optimization to coexist efficiently with routers that perform
"ingress filtering" [23].
- In Mobile IPv6, the mobile node does not have to tunnel multicast
packets to its home agent.
- The movement detection mechanism in Mobile IPv6 provides
bidirectional confirmation of a mobile node's ability to
communicate with its default router in its current location.
- 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 2]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- 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.
- The use of IPv6 encapsulation (and the routing header) removes
the need in Mobile IPv6 to manage "tunnel soft state".
- 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 RFC 2119 [2].
3.1. General Terms
IP Internet Protocol Version 6 (IPv6).
node A device that implements IP.
router A node that forwards IP packets not explicitly
addressed to itself.
unicast routable address
An identifier for a single interface such that
a packet sent to it from another IPv6 subnet is
delivered to the interface identified by that
address. Accordingly, a unicast routable address must
have either a global or site-local scope (but not
link-local).
host Any node that is not a router.
link A communication facility or medium over which nodes
can communicate at the link layer, such as an Ethernet
(simple or bridged). A link is the layer immediately
below IP.
interface A node's attachment to a link.
subnet prefix
A bit string that consists of some number of initial
bits of an IP address.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 3]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
interface identifier
A number used to identify a node's interface on a
link. The interface identifier is the remaining
low-order bits in the node's IP address after the
subnet prefix.
link-layer address
A link-layer identifier for an interface, such as
IEEE 802 addresses on Ethernet links.
packet An IP header plus payload.
security association
A security object shared between two nodes which
includes the data mutually agreed on for operation of
some cryptographic algorithm (typically including a
key).
security policy database
A database of rules that describe what security
associations should be applied for different kinds of
packets.
destination option
Destination options are carried by the IPv6
Destination Options extension header. 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 other
intermediate routing nodes. 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 '|'
indicate bytewise concatenation, as in A | B. This
concatenation requires that all of the bytes of the
datum A appear first in the result, followed by all of
the bytes of the datum B.
First (size, input)
Some formulas in this specification use a functional
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 4]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
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".
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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 5]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
prefixes), the one registered with the mobile node's
home agent is called its "primary" care-of address.
home agent
A router on a mobile node's home link with which the
mobile node has registered its current care-of address.
While the mobile node is away from home, the home agent
intercepts packets on the home link destined to the
mobile node's home address, encapsulates them, and
tunnels them to the mobile node's registered care-of
address.
binding The association of the home address of a mobile node
with a care-of address for that mobile node, along with
the remaining lifetime of that association.
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 procedure
A binding procedure is initiated by the mobile node to
inform either a correspondent node or the mobile node's
home agent of the current binding of the mobile node.
binding authorization
Binding procedure needs to be authorized to allow the
recipient to believe that the sender has the right to
specify a new binding.
return routability procedure
The return routability procedure authorizes binding
procedures by the use of a cryptographic token exchange.
correspondent binding procedure
A return routability procedure followed by a
binding procedure, run between the mobile node and a
correspondent node.
home binding procedure
A binding procedure between the mobile node and its home
agent, authorized by the use of IPsec.
nonce Nonces are random numbers used internally by the
correspondent node in the creation of keygen tokens
related to the return routability procedure. The nonces
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 6]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
are not specific to a mobile node, and are kept secret
within the correspondent node.
nonce index
A nonce index is used to indicate which nonces have
been used when creating keygen token values, without
revealing the nonces themselves.
cookie A cookie is a random number used by a mobile nodes 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 7]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
a mobile node is at home, packets addressed to its home address are
routed to the mobile node's home link, using conventional Internet
routing mechanisms.
While a mobile node is attached to some foreign link away from home,
it is also addressable at one or more care-of addresses. A care-of
address is an IP address associated with a mobile node that has the
subnet prefix of a particular foreign link. The mobile node can
acquire its care-of address through conventional IPv6 stateless or
stateful auto-configuration mechanisms. 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
and the home agent is specified in Sections 11 and 10, respectively.
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 binding procedure. As
a part of this procedure, a return routability test is performed in
order to authorize the establishment of the binding. The operation
of the correspondent node is specified in Section 9.
There are two possible modes for communications between the mobile
node and a correspondent node. The first mode, bidirectional
tunneling, does not require Mobile IPv6 support from the
correspondent node and is available even if the mobile node has not
registered its current binding with the correspondent node. Packets
from the correspondent node are routed to the home agent and then
tunneled to the mobile node. Packets to the correspondent node are
tunneled from the mobile node to the home agent ("reverse tunneled")
and then routed normally from the home network to the correspondent
node. In this mode, the home agent uses proxy Neighbor Discovery
to intercept any IPv6 packets addressed to the mobile node's home
address (or home addresses) on the home link. Each intercepted
packet is tunneled to the mobile node's primary care-of address.
This tunneling is performed using IPv6 encapsulation [15].
The second mode, "route optimization", requires the mobile node to
register its current binding at the correspondent node. Packets
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 8]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 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 "prefix
discovery" mechanism. These mechanisms are described in Sections 6.5
through 6.8.
4.2. New IPv6 Protocol
Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header
(see Section 6.1). This Header is used to carry the following
messages:
Home Test Init
Home Test
Care-of Test Init
Care-of Test
These four messages are used to initiate 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 9]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
The format of the messages are defined in Sections 6.1.3
through 6.1.6.
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". The Binding Update message
is described in detail in Section 6.1.7.
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 Acknowledgement is described in
detail in Section 6.1.8.
Binding Refresh Request
A Binding Refresh Request is used 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. The Binding Refresh Request is
described in detail in Section 6.1.2.
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. This message is
described in detail in Section 6.1.9.
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
Sections 10.5 and 11.4.1, the following two new ICMP message types
are used for home agent address discovery:
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 10]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- Home Agent Address Discovery Request, described in Section 6.5.
- 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:
- Mobile Prefix Solicitation, described in Section 6.7.
- 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 sent in the Binding Update
expires. See Section 11.1.
Home Agents List
Home agents need to know which other home agents are on the
same link. This information is stored in the Home Agents List,
as described in more detail in Section 10.1. The list is used
for informing mobile nodes during dynamic home agent address
discovery.
4.6. Site-Local Addressability
Mobile nodes are free to move from site to site, but the use of
site-local addresses must be carefully managed. When a mobile node
or home agent address is site-local, then packets that use those
address need to stay within the site. The mobile node SHOULD use
such addresses only when it somehow has a guarantee - for instance,
by configuration - that it is safe to do so. Thus, a mobile node MAY
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 11]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
use a site-local home address for roaming within a site, but not for
roaming to another site. This is true even though the mobile node
may be able to obtain a globally addressable care-of address at the
new site.
If a mobile node or home agent has a global IPv6 address available,
it SHOULD be selected for use with Mobile IP signaling, in order to
make the greatest chance for success in case the mobile node might
move to a different site.
Operations affecting multi-sited IPv6 nodes are not completely
understood, especially when mobility management is involved. For
this reason, home agents SHOULD NOT be multi-sited. Similarly,
a mobile node that uses site-local home, care-of, or home agent
addresses SHOULD NOT be multi-sited.
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, and the protection of tunnels, home address
information, and routing instructions in 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.
5.1. Binding Updates to Home Agents
The mobile node and the home agent must have a security association
to protect this signaling. Authentication Header (AH) or
Encapsulating Security Payload (ESP) MUST be used. For ESP, a
non-null authentication algorithm MUST be applied.
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 checking that the given home address has been used with
the right security association. Such a check can be provided in
IPsec processing, by having the security policy database entries
unequivocally identify a single security association for any given
home address and home agent. The check may also be provided as
a part of Mobile IPv6 processing, if information about the used
security association is available in there. In any case, it is
necessary that the home address of the mobile node is visible in
the Binding Updates and Acknowledgements. The home address is used
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 12]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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. Automatic
key management with IKE [9] MAY be supported. When dynamic keying
is used, either the security policy database entries or the MIPv6
processing MUST unequivocally identify the IKE phase 1 credentials
which can be used to create security associations for a particular
home address.
Reference [24] is an informative description and example of using
IPsec to protect the communications between the mobile node and the
home agent.
5.2. Binding Updates to Correspondent Nodes
Binding Updates to correspondent nodes can be protected by using
a binding management key, Kbm. Kbm may be 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. Finally, Sections 5.2.7 and 5.2.8 discuss some
additional issues.
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 avoid 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, Perkins, Arkko Expires 29 April 2003 [Page 13]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
- 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.
- The "home keygen token" and "care-of keygen token" are 64-bit
values sent by the correspondent node to the mobile node via the
home agent (via the Home Test message) and the care-of address
(by the Care-of Test message), respectively.
The mobile node should use a newly generated random number for each
request that carries a home init or care-of init cookie. 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 14]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 [19]. Message Authentication Codes (MACs) are computed using
HMAC_SHA1 [25, 19]. 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.
Figure 1 shows the message flow for the return routability
procedures.
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 to acquire the home keygen token. The
contents of the message can be summarized as follows:
Source Address = home address
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 15]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Mobile node Home agent Correspondent node
| |
| Home Test Init (HoTI) | |
|------------------------->|------------------------->|
| | |
| Care-of Test Init (CoTI) |
|---------------------------------------------------->|
| |
| | Home Test (HoT) |
|<-------------------------|<-------------------------|
| | |
| Care-of Test (CoT) |
|<----------------------------------------------------|
| |
Figure 1: Message Flow for Return Routability Address Testing
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 mobile node remembers these cookie
values to obtain some assurance that its protocol messages are
being processed by the desired correspondent node.
Care-of Test Init
The mobile node sends a Care-of Test Init message to the
correspondent node 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 16]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Home Test
The Home Test message is sent in response to a Home Test Init
message. The contents of the message are:
Source Address = correspondent
Destination Address = home address
Parameters:
- home init cookie
- home keygen token
- home nonce index
When the correspondent node receives the Home Test Init
message, it generates a home keygen token as follows:
home keygen token :=
First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0)))
where | denotes concatenation. The final "0" inside the
HMAC_SHA1 function is a single zero octet, used to distinguish
home and care-of cookies from each other.
The home keygen token is formed from the first 64 bits of
the MAC. The home keygen token tests that the mobile can
receive 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, it is important
that the data passed between the home agent and the mobile node
be immune from inspection and passive attack. Such protection
can be gained by encrypting the home keygen token as it is
tunneled from the home agent to the mobile node.
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 17]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Care-of Test
This message is sent in response to a Care-of Test Init
message. The contents of the message are:
Source Address = correspondent
Destination Address = care-of address
Parameters:
- care-of init cookie
- care-of keygen token
- care-of nonce index
The correspondent node sends a challenge also to the mobile's
care-of address. 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 from 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 by setting the care-of address equal to the home address
(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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 18]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
mobile node. The correspondent node does not maintain the value for
the binding management key Kbm; it creates Kbm when given the nonce
indices and the mobile node's addresses.
5.2.6. Authorizing Binding Management Messages
After the mobile node has created the binding management key (Kbm),
it can supply a verifiable Binding Update to the correspondent
node. This section provides an overview of this binding procedure.
Figure 2 shows the message flow. The Binding Update creates a
binding, and the Binding Acknowledgement is optional.
Mobile node Correspondent node
| |
| Binding Update (BU) |
|---------------------------------------------->|
| (MAC, seq#, nonce indices, care-of address) |
| |
| |
| Binding Acknowledgement (BA) (if sent) |
|<----------------------------------------------|
| (MAC, seq#, status) |
Figure 2: Message Flow for Establishing Binding at
the Correspondent Node
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 or in 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)
- HMAC_SHA1 (Kbm, (care-of address | CN address | BU))
The Binding Update may contain a Nonce Indices option,
indicating to the correspondent node which home and care-of
nonces to use to recompute Kbm, the binding management key.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 19]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
The MAC is computed as described in Section 6.2.6, using the
correspondent node's address as the destination address and the
Binding Update message itself as the Mobility Header 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 optionally 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)
- HMAC_SHA1 (Kbm, (care-of address | CN address | BA))
The Binding Acknowledgement contains the same sequence number
as the Binding Update. The MAC is computed as described in
Section 6.2.6, using the correspondent node's address as the
destination address and the message itself as the Mobility
Header Data.
Bindings established with correspondent nodes using keys created
by way of the return routability procedure MUST NOT exceed
MAX_RR_BINDING_LIFE 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 6.2.4). 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.
The care-of address may be set equal to the home address in order to
delete a previously established binding In this case, generation of
the binding management key depends exclusively on the home keygen
token (Section 5.2.5).
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)
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 20]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
acceptable for at least MAX_TOKEN_LIFE 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_LIFE 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_LIFE / 30)
nonces. This results in tokens being acceptable MAX_TOKEN_LIFE
to MAX_NONCE_LIFE 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 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 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_LIFE seconds, the mobile node MAY use them beyond a single
run of the return routability procedure until MAX_TOKEN_LIFE expires.
After this the mobile node SHOULD NOT use the tokens. A fast moving
mobile node may reuse a recent home keygen token from a correspondent
node when moving to a new location, and just acquire a new care-of
keygen token to show routability in the new location.
While this does not save the number of round-trips due to the
simultaneous processing of home and care-of return routability tests,
there are fewer messages being exchanged, and a potentially long
round-trip through the home agent is avoided. Consequently, this
optimization is often useful. A mobile node that has multiple home
addresses, may also use the same care-of keygen token for Binding
Updates concerning all of these addresses.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 21]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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. The correspondent node can, for instance, change the nonce
often enough to ensure that the nonces used when removed entries
were created are no longer valid. If many such deletions occur
the correspondent node can batch them together to avoid having to
increment the nonce index too often.
5.3. Dynamic Home Agent Address Discovery
No security is required for dynamic home agent address discovery.
5.4. Prefix Discovery
The mobile node and the home agent must have a security association
to protect prefix discovery. IPsec AH or ESP SHOULD be supported and
used for integrity protection. For ESP, a non-null authentication
algorithm MUST be applied.
5.5. Payload Packets
Payload packets exchanged with mobile nodes can be protected in the
usual manner, in the same way as stationary hosts can protect them.
However, Mobile IPv6 introduces the Home Address destination option,
a routing header, and tunneling headers in the payload packets. In
the following we define the security measures taken to protect these,
and to prevent their use in attacks against other parties.
This specification limits the use of the Home Address destination
option to the situation where the correspondent node already has a
Binding Cache entry for the given home address. This avoids the use
of the Home Address option in attacks described in Section 14.1.
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 14.1.
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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 22]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
node (Binding Updates sent to the home agents are secure). These
measures protect the tunnels against vulnerabilities discussed in
Section 14.1.
For traffic tunneled via the home agent, additional IPsec AH or ESP
encapsulation MAY be supported and used.
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.
6.1.1. Format
The Mobility Header is identified by a Next Header value of TBD <To
be assigned by IANA> 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 specification
of piggybacking binding messages on payload packets (see
Section B.1).
Implementations conforming to this specification SHOULD set the
payload protocol type to IPPROTO_NONE (59 decimal).
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 23]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 Sections 6.1.2
to 6.1.9. An unrecognized MH Type field causes an error
indication to be sent.
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 [11]. The Next Header value used in the
pseudo-header is TBD <To be assigned by IANA>. 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 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 [26].
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 24]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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
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 is used to request a
mobile node's binding from the mobile node. It is sent 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 25]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The encoding and format
of defined options are described in Section 6.2. The receiver
MUST ignore and skip any options which it does not understand.
There MAY be additional information, associated with this
Binding Refresh Request message, that need not be present in
all Binding 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 26]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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. Contains
one 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 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 policy data base. 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 .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 27]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains
one or more TLV-encoded mobility options. The receiver MUST
ignore and skip any options which it does not understand. 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 Nonce +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 28]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
Home Keygen Nonce
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. Contains
one 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,
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 29]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 Nonce +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. 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.
Care-of Init Cookie
64-bit field which contains the care-of init cookie.
Care-of Keygen Nonce
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. Contains
one 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 30]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 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|S|D|L| 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.
Home Registration (H)
The Home Registration (H) bit is set by the sending mobile
node to request that the receiving node should act as this
node's home agent. The destination of the packet carrying this
message MUST be that of a router sharing the same subnet prefix
as the home address of the mobile node in the binding.
Single Address Only (S)
If this bit is set, the mobile node requests that the home
agent make no changes to any other Binding Cache entry except
for the particular one containing the home address specified
in the Home Address destination option. This disables home
agent processing for other related addresses, as is described
in Section 10.3.1.
Duplicate Address Detection (D)
The Duplicate Address Detection (D) bit is set by the sending
mobile node to request that the receiving node (the mobile
node's home agent) perform Duplicate Address Detection [13]
on the mobile node's home link for the home address in this
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 31]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
binding. This bit is only valid when the Home Registration (H)
and Acknowledge (A) bits are also set, and MUST NOT be set
otherwise.
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 (IID) as the mobile node's link-local address.
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 number used by the receiving node to sequence Binding
Updates and by the sending node to match a returned Binding
Acknowledgement with this Binding Update.
Lifetime
16-bit unsigned integer. The number of time units remaining
before the binding MUST be considered expired. A value of
all one bits (0xffff) indicates infinity. A value of zero
indicates that the Binding Cache entry for the mobile node MUST
be deleted. One time unit is 4 seconds.
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The encoding and format
of defined options are described in Section 6.2. The receiver
MUST ignore and skip any options which it does not understand.
The following options are valid in a Binding Update:
- Binding Authorization Data option
- Nonce Indices option.
- Alternate Care-of Address option
If no options are present in this message, 4 bytes of padding is
necessary and the Header Len field will be set to 1.
The care-of address MUST be a unicast routable address. Binding
Updates for a care-of address which is not a unicast routable address
MUST be silently discarded.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 32]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
The deletion of a binding can be indicated by setting the Lifetime
field to 0 or by setting the care-of address equal to the home
address. In either case, generation of the binding management
key depends exclusively on the home keygen token (Section 5.2.5).
Correspondent nodes SHOULD NOT expire 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 deallocated 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 an Binding Error message is sent to the mobile
node (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
Sections 9.5.4 and 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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
These fields are unused. They MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
Status
8-bit unsigned integer indicating the disposition of the
Binding Update. Values of the Status field less than 128
indicate that the Binding Update was accepted by the receiving
node. 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:
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 33]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
0 Binding Update accepted
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
Up-to-date values of the Status field are to be specified in
the IANA registry of assigned numbers [18].
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.
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. A value of all one bits (0xffff) indicates
infinity.
The value of this field is undefined if the Status field
indicates that the Binding Update was rejected.
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
or more TLV-encoded mobility options. The encoding and format
of defined options are described in Section 6.2. The receiver
MUST ignore and skip any options which it does not understand.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 34]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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
- Binding Refresh Advice option
If no options are present in this message, 4 bytes of padding is
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.
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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 35]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Reserved
A 8-bit field reserved for future use. The value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.
Home Address
The home address that was contained in the Home Address
destination option. The mobile node uses this information to
determine which binding does not exist, in cases where the
mobile node has several home addresses.
Mobility Options
Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. Contains one
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 one 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 6.2.6) MUST be the last option and MUST NOT have
trailing padding. Otherwise, options can be placed in any order.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 36]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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, Perkins, Arkko Expires 29 April 2003 [Page 37]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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. Option data MUST be
ignored by the receiver.
6.2.4. 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 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Alternate Care-of Address 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 38]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
6.2.5. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Nonce Indices option is valid only in the Binding Update message,
and only when present together with an Binding Authorization Data
option.
The Home Nonce Index field tells the correspondent node that receives
the message which of its stored random nonce values is to be used to
produce the home keygen token to authorize the Binding Update.
The Care-of Nonce Index field tells the correspondent node that
receives the message which of its stored random nonce values is to
be used to produce the care-of keygen token to authorize the Binding
Update.
6.2.6. Binding Authorization Data
The Binding Authorization Data option has an alignment requirement of
8n+2. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 5 | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Authenticator |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Binding Authorization Data option is valid in the Binding Update
and Binding Acknowledgment.
The Option Length field contains the length of the authenticator in
octets.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 39]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 depend on the used
authorization procedure.
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 | final dest | Mobility Header Data
Authenticator = First (96, HMAC_SHA1 (Kbm, Mobility Data))
Where | denotes concatenation and "final dest" is the IPv6 address
of the final destination of the packet. "Mobility Header 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).
The first 96 bits from the MAC result are used as the Authenticator
field. Note that, if the message is sent to a destination which is
itself mobile, the "final dest" address may not be the address found
in the Destination Address field of the IPv6 header; instead the
address of the true destination (e.g., its home address) should be
used.
6.2.7. 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 = 6 | 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
how long before the mobile node SHOULD send a new home registration
to the home agent. The Refresh Interval MUST be set to indicate
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 40]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
a smaller time interval than the Lifetime value of the Binding
Acknowledgement.
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.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Header Ext Len | 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.
IPv6 requires that options appearing in a Hop-by-Hop Options
header or Destination Options header be aligned in a packet so that
multi-octet values within the Option Data field of each option fall
on natural boundaries (i.e., fields of width n octets are placed at
an integer multiple of n octets from the start of the header, for
n = 1, 2, 4, or 8) [11]. The alignment requirement [11] for the Home
Address option is 8n+6.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 41]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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:
- Any IPv6 node that does not recognize the Option Type must
discard the packet.
- 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; otherwise, for multicast addresses, the ICMP
message MUST NOT be sent.
- The data within the option cannot change en-route to the packet's
final destination.
The Home Address option MUST be placed as follows:
- After the routing header, if that header is present
- Before the Fragment Header, if that header is present
- Before the AH Header or ESP Header, if either one of those
headers is present
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 42]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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).
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].
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 43]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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. Initialized to zero for transmission,
and ignored on reception.
Home Address
The Home Address of the destination Mobile Node.
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 [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.
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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 44]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
the Home Agent Address Discovery Request message to the Mobile IPv6
Home-Agents anycast address for its own home subnet prefix [16].
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
150 <To Be Assigned by IANA>
Code
0
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 MUST be one of the mobile node's current care-of
addresses. The home agent MUST then return the Home Agent Address
Discovery Reply message directly to the Source Address chosen by the
mobile node. Note that, at the time of performing this dynamic home
agent address discovery procedure, it is likely that the mobile node
is not registered with any home agent within the specified anycast
group.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 45]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
151 <To Be Assigned by IANA>
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 present in the list is
indicated by the remaining length of the IPv6 packet carrying
the Home Agent Address Discovery Reply message.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 46]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
Source Address
The mobile node's care-of address.
Destination Address
The address of the mobile node's home agent. This home agent
must be on the link which the mobile node wishes to learn
prefix information about.
Hop Limit
Set to an initial hop limit value, similarly to any other
unicast packet sent by the mobile node.
Destination Option:
A Home Address destination option MUST be included.
AH or ESP header:
IPsec headers SHOULD be supported and used as described in
Section 5.4.
ICMP Fields:
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 47]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Type
152 <To Be Assigned by IANA>
Code
0
Checksum
The ICMP checksum [14].
Identifier
An identifier to aid in matching a future Mobile Prefix
Advertisement to this Mobile Prefix Solicitation.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 48]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 | 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.
Routing header:
A type 2 routing header MUST be included.
AH or ESP header:
IPsec headers SHOULD be supported and used as described in
Section 5.4.
ICMP Fields:
Type
153 <To Be Assigned by IANA>
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 49]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Code
0
Checksum
The ICMP checksum [14].
Identifier
An identifier to aid in matching this Mobile Prefix
Advertisement to a previous Mobile Prefix Solicitation.
Options:
Prefix Information
Each message contains one or more Prefix Information options.
Each option carries the prefix(es) that the mobile node should
use to configure its home address(es). Section 10.6 describes
which prefixes should be advertised to the mobile node.
The Prefix Information option is defined in Section 4.6.2
of [12], with modifications defined in Section 7.2 of this
specification. The home agent MUST use this modified Prefix
Information option to send the aggregate list of home network
prefixes as defined in Section 10.6.1.
The Mobile Prefix Advertisement sent by the home agent MAY include
the Source Link-layer Address option defined in RFC 2461 [12], or the
Advertisement Interval option specified in Section 7.3.
Future versions of this protocol may define new option types. Mobile
nodes MUST silently ignore any options they do not recognize and
continue processing the message.
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 50]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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:
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 51]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 (Sections 10.5 and 11.4.1).
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:
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, in addition to advertising the indicated prefix,
contains a complete IP address assigned to the sending router.
This router IP address has the same scope and conforms to the
same lifetime values as the advertised prefix. This use of
the Prefix field is compatible with its use in advertising
the prefix itself, since Prefix Advertisement uses only the
leading number Prefix bits specified by the Prefix Length
field. Interpretation of this flag bit is thus independent
of the processing required for the On-Link (L) and Autonomous
Address-Configuration (A) flag bits.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 52]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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]. In this case, at
least one (not all) of these multiple Advertisements being sent needs
to satisfy the above requirement.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 53]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
7.3. New Advertisement Interval Option Format
Mobile IPv6 defines a new Advertisement Interval option, used in
Router Advertisement messages to advertise the interval at which the
sending router sends unsolicited multicast Router Advertisements.
The format of the Advertisement Interval option is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertisement Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
7
Length
8-bit unsigned integer. The length of the option (including
the type and length fields) in units of 8 octets. The value of
this field MUST be 1.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Advertisement Interval
32-bit unsigned integer. The maximum time, in milliseconds,
between successive unsolicited router Router Advertisement
messages sent by this router on this network interface. Using
the conceptual router configuration variables defined by
Neighbor Discovery [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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 54]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
Home Agent Preference
16-bit signed, two's complement integer. The preference for
the home agent sending this Router Advertisement, for use in
ordering the addresses returned to a mobile node in the Home
Agent Addresses field of a Home Agent Address Discovery Reply
message. Higher values mean more preferable. If this option
is not included in a Router Advertisement in which the Home
Agent (H) bit is set, the preference value for this home agent
SHOULD be considered to be 0. Values greater than 0 indicate a
home agent more preferable than this default value, and values
less than 0 indicate a less preferable home agent.
The manual configuration of the Home Agent Preference value
is described in Section 8.4. 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,
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 55]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 56]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
Mobile IPv6 relaxes this limit such that routers MAY send unsolicited
multicast Router Advertisements more frequently. This is important
on network interfaces where the router is expecting to provide
service to visiting mobile nodes (e.g., wireless network interfaces),
or on which it is serving as a home agent to one or more mobile
nodes (who may return home and need to hear its Advertisements).
Such routers SHOULD be configured with a smaller MinRtrAdvInterval
value and MaxRtrAdvInterval value, to allow sending of unsolicited
multicast Router Advertisements more often. Recommended values for
these limits are:
- MinRtrAdvInterval 0.05 seconds
- MaxRtrAdvInterval 1.5 seconds
Use of these modified limits MUST be configurable, and specific
knowledge of the type of network interface in use SHOULD be taken
into account in configuring these limits for each network interface.
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 MaxRtrAdvInterval in such a case can be set to some high value.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 57]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
When sending unsolicited multicast Router Advertisements more
frequently than the standard limit on unsolicited multicast
Advertisement frequency, the sending router need not include all
options in each of these Advertisements, but it SHOULD include at
least one Prefix Information option with the Router Address (R) bit
set (Section 7.2) in each.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 58]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
7.6. Changes to Sending Router Solicitations
In addition to the limit on routers sending unsolicited multicast
Router Advertisement messages (Section 7.5), Neighbor Discovery
defines limits on nodes sending Router Solicitation messages, such
that a node SHOULD send no more than 3 Router Solicitations, and that
these 3 transmissions SHOULD be spaced at least 4 seconds apart.
However, these limits prevent a mobile node from finding a new
default router (and thus a new care-of address) quickly as it moves
about.
Mobile IPv6 relaxes this limit such that, while a mobile node is away
from home, it MAY send Router Solicitations more frequently. The
following limits for sending Router Solicitations are recommended for
mobile nodes while away from home:
- A mobile node that is not configured with any current care-of
address (e.g., the mobile node has moved since its previous
care-of address was configured), MAY send more than the defined
Neighbor Discovery limit of MAX_RTR_SOLICITATIONS Router
Solicitations.
- The rate at which a mobile node sends Router Solicitations MUST
be limited, although a mobile node MAY send Router Solicitations
more frequently than the defined Neighbor Discovery limit of
RTR_SOLICITATION_INTERVAL seconds. The minimum interval MUST
be configurable, and specific knowledge of the type of network
interface in use SHOULD be taken into account in configuring this
limit for each network interface. A recommended minimum interval
is 1 second.
- After sending at most MAX_RTR_SOLICITATIONS Router Solicitations,
a mobile node MUST reduce the rate at which it sends subsequent
Router Solicitations. Subsequent Router Solicitations SHOULD
be sent using a binary exponential back-off mechanism, doubling
the interval between consecutive Router Solicitations, up to a
maximum interval. The maximum interval MUST be configurable and
SHOULD be chosen appropriately based on the characteristics of
the type of network interface in use.
- While still searching for a new default router and care-of
address, a mobile node MUST NOT increase the rate at which it
sends Router Solicitations unless it has received a positive
indication (such as from lower network layers) that it has moved
to a new link. After successfully acquiring a new care-of
address, the mobile node SHOULD also increase the rate at which
it will send Router Solicitations when it next begins searching
for a new default router and care-of address.
- A mobile node that is currently configured with a care-of address
SHOULD NOT send Router Solicitations to the default router
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 59]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
on its current link, until its movement detection algorithm
(Section 11.5.1) determines that it has moved and that its
current care-of address might no longer be valid.
7.7. Changes to Duplicate Address Detection
Upon failing Duplicate Address Detection, [13] requires IPv6 nodes to
stop using the address and wait for reconfiguration. In addition, if
the failed address was a link-local address formed from an interface
identifier, the interface should be disabled.
Mobile IPv6 extends this behavior as follows. Upon failing Duplicate
Address Detection while away from home, the mobile node SHOULD stop
using the address on this interface until the mobile node moves to
another link. The mobile node SHOULD NOT wait for reconfiguration or
disable the interface.
The mobile node MUST NOT discard the home address based on a failure
of a link-local address with the same interface identifier. Instead,
the mobile node SHOULD generate a new random interface identifier and
use it for assigning itself a new link-local address. In order to do
this, the mobile node applies to the link-local address the procedure
described in [17] for global addresses. At most 5 consecutive
attempts SHOULD be performed to generate such addresses and test
them through Duplicate Address Detection. If after these attempts
no unique address was found, the mobile node SHOULD log a system
error and give up attempting to find a link-local address on that
interface, until the node moves to a new link.
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:
- All IPv6 nodes.
- All IPv6 nodes with support for route optimization.
- All IPv6 routers.
- All Mobile IPv6 home agents.
- All Mobile IPv6 mobile nodes.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 60]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 requirements
for such nodes, and standard IPv6 techniques are sufficient.
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, beneficial for robustness and
reduction of jitter and latency, and necessary to avoid congestion
in the home network. The following requirements apply to all
correspondent nodes that support route optimization:
- The node MUST be able validate a Home Address option using an
existing Binding Cache entry, as described in Section 9.3.1.
- 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.
- Unless the correspondent node is also acting as a mobile node, it
MUST ignore type 2 routing headers and drop all packets that it
has received with such headers.
- The node SHOULD be able to interpret ICMP messages as described
in Section 9.3.4.
- The node MUST be able to send Binding Error messages as described
in Section 9.3.3.
- The node MUST be able to process Mobility Headers as described in
Section 9.2.
- The node MUST be able to participate in a return routability
procedure (Section 9.4).
- The node MUST be able to process Binding Update messages
(Section 9.5).
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 61]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- The node MUST be able to return a Binding Acknowledgement
(Section 9.5.4).
- The node MUST be able to maintain a Binding Cache of the
bindings received in accepted Binding Updates, as described in
Sections 9.1 and 9.6.
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:
- 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 MUST be configurable.
- Every IPv6 router SHOULD be able to support sending unsolicited
multicast Router Advertisements at the faster rate described in
Section 7.5. The use of this faster rate MUST be configurable.
- Each router SHOULD include at least one prefix with the Router
Address (R) bit set and with its full IP address in its Router
Advertisements (as described in Section 7.2).
- Filtering routers 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:
- 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 (Sections 10.1 and 10.3.1).
- 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).
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 62]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- 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).
- 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 10.4.3).
- The node MUST be able to process Mobility Headers as described in
Section 10.2.
- Every home agent MUST be able to return a Binding Acknowledgement
in response to a Binding Update (Section 10.3.1).
- 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
Sections 10.1 and 10.5.1.
- Every home agent MUST be able to accept packets addressed to
the Mobile IPv6 Home-Agents anycast address for the subnet
on which it is serving as a home agent [16], and MUST be
able to participate in dynamic home agent address discovery
(Section 10.5).
- 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).
- Every home agent SHOULD support sending ICMP Mobile Prefix
Advertisements (Section 6.8), and SHOULD respond to Mobile Prefix
Solicitations (Section 6.7). 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.
- Every home agent MUST support IPsec ESP for protection of packets
belonging to the return routability procedure (Section 10.4.4).
8.5. IPv6 Mobile Nodes
Finally, the following requirements apply to all IPv6 nodes capable
of functioning as mobile nodes:
- The node MUST maintain a Binding Update List (Section 11.1).
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 63]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- 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).
- The node MUST be able to perform IPv6 encapsulation and
decapsulation [15].
- The node MUST be able to process type 2 routing header as defined
in Sections 6.4 and 11.3.3.
- The node MUST support receiving a Binding Error message
(Section 11.7.5).
- The node SHOULD support receiving ICMP errors (Section 11.3.4).
- The node MUST support movement detection, care-of address
formation, and returning home (Section 11.5).
- The node MUST be able to process Mobility Headers as described in
Section 11.2.
- The node MUST support the return routability procedure
(Section 11.6).
- The node MUST be able to send Binding Updates, as specified in
Sections 11.7.1 and 11.7.2.
- The node MUST be able to receive and process Binding
Acknowledgements, as specified in Section 11.7.3.
- The node MUST support receiving a Binding Refresh Request
(Section 6.1.2), by responding with a Binding Update.
- The node MUST support receiving Mobile Prefix Advertisements
(Section 11.4.3) and reconfiguring its home address based on the
prefix information contained therein.
- The node SHOULD support use of the dynamic home agent address
discovery mechanism, as described in Section 11.4.1.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 64]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 IPv6 addresses. The
Binding Cache MAY be implemented in any manner consistent with the
external behavior described in this document, for example by being
combined with the node's Destination Cache as maintained by Neighbor
Discovery [12]. When sending a packet, the Binding Cache is searched
before the Neighbor Discovery conceptual Destination Cache [12].
That is, any Binding Cache entry for this destination SHOULD take
precedence over any Destination Cache entry for the same destination.
Each Binding Cache entry conceptually contains the following fields:
- The home address of the mobile node for which this is the Binding
Cache entry. This field is used as the key for searching the
Binding Cache for the destination address of a packet being sent.
If the destination address of the packet matches the home address
in the Binding Cache entry, this entry SHOULD be used in routing
that packet.
- The care-of address for the mobile node indicated by the home
address field in this Binding Cache entry. If the destination
address of a packet being routed by a node matches the home
address in this entry, the packet SHOULD be routed to this
care-of address. This is described in Section 9.3.2 for packets
originated by this node.
- A lifetime value, indicating the remaining lifetime for this
Binding Cache entry. The lifetime value is initialized from
the Lifetime field in the Binding Update that created or last
modified this Binding Cache entry. Once the lifetime of this
entry expires, the entry MUST be deleted from the Binding Cache.
- A flag indicating whether or not this Binding Cache entry is a
home registration entry.
- The maximum value of the Sequence Number field received in
previous Binding Updates for this mobile node home address. The
Sequence Number field is 16 bits long. Sequence Number values
MUST be compared modulo 2**16 as explained in Section 9.5.1.
- 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 65]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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:
1. The MH Type field MUST have a known value (Section 6.1.1).
Otherwise, the node MUST discard the message and SHOULD issue a
Binding Error message as described in Section 9.3.3, with Status
field set to 2 (unrecognized MH Type value).
2. The Payload Proto field MUST be IPPROTO_NONE (59 decimal).
Otherwise, the node MUST silently discard the message.
3. The checksum must be verified as per Section 6.1. Otherwise, the
node MUST silently discard the message.
Subsequent checks depend on the particular Mobility Header, as
specified in Sections 9.4 and 9.5.
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 Destination Option
If the correspondent node has a Binding Cache entry for the home
address of a mobile node, packets sent by the mobile node MAY include
a Home Address destination option.
Packets containing a Home Address option MUST be dropped if the given
home address is not a unicast routable address.
Packets containing a Home Address option MUST also be dropped if
there is no corresponding Binding Cache entry for the given home
address. A corresponding Binding Cache entry MUST have the currently
registered care-of address equal to the source address of the packet.
These tests MUST NOT be done for packets that contain a Binding
Update and a Home Address option.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 66]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
If the packet is dropped due the above tests, the correspondent node
SHOULD 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 to process the packet without the knowledge that it
came originally from a care-of address or that a Home Address option
was used.
No additional authentication of the Home Address option is
required, except that if the IPv6 header of a packet is covered
by authentication, then that authentication MUST also cover the
Home Address option; this coverage is achieved automatically by the
definition of the Option Type code for the Home Address option, since
it indicates that the data within the option cannot change en-route
to the packet's final destination, and thus the option is included in
the authentication computation. By requiring that any authentication
of the IPv6 header also cover the Home Address option, the security
of the Source Address field in the IPv6 header is not compromised by
the presence of a Home Address option. When attempting to verify
authentication data in a packet that contains a Home Address option,
the receiving node MUST make the calculation as if the care-of
address were present in the Home Address option, and the home address
were present in the source IPv6 address field of the IPv6 header.
This conforms with the calculation specified in Section 11.3.2.
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. Assuming there are no additional routing
headers in this packet beyond those needed by Mobile IPv6, the mobile
node sets the fields in the packet's IPv6 header and routing header
as follows:
- The Destination Address in the packet's IPv6 header is set to the
mobile node's home address (the original destination address to
which the packet was being sent).
- 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 67]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
The IP layer will insert the routing header before performing IPsec
processing. The IPsec Security Policy Database will be consulted
based on the IP source address and the destination address (which
will be the mobile node's home address). 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
Sections 9.2 and 9.3.1 describe error conditions that lead to a need
to send a Binding Error message.
A Binding Error message is sent 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.
Binding Error messages are subject to rate limiting in the same
manner as is done for ICMPv6 messages [14].
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 68]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
9.4. Return Routability Procedure
This subsection specifies actions taken by a correspondent node
during the return routability procedure.
9.4.1. Receiving Home Test Init Messages
Upon receiving a Home Test Init message, the correspondent node
verifies the following:
- The Header Len field in the Mobility Header MUST NOT be less than
the length specified in Section 6.1.3.
- 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 69]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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:
- The Header Len field in the Mobility Header MUST NOT be less than
the length specified in Section 6.1.4.
- 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. Note that the Home Test message is always
sent to the home address of the mobile node, even when there is an
existing binding for the mobile node.
9.4.4. Sending Care-of Test Messages
The correspondent node creates a care-of nonce 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:
- Binding Update
- Binding Refresh Request
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 70]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- Binding Acknowledgement
- 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:
- The packet MUST contain a Home Address option with a unicast
routable home address, unless the Source Address is the home
address of the mobile node
- The Header Len field in the Mobility Header is no less than the
length specified in Section 6.1.7.
- The Sequence Number field in the Binding Update is greater than
the Sequence Number received in the previous Binding Update for
this home address, if any.
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 32767 values, inclusive. For example, if the last
received sequence number was 15, then messages with sequence
numbers 0 through 15, as well as 32784 through 65535, would be
considered less than or equal.
When the return routability procedure is used to enable the
establishment of nonce indices as inputs to the creation of the
binding key Kbm, the following are also required:
- 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.
- 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.
When using Kbm for validating the Binding Update, the following are
required:
- 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 71]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
have been specified by including an Alternate Care-of Address
mobility option in the Binding Update. When such message is
received and the return routability procedure is used as an
authorization method, the correspondent node MUST verify the
authenticator by using the address within the Alternate Care-of
Address in the calculations.
- The Binding Authorization Data mobility option MUST be the last
option and MUST NOT have trailing padding.
- The Home Registration (H) bit MUST NOT be set.
If the mobile node sends a sequence number which is not greater than
the sequence number from the last successful Binding Update, then the
receiving node MUST send back a Binding Acknowledgement with status
code 135, and the last accepted sequence number in the Sequence
Number field of the Binding Acknowledgement.
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.
Packets carrying Binding Updates that fail to satisfy all of these
tests for any reason other than insufficiency of the Sequence Number
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:
- If the Lifetime specified in the Binding Update is nonzero and
the specified care-of address is not equal to the home address
for the binding, then this is a request to cache a binding for
the mobile node. If the Home Registration (H) bit is set in the
Binding Update, the Binding Update is processed according to the
procedure specified in Section 10.3.1; otherwise, it is processed
according to the procedure specified in Section 9.5.2.
- If the Lifetime specified in the Binding Update is zero or the
specified care-of address matches the home address for the
binding, then this is a request to delete the mobile node's
cached binding. The update MUST include a valid home nonce index
(the care-of nonce index MUST be ignored by the correspondent
node). In this case, generation of the binding management key
depends 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:
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 72]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- If the Alternate Care-of Address option is present, the care-of
address is the address in that option.
- 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:
- If the Home Address destination option is present, the home
address is the address in that option.
- Otherwise, the home address is the Source Address field in the
packet's IPv6 header. This implies that the mobile node is at
home and is about to perform de-registration.
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 mobile node's binding, for which the Home
Registration (H) bit is not set in the Binding Update.
In this case, the receiving node SHOULD create a new entry in its
Binding Cache for this mobile node, or update its existing Binding
Cache entry for this mobile node, if such an entry already exists.
The 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.
The Sequence Number value received from a mobile node in a Binding
Update is stored by a correspondent node in its Binding Cache entry
for that mobile node. If the receiving correspondent node has no
Binding Cache entry for the sending mobile node, it MUST accept any
Sequence Number value in a received Binding Update from this mobile
node.
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 73]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 mobile node's binding from its Binding
Cache, for which the Home Registration (H) bit is not set in the
Binding Update.
Any existing binding for the mobile node MUST be deleted. A Binding
Cache entry for the mobile node MUST NOT be created in response to
receiving the Binding Update.
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.
9.5.4. Sending Binding Acknowledgements
A Binding Acknowledgement may be sent to indicate receipt of a
Binding Update as follows:
- If the Binding Update was silently discarded as described in
Section 9.5.1, a Binding Acknowledgement MUST NOT be sent.
- Otherwise, if the Acknowledge (A) bit set is set in the Binding
Update, a Binding Acknowledgement MUST be sent.
- Otherwise, if the node rejects the Binding Update, a Binding
Acknowledgement MUST be sent.
- Otherwise, if the node accepts the Binding Update, a 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 [18].
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 74]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
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.
Entries in a node's Binding Cache MUST be deleted when their lifetime
expires.
9.5.5. Sending Binding Refresh Requests
If a Binding Cache entry being deleted is still in active use
in sending packets to a mobile node, the next packet sent to the
mobile node will be routed normally to the mobile node's home link.
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. The Binding Refresh Request
message is sent in the same way as any packet addressed to the mobile
node (Section 9.3.2).
The correspondent node MAY retransmit Binding Refresh Request
messages provided that rate limitation is applied. The correspondent
node SHOULD 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 75]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
within its Binding Cache, except that any entry marked as a home
registration (Section 10.3.1) MUST NOT be deleted from the cache
until the expiration of its lifetime period. When such home
registration entries are deleted, the home agent MUST also cease
intercepting packets on the mobile node's home link addressed to
the mobile node (Section 10.4.1), just as if the mobile node had
de-registered its primary care-of address (see Section 10.3.2).
When attempting to add a new home registration entry in response
to a Binding Update with the Home Registration (H) bit set, if
no sufficient space can be found, the home agent MUST reject the
Binding Update. Furthermore, the home agent MUST return a Binding
Acknowledgement to the sending mobile node, in which the Status field
is set to 130 (insufficient resources).
A node MAY choose to drop any entry already in its Binding Cache,
other than home registration entries, in order to make space for
a new entry. For example, a "least-recently used" (LRU) strategy
for cache entry replacement among entries not marked as home
registrations is likely to work well unless the size of the Binding
Cache is substantially insufficient.
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.
10. Home Agent Operation
10.1. Conceptual Data Structures
Each home agent MUST maintain a Binding Cache and Home Agents List.
The rules for maintaining a Binding Cache are same for home
agents and correspondent nodes, and have already been described in
Section 9.1.
The Home Agents List is maintained by each home agent, recording
information about each router on the same link which 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 thus 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 76]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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:
- The link-local IP address of a home agent on the link. This
address is learned through the Source Address of the Router
Advertisements received from the router [12].
- 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, 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].
- The remaining lifetime of this Home Agents List entry. If a Home
Agent Information Option is present in a Router Advertisement
received from a home agent, the lifetime of the Home Agents List
entry representing that home agent is initialized from the Home
Agent Lifetime field in the option; otherwise, the lifetime is
initialized from the Router Lifetime field in the received Router
Advertisement. If Home Agents List entry lifetime reaches zero,
the entry MUST be deleted from the Home Agents List.
- The preference for this home agent; higher values indicate a more
preferable home agent. The preference value is taken from the
Home Agent Preference field 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. This includes authorization of
the particular node to control a particular home address, as the home
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 77]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
address unequivocally identifies 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:
- If the node is not a router that implements home agent
functionality, then the node MUST reject the Binding Update
and MUST return a Binding Acknowledgement to the mobile node,
in which the Status field is set to 131 (home registration not
supported).
- Else, if the home address for the binding (the Home Address field
in the packet's Home Address option) is not an on-link IPv6
address with respect to the home agent's current Prefix List,
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).
- Else, if the home agent chooses to reject the Binding Update for
any other reason (e.g., insufficient resources to serve another
mobile node as a home agent), then the home agent SHOULD return a
Binding Acknowledgement to the mobile node, in which the Status
field is set to an appropriate value to indicate the reason for
the rejection.
- A Home Address destination option MUST be present in the message.
- Finally, if the Duplicate Address Detection (D) bit is set in the
Binding Update, this home agent MUST perform Duplicate Address
Detection [13] on the mobile node's home link for the link-local
address associated with the home address in this binding, before
returning the Binding Acknowledgement. This ensures that no
other node on the home link was using the mobile node's home
address when the Binding Update arrived.
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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 78]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
the Binding Cache (Section 9.6) and MUST NOT be removed from the
Binding Cache until the expiration of the Lifetime period.
Normal processing for Duplicate Address Detection specifies that, in
certain cases, the node SHOULD delay sending the initial Neighbor
Solicitation of Duplicate Address Detection by a random delay
between 0 and MAX_RTR_SOLICITATION_DELAY [12, 13]. However, when
the Duplicate Address Detection (D) bit instructs the home agent
to perform Duplicate Address Detection, the home agent SHOULD NOT
perform such a delay. If this Duplicate Address Detection fails,
then the home agent MUST reject the Binding Update and MUST return a
Binding Acknowledgement to the mobile node, in which the Status field
is set to 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 home address will
continue to be kept unique by the home agent at least as long as the
lifetime granted for that home address binding is not over.
If the Single Address Only (S) bit in the Binding Update is zero,
the home agent creates Binding Cache entries for each of possibly
several home addresses. The set of such home addresses is formed
by replacing the routing prefix for the given home address with
all other routing prefixes on the mobile node's home link that are
supported by the home agent processing the Binding Update. The home
agent creates such a separate primary care-of address registration
for each such home address. Note that the same considerations for
Duplicate Address Detection apply for each affected home address.
The value of the Single Address Only (S) bit field is examined only
for new registrations. Its value is ignored on de-registrations and
re-registrations of the same addresses.
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 settings of the Single Address Only
(S) and Link-Local Address Compatibility (L) bits, as follows:
- L=0: Defend the given address. The Single Address Only (S) bit
is ignored in this case since we cannot derive other on-link
addresses without knowing the interface identifier.
- L=1 and S=0: Defend all non link-local unicast addresses
possible on link and the derived link-local.
- L=1 and S=1: Defend both the given non link-local unicast (home)
address and the derived link-local.
The lifetime of the Binding Cache entry depends on a number of
factors:
- The lifetime for the Binding Cache entry MUST NOT be greater than
the Lifetime value specified in the Binding Update.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 79]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- 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 for this
prefix [12].
- However, if the Single Address Only (S) bit field in the Binding
Update is zero, the lifetime for that Binding Cache entry MUST
NOT be greater than the minimum remaining valid lifetime for all
subnet prefixes on the mobile node's home link. If the value of
the Lifetime field specified by the mobile node in its Binding
Update is greater than this prefix lifetime, the home agent MUST
decrease the binding lifetime to less than or equal to the prefix
valid lifetime.
- The home agent MAY further decrease the specified lifetime for
the binding, for example based on a local policy. The resulting
lifetime is stored by the home agent in the Binding Cache entry,
and this Binding Cache entry MUST be deleted by the home agent
after the expiration of this lifetime.
Regardless of the setting of the Acknowledge (A) bit in the Binding
Update, the home agent MUST return a Binding Acknowledgement to the
mobile node, constructed as follows:
- The Status field MUST be set to a value 0, indicating success.
- The Sequence Number field MUST be copied from the Sequence Number
given in the Binding Update.
- The Lifetime field MUST be set to the remaining lifetime for the
binding as set by the home agent in its home registration Binding
Cache entry for the mobile node, as described above.
- 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 sooner than
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 Update.
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 80]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 10.4.3. 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 Update is validated and authorized in the manner described
in the previous section. This section describes the processing of a
valid Binding Update that requests the receiving node to no longer
serve as its home agent, de-registering its primary care-of address.
To begin processing the Binding Update, the home agent MUST perform
the following test:
- If the receiving node has no entry marked as a home registration
in its Binding Cache for this mobile node, then this node
MUST reject the Binding Update and SHOULD return a Binding
Acknowledgement to the mobile node, in which the Status field is
set to 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:
- The Status field MUST be set to a value 0, indicating success.
- The Sequence Number field MUST be copied from the Sequence Number
given in the Binding Update.
- The Lifetime field MUST be set to zero.
- The 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 81]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 same link-layer address as the Binding
Update came from.
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, and MUST tunnel each intercepted packet
to the mobile node using IPv6 encapsulation [15].
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. Specifically, the home
agent performs the following steps:
1. The home agent examines the value of the Single Address Only (S)
bit in the received Binding Update. If this bit is nonzero, the
next step is carried out only for the individual home address
specified for this binding. If, instead, this bit is zero, then
the next step is carried out for one address for each one of the
subnet prefixes currently considered by the home agent to be
on-link the mobile node. Each address is formed by replacing,
in turn, the configured subnet prefix in the mobile node's home
address. For this purpose, the set of on-link prefixes includes
both the link-local and site-local prefix.
2. For each specific IP address for the mobile node determined
in the first step above, 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.
All fields in each such Neighbor Advertisement message SHOULD be
set in the same way they would be set by the mobile node itself
if sending this Neighbor Advertisement while at home [12], with
the following exceptions:
- The Target Address in the Neighbor Advertisement MUST be set
to the specific IP address for the mobile node.
- The Advertisement MUST include a Target Link-layer Address
option specifying the home agent's link-layer address.
- The Router (R) bit in the Advertisement MUST be set to zero.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 82]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- The Solicited Flag (S) in the Advertisement MUST NOT be set,
since it was not solicited by any Neighbor Solicitation.
- The Override Flag (O) in the Advertisement MUST be set,
indicating that the Advertisement SHOULD override any
existing Neighbor Cache entry at any node receiving it.
Any node on the home link receiving one of the Neighbor Advertisement
messages described above will thus update its Neighbor Cache to
associate the mobile node's address with the home agent's link
layer address, causing it to transmit any future packets normally
destined to the mobile node to the mobile node's home agent. Since
multicasting on the local link (such as Ethernet) is typically
not guaranteed to be reliable, the home agent MAY retransmit
this Neighbor Advertisement message up to MAX_ADVERT_REXMIT (see
Section 12) times to increase its reliability. It is still possible
that some nodes on the home link will not receive any of these
Neighbor Advertisements, but these nodes will eventually be able
to detect the link-layer address change for the mobile node's home
address, through use of Neighbor Unreachability Detection [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's home address.
In order to intercept packets in this way, the home agent MUST
act as a proxy for this mobile node, and reply to any received
Neighbor Solicitations for it. When a home agent receives a Neighbor
Solicitation, it MUST check if the Target Address specified in the
message matches the home address of any mobile node for which it
has a Binding Cache entry marked as a home registration. Note that
Binding Update with the Single Address Only (S) bit set to zero will
result in multiple Binding Cache entries, so checks on all these
entries necessarily include all possible home addresses for the
mobile node.
If such an entry exists in the home agent's Binding Cache, the
home agent MUST reply to the Neighbor Solicitation 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 IPv6 home address, and allows the
home agent to defend these addresses on the home link for Duplicate
Address Detection [12].
10.4.2. Tunneling Intercepted Packets to a Mobile Node
For any packet sent to a mobile node from the mobile node's home
agent (for which the home agent is the original sender of the
packet), the home agent is operating as a correspondent node of
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 83]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 (including addresses formed from other on-link prefixes, if
the Single Address Only (S) bit was zero in the Binding Update), 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
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, such a packet MUST
be discarded, and the home agent SHOULD return an ICMP Destination
Unreachable, Code 3, message to the packet's Source Address (unless
this Source Address is a multicast address). Packets addressed to
the mobile node's site-local address SHOULD be tunneled to the mobile
node by default, but this behavior MUST be configurable to disable
it; currently, the exact definition and semantics of a "site" and a
site-local address are incompletely defined in IPv6, and this default
behavior might change at some point in the future.
Tunneling of multicast packets to a mobile node follows similar
limitations to those defined above for unicast packets addressed to
the mobile node's link-local and site-local addresses. Multicast
packets addressed to a multicast address with link-local scope [3],
to which the mobile node is subscribed, MUST NOT be tunneled
to the mobile node; such packets SHOULD be silently discarded
(after delivering to other local multicast recipients). Multicast
packets addressed to a multicast address with scope larger
than link-local but smaller than global (e.g., site-local and
organization-local [3]), to which the mobile node is subscribed,
SHOULD be tunneled to the mobile node by default. This behavior MUST
be configurable to allow changing or disabling it. Note that this
default behavior might change at some point in the future as the
definition of these scopes become more completely defined in IPv6.
Before tunneling a packet to the mobile node, the home agent MUST
perform any IPsec processing as indicated by the security policy data
base.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 84]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
10.4.3. Handling Reverse Tunneled Packets from a Mobile Node
Unless a binding has been established between the mobile node and a
correspondent node, traffic from the mobile node to the correspondent
node goes through a reverse tunnel. Home agents MUST support reverse
tunneling as follows:
- The tunneled traffic arrives to the home agent using IPv6
encapsulation [15].
- The tunnel entry point is the primary care-of address as
registered with the home agent and the tunnel exit point is the
home agent.
- When a home agent decapsulates a tunneled packet from the mobile
node, the home agent 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.
Reverse tunneled packets MAY be discarded unless accompanied by a
valid AH or ESP header, depending on the security policies used by
the home agent. The support for authenticated reverse tunneling
allows the home agent to protect the home network and correspondent
nodes from malicious nodes masquerading as a mobile node, even if
they know the current location of the real mobile node.
10.4.4. 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 to 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 isn't necessary
to distinguish between different kinds of packets within the return
routability procedure.
The security association between the home agent and the mobile node
MUST change its destination address (tunnel gateway address) when the
care-of address for the mobile node changes [24].
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. These messages and the return routability messages employ the
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 85]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
same protocol from the point of view of the security policy database,
the Mobility Header. The security policy database entries MUST be
defined as if they were specifically for the tunnel interface between
the mobile node and the home agent. 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.
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:
- If the Home Agent (H) bit in the Router Advertisement is not set,
check to see if the sending node has an entry in the current Home
Agents List. If it does, delete the corresponding entry. In any
case all of the following steps are skipped.
- Otherwise, extract the Source Address from the IP header of the
Router Advertisement. This is the link-local IP address on this
link of the home agent sending this Advertisement [12].
- 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 86]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- 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.
- If the link-local address of the home agent sending this
Advertisement is already present in this home agent's Home
Agents List and the received home agent lifetime value is zero,
immediately delete this entry in the Home Agents List.
- Otherwise, if the link-local address of the home agent sending
this Advertisement is already present in the receiving home
agent's Home Agents List, reset its lifetime and preference to
the values determined above.
- If the link-local address of the home agent sending this
Advertisement 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.
- 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
such a Home Agent Address Discovery Request message that is serving
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:
- The Home Agent Addresses field SHOULD contain one global IP
address for each home agent currently listed in this home agent's
own Home Agents List (Section 10.1). However, if this home
agent's own global IP address would be placed as the first entry
in the list (as described below), then this home agent SHOULD NOT
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 87]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
include its own address in the Home Agent Addresses field in the
Reply message. Not placing this home agent's own IP address in
the list will cause the receiving mobile node to consider this
home agent as the most preferred home agent; otherwise, this home
agent will be considered to be preferred in its order given by
its place in the list returned.
- The IP addresses in the Home Agent Addresses field SHOULD
be listed in order of decreasing preference 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).
- Among home agents with equal preference, their IP addresses
in the Home Agent Addresses field SHOULD be listed in an
order randomized with respect to other home agents with equal
preference, each time a Home Agent Address Discovery Reply
message is returned by this home agent.
- For each entry in this home agent's Home Agents List, if more
than one global IP address is associated with this list entry,
then one of these global IP addresses SHOULD be selected to
include in the Home Agent Addresses field in the Reply message.
The selected global IP address for each home agent to include in
forming the Home Agent Addresses field in the Reply message MUST
be the global IP address of the respective home agent sharing a
prefix with the Destination IP address of the Request message.
If no such global IP address is known for some home agent, an
entry for that home agent MUST NOT be included in the Home Agent
Addresses field in the Reply message.
- 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]).
- If the Reply message packet must be truncated to fit within the
minimum IPv6 MTU, and the home agent sending the message is
not the highest priority, then its address MUST appear in the
list sent to avoid implying that it is the highest priority.
Therefore, if this home agent would not appear in the truncated
list because it is of lower priority than the last entry, this
home agent's address must be substituted for the last entry.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 88]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
10.6. Sending Prefix Information to the Mobile Node
10.6.1. Aggregate 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 with Prefix Information Options, which give
the valid lifetime and preferred lifetime for available prefixes on
the home link.
A mobile node on a remote network SHOULD autoconfigure all of the
global IP addresses, which it would autoconfigure if it were attached
to its home network and which are from prefixes served by home
agents. Site-local addresses MAY be autoconfigured if the mobile
node is roaming in a network on the same site as its home addresses.
Site-local addresses and addresses not served by a home agent MUST
NOT be autoconfigured, since they are unusable in the remote network.
To support this, the home agent monitors prefixes advertised by
itself and other home agents routers on the home link, and passes
this aggregated list of relevant subnet prefixes on to the mobile
node in Mobile Prefix Advertisements.
The home agent SHOULD construct the aggregate list of home subnet
prefixes as follows:
- Copy prefix information defined in the home agent's AdvPrefixList
on the home subnet's interfaces to the aggregate list. Also
apply any changes made to the AdvPrefixList on the home agent to
the aggregate list.
- Check valid prefixes received in Router Advertisements from the
home network for consistency with the home agent's AdvPrefixList,
as specified in Section 6.2.7 of RFC 2461 [12]. Do not update
the aggregate list with any information from received prefixes
that fail this check.
- For Router Advertisements which have the Home Agent (H) bit
set, check valid prefixes that are not yet in the aggregate
list. If a Prefix Information option has the autonomous address
configuration (A) flag set and the prefix length is valid
for address autoconfiguration on the home subnet, add these
advertisements and preserve the on-link (L) flag value. Clear
the Router Address (R) flag and zero the interface-id portion of
the prefix field to prevent mobile nodes from treating another
router's interface address as belonging to the home agent. Treat
the lifetimes of these prefixes as decrementing in real time, as
defined in Section 6.2.7 of RFC 2461 [12].
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 89]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- Do not perform consistency checks on valid prefixes received
in Router Advertisements on the home network that do not exist
in the home agent's AdvPrefixList. Instead, if the prefixes
already exist in the aggregate list, update the prefix lifetime
fields in the aggregate list according to the rules specified for
hosts in Section 6.3.4 of RFC 2461 [12] and Section 5.5.3 of RFC
2462 [13].
- If the L flag is set on valid prefixes received in a Router
Advertisement, and that prefix already exists in the aggregate
list, set the flag in the aggregate list. Ignore the flag if it
is clear.
- Delete prefixes from the aggregate list when their valid
lifetimes expire.
The home agent uses the information in the aggregate list to
construct Mobile Prefix Advertisements. It may be possible to
construct an aggregate list by combining information contained in the
home agent's AdvPrefixList and its Home Agents List used for Dynamic
Home Agent Address Discovery (Section 11.4.1).
10.6.2. Scheduling Prefix Deliveries to the Mobile Node
A home agent serving a mobile node will schedule the delivery of new
prefix information to that mobile node when any of the following
conditions occur:
MUST:
- The valid or preferred lifetime or the state of the flags changes
for the prefix of the mobile node's registered home address.
- The mobile node requests the information with a Mobile Prefix
Solicitation (see Section 11.4.2).
MAY:
- A new prefix is added to the aggregate list.
- The valid or preferred lifetime or the state of the flags changes
for a prefix which is not used in any Binding Cache entry for
this mobile node.
The home agent uses the following algorithm to determine when to send
prefix information to the mobile node.
- If the mobile node has not received the prefix information within
the last HomeRtrAdvInterval (see Section 12) seconds, then
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 90]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
transmit the prefix information. This MAY be done according to a
periodically scheduled transmission.
- If a mobile node sends a solicitation, answer right away.
- If a prefix in the aggregate list that matches the mobile node's
home registration is added, or if its information changes in
any way that does not cause the mobile node's address to go
deprecated, ensure that a transmission is scheduled (as described
below), and calculate RAND_ADV_DELAY in order to randomize the
time at which the transmission is scheduled.
- If a home registration expires, cancel any scheduled
advertisements to the mobile node.
The aggregate list is sent in its entirety in all cases.
Suppose that the home agent already has scheduled the transmission
of a Mobile Prefix Advertisement to the mobile node. The home agent
deletes the previously scheduled transmission event and schedules
another advertisement to the mobile node.
Otherwise, the home agent computes a fresh value for RAND_ADV_DELAY,
the offset from the current time for the scheduled transmission
as follows. First calculate the maximum delay for the scheduled
Advertisement:
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))
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, if needed 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, up until a maximum of
PREFIX_ADV_RETRIES attempts (see Section 12). 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 91]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
if not all PREFIX_ADV_RETRIES have been retransmitted. If the
mobile node sends another Binding Update without returning home in
the meantime, the home agent SHOULD again begin transmitting the
unsolicited Advertisement.
If some condition as described above occurs on the home link 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
discards the old Advertisement.
10.6.3. Sending Advertisements to the Mobile Node
When sending a Mobile Prefix Advertisement to the mobile node, the
home agent MUST construct the packet as follows:
- The Source Address in the packet's IPv6 header MUST be set to
the home agent's IP address to which the mobile node addressed
its current home registration, or its default global home agent
address if no binding exists.
- If 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.
- A type 2 routing header MUST be included with the mobile node's
home address.
- IPsec headers SHOULD be supported and used.
- The home agent MUST send the packet as it would any other unicast
IPv6 packet that it originates.
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 be no greater than the
remaining valid lifetime for the subnet prefix in the mobile node's
home address. This limit on the binding lifetime serves to prohibit
use of a mobile node's home address after it becomes invalid.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 92]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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, for which the Lifetime sent in that
Binding Update has not yet expired. The Binding Update List includes
all bindings sent by the mobile node 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 any manner consistent with the external behavior
described in this document.
Each Binding Update List entry conceptually contains the following
fields:
- The IP address of the node to which a Binding Update was sent.
If the Binding Update was successfully received by that node
(e.g., not lost by the network), a Binding Cache entry may have
been created or updated based on this Binding Update. The
Binding Cache entry may still exist, if that node has not deleted
the entry before its expiration for some reason.
- The home address for which that Binding Update was sent.
- The care-of address sent in that Binding Update. This value
is necessary for the mobile node to determine if it has sent a
Binding Update giving its new care-of address to this destination
after changing its care-of address.
- The initial value of the Lifetime field sent in that Binding
Update.
- The remaining lifetime of that binding. This lifetime is
initialized from the Lifetime value sent in the Binding Update
and is decremented until it reaches zero, at which time this
entry MUST be deleted from the Binding Update List.
- The maximum value of the Sequence Number field sent in previous
Binding Updates to this destination. The Sequence Number field
is 16 bits long, and all comparisons between Sequence Number
values MUST be performed modulo 2**16 (see Section 9.5.1).
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 93]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- The time at which a Binding Update was last sent to this
destination, as needed to implement the rate limiting restriction
for sending Binding Updates.
- The state of any retransmissions needed for this Binding Update,
if the Acknowledge (A) bit was set in this Binding Update. This
state includes the time remaining until the next retransmission
attempt for the Binding Update, and the current state of the
exponential back-off mechanism for retransmissions.
- A flag 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 11.3.4.
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.
- The time at which a Home Test Init or Care-of Test Init message
was last sent to this destination, as needed to implement the
rate limiting restriction for the return routability procedure.
- The state of any retransmissions needed for this return
routability procedure. This state includes the time remaining
until the next retransmission attempt and the current state of
the exponential back-off mechanism for retransmissions.
- Cookie values used the Home Test Init and Care-of Test Init
messages.
- Home and care-of keygen tokens received from the correspondent
node.
- Home and care-of nonce indices received from the correspondent
node.
- The time at which each of the tokens and nonces was received
from this 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 94]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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:
- 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 send the packets directly to the correspondent node.
Otherwise, if a binding does not exist, the mobile node MUST use
reverse tunneling. Detailed operation for both of these cases is
described later in this section.
- 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. An example of this type of communication might
be DNS queries sent by the mobile node [27, 28]. 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 mobile node may send packets to the correspondent node
that includes the home address destination option directly
to the correspondent node only if the mobile node is aware
that the correspondent node already has a Binding Cache entry
for the mobile node's home address. Section 9.3.1 specifies
the rules for Home Address Destination Option Processing at a
correspondent node. The mobile node needs to ensure that there
exists a Binding Cache entry for its home address so that the
correspondent node can process the packet.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 95]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- While not at its home link, the mobile node MUST NOT use its home
address (or the home address destination option) in Neighbor
Discovery messages on the visited link. The mobile node also
MUST NOT use its home address when communicating with link-local
or site-local peers on the visited link, if the scope of the home
address is larger than the scope of the peer's address.
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 any 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:
direct delivery
This is manner of delivering packets does not require going
through the home network, and typically will enable faster and
more reliable transmission. A mobile node SHOULD arrange to
supply the home address in a Home Address option, and allowing
the IPv6 header's Source Address field to be set to one of the
mobile node's care-of addresses; the correspondent node will
then use the address supplied in the Home Address option to
serve the function traditionally done by the Source IP address
in the IPv6 header. The mobile node's home address is then
supplied to higher protocol layers and applications.
Specifically:
- Construct the packet using the mobile node's home address
as the packet's Source Address, in the same way as if the
mobile node were at home. This 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.
- Change the Source Address field in the packet's IPv6 header
to one of the mobile node's care-of addresses. This will
typically be the mobile node's current primary care-of
address, but MUST be a care-of address with a subnet prefix
that is on-link on the network interface on which the
mobile node will transmit the packet.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 96]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 [23].
reverse tunneling
This is the mechanism which tunnels the packets via the home
agent. It isn't as efficient as the above mechanism, but is
needed if there is no binding yet with the correspondent node.
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.
Reverse tunneled packets MAY be protected using a AH or ESP
header, depending on the security policies used by the home
agent. The support for encrypted reverse tunneling allows
mobile nodes to defeat certain kinds of traffic analysis, and
provides a mechanism by which routers on the home network can
distinguish authorized traffic from other possibly malicious
traffic.
11.3.2. 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):
- 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.
- 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].
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 97]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- 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.
- Since the mobile node is away from home, the mobile is either
using reverse tunneling or route optimization to reach the
correspondent node.
If reverse tunneling is used, the packet is constructed in the
normal manner and then tunneled through the home agent.
If route optimization is in use, the mobile node inserts a Home
Address destination option into the packet, replacing the Source
Address in the packet's IP header with a care-of address suitable
for the link on which the packet is being sent, as described in
Section 11.3.1. 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. The AH authentication data MUST be calculated
as if the following were true:
* the IPv6 source address in the IPv6 header contains the
mobile node's home address,
* the Home Address field of the Home Address destination option
(Section 6.3) contains the new care-of address.
- This allows, but does not require, the receiver of the packet
containing a Home Address destination option to exchange the two
fields of the incoming packet, simplifying processing for all
subsequent packet headers. 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 towards 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 as
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 98]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
the automated key management protocol [9, 4], such problems can be
avoided by the following requirements:
- 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).
- 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 exchange,
giving the mobile node's home address as the initiator of the
Security Association [7].
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 three methods:
- Packets sent by a correspondent node that does not have a Binding
Cache entry for the mobile node, will be tunneled to the mobile
node via its home agent.
- Packets sent by a correspondent node that has a Binding Cache
entry for the mobile node that contains the mobile node's current
care-of address, will be sent by the correspondent node using
a type 2 routing header. The packet will be addressed to the
mobile node's care-of address, with the final hop in the routing
header directing the packet to the mobile node's home address;
the processing of this last hop of the routing header is entirely
internal to the mobile node, since the care-of address and home
address are both addresses within the mobile node.
For packets received by the first of these methods, the mobile node
MUST check that the IPv6 source address of the tunneled packet is the
IP address of its home agent.
For packets received by either the first or last of these three
methods, the mobile node SHOULD send a Binding Update to the original
sender of the packet, as described in Section 11.7.2, 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 above (using a type 2
routing header), the following rules will result in the packet being
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 99]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
processed normally by upper-layer protocols within the mobile node,
as if it had been addressed to the mobile node's home address.
A node receiving a packet addressed to itself (i.e., one of the
node's addresses is in the IPv6 destination field) follows the next
header chain of headers and processes them. When it encounters
a type 2 routing header during this processing it performs the
following checks. If any of these checks fail the node MUST silently
discard the packet.
- The length field in the routing header is exactly 2.
- The segments left field in the routing header is either 0 or 1.
(Values on the wire are always 1. 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.)
- 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, and resubmits the packet to IP
for processing the next header. Conceptually this follows the same
model as in RFC 2460. However, in the case of type 2 routing header
this can be simplified since it is known that the packet will not be
forwarded to a different node.
The definition of AH requires the sender to calculate the AH
integrity check value of a routing header in a way as it appears in
the receiver after it has processed the header. Since IPsec headers
follow the routing header, any IPsec processing will operate on
the packet with the home address in the IP destination field and
segments left being zero. Thus, the AH calculations at the sender
and receiver will have an identical view of the packet.
11.3.4. Receiving ICMP Error Messages
Any node that doesn't 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.
Correspondent nodes who have participated in the return routability
procedure MUST implement the ability to correctly process received
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 100]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
11.3.5. Routing Multicast Packets
A mobile node that is connected to its home link functions in the
same way as any other (stationary) node. Thus, when it is at home,
a mobile node functions identically to other multicast senders and
receivers. This section therefore describes the behavior of a mobile
node that is not on its home link.
In order to receive packets sent to some multicast group, a mobile
node must join that multicast group. One method by which a mobile
node MAY join the group is via a (local) multicast router on the
foreign link being visited. The mobile node SHOULD use one of its
care-of addresses that shares a subnet prefix with the multicast
router, as the source IPv6 address of its multicast group membership
control messages. The mobile node MUST NOT use the Home Address
destination option when sending MLD packets [29]
Alternatively, a mobile node MAY join multicast groups via a
bi-directional tunnel to its home agent. The mobile node tunnels its
multicast group membership control packets to its home agent, and the
home agent forwards multicast packets down the tunnel to the mobile
node.
A mobile node that wishes to send packets to a multicast group also
has two options:
1. Send directly on the foreign link being visited.
The application is aware of the care-of address and uses it for
multicast traffic just like any other stationary address. The
mobile node MUST NOT use Home Address destination option in such
traffic.
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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 101]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 CCoA 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 reverse tunnelling can ensure that multicast trees
are independent of the mobile nodes movement, in some case such
tunnelling 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 and is therefore bandwidth inefficient compared to
the native multicast forwarding in the foreign multicast system.
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, giving this home agent's own
global unicast IP address along with a list of the global unicast IP
address of each other home agent operating on the home link.
The mobile node, upon receiving this Home Agent Address Discovery
Reply message, MAY then send its home registration Binding Update to
the home agent address given as the IP Source Address of the packet
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 102]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
carrying the Reply message or to any of the unicast IP addresses
listed in the Home Agent Addresses field in the Reply. For example,
if necessary, the mobile node MAY attempt its home registration
with each of these home agents, in turn, by sending each a Binding
Update and waiting for the matching Binding Acknowledgement, until
its registration is accepted by one of these home agents. The mobile
node MUST, however, wait at least 1.5 times longer than (RetransTimer
* DupAddrDetectTransmits) 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 in the order listed in the Home Agent
Addresses field in the received Home Agent Address Discovery Reply
message. If the home agent identified by the Source Address field in
the IP header of the packet carrying the Home Agent Address Discovery
Reply message is not listed in the Home Agent Addresses field in the
Reply, it SHOULD be tried before the first address given in the list;
otherwise, it SHOULD be tried in its listed order.
If the mobile node has a current registration with some home agent
on its home link (the Lifetime for that registration has not yet
expired), then the mobile node MUST attempt any new registration
first with that home agent. If that registration attempt fails
(e.g., times out or is rejected), the mobile node SHOULD then
reattempt this registration with another home agent on its home link.
If the mobile node knows of no other suitable home agent, then it MAY
attempt the dynamic home agent address discovery mechanism described
above.
If, after a mobile node transmits a Home Agent Address Discovery
Request message to the Home Agents Anycast address, it does not
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 sends a Mobile Prefix Solicitation to its home agent
in an attempt to acquire fresh routing prefix information. The
new information also enables the mobile node to participate in
renumbering operations affecting the home network, as described in
Section 10.6.
The mobile node MUST use the Home Address destination option to carry
its home address and SHOULD use IPsec to protect the solicitation.
The mobile node SHOULD send a Solicitation to the home agent when
its home address will become invalid within MaxRtrAdvInterval
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 103]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
seconds, where this value is acquired in a previous Mobile Prefix
Advertisement from the home agent. If no such value is known, the
value MAX_PFX_ADV_DELAY seconds is used instead (see Section 12).
This solicitation follows the same retransmission rules specified for
Router Solicitations [12], except that the initial retransmission
interval is specified to be INITIAL_SOLICIT_TIMER (see Section 12).
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.
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:
- 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 104]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- The packet MUST have a type 2 routing header and SHOULD be
protected by an IPsec header as described in Sections 5.4
and 6.8.
Any received Mobile Prefix Advertisement not meeting this test MUST
be silently discarded. For advertisements that do not contain the
same ICMP Identifier value as in a recently sent solicitation, the
mobile node MUST send a solicitation and expect an advertisement with
a matching Identifier before further processing.
For an accepted Mobile Prefix Advertisement, the mobile node MUST
process the Prefix Information Options as if they arrived in a
Router Advertisement on the mobile node's home link [12]. Such
processing may result in the mobile node configuring a new home
address, although due to separation between preferred lifetime and
valid lifetime, such changes should not affect most communication
by the mobile node, in the same way as for nodes that are at home.
In this case, the mobile node MUST return a Binding Update, which
will be viewed by the home agent as an acknowledgement of the
corresponding Mobile Prefix Advertisement, which it can cease
transmitting. In addition, if the method used for this new home
address configuration would require the mobile node to perform
Duplicate Address Detection [13] for the new address if the mobile
node were located at home, then the mobile node MUST set the
Duplicate Address Detection (D) bit in this Binding Update to its
home agent, to request the home agent to perform this Duplicate
Address Detection on behalf of the mobile node.
11.5. Movement
11.5.1. Movement Detection
The primary movement detection mechanism for Mobile IPv6 defined
in this section uses the facilities of IPv6 Neighbor Discovery,
including Router Discovery and Neighbor Unreachability Detection.
The mobile node SHOULD supplement this mechanism with other
information whenever it is available to the mobile node (e.g.,
from lower protocol layers). The description here is based on the
conceptual model of the organization and data structures defined by
Neighbor Discovery [12].
Mobile nodes SHOULD use Router Discovery to discover new routers
and on-link subnet prefixes; a mobile node MAY send Router
Solicitations, or MAY wait for unsolicited (periodic) multicast
Router Advertisements, as specified for Router Discovery [12]. Based
on received Router Advertisements, a mobile node maintains an entry
in its Default Router List for each router, and an entry in its
Prefix List for each subnet prefix that it currently considers to be
on-link. Each entry in these lists has an associated invalidation
timer value. While away from home, a mobile node typically selects
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 105]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
one default router and one subnet prefix to use as the subnet
prefix in its primary care-of address. A mobile node MAY also have
associated additional care-of addresses, using other subnet prefixes
from its Prefix List. The method by which a mobile node selects
and forms a care-of address from the available subnet prefixes is
described in Section 11.5.2. The mobile node registers its primary
care-of address with its home agent, as described in Section 11.7.1.
While a mobile node is away from home, it is important for the mobile
node to quickly detect when its default router becomes unreachable.
When this happens, the mobile node SHOULD switch to a new default
router and potentially to a new primary care-of address. If, on the
other hand, the mobile node becomes unreachable from its default
router, it should attempt to become reachable through some other
router. To detect when its default router becomes unreachable, a
mobile node SHOULD use Neighbor Unreachability Detection.
For a mobile node to detect when it has become unreachable from its
default router, the mobile node cannot efficiently rely on Neighbor
Unreachability Detection alone, since the network overhead would
be prohibitively high in many cases. Instead, when a mobile node
receives any IPv6 packets from its current default router at all,
irrespective of the source IPv6 address, it SHOULD use that as an
indication that it is still reachable from the router.
Since the router SHOULD be sending periodic unsolicited multicast
Router Advertisements, the mobile node will have frequent opportunity
to check if it is still reachable from its default router, even
in the absence of other packets to it from the router. If Router
Advertisements that the mobile node receives include an Advertisement
Interval option, the mobile node MAY use its Advertisement Interval
field as an indication of the frequency with which it SHOULD expect
to continue to receive future Advertisements from that router. This
field specifies the minimum rate (the maximum amount of time between
successive Advertisements) that the mobile node SHOULD expect. If
this amount of time elapses without the mobile node receiving any
Advertisement from this router, the mobile node can be sure that at
least one Advertisement sent by the router has been lost. It is
thus possible for the mobile node to implement its own policy for
determining the number of Advertisements from its current default
router it is willing to tolerate losing before deciding to switch to
a different router from which it may currently be correctly receiving
Advertisements.
On some types of network interfaces, the mobile node MAY also
supplement this monitoring of Router Advertisements, by setting its
network interface into "promiscuous" receive mode, so that it is able
to receive all packets on the link, including those not addressed to
it at the link layer (i.e., disabling link-level address filtering).
The mobile node will then be able to detect any packets sent by the
router, in order to detect reachability from the router. This use of
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 106]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
promiscuous mode may be useful on very low bandwidth (e.g., wireless)
links, but its use MUST be configurable on the mobile node since it
is likely to consume additional energy resources.
If the above means do not provide indication that the mobile node
is still reachable from its current default router (for instance,
the mobile node receives no packets from the router for a period of
time), then the mobile node SHOULD attempt to actively probe the
router with Neighbor Solicitations, even if it is not otherwise
actively sending packets to the router. If it receives a solicited
Neighbor Advertisement in response from the router, then the mobile
node can deduce that it is still reachable. It is expected that the
mobile node will in most cases be able to determine its reachability
from the router by listening for packets from the router as described
above, and thus, such extra Neighbor Solicitation probes should
rarely be necessary.
With some types of networks, indications about link-layer mobility
might be obtained from lower-layer protocol or device driver software
within the mobile node. However, all link-layer mobility indications
from lower layers do not necessarily indicate a movement of the
mobile node to a new link, such that the mobile node would need to
switch to a new default router and primary care-of address. For
example, movement of a mobile node from one cell to another in
many wireless LANs can be made transparent to the IP level through
use of a link-layer "roaming" protocol, as long as the different
wireless LAN cells all operate as part of the same IP link with
the same subnet prefix. Upon lower-layer indication of link-layer
mobility, the mobile node MAY send Router Solicitations to determine
if additional on-link subnet prefixes are available on its new link.
Such lower-layer information might also be useful to a mobile node in
deciding to switch its primary care-of address to one of the other
care-of addresses it has formed from the on-link subnet prefixes
currently available through different routers from which the mobile
node is reachable. For example, a mobile node MAY use signal
strength or signal quality information (with suitable hysteresis) for
its link with the available routers to decide when to switch to a new
primary care-of address using that router rather than its current
default router (and current primary care-of address). Even though
the mobile node's current default router may still be reachable in
terms of Neighbor Unreachability Detection, the mobile node MAY use
such lower-layer information to determine that switching to a new
default router would provide a better connection.
11.5.2. Forming New Care-of Addresses
After detecting that it has moved from one link to another (i.e., its
current default router has become unreachable and it has discovered
a new default router), a mobile node SHOULD form a new primary
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 107]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
care-of address using one of the on-link subnet prefixes advertised
by the new router. A mobile node MAY form a new primary care-of
address at any time, except that it MUST NOT do so too frequently.
Specifically, a mobile node MUST NOT send a Binding Update about a
new care-of address to its home agent (which is required to register
the new address as its primary care-of address) more often than once
per MAX_UPDATE_RATE seconds.
In addition, after discovering a new on-link subnet prefix, a mobile
node MAY form a new (non-primary) care-of address using that subnet
prefix, even when it has not switched to a new default router. A
mobile node can have only one primary care-of address at a time
(which is registered with its home agent), but it MAY have an
additional care-of address for any or all of the prefixes on its
current link. Furthermore, since a wireless network interface may
actually allow a mobile node to be reachable on more than one link at
a time (i.e., within wireless transmitter range of routers on more
than one separate link), a mobile node MAY have care-of addresses
on more than one link at a time. The use of more than one care-of
address at a time is described in Section 11.5.3.
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 [30]) Address Autoconfiguration. If a mobile node needs to
send packets as part of the method of address autoconfiguration,
it MUST use an IPv6 link-local address rather than its own IPv6
home address as the Source Address in the IPv6 header of each such
autoconfiguration packet.
In some cases, a mobile node may already know a (constant) IPv6
address that has been assigned to it for its use only while
visiting a specific foreign link. For example, a mobile node may be
statically configured with an IPv6 address assigned by the system
administrator of some foreign link, for its use while visiting that
link. If so, rather than using Address Autoconfiguration to form a
new care-of address using this subnet prefix, the mobile node MAY use
its own pre-assigned address as its care-of address on this link.
A mobile node, after forming a new care-of address, MAY begin
using the new care-of address without performing Duplicate Address
Detection. Furthermore, the mobile node MAY continue using the
address without performing Duplicate Address Detection, although
it SHOULD in most cases. begin Duplicate Address Detection
asynchronously when it begins use of the address. This allows the
Duplicate Address Detection procedure to complete in parallel with
normal communication using the address, avoiding major delays for
some applications.
In addition, normal processing for Duplicate Address Detection
specifies that, in certain cases, the node SHOULD delay sending the
initial Neighbor Solicitation message of Duplicate Address Detection
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 108]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY [12, 13];
however, in this case, the mobile node SHOULD NOT perform such a
delay in its use of Duplicate Address Detection, unless the mobile
node is initializing after rebooting.
11.5.3. 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. A
mobile node SHOULD select a primary care-of address from among those
care-of addresses it has formed using any of these subnet prefixes,
based on the movement detection mechanism in use, as described in
Section 11.5.1. 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 [30], the mobile
node may not wish to release the address immediately upon switching
to a new primary care-of address.
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 defend the mobile node's home address
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 109]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
for Duplicate Address Detection (DAD). In particular, a Neighbor
Solicitation from the mobile node using its home address as the
Source Address would be detected by the home agent as a duplicate
address. In many cases, Neighbor Solicitation by the mobile node
for the home agent's address will not be necessary, since the mobile
node may have already learned the home agent's link-layer address,
for example from a Source Link-Layer Address option in the Router
Advertisement from which it learned that its home address was on-link
and that the mobile node had thus returned home.
If the mobile node does Neighbor Solicitation to learn the home
agent's link-layer address, in this special case of the mobile node
returning home, the mobile node MUST 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 home agent's IPv6 address, which is
known to the mobile node. The destination IP address MUST be set to
the Solicited-Node multicast address [3]. The home agent will be
unable to distinguish this solicitation from a similar packet that
would only be used for DAD, and it will respond as if for DAD. The
home agent will send a multicast Neighbor Advertisement back to the
mobile node with the Solicited flag (S) set to zero. The mobile node
SHOULD accept this advertisement, and set the state of the Neighbor
Cache entry for the home agent to REACHABLE.
The mobile node then sends its Binding Update using the home agent's
link-layer address, instructing its home agent to no longer serve
as a home agent for it. By processing this Binding Update, the
home agent will cease defending the mobile node's home address for
Duplicate Address Detection and will no longer respond to Neighbor
Solicitations for the mobile node's home address. The mobile node
is then the only node on the link receiving packets at the mobile
node's home address. In addition, when returning home prior to the
expiration of a current binding for its home address, and configuring
its home address on its network interface on its home link, the
mobile node MUST NOT perform Duplicate Address Detection on its own
home address, in order to avoid confusion or conflict with its home
agent's use of the same address. If the mobile node returns home
after the bindings for all of its care-of addresses have expired,
then it SHOULD perform DAD. It SHOULD also perform DAD for addresses
which may have been registered with 'D' and 'S' bits set to one.
After the Mobile Node sends the Binding Update, the Home Agent MUST
remove the Proxy Neighbor Cache entry for the Mobile Node and MAY
learn its link-layer address based on the link-layer packet or cached
information, or if that is not available, it SHOULD send a Neighbor
Solicitation with the target address equal to the Binding Update's
source IP address. The Mobile Node MUST then reply with a unicast
Neighbor Advertisement to the Home Agent with its link-layer address.
While the Mobile Node is waiting for a Binding Acknowledgement, it
MUST NOT respond to any Neighbor Solicitations for its Home Address
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 110]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
other than those originating from the IP address to which it sent the
Binding Update.
After receiving the Binding Acknowledgement for its Binding Update to
its home agent, the mobile node MUST multicast onto the home link (to
the all-nodes multicast address) a Neighbor Advertisement [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 up to MAX_ADVERT_REXMIT times to increase
their reliability. It is still possible that some nodes on the home
link will not receive any of these Neighbor Advertisements, but these
nodes will eventually be able to recover through use of Neighbor
Unreachability Detection [12].
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 Home and Care-of Test Init Messages
A mobile node that initiates a return routability procedure MUST
send (in parallel) a Home Test Init message and a Care-of Test Init
messages. However, if the mobile node has recently received 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 and has
previously visited the same care-of address so that it also has a
recent care-of keygen token. If the mobile node sets the Lifetime to
zero or the care-of address in the Binding Update equal to its home
address - such as when returning home - it MUST use the home keygen
token and nonce index by itself (without a care-of keygen token and
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 111]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
nonce index). 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:
- The IP address of the node to which the message was sent.
- 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.
- The time at which each of these messages was sent.
- 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 List entries.
11.6.2. Receiving Return Routability Messages
Upon receiving a packet carrying a Home Test message, a mobile node
MUST validate the packet according to the following tests:
- The Header Len field in the Mobility Header is greater than or
equal to the length specified in Section 6.1.5.
- The Source Address of the packet belongs to a correspondent
node for which the mobile node has a Binding Update List entry
with a state indicating that return routability procedure is in
progress. Note that there may be multiple such entries.
- The Binding Update List indicates that no home keygen token has
been received yet.
- The Destination Address of the packet has the home address of the
mobile node, and the packet has been received in a tunnel from
the home agent.
- 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 112]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 additional messages.
Upon receiving a packet carrying a Care-of Test message, a mobile
node MUST validate the packet according to the following tests:
- The Header Len field in the Mobility Header is greater than or
equal to the length specified in Section 6.1.6.
- The Source Address of the packet belongs to a correspondent
node for which the mobile node has a Binding Update List entry
with a state indicating that return routability procedure is in
progress. Note that there may be multiple such entries.
- The Binding Update List indicates that no care-of keygen token
has been received yet.
- The Destination Address of the packet is the current care-of
address of the mobile node.
- 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 additional messages.
If after receiving either the Home Test or the Care-of Test message
and performing the above actions, the Binding Update List entry has
both the home and the care-of 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 10.4.4.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 113]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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
Sections 11.5.1 and 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 MUST
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.
In both of these situations, the mobile node sends a packet to its
home agent containing a Binding Update, with the packet constructed
as follows:
- The Home Registration (H) bit MUST be set in the Binding Update.
- The Acknowledge (A) bit MUST be set in the Binding Update.
- The packet MUST contain a Home Address destination option, giving
the mobile node's home address for the binding.
- 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 MAY be included when the mobile node so desires, and
MUST be included if the mobile node cannot be assured that the
IPsec AH protocol is used to secure the Binding Update. The ESP
protocol will not be able to protect care-of addresses in the
IPv6 header. Mobile IPv6 implementations which are unaware of
how IPsec secures their messaging will therefore need to use the
Alternate Care-of Address option.
- The Single Address Only (S) bit is cleared to request a binding
for all home addresses of the mobile node. These addresses are
based on the interface identifier of the home address indicated
in the Binding Update, and all on-link subnet prefixes on the
home link. When this bit is cleared, the Link-Local Address
Compatibility (L) bit MUST be set.
If the mobile node desires that only a single home address should
be affected by this Binding Update, the Single Address Only (S)
bit is set to 1.
The value of the Single Address Only (S) bit MUST be set
equivalently for subsequent de-registrations and re-registrations
with the same addresses.
- If the mobile node's link-local address has the same interface
identifier as the home address for which it is supplying a new
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 114]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
care-of address, then the mobile node SHOULD set the Link-Local
Address Compatibility (L) bit.
- If the home address was generated using RFC 3041 [17], then the
link local address is unlikely to have a compatible interface
identifier. In this case, the mobile node MUST set the
Single Address Only (S) bit and clear the Link-Local Address
Compatibility (L) bit.
- The value specified in the Lifetime field SHOULD be less than
or equal to the remaining lifetime of the home address and the
care-of address specified for the binding.
The Acknowledge (A) bit in the Binding Update requests the home agent
to return a Binding Acknowledgement in response to this Binding
Update. As described in Section 6.1.8, the mobile node SHOULD
retransmit this Binding Update to its home agent until it receives
a matching Binding Acknowledgement. Once reaching a retransmission
timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart
the process of delivering the Binding Update, but trying instead the
next home agent 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.
Depending on the value of the Single Address Only (S) bit in the
Binding Update, the home agent is requested to serve either a single
home address or all home addresses for the mobile node. Until the
lifetime of this registration expires, the home agent considers
itself the home agent for each such home address of the mobile node.
As the set of on-link subnet prefixes on the home link changes over
time, the home agent changes the set of home addresses for this
mobile node for which it is serving as the home agent.
Each Binding Update MUST be authenticated as coming from the right
mobile node, as defined in Section 5.1. 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 right 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 right Sequence Number value, it may start at any
value. If the home agent rejects the value, it sends back a Binding
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 115]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Acknowledgement with 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 using a different
interface identifier, then the mobile node SHOULD send an additional
packet containing a Binding Update to its home agent to register the
care-of address for each such other home address (or set of home
addresses sharing an interface identifier).
While the mobile node is away from home, it relies on the home
agent to participate in Duplicate Address Detection (DAD) to defend
its home address against stateless autoconfiguration performed by
another node. Therefore, the mobile node SHOULD set the Duplicate
Address Detection (D) bit based on any requirements for DAD that
would apply to the mobile node if it were at home [12, 13]. If the
mobile node's recent Binding Update was accepted by the home agent,
and the lifetime for that Binding Update has not yet expired, the
mobile node SHOULD NOT set the Duplicate Address Detection (D) bit in
the new Binding Update; the home agent will already be defending the
home address(es) of the mobile node and does not need to perform DAD
again.
The home agent will only perform DAD for the mobile node's home
address when the mobile node has supplied a valid binding between
its home address and a care-of address. If some time elapses during
which the mobile node has no binding at the home agent, it might
be possible for another node to autoconfigure the mobile node's
home address. Therefore, the mobile node MUST treat creation of
a new binding with the home agent using an existing home address
the same as creation of a new home address. In the unlikely event
that the mobile node's home address is autoconfigured as the IPv6
address of another network node on the home network, the home agent
will reply to the mobile node's subsequent Binding Update with a
Binding Acknowledgement containing a Status of 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 care-of
addresses for its other home addresses, if any. The mobile node MAY
also attempt to acquire a new home address to replace the one for
which Status 134 was received, for instance by using the techniques
described in Appendix B.5.
11.7.2. Correspondent Binding Procedure
When the mobile node is assured that its home address is valid, it
MAY at any time initiate a correspondent binding procedure with
the purpose of allowing the correspondent node to cache the mobile
node's current care-of address. The mobile node is responsible for
the initiation and completion of this procedure, as well as any
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 116]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
retransmissions that may be needed (subject to the rate limiting
defined in Section 11.8).
This section defines the rules that the mobile node must follow when
performing the correspondent binding procedure.
The mobile node can be assured that its home address is still
valid, for example, by the home agent's use the Duplicate Address
Detection (D) bit of Binding Updates (see Section 10.3.1). In any
Binding Update sent by a mobile node, the care-of address (either the
Source Address in the packet's IPv6 header or the Care-of Address in
the Alternate Care-of Address mobility option of the Binding Update)
MUST be set to one of the care-of addresses currently in use by the
mobile node or to the mobile node's home address. A mobile node MAY
set the care-of address differently for sending Binding Updates to
different correspondent nodes.
A mobile node MAY choose to keep its location private from
certain correspondent nodes, and thus need not initiate the
return routability procedure, or send new Binding Updates to those
correspondents. A mobile node MAY also send a Binding Update to
such a correspondent node to instruct it to delete any existing
binding for the mobile node from its Binding Cache, as described in
Section 6.1.7. However, all Binding Updates to the correspondent
node require the successful completion of the return routability
procedure first, as no other IPv6 nodes are authorized to send
Binding Updates on behalf of a mobile node.
If set to one of the mobile node's current care-of addresses (the
care-of address given MAY differ from the mobile node's primary
care-of address), the Binding Update requests the correspondent node
to create or update an entry for the mobile node in the correspondent
node's Binding Cache in order to record this care-of address for use
in sending future packets to the mobile node. In this case, the
value specified in the Lifetime field sent in the Binding Update
SHOULD be less than or equal to the remaining lifetime of the home
address and the care-of address specified for the binding.
If the care-of address is set to the mobile node's home address
or the Lifetime field set to zero, the Binding Update requests
the correspondent node to delete any existing Binding Cache entry
that it has for the mobile node. 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.
After the mobile node has sent a Binding Update to its home
agent to register a new primary care-of address (as described in
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 117]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Section 11.7.1), the mobile node SHOULD send a Binding Update to each
other node for which an entry exists in the mobile node's Binding
Update List, as detailed below. Typically this requires starting a
return routability procedure. Upon successful return routability
procedure and after receiving a successful Binding Acknowledgement
from the Home Agent, a Binding Update is sent to all other nodes.
Thus, other relevant nodes are generally kept updated about the
mobile node's binding and can send packets directly to the mobile
node using the mobile node's current care-of address.
The mobile node, however, need not initiate these actions immediately
after configuring a new care-of address. For example, the mobile
node MAY delay initiating the return routability procedure to any
correspondent node for a short period of time, if it isn't certain
that there is any significant traffic to the correspondent node.
In addition, when a mobile node receives a packet for which the
mobile node can deduce that the original sender of the packet either
has no Binding Cache entry for the mobile node, or a stale entry
for the mobile node in its Binding Cache, the mobile node SHOULD
initiate a return routability procedure with the sender, in order to
finally update the sender's Binding Cache with the current care-of
address (subject to the rate limiting defined in Section 11.8). In
particular, the mobile node SHOULD initiate a return routability
procedure in response to receiving a packet that meets all of the
following tests:
- The packet was tunneled using IPv6 encapsulation.
- The Destination Address in the tunnel (outer) IPv6 header is
equal to any of the mobile node's care-of addresses.
- The Destination Address in the original (inner) IPv6 header is
equal to one of the mobile node's home addresses.
- The Source Address in the tunnel (outer) IPv6 header differs from
the Source Address in the original (inner) IPv6 header.
The destination address to which the procedure should be initiated to
in response to receiving a packet meeting all of the above tests is
the Source Address in the original (inner) IPv6 header of the packet.
The home address for which this Binding Update is sent should be the
Destination Address of the original (inner) packet.
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 MAY request an acknowledgement by setting the Acknowledge
(A) bit in the Binding Update. In this case, however, the mobile
node SHOULD NOT continue to retransmit the Binding Update once the
retransmission timeout period has reached MAX_BINDACK_TIMEOUT.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 118]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
The mobile node SHOULD create a Binding Update as follows:
- The Source Address of the IPv6 header MUST contain the current
care-of address of the mobile node.
- The Destination Address of the IPv6 header MUST contain the
address of the correspondent node.
- The Mobility Header is constructed according to rules in
Section 6.1.7 and 5.2.6, including the Binding Authorization Data
(calculated as defined in Section 6.2.6) and possibly the Nonce
Indices mobility options.
- 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 a Sequence Number greater than the Sequence
Number value sent in the previous Binding Update (if any) to the same
destination address modulo 2**16, as 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.
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:
- The packet meets the authentication requirements for Binding
Acknowledgements, defined in Sections 6.1.8 and 5. That is,
if the Binding Update was sent to the home agent, underlying
IPsec protection is used. If the Binding Update was sent to
the correspondent node, the Binding Authorization Data mobility
option MUST be present and have a valid value.
- The Binding Authorization Data mobility option, if present, MUST
be the last option and MUST not have trailing padding.
- The Header Len field in the Binding Acknowledgement is greater
than or equal to the length specified in Section 6.1.8.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 119]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- 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:
- If the Status field indicates that the Binding Update was
accepted (the Status field is less than 128), then the mobile
node MUST update the corresponding entry in its Binding Update
List to indicate that the Binding Update has been acknowledged;
the mobile node MUST then stop retransmitting the Binding Update.
In addition, if the value specified in the Lifetime field in the
Binding Acknowledgement is less than the Lifetime value sent
in the Binding Update being acknowledged, then the mobile node
MUST subtract the difference between these two Lifetime values
from the remaining lifetime for the binding as maintained in the
corresponding Binding Update List entry (with a minimum value
for the Binding Update List entry lifetime of 0). That is, if
the Lifetime value sent in the Binding Update was L_update, the
Lifetime value received in the Binding Acknowledgement was L_ack,
and the current remaining lifetime of the Binding Update List
entry is L_remain, then the new value for the remaining lifetime
of the Binding Update List entry should be
max((L_remain - (L_update - L_ack)), 0)
where max(X, Y) is the maximum of X and Y. The effect of this
step is to correctly manage the mobile node's view of the
binding's remaining lifetime (as maintained in the corresponding
Binding Update List entry) so that it correctly counts down from
the Lifetime value given in the Binding Acknowledgement, but with
the timer countdown beginning at the time that the Binding Update
was sent.
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.
- If the Status field indicates that the Binding Update was
rejected (the Status field is greater than or equal to 128), then
the mobile node MUST delete the corresponding Binding Update List
entry, and it MUST also stop retransmitting the Binding Update.
Optionally, the mobile node MAY then take steps to correct the
cause of the error and retransmit the Binding Update (with a new
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 120]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Sequence Number value), subject to the rate limiting restriction
specified in Section 11.8.
The treatment of a Binding Refresh Advice mobility option within the
Binding Acknowledgement depends on the 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 Refresh Interval field in the option as a suggestion that
it SHOULD attempt to refresh its home registration at the indicated
shorter interval.
11.7.4. Receiving Binding Refresh Requests
When a mobile node receives a packet containing a Binding Refresh
Request message and there already exists a Binding Update List entry
for the source of the Binding Refresh Request, it MAY start a return
routability procedure. The mobile node MAY also choose to either
ignore the Binding Refresh Request or to delete its binding from the
sender of the Binding Refresh Request. Note that the mobile node
SHOULD NOT respond Binding Refresh Requests from previously unknown
correspondent nodes due to Denial-of-Service concerns.
If the return routability procedure completes successfully, a
Binding Update message SHOULD be sent as described in Section 11.7.2.
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 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.
Instead, if the mobile node chooses to delete its binding from the
sender of the Binding Refresh Request, the mobile node SHOULD return
a Binding Update to the sender with the Lifetime specified as zero
and specify a Care-of Address that matches the home address for the
binding.
11.7.5. Receiving Binding Error Messages
When a mobile node receives a packet containing a Binding Error
message, it should first check if the mobile node has a Binding
Update List entry for the source of the Binding Error message. If
the mobile node does not have such entry, it MUST ignore the message.
This is necessary to prevent a waste of resources on e.g. return
routability procedure due to spoofed Binding Error messages.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 121]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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:
- If the mobile node does have a Binding Update List entry but
has recent upper layer progress information that indicates
communications with the correspondent node are progressing, it
MAY ignore the message. This can be done in order to limit the
damage that spoofed Binding Error messages can cause to ongoing
communications.
- If the mobile node does have a Binding Update List entry but
no upper layer progress information, it MUST remove the entry
and route further communications through the home agent. It
MAY also optionally start a return routability procedure (see
Section 5.2).
If the message Status field was 2 (unrecognized MH Type value), the
mobile node should perform one of the following two actions:
- If the mobile node is not expecting an acknowledgement or
response from the correspondent node, the mobile node SHOULD
ignore this message.
- Otherwise, the mobile node SHOULD cease the use of any extensions
to this specification. If no extensions had been used, the
mobile node should cease the attempt to use route optimization.
11.8. Retransmissions and Rate Limiting
The mobile node is responsible for retransmissions and rate limiting
in the return routability and binding procedures.
When the mobile node sends a 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:
- If the mobile node is sending a Binding Update and it does not
have an existing binding at the home agent, it SHOULD use a value
for the initial retransmission timer that is at least 1.5 times
longer than (RetransTimer * DupAddrDetectTransmits). This value
is likely to be substantially longer than the otherwise specified
value of INITIAL_BINDACK_TIMEOUT (see Section 12) that would be
used by the mobile node. This longer retransmission interval
will allow the home agent to complete the DAD procedure which is
mandated in this case, as detailed in Section 11.7.1.
- Otherwise, the mobile node should use the specified value of
INITIAL_BINDACK_TIMEOUT for the initial retransmission timer.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 122]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 sufficient amount of time to
answer 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 often than
once per MAX_UPDATE_RATE seconds.
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
HomeRtrAdvInterval 3,600 seconds
DHAAD_RETRIES 3 retransmissions
INITIAL_BINDACK_TIMEOUT 1 second
INITIAL_DHAAD_TIMEOUT 2 seconds
INITIAL_SOLICIT_TIMER 2 seconds
MAX_ADVERT_REXMIT 3 transmissions
MAX_BINDACK_TIMEOUT 256 seconds
MaxMobPfxAdvInterval 86,400 seconds
MAX_NONCE_LIFE 240 seconds
MAX_TOKEN_LIFE 210 seconds
MAX_RR_BINDING_LIFE 420 seconds
MAX_UPDATE_RATE once per second
MinDelayBetweenRAs 0.05 seconds
MinMobPfxAdvInterval 600 seconds
PREFIX_ADV_RETRIES 3 retransmissions
PREFIX_ADV_TIMEOUT 5 seconds
SLOW_UPDATE_RATE once per 10 second interval
The value MinDelayBetweenRAs overrides the value of the protocol
constant MIN_DELAY_BETWEEN_RAS, as specified in RFC 2461 [12].
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 123]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
13. IANA Considerations
This document defines a new IPv6 protocol, the Mobility Header,
described in Section 6.1. This protocol must be assigned a protocol
number. The MH Type field in the Mobility Header is used to indicate
a particular type of a message. The current message types are
described in Sections 6.1.2 through 6.1.9, and include the following:
0 Binding Refresh Request
1 Home Test Init
2 Care-of Test Init
3 Home Test
4 Care-of Test
5 Binding Update
6 Binding Acknowledgement
7 Binding Error
Future values of the MH Type can be allocated using standards
action [10].
Furthermore, each mobility message may contain mobility options as
described in Section 6.2. The current mobility options are defined
in Sections 6.2.2 through 6.2.7, and include the following:
0 Pad1
1 PadN
3 Alternate Care-of Address
4 Nonce Indices
5 Authorization Data
6 Binding Refresh Advice
Future values of the Option Type can be allocated using standards
action [10].
This document also defines a new IPv6 destination option, the Home
Address option, described in Section 6.3. This option must be
assigned an Option Type value.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 124]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
This document also defines a new IPv6 type 2 routing header,
described in Section 6.4. The value 2 is to be allocated by IANA
when this specification becomes an RFC.
In addition, this document defines four ICMP message types, two used
as part of the dynamic home agent address discovery mechanism and
two used in lieu of Router Solicitations and Advertisements when the
mobile node is away from the home link:
- The Home Agent Address Discovery Request message, described in
Section 6.5;
- The Home Agent Address Discovery Reply message, described in
Section 6.6;
- The Mobile Prefix Solicitation, described in Section 6.7; and
- The Mobile Prefix Advertisement, described in Section 6.8.
This document also defines two new Neighbor Discovery [12] options,
which must be assigned Option Type values within the option numbering
space for Neighbor Discovery messages:
- The Advertisement Interval option, described in Section 7.3; and
- The Home Agent Information option, described in Section 7.4.
14. Security Considerations
14.1. 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:
1. 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 125]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 doesn't 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 towards
networks instead of nodes. Further variations of this threat are
described elsewhere [31, 32].
An attacker might also attempt to disrupt a mobile node's
communications by replaying a Binding Update that the node had
sent earlier. If the old Binding Update was accepted, packets
destined for the mobile node would be sent to its old location
and not its current location.
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.
2. Threats associated with payload packets: Payload packets
exchanged with mobile nodes are exposed to similar threats as
regular IPv6 traffic is. 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 126]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
address appears in the option. In this case, ingress filtering
would not catch the forged "return address" [33] [34].
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 be able forge also packets that appear to come directly
from the mobile node. This is a 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.
3. Threats associated with dynamic home agent and prefix discovery.
4. 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 in the light of being applicable to
the whole Internet.
14.2. Features
This specification provides a number of security features designed to
mitigate or alleviate the threats listed above. The main security
features are the following:
- Reverse Tunneling as a mandatory feature.
- Protection of Binding Updates sent to home agents.
- Protection of Binding Updates sent to correspondent nodes.
- Protection against reflection attacks that use the Home Address
destination option.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 127]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- Protection of tunnels between the mobile node and the home agent.
- Closing routing header vulnerabilities.
- 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 expected to be naturally subject to the
network administration of the home domain.
Thus, they can and are supposed to have a strong 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 14.3 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 these 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 14.4 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
Sections 14.7, 14.8, and 14.9.
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 14.4.5.
14.3. Binding Updates to Home Agent
Signaling between the mobile node and the home agent requires message
integrity, correct ordering and replay protection. This is necessary
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 128]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
to assure the home agent that a Binding Update is from a legitimate
mobile node.
IPsec AH or ESP protects the integrity of the Binding Updates and
Binding Acknowledgements, by securing mobility messages between the
mobile node and the home agent. For ESP, a non-null authentication
algorithm MUST be applied.
However, IPsec can easily provide replay protection only if dynamic
security association establishment is used. This may not always be
possible, and manual keying would be preferred in some cases. IPsec
also does not guarantee correct ordering of packets, only that they
have not been replayed. Because of this, sequence numbers with the
Mobile IPv6 messages ensure correct ordering (see Section 5.1).
However, if a home agent reboots and loses its state regarding the
sequence numbers, replay attacks become possible. he use of a key
management mechanism together with IPsec can be used to prevent such
replay attacks.
A sliding window scheme is used for the sequence numbers. The
protection against replays and reordering attacks without a key
management mechanism works when the attacker remembers up to a
maximum of 2**15 Binding Updates.
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 where forwarding from a previous care-of address is used,
a router in the visited network must act as a temporary home agent
for the mobile node. Nevertheless, the same security requirements
apply in this case. That is, a pre-arranged security association
must exist even with the temporary home agent. This limits the use
of the forwarding feature to those networks where such arrangements
are practical.
Note that the use of a single pair of manually keyed security
associations conflicts with the generation of a new home
addresses [17] for the mobile node, or with the adoption of a
new home prefix. This is because IPsec SAs 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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 129]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
or automatic procedures to establish new certificates. Therefore,
this specification limits restricts the generation of new home
addresses (for any reason) to those situations where there already
exists a security association or certificate for the new address.
(Section B.4 lists the improvement of security for new addresses as
one of the future developments for Mobile IPv6.)
14.4. Binding Updates to Correspondent Nodes
14.4.1. Overview
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 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:
- 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.
- The eventual Binding Update is cryptographically bound to the
tokens supplied in the exchanged messages.
- 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 as the request was
sent from.
- The correspondent node operates in a stateless manner until it
receives a fully authorized Binding Update.
- Some additional protection is provided by encrypting the tunnels
between the mobile node and home agent with IPsec ESP. As the
tunnel transports also the nonce exchanges, this limits the
ability of attackers to see these nonces. For instance, this
prevents attacks launched from the mobile node's current foreign
link where no link-layer confidentiality is available.
For further information about the design rationale of the return
routability procedure, see [31, 32, 35, 34]. The used mechanisms
have been adopted from these documents.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 130]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
14.4.2. Offered Protection
This 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 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 Mobile IPv6 is in use.
This procedure also protects against Denial-of-Service attacks in
which the attacker pretends to be a mobile, but uses the victim's
address as the care of address. This would cause the correspondent
node to send the victim some unexpected traffic. The procedure
defends against these attacks by requiring at least 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.
The Binding Acknowledgement is not authenticated in other ways than
including the right sequence number in the reply.
14.4.3. 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 in 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 MitM. (Since IPv6 communications are relatively
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 131]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
well protected against off-path attackers, it is important that
Mobile IPv6 prevents off-path attacks as well.)
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 same as those as in IPv6,
but Masquerade and MitM attacks can now be launched also 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 stay during the lifetime of the binding. This
happens even if the attacker moves away from the link. In regular
IPv6, the attacker generally has to be 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, MitM,
and DoS 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 attacker faces
in these attacks is getting to 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 probe
messages.
In conclusion, we can state the following main results from this
comparison:
- Return routability procedure prevents any off-path attacks beyond
those that are already possible in regular IPv6. This is the
most important result, and prevents attackers from the Internet
from exploiting any vulnerabilities.
- 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.
- 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_LIFE seconds after the last
routability check has been performed, and limits the duration of
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 132]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
a binding to at most MAX_RR_BINDING_LIFE seconds. With these
limitation, attackers cannot take practical advantages of this
vulnerability. This limited vulnerability can also be compared
to similar vulnerabilities in IPv6 Neighbor Discovery, with
Neighbor Cache entries having a limited lifetime.
- There are some other minor differences, such as an effect
to the DoS vulnerabilities. These can be considered to be
insignificant.
- 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 make the return
routability procedure the easiest route for attackers. For this
reason, this specification should have a protection mechanism for
selecting between return routability and potential other future
mechanisms.
For a more in-depth discussion of these issues, see [34].
14.4.4. Return Routability Replays
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 would not verify after such 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.
14.4.5. Return Routability Denial-of-Service
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
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 133]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
Nevertheless, as [31] 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 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 MUST allow route
optimization to be administratively enabled or disabled. The default
SHOULD be enabled.
14.5. 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. Attackers
will not be able to learn much from this information, however, and
mobile nodes cannot be tricked into using wrong home agents as all
other communication with the home agents is secure.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 134]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
14.6. Prefix Discovery
The prefix discovery function may leak interesting information
about network topology and prefix lifetimes to eavesdroppers,
and for this reason requests for this information have 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.
14.7. 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 the outer IP address
corresponds to the current location of the mobile node. This
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 also send spoofed packets without using a tunnel.
Home agents and mobile nodes may use IPsec AH or ESP to protect
payload packets tunneled between themselves. This is useful to
protect communications against attackers on the path of the tunnel.
When site local home address are used, reverse tunneling can be used
to send site local traffic from another location. Administrators
should be aware of this when allowing such home addresses. In
particular, the outer IP address check described above is not
sufficient against all attackers. The use of encrypted tunnels is
particularly useful for this kind of home addresses.
14.8. 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. Ingress filtering [23] works therefore 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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 135]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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 used when a binding has already been established
with the participation of the node at the home address, as described
in Sections 5.5 and 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 as the mobile node
sends traffic from.
No special authentication of the Home Address option is required
beyond the above, except that if the IPv6 header of a packet is
covered by authentication, then that authentication MUST also cover
the Home Address option; this coverage is achieved automatically by
the definition of the Option Type code for the Home Address option
(Section 6.3), since it indicates that the option is included in the
authentication computation. Thus, even when authentication is used
in the IPv6 header, the security of the Source Address field in the
IPv6 header is not compromised by the presence of a Home Address
option. Without authentication of the packet, then any field in the
IPv6 header, including the Source Address field, and any other parts
of the packet, including the Home Address option, can be forged or
modified in transit. In this case, the contents of the Home Address
option is no more suspect than any other part of the packet.
14.9. 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.
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.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 136]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
This implies that a device which implements filtering of packets
should be able to distinguish between a type 2 routing header and
other routing headers, as required in Section 8.3. This is necessary
in order to allow Mobile IPv6 traffic while still having the option
to filter out other uses of routing headers.
Contributors
Tuomas Aura, Mike Roe, Greg O'Shea (Microsoft), Pekka Nikander
(Ericsson), Erik Nordmark (Sun Microsystems), and Michael Thomas
(Cisco) worked on the return routability protocols which eventually
led to the procedures used in this protocol. The procedures
described in [32] 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 (Sun Microsystems) and Pekka
Nikander (Ericsson), who have contributed volumes of text to this
specification.
Acknowledgements
We would like to thank the members of the Mobile IP and IPng Working
Groups for their comments and suggestions on this work. We would
particularly like to thank (in alphabetical order) Fred Baker
(Cisco), Josh Broch (Carnegie Mellon University), Samita Chakrabarti
(Sun Microsystems), Robert Chalmers (University of California, Santa
Barbara), Noel Chiappa (MIT), Vijay Devarapalli (Nokia Research
Center), Rich Draves (Microsoft Research), Francis Dupont (ENST
Bretagne), Thomas Eklund (Xelerated), Jun-Ichiro Itojun Hagino (IIJ
Research Laboratory), Brian Haley (Compaq), John Ioannidis (AT & T
Labs Research), James Kempf (DoCoMo), Rajeev Koodli (Nokia), Krishna
Kumar (IBM Research), T.J. Kniveton (Nokia Research), Joe Lau (HP),
Jiwoong Lee (KTF), Aime Le Rouzic (Bull S.A.), Vesa-Matti Mantyla
(Ericsson), Kevin Miles (Cisco), Glenn Morrow (Nortel Networks),
Thomas Narten (IBM), Karen Nielsen (Ericsson Telebit), Simon Nybroe
(Ericsson Telebit), David Oran (Cisco), Brett Pentland (Monash
University), Lars Henrik Petander (HUT), Basavaraj Patil (Nokia),
Mohan Parthasarathy (Tahoe Networks), Alexandru Petrescu (Motorola),
Mattias Petterson (Ericsson), Ken Powell (HP), Phil Roberts
(Megisto), Patrice Romand (Bull S.A.), Jeff Schiller (MIT), Pekka
Savola (Netcore), Arvind Sevalkar (Intinfotech), Keiichi Shima (IIJ
Research Laboratory), Tom Soderlund (Nokia Research), Hesham Soliman
(Ericsson), Jim Solomon (RedBack Networks), Tapio Suihko (Technical
Research Center of Finland), Dave Thaler (Microsoft), Benny Van Houdt
(University of Antwerp), Jon-Olov Vatn (KTH), Vladislav Yasevich
(HP), Alper Yegin (DoCoMo), and Xinhua Zhao (Stanford University) for
their detailed reviews of earlier versions of this document. Their
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 137]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
suggestions have helped to improve both the design and presentation
of the protocol.
We would also like to thank the participants in the Mobile IPv6
testing event held at Nancy, France, September 15-17, 1999, for their
valuable feedback as a result of interoperability testing of four
Mobile IPv6 implementations coming from four different organizations:
Bull, Ericsson Research and Ericsson Telebit, NEC, and INRIA.
Further, we would like to thank the feedback from the implementors
who participated in the Mobile IPv6 interoperability testing
at Connectathons 2000, 2001, and 2002 in San Jose, California.
Similarly, we would like to thank the participants at the ETSI
interoperability testing at ETSI, in Sophia Antipolis, France, during
October 2-6, 2000, including teams from Compaq, Ericsson, INRIA,
Nokia, and Technical University of Helsinki.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 138]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
References
[1] D. Eastlake, 3rd, S. Crocker, and J. Schiller. Randomness
Recommendations for Security. Request for Comments
(Informational) 1750, Internet Engineering Task Force, December
1994.
[2] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. Request for Comments (Best Current Practice) 2119,
Internet Engineering Task Force, March 1997.
[3] R. Hinden and S. Deering. IP Version 6 Addressing Architecture.
Request for Comments (Proposed Standard) 2373, Internet
Engineering Task Force, July 1998.
[4] S. Kent and R. Atkinson. Security Architecture for the Internet
Protocol. Request for Comments (Proposed Standard) 2401,
Internet Engineering Task Force, November 1998.
[5] S. Kent and R. Atkinson. IP Authentication Header. Request for
Comments (Proposed Standard) 2402, Internet Engineering Task
Force, November 1998.
[6] S. Kent and R. Atkinson. IP Encapsulating Security Payload
(ESP). Request for Comments (Proposed Standard) 2406, Internet
Engineering Task Force, November 1998.
[7] D. Piper. The Internet IP Security Domain of Interpretation for
ISAKMP. Request for Comments (Proposed Standard) 2407, Internet
Engineering Task Force, November 1998.
[8] D. Maughan, M. Schertler, M. Schneider, and J. Turner. Internet
Security Association and Key Management Protocol (ISAKMP).
Request for Comments (Proposed Standard) 2408, Internet
Engineering Task Force, November 1998.
[9] D. Harkins and D. Carrel. The Internet Key Exchange (IKE).
Request for Comments (Proposed Standard) 2409, Internet
Engineering Task Force, November 1998.
[10] T. Narten and H. Alvestrand. Guidelines for Writing an IANA
Considerations Section in RFCs. Request for Comments (Best
Current Practice) 2434, Internet Engineering Task Force, October
1998.
[11] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. Request for Comments (Draft Standard) 2460,
Internet Engineering Task Force, December 1998.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 139]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
[12] T. Narten, E. Nordmark, and W. Simpson. Neighbor Discovery for
IP Version 6 (IPv6). Request for Comments (Draft Standard)
2461, Internet Engineering Task Force, December 1998.
[13] S. Thomson and T. Narten. IPv6 Stateless Address
Autoconfiguration. Request for Comments (Draft Standard) 2462,
Internet Engineering Task Force, December 1998.
[14] A. Conta and S. Deering. Internet Control Message Protocol
(ICMPv6) for the Internet Protocol version 6 (IPv6)
specification. Request for Comments (Draft Standard) 2463,
Internet Engineering Task Force, December 1998.
[15] A. Conta and S. Deering. Generic Packet Tunneling in IPv6
Specification. Request for Comments (Proposed Standard) 2473,
Internet Engineering Task Force, December 1998.
[16] D. Johnson and S. Deering. Reserved IPv6 Subnet Anycast
Addresses. Request for Comments (Proposed Standard) 2526,
Internet Engineering Task Force, March 1999.
[17] T. Narten and R. Draves. Privacy Extensions for Stateless
Address Autoconfiguration in IPv6. Request for Comments
(Proposed Standard) 3041, Internet Engineering Task Force,
January 2001.
[18] Editor J. Reynolds. Assigned Numbers: RFC 1700 is Replaced by
an On-line Database. Request for Comments (Informational) 3232,
Internet Engineering Task Force, January 2002.
[19] National Institute of Standards and Technology. Secure hash
standard. Technical Report NIST FIPS PUB 180-1, U.S. Department
of Commerce, April 1995.
[20] C. Perkins. IP Mobility Support. Request for Comments
(Proposed Standard) 2002, Internet Engineering Task Force,
October 1996.
[21] C. Perkins. IP Encapsulation within IP. Request for Comments
(Proposed Standard) 2003, Internet Engineering Task Force,
October 1996.
[22] C. Perkins. Minimal Encapsulation within IP. Request for
Comments (Proposed Standard) 2004, Internet Engineering Task
Force, October 1996.
[23] P. Ferguson and D. Senie. Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP source address
spoofing. Request for Comments (Informational) 2267, Internet
Engineering Task Force, January 1998.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 140]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
[24] Jari Arkko, Vijay Devarapalli, and Francis Dupont. Using IPsec
to Protect Mobile IPv6 signaling between Mobile Nodes and Home
Agents (work in progress). Internet Draft, Internet Engineering
Task Force, October 2002.
[25] H. Krawczyk, M. Bellare, and R. Canetti. HMAC: Keyed-Hashing
for Message Authentication. Request for Comments
(Informational) 2104, Internet Engineering Task Force,
February 1997.
[26] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. Request for Comments (Proposed Standard) 1883,
Internet Engineering Task Force, December 1995.
[27] P. V. Mockapetris. Domain names - concepts and facilities.
Request for Comments (Standard) 1034, Internet Engineering Task
Force, November 1987.
[28] P. V. Mockapetris. Domain names - implementation and
specification. Request for Comments (Standard) 1035, Internet
Engineering Task Force, November 1987.
[29] S. Deering, W. Fenner, and B. Haberman. Multicast Listener
Discovery (MLD) for IPv6. Request for Comments (Proposed
Standard) 2710, Internet Engineering Task Force, October 1999.
[30] J. Bound, C. Perkins, M. Carney, and R. Droms. Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) (work in progress).
Internet Draft, Internet Engineering Task Force, January 2001.
[31] Tuomas Aura and Jari Arkko. MIPv6 BU Attacks and Defenses (work
in progress). Internet Draft, Internet Engineering Task Force,
February 2002.
[32] Michael Roe, Greg O'Shea, Tuomas Aura, and Jari Arkko.
Authentication of Mobile IPv6 Binding Updates and
Acknowledgments (work in progress). Internet Draft,
Internet Engineering Task Force, February 2002.
[33] Pekka Savola. Security of IPv6 Routing Header and Home
Address Options (work in progress). Internet Draft, Internet
Engineering Task Force, November 2001.
[34] Erik Nordmark, Gabriel Montenegro, Pekka Nikander, and
Jari Arkko. Mobile IPv6 Security Design Rationale (work in
progress). Internet Draft, Internet Engineering Task Force,
2002.
[35] Erik Nordmark. Securing MIPv6 BUs using Return Routability
(BU3WAY) (work in progress). Internet Draft, Internet
Engineering Task Force, November 2001.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 141]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
References [1] through [19] are normative and others are informative.
A. Changes from Previous Version of the Draft
This appendix briefly lists some of the major changes in this
draft relative to the previous version of this same draft,
draft-ietf-mobileip-ipv6-18.txt:
A.1. Changes from Draft Version 18
- The draft no longer requires Home Address option and Binding
Error support from all nodes. Similarly, we no longer support
Home Address options protected solely using IPsec (tracked issues
53 and 54).
- Dynamic home agent address advertisement optimizations for
excluding the sender's own address have been aligned with the
priority mechanism (tracked issue 56).
- Units for Binding Update and Acknowledgement lifetimes have been
aligned, and Status code values are now consistent across the
document (tracked issue 58, 91).
- The ability to use link-local and site-local care-of addresses,
home agent addresses, and home addresses has been clarified
(tracked issues 62 and 94).
- Clarified the kind of multicast support provided in the base
Mobile IPv6 specification (tracked issue 63).
- Inconsistencies on using routing headers and Binding
Acknowledgment have been removed (tracked issue 65).
- Semantics for de-registration with the Single Address Only (S)
bit have been specified (tracked issue 66).
- More exact rules for how to use IPsec between the mobile node
and home agent have been provided in this draft as well as in a
separate informative draft (tracked issue 69).
- Rules for when the Alternate Care-of Address mobility option is
needed have been clarified (tracked issue 70).
- Forwarding from previous care-of address has be deprecated
(tracked issue 72).
- New values for MaxRtAdvInterval have been provided (tracked issue
73).
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 142]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- The rules on how care-of address can be used for some
communications have been clarified (tracked issue 74)
- State machine description has been removed and only the normative
text remains (tracked issue 76).
- Rules for processing Mobility Header messages have been clarified
(tracked issue 77).
- Rules on how to not use Home Address destination option in
Neighbor Discovery packets have been introduced (tracked issue
78).
- Behavior after an address collision has been specified (tracked
issue 79).
- There are no longer specific rules for re-starting return
routability procedure after a Binding Refresh Request has been
received (tracked issue 82).
- It is no longer required to clear the contents of the Binding
Cache upon reboot (tracked issue 83).
- Rules for filling the Home Address field within the Binding Error
message have been clarified (tracked issue 85).
- Binding Acknowledgement length and padding values have been
corrected (tracked issue 87).
- MIN_DELAY_BETWEEN_RAS has been redefined (tracked issue 88).
- The MH Type field has been shortened to 8 bits and MH Length no
longer includes the first 8 bytes (tracked issues 89 and 93).
- It has been clarified that the Home Address option may be used
within the Mobility Header checksum calculation. Also Mobility
Header is considered as an upper layer protocol for the purposes
of checksum calculation (tracked issues 90 and 111).
- Reflection attacks using Binding Acknowledgements have been
prevented (tracked issue 92).
- References to routing headers indicate the type (tracked issue
95).
- The rules for when new nonces are needed have been clarified, as
has the rules for (re-)using keygen tokens (tracked issues 96,
103).
- Binding Refresh Advice type number has been corrected (tracked
issue 97).
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 143]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- Keygen tokens are now produced with a different formula for home
and care-of tokens (tracked issue 98).
- Binding Acknowledgements with Status code 136-138 are no longer
authenticated (tracked issue 99).
- New requirements have been placed to Section 8.
- The coverage of the Authenticator has been clarified (tracked
issue 106).
- Rules for registering home bindings with the Link-Local Address
Compatibility (L) bit have been improved (tracked issue 108).
- Type 0 routing headers has been specified as orthogonal to type 2
usage (tracked issue 109).
- The inclusion of nonce indices has been made mandatory when
return routability is the authorization method for correspondent
bindings (tracked issue 113).
- Invalid Home and Care-of Test Init messages have to be silently
discarded (tracked issue 114).
- The Binding Authorization Data mobility option is required to be
the last one (tracked issue 115).
- The use of zero lifetime and home addresses in de-registration
and Binding in Refresh Request responses has been clarified
(tracked issue 116).
- Home keygen tokens are now sufficient for de-registration
(tracked issue 117).
- A new Status code has been added to signal the expiry of both
nonces (tracked issue 118).
- Kbm length has been changed to 20 bytes (tracked issue 119).
- Unique Identifier mobility option has been removed (tracked issue
121).
- The security mechanisms and requirements for dynamic home agent
address and prefix discovery have been included (tracked issues
123 and 124).
- Processing order for route headers has been corrected (tracked
issue 125).
- Rate-limiting and retransmission procedures have been combined
and simplified (tracked issues 126 and 136).
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 144]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- The allowed start time for return routability procedure has been
specified (tracked issue 127).
- Rules for regenerating nonces and Kcn have been changed to
accommodate situations where these values have not been used at
all (tracked issue 131).
- The correspondent node's address which is used in Binding
Authorization Data calculation has been specified to take in
account Home Address destination option (tracked issue 133).
- The matching rules for Home and Care-of Test messages against
sent Init messages have been specified (tracked issue 138).
- Rules for when Home Address destination option may appear in
Binding Updates have been changed and made consistent (tracked
issue 139).
- Authenticator calculation shall precede checksum calculation
(tracked issue 140).
- Rules for sending Binding Acknowledgement errors have been made
consistent (tracked issue 142).
- Invalid authenticator and route optimization not desired Status
values have been removed, and values higher than these have been
renumbered (tracked issues 100).
- The acknowledgement for Mobile Prefix Advertisements is now
Mobile Prefix Solicitation, and not a Binding Update (tracked
issue 144).
- Multiple tries to different home agents are now timed in a manner
that does not cause problems for Duplicate Address Detection
(tracked issue 145).
- Correspondent node binding updates can be secured with also
pre-configured binding management key in addition to return
routability (tracked issue 146).
- Router Advertisement and prefix rules have been clarified
(tracked issue 147).
- Requirements section has been completed to include all necessary
requirements (tracked issue 148).
- Implementations have been given the freedom to implement the
security association - home address check either in the security
policy data base or in the mobile IPv6 code (tracked issue 149).
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 145]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
- A procedure has been provided to deal with failed
de-registration, to ensure that the Binding Acknowledgement still
reaches the mobile node (tracked issue 150).
- Binding Error messages are now sent only to unicast addresses
(tracked issue 151).
- Mobile nodes are now expected to limit their requested bindings
to valid, not preferred, lifetime (tracked issue 152).
- Acknowledgements are now recommended for correspondent bindings
(tracked issue 153).
- A large number of editorial modifications have been performed,
including some restructuring of the document. Some of these
modifications have been tracked as issues 52, 55, 57, 59, 64, 67,
84, 86, 102, 104, 107, 112, 120, 122, 128, 130, 137.
B. Future Extensions
B.1. Piggybacking
This document does not specify how to piggyback payload packets on
the binding related messages. However, it is envisioned that this
can be specified in a separate document when currently discussed
issues such as the interaction between piggybacking and IPsec are
fully resolved (see also Section B.3). The return routability
messages can indicate support for piggybacking with a new mobility
option.
B.2. Triangular Routing and Unverified Home Addresses
Due to the concerns about opening reflection attacks with the Home
Address destination option, this specification requires that this
option must be verified against the Binding Cache, i.e., there must
be a Binding Cache entry for the Home Address and Care-of Address.
Future extensions may be specified that allow the use of unverified
Home Address destination options in ways that do not introduce
security issues.
B.3. New Authorization Methods beyond Return Routability
While the return routability procedure provides a good level
of security, there exists methods that have even higher levels
of security. Secondly, as discussed in Section 14.4, future
enhancements of IPv6 security may cause a need to improve also the
security of the return routability procedure. Using IPsec as the
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 146]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
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.
B.4. Security and 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.
B.5. Remote Home Address Configuration
The method for initializing a mobile node's home addresses on
power-up or after an extended period of being disconnected from
the network is beyond the scope of this specification. Whatever
procedure is used should result in the mobile node having the same
stateless or stateful (e.g., DHCPv6) home address autoconfiguration
information it would have if it were attached to the home network.
Due to the possibility that the home network could be renumbered
while the mobile node is disconnected, a robust mobile node would not
rely solely on storing these addresses locally.
Such a mobile node could initialize by using the following procedure:
1. Generate a care-of address.
2. Query DNS for the home network's mobile agent anycast address.
3. Send a Home Agent Address Discovery Request message to the home
network.
4. Receive Home Agent Address Discovery Reply.
5. Select the most preferred home agent and establish a security
association between the mobile node's current care-of address and
the home agent for temporary use during initialization only.
6. Send a Home Prefix Solicitation with the Request All Prefixes
flag set to the home agent from the mobile node's care-of
address.
7. Receive a Home Prefix Advertisement from the home agent, follow
stateless address autoconfiguration rules to configure home
addresses for prefixes received.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 147]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
8. Create a security association between the mobile node's home
address and the home agent.
9. Send a Binding Update(s) to the home agent to register the mobile
node's home addresses.
10. Receive Binding Acknowledgement(s) then begin normal
communications.
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 148]
INTERNET-DRAFT Mobility Support in IPv6 29 Oct 2002
Chairs' Addresses
The Working Group can be contacted via its current chairs:
Basavaraj Patil Phil Roberts
Nokia Corporation Megisto Corp.
6000 Connection Drive Suite 120
M/S M8-540 20251 Century Blvd
Irving, TX 75039 Germantown MD 20874
USA USA
Phone: +1 972-894-6709 Phone: +1 847-202-9314
Fax : +1 972-894-5349 Email: PRoberts@MEGISTO.com
EMail: Raj.Patil@nokia.com
Authors' Addresses
Questions about this document can also be directed to the authors:
David B. Johnson Charles E. Perkins
Rice University Nokia Research Center
Dept. of Computer Science, MS 132
6100 Main Street 313 Fairchild Drive
Houston, TX 77005-1892 Mountain View, CA 94043
USA USA
Phone: +1 713 348-3063 Phone: +1 650 625-2986
Fax: +1 713 348-5930 Fax: +1 650 625-2502
E-mail: dbj@cs.rice.edu E-mail: charliep@iprg.nokia.com
Jari Arkko
Ericsson
Jorvas 02420
Finland
Phone: +358 40 5079256
E-mail: jari.arkko@ericsson.com
Johnson, Perkins, Arkko Expires 29 April 2003 [Page 149]
