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IETF Mobile IP Working GroupD. Johnson
Internet-DraftRice University
Expires: July 21, 2003C. Perkins
 Nokia Research Center
 J. Arkko
 Ericsson
 January 20, 2003

Mobility Support in IPv6
draft-ietf-mobileip-ipv6-20.txt

Status of this Memo

This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.

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 Internet-Draft will expire on July 21, 2003.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

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.



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Table of Contents


1. Introduction
2. Comparison with Mobile IP for IPv4
3. Terminology
     3.1 General Terms
     3.2 Mobile IPv6 Terms
4. Overview of Mobile IPv6
     4.1 Basic Operation
     4.2 New IPv6 Protocol
     4.3 New IPv6 Destination Option
     4.4 New IPv6 ICMP Messages
     4.5 Conceptual Data Structure Terminology
     4.6 Site-Local Addressability
5. Overview of Mobile IPv6 Security
     5.1 Binding Updates to Home Agents
     5.2 Binding Updates to Correspondent Nodes
            5.2.1 Node Keys
            5.2.2 Nonces
            5.2.3 Cookies and Tokens
            5.2.4 Cryptographic Functions
            5.2.5 Return Routability Procedure
            5.2.6 Authorizing Binding Management Messages
            5.2.7 Updating Node Keys and Nonces
            5.2.8 Preventing Replay Attacks
     5.3 Dynamic Home Agent Address Discovery
     5.4 Prefix Discovery
     5.5 Payload Packets
6. New IPv6 Protocol, Message Types, and Destination Option
     6.1 Mobility Header
            6.1.1 Format
            6.1.2 Binding Refresh Request Message
            6.1.3 Home Test Init Message
            6.1.4 Care-of Test Init Message
            6.1.5 Home Test Message
            6.1.6 Care-of Test Message
            6.1.7 Binding Update Message
            6.1.8 Binding Acknowledgement Message
            6.1.9 Binding Error Message
     6.2 Mobility Options
            6.2.1 Format
            6.2.2 Pad1
            6.2.3 PadN
            6.2.4 Binding Refresh Advice
            6.2.5 Alternate Care-of Address
            6.2.6 Nonce Indices
            6.2.7 Binding Authorization Data
     6.3 Home Address Option
     6.4 Type 2 Routing Header
            6.4.1 Format
     6.5 ICMP Home Agent Address Discovery Request Message
     6.6 ICMP Home Agent Address Discovery Reply Message
     6.7 ICMP Mobile Prefix Solicitation Message Format
     6.8 ICMP Mobile Prefix Advertisement Message Format
7. Modifications to IPv6 Neighbor Discovery
     7.1 Modified Router Advertisement Message Format
     7.2 Modified Prefix Information Option Format
     7.3 New Advertisement Interval Option Format
     7.4 New Home Agent Information Option Format
     7.5 Modified Neighbor Solicitation Message Format
     7.6 Changes to Sending Router Advertisements
     7.7 Changes to Duplicate Address Detection
8. Requirements for Types of IPv6 Nodes
     8.1 All IPv6 Nodes
     8.2 IPv6 Nodes with Support for Route Optimization
     8.3 All IPv6 Routers
     8.4 IPv6 Home Agents
     8.5 IPv6 Mobile Nodes
9. Correspondent Node Operation
     9.1 Conceptual Data Structures
     9.2 Processing Mobility Headers
     9.3 Packet Processing
            9.3.1 Receiving Packets with Home Address Option
            9.3.2 Sending Packets to a Mobile Node
            9.3.3 Sending Binding Error Messages
            9.3.4 Receiving ICMP Error Messages
     9.4 Return Routability Procedure
            9.4.1 Receiving Home Test Init Messages
            9.4.2 Receiving Care-of Test Init Messages
            9.4.3 Sending Home Test Messages
            9.4.4 Sending Care-of Test Messages
     9.5 Processing Bindings
            9.5.1 Receiving Binding Updates
            9.5.2 Requests to Cache a Binding
            9.5.3 Requests to Delete a Binding
            9.5.4 Sending Binding Acknowledgements
            9.5.5 Sending Binding Refresh Requests
     9.6 Cache Replacement Policy
10. Home Agent Operation
     10.1 Conceptual Data Structures
     10.2 Processing Mobility Headers
     10.3 Processing Bindings
            10.3.1 Primary Care-of Address Registration
            10.3.2 Primary Care-of Address De-Registration
     10.4 Packet Processing
            10.4.1 Intercepting Packets for a Mobile Node
            10.4.2 Tunneling Intercepted Packets
            10.4.3 Multicast Membership Control
            10.4.4 Stateful Address Autoconfiguration
            10.4.5 Handling Reverse Tunneled Packets
            10.4.6 Protecting Return Routability Packets
     10.5 Dynamic Home Agent Address Discovery
            10.5.1 Receiving Router Advertisement Messages
     10.6 Sending Prefix Information to the Mobile Node
            10.6.1 Aggregate List of Home Network Prefixes
            10.6.2 Scheduling Prefix Deliveries
            10.6.3 Sending Advertisements
            10.6.4 Lifetimes for Changed Prefixes
11. Mobile Node Operation
     11.1 Conceptual Data Structures
     11.2 Processing Mobility Headers
     11.3 Packet Processing
            11.3.1 Sending Packets While Away from Home
            11.3.2 Interaction with Outbound IPsec Processing
            11.3.3 Receiving Packets While Away from Home
            11.3.4 Routing Multicast Packets
            11.3.5 Receiving ICMP Error Messages
            11.3.6 Receiving Binding Error Messages
     11.4 Home Agent and Prefix Management
            11.4.1 Dynamic Home Agent Address Discovery
            11.4.2 Sending Mobile Prefix Solicitations
            11.4.3 Receiving Mobile Prefix Advertisements
     11.5 Movement
            11.5.1 Movement Detection
            11.5.2 Forming New Care-of Addresses
            11.5.3 Using Multiple Care-of Addresses
            11.5.4 Returning Home
     11.6 Return Routability Procedure
            11.6.1 Sending Test Init Messages
            11.6.2 Receiving Test Messages
            11.6.3 Protecting Return Routability Packets
     11.7 Processing Bindings
            11.7.1 Sending Binding Updates to the Home Agent
            11.7.2 Correspondent Binding Procedure
            11.7.3 Receiving Binding Acknowledgements
            11.7.4 Receiving Binding Refresh Requests
     11.8 Retransmissions and Rate Limiting
12. Protocol Constants
13. Protocol Configuration Variables
14. IANA Considerations
15. Security Considerations
     15.1 Threats
     15.2 Features
     15.3 Binding Updates to Home Agent
     15.4 Binding Updates to Correspondent Nodes
            15.4.1 Overview
            15.4.2 Achieved Security Properties
            15.4.3 Comparison to Regular IPv6 Communications
            15.4.4 Return Routability Replays
            15.4.5 Return Routability Denial-of-Service
     15.5 Dynamic Home Agent Address Discovery
     15.6 Prefix Discovery
     15.7 Tunneling via the Home Agent
     15.8 Home Address Option
     15.9 Type 2 Routing Header
16. Contributors
17. Acknowledgements
     § Normative References
     § Informative References
     § Authors' Addresses
A. Changes from Previous Version of the Draft
B. Future Extensions
     B.1 Piggybacking
     B.2 Triangular Routing
     B.3 New Authorization Methods
     B.4 Dynamically Generated Home Addresses
     B.5 Remote Home Address Configuration
     § Intellectual Property and Copyright Statements



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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:



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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)[23][24][25], 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:



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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.
interface identifier

A number used to identify a node's interface on a link. The interface identifier is the remaining low-order bits in the node's IP address after the subnet prefix.
link-layer address

A link-layer identifier for an interface, such as IEEE 802 addresses on Ethernet links.
packet

An IP header plus payload.
security association

An IPsec security association is a simplex "connection" that affords security services to the traffic carried by it. Security services are afforded to a security association by the use of the AH and ESP protocols.
security policy database

A database that specifies what security services are to be offered to IP packets and in what fashion.
destination option

Destination options are carried by the IPv6 Destination Options extension header. Destination options include optional information that need be examined only by the IPv6 node given as the destination address in the IPv6 header, not by other intermediate routing nodes. Mobile IPv6 defines one new destination option, the Home Address destination option (see Home Address Option).
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 Type 2 Routing Header.
'|' (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 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 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 Mobility Header).
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 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.



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4. Overview of Mobile IPv6

4.1 Basic Operation

A mobile node is always expected to be addressable at its home address, whether it is currently attached to its home link or is away from home. The "home address" is an IP address assigned to the mobile node within its home subnet prefix on its home link. While a mobile node is at home, packets addressed to its home address are routed to the mobile node's home link, using conventional Internet routing mechanisms.

While a mobile node is attached to some foreign link away from home, it is also addressable at one or more care-of addresses. A care-of address is an IP address associated with a mobile node that has the subnet prefix of a particular foreign link. The mobile node can acquire its care-of address through conventional IPv6 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 is specified in Mobile Node Operation, and the operation of the home agent is specified in Home Agent Operation.

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 Correspondent Node Operation.

There are two possible modes for communications between the mobile node and a correspondent node. The first mode, bidirectional tunneling, does not require Mobile IPv6 support from the correspondent node and is available even if the mobile node has not registered its current binding with the correspondent node. Packets from the correspondent node are routed to the home agent and then tunneled to the mobile node. Packets to the correspondent node are tunneled from the mobile node to the home agent ("reverse tunneled") and then routed normally from the home network to the correspondent node. In this mode, the home agent uses proxy Neighbor Discovery to intercept any IPv6 packets addressed to the mobile node's home address (or home addresses) on the home link. Each intercepted packet is tunneled to the mobile node's primary care-of address. This tunneling is performed using IPv6 encapsulation[15].

The second mode, "route optimization", requires the mobile node to register its current binding at the correspondent node. Packets from the correspondent node can be routed directly to the care-of address of the mobile node. When sending a packet to any IPv6 destination, the correspondent node checks its cached bindings for an entry for the packet's destination address. If a cached binding for this destination address is found, the node uses a new type of IPv6 routing header[11] (see Type 2 Routing Header) 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 Type 2 Routing Header) 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 Home Address Option) 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 starting from ICMP Home Agent Address Discovery Request Message.

4.2 New IPv6 Protocol

Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header (see Mobility Header). 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 Return Routability Procedure.
Binding Update

A Binding Update is used by a mobile node to notify a correspondent node or the mobile node's home agent of its current binding. The Binding Update sent to the mobile node's home agent to register its primary care-of address is marked as a "home registration".
Binding Acknowledgement

A Binding Acknowledgement is used to acknowledge receipt of a Binding Update, if an acknowledgement was requested in the Binding Update.
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.
Binding Error

The Binding Error is used by the correspondent node to signal an error related to mobility, such as an inappropriate attempt to use the Home Address destination option without an existing binding.

4.3 New IPv6 Destination Option

Mobile IPv6 defines a new IPv6 destination option, the Home Address destination option. This option is described in detail in Home Address Option.

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 Dynamic Home Agent Address Discovery and Dynamic Home Agent Address Discovery, the following two new ICMP message types are used for home agent address discovery:

The next two message types are used for network renumbering and address configuration on the mobile node, as described in Sending Prefix Information to the Mobile Node:

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 Conceptual Data Structures) and "home registration" entries (see Conceptual Data Structures).
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 Conceptual Data Structures.
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 Conceptual Data Structures. The list is used for informing mobile nodes during dynamic home agent address discovery.

4.6 Site-Local Addressability

This specification requires that home and care-of addresses MUST be unicast routable addresses. Site-local addresses may be usable on networks that are not connected to the Internet, but this specification does not define when such usage is safe and when not. Mobile nodes may not be aware of which site they are currently on, it is hard to prevent accidental attachment to other sites, and ambiguity of site-local addresses can cause problems the home and visited networks use the same addresses. Therefore, site-local addresses SHOULD NOT be used as home or care-of addresses.



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5. Overview of Mobile IPv6 Security

This specification provides a number of security features. These include the protection of Binding Updates both to home agents and correspondent nodes, the protection of prefix discovery, and the protection of the mechanisms that Mobile IPv6 uses for transporting data packets.

Binding Updates are protected by the use of IPsec extension headers, or by the use of the Binding Authorization Data option. This option employs a binding management key, Kbm, which can be established through the return routability procedure. Prefix discovery is protected through the use of IPsec extension headers. Mechanisms related to transporting payload packets - such as the Home Address destination option and type 2 routing header - have been specified in a manner which restricts their use in attacks.

5.1 Binding Updates to Home Agents

The mobile node and the home agent MUST use an IPsec security association to protect the integrity and authenticity of the Binding Updates and Acknowledgements. Both the mobile nodes and the home agents SHOULD use the Encapsulating Security Payload (ESP)[6] header in transport mode and MUST use a non-NULL payload authentication algorithm to provide data origin authentication, connectionless integrity and optional anti-replay protection. Note that Authentication Header (AH)[5] is also possible but for brevity not discussed in this specification.

In order to protect messages exchanged between the mobile node and the home agent with IPsec, appropriate security policy database entries must be created. A mobile node must be prevented from using its security association to send a Binding Update on behalf of another mobile node using the same home agent. This MUST be achieved by having the home agent check that the given home address has been used with the right security association. Such a check is provided in the IPsec processing, by having the security policy database entries unequivocally identify a single security association for any given home address and home agent. In order to make this possible, it is necessary that the home address of the mobile node is visible in the Binding Updates and Acknowledgements. The home address is used in these packets as a source or destination, or in the Home Address Destination option or the type 2 routing header.

As with all IPsec security associations in this specification, manual configuration of security associations MUST be supported. The used shared secrets MUST be random and unique for different mobile nodes, and MUST be distributed off-line to the mobile nodes.

Automatic key management with IKE[9] MAY be supported. When IKE is used, either the security policy database entries or the MIPv6 processing MUST unequivocally identify the IKE phase 1 credentials which can be used to authorize the creation of security associations for a particular home address. How these mappings are maintained is outside the scope of this specification, but they may be maintained, for instance, as a locally administered table in the home agent. If the phase 1 identity is a FQDN, secure forms of DNS may also be used.

Interaction with Outbound IPsec Processing discusses how IKE connections to the home agent need a careful treatment of the addresses used for transporting IKE. This is necessary to ensure that a Binding Update is not needed before the IKE exchange which is needed for securing the Binding Update.

When IKE version 1 is used with preshared secret authentication between the mobile node and the home agent, aggressive mode MUST be used. Similarly, the ID_IPV6_ADDR Identity Payload MUST NOT be used in IKEv1 phase 1.

Reference[21] contains a more detailed description and examples on using IPsec to protect the communications between the mobile node and the home agent.

5.2 Binding Updates to Correspondent Nodes

The protection of Binding Updates sent to correspondent nodes does not require the configuration of security associations or the existence of an authentication infrastructure between the mobile nodes and correspondent nodes. Instead, a method called the return routability procedure is used to assure that the right mobile node is sending the message. This method does not protect against attackers who are on the path between the home network and the correspondent node. However, attacker in such a location are capable of performing the same attacks even without Mobile IPv6. The main advantage of the return routability procedure is that it limits the potential attackers to those having an access to one specific path in the Internet, and avoids forged Binding Updates from anywhere else in the Internet. For a more in depth explanation of the security properties of the return routability procedure, see Security Considerations.

The integrity and authenticity of the Binding Updates messages to correspondent nodes are protected by using a keyed-hash algorithm. The binding management key, Kbm, is used to key the hash algorithm for this purpose. Kbm is established using data exchanged during the return routability procedure. The data exchange is accomplished by use of node keys, nonces, cookies, tokens, and certain cryptographic functions. Return Routability Procedure outlines the basic return routability procedure. Authorizing Binding Management Messages shows how the results of this procedure are used to authorize a Binding Update to a correspondent node.

5.2.1 Node Keys

Each correspondent node has a secret key, Kcn, called the "node key", which it uses to produce the keygen tokens sent to the mobile nodes. The node key MUST be a random number, 20 octets in length. The node key allows the correspondent node to verify that the keygen tokens used by the mobile node in authorizing a Binding Update are indeed its own. This key MUST NOT be shared with any other entity.

A correspondent node MAY generate a fresh node key at any time; this avoids the need for secure persistent key storage. Procedures for optionally updating the node key are discussed later in Updating Node Keys and Nonces.

5.2.2 Nonces

Each correspondent node also generates nonces at regular intervals. The nonces should be generated by using a random number generator that is known to have good randomness properties[1]. A correspondent node may use the same Kcn and nonce with all the mobiles it is in communication with.

Each nonce is identified by a nonce index. When a new nonce is generated, it must be associated with a new nonce index; this may be done, for example, by incrementing the value of the previous nonce index, if the nonce index is used as an array pointer into a linear array of nonces. However, there is no requirement that nonces be stored that way, or that the values of subsequent nonce indices have any particular relationship to each other. The index value is communicated in the protocol, so that if a nonce is replaced by new nonce during the run of a protocol, the correspondent node can distinguish messages that should be checked against the old nonce from messages that should be checked against the new nonce. Strictly speaking, indices are not necessary in the authentication, but allow the correspondent node to efficiently find the nonce value that it used in creating a keygen token.

Correspondent nodes keep both the current nonce and a small set of valid previous nonces whose lifetime has not yet expired. Expired values MUST be discarded, and messages using stale or unknown indices will be rejected.

The specific nonce index values cannot be used by mobile nodes to determine the validity of the nonce. Expected validity times for the nonces values and the procedures for updating them are discussed later in Updating Node Keys and Nonces.

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 mobile node should set the home init or care-of init cookie to a newly generated random number in every Home or Care-of Test Init message it sends. The cookies are used to verify that the Home Test or Care-of Test message matches the Home Test Init or Care-of Test Init message, respectively. These cookies also serve to ensure that parties who have not seen the request cannot spoof responses.

Home and care-of keygen tokens are produced by the correspondent node based on its currently active secret key (Kcn) and nonces, as well as the home or care-of address (respectively). A keygen token is valid as long as both the secret key (Kcn) and the nonce used to create it are valid.

5.2.4 Cryptographic Functions

In this specification, the function used to compute hash values is SHA1[20]. Message Authentication Codes (MACs) are computed using HMAC_SHA1[26][20]. HMAC_SHA1(K,m) denotes such a MAC computed on message m with key K.

5.2.5 Return Routability Procedure

The Return Routability Procedure enables the correspondent node to obtain some reasonable assurance that the mobile node is in fact addressable at its claimed care-of address as well as at its home address. Only with this assurance is the correspondent node able to accept Binding Updates from the mobile node which would then instruct the correspondent node to direct that mobile node's data traffic to its claimed care-of address.

This is done by testing whether packets addressed to the two claimed addresses are routed to the mobile node. The mobile node can pass the test only if it is able to supply proof that it received certain data (the "keygen tokens") which the correspondent node sends to those addresses. These data are combined by the mobile node into a binding management key, denoted Kbm.

The below figure shows the message flow for the return routability procedure.

 Mobile node                 Home agent           Correspondent node
      |                                                     |
      |  Home Test Init (HoTI)   |                          |
      |------------------------->|------------------------->|
      |                          |                          |
      |  Care-of Test Init (CoTI)                           |
      |---------------------------------------------------->|
      |                                                     |
      |                          |  Home Test (HoT)         |
      |<-------------------------|<-------------------------|
      |                          |                          |
      |                             Care-of Test (CoT)      |
      |<----------------------------------------------------|
      |                                                     |

The Home and Care-of Test Init messages are sent at the same time. The procedure requires very little processing at the correspondent node, and the Home and Care-of Test messages can be returned quickly, perhaps nearly simultaneously. These four messages form the return routability procedure.

Home Test Init



A mobile node sends a Home Test Init message to the correspondent node to acquire the home keygen token. The contents of the message can be summarized as follows:
  • Source Address = home address
  • Destination Address = correspondent
  • Parameters:
    • home init cookie



The Home Test Init message conveys the mobile node's home address to the correspondent node. The mobile node also sends along a home init cookie that the correspondent node must return later. The Home Test Init message is reverse tunneled through the home agent. (The headers and addresses related to reverse tunneling have been omitted from the above discussion of the message contents.) The mobile node remembers these cookie values to obtain some assurance that its protocol messages are being processed by the desired correspondent node.

Care-of Test Init



The mobile node sends a Care-of Test Init message to the correspondent node to acquire the care-of keygen token. The contents of this message can be summarized as follows:
  • Source Address = care-of address
  • Destination Address = correspondent
  • Parameters:
    • care-of init cookie



The Care-of Test Init message conveys the mobile node's care-of address to the correspondent node. The mobile node also sends along a care-of init cookie that the correspondent node must return later. The Care-of Test Init message is sent directly to the correspondent node.

Home Test



The Home Test message is sent in response to a Home Test Init message. 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, the data passed between the home agent and the mobile node can be made immune to inspection and passive attacks. Such protection can be gained by encrypting the home keygen token as it is tunneled from the home agent to the mobile node as specified in Protecting Return Routability Packets. The security properties of this additional security are discussed in Overview. The home init cookie from the mobile node is returned in the Home Test message, to ensure that the message comes from a node on the route between the home agent and the correspondent node. The home nonce index is delivered to the mobile node to later allow the correspondent node to efficiently find the nonce value that it used in creating the home keygen token.

Care-of Test



This message is sent in response to a Care-of Test Init message. The contents of the message are:
  • Source Address = correspondent
  • Destination Address = care-of address
  • Parameters:
    • care-of init cookie
    • care-of keygen token
    • care-of nonce index



When the correspondent node receives the Care-of Test Init message, it generates a care-of keygen token as follows:

    care-of keygen token :=	
       First (64, HMAC_SHA1 (Kcn, (care-of address | nonce | 1)))

Here, the final "1" inside the HMAC_SHA1 function is a single octet containing the hex value 0x01, and is used to distinguish home and care-of cookies from each other. The keygen token is formed from the first 64 bits of the MAC, and sent directly to the mobile node at its care-of address. The care-of init cookie from the 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 (Binding Update Message). In this case, the care-of keygen token is not used. Instead, the binding management key is generated as follows:

    Kbm = SHA1(home keygen token)

Note that the correspondent node does not create any state specific to the mobile node, until it receives the Binding Update from that mobile node. The correspondent node does not maintain the value for the binding management key Kbm; it creates Kbm when given the nonce indices and the mobile node's addresses.

5.2.6 Authorizing Binding Management Messages

After the mobile node has created the binding management key (Kbm), it can supply a verifiable Binding Update to the correspondent node. This section provides an overview of this binding procedure. The below figure 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)              |
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 contains a Nonce Indices option, indicating to the correspondent node which home and care-of nonces to use to recompute Kbm, the binding management key. The MAC is computed as described in Binding Authorization Data, 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 Binding Authorization Data, 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 Protocol Constants).

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 Alternate Care-of Address). When such a message is sent to the correspondent node and the return routability procedure is used as the authorization method, the Care-of Test Init and Care-of Test messages MUST have been performed for the address in the Alternate Care-of Address option (not the Source Address). The nonce indices and MAC value MUST be based on information gained in this test.

Binding Updates may also be sent to delete a previously established binding. In this case, generation of the binding management key depends exclusively on the home keygen token and the care-of nonce index is ignored.

5.2.7 Updating Node Keys and Nonces

Correspondent nodes generate nonces at regular intervals. It is recommended to keep each nonce (identified by a nonce index) acceptable for at least MAX_TOKEN_LIFE seconds (see Protocol Constants) 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 Protocol Constants) 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 a time ago the nonces were used for the first time, and does not generate new nonces on every return routability request.

Due to resource limitations, rapid deletion of bindings, or reboots the correspondent node may not in all cases recognize the nonces that the tokens were based on. If a nonce index is unrecognized, the correspondent node replies with an an error code in the Binding Acknowledgement (either 136, 137, or 138 as discussed in Binding Acknowledgement Message). 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.

5.2.8 Preventing Replay Attacks

The return routability procedure also protects the participants against replayed Binding Updates through the use of the sequence number and a MAC. Care must be taken when removing bindings at the correspondent node, however. Correspondent nodes must retain bindings and the associated sequence number information at least as long as the nonces used in the authorization of the binding are still valid. Alternatively, if memory is very constrained, the correspondent node MAY invalidate the nonces that were used for the binding being deleted (or some larger group of nonces that they belong to). This may, however, impact the ability to accept Binding Updates from mobile nodes that have recently received keygen tokens. This alternative is therefore recommended only as a last measure.

5.3 Dynamic Home Agent Address Discovery

No security is required for dynamic home agent address discovery.

5.4 Prefix Discovery

The mobile node and the home agent SHOULD use an IPsec security association to protect the integrity and authenticity of the Mobile Prefix Solicitations and Advertisements. Both the mobile nodes and the home agents SHOULD use the Encapsulating Security Payload (ESP) header in transport mode with a non-NULL payload authentication algorithm to provide data origin authentication, connectionless integrity and optional anti-replay protection.

5.5 Payload Packets

Payload packets exchanged with mobile nodes can be protected in the usual manner, in the same way as stationary hosts can protect them. However, Mobile IPv6 introduces the Home Address destination option, a routing header, and tunneling headers in the payload packets. In the following we define the security measures taken to protect these, and to prevent their use in attacks against other parties.

This specification limits the use of the Home Address destination option to the situation where the correspondent node already has a Binding Cache entry for the given home address. This avoids the use of the Home Address option in attacks described in Threats.

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 Threats.

Tunnels between the mobile node and the home agent are protected by ensuring proper use of source addresses, and optional cryptographic protection. The mobile node verifies that the outer IP address corresponds to its home agent. The home agent verifies that the outer IP address corresponds to the current location of the mobile node (Binding Updates sent to the home agents are secure). The home agent identifies the mobile node through the source address of the inner packet.(Typically, this is the home address of the mobile node, but it can also be a link-local address, as discussed in Tunneling Intercepted Packets. To recognize the latter type of addresses, the home agent requires that the Link-Local Address Compatibility (L) was set in the Binding Update.) These measures protect the tunnels against vulnerabilities discussed in Threats.

For traffic tunneled via the home agent, additional IPsec ESP encapsulation MAY be supported and used. If multicast group membership control protocols or stateful address autoconfiguration protocols are supported, payload data protection MUST be supported.



 TOC 

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 Piggybacking). Implementations conforming to this specification SHOULD set the payload protocol type to IPPROTO_NONE (59 decimal).
Header Len

8-bit unsigned integer, representing the length of the Mobility Header in units of 8 octets, excluding the first 8 octets.

The length of the Mobility Header MUST be a multiple of 8 octets.
MH Type

8-bit selector. Identifies the particular mobility message in question. Current values are specified in Binding Refresh Request Message and onward. 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 RFC 2460[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 Sending Packets While Away from Home 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 [22]. The Mobility Header is considered as the upper layer protocol for the purposes of calculating the pseudo-header. The Upper-Layer Packet Length field in the pseudo-header MUST be set to the total length of the Mobility Header. For computing the checksum, the checksum field is set to zero.
Message Data

A variable length field containing the data specific to the indicated Mobility Header type.

Mobile IPv6 also defines a number of "mobility options" for use within these messages; if included, any options MUST appear after the fixed portion of the message data specified in this document. The presence of such options will be indicated by the Header Len field within the message. When the Header Len value is greater than the length required for the message specified here, the remaining octets are interpreted as mobility options. These options include padding options that can be used to ensure that other options are aligned properly, and that the total length of the message is divisible by 8. The encoding and format of defined options are described in Mobility Options.

Alignment requirements for the Mobility Header are the same as for any IPv6 protocol Header. That is, they MUST be aligned on an 8-octet boundary.

6.1.2 Binding Refresh Request Message

The Binding Refresh Request (BRR) message requests a mobile node to update its mobility binding. This message is sent by correspondent nodes according to the rules in Sending Binding Refresh Requests. When a mobile node receives a packet containing a Binding Refresh Request message it processes the message according to the rules in Receiving Binding Refresh Requests.

The Binding Refresh Request message uses the MH Type value 0. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved

16-bit field reserved for future use. The value MUST be initialized to zero by the sender, and MUST be ignored by the receiver.
Mobility Options

Variable-length field of such length that the complete Mobility Header is an integer multiple of 8 octets long. Contains zero or more TLV-encoded mobility options. The encoding and format of defined options are described in Mobility Options. 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 Sending Test Init Messages). The Home Test Init message uses the MH Type value 1. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                       Home Init Cookie                        +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                       Mobility Options                        .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved

16-bit field reserved for future use. This value MUST be initialized to zero by the sender, and MUST be ignored by the receiver.
Home Init Cookie

64-bit field which contains a random value, the home init cookie.
Mobility Options

Variable-length field of such length that the complete Mobility Header is an integer multiple of 8 octets long. Contains zero or more TLV-encoded mobility options. The receiver MUST ignore and skip any options which it does not understand. This specification does not define any options valid for the Home Test 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 Sending Test Init Messages). The Care-of Test Init message uses the MH Type value 2. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                      Care-of Init Cookie                      +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility Options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved

16-bit field reserved for future use. The value MUST be initialized to zero by the sender, and MUST be ignored by the receiver.
Care-of Init Cookie

64-bit field which contains a random value, the care-of init cookie.
Mobility Options

Variable-length field of such length that the complete Mobility Header is an integer multiple of 8 octets long. Contains zero or more TLV-encoded mobility options. The receiver MUST ignore and skip any options which it does not understand. This specification does not define any options valid for the Care-of Test Init message.

If no actual options are present in this message, no padding is necessary and the Header Len field will be set to 1.

6.1.5 Home Test Message

The Home Test (HoT) message is a response to the Home Test Init message, and is sent from the correspondent node to the mobile node (see Return Routability Procedure). The Home Test message uses the MH Type value 3. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |       Home Nonce Index        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                        Home Init Cookie                       +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                       Home Keygen Token                       +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Nonce Index

This field will be echoed back by the mobile node to the correspondent node in a subsequent Binding Update.
Home Init Cookie

64-bit field which contains the home init cookie.
Home Keygen Token

This field contains the 64 bit home keygen token used in the return routability procedure.
Mobility Options

Variable-length field of such length that the complete Mobility Header is an integer multiple of 8 octets long. Contains zero or more TLV-encoded mobility options. The receiver MUST ignore and skip any options which it does not understand. This specification does not define any options valid for the Home Test message.

If no actual options are present in this message, no padding is necessary and the Header Len field will be set to 2.

6.1.6 Care-of Test Message

The Care-of Test (CoT) message is a response to the Care-of Test Init message, and is sent from the correspondent node to the mobile node (see Receiving Test Messages). The Care-of Test message uses the MH Type value 4. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |      Care-of Nonce Index      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                      Care-of Init Cookie                      +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                     Care-of Keygen Token                      +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility Options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Care-of Nonce Index

This value will be echoed back by the mobile node to the correspondent node in a subsequent Binding Update.
Care-of Init Cookie

64-bit field which contains the care-of init cookie.
Care-of Keygen Token

This field contains the 64 bit care-of keygen token used in the return routability procedure.
Mobility Options

Variable-length field of such length that the complete Mobility Header is an integer multiple of 8 octets long. Contains zero or more TLV-encoded mobility options. The receiver MUST ignore and skip any options which it does not understand. This specification does not define any options valid for the Care-of Test message.

If no actual options are present in this message, no padding is necessary and the Header Len field will be set to 2.

6.1.7 Binding Update Message

The Binding Update (BU) message is used by a mobile node to notify other nodes of a new care-of address for itself. Binding Updates are sent as described in Sending Binding Updates to the Home Agent and Correspondent Binding Procedure.

The Binding Update uses the MH Type value 5. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |          Sequence #           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |A|H|L|K|        Reserved       |           Lifetime            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Acknowledge (A)

The Acknowledge (A) bit is set by the sending mobile node to request a Binding Acknowledgement (Binding Acknowledgement Message) 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.
Link-Local Address Compatibility (L)

The Link-Local Address Compatibility (L) bit is set when the home address reported by the mobile node has the same interface identifier as the mobile node's link-local address.
Key Management Mobility Capability (K)

If this bit is reset, the protocol used for establishing the IPsec security associations between the mobile node and the home agent does not survive movements. It may then have to be rerun. (Note that the IPsec security associations themselves are expected to survive movements.) If manual IPsec configuration is used, the bit MUST be set to 1.

This bit is valid only in Binding Updates sent to the home agent. Correspondent nodes MUST ignore this bit.
Reserved

These fields are unused. They MUST be initialized to zero by the sender and MUST be ignored by the receiver.
Sequence #

A 16-bit unsigned integer used by the receiving node to sequence Binding Updates and by the sending node to match a returned Binding Acknowledgement with this Binding Update.
Lifetime

16-bit unsigned integer. The number of time units remaining before the binding MUST be considered expired. A value of 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 Mobility Options. 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.

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 (Return Routability Procedure). 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 Binding Error Message); 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 (Binding Update Message). This packet is sent as described in Sending Binding Acknowledgements and Primary Care-of Address Registration.

The Binding Acknowledgement has the MH Type value 6. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:

                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |    Status     |K|  Reserved   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Sequence #          |           Lifetime            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Mobility options                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Key Management Mobility Capability (K)

If this bit is reset, the protocol used by the home agent for establishing the IPsec security associations between the mobile node and the home agent does not survive movements. It may then have to be rerun. (Note that the IPsec security associations themselves are expected to survive movements.)

Correspondent nodes MUST set the K bit to 0.
Reserved

These fields are unused. They MUST be initialized to zero by the sender and MUST be ignored by the receiver.
Status

8-bit unsigned integer indicating the disposition of the Binding Update. Values of the Status field less than 128 indicate that the Binding Update was accepted by the receiving node. Values greater than or equal to 128 indicate that the Binding Update was rejected by the receiving node. The following Status values are currently defined:
0
Binding Update accepted
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[19].

Sequence #

The Sequence Number in the Binding Acknowledgement is copied from the Sequence Number field in the Binding Update. It is used by the mobile node in matching this Binding Acknowledgement with an outstanding Binding Update.
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 Mobility Options. The receiver MUST ignore and skip any options which it does not understand.

There MAY be additional information, associated with this Binding Acknowledgement, that need not be present in all Binding Acknowledgements sent. Mobility options allow future extensions to the format of the Binding Acknowledgement to be defined. The following options are valid for the Binding Acknowledgement:
  • Binding Authorization Data option
  • 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 Sending Binding Error Messages 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

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 zero or more TLV-encoded mobility options. The receiver MUST ignore and skip any options which it does not understand.

There MAY be additional information, associated with this Binding Error message, that need not be present in all Binding Error messages sent. Mobility options allow future extensions to the format of the format of the Binding Error message to be defined. The encoding and format of defined options are described in Mobility Options. 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 Mobility Header.

The presence of such options will be indicated by the Header Len of the Mobility Header. If included, the Binding Authorization Data option (Binding Authorization Data) MUST be the last option and MUST NOT have trailing padding. Otherwise, options can be placed in any order.

6.2.1 Format

Mobility options are encoded within the remaining space of the Message Data field of a mobility message, using a type-length-value (TLV) format as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  | Option Length |   Option Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.

The Pad1 option is used to insert one octet of padding in the Mobility Options area of a Mobility Header. If more than one octet of padding is required, the PadN option, described next, should be used rather than multiple Pad1 options.

6.2.3 PadN

The PadN option does not have any alignment requirements. Its format is as follows:

     0                   1
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
    |   Type = 1    | Option Length | Option Data
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

The PadN option is used to insert two or more octets of padding in the Mobility Options area of a mobility message. For N octets of padding, the Option Length field contains the value N-2, and the Option Data consists of N-2 zero-valued octets. PadN Option data MUST be ignored by the receiver.

6.2.4 Binding Refresh Advice

The Binding Refresh Advice option has an alignment requirement of 2n. Its format is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |   Type = 2    |   Length = 2  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Refresh Interval        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Binding Refresh Advice option is only valid in the Binding Acknowledgement, and only on Binding Acknowledgements sent from the mobile node's home agent in reply to a home registration. The Refresh Interval is measured in units of four seconds, and indicates how long before the mobile node SHOULD send a new home registration to the home agent. The Refresh Interval MUST be set to indicate a smaller time interval than the Lifetime value of the Binding Acknowledgement.

6.2.5 Alternate Care-of Address

The Alternate Care-of Address option has an alignment requirement of 8n+6. Its format is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |   Type = 3    |  Length = 16  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                   Alternate Care-of Address                   +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.

6.2.6 Nonce Indices

The Nonce Indices option has an alignment requirement of 2n. Its format is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |   Type = 4    |   Length = 4  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Home Nonce Index      |     Care-of Nonce Index       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Nonce Indices option is valid only in the Binding Update message sent to a correspondent node, and only when present together with a Binding Authorization Data option. When the correspondent node authorizes the Binding Update, it needs to produce home and care-of keygen tokens from its stored random nonce values.

The Home Nonce Index field tells the correspondent node which nonce value to use when producing the home keygen token.

The Care-of Nonce Index field is ignored in requests to delete a binding. Otherwise, it tells the correspondent node which nonce value to use when producing the care-of keygen token.

6.2.7 Binding Authorization Data

The Binding Authorization Data option does not have any alignment requirements. 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.

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 Return Routability Procedure).

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.

6.3 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
                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   |  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.

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:

The Home Address option MUST be placed as follows:

For each IPv6 packet header, the Home Address Option MUST NOT appear more than once. However, an encapsulated packet[15] MAY contain a separate Home Address option associated with each encapsulating IP header.

The inclusion of a Home Address destination option in a packet affects the receiving node's processing of only this single packet. No state is created or modified in the receiving node as a result of receiving a Home Address option in a packet. In particular, the presence of a Home Address option in a received packet MUST NOT alter the contents of the receiver's Binding Cache and MUST NOT cause any changes in the routing of subsequent packets sent by this receiving node.

6.4 Type 2 Routing Header

Mobile IPv6 defines a new routing header variant, the type 2 routing header, to allow the packet to be routed directly from a correspondent to the mobile node's care-of address. The mobile node's care-of address is inserted into the IPv6 Destination Address field. Once the packet arrives at the care-of address, the mobile node retrieves its home address from the routing header, and this is used as the final destination address for the packet.

The new routing header uses a different type than defined for "regular" IPv6 source routing, enabling firewalls to apply different rules to source routed packets than to Mobile IPv6. This routing header type (type 2) is restricted to carry only one IPv6 address. All IPv6 nodes which process this routing header MUST verify that the address contained within is the node's own home address in order to prevent packets from being forwarded outside the node. The IP address contained in the routing header, since it is the mobile node's home address, MUST be a unicast routable address. Furthermore, if the scope of the home address is smaller than the scope of the care-of address, the mobile node MUST discard the packet (see Site-Local Addressability).

6.4.1 Format

The type 2 routing header has the following format:

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Next Header  | Hdr Ext Len=2 | Routing Type=2|Segments Left=1|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Reserved                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                         Home Address                          +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header

8-bit selector. Identifies the type of header immediately following the routing header. Uses the same values as the IPv6 Next Header field[11].
Hdr Ext Len

2 (8-bit unsigned integer); length of the routing header in 8-octet units, not including the first 8 octets
Routing Type

2 (8-bit unsigned integer).
Segments Left

1 (8-bit unsigned integer).
Reserved

32-bit reserved field. 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 RFC 2460[11]. The type 2 routing header defined for Mobile IPv6 follows the same ordering as other routing headers. If both a type 0 and a type 2 routing header are present, the type 2 routing header should follow the other routing header. A packet containing such nested encapsulation should be created as if the inner (type 2) routing header was constructed first and then treated as an original packet by the outer (type 2) routing header construction process.

In addition, the general procedures defined by IPv6 for routing headers suggest that a received routing header MAY be automatically "reversed" to construct a routing header for use in any response packets sent by upper-layer protocols, if the received packet is authenticated [6]. This MUST NOT be done automatically for type 2 routing headers.

6.5 ICMP Home Agent Address Discovery Request Message

The ICMP Home Agent Address Discovery Request message is used by a mobile node to initiate the dynamic home agent address discovery mechanism, as described in Dynamic Home Agent Address Discovery. The mobile node sends the Home Agent Address Discovery Request message to the Mobile IPv6 Home-Agents anycast address[16] for its own home subnet prefix.

    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 is typically one of the mobile node's current care-of addresses. At the time of performing this dynamic home agent address discovery procedure, it is likely that the mobile node is not registered with any home agent. Therefore, neither the nature of the address nor the identity of the mobile node can be established at this time. The home agent MUST then return the Home Agent Address Discovery Reply message directly to the Source Address chosen by the mobile node.

6.6 ICMP Home Agent Address Discovery Reply Message

The ICMP Home Agent Address Discovery Reply message is used by a home agent to respond to a mobile node that uses the dynamic home agent address discovery mechanism, as described in Dynamic Home Agent Address Discovery.

    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.

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.

ESP header:


IPsec headers SHOULD be supported and used as described in Prefix Discovery.

ICMP Fields:

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.

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 Sending Prefix Information to the Mobile Node.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Identifier           |M|O|        Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Fields:

Source Address

The home agent's address as the mobile node would expect to see it (i.e., same network prefix).
Destination Address

If this message is a response to a Mobile Prefix Solicitation, this field contains the Source Address field from that packet. For unsolicited messages, the mobile node's care-of address SHOULD be used. Note that unsolicited messages can only be sent if the mobile node is currently registered with the home agent.

Routing header:


A type 2 routing header MUST be included.

ESP header:


IPsec headers SHOULD be supported and used as described in Prefix Discovery.

ICMP Fields:

Type

153 <To Be Assigned by IANA>
Code

0
Checksum

The ICMP checksum[14].
Identifier

An identifier to aid in matching this Mobile Prefix Advertisement to a previous Mobile Prefix Solicitation.
M

1-bit Managed Address Configuration flag. When set, hosts use the administered (stateful) protocol for address autoconfiguration in addition to any addresses autoconfigured using stateless address autoconfiguration. The use of this flag is described in [12][13].
O

1-bit Other Stateful Configuration flag. When set, hosts use the administered (stateful) protocol for autoconfiguration of other (non-address) information. The use of this flag is described in [12][13].
Reserved

This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.

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). Sending Prefix Information to the Mobile Node describes which prefixes should be advertised to the mobile node.

The Prefix Information option is defined in Section 4.6.2 of RFC 2461[12], with modifications defined in Modified Prefix Information Option Format 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 Aggregate List of Home Network Prefixes.

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.

The M and O bits MUST be reset if the Home Agent DHCPv6 support is not provided. If such support is provided then they are set in concert with the home network's administrative settings.



 TOC 

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.

7.2 Modified Prefix Information Option Format

Mobile IPv6 requires knowledge of a router's global address in building a Home Agents List as part of the dynamic home agent address discovery mechanism.

However, Neighbor Discovery[12] only advertises a router's link-local address, by requiring this address to be used as the IP Source Address of each Router Advertisement.

Mobile IPv6 extends Neighbor Discovery to allow a router to advertise its global address, by the addition of a single flag bit in the format of a Prefix Information option for use in Router Advertisement messages. The format of the Prefix Information option is as follows:

    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.
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.

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 Movement Detection.

This option MUST be silently ignored for other Neighbor Discovery messages.

7.4 New Home Agent Information Option Format

Mobile IPv6 defines a new Home Agent Information option, used in Router Advertisements sent by a home agent to advertise information specific to this router's functionality as a home agent. The format of the Home Agent Information option is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Home Agent Preference     |      Home Agent Lifetime      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type

8
Length

8-bit unsigned integer. The length of the option (including the type and length fields) in units of 8 octets. The value of this field MUST be 1.
Reserved

This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
Home Agent Preference

16-bit unsigned integer. The preference for the home agent sending this Router Advertisement, for use in ordering the addresses returned to a mobile node in the Home Agent Addresses field of a Home Agent Address Discovery Reply message. Higher values mean more preferable. If this option is not included in a Router Advertisement in which the Home Agent (H) bit is set, the preference value for this home agent MUST be considered to be 0. Greater values indicate a more preferable home agent than lower values.

The manual configuration of the Home Agent Preference value is described in IPv6 Home Agents. In addition, the sending home agent MAY dynamically set the Home Agent Preference value, for example basing it on the number of mobile nodes it is currently serving or on its remaining resources for serving additional mobile nodes; such dynamic settings are beyond the scope of this document. Any such dynamic setting of the Home Agent Preference, however, MUST set the preference appropriately, relative to the default Home Agent Preference value of 0 that may be in use by some home agents on this link (i.e., a home agent not including a Home Agent Information option in its Router Advertisements will be considered to have a Home Agent Preference value of 0).
Home Agent Lifetime

16-bit unsigned integer. The lifetime associated with the home agent in units of seconds. The default value is the same as the Router Lifetime, as specified in the main body of the Router Advertisement. The maximum value corresponds to 18.2 hours. A value of 0 MUST NOT be used. The Home Agent Lifetime applies only to this router's usefulness as a home agent; it does not apply to information contained in other message fields or options.

Home agents MAY include this option in their Router Advertisements. This option MUST NOT be included in a Router Advertisement in which the Home Agent (H) bit (see Modified Router Advertisement Message Format) 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.

7.5 Modified Neighbor Solicitation Message Format

Mobile nodes may need to send Neighbor Solicitations to their home agent when the home agent still has a binding for them. As the home agent defends the mobile node's addresses on the home link, the mobile node can not use its own addresses until it it successfully deletes the binding. However, in order to do this it must send a Binding Update to the home agent, and possibly find its link-layer address.

The modified Neighbor Solicitation message allows this to be done with the IP Source Address set to the unspecified address and the ICMP Code field set to 1. The modified format MUST NOT be used except for the purpose of discovering the link-layer address of a home agent when the mobile node is returning home (Returning Home).

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Reserved                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                       Target Address                          +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-

This format represents the following changes over that originally specified for Neighbor Discovery[12]:

IP Fields:

Source Address

The unspecified address.

ICMP Fields:

Code

1

Upon receiving a Neighbor Solicitation message from the unspecified address with the Code field set to 1, home agents MUST process this message as described in Section 7.2.3 of RFC 2461[12]. Such messages MUST NOT be considered as a sign that the sending node is performing Duplicate Address Detection[13].

7.6 Changes to Sending Router Advertisements

The Neighbor Discovery protocol specification[12] limits routers to a minimum interval of 3 seconds between sending unsolicited multicast Router Advertisement messages from any given network interface (limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that:

"Routers generate Router Advertisements frequently enough that hosts will learn of their presence within a few minutes, but not frequently enough to rely on an absence of advertisements to detect router failure; a separate Neighbor Unreachability Detection algorithm provides failure detection."

This limitation, however, is not suitable to providing timely movement detection for mobile nodes. Mobile nodes detect their own movement by learning the presence of new routers as the mobile node moves into wireless transmission range of them (or physically connects to a new wired network), and by learning that previous routers are no longer reachable. Mobile nodes MUST be able to quickly detect when they move to a link served by a new router, so that they can acquire a new care-of address and send Binding Updates to register this care-of address with their home agent and to notify correspondent nodes as needed.

One method which can provide for faster movement detection, is to increase the rate at which unsolicited Router Advertisements are sent. Mobile IPv6 relaxes this limit such that routers MAY send unsolicited multicast Router Advertisements more frequently. This method can be applied where the router is expecting to provide service to visiting mobile nodes (e.g., wireless network interfaces), or on which it is serving as a home agent to one or more mobile nodes (who may return home and need to hear its Advertisements).

Routers supporting mobility SHOULD be able to be configured with a smaller MinRtrAdvInterval value and MaxRtrAdvInterval value to allow sending of unsolicited multicast Router Advertisements more often. The minimum allowed values are:

In the case where the minimum intervals and delays are used, the mean time between unsolicited multicast router advertisements is 50ms. Use of these modified limits MUST be configurable (see also the configuration variable MinDelayBetweenRas in Protocol Configuration Variables which may also have to be modified accordingly). Systems where these values are available MUST NOT default to them, and SHOULD default to values specified in RFC 2461. Knowledge of the type of network interface and operating environment SHOULD be taken into account in configuring these limits for each network interface. This is important with some wireless links, where increasing the frequency of multicast beacons can cause considerable overhead. Routers SHOULD adhere to the intervals specified in RFC 2461[12], if this overhead is likely to cause service degradation.

Additionally, the possible low values of MaxRtrAdvInterval may cause some problems with movement detection in some mobile nodes. To ensure that this is not a problem, Routers SHOULD add 20ms to any Advertisement Intervals sent in RAs, which are below 200 ms, in order to account for scheduling granularities on both the MN and the Router.

Note that multicast Router Advertisements are not always required in certain wireless networks that have limited bandwidth. Mobility detection or link changes in such networks may be done at lower layers. Router advertisements in such networks SHOULD be sent only when solicited. In such networks it SHOULD be possible to disable unsolicited multicast Router Advertisements on specific interfaces. The MinRtrAdvInterval and MaxRtrAdvInterval in such a case can be set to some high values.

When sending unsolicited multicast Router Advertisements more frequently than the limit specified in RFC 2461[12], 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 (Modified Prefix Information Option Format) in each.

Home agents MUST include the Source Link-Layer Address option in all Router Advertisements they send.

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 nodes that wish to avoid this situation MAY use temporary link-local addresses as follows. The mobile node SHOULD generate a random interface identifier and use it for assigning itself a link-local address. In order to do this, the mobile node applies to the link-local address the procedure described in RFC 3041[18] 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.



 TOC 

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:

It is outside the scope of this specification to specify which of these groups are mandatory in IPv6. We only describe what is mandatory for a node that supports, for instance, route optimization. Other specifications are expected to define the extent of IPv6.

8.1 All IPv6 Nodes

Any IPv6 node may at any time be a correspondent node of a mobile node, either sending a packet to a mobile node or receiving a packet from a mobile node. There are no Mobile IPv6 specific MUST requirements for such nodes, and basic IPv6 techniques are sufficient. If a mobile node attempts to set up route optimization with a node with only basic IPv6 support, an ICMP error will signal that the node does not support such optimizations, and communications will flow through the home agent.

8.2 IPv6 Nodes with Support for Route Optimization

Nodes that implement route optimization are a subset of all IPv6 nodes on the Internet. The ability of a correspondent node to participate in route optimization is essential for the efficient operation of the IPv6 Internet, for the following reasons:

These effects combine to enable much better performance and robustness for communications between mobile nodes and IPv6 correspondent nodes.

The following requirements apply to all correspondent nodes that support route optimization:

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:

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:

8.5 IPv6 Mobile Nodes

Finally, the following requirements apply to all IPv6 nodes capable of functioning as mobile nodes:



 TOC 

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:

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:

Subsequent checks depend on the particular Mobility Header.

9.3 Packet Processing

This section describes how the correspondent node sends packets to the mobile node, and receives packets from it.

9.3.1 Receiving Packets with Home Address Option

Packets containing a Home Address option MUST be dropped if the given home address is not a unicast routable address.

Mobile nodes are expected to include a Home Address destination option in a packet they believe the correspondent node has a Binding Cache entry for the home address of a mobile node. Packets containing a Home Address option MUST be dropped if there is no corresponding Binding Cache entry. A corresponding Binding Cache entry MUST have the same home address as appears in the Home Address destination option, and the currently registered care-of address MUST be equal to the source address of the packet. These tests MUST NOT be done for packets that contain a Home Address option and a Binding Update, or for IPsec AH or ESP packets.

If the packet is dropped due the above tests, the correspondent node SHOULD send the Binding Error message as described in Sending Binding Error Messages. The Status field in this message should be set to 1 (unknown binding for Home Address destination option).

The correspondent node MUST process the option in a manner consistent with exchanging the Home Address field from the Home Address option into the IPv6 header and replacing the original value of the Source Address field there. After all IPv6 options have been processed, it MUST be possible for upper layers to process the packet without the knowledge that it came originally from a care-of address or that a Home Address option was used.

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 calculate the authentication data as if the following were true: The Home Address option contains the care-of address, and the source IPv6 address field of the IPv6 header contains the home address. This conforms with the calculation specified in Interaction with Outbound IPsec Processing.

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. When calculating authentication data in a packet that contains a type 2 routing header, the correspondent node MUST calculate the authentication data as if the following were true: The routing header contains the care-of address, the destination IPv6 address field of the IPv6 header contains the home address, and the Segments Left field is zero. The IPsec Security Policy Database look MUST based on the mobile node's home address.

For instance, assuming there are no additional routing headers in this packet beyond those needed by Mobile IPv6, the correspondent node could set the fields in the packet's IPv6 header and routing header as follows:

The IP layer will insert the routing header before performing any necessary IPsec processing. Once all IPsec processing has been performed, the node swaps the IPv6 destination field with the Home Address field in the routing header, sets the Segments Left field to one, and sends the packet. This ensures the AH calculation is done on the packet in the form it will have on the receiver after advancing the routing header.

Following the definition of a type 2 routing header in Type 2 Routing Header, 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

Processing Mobility Headers and Receiving Packets with Home Address Option 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].

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:

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 Binding Updates to Correspondent Nodes. 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.

Sending Home Test Messages 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:

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 Receiving Home Test Init Messages.

Sending Care-of Test Messages 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 (Home Test Message) 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 (Care-of Test Message) 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:

9.5.1 Receiving Binding Updates

Before accepting a Binding Update, the receiving node MUST validate the Binding Update according to the following tests:

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:

When using Kbm for validating the Binding Update, the following are required:

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.

For 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:

The specified care-of address MUST be determined as follows:

The home address for the binding MUST be determined as follows:

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.

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 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 Binding Acknowledgement Message and in the IANA registry of assigned numbers[19].

If the Status field in the Binding Acknowledgement contains the value 136 (expired home nonce index), 137 (expired care-of nonce index), or 138 (expired nonces) then the message MUST NOT include the Binding Authorization Data mobility option. Otherwise, the Binding Authorization Data mobility option MUST be included, and MUST meet the specific authentication requirements for Binding Acknowledgements as defined in Binding Updates to Correspondent Nodes.

If the Source Address field of the IPv6 header that carried the Binding Update does not contain a unicast address, the Binding Acknowledgement MUST NOT be sent, and the Binding Update packet MUST be silently discarded. Otherwise, the acknowledgement MUST be sent to the Source Address. Unlike the treatment of regular packets, this addressing procedure does not use information from the Binding Cache. However, a routing header is needed in some cases. If the Source Address is the home address of the mobile node, i.e., the Binding Update did not contain a Home Address destination option, then the Binding Acknowledgement MUST be sent to that address, and the routing header MUST NOT be used. Otherwise, the Binding Acknowledgement MUST be sent using a type 2 routing header which contains the mobile node's home address.

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 (Sending Packets to a Mobile Node).

The correspondent node MAY retransmit Binding Refresh Request messages provided that rate limitation is applied. The correspondent node MUST stop retransmitting when it receives a Binding Update.

9.6 Cache Replacement Policy

Conceptually, a node maintains a separate timer for each entry in its Binding Cache. When creating or updating a Binding Cache entry in response to a received and accepted Binding Update, the node sets the timer for this entry to the specified Lifetime period. Any entry in a node's Binding Cache MUST be deleted after the expiration of the Lifetime specified in the Binding Update from which the entry was created or last updated.

Each node's Binding Cache will, by necessity, have a finite size. A node MAY use any reasonable local policy for managing the space within its Binding Cache, except that any entry marked as a home registration (Primary Care-of Address Registration) 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 (Intercepting Packets for a Mobile Node), just as if the mobile node had de-registered its primary care-of address (see Primary Care-of Address De-Registration).

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.



 TOC 

10. Home Agent Operation

10.1 Conceptual Data Structures

Each home agent MUST maintain a Binding Cache and Home Agents List.

The rules for maintaining a Binding Cache are the same for home agents and correspondent nodes, and have already been described in Conceptual Data Structures.

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.

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:

10.2 Processing Mobility Headers

All IPv6 home agents MUST observe the rules described in Processing Mobility Headers 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 Receiving Binding Updates. Furthermore, it MUST authenticate the Binding Update as described in Binding Updates to Home Agents. An authorization step specific for the home agent is also needed to ensure that only the right node can control a particular home address. This is provided through the home address unequivocally identifying the security association that must be used.

This section describes the processing of a valid and authorized Binding Update, when it requests the registration of the mobile node's primary care-of address.

To begin processing the Binding Update, the home agent MUST perform the following sequence of tests:

If home agent accepts the Binding Update, it MUST then create a new entry in its Binding Cache for this mobile node, or update its existing Binding Cache entry, if such an entry already exists. The Home Address field as received in the Home Address option provides the home address of the mobile node.

The home agent MUST mark this Binding Cache entry as a home registration to indicate that the node is serving as a home agent for this binding. Binding Cache entries marked as a home registration MUST be excluded from the normal cache replacement policy used for the Binding Cache (Cache Replacement Policy) and MUST NOT be removed from the Binding Cache until the expiration of the Lifetime period.

Unless this home agent already has a binding for the given home address, the home agent MUST perform Duplicate Address Detection[13] on the mobile node's home link before returning the Binding Acknowledgement. This ensures that no other node on the home link was using the mobile node's home address when the Binding Update arrived. If this Duplicate Address Detection fails for the given home address or an associated link local address, then the home agent MUST reject the complete Binding Update and MUST return a Binding Acknowledgement to the mobile node, in which the Status field is set to 134 (Duplicate Address Detection failed). When the home agent sends a successful Binding Acknowledgement to the mobile node, the home agent assures to the mobile node that its address(es) will continue to be kept unique by the home agent at least as long as the lifetime granted for the bindings is not over.

The specific addresses which are to be tested before accepting the Binding Update, and later to be defended by performing Duplicate Address Detection, depend on the setting of the Link-Local Address Compatibility (L) bit, as follows:

The lifetime of the Binding Cache entry depends on a number of factors:

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 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 Sending Binding Acknowledgements.

In addition, the home agent MUST follow the procedure defined in Intercepting Packets for a Mobile Node 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 Handling Reverse Tunneled Packets. Finally, the home agent MUST also propagate new home network prefixes, as described in Sending Prefix Information to the Mobile Node.

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 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:

In addition, the home agent MUST stop intercepting packets on the mobile node's home link that are addressed to the mobile node (Intercepting Packets for a Mobile Node).

The rules for selecting the Destination IP address (and, if required, routing header construction) for the Binding Acknowledgement to the mobile node are the same as in the previous section. When the Status field in the Binding Acknowledgement is greater than or equal to 128 and the Source Address of the Binding Update is on the home link, the home agent MUST send it to the mobile node's link layer address (retrieved either from the Binding Update or through Neighbor Solicitation).

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. For the home address specified in the Binding Update, the home agent sends a Neighbor Advertisement message[12] to the all-nodes multicast address on the home link, to advertise the home agent's own link-layer address for this IP address on behalf of the mobile node.

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[12] while at home, with the following exceptions:

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_NEIGHBOR_ADVERTISEMENT (see [12]) times to increase its reliability. It is still possible that some nodes on the home link will not receive any of 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.

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

For any packet sent to a mobile node from the mobile node's home agent (for which the home agent is the original sender of the packet), the home agent is operating as a correspondent node of the mobile node for this packet and the procedures described in Sending Packets to a Mobile Node apply. The home agent then uses a routing header to route the packet to the mobile node by way of the primary care-of address in the home agent's Binding Cache.

While the mobile node is away from home, the home agent intercepts any packets on the home link addressed to the mobile node's home address, as described in Intercepting Packets for a Mobile Node. 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 NOT be tunneled to the mobile node by default, but this behavior MUST be configurable to enable 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.

Interception and tunneling of the following multicast addressed packets on the home network are only done if the home agent supports multicast group membership control messages from the mobile node as described in the next section. Tunneling of multicast packets to a mobile node follows similar limitations to those defined above for unicast packets addressed to the mobile node's link-local and site-local addresses. Multicast packets addressed to a multicast address with link-local scope[3], to which the mobile node is subscribed, MUST NOT be tunneled to the mobile node; 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 NOT be tunneled to the mobile node by default. This behavior MUST be configurable to allow it to be enabled. 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. Multicast packets addressed with a global scope to which the mobile node has successfully subscribed MUST be tunneled to the mobile node.

Before tunneling a packet to the mobile node, the home agent MUST perform any IPsec processing as indicated by the security policy data base.

10.4.3 Multicast Membership Control

This section is a prerequisite for the multicast data packet forwarding described in the previous section. If this support is not provided, multicast group membership control messages are silently ignored.

In order to tunnel forward multicast data packets from the home network to all the proper mobile nodes the home agent SHOULD be capable of receiving tunneled multicast group membership control information from the mobile node in order to determine which groups the mobile node has subscribed to. These multicast group membership messages are Listener Report messages specified MLD[17] or in other protocols such as[35].

The messages are issued by the mobile node but sent through the reverse tunnel to the home agent. These messages are issued whenever the mobile node decides to enable reception of packets for a multicast group or in response to an MLD Query from the home agent. The mobile node will also issue multicast group control messages to disable reception of multicast packets when it is no longer interested in receiving multicasts for a particular group.

To obtain the mobile node's current multicast group membership the home agent must periodically transmit MLD Query messages through the tunnel to the mobile node. These MLD periodic transmissions will ensure the home agent has an accurate record of the groups in which the mobile node is interested despite packet losses of the mobile node's MLD group membership messages.

All MLD packets are sent directly between the mobile node and the home agent. Since these packets all contain a link-local source address, are destined to a link-scope multicast address, and have a hop limit of 1 there is no direct forwarding of such packets between the home network and the mobile node. The MLD packets between the mobile node and the home agent are encapsulated within the same tunnel header used for other packet flows between the mobile node and home agent.

Note that at this time, even though a link-local source is used on MLD packets, no functionality depends on these addresses being unique, nor do they elicit direct responses. All MLD messages are sent to multicast destinations. To avoid ambiguity on the home agent due to mobile nodes which may choose identical link-local source addresses for their MLD function it is necessary for the home agent to identify which mobile node was actually the issuer of a particular MLD message. This may be accomplished by noting which tunnel such an MLD arrived by, which IPsec SA was used, or by other distinguishing means.

This specification puts no requirement on how the functions in this section and the multicast forwarding in Tunneling Intercepted Packets are to be achieved. At the time of this writing it was thought that a full IPv6 multicast router function would be necessary on the home agent, but it may be possible to achieve the same effects through a "proxy MLD" application coupled with kernel multicast forwarding. This may be the subject of future specifications.

10.4.4 Stateful Address Autoconfiguration

This section describes how home agents support the use of stateful address autoconfiguration mechanisms such as DHCPv6[29] from the mobile nodes. If this support is not provided, then the M and O bits must remain reset on the Mobile Prefix Advertisement Messages. Any mobile node which issues autoconfiguration queries for servers without this support will not receive a response.

If DHCPv6 is used, packets are sent with link-local source addresses either to a link-scope multicast address or a link-local address. Mobile nodes desiring to locate a DHCPv6 service may reverse tunnel standard DHCPv6 packets to the home agent. Since these link-scope packets can not be forwarded onto the home network it is necessary for the home agent to either implement a DHCPv6 relay agent or a DHCPv6 server function itself. The arriving tunnel or IPsec SA of DHCPv6 link-scope messages from the mobile node must be noted so that DHCPv6 responses may be sent back to the appropriate mobile node. DHCPv6 messages sent to the mobile node with a link-local destination must be tunneled within the same tunnel header used for other packet flows.

10.4.5 Handling Reverse Tunneled Packets

Unless a binding has been established between the mobile node and a correspondent node, traffic from the mobile node to the correspondent node goes through a reverse tunnel. Home agents MUST support reverse tunneling as follows:

Reverse tunneled packets MAY be discarded unless accompanied by a valid 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.

10.4.6 Protecting Return Routability Packets

The return routability procedure described in Return Routability Procedure 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.

Security associations are needed to provide this protection. When the care-of address for the mobile node changes, special treatment is needed for the next packets sent using these security associations. The home agent MUST set the new care-of address as the destination address of these packets, as if the destination gateway address in the security association had changed[21].

The above protection SHOULD be used with all mobile nodes. The use is controlled by configuration of the IPsec security policy database both at the mobile node and at the home agent.

As described earlier, the Binding Update and Binding Acknowledgement messages require protection between the home agent and the mobile node. The Mobility Header protocol carries both these messages as well as the return routability messages. From the point of view of the security policy database these messages are indistinguishable. 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 Dynamic Home Agent Address Discovery. 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:

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 Dynamic Home Agent Address Discovery, 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:

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.

The messages relayed to the mobile node are the ones learned via Neighbor Discovery on the home link. The prefix options are processed as defined in [12][13].

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:

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 (Dynamic Home Agent Address Discovery).

10.6.2 Scheduling Prefix Deliveries

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:

SHOULD:

MAY:

The home agent uses the following algorithm to determine when to send prefix information to the mobile node.

The aggregate list is sent in its entirety in all cases.

If the home agent already has scheduled the transmission of a Mobile Prefix Advertisement to the mobile node, the home agent replaces the advertisement with a new one, to be sent at the scheduled time.

Otherwise, the home agent computes a fresh value for RAND_ADV_DELAY, 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 Protocol Constants. Then compute the final delay for the advertisement:


  RAND_ADV_DELAY = MinMobPfxAdvInterval + 
        (rand() % abs(MaxScheduleDelay - MinMobPfxAdvInterval))

Here rand() returns a random integer value in the range of 0 to the maximum possible integer value. This computation is expected to alleviate bursts of advertisements when prefix information changes. In addition, a home agent MAY further reduce the rate of packet transmission by further delaying individual advertisements, 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 Protocol Constants) before the first retransmission, and double the retransmission wait time for every succeeding retransmission, up until a maximum of PREFIX_ADV_RETRIES attempts (see Protocol Constants). If the mobile node's bindings expire before the matching Binding Update has been received, then the home agent MUST NOT attempt any more retransmissions, even if not all PREFIX_ADV_RETRIES have been retransmitted. 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 and causes another Prefix Advertisement to be sent to the mobile node, before the mobile node acknowledges a previous transmission, the home agent SHOULD combine any Prefix Information options in the unacknowledged Mobile Prefix Advertisement into a new Advertisement. The home agent discards the old Advertisement.

10.6.3 Sending Advertisements

When sending a Mobile Prefix Advertisement to the mobile node, the home agent MUST construct the packet as follows:

10.6.4 Lifetimes for Changed Prefixes

As described in Primary Care-of Address Registration, 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.



 TOC 

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 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.

11.2 Processing Mobility Headers

All IPv6 mobile nodes MUST observe the rules described in Processing Mobility Headers when processing Mobility Headers.

11.3 Packet Processing

11.3.1 Sending Packets While Away from Home

While a mobile node is away from home, it continues to use its home address, as well as also using one or more care-of addresses. When sending a packet while away from home, a mobile node MAY choose among these in selecting the address that it will use as the source of the packet, as follows:

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 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 an address assigned to the interface on the link being used.



By using the care-of address as the Source Address in the IPv6 header, with the mobile node's home address instead in the Home Address option, the packet will be able to safely pass through any router implementing ingress filtering[27].

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.

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 Sending Packets While Away from Home):

When an automated key management protocol is used to create new security associations for a peer, it is important to ensure that the peer can send the key management protocol packets to the mobile node. This may not be possible if the peer is the home agent of the mobile node, and the purpose of the security associations would be to send a Binding Update to the home agent. Packets addressed to the home address of the mobile node cannot be used before the Binding Update has been processed. For the default case of using IKE[9] as the automated key management protocol, such problems can be avoided by the following requirements when communicating with its home agent:

The Key Management Mobility Capability (K) bit in Binding Updates and Acknowledgements can be used avoid the need to rerun IKE upon movements.

11.3.3 Receiving Packets While Away from Home

While away from home, a mobile node will receive packets addressed to its home address, by one of three methods:

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. In this method the mobile node SHOULD also send a Binding Update to the original sender of the packet, as described in Correspondent Binding Procedure, subject to the rate limiting defined in Retransmissions and Rate Limiting. The mobile node MUST also process the received packet in the manner defined for IPv6 encapsulation[15], which will result in the encapsulated (inner) packet being processed normally by upper-layer protocols within the mobile node, as if it had been addressed (only) to the mobile node's home address.

For packets received by the second method, the following rules will result in the packet being processed normally by upper-layer protocols within the mobile node, as if it had been addressed to the mobile node's home address.

A node receiving a packet addressed to itself (i.e., one of the node's addresses is in the IPv6 destination field) follows the next header chain of headers and processes them. When it encounters a type 2 routing header during this processing it performs the following checks. If any of these checks fail the node MUST silently discard the packet.

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 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. In this case, the mobile node MUST NOT use its home address or the Home Address destination option when sending MLD packets[17]

Alternatively, a mobile node MAY join multicast groups via a bi-directional tunnel to its home agent. The mobile node tunnels its multicast group membership control packets (such as those defined in [17] or in [35]) to its home agent, and the home agent forwards multicast packets down the tunnel to the mobile node. A mobile node MUST NOT tunnel multicast group membership control packets until (1) the mobile node has a binding in place at the home agent, and (2) the latter sends at least one such multicast group membership control packet via the tunnel. Once this condition is true, the mobile node SHOULD assume it does not change as long as the binding does not expire.

A mobile node that wishes to send packets to a multicast group also has two options:

  1. Send directly on the foreign link being visited.

    The application is aware of the care-of address and uses it 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 associated multicast tree is specific to that source location and any change of location and source address will invalidate the source specific tree or branch and the application context of the other multicast group members.

This specification does not provide mechanisms to enable such local multicast session to survive hand-off, and to seamlessly continue from a new care-of address on each new foreign link. Any such mechanism, developed as an extension to this specification, needs to take into account the impact of fast moving mobile nodes on the Internet multicast routing protocols and their ability to maintain the integrity of source specific multicast trees and branches.

While the use of reverse tunneling can ensure that multicast trees are independent of the mobile nodes movement, in some case such tunneling can have adverse affects. The latency of specific types of multicast applications such as multicast based discovery protocols will be affected when the round-trip time between the foreign subnet and the home agent is significant compared to that of the topology to be discovered. In addition, the delivery tree from the home agent in such circumstances relies on unicast encapsulation from the agent to the mobile node and is therefore bandwidth inefficient compared to the native multicast forwarding in the foreign multicast system.

11.3.5 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. New Binding Update List entries MUST NOT be created as a result of receiving ICMP messages.

Correspondent nodes that have participated in the return routability procedure MUST implement the ability to correctly process received packets containing a Home Address destination option. Therefore, correctly implemented correspondent nodes should always be able to recognize Home Address options. If a mobile node receives an ICMP Parameter Problem, Code 2, message from some node indicating that it does not support the Home Address option, the mobile node SHOULD log the error and then discard the ICMP message.

11.3.6 Receiving Binding Error Messages

When a mobile node receives a packet containing a Binding Error message, it should first check if the mobile node has a Binding Update List entry for the source of the Binding Error message. If the mobile node does not have such an entry, it MUST ignore the message. This is necessary to prevent a waste of resources on e.g. return routability procedure due to spoofed Binding Error messages.

Otherwise, if the message Status field was 1 (unknown binding for Home Address destination option), the mobile node should perform one of the following two actions:

If the message Status field was 2 (unrecognized MH Type value), the mobile node should perform one of the following two actions:

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 Sending Binding Updates to the Home Agent, 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 Dynamic Home Agent Address Discovery, the home agent on its home link that receives this Request message will return an ICMP Home Agent Address Discovery Reply message, giving the global unicast IP addresses for the home agents operating on the home link

The mobile node, upon receiving this Home Agent Address Discovery Reply message, MAY then send its home registration Binding Update to any of the unicast IP addresses listed in the Home Agent Addresses field in the Reply. For example, the mobile node MAY attempt its home registration to each of these addresses, in turn, until its registration is accepted. The mobile node sends a Binding Update to an address and waits for the matching Binding Acknowledgement, moving on to the next address if there is no response. The mobile node MUST, however, wait at least InitialBindackTimeoutFirstReg seconds (see Protocol Configuration Variables) 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 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 Protocol Constants) 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 Protocol Constants) 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 Sending Prefix Information to the Mobile Node.

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 MUST set the Identifier field in the ICMP header to a random value.

The mobile node SHOULD send a Solicitation to the home agent when its home address will become invalid within MaxRtrAdvInterval seconds, where this value is acquired in a previous Mobile Prefix Advertisement from the home agent. If no such value is known, the value MAX_PFX_ADV_DELAY seconds is used instead (see Protocol Constants).

As described in Correspondent Binding Procedure, 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 Scheduling Prefix Deliveries) 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

Sending Prefix Information to the Mobile Node 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:

Any received Mobile Prefix Advertisement not meeting these tests MUST be silently discarded.

If the advertisement was unsolicited, the mobile node SHOULD send a Mobile Prefix Solicitation.

For an accepted Mobile Prefix Advertisement, the mobile node MUST process Managed Address Configuration (M), Other Stateful Configuration (O), and the Prefix Information Options as if they arrived in a Router Advertisement[12] on the mobile node's home link. (This specification does not, however, describe how to acquire home addresses through stateful protocols.) Such processing may result in the mobile node configuring a new home address, although due to separation between preferred lifetime and valid lifetime, such changes should not affect most communication by the mobile node, in the same way as for nodes that are at home.

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 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 Forming New Care-of Addresses. The mobile node registers its primary care-of address with its home agent, as described in Sending Binding Updates to the Home Agent.

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 sends periodic unsolicited multicast Router Advertisements, the mobile node will have an 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 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 SHOULD send Router Solicitations to determine if additional on-link subnet prefixes are available on its new link. The mobile node SHOULD also mark its link-local address as tentative, and follow standard Duplicate Address Detection procedures[13].

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 generate a new primary care-of address using normal IPv6 mechanisms. A mobile node MAY form a new primary care-of address at any time, but a mobile node MUST NOT send a Binding Update about a new care-of address to its home agent more than MAX_UPDATE_RATE times within a second.

In addition, a mobile node MAY form new non-primary care-of addresses even when it has not switched to a new default router. A mobile node can have only one primary care-of address at a time (which is registered with its home agent), but it MAY have an additional care-of address for any or all of the prefixes on its current link. Furthermore, since a wireless network interface may actually allow a mobile node to be reachable on more than one link at a time (i.e., within wireless transmitter range of routers on more than one separate link), a mobile node MAY have care-of addresses on more than one link at a time. The use of more than one care-of address at a time is described in Using Multiple Care-of Addresses.

As described in Overview of Mobile IPv6, in order to form a new care-of address, a mobile node MAY use either stateless[13] or stateful (e.g., DHCPv6[29]) Address Autoconfiguration. If a mobile node needs to 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.

RFC 2462[13] specifies that in normal processing for Duplicate Address Detection, the node SHOULD delay sending the initial Neighbor Solicitation message by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY. Since delaying DAD can result in significant delays in configuring a new care of address when the Mobile Node moves to a new link, the Mobile Node preferably SHOULD NOT delay DAD when configuring a new care of address. The Mobile Node SHOULD delay according to the mechanisms specified in RFC 2462 if the Mobile IP stack cannot distinguish between the normal process of moving to a new link and reinitializing the interface, or if the link layer does not include adequate local collision and congestion control mechanisms.

11.5.3 Using Multiple Care-of Addresses

As described in Forming New Care-of Addresses, a mobile node MAY use more than one care-of address at a time. Particularly in the case of many wireless networks, a mobile node effectively might be reachable through multiple links at the same time (e.g., with overlapping wireless cells), on which different on-link subnet prefixes may exist. The mobile node MUST ensure that its primary care-of address always has a prefix that is considered on-link by its current default router, i.e., advertised by its current default router in a solicited Router Advertisement. 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 Sending Binding Updates to the Home Agent.

To assist with smooth handovers, a mobile node SHOULD retain its previous primary care-of address as a (non-primary) care-of address, and SHOULD still accept packets at this address, even after registering its new primary care-of address with its home agent. This is reasonable, since the mobile node could only receive packets at its previous primary care-of address if it were indeed still connected to that link. If the previous primary care-of address was allocated using stateful Address Autoconfiguration[29], the mobile node may not wish to release the address immediately upon switching to a new primary care-of address.

Whenever a mobile node determines that it is no longer reachable through a given link, it SHOULD invalidate all care-of addresses associated with address prefixes that it discovered from routers on the unreachable link which are not in the current set of address prefixes advertised by the (possibly new) current default router.

11.5.4 Returning Home

A mobile node detects that it has returned to its home link through the movement detection algorithm in use (Movement Detection), when the mobile node detects that its home subnet prefix is again on-link. The mobile node SHOULD then send a Binding Update to its home agent, to instruct its home agent to no longer intercept or tunnel packets for it. In this home registration, the mobile node MUST set the Acknowledge (A) and Home Registration (H) bits, set the Lifetime field to zero, and set the care-of address for the binding to the mobile node's own home address. The mobile node MUST use its home address as the source address in the Binding Update.

When sending this Binding Update to its home agent, the mobile node must be careful in how it uses Neighbor Solicitation[12] (if needed) to learn the home agent's link-layer address, since the home agent will be currently configured to intercept packets to the mobile node's home address for Duplicate Address Detection (DAD). In particular, the mobile node is unable to use its home address as the Source Address in the Neighbor Solicitation until the home agent stops defending the home address.

Neighbor Solicitation by the mobile node for the home agent's address will normally not be necessary, since the mobile node has already learned the home agent's link-layer address from a Source Link-Layer Address option in a Router Advertisement. However, if there are multiple home agents it may still be necessary to send a solicitation. In this special case of the mobile node returning home, the mobile node MUST multicast the packet, and in addition set the Source Address of this Neighbor Solicitation to the unspecified address (0:0:0:0:0:0:0:0). The target of the Neighbor Solicitation MUST be set to the 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 ICMP Code field MUST be set to 1 in order to distinguish this solicitation from a similar packet that would only be used for DAD, as described in Modified Neighbor Solicitation Message Format. The home agent will send a multicast Neighbor Advertisement back to the mobile node with the Solicited flag (S) set to zero. In any case, the mobile node SHOULD record the information from the Source Link-Layer Address option or from the advertisement, and set the state of the Neighbor Cache entry for the home agent to REACHABLE.

The mobile node then sends its Binding Update using the home agent's link-layer address, instructing its home agent to no longer serve as a home agent for it. By processing this Binding Update, the home agent will cease defending the mobile node's home address for Duplicate Address Detection and will no longer respond to Neighbor Solicitations for the mobile node's home address. The mobile node is then the only node on the link receiving packets at the mobile node's home address. In addition, when returning home prior to the expiration of a current binding for its home address, and configuring its home address on its network interface on its home link, the mobile node MUST NOT perform Duplicate Address Detection on its own home address, in order to avoid confusion or conflict with its home agent's use of the same address. If the mobile node returns home after the bindings for all of its care-of addresses have expired, then it SHOULD perform DAD.

After the Mobile Node sends the Binding Update, it MUST be prepared to reply to Neighbor Solicitations from its home agent. The replies MUST be sent using a unicast Neighbor Advertisement to the home agent's link-layer address.

After receiving the Binding Acknowledgement for its Binding Update to its home agent, the mobile node MUST multicast onto the home link (to the all-nodes multicast address) a Neighbor Advertisement[12], to advertise the mobile node's own link-layer address for its own home address. The Target Address in this Neighbor Advertisement MUST be set to the mobile node's home address, and the Advertisement MUST include a Target Link-layer Address option specifying the mobile node's link-layer address. The mobile node MUST multicast such a Neighbor Advertisement for each of its home addresses, as defined by the current on-link prefixes, including its link-local address and site-local address. The Solicited Flag (S) in these Advertisements MUST NOT be set, since they were not solicited by any Neighbor Solicitation. The Override Flag (O) in these Advertisements MUST be set, indicating that the Advertisements SHOULD override any existing Neighbor Cache entries at any node receiving them.

Since multicasting on the local link (such as Ethernet) is typically not guaranteed to be reliable, the mobile node MAY retransmit these Neighbor Advertisements[12] up to MAX_NEIGHBOR_ADVERTISEMENT times to increase their reliability. It is still possible that some nodes 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. Correspondent Binding Procedure describes the rules when the return routability procedure needs to be initiated.

11.6.1 Sending Test Init Messages

A mobile node that initiates a return routability procedure MUST send (in parallel) a Home Test Init message and a Care-of Test Init messages. However, if the mobile node has recently received 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 intends to send a Binding Update with the Lifetime set to zero or the care-of address equal to its home address - such as when returning home - sending a Home Test Init message is sufficient. In this case, generation of the binding management key depends exclusively on the home keygen token (Return Routability Procedure).

A Home Test Init message MUST be created as described in Home Test Init Message. A Care-of Test Init message MUST be created as described in Care-of Test Init Message. 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:

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 Test Messages

Upon receiving a packet carrying a Home Test message, a mobile node MUST validate the packet according to the following tests:

Any Home Test message not satisfying all of these tests MUST be silently ignored. Otherwise, the mobile node MUST record the Home Nonce Index and home keygen token in the Binding Update List. If the Binding Update List entry does not have a care-of keygen token, the mobile node SHOULD continue waiting for additional messages.

Upon receiving a packet carrying a Care-of Test message, a mobile node MUST validate the packet according to the following tests:

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 Correspondent Binding Procedure.

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 Protecting Return Routability Packets.

When IPsec is used to protect return routability signaling or payload packets, the mobile node MUST set the source address it uses for the outgoing tunnel packets to the current primary care-of address. The mobile node starts to use a new primary care-of address immediately after sending a Binding Update to the home agent to register this new address.

11.7 Processing Bindings

11.7.1 Sending Binding Updates to the Home Agent

After deciding to change its primary care-of address as described in Movement Detection and Forming New Care-of Addresses, 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 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 Binding Acknowledgement Message, 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 Dynamic Home Agent Address Discovery). 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 Retransmissions and Rate Limiting for information about retransmitting Binding Updates.

With the Binding Update, the mobile node requests the home agent to serve as the home agent for the given home address. Until the lifetime of this registration expires, the home agent considers itself the home agent for this home address.

Each Binding Update MUST be authenticated as coming from the right mobile node, as defined in Binding Updates to Home Agents. 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 Correspondent Binding Procedure.

The last Sequence Number value sent to the home agent in a Binding Update is stored by the mobile node. If the sending mobile node has no knowledge of the right Sequence Number value, it may start at any value. If the home agent rejects the value, it sends back a Binding Acknowledgement with status code 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).

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 Remote Home Address Configuration.

11.7.2 Correspondent Binding Procedure

When the mobile node is assured that its home address is valid, it can initiate a correspondent binding procedure with the purpose of allowing the correspondent node to cache the mobile node's current care-of address. This procedure consists of the return routability procedure followed by a binding procedure.

This section defines when to initiate the correspondent binding procedure, and rules to follow when performing it.

After the mobile node has sent a Binding Update to its home agent to register a new primary care-of address (as described in Sending Binding Updates to the Home Agent), the mobile node SHOULD initiate a correspondent binding procedure for each node that already appears in the mobile node's Binding Update List. This is necessary in order to ensure that correspondent nodes do not have invalid information about the current location of the mobile node. The initiated procedures can be used to either update or delete binding information in the correspondent node.

For nodes that do not appear in the mobile node's Binding Update List, the mobile node MAY initiate a correspondent binding procedure at any time after sending the Binding Update to its home agent. Considerations regarding when (and if) to initiate the procedure depend on the specific movement and traffic patterns of the mobile node and are outside the scope of this document.

In addition, when a mobile node receives a packet for which the mobile node can deduce that the original sender of the packet has a stale Binding Cache entry for the mobile node, the mobile node SHOULD initiate a correspondent binding procedure. In addition, the mobile node MAY initiate the procedure in response to receiving a packet that meets all of the following tests:

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. If a Home Address destination option is present in the packet (inner IPv6 header), the source address MUST be the home address in the home address option. The home address used in the procedure should be the Destination Address of the original (inner) packet.

A mobile node MAY also choose to keep its location private from certain correspondent nodes, and thus need not initiate the correspondent binding procedure.

Upon successfully completing the return routability procedure, and after receiving a successful Binding Acknowledgement from the Home Agent, a Binding Update can be sent to the correspondent node.

In any Binding Update sent by a mobile node, the care-of address (either the Source Address in the packet's IPv6 header or the Care-of Address in the Alternate Care-of Address mobility option of the Binding Update) MUST be set to one of the care-of addresses currently in use by the mobile node or to the mobile node's home address. A mobile node MAY set the care-of address differently for sending Binding Updates to different correspondent nodes.

A mobile node MAY also send a Binding Update to such a correspondent node to instruct it to delete any existing binding for the mobile node from its Binding Cache, as described in Binding Update Message. Even in this case a successful completion of the return routability procedure is required first.

If set to one of the mobile node's current care-of addresses, 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. The care-of address given in the Binding Update MAY differ from the mobile node's primary care-of address.

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 (Return Routability Procedure). The care-of nonce index SHOULD be set to zero in this case. In keeping with the Binding Update creation rules below, the care-of address MUST be set to the home address if the mobile node is at home, or to the current care-of address if it is away from home.

If the mobile node wants to ensure that its new care-of address has been entered into a correspondent node's Binding Cache, the mobile node 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.

The mobile node SHOULD create a Binding Update as follows:

Each Binding Update MUST a Sequence Number greater than the Sequence Number value sent in the previous Binding Update to the same destination address (if any). The sequence numbers are compared modulo 2**16, as described in Receiving Binding Updates. There is no requirement, however, that the Sequence Number value strictly increase by 1 with each new Binding Update sent or received, as long as the value stays within the window. The last Sequence Number value sent to a destination in a Binding Update is stored by the mobile node in its Binding Update List entry for that destination. If the sending mobile node has no Binding Update List entry, the Sequence Number SHOULD start at a random value. The mobile node MUST NOT use the same Sequence Number in two different Binding Updates to the same correspondent node, even if the Binding Updates provide different care-of addresses.

The mobile node is responsible for the completion of the correspondent binding procedure, as well as any retransmissions that may be needed (subject to the rate limiting defined in Retransmissions and Rate Limiting).

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:

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:

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.

If the acknowledgement came from the home agent, the mobile node examines the value of the Key Management Mobility Capability (K) bit. If this bit is not set, the mobile node SHOULD discard key management protocol connections, if any, to the home agent. The mobile node MAY also initiate a new key management connection.

If this bit is set, the mobile node SHOULD move its own endpoint in the key management protocol connections to the home agent, if any. The mobile node's new endpoint should be the new care-of address. For an IKE phase 1 connection, this means packets sent to this address with the original ISAKMP cookies are accepted.

11.7.4 Receiving Binding Refresh Requests

When a mobile node receives a packet containing a Binding Refresh Request message and there already exists a Binding Update List entry for the source of the Binding Refresh Request, it SHOULD start a return routability procedure. The mobile node MAY also choose to ignore the Binding Refresh Request. Also, the mobile node can at any time delete its binding from a correspondent node.

Note that the mobile node SHOULD NOT respond to 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 Correspondent Binding Procedure. 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.

11.8 Retransmissions and Rate Limiting

The mobile node is responsible for retransmissions and rate limiting in the return routability and binding procedures and for solicited prefix discovery.

When the mobile node sends a Mobile Prefix Solicitation, Home Test Init, Care-of Test Init or Binding Update for which it expects a response, the mobile node has to determine a value for the initial retransmission timer:

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 than MAX_UPDATE_RATE times within a second.

Retransmitted Binding Updates MUST use a Sequence Number value greater than that used for the previous transmission of this Binding Update. Retransmitted Home Test Init and Care-of Test Init messages MUST use new cookie values.



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12. Protocol Constants

 DHAAD_RETRIES                   4 retransmissions  
 INITIAL_BINDACK_TIMEOUT         1 second   
 INITIAL_DHAAD_TIMEOUT           3 seconds   
 INITIAL_SOLICIT_TIMER           3 seconds   
 MAX_BINDACK_TIMEOUT             32 seconds   
 MAX_NONCE_LIFE                  240 seconds    
 MAX_TOKEN_LIFE                  210 seconds    
 MAX_RR_BINDING_LIFE             420 seconds   
 MAX_UPDATE_RATE                 3 times   
 PREFIX_ADV_RETRIES              3 retransmissions  
 PREFIX_ADV_TIMEOUT              3 seconds   


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13. Protocol Configuration Variables

 HomeRtrAdvInterval              Default: 3,600 seconds  
 MaxMobPfxAdvInterval            Default: 86,400 seconds  
 MinDelayBetweenRAs              Default: 3 seconds, 
                                 Min: 0.03 seconds   
 MinMobPfxAdvInterval            Default: 600 seconds   
 InitialBindackTimeoutFirstReg   Default: 1.5 seconds 

Home agents MUST allow the first four variables to be configured by system management, and mobile nodes MUST allow the last variable to be configured by system management.

The default value for InitialBindackTimeoutFirstReg has been calculated as 1.5 times the default value of RetransTimer[12] times the default value of DupAddrDetectTransmits[13].

The value MinDelayBetweenRAs overrides the value of the protocol constant MIN_DELAY_BETWEEN_RAS, as specified in RFC 2461[12]. This variable SHOULD be set to MinRtrAdvInterval, if MinRtrAdvInterval is less than 3 seconds.



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14. IANA Considerations

This document defines a new IPv6 protocol, the Mobility Header, described in Mobility Header. This protocol must be assigned a protocol number.

This document also creates a new name space "Mobility Header Type", for the MH Type field in the Mobility Header. The current message types are described starting from Binding Refresh Request Message, and are the following:

0
Binding Refresh Request
1
Home Test Init
2
Care-of Test Init
3
Home Test
4
Care-of Test
5
Binding Update
6
Binding Acknowledgement
7
Binding Error

Future values of the MH Type can be allocated using standards action[10].

Furthermore, each mobility message may contain mobility options as described in Mobility Options. This document defines a new name space "Mobility Option" to identify these options. The current mobility options are defined starting from Pad1 and are the following:

0
Pad1
1
PadN
2
Binding Refresh Advice
3
Alternate Care-of Address
4
Nonce Indices
5
Authorization Data

Future values of the Option Type can be allocated using standards action[10].

This document also defines a new IPv6 destination option, the Home Address option, described in Home Address Option. This option has already been assigned the Option Type value 0xC9.

This document also defines a new IPv6 type 2 routing header, described in Type 2 Routing Header. 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. These messages must be assigned ICMPv6 type numbers from the informational message range:

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:



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15. Security Considerations

15.1 Threats

Any mobility solution must protect itself against misuses of the mobility features and mechanisms. In Mobile IPv6, most of the potential threats are concerned with false Bindings, usually resulting in Denial-of-Service attacks. Some of the threats also pose potential for Man-in-the-Middle, Hijacking, Confidentiality, and Impersonation attacks. The main threats this protocol protects against are the following:

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.

It should also be noted that some additional threats result from movements as such, even without the involvement of mobility protocols. Mobile nodes must be capable to defend themselves in the networks that they visit, as typical perimeter defenses applied in the home network no longer protect them.

15.2 Features

This specification provides a series of features designed to mitigate the risk introduced by the threats listed above. The main security features are the following:

The support for encrypted reverse tunneling (see Sending Packets While Away from Home) 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 security association that can be used to reliably authenticate the exchanged messages. See Binding Updates to Home Agents for the description of the protocol mechanisms, and Binding Updates to Home Agent below for a discussion of the resulting level of security.

It is expected that Mobile IPv6 route optimization will be used on a global basis between nodes belonging to different administrative domains. It would be a very demanding task to build an authentication infrastructure on this scale. Furthermore, a traditional authentication infrastructure cannot be easily used to authenticate IP addresses, because IP addresses can change often. It is not sufficient to just authenticate the mobile nodes. Authorization to claim the right to use an address is needed as well. Thus, an "infrastructureless" approach is necessary. The chosen infrastructureless method is described in Binding Updates to Correspondent Nodes and Binding Updates to Correspondent Nodes 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 Tunneling via the Home Agent, Home Address Option, and Type 2 Routing Header.

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 Return Routability Denial-of-Service.

15.3 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 to assure the home agent that a Binding Update is from a legitimate mobile node.

IPsec ESP protects the integrity of the Binding Updates and Binding Acknowledgements, by securing mobility messages between the mobile node and the home agent.

However, IPsec can easily provide replay protection only if dynamic security association establishment is used. 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 Binding Updates to Home Agents). However, if a home agent reboots and loses its state regarding the sequence numbers, replay attacks become possible. The 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 the use of a single pair of manually keyed security associations conflicts with the generation of a new home addresses[18] for the mobile node, or with the adoption of a new home subnet prefix. This is because IPsec security associations are bound to the used addresses. While certificate-based automatic keying alleviates this problem to an extent, it is still necessary to ensure that a given mobile node cannot send Binding Updates for the address of another mobile node. In general, this leads to the inclusion of home addresses in certificates in the Subject AltName field. This again limits the introduction of new addresses without either manual or automatic procedures to establish new certificates. Therefore, this specification restricts the generation of new home addresses (for any reason) to those situations where there already exists a security association or certificate for the new address. (Dynamically Generated Home Addresses lists the improvement of security for new addresses as one of the future developments for Mobile IPv6.)

15.4 Binding Updates to Correspondent Nodes

15.4.1 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. Binding Updates to Correspondent Nodes describes the return routability procedure in detail. The procedure uses the following principles:

For further information about the design rationale of the return routability procedure, see [28][33][32][31]. The used mechanisms have been adopted from these documents.

15.4.2 Achieved Security Properties

The return routability procedure protects Binding Updates against all attackers who are unable to monitor the path between the home agent and the correspondent node. The procedure does not defend against attackers who can monitor this path. Note that such attackers are in any case able to mount an active attack against the mobile node when 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.

15.4.3 Comparison to Regular IPv6 Communications

This section discusses the protection offered by the return routability method by comparing it to the security of regular IPv6 communications. We will divide vulnerabilities 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 Man-in-the-Middle.

Next, we will consider the vulnerabilities that exist when IPv6 is used together with Mobile IPv6 and the return routability procedure. On the local link the vulnerabilities are same as those as in IPv6, but Masquerade and Man-in-the-Middle 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 contrast, an attacker who uses only plain IPv6 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, Man-in-the-Middle, and Denial-of-Service attacks can be launched with just the interception of a few packets, whereas in regular IPv6 it is necessary to intercept every packet. The effect of the attacks is the same regardless of the method, however. In any case, the most difficult task 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:

For a more in-depth discussion of these issues, see [31].

15.4.4 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 Preventing Replay Attacks ensure that this cannot happen.

15.4.5 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 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 [28] 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 be capable of administratively disabling route optimization.

15.5 Dynamic Home Agent Address Discovery

The dynamic home agent address discovery function could be used to learn the addresses of home agents in the home network. 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.

15.6 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.

15.7 Tunneling via the Home Agent

Tunnels between the mobile node and the home agent can be protected by ensuring proper use of source addresses, and optional cryptographic protection. These procedures are discussed in Payload Packets.

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 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.

15.8 Home Address Option

When the mobile node sends packets directly to the correspondent node, the Source Address field of the packet's IPv6 header is the care-of address. Ingress filtering[27] 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.

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 Payload Packets and Home Address Option. 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 (Home Address Option), 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.

15.9 Type 2 Routing Header

The definition of the type 2 routing header is described in Type 2 Routing Header. 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.

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 All IPv6 Routers. This is necessary in order to allow Mobile IPv6 traffic while still having the option to filter out other uses of routing headers.



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16. Contributors

Tuomas Aura, Mike Roe, Greg O'Shea (Microsoft Research, Cambdridge), 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 [33] 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.



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17. Acknowledgements

We would like to thank the members of the Mobile IP and IPng Working Groups for their comments and suggestions on this work. We would particularly like to thank (in alphabetical order) Fred Baker (Cisco), Josh Broch (Carnegie Mellon University), Samita Chakrabarti (Sun Microsystems), Robert Chalmers (University of California, Santa Barbara), Noel Chiappa (MIT), Greg Daley (Monash University), 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), Marc Hasson (Mentat), 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), Carl E. Williams (MCSR Laboratories), Vladislav Yasevich (HP), Alper Yegin (DoCoMo), and Xinhua Zhao (Stanford University) for their detailed reviews of earlier versions of this document. Their suggestions have helped to improve both the design and presentation of the protocol.

We would also like to thank the participants in the Mobile IPv6 testing event held at Nancy, France, September 15-17, 1999, for their valuable feedback as a result of interoperability testing of four Mobile IPv6 implementations coming from four different organizations: Bull, 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.



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Normative References

[1] Eastlake, D., Crocker, S. and J. Schiller, "Randomness Recommendations for Security", RFC 1750, December 1994.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[3] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 2373, July 1998 (TXT, HTML, XML).
[4] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998 (TXT, HTML, XML).
[5] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, November 1998 (TXT, HTML, XML).
[6] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998 (TXT, HTML, XML).
[7] Piper, D., "The Internet IP Security Domain of Interpretation for ISAKMP", RFC 2407, November 1998 (TXT, HTML, XML).
[8] Maughan, D., Schertler, M., Schneider, M. and J. Turner, "Internet Security Association and Key Management Protocol (ISAKMP)", RFC 2408, November 1998 (TXT, HTML, XML).
[9] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998 (TXT, HTML, XML).
[10] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998 (TXT, HTML, XML).
[11] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998 (TXT, HTML, XML).
[12] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998 (TXT, HTML, XML).
[13] Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998 (TXT, HTML, XML).
[14] Conta, A. and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 2463, December 1998 (TXT, HTML, XML).
[15] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, December 1998 (TXT, HTML, XML).
[16] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast Addresses", RFC 2526, March 1999.
[17] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener Discovery (MLD) for IPv6", RFC 2710, October 1999.
[18] Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[19] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On-line Database", RFC 3232, January 2002.
[20] National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-1, April 1995.
[21] Arkko, J., Devarapalli, V. and F. Dupont, "Using IPsec to Protect Mobile IPv6 Signaling betweenMobile Nodes and Home Agents", draft-ietf-mobileip-mipv6-ha-ipsec-01 (work in progress), October 2002.


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Informative References

[22] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 1883, December 1995.
[23] Perkins, C., "IP Mobility Support", RFC 2002, October 1996.
[24] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996 (TXT, HTML, XML).
[25] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, October 1996 (TXT, HTML, XML).
[26] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
[27] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", RFC 2267, January 1998 (TXT, HTML, XML).
[28] Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses", draft-aura-mipv6-bu-attacks-01 (work in progress), March 2002.
[29] Droms, R., "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress), November 2002.
[30] Draves, R., "Default Address Selection for IPv6", draft-ietf-ipv6-default-addr-select-09 (work in progress), August 2002.
[31] Nikander, P., Nordmark, E., Montenegro, G. and J. Arkko, "Mobile IPv6 Security Design Rationale", draft-nikander-mipv6-design-rationale-00.txt (work in progress), February 2003.
[32] Nordmark, E., "Securing MIPv6 BUs using return routability (BU3WAY)", draft-nordmark-mobileip-bu3way-00 (work in progress), November 2001.
[33] Roe, M., Aura, T., O'Shea, G. and J. Arkko, "Authentication of Mobile IPv6 Binding Updates and Acknowledgments", draft-roe-mobileip-updateauth-02 (work in progress), March 2002.
[34] Savola, P., "Security of IPv6 Routing Header and Home Address Options", draft-savola-ipv6-rh-ha-security-03 (work in progress), December 2002.
[35] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", draft-vida-mld-v2-06 (work in progress), December 2002.


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Authors' Addresses

  David B. Johnson
  Rice University
  Dept. of Computer Science, MS 132
  6100 Main Street
  Houston TX 77005-1892
  USA
EMail:  dbj@cs.rice.edu
  
  Charles E. Perkins
  Nokia Research Center
  313 Fairchild Drive
  Mountain View CA 94043
  USA
EMail:  charliep@iprg.nokia.com
  
  Jari Arkko
  Ericsson
  Jorvas 02420
  Finland
EMail:  jari.arkko@ericsson.com


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Appendix 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-ipv6-19.txt:



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Appendix 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 New Authorization Methods). The return routability messages can indicate support for piggybacking with a new mobility option.

B.2 Triangular Routing

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

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 Binding Updates to Correspondent Nodes, future enhancements of IPv6 security may cause a need to improve also the security of the return routability procedure. Using IPsec as the sole method for authorizing Binding Updates to correspondent nodes is also possible. The protection of the Mobility Header for this purpose is easy, though one must ensure that the IPsec SA was created with appropriate authorization to use the home address referenced in the Binding Update. For instance, a certificate used by IKE to create the security association might contain the home address. A future specification may specify how this is done.

B.4 Dynamically Generated Home Addresses

A future version of this specification may include functionality that allows the generation of new home addresses without requiring pre-arranged security associations or certificates even for the new addresses.

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 an anycast address associated with the FQDN of the home agent(s).
  3. Perform home agent address discovery, and select a home agent.
  4. Configure one home address based on the selected home agent's subnet prefix and the interface identifier of the mobile node.
  5. Create security associations and security policy database entries for protecting the traffic between the selected home address and home agent.
  6. Perform a home registration to the selected home agent.
  7. Perform prefix discovery.
  8. Make a decision if further home addresses need to be configured.

This procedure is restricted to those situations where the home prefix is 64 bits and the mobile node knows its own interface identifier of also 64 bits.



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Intellectual Property Statement

Full Copyright Statement

Acknowledgement