    Internet Engineering Task Force (IETF)                   C. Perkins, Ed.
    Request for Comments: 6275                                 Tellabs, Inc.
    Obsoletes: 3775                                               D. Johnson
    Category: Standards Track                                Rice University
    ISSN: 2070-1721                                                 J. Arkko
                                                                    Ericsson
                                                                   July 2011

                            Mobility Support in IPv6

    Abstract

       This document specifies Mobile IPv6, a protocol that allows nodes to
       remain reachable while moving around in the IPv6 Internet.  Each
       mobile node is always identified by its home address, regardless of
       its current point of attachment to the Internet.  While situated away
       from its home, a mobile node is also associated with a care-of
       address, which provides information about the mobile node's current
       location.  IPv6 packets addressed to a mobile node's home address are
       transparently routed to its care-of address.  The protocol enables
       IPv6 nodes to cache the binding of a mobile node's home address with
       its care-of address, and to then send any packets destined for the
       mobile node directly to it at this care-of address.  To support this
       operation, Mobile IPv6 defines a new IPv6 protocol and a new
       destination option.  All IPv6 nodes, whether mobile or stationary,
       can communicate with mobile nodes.  This document obsoletes RFC 3775.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc6275.

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    Copyright Notice

       Copyright (c) 2011 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

       This document may contain material from IETF Documents or IETF
       Contributions published or made publicly available before November
       10, 2008.  The person(s) controlling the copyright in some of this
       material may not have granted the IETF Trust the right to allow
       modifications of such material outside the IETF Standards Process.
       Without obtaining an adequate license from the person(s) controlling
       the copyright in such materials, this document may not be modified
       outside the IETF Standards Process, and derivative works of it may
       not be created outside the IETF Standards Process, except to format
       it for publication as an RFC or to translate it into languages other
       than English.

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    RFC 6275                Mobility Support in IPv6               July 2011

    Table of Contents

       1. Introduction ....................................................7
       2. Comparison with Mobile IP for IPv4 ..............................8
       3. Terminology .....................................................9
          3.1. General Terms ..............................................9
          3.2. Mobile IPv6 Terms .........................................11
       4. Overview of Mobile IPv6 ........................................15
          4.1. Basic Operation ...........................................15
          4.2. New IPv6 Protocol .........................................17
          4.3. New IPv6 Destination Option ...............................18
          4.4. New IPv6 ICMP Messages ....................................19
          4.5. Conceptual Data Structure Terminology .....................19
          4.6. Unique-Local Addressability ...............................20
       5. Overview of Mobile IPv6 Security ...............................20
          5.1. Binding Updates to Home Agents ............................21
          5.2. Binding Updates to Correspondent Nodes ....................22
               5.2.1. Node Keys ..........................................22
               5.2.2. Nonces .............................................23
               5.2.3. Cookies and Tokens .................................23
               5.2.4. Cryptographic Functions ............................24
               5.2.5. Return Routability Procedure .......................24
               5.2.6. Authorizing Binding Management Messages ............28
               5.2.7. Updating Node Keys and Nonces ......................30
               5.2.8. Preventing Replay Attacks ..........................32
               5.2.9. Handling Interruptions to Return Routability .......32
          5.3. Dynamic Home Agent Address Discovery ......................33
          5.4. Mobile Prefix Discovery ...................................33
          5.5. Payload Packets ...........................................33
       6. New IPv6 Protocol, Message Types, and Destination Option .......34
          6.1. Mobility Header ...........................................34
               6.1.1. Format .............................................34
               6.1.2. Binding Refresh Request Message ....................36
               6.1.3. Home Test Init Message .............................37
               6.1.4. Care-of Test Init Message ..........................38
               6.1.5. Home Test Message ..................................39
               6.1.6. Care-of Test Message ...............................41
               6.1.7. Binding Update Message .............................42
               6.1.8. Binding Acknowledgement Message ....................44
               6.1.9. Binding Error Message ..............................47
          6.2. Mobility Options ..........................................48
               6.2.1. Format .............................................49
               6.2.2. Pad1 ...............................................49
               6.2.3. PadN ...............................................50
               6.2.4. Binding Refresh Advice .............................50
               6.2.5. Alternate Care-of Address ..........................51
               6.2.6. Nonce Indices ......................................52
               6.2.7. Binding Authorization Data .........................52

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          6.3. Home Address Option .......................................54
          6.4. Type 2 Routing Header .....................................55
               6.4.1. Format .............................................56
          6.5. ICMP Home Agent Address Discovery Request Message .........57
          6.6. ICMP Home Agent Address Discovery Reply Message ...........58
          6.7. ICMP Mobile Prefix Solicitation Message Format ............60
          6.8. ICMP Mobile Prefix Advertisement Message Format ...........61
       7. Modifications to IPv6 Neighbor Discovery .......................64
          7.1. Modified Router Advertisement Message Format ..............64
          7.2. Modified Prefix Information Option Format .................65
          7.3. New Advertisement Interval Option Format ..................66
          7.4. New Home Agent Information Option Format ..................67
          7.5. Changes to Sending Router Advertisements ..................69
       8. Requirements for Types of IPv6 Nodes ...........................71
          8.1. All IPv6 Nodes ............................................71
          8.2. IPv6 Nodes with Support for Route Optimization ............72
          8.3. All IPv6 Routers ..........................................73
          8.4. IPv6 Home Agents ..........................................74
          8.5. IPv6 Mobile Nodes .........................................75
       9. Correspondent Node Operation ...................................76
          9.1. Conceptual Data Structures ................................76
          9.2. Processing Mobility Headers ...............................78
          9.3. Packet Processing .........................................78
               9.3.1. Receiving Packets with Home Address Option .........78
               9.3.2. Sending Packets to a Mobile Node ...................79
               9.3.3. Sending Binding Error Messages .....................81
               9.3.4. Receiving ICMP Error Messages ......................81
          9.4. Return Routability Procedure ..............................82
               9.4.1. Receiving Home Test Init Messages ..................82
               9.4.2. Receiving Care-of Test Init Messages ...............82
               9.4.3. Sending Home Test Messages .........................83
               9.4.4. Sending Care-of Test Messages ......................83
          9.5. Processing Bindings .......................................83
               9.5.1. Receiving Binding Updates ..........................83
               9.5.2. Requests to Cache a Binding ........................86
               9.5.3. Requests to Delete a Binding .......................86
               9.5.4. Sending Binding Acknowledgements ...................87
               9.5.5. Sending Binding Refresh Requests ...................88
          9.6. Cache Replacement Policy ..................................88
       10. Home Agent Operation ..........................................89
          10.1. Conceptual Data Structures ...............................89
          10.2. Processing Mobility Headers ..............................90
          10.3. Processing Bindings ......................................90
               10.3.1. Primary Care-of Address Registration ..............90
               10.3.2. Primary Care-of Address De-Registration ...........94
          10.4. Packet Processing ........................................96
               10.4.1. Intercepting Packets for a Mobile Node ............96
               10.4.2. Processing Intercepted Packets ....................98

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               10.4.3. Multicast Membership Control ......................99
               10.4.4. Stateful Address Autoconfiguration ...............100
               10.4.5. Handling Reverse-Tunneled Packets ................100
               10.4.6. Protecting Return Routability Packets ............101
          10.5. Dynamic Home Agent Address Discovery ....................102
               10.5.1. Receiving Router Advertisement Messages ..........102
          10.6. Sending Prefix Information to the Mobile Node ...........104
               10.6.1. List of Home Network Prefixes ....................104
               10.6.2. Scheduling Prefix Deliveries .....................105
               10.6.3. Sending Advertisements ...........................107
               10.6.4. Lifetimes for Changed Prefixes ...................108
       11. Mobile Node Operation ........................................108
          11.1. Conceptual Data Structures ..............................108
          11.2. Processing Mobility Headers .............................110
          11.3. Packet Processing .......................................110
               11.3.1. Sending Packets While Away from Home .............110
               11.3.2. Interaction with Outbound IPsec Processing .......113
               11.3.3. Receiving Packets While Away from Home ...........115
               11.3.4. Routing Multicast Packets ........................117
               11.3.5. Receiving ICMP Error Messages ....................118
               11.3.6. Receiving Binding Error Messages .................119
          11.4. Home Agent and Prefix Management ........................120
               11.4.1. Dynamic Home Agent Address Discovery .............120
               11.4.2. Sending Mobile Prefix Solicitations ..............121
               11.4.3. Receiving Mobile Prefix Advertisements ...........121
          11.5. Movement ................................................123
               11.5.1. Movement Detection ...............................123
               11.5.2. Home Link Detection ..............................125
               11.5.3. Forming New Care-of Addresses ....................126
               11.5.4. Using Multiple Care-of Addresses .................127
               11.5.5. Returning Home ...................................127
          11.6. Return Routability Procedure ............................130
               11.6.1. Sending Test Init Messages .......................130
               11.6.2. Receiving Test Messages ..........................131
               11.6.3. Protecting Return Routability Packets ............132
          11.7. Processing Bindings .....................................132
               11.7.1. Sending Binding Updates to the Home Agent ........132
               11.7.2. Correspondent Registration .......................135
               11.7.3. Receiving Binding Acknowledgements ...............138
               11.7.4. Receiving Binding Refresh Requests ...............140
          11.8. Retransmissions and Rate Limiting .......................141
       12. Protocol Constants ...........................................142
       13. Protocol Configuration Variables .............................142
       14. IANA Considerations ..........................................143
       15. Security Considerations ......................................146
          15.1. Threats .................................................146
          15.2. Features ................................................148
          15.3. Binding Updates to Home Agent ...........................150

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          15.4. Binding Updates to Correspondent Nodes ..................152
               15.4.1. Overview .........................................153
               15.4.2. Achieved Security Properties .....................153
               15.4.3. Comparison to Regular IPv6 Communications ........154
               15.4.4. Replay Attacks ...................................156
               15.4.5. Denial-of-Service Attacks ........................156
               15.4.6. Key Lengths ......................................157
          15.5. Dynamic Home Agent Address Discovery ....................158
          15.6. Mobile Prefix Discovery .................................159
          15.7. Tunneling via the Home Agent ............................159
          15.8. Home Address Option .....................................160
          15.9. Type 2 Routing Header ...................................161
          15.10. SHA-1 Secure Enough for Mobile IPv6 Control Messages ...161
       16. Contributors .................................................162
       17. Acknowledgements .............................................162
       18. References ...................................................162
          18.1. Normative References ....................................162
          18.2. Informative References ..................................164
       Appendix A. Future Extensions ....................................166
          A.1. Piggybacking .............................................166
          A.2. Triangular Routing .......................................166
          A.3. New Authorization Methods ................................166
          A.4. Neighbor Discovery Extensions ............................166
       Appendix B. Changes since RFC 3775 ...............................167

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    RFC 6275                Mobility Support in IPv6               July 2011

    1.  Introduction

       This document specifies a protocol that allows nodes to remain
       reachable while moving around in the IPv6 Internet.  Without specific
       support for mobility in IPv6 [6], packets destined to a mobile node
       would not be able to reach it while the mobile node is away from its
       home link.  In order to continue communication in spite of its
       movement, a mobile node could change its IP address each time it
       moves to a new link, but the mobile node would then not be able to
       maintain transport and higher-layer connections when it changes
       location.  Mobility support in IPv6 is particularly important, as
       mobile computers are likely to account for a majority or at least a
       substantial fraction of the population of the Internet during the
       lifetime of IPv6.

       The protocol defined in this document, known as Mobile IPv6, allows a
       mobile node to move from one link to another without changing the
       mobile node's "home address".  Packets may be routed to the mobile
       node using this address regardless of the mobile node's current point
       of attachment to the Internet.  The mobile node may also continue to
       communicate with other nodes (stationary or mobile) after moving to a
       new link.  The movement of a mobile node away from its home link is
       thus transparent to transport and higher-layer protocols and
       applications.

       The Mobile IPv6 protocol is just as suitable for mobility across
       homogeneous media as for mobility across heterogeneous media.  For
       example, Mobile IPv6 facilitates node movement from one Ethernet
       segment to another as well as it facilitates node movement from an
       Ethernet segment to a wireless LAN cell, with the mobile node's IP
       address remaining unchanged in spite of such movement.

       One can think of the Mobile IPv6 protocol as solving the network-
       layer mobility management problem.  Some mobility management
       applications -- for example, handover among wireless transceivers,
       each of which covers only a very small geographic area -- have been
       solved using link-layer techniques.  For example, in many current
       wireless LAN products, link-layer mobility mechanisms allow a
       "handover" of a mobile node from one cell to another, re-establishing
       link-layer connectivity to the node in each new location.

       Mobile IPv6 does not attempt to solve all general problems related to
       the use of mobile computers or wireless networks.  In particular,
       this protocol does not attempt to solve:

       o  Handling links with unidirectional connectivity or partial
          reachability, such as the hidden terminal problem where a host is
          hidden from only some of the routers on the link.

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       o  Access control on a link being visited by a mobile node.

       o  Local or hierarchical forms of mobility management (similar to
          many current link-layer mobility management solutions).

       o  Assistance for adaptive applications.

       o  Mobile routers.

       o  Service discovery.

       o  Distinguishing between packets lost due to bit errors versus
          network congestion.

       This document obsoletes RFC 3775.  Issues with the original document
       have been observed during the integration, testing, and deployment of
       RFC 3775.  A more detailed list of the changes since RFC 3775 may be
       found in Appendix B.

    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) [32] [25] [26], and from the opportunities
       provided by IPv6.  Mobile IPv6 thus shares many features with Mobile
       IPv4, but is integrated into IPv6 and offers many other improvements.
       This section summarizes the major differences between Mobile IPv4 and
       Mobile IPv6:

       o  There is no need to deploy special routers as "foreign agents", as
          in Mobile IPv4.  Mobile IPv6 operates in any location without any
          special support required from the local router.

       o  Support for route optimization is a fundamental part of the
          protocol, rather than a nonstandard set of extensions.

       o  Mobile IPv6 route optimization can operate securely even without
          pre-arranged security associations.  It is expected that route
          optimization can be deployed on a global scale between all mobile
          nodes and correspondent nodes.

       o  Support is also integrated into Mobile IPv6 for allowing route
          optimization to coexist efficiently with routers that perform
          "ingress filtering" [27].

       o  The IPv6 Neighbor Unreachability Detection ensures symmetric
          reachability between the mobile node and its default router in the
          current location.

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    RFC 6275                Mobility Support in IPv6               July 2011

       o  Most packets sent to a mobile node while away from home in Mobile
          IPv6 are sent using an IPv6 routing header rather than IP
          encapsulation, reducing the amount of resulting overhead compared
          to Mobile IPv4.

       o  Mobile IPv6 is decoupled from any particular link layer, as it
          uses IPv6 Neighbor Discovery [18] instead of the Address
          Resolution Protocol (ARP).  This also improves the robustness of
          the protocol.

       o  The use of IPv6 encapsulation (and the routing header) removes the
          need in Mobile IPv6 to manage "tunnel soft state".

       o  The dynamic home agent address discovery mechanism in Mobile IPv6
          returns a single reply to the mobile node.  The directed broadcast
          approach used in IPv4 returns separate replies from each home
          agent.

    3.  Terminology

       The key words "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 be
          either a global IPv6 address or a unique local IPv6 address.

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       host

          Any node that is not a router.

       link

          A communication facility or medium over which nodes can
          communicate at the link layer, such as an Ethernet (simple or
          bridged).  A link is the layer immediately below IP.

       interface

          A node's attachment to a link.

       subnet prefix

          A bit string that consists of some number of initial bits of an IP
          address.

       interface identifier

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

       link-layer address

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

       packet

          An IP header plus payload.

       security association

          An IPsec security association is a cooperative relationship formed
          by the sharing of cryptographic keying material and associated
          context.  Security associations are simplex.  That is, two
          security associations are needed to protect bidirectional traffic
          between two nodes, one for each direction.

       security policy database

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

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       destination option

          Destination options are carried by the IPv6 Destination Options
          extension header.  Destination options include optional
          information that need be examined only by the IPv6 node given as
          the destination address in the IPv6 header, not by routers in
          between.  Mobile IPv6 defines one new destination option, the Home
          Address destination option (see Section 6.3).

       routing header

          A routing header may be present as an IPv6 header extension, and
          indicates that the payload has to be delivered to a destination
          IPv6 address in some way that is different from what would be
          carried out by standard Internet routing.  In this document, use
          of the term "routing header" typically refers to use of a type 2
          routing header, as specified in Section 6.4.

       "|" (concatenation)

          Some formulas in this specification use the symbol "|" to indicate
          bytewise concatenation, as in A | B.  This concatenation requires
          that all of the octets of the datum A appear first in the result,
          followed by all of the octets of the datum B.

       First (size, input)

          Some formulas in this specification use a functional form "First
          (size, input)" to indicate truncation of the "input" data so that
          only the first "size" bits remain to be used.

    3.2.  Mobile IPv6 Terms

       These terms are intended to be compatible with the definitions given
       in RFC 3753 [40].  However, if there is any conflict, the definitions
       given here should be considered to supersede those in RFC 3753.

       home address

          A unicast routable address assigned to a mobile node, used as the
          permanent address of the mobile node.  This address is within the
          mobile node's home link.  Standard IP routing mechanisms will
          deliver packets destined for a mobile node's home address to its
          home link.  Mobile nodes can have multiple home addresses, for
          instance, when there are multiple home prefixes on the home link.

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       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".

       Layer 2 (L2) handover

          A process by which the mobile node changes from one link-layer
          connection to another.  For example, a change of wireless access
          point is an L2 handover.

       Layer 3 (L3) handover

          Subsequent to an L2 handover, a mobile node detects a change in an
          on-link subnet prefix that would require a change in the primary
          care-of address.  For example, a change of access router
          subsequent to a change of wireless access point typically results
          in an L3 handover.

       correspondent node

          A peer node with which a mobile node is communicating.  The
          correspondent node may be either mobile or stationary.

       foreign subnet prefix

          Any IP subnet prefix other than the mobile node's home subnet
          prefix.

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       foreign link

          Any link other than the mobile node's home link.

       care-of address

          A unicast routable address associated with a mobile node while
          visiting a foreign link; the subnet prefix of this IP address is a
          foreign subnet prefix.  Among the multiple care-of addresses that
          a mobile node may have at any given time (e.g., with different
          subnet prefixes), the one registered with the mobile node's home
          agent for a given home address is called its "primary" care-of
          address.

       home agent

          A router on a mobile node's home link with which the mobile node
          has registered its current care-of address.  While the mobile node
          is away from home, the home agent intercepts packets on the home
          link destined to the mobile node's home address, encapsulates
          them, and tunnels them to the mobile node's registered care-of
          address.

       binding

          The association of the home address of a mobile node with a
          care-of address for that mobile node, along with the remaining
          lifetime of that association.

       registration

          The process during which a mobile node sends a Binding Update to
          its home agent or a correspondent node, causing a binding for the
          mobile node to be registered.

       mobility message

          A message containing a Mobility Header (see Section 6.1).

       binding authorization

          Correspondent registration needs to be authorized to allow the
          recipient to believe that the sender has the right to specify a
          new binding.

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       return routability procedure

          The return routability procedure authorizes registrations by the
          use of a cryptographic token exchange.

       correspondent registration

          A return routability procedure followed by a registration, run
          between the mobile node and a correspondent node.

       home registration

          A registration between the mobile node and its home agent,
          authorized by the use of IPsec.

       nonce

          Nonces are random numbers used internally by the correspondent
          node in the creation of keygen tokens related to the return
          routability procedure.  The nonces are not specific to a mobile
          node, and are kept secret within the correspondent node.

       nonce index

          A nonce index is used to indicate which nonces have been used when
          creating keygen token values, without revealing the nonces
          themselves.

       cookie

          A cookie is a random number used by a mobile node to prevent
          spoofing by a bogus correspondent node in the return routability
          procedure.

       care-of init cookie

          A cookie sent to the correspondent node in the Care-of Test Init
          message, to be returned in the Care-of Test message.

       home init cookie

          A cookie sent to the correspondent node in the Home Test Init
          message, to be returned in the Home Test message.

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       keygen token

          A keygen token is a number supplied by a correspondent node in the
          return routability procedure to enable the mobile node to compute
          the necessary binding management key for authorizing a Binding
          Update.

       care-of keygen token

          A keygen token sent by the correspondent node in the Care-of Test
          message.

       home keygen token

          A keygen token sent by the correspondent node in the Home Test
          message.

       binding management key (Kbm)

          A binding management key (Kbm) is a key used for authorizing a
          binding cache management message (e.g., Binding Update or Binding
          Acknowledgement).  Return routability provides a way to create a
          binding management key.

    4.  Overview of Mobile IPv6

    4.1.  Basic Operation

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

       While a mobile node is attached to some foreign link away from home,
       it is also addressable at one or more care-of addresses.  A care-of
       address is an IP address associated with a mobile node that has the
       subnet prefix of a particular foreign link.  The mobile node can
       acquire its care-of address through conventional IPv6 mechanisms,
       such as stateless or stateful auto-configuration.  As long as the
       mobile node stays in this location, packets addressed to this care-of
       address will be routed to the mobile node.  The mobile node may also
       accept packets from several care-of addresses, such as when it is
       moving but still reachable at the previous link.

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       The association between a mobile node's home address and care-of
       address is known as a "binding" for the mobile node.  While away from
       home, a mobile node registers its primary care-of address with a
       router on its home link, requesting this router to function as the
       "home agent" for the mobile node.  The mobile node performs this
       binding registration by sending a "Binding Update" message to the
       home agent.  The home agent replies to the mobile node by returning a
       "Binding Acknowledgement" message.  The operation of the mobile node
       is specified in Section 11, and the operation of the home agent is
       specified in Section 10.

       Any node communicating with a mobile node is referred to in this
       document as a "correspondent node" of the mobile node, and may itself
       be either a stationary node or a mobile node.  Mobile nodes can
       provide information about their current location to correspondent
       nodes.  This happens through the correspondent registration.  As a
       part of this procedure, a return routability test is performed in
       order to authorize the establishment of the binding.  The operation
       of the correspondent node is specified in Section 9.

       There are two possible modes for communications between the mobile
       node and a correspondent node.  The first mode, bidirectional
       tunneling, does not require Mobile IPv6 support from the
       correspondent node and is available even if the mobile node has not
       registered its current binding with the correspondent node.  Packets
       from the correspondent node are routed to the home agent and then
       tunneled to the mobile node.  Packets to the correspondent node are
       tunneled from the mobile node to the home agent ("reverse tunneled")
       and then routed normally from the home network to the correspondent
       node.  In this mode, the home agent uses proxy Neighbor Discovery to
       intercept any IPv6 packets addressed to the mobile node's home
       address (or home addresses) on the home link.  Each intercepted
       packet is tunneled to the mobile node's primary care-of address.
       This tunneling is performed using IPv6 encapsulation [7].

       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 [6] (see Section 6.4) to route the packet to the
       mobile node by way of the care-of address indicated in this binding.

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       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 temporary failures of the home agent or
       networks on the path to or from the home agent is reduced.

       When routing packets directly to the mobile node, the correspondent
       node sets the Destination Address in the IPv6 header to the care-of
       address of the mobile node.  A new type of IPv6 routing header (see
       Section 6.4) is also added to the packet to carry the desired home
       address.  Similarly, the mobile node sets the Source Address in the
       packet's IPv6 header to its current care-of addresses.  The mobile
       node adds a new IPv6 "Home Address" destination option (see
       Section 6.3) to carry its home address.  The inclusion of home
       addresses in these packets makes the use of the care-of address
       transparent above the network layer (e.g., at the transport layer).

       Mobile IPv6 also provides support for multiple home agents, and a
       limited support for the reconfiguration of the home network.  In
       these cases, the mobile node may not know the IP address of its own
       home agent, and even the home subnet prefixes may change over time.
       A mechanism known as "dynamic home agent address discovery" allows a
       mobile node to dynamically discover the IP address of a home agent on
       its home link, even when the mobile node is away from home.  Mobile
       nodes can also learn new information about home subnet prefixes
       through the "mobile prefix discovery" mechanism.  These mechanisms
       are described starting in Section 6.5.

       This document is written under the assumption that the mobile node is
       configured with the home prefix for the mobile node to be able to
       discover a home agent and configure a home address.  This might be
       limiting in deployments where the home agent and the home address for
       the mobile node need to be assigned dynamically.  Additional
       mechanisms have been specified for the mobile node to dynamically
       configure a home agent, a home address, and the home prefix.  These
       mechanisms are described in "Mobile IPv6 Bootstrapping in Split
       Scenario" [22] and "MIP6-bootstrapping for the Integrated Scenario"
       [36].

    4.2.  New IPv6 Protocol

       Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header
       (see Section 6.1).  This header is used to carry the following
       messages:

       Home Test Init

       Home Test

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       Care-of Test Init

       Care-of Test

          These four messages are used to perform the return routability
          procedure from the mobile node to a correspondent node.  This
          ensures authorization of subsequent Binding Updates, as described
          in Section 5.2.5.

       Binding Update

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

       Binding Acknowledgement

          A Binding Acknowledgement is used to acknowledge receipt of a
          Binding Update, if an acknowledgement was requested in the Binding
          Update (e.g., the Binding Update was sent to a home agent), or an
          error occurred.

       Binding Refresh Request

          A Binding Refresh Request is used by a correspondent node to
          request that a mobile node 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.
          The Binding Error message is also used by the home agent to signal
          an error to the mobile node, if it receives an unrecognized
          Mobility Header Message Type from the mobile node.

    4.3.  New IPv6 Destination Option

       Mobile IPv6 defines a new IPv6 destination option, the Home Address
       destination option.  This option is described in detail in
       Section 6.3.

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    4.4.  New IPv6 ICMP Messages

       Mobile IPv6 also introduces four new ICMP message types, two for use
       in the dynamic home agent address discovery mechanism, and two for
       renumbering and mobile configuration mechanisms.  As described in
       Sections 10.5 and 11.4.1, the following two new ICMP message types
       are used for home agent address discovery:

       o  Home Agent Address Discovery Request, described in Section 6.5.

       o  Home Agent Address Discovery Reply, described in Section 6.6.

       The next two message types are used for network renumbering and
       address configuration on the mobile node, as described in
       Section 10.6:

       o  Mobile Prefix Solicitation, described in Section 6.7.

       o  Mobile Prefix Advertisement, described in Section 6.8.

    4.5.  Conceptual Data Structure Terminology

       This document describes the Mobile IPv6 protocol in terms of the
       following conceptual data structures:

       Binding Cache

          A cache of bindings for other nodes.  This cache is maintained by
          home agents and correspondent nodes.  The cache contains both
          "correspondent registration" entries (see Section 9.1) and "home
          registration" entries (see Section 10.1).

       Binding Update List

          This list is maintained by each mobile node.  The list has an item
          for every binding that the mobile node has or is trying to
          establish with a specific other node.  Both correspondent and home
          registrations are included in this list.  Entries from the list
          are deleted as the lifetime of the binding expires.  See
          Section 11.1.

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       Home Agents List

          Home agents need to know which other home agents are on the same
          link.  This information is stored in the Home Agents List, as
          described in more detail in Section 10.1.  The list is used for
          informing mobile nodes during dynamic home agent address
          discovery.

    4.6.  Unique-Local Addressability

       This specification requires that home and care-of addresses MUST be
       unicast routable addresses.  Unique-local IPv6 unicast addresses
       (ULAs, RFC 4193 [15]) may be usable on networks that use such non-
       globally routable addresses, but this specification does not define
       when such usage is safe and when it is not.  Mobile nodes may not be
       able to distinguish between their home site and the site at which
       they are currently located.  This can make it hard to prevent
       accidental attachment to other sites, because the mobile node might
       use the ULA at another site, which could not be used to successfully
       send packets to the mobile node's home agent (HA).  This would result
       in unreachability between the mobile node (MN) and the HA, when
       unique-local IPv6 routable addresses are used as care-of addresses.
       Similarly, CNs outside the MN's own site will not be reachable when
       ULAs are used as home addresses.  Therefore, unique-local IPv6
       unicast addresses SHOULD NOT be used as home or care-of addresses
       when other address choices are available.  If such addresses are
       used, however, according to RFC 4193 [15], they are treated as any
       global unicast IPv6 address so, for the remainder of this
       specification, use of unique-local IPv6 unicast addresses is not
       differentiated from other globally unique IPv6 addresses.

    5.  Overview of Mobile IPv6 Security

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

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

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    5.1.  Binding Updates to Home Agents

       The mobile node and the home agent MUST use an IPsec security
       association to protect the integrity and authenticity of the Binding
       Updates and Acknowledgements.  Both the mobile nodes and the home
       agents MUST support and SHOULD use the Encapsulating Security Payload
       (ESP) [5] 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) [4] is also possible but for brevity
       not discussed in this specification.

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

       As with all IPsec security associations in this specification, manual
       configuration of security associations MUST be supported.  The shared
       secrets used MUST be random and unique for different mobile nodes,
       and MUST be distributed off-line to the mobile nodes.  Automatic key
       management with the Internet Key Exchange Protocol version 2 (IKEv2)
       [24] MAY be supported as described in [20].

       Section 11.3.2 discusses how IKEv2 connections to the home agent need
       a careful treatment of the addresses used for transporting IKEv2.
       This is necessary to ensure that a Binding Update is not needed
       before the IKEv2 exchange that is needed for securing the Binding
       Update.

       More detailed descriptions and examples using IPsec to protect
       communications between the mobile node and the home agent have been
       published [12][20].

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    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 ensure that the right mobile node is
       sending the message.  This method does not protect against attackers
       who are on the path between the home network and the correspondent
       node.  However, attackers in such a location are capable of
       performing the same attacks even without Mobile IPv6.  The main
       advantage of the return routability procedure is that it limits the
       potential attackers to those having an access to one specific path in
       the Internet, and avoids forged Binding Updates from anywhere else in
       the Internet.  For a more in-depth explanation of the security
       properties of the return routability procedure, see Section 15.
       Also, consult [43].

       The integrity and authenticity of the Binding Update 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.  Section 5.2.5 outlines the basic return routability
       procedure.  Section 5.2.6 shows how the results of this procedure are
       used to authorize a Binding Update to a correspondent node.

    5.2.1.  Node Keys

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

       A correspondent node MAY generate a fresh node key at any time; this
       avoids the need for secure persistent key storage.  Procedures for
       optionally updating the node key are discussed later in
       Section 5.2.7.

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    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 [14].  A
       correspondent node may use the same Kcn and nonce with all the mobile
       nodes with which it is in communication.

       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 a new
       nonce during the run of a protocol, the correspondent node can
       distinguish messages that should be checked against the old nonce
       from messages that should be checked against the new nonce.  Strictly
       speaking, indices are not necessary in the authentication, but allow
       the correspondent node to efficiently find the nonce value that it
       used in creating a keygen token.

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

       The specific nonce index values cannot be used by mobile nodes to
       determine the validity of the nonce.  Expected validity times for the
       nonces values and the procedures for updating them are discussed
       later in Section 5.2.7.

       A nonce is an octet string of any length.  The recommended length is
       64 bits.

    5.2.3.  Cookies and Tokens

       The return routability address test procedure uses cookies and keygen
       tokens as opaque values within the test init and test messages,
       respectively.

       o  The "home init cookie" and "care-of init cookie" are 64-bit values
          sent to the correspondent node from the mobile node, and later
          returned to the mobile node.  The home init cookie is sent in the
          Home Test Init message, and returned in the Home Test message.
          The care-of init cookie is sent in the Care-of Test Init message,
          and returned in the Care-of Test message.

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       o  The "home keygen token" and "care-of keygen token" are 64-bit
          values sent by the correspondent node to the mobile node via the
          home agent (via the Home Test message) and the care-of address (by
          the Care-of Test message), respectively.

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

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

    5.2.4.  Cryptographic Functions

       By default in this specification, the function used to compute hash
       values is SHA-1 [11], which is considered to offer sufficient
       protection for Mobile IPv6 control messages (see Section 15.10).
       Message Authentication Codes (MACs) are then computed using HMAC_SHA1
       [1][11].  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") that the correspondent node sends to those
       addresses.  These data are combined by the mobile node into a binding
       management key, denoted Kbm.

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

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

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

       Home Test Init

          A mobile node sends a Home Test Init message to the correspondent
          node (via the home agent) to acquire the home keygen token.  The
          contents of the message can be summarized as follows:

          *  Source Address = home address

          *  Destination Address = correspondent

          *  Parameters:

             +  home init cookie

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

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       Care-of Test Init

          The mobile node sends a Care-of Test Init message to the
          correspondent node (directly, not via the home agent) to acquire
          the care-of keygen token.  The contents of this message can be
          summarized as follows:

          *  Source Address = care-of address

          *  Destination Address = correspondent

          *  Parameters:

             +  care-of init cookie

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

       Home Test

          The Home Test message is sent in response to a Home Test Init
          message.  It is sent via the home agent.  The contents of the
          message are:

          *  Source Address = correspondent

          *  Destination Address = home address

          *  Parameters:

             +  home init cookie

             +  home keygen token

             +  home nonce index

          When the correspondent node receives the Home Test Init message,
          it generates a home keygen token as follows:

           home keygen token :=
                First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0)))

       where | denotes concatenation.  The final "0" inside the HMAC_SHA1
       function is a single zero octet, used to distinguish home and care-of
       cookies from each other.

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       The home keygen token is formed from the first 64 bits of the MAC.
       The home keygen token tests that the mobile node can receive messages
       sent to its home address.  Kcn is used in the production of home
       keygen token in order to allow the correspondent node to verify that
       it generated the home and care-of nonces, without forcing the
       correspondent node to remember a list of all tokens it has handed
       out.

       The Home Test message is sent to the mobile node via the home
       network, where it is presumed that the home agent will tunnel the
       message to the mobile node.  This means that the mobile node needs to
       already have sent a Binding Update to the home agent, so that the
       home agent will have received and authorized the new care-of address
       for the mobile node before the return routability procedure.  For
       improved security, the data passed between the home agent and the
       mobile node is made immune to inspection and passive attacks.  Such
       protection is gained by encrypting the home keygen token as it is
       tunneled from the home agent to the mobile node as specified in
       Section 10.4.6.  The security properties of this additional security
       are discussed in Section 15.4.1.

       The home init cookie from the mobile node is returned in the Home
       Test message, to ensure that the message comes from a node on the
       route between the home agent and the correspondent node.

       The home nonce index is delivered to the mobile node to later allow
       the correspondent node to efficiently find the nonce value that it
       used in creating the home keygen token.

       Care-of Test

          This message is sent in response to a Care-of Test Init message.
          This message is not sent via the home agent; it is sent directly
          to the mobile node.  The contents of the message are:

          *  Source Address = correspondent

          *  Destination Address = care-of address

          *  Parameters:

             +  care-of init cookie

             +  care-of keygen token

             +  care-of nonce index

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

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

       Here, the final "1" inside the HMAC_SHA1 function is a single octet
       containing the hex value 0x01, and is used to distinguish home and
       care-of cookies from each other.  The keygen token is formed from the
       first 64 bits of the MAC, and sent directly to the mobile node at its
       care-of address.  The care-of init cookie from the Care-of Test Init
       message is returned to ensure that the message comes from a node on
       the route to the correspondent node.

       The care-of nonce index is provided to identify the nonce used for
       the care-of keygen token.  The home and care-of nonce indices MAY be
       the same, or different, in the Home and Care-of Test messages.

       When the mobile node has received both the Home and Care-of Test
       messages, the return routability procedure is complete.  As a result
       of the procedure, the mobile node has the data it needs to send a
       Binding Update to the correspondent node.  The mobile node hashes the
       tokens together to form a 20-octet binding key Kbm:

           Kbm = SHA-1 (home keygen token | care-of keygen token)

       A Binding Update may also be used to delete a previously established
       binding (Section 6.1.7).  In this case, the care-of keygen token is
       not used.  Instead, the binding management key is generated as
       follows:

           Kbm = SHA-1(home keygen token)

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

    5.2.6.  Authorizing Binding Management Messages

       After the mobile node has created the binding management key (Kbm),
       it can supply a verifiable Binding Update to the correspondent node.
       This section provides an overview of this registration.  The figure
       below shows the message flow.

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         Mobile node                                Correspondent node
              |                                               |
              |             Binding Update (BU)               |
              |---------------------------------------------->|
              |  (MAC, seq#, nonce indices, care-of address)  |
              |                                               |
              |                                               |
              |    Binding Acknowledgement (BA) (if sent)     |
              |<----------------------------------------------|
              |              (MAC, seq#, status)              |

       Binding Update

          To authorize a Binding Update, the mobile node creates a binding
          management key Kbm from the keygen tokens as described in the
          previous section.  The contents of the Binding Update include the
          following:

          *  Source Address = care-of address

          *  Destination Address = correspondent

          *  Parameters:

             +  home address (within the Home Address destination option if
                different from the Source Address)

             +  sequence number (within the Binding Update message header)

             +  home nonce index (within the Nonce Indices option)

             +  care-of nonce index (within the Nonce Indices option)

             +  First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
                | BU)))

          The Binding Update contains a Nonce Indices option, indicating to
          the correspondent node which home and care-of nonces to use to
          recompute Kbm, the binding management key.  The MAC is computed as
          described in Section 6.2.7, using the correspondent node's address
          as the destination address and the Binding Update message itself
          ("BU" above) as the Mobility Header (MH) Data.

          Once the correspondent node has verified the MAC, it can create a
          Binding Cache entry for the mobile.

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       Binding Acknowledgement

          The Binding Update is in some cases acknowledged by the
          correspondent node.  The contents of the message are as follows:

          *  Source Address = correspondent

          *  Destination Address = care-of address

          *  Parameters:

             +  sequence number (within the Binding Update message header)

             +  First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
                | BA)))

          The Binding Acknowledgement contains the same sequence number as
          the Binding Update.  The MAC is computed as described in
          Section 6.2.7, using the correspondent node's address as the
          destination address and the message itself ("BA" above) as the MH
          Data.

       Bindings established with correspondent nodes using keys created by
       way of the return routability procedure MUST NOT exceed
       MAX_RR_BINDING_LIFETIME seconds (see Section 12).

       The value in the Source Address field in the IPv6 header carrying the
       Binding Update is normally also the care-of address that is used in
       the binding.  However, a different care-of address MAY be specified
       by including an Alternate Care-of Address mobility option in the
       Binding Update (see Section 6.2.5).  When such a message is sent to
       the correspondent node and the return routability procedure is used
       as the authorization method, the Care-of Test Init and Care-of Test
       messages MUST have been performed for the address in the Alternate
       Care-of Address option (not the Source Address).  The nonce indices
       and MAC value MUST be based on information gained in this test.

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

    5.2.7.  Updating Node Keys and Nonces

       Correspondent nodes generate nonces at regular intervals.  It is
       recommended to keep each nonce (identified by a nonce index)
       acceptable for at least MAX_TOKEN_LIFETIME seconds (see Section 12)
       after it has been first used in constructing a return routability

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       message response.  However, the correspondent node MUST NOT accept
       nonces beyond MAX_NONCE_LIFETIME seconds (see Section 12) after the
       first use.  As the difference between these two constants is 30
       seconds, a convenient way to enforce the above lifetimes is to
       generate a new nonce every 30 seconds.  The node can then continue to
       accept tokens that have been based on the last 8 (MAX_NONCE_LIFETIME
       / 30) nonces.  This results in tokens being acceptable
       MAX_TOKEN_LIFETIME to MAX_NONCE_LIFETIME seconds after they have been
       sent to the mobile node, depending on whether the token was sent at
       the beginning or end of the first 30-second period.  Note that the
       correspondent node may also attempt to generate new nonces on demand,
       or only if the old nonces have been used.  This is possible, as long
       as the correspondent node keeps track of how long a time ago the
       nonces were used for the first time, and does not generate new nonces
       on every return routability request.

       Due to resource limitations, rapid deletion of bindings, or reboots
       the correspondent node may not in all cases recognize the nonces that
       the tokens were based on.  If a nonce index is unrecognized, the
       correspondent node replies with an error code in the Binding
       Acknowledgement (either 136, 137, or 138 as discussed in
       Section 6.1.8).  The mobile node can then retry the return
       routability procedure.

       An update of Kcn SHOULD be done at the same time as an update of a
       nonce, so that nonce indices can identify both the nonce and the key.
       Old Kcn values have to be therefore remembered as long as old nonce
       values.

       Given that the tokens are normally expected to be usable for
       MAX_TOKEN_LIFETIME seconds, the mobile node MAY use them beyond a
       single run of the return routability procedure until
       MAX_TOKEN_LIFETIME expires.  After this the mobile node SHOULD NOT
       use the tokens.  A fast moving mobile node MAY reuse a recent home
       keygen token from a correspondent node when moving to a new location,
       and just acquire a new care-of keygen token to show routability in
       the new location.

       While this does not save the number of round-trips due to the
       simultaneous processing of home and care-of return routability tests,
       there are fewer messages being exchanged, and a potentially long
       round-trip through the home agent is avoided.  Consequently, this
       optimization is often useful.  A mobile node that has multiple home
       addresses MAY also use the same care-of keygen token for Binding
       Updates concerning all of these addresses.

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    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.

    5.2.9.  Handling Interruptions to Return Routability

       In some scenarios, such as simultaneous mobility, where both
       correspondent host and mobile host move at the same time, or in the
       case where the correspondent node reboots and loses data, route
       optimization may not complete, or relevant data in the binding cache
       might be lost.

       o  Return Routability signaling MUST be sent to the correspondent
          node's home address if it has one (i.e., not to the correspondent
          nodes care-of address if the correspondent node is also mobile).

       o  If Return Routability signaling timed out after MAX_RO_FAILURE
          attempts, the mobile node MUST revert to sending packets to the
          correspondent node's home address through its home agent.

       The mobile node may run the bidirectional tunneling in parallel with
       the return routability procedure until it is successful.  Exponential
       backoff SHOULD be used for retransmission of return routability
       messages.

       The return routability procedure may be triggered by movement of the
       mobile node or by sustained loss of end-to-end communication with a
       correspondent node (e.g., based on indications from upper layers)
       that has been using a route optimized connection to the mobile node.
       If such indications are received, the mobile node MAY revert to
       bidirectional tunneling while restarting the return routability
       procedure.

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    5.3.  Dynamic Home Agent Address Discovery

       Dynamic home agent address discovery has been designed for use in
       deployments where security is not needed.  For this reason, no
       security solution is provided in this document for dynamic home agent
       address discovery.

    5.4.  Mobile Prefix Discovery

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

    5.5.  Payload Packets

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

       This specification limits the use of the Home Address destination
       option to the situation where the correspondent node already has a
       Binding Cache entry for the given home address.  This avoids the use
       of the Home Address option in attacks described in Section 15.1.

       Mobile IPv6 uses a type of routing header specific to Mobile IPv6.
       This type provides the necessary functionality but does not open
       vulnerabilities discussed in Section 15.1 and RFC 5095 [45].

       Tunnels between the mobile node and the home agent are protected by
       ensuring proper use of source addresses, and optional cryptographic
       protection.  The mobile node verifies that the outer IP address
       corresponds to its home agent.  The home agent verifies that the
       outer IP address corresponds to the current location of the mobile
       node (Binding Updates sent to the home agents are secure).  The home
       agent identifies the mobile node through the source address of the
       inner packet.  (Typically, this is the home address of the mobile
       node, but it can also be a link-local address, as discussed in
       Section 10.4.2.  To recognize the latter type of addresses, the home

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       agent requires that the Link-Local Address Compatibility (L) was set
       in the Binding Update.)  These measures protect the tunnels against
       vulnerabilities discussed in Section 15.1.

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

    6.  New IPv6 Protocol, Message Types, and Destination Option

    6.1.  Mobility Header

       The Mobility Header is an extension header used by mobile nodes,
       correspondent nodes, and home agents in all messaging related to the
       creation and management of bindings.  The subsections within this
       section describe the message types that may be sent using the
       Mobility Header.

       Mobility Header messages MUST NOT be sent with a type 2 routing
       header, except as described in Section 9.5.4 for Binding
       Acknowledgement.  Mobility Header messages also MUST NOT be used with
       a Home Address destination option, except as described in Sections
       11.7.1 and 11.7.2 for Binding Update.  Binding Update List or Binding
       Cache information (when present) for the destination MUST NOT be used
       in sending Mobility Header messages.  That is, Mobility Header
       messages bypass both the Binding Cache check described in
       Section 9.3.2 and the Binding Update List check described in
       Section 11.3.1 that are normally performed for all packets.  This
       applies even to messages sent to or from a correspondent node that is
       itself a mobile node.

    6.1.1.  Format

       The Mobility Header is identified by a Next Header value of 135 in
       the immediately preceding header, and has the following format:

           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           | Payload Proto |  Header Len   |   MH Type     |   Reserved    |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |           Checksum            |                               |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
           |                                                               |
           .                                                               .
           .                       Message Data                            .
           .                                                               .
           |                                                               |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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       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 [6].

          This field is intended to be used by a future extension (see
          Appendix A.1).

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

       Header Len

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

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

       MH Type

          8-bit selector.  Identifies the particular mobility message in
          question.  Current values are specified in Section 6.1.2 and
          onward.  An unrecognized MH Type field causes an error indication
          to be sent.

       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 [6].  The Next Header value used in the
          pseudo-header is 135.  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.

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          Note that the procedures of calculating upper-layer checksums
          while away from home described in Section 11.3.1 apply even for
          the Mobility Header.  If a mobility message has a Home Address
          destination option, then the checksum calculation uses the home
          address in this option as the value of the IPv6 Source Address
          field.  The type 2 routing header is treated as explained in [6].

          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
       Section 6.2.

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

    6.1.2.  Binding Refresh Request Message

       The Binding Refresh Request (BRR) message requests a mobile node to
       update its mobility binding.  This message is sent by correspondent
       nodes according to the rules in Section 9.5.5.  When a mobile node
       receives a packet containing a Binding Refresh Request message it
       processes the message according to the rules in Section 11.7.4.

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

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                                           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                           |          Reserved             |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                                                               |
           .                                                               .
           .                        Mobility Options                       .
           .                                                               .
           |                                                               |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Reserved

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

       Mobility Options

          Variable-length field of such length that the complete Mobility
          Header is an integer multiple of 8 octets long.  This field
          contains zero or more TLV-encoded mobility options.  The encoding
          and format of defined options are described in Section 6.2.  The
          receiver MUST ignore and skip any options that it does not
          understand.

          There MAY be additional information, associated with this Binding
          Refresh Request message that need not be present in all Binding
          Refresh Request messages sent.  Mobility options allow future
          extensions to the format of the Binding Refresh Request message to
          be defined.  This specification does not define any options valid
          for the Binding Refresh Request message.

       If no actual options are present in this message, no padding is
       necessary and the Header Len field will be set to 0.

    6.1.3.  Home Test Init Message

       A mobile node uses the Home Test Init (HoTI) message to initiate the
       return routability procedure and request a home keygen token from a
       correspondent node (see Section 11.6.1).  The Home Test Init message
       uses the MH Type value 1.  When this value is indicated in the MH
       Type field, the format of the Message Data field in the Mobility
       Header is as follows:

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                                           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                           |           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 that contains a random value, the home init cookie.

       Mobility Options

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

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

       This message is tunneled through the home agent when the mobile node
       is away from home.  Such tunneling SHOULD employ IPsec ESP in tunnel
       mode between the home agent and the mobile node.  This protection is
       indicated by the IPsec security policy database.  The protection of
       Home Test Init messages is unrelated to the requirement to protect
       regular payload traffic, which MAY use such tunnels as well.

    6.1.4.  Care-of Test Init Message

       A mobile node uses the Care-of Test Init (CoTI) message to initiate
       the return routability procedure and request a care-of keygen token
       from a correspondent node (see Section 11.6.1).  The Care-of Test

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       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 that 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.  This field
          contains zero or more TLV-encoded mobility options.  The receiver
          MUST ignore and skip any options that it does not understand.
          This specification does not define any options valid for the
          Care-of Test Init message.

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

    6.1.5.  Home Test Message

       The Home Test (HoT) message is a response to the Home Test Init
       message, and is sent from the correspondent node to the mobile node
       (see Section 5.2.5).  The Home Test message uses the MH Type value 3.
       When this value is indicated in the MH Type field, the format of the
       Message Data field in the Mobility Header is as follows:

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                                           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                           |       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 that 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.  This field
          contains zero or more TLV-encoded mobility options.  The receiver
          MUST ignore and skip any options that 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.

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    6.1.6.  Care-of Test Message

       The Care-of Test (CoT) message is a response to the Care-of Test Init
       message, and is sent from the correspondent node to the mobile node
       (see Section 11.6.2).  The Care-of Test message uses the MH Type
       value 4.  When this value is indicated in the MH Type field, the
       format of the Message Data field in the Mobility Header is as
       follows:

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

       Care-of Nonce Index

          This value will be echoed back by the mobile node to the
          correspondent node in a subsequent Binding Update.

       Care-of Init Cookie

          64-bit field that contains the care-of init cookie.

       Care-of Keygen Token

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

       Mobility Options

          Variable-length field of such length that the complete Mobility
          Header is an integer multiple of 8 octets long.  This field
          contains zero or more TLV-encoded mobility options.  The receiver

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          MUST ignore and skip any options that 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 Sections 11.7.1 and 11.7.2.

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

                                           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                           |          Sequence #           |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |A|H|L|K|        Reserved       |           Lifetime            |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                                                               |
           .                                                               .
           .                        Mobility Options                       .
           .                                                               .
           |                                                               |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Acknowledge (A)

          The Acknowledge (A) bit is set by the sending mobile node to
          request a Binding Acknowledgement (Section 6.1.8) be returned upon
          receipt of the Binding Update.

       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.

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       Key Management Mobility Capability (K)

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

          This bit is valid only in Binding Updates sent to the home agent,
          and MUST be cleared in other Binding Updates.  Correspondent nodes
          MUST ignore this bit.

       Reserved

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

       Sequence #

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

       Lifetime

          16-bit unsigned integer.  The number of time units remaining
          before the binding MUST be considered expired.  A value of zero
          indicates that the Binding Cache entry for the mobile node MUST be
          deleted.  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.  This field
          contains zero or more TLV-encoded mobility options.  The encoding
          and format of defined options are described in Section 6.2.  The
          receiver MUST ignore and skip any options that it does not
          understand.

          The following options are valid in a Binding Update:

          *  Binding Authorization Data option (this option is mandatory in
             Binding Updates sent to a correspondent node)

          *  Nonce Indices option

          *  Alternate Care-of Address option

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       If no options are present in this message, 4 octets of padding are
       necessary and the Header Len field will be set to 1.

       The care-of address is specified either by the Source Address field
       in the IPv6 header or by the Alternate Care-of Address option, if
       present.  The care-of address MUST be a unicast routable address.
       IPv6 Source Address MUST be a topologically correct source address.
       Binding Updates for a care-of address that is not a unicast routable
       address MUST be silently discarded.

       The deletion of a binding MUST be indicated by setting the Lifetime
       field to 0.  In deletion, the generation of the binding management
       key depends exclusively on the home keygen token, as explained in
       Section 5.2.5.

       Correspondent nodes SHOULD NOT delete the Binding Cache entry before
       the lifetime expires, if any application hosted by the correspondent
       node is still likely to require communication with the mobile node.
       A Binding Cache entry that is de-allocated prematurely might cause
       subsequent packets to be dropped from the mobile node, if they
       contain the Home Address destination option.  This situation is
       recoverable, since a Binding Error message is sent to the mobile node
       (see Section 6.1.9); however, it causes unnecessary delay in the
       communications.

    6.1.8.  Binding Acknowledgement Message

       The Binding Acknowledgement is used to acknowledge receipt of a
       Binding Update (Section 6.1.7).  This packet is sent as described in
       Sections 9.5.4 and 10.3.1.

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

                                           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                           |    Status     |K|  Reserved   |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |           Sequence #          |           Lifetime            |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                                                               |
           .                                                               .
           .                        Mobility Options                       .
           .                                                               .
           |                                                               |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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       Status

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

               0  Binding Update accepted

               1  Accepted but prefix discovery necessary

             128  Reason unspecified

             129  Administratively prohibited

             130  Insufficient resources

             131  Home registration not supported

             132  Not home subnet

             133  Not home agent for this mobile node

             134  Duplicate Address Detection failed

             135  Sequence number out of window

             136  Expired home nonce index

             137  Expired care-of nonce index

             138  Expired nonces

             139  Registration type change disallowed

             174  Invalid Care-of Address

          Up-to-date values of the Status field are to be specified in the
          IANA registry of assigned numbers [30].

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       Key Management Mobility Capability (K)

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

          Correspondent nodes MUST set the K bit to 0.

       Reserved

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

       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.

          The value of this field is undefined if the Status field indicates
          that the Binding Update was rejected.

       Mobility Options

          Variable-length field of such length that the complete Mobility
          Header is an integer multiple of 8 octets long.  This field
          contains zero or more TLV-encoded mobility options.  The encoding
          and format of defined options are described in Section 6.2.  The
          receiver MUST ignore and skip any options that 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:

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          *  Binding Authorization Data option (this option is mandatory in
             Binding Acknowledgements sent by a correspondent node, except
             where otherwise noted in Section 9.5.4)

          *  Binding Refresh Advice option

       If no options are present in this message, 4 octets of padding are
       necessary and the Header Len field will be set to 1.

    6.1.9.  Binding Error Message

       The Binding Error (BE) message is used by the correspondent node to
       signal an error related to mobility, such as an inappropriate attempt
       to use the Home Address destination option without an existing
       binding; see Section 9.3.3 for details.

       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

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       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.

       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.  This field
          contains zero or more TLV-encoded mobility options.  The receiver
          MUST ignore and skip any options that 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 Binding Error message to be defined.  The encoding
          and format of defined options are described in Section 6.2.  This
          specification does not define any options valid for the Binding
          Error message.

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

    6.2.  Mobility Options

       Mobility messages can include zero or more mobility options.  This
       allows optional fields that may not be needed in every use of a
       particular Mobility Header, as well as future extensions to the
       format of the messages.  Such options are included in the Message
       Data field of the message itself, after the fixed portion of the
       message data specified in the message subsections of Section 6.1.

       The presence of such options will be indicated by the Header Len of
       the Mobility Header.  If included, the Binding Authorization Data
       option (Section 6.2.7) MUST be the last option and MUST NOT have
       trailing padding.  Otherwise, options can be placed in any order.

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    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 that are currently
       defined for use in the Mobility Header.

       Implementations MUST silently ignore any mobility options that they
       do not understand.

       Mobility options may have alignment requirements.  Following the
       convention in IPv6, these options are aligned in a packet so that
       multi-octet values within the Option Data field of each option fall
       on natural boundaries (i.e., fields of width n octets are placed at
       an integer multiple of n octets from the start of the header, for n =
       1, 2, 4, or 8) [6].

    6.2.2.  Pad1

       The Pad1 option does not have any alignment requirements.  Its format
       is as follows:

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            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

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       remaining time until the mobile node SHOULD send a new home
       registration to the home agent.  The Refresh Interval MUST be set to
       indicate a smaller time interval than the Lifetime value of the
       Binding Acknowledgement.

    6.2.5.  Alternate Care-of Address

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

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

       Normally, a Binding Update specifies the desired care-of address in
       the Source Address field of the IPv6 header.  However, this is not
       possible in some cases, such as when the mobile node wishes to
       indicate a care-of address that it cannot use as a topologically
       correct source address (Sections 6.1.7 and 11.7.2) or when the used
       security mechanism does not protect the IPv6 header (Section 11.7.1).

       The Alternate Care-of Address option is provided for these
       situations.  This option is valid only in Binding Update.  The
       Alternate Care-of Address field contains an address to use as the
       care-of address for the binding, rather than using the Source Address
       of the packet as the care-of address.

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    6.2.6.  Nonce Indices

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

            0                   1                   2                   3
            0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                           |   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 alignment
       requirements as such.  However, since this option must be the last
       mobility option, an implicit alignment requirement is 8n + 2.  The
       format of this option is as follows:

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

       The Binding Authorization Data option is valid in the Binding Update
       and Binding Acknowledgement.

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       The Option Length field contains the length of the authenticator in
       octets.

       The Authenticator field contains a cryptographic value that can be
       used to determine that the message in question comes from the right
       authority.  Rules for calculating this value depends on the used
       authorization procedure.

       For the return routability procedure, this option can appear in the
       Binding Update and Binding Acknowledgements.  Rules for calculating
       the Authenticator value are the following:

         Mobility Data = care-of address | correspondent | MH Data
         Authenticator = First (96, HMAC_SHA1 (Kbm, Mobility Data))

       Where | denotes concatenation.  "Care-of address" is the care-of
       address that will be registered for the mobile node if the Binding
       Update succeeds, or the home address of the mobile node if this
       option is used in de-registration.  Note also that this address might
       be different from the source address of the Binding Update message,
       if the Alternative Care-of Address mobility option is used, or when
       the lifetime of the binding is set to zero.

       The "correspondent" is the IPv6 address of the correspondent node.
       Note that, if the message is sent to a destination that is itself
       mobile, the "correspondent" address may not be the address found in
       the Destination Address field of the IPv6 header; instead, the home
       address from the type 2 Routing header should be used.

       "MH Data" is the content of the Mobility Header, excluding the
       Authenticator field itself.  The Authenticator value is calculated as
       if the Checksum field in the Mobility Header was zero.  The Checksum
       in the transmitted packet is still calculated in the usual manner,
       with the calculated Authenticator being a part of the packet
       protected by the Checksum.  Kbm is the binding management key, which
       is typically created using nonces provided by the correspondent node
       (see Section 9.4).  Note that while the contents of a potential Home
       Address destination option are not covered in this formula, the rules
       for the calculation of the Kbm do take the home address in account.
       This ensures that the MAC will be different for different home
       addresses.

       The first 96 bits from the MAC result are used as the Authenticator
       field.

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    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.

       The alignment requirement [6] 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 [6]; for the Home Address
       option, these three bits are set to 110.  This indicates the
       following processing requirements:

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       o  Any IPv6 node that does not recognize the Option Type must discard
          the packet, and if the packet's Destination Address was not a
          multicast address, return an ICMP Parameter Problem, Code 2,
          message to the packet's Source Address.  The Pointer field in the
          ICMP message SHOULD point at the Option Type field.  Otherwise,
          for multicast addresses, the ICMP message MUST NOT be sent.

       o  The data within the option cannot change en route to the packet's
          final destination.

       The Home Address option MUST be placed as follows:

       o  After the routing header, if that header is present

       o  Before the Fragment Header, if that header is present

       o  Before the AH Header or ESP Header, if either one of those headers
          is present

       For each IPv6 packet header, the Home Address option MUST NOT appear
       more than once.  However, an encapsulated packet [7] 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 that process this routing header MUST verify that the

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       address contained within is the node's own home address in order to
       prevent packets from being forwarded outside the node.  The IP
       address contained in the routing header, since it is the mobile
       node's home address, MUST be a unicast routable address.
       Furthermore, if the scope of the home address is smaller than the
       scope of the care-of address, the mobile node MUST discard the packet
       (see Section 4.6).

    6.4.1.  Format

       The type 2 routing header has the following format:

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

       Next Header

          8-bit selector.  Identifies the type of header immediately
          following the routing header.  Uses the same values as the IPv6
          Next Header field [6].

       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).

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       Reserved

          32-bit reserved field.  The value MUST be initialized to zero by
          the sender, and MUST be ignored by the receiver.

       Home Address

          The home address of the destination mobile node.

       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 [6].  The type 2 routing header defined
       for Mobile IPv6 follows the same ordering as other routing headers.
       If another routing header is present along with a type 2 routing
       header, 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 header construction process
       for the other routing header.

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

    6.5.  ICMP Home Agent Address Discovery Request Message

       The ICMP Home Agent Address Discovery Request message is used by a
       mobile node to initiate the dynamic home agent address discovery
       mechanism, as described in Section 11.4.1.  The mobile node sends the
       Home Agent Address Discovery Request message to the Mobile IPv6 Home-
       Agents anycast address [8] for its own home subnet prefix.  (Note
       that the currently defined anycast addresses may not work with all
       prefix lengths other than those defined in RFC 4291 [16] [37].)

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

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       Type

          144

       Code

          0

       Checksum

          The ICMP checksum [17].

       Identifier

          An identifier to aid in matching Home Agent Address Discovery
          Reply messages to this Home Agent Address Discovery Request
          message.

       Reserved

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

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

    6.6.  ICMP Home Agent Address Discovery Reply Message

       The ICMP Home Agent Address Discovery Reply message is used by a home
       agent to respond to a mobile node that uses the dynamic home agent
       address discovery mechanism, as described in Section 10.5.

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           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

          145

       Code

          0

       Checksum

          The ICMP checksum [17].

       Identifier

          The identifier from the invoking Home Agent Address Discovery
          Request message.

       Reserved

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

       Home Agent Addresses

          A list of addresses of home agents on the home link for the mobile
          node.  The number of addresses presented in the list is indicated
          by the remaining length of the IPv6 packet carrying the Home Agent
          Address Discovery Reply message.

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    6.7.  ICMP Mobile Prefix Solicitation Message Format

       The ICMP Mobile Prefix Solicitation message is sent by a mobile node
       to its home agent while it is away from home.  The purpose of the
       message is to solicit a Mobile Prefix Advertisement from the home
       agent, which will allow the mobile node to gather prefix information
       about its home network.  This information can be used to configure
       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 that the mobile node wishes to learn prefix
          information about.

       Hop Limit

          Set to an initial hop limit value, similarly to any other unicast
          packet sent by the mobile node.

       Destination Option:

          A Home Address destination option MUST be included.

       ESP header:

          IPsec headers MUST be supported and SHOULD be used as described in
          Section 5.4.

       ICMP Fields:

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       Type

          146

       Code

          0

       Checksum

          The ICMP checksum [17].

       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.

       The Mobile Prefix Solicitation messages may have options.  These
       options MUST use the option format defined in Neighbor Discovery (RFC
       4861 [18]).  This document does not define any option types for the
       Mobile Prefix Solicitation message, but future documents may define
       new options.  Home agents MUST silently ignore any options they do
       not recognize and continue processing the message.

    6.8.  ICMP Mobile Prefix Advertisement Message Format

       A home agent will send a Mobile Prefix Advertisement to a mobile node
       to distribute prefix information about the home link while the mobile
       node is traveling away from the home network.  This will occur in
       response to a Mobile Prefix Solicitation with an Advertisement, or by
       an unsolicited Advertisement sent according to the rules in
       Section 10.6.

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

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       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 MUST be supported and SHOULD be used as described in
          Section 5.4.

       ICMP Fields:

       Type

          147

       Code

          0

       Checksum

          The ICMP checksum [17].

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       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 [18]
          [19].

       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
          [18] [19].

       Reserved

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

       The Mobile Prefix Advertisement messages may have options.  These
       options MUST use the option format defined in Neighbor Discovery (RFC
       4861 [18]).  This document defines one option that may be carried in
       a Mobile Prefix Advertisement message, but future documents may
       define new options.  Mobile nodes MUST silently ignore any options
       they do not recognize and continue processing the message.

       Prefix Information

          Each message contains one or more Prefix Information options.
          Each option carries the prefix(es) that the mobile node should use
          to configure its home address(es).  Section 10.6 describes which
          prefixes should be advertised to the mobile node.

          The Prefix Information option is defined in Section 4.6.2 of
          Neighbor Discovery (RFC 4861 [18]), with modifications defined in
          Section 7.2 of this specification.  The home agent MUST use this
          modified Prefix Information option to send home network prefixes
          as defined in Section 10.6.1.

       If the Advertisement is sent in response to a Mobile Prefix
       Solicitation, the home agent MUST copy the Identifier value from that
       message into the Identifier field of the Advertisement.

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       The home agent MUST NOT send more than one Mobile Prefix
       Advertisement message per second to any mobile node.

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

    7.  Modifications to IPv6 Neighbor Discovery

    7.1.  Modified Router Advertisement Message Format

       Mobile IPv6 modifies the format of the Router Advertisement message
       [18] 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 [18]:

       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.

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    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 [18] 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 [18]:

       Router Address (R)

          1-bit router address flag.  When set, indicates that the Prefix
          field contains a complete IP address assigned to the sending
          router.  The indicated prefix is given by the first Prefix Length
          bits of the Prefix field.  The router IP address has the same
          scope and conforms to the same lifetime values as the advertised
          prefix.  This use of the Prefix field is compatible with its use
          in advertising the prefix itself, since Prefix Advertisement uses

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          only the leading bits.  Interpretation of this flag bit is thus
          independent of the processing required for the On-Link (L) and
          Autonomous Address-Configuration (A) flag bits.

       Reserved1

          Reduced from a 6-bit field to a 5-bit field to account for the
          addition of the above bit.

       In a Router Advertisement, a home agent MUST, and all other routers
       MAY, include at least one Prefix Information option with the Router
       Address (R) bit set.  Neighbor Discovery (RFC 4861 [18]) specifies
       that, when 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 Neighbor
       Discovery options.  Also, when sending unsolicited multicast Router
       Advertisements more frequently than the limit specified in RFC 4861,
       the sending router need not include all options in each of these
       Advertisements.  However, in both of these cases the router SHOULD
       include at least one Prefix Information option with the Router
       Address (R) bit set in each such advertisement, if this bit is set in
       some advertisement sent by the router.

       In addition, the following requirement can assist mobile nodes in
       movement detection.  Barring changes in the prefixes for the link,
       routers that send multiple Router Advertisements with the Router
       Address (R) bit set in some of the included Prefix Information
       options SHOULD provide at least one option and router address that
       stays the same in all of the Advertisements.

    7.3.  New Advertisement Interval Option Format

       Mobile IPv6 defines a new Advertisement Interval option, used in
       Router Advertisement messages to advertise the interval at which the
       sending router sends unsolicited multicast Router Advertisements.
       The format of the Advertisement Interval option is as follows:

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

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       Type

          7

       Length

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

       Reserved

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

       Advertisement Interval

          32-bit unsigned integer.  The maximum time, in milliseconds,
          between successive unsolicited Router Advertisement messages sent
          by this router on this network interface.  Using the conceptual
          router configuration variables defined by Neighbor Discovery [18],
          this field MUST be equal to the value MaxRtrAdvInterval, expressed
          in milliseconds.

       Routers MAY include this option in their Router Advertisements.  A
       mobile node receiving a Router Advertisement containing this option
       SHOULD utilize the specified Advertisement Interval for that router
       in its movement detection algorithm, as described in Section 11.5.1.

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

    7.4.  New Home Agent Information Option Format

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

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

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       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 Section 8.4.  In addition, the sending home agent MAY
          dynamically set the Home Agent Preference value, for example,
          basing it on the number of mobile nodes it is currently serving or
          on its remaining resources for serving additional mobile nodes;
          such dynamic settings are beyond the scope of this document.  Any
          such dynamic setting of the Home Agent Preference, however, MUST
          set the preference appropriately, relative to the default Home
          Agent Preference value of 0 that may be in use by some home agents
          on this link (i.e., a home agent not including a Home Agent
          Information option in its Router Advertisements will be considered
          to have a Home Agent Preference value of 0).

       Home Agent Lifetime

          16-bit unsigned integer.  The lifetime associated with the home
          agent in units of seconds.  The default value is the same as the
          Router Lifetime, as specified in the main body of the Router
          Advertisement.  The maximum value corresponds to 18.2 hours.  A

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          value of 0 MUST NOT be used.  The Home Agent Lifetime applies only
          to this router's usefulness as a home agent; it does not apply to
          information contained in other message fields or options.

       Home agents MAY include this option in their Router Advertisements.
       This option MUST NOT be included in a Router Advertisement in which
       the Home Agent (H) bit (see Section 7.1) is not set.  If this option
       is not included in a Router Advertisement in which the Home Agent (H)
       bit is set, the lifetime for this home agent MUST be considered to be
       the same as the Router Lifetime in the Router Advertisement.  If
       multiple Advertisements are being sent instead of a single larger
       unsolicited multicast Router 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.  Changes to Sending Router Advertisements

       The Neighbor Discovery protocol specification [18] 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.

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       One method that 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:

       o  MinRtrAdvInterval 0.03 seconds

       o  MaxRtrAdvInterval 0.07 seconds

       In the case where the minimum intervals and delays are used, the mean
       time between unsolicited multicast Router Advertisements is 50 ms.
       Use of these modified limits MUST be configurable (see also the
       configuration variable MinDelayBetweenRas in Section 13 that 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 Neighbor Discovery (RFC 4861 [18]).  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 4861 [18], if
       this overhead is likely to cause service degradation.

       Additionally, the possible low values of MaxRtrAdvInterval may cause
       some problems with movement detection in some mobile nodes.  To
       ensure that this is not a problem, Routers SHOULD add 20 ms to any
       Advertisement Intervals sent in RAs that 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.

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       Home agents MUST include the Source Link-Layer Address option in all
       Router Advertisements they send.  This simplifies the process of
       returning home, as discussed in Section 11.5.5.

       Note that according to Neighbor Discovery (RFC 4861 [18]),
       AdvDefaultLifetime is by default based on the value of
       MaxRtrAdvInterval.  AdvDefaultLifetime is used in the Router Lifetime
       field of Router Advertisements.  Given that this field is expressed
       in seconds, a small MaxRtrAdvInterval value can result in a zero
       value for this field.  To prevent this, routers SHOULD keep
       AdvDefaultLifetime in at least one second, even if the use of
       MaxRtrAdvInterval would result in a smaller value.

    8.  Requirements for Types of IPv6 Nodes

       Mobile IPv6 places some special requirements on the functions
       provided by different types of IPv6 nodes.  This section summarizes
       those requirements, identifying the functionality each requirement is
       intended to support.

       The requirements are set for the following groups of nodes:

       o  All IPv6 nodes.

       o  All IPv6 nodes with support for route optimization.

       o  All IPv6 routers.

       o  All Mobile IPv6 home agents.

       o  All Mobile IPv6 mobile nodes.

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

    8.1.  All IPv6 Nodes

       Any IPv6 node may at any time be a correspondent node of a mobile
       node, either sending a packet to a mobile node or receiving a packet
       from a mobile node.  There are no Mobile IPv6 specific MUST
       requirements for such nodes, and basic IPv6 techniques are
       sufficient.  If a mobile node attempts to set up route optimization
       with a node with only basic IPv6 support, an ICMP error will signal
       that the node does not support such optimizations (Section 11.3.5),
       and communications will flow through the home agent.

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       An IPv6 node MUST NOT support the Home Address destination option,
       type 2 routing header, or the Mobility Header unless it fully
       supports the requirements listed in the next sections for either
       route optimization, mobile node, or home agent functionality.

    8.2.  IPv6 Nodes with Support for Route Optimization

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

       o  Avoidance of congestion in the home network, and enabling the use
          of lower-performance home agent equipment even for supporting
          thousands of mobile nodes.

       o  Reduced network load across the entire Internet, as mobile devices
          begin to predominate.

       o  Reduction of jitter and latency for the communications.

       o  Greater likelihood of success for Quality of Service (QoS)
          signaling as tunneling is avoided and, again, fewer sources of
          congestion.

       o  Improved robustness against network partitions, congestion, and
          other problems, since fewer routing path segments are traversed.

       These effects combine to enable much better performance and
       robustness for communications between mobile nodes and IPv6
       correspondent nodes.  Route optimization introduces a small amount of
       additional state for the peers, some additional messaging, and up to
       1.5 round-trip delays before it can be turned on.  However, it is
       believed that the benefits far outweigh the costs in most cases.
       Section 11.3.1 discusses how mobile nodes may avoid route
       optimization for some of the remaining cases, such as very short-term
       communications.

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

       o  The node MUST be able to validate a Home Address option using an
          existing Binding Cache entry, as described in Section 9.3.1.

       o  The node MUST be able to insert a type 2 routing header into
          packets to be sent to a mobile node, as described in
          Section 9.3.2.

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       o  Unless the correspondent node is also acting as a mobile node, it
          MUST ignore type 2 routing headers and silently discard all
          packets that it has received with such headers.

       o  The node SHOULD be able to interpret ICMP messages as described in
          Section 9.3.4.

       o  The node MUST be able to send Binding Error messages as described
          in Section 9.3.3.

       o  The node MUST be able to process Mobility Headers as described in
          Section 9.2.

       o  The node MUST be able to participate in a return routability
          procedure (Section 9.4).

       o  The node MUST be able to process Binding Update messages
          (Section 9.5).

       o  The node MUST be able to return a Binding Acknowledgement
          (Section 9.5.4).

       o  The node MUST be able to maintain a Binding Cache of the bindings
          received in accepted Binding Updates, as described in Sections 9.1
          and 9.6.

       o  The node SHOULD allow route optimization to be administratively
          enabled or disabled.  The default SHOULD be enabled.

    8.3.  All IPv6 Routers

       All IPv6 routers, even those not serving as a home agent for Mobile
       IPv6, have an effect on how well mobile nodes can communicate:

       o  Every IPv6 router SHOULD be able to send an Advertisement Interval
          option (Section 7.3) in each of its Router Advertisements [18], to
          aid movement detection by mobile nodes (as in Section 11.5.1).
          The use of this option in Router Advertisements SHOULD be
          configurable.

       o  Every IPv6 router SHOULD be able to support sending unsolicited
          multicast Router Advertisements at the faster rate described in
          Section 7.5.  If the router supports a faster rate, the used rate
          MUST be configurable.

       o  Each router SHOULD include at least one prefix with the Router
          Address (R) bit set and with its full IP address in its Router
          Advertisements (as described in Section 7.2).

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       o  Routers supporting filtering packets with routing headers SHOULD
          support different rules for type 0 and type 2 routing headers (see
          Section 6.4) so that filtering of source routed packets (type 0)
          will not necessarily limit Mobile IPv6 traffic that is delivered
          via type 2 routing headers.

    8.4.  IPv6 Home Agents

       In order for a mobile node to operate correctly while away from home,
       at least one IPv6 router on the mobile node's home link must function
       as a home agent for the mobile node.  The following additional
       requirements apply to all IPv6 routers that serve as a home agent:

       o  Every home agent MUST be able to maintain an entry in its Binding
          Cache for each mobile node for which it is serving as the home
          agent (Sections 10.1 and 10.3.1).

       o  Every home agent MUST be able to intercept packets (using proxy
          Neighbor Discovery [18]) addressed to a mobile node for which it
          is currently serving as the home agent, on that mobile node's home
          link, while the mobile node is away from home (Section 10.4.1).

       o  Every home agent MUST be able to encapsulate [7] such intercepted
          packets in order to tunnel them to the primary care-of address for
          the mobile node indicated in its binding in the home agent's
          Binding Cache (Section 10.4.2).

       o  Every home agent MUST support decapsulating [7] reverse-tunneled
          packets sent to it from a mobile node's home address.  Every home
          agent MUST also check that the source address in the tunneled
          packets corresponds to the currently registered location of the
          mobile node (Section 10.4.5).

       o  The node MUST be able to process Mobility Headers as described in
          Section 10.2.

       o  Every home agent MUST be able to return a Binding Acknowledgement
          in response to a Binding Update (Section 10.3.1).

       o  Every home agent MUST maintain a separate Home Agents List for
          each link on which it is serving as a home agent, as described in
          Sections 10.1 and 10.5.1.

       o  Every home agent MUST be able to accept packets addressed to the
          Mobile IPv6 Home-Agents anycast address [8] for the subnet on
          which it is serving as a home agent, and MUST be able to
          participate in dynamic home agent address discovery
          (Section 10.5).

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       o  Every home agent SHOULD support a configuration mechanism to allow
          a system administrator to manually set the value to be sent by
          this home agent in the Home Agent Preference field of the Home
          Agent Information Option in Router Advertisements that it sends
          (Section 7.4).

       o  Every home agent SHOULD support sending ICMP Mobile Prefix
          Advertisements (Section 6.8), and SHOULD respond to Mobile Prefix
          Solicitations (Section 6.7).  If supported, this behavior MUST be
          configurable, so that home agents can be configured to avoid
          sending such Prefix Advertisements according to the needs of the
          network administration in the home domain.

       o  Every home agent MUST support IPsec ESP for protection of packets
          belonging to the return routability procedure (Section 10.4.6).

       o  Every home agent SHOULD support the multicast group membership
          control protocols as described in Section 10.4.3.  If this support
          is provided, the home agent MUST be capable of using it to
          determine which multicast data packets to forward via the tunnel
          to the mobile node.

       o  Home agents MAY support stateful address autoconfiguration for
          mobile nodes as described in Section 10.4.4.

    8.5.  IPv6 Mobile Nodes

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

       o  The node MUST maintain a Binding Update List (Section 11.1).

       o  The node MUST support sending packets containing a Home Address
          option (Section 11.3.1), and follow the required IPsec interaction
          (Section 11.3.2).

       o  The node MUST be able to perform IPv6 encapsulation and
          decapsulation [7].

       o  The node MUST be able to process type 2 routing header as defined
          in Sections 6.4 and 11.3.3.

       o  The node MUST support receiving a Binding Error message
          (Section 11.3.6).

       o  The node MUST support receiving ICMP errors (Section 11.3.5).

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       o  The node MUST support movement detection, care-of address
          formation, and returning home (Section 11.5).

       o  The node MUST be able to process Mobility Headers as described in
          Section 11.2.

       o  The node MUST support the return routability procedure
          (Section 11.6).

       o  The node MUST be able to send Binding Updates, as specified in
          Sections 11.7.1 and 11.7.2.

       o  The node MUST be able to receive and process Binding
          Acknowledgements, as specified in Section 11.7.3.

       o  The node MUST support receiving a Binding Refresh Request
          (Section 6.1.2), by responding with a Binding Update.

       o  The node MUST support receiving Mobile Prefix Advertisements
          (Section 11.4.3) and reconfiguring its home address based on the
          prefix information contained therein.

       o  The node SHOULD support use of the dynamic home agent address
          discovery mechanism, as described in Section 11.4.1.

       o  The node MUST allow route optimization to be administratively
          enabled or disabled.  The default SHOULD be enabled.

       o  The node MAY support the multicast address listener part of a
          multicast group membership protocol as described in
          Section 11.3.4.  If this support is provided, the mobile node MUST
          be able to receive tunneled multicast packets from the home agent.

       o  The node MAY support stateful address autoconfiguration mechanisms
          such as DHCPv6 [31] on the interface represented by the tunnel to
          the home agent.

    9.  Correspondent Node Operation

    9.1.  Conceptual Data Structures

       IPv6 nodes with route optimization support maintain a Binding Cache
       of bindings for other nodes.  A separate Binding Cache SHOULD be
       maintained by each IPv6 node for each of its unicast routable
       addresses.  The Binding Cache MAY be implemented in any manner
       consistent with the external behavior described in this document, for
       example, by being combined with the node's Destination Cache as

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       maintained by Neighbor Discovery [18].  When sending a packet, the
       Binding Cache is searched before the Neighbor Discovery conceptual
       Destination Cache [18].

       Each Binding Cache entry conceptually contains the following fields:

       o  The home address of the mobile node for which this is the Binding
          Cache entry.  This field is used as the key for searching the
          Binding Cache for the destination address of a packet being sent.

       o  The care-of address for the mobile node indicated by the home
          address field in this Binding Cache entry.

       o  A lifetime value, indicating the remaining lifetime for this
          Binding Cache entry.  The lifetime value is initialized from the
          Lifetime field in the Binding Update that created or last modified
          this Binding Cache entry.  A correspondent node MAY select a
          smaller lifetime for the Binding Cache entry, and supply that
          value to the mobile node in the Binding Acknowledgment message.

       o  A flag indicating whether or not this Binding Cache entry is a
          home registration entry (applicable only on nodes that support
          home agent functionality).

       o  The maximum value of the Sequence Number field received in
          previous Binding Updates for this home address.  The Sequence
          Number field is 16 bits long.  Sequence Number values MUST be
          compared modulo 2**16 as explained in Section 9.5.1.

       o  Usage information for this Binding Cache entry.  This is needed to
          implement the cache replacement policy in use in the Binding
          Cache.  Recent use of a cache entry also serves as an indication
          that a Binding Refresh Request should be sent when the lifetime of
          this entry nears expiration.

       Binding Cache entries not marked as home registrations MAY be
       replaced at any time by any reasonable local cache replacement policy
       but SHOULD NOT be unnecessarily deleted.  The Binding Cache for any
       one of a node's IPv6 addresses may contain at most one entry for each
       mobile node home address.  The contents of a node's Binding Cache
       MUST NOT be changed in response to a Home Address option in a
       received packet.

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    9.2.  Processing Mobility Headers

       Mobility Header processing MUST observe the following rules:

       o  The checksum must be verified as per Section 6.1.  If invalid, the
          node MUST silently discard the message.

       o  The MH Type field MUST have a known value (Section 6.1.1).
          Otherwise, the node MUST discard the message and issue a Binding
          Error message as described in Section 9.3.3, with the Status field
          set to 2 (unrecognized MH Type value).

       o  The Payload Proto field MUST be IPPROTO_NONE (59 decimal).
          Otherwise, the node MUST discard the message and SHOULD send ICMP
          Parameter Problem, Code 0, directly to the Source Address of the
          packet as specified in RFC 4443 [17].  Thus, no Binding Cache
          information is used in sending the ICMP message.  The Pointer
          field in the ICMP message SHOULD point at the Payload Proto field.

       o  The Header Len field in the Mobility Header MUST NOT be less than
          the length specified for this particular type of message in
          Section 6.1.  Otherwise, the node MUST discard the message and
          SHOULD send ICMP Parameter Problem, Code 0, directly to the Source
          Address of the packet as specified in RFC 4443 [17].  (The Binding
          Cache information is again not used.)  The Pointer field in the
          ICMP message SHOULD point at the Header Len field.

       Subsequent checks depend on the particular Mobility Header.

    9.3.  Packet Processing

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

    9.3.1.  Receiving Packets with Home Address Option

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

       Mobile nodes can include a Home Address destination option in a
       packet if they believe the correspondent node has a Binding Cache
       entry for the home address of a mobile node.  If the Next Header
       value of the Destination Option is one of the following: {50 (ESP),
       51 (AH), 135 (Mobility Header)}, the packet SHOULD be processed
       normally.  Otherwise, the packet MUST be dropped if there is no
       corresponding Binding Cache entry.  A corresponding Binding Cache

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       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.

       If the packet is dropped due to the above tests, the correspondent
       node MUST send the Binding Error message as described in
       Section 9.3.3.  The Status field in this message should be set to 1
       (unknown binding for Home Address destination option).

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

       The use of IPsec Authentication Header (AH) for the Home Address
       option is not required, except that if the IPv6 header of a packet is
       covered by AH, then the authentication MUST also cover the Home
       Address option; this coverage is achieved automatically by the
       definition of the Option Type code for the Home Address option, since
       it indicates that the data within the option cannot change en route
       to the packet's final destination, and thus the option is included in
       the AH computation.  By requiring that any authentication of the IPv6
       header also cover the Home Address option, the security of the Source
       Address field in the IPv6 header is not compromised by the presence
       of a Home Address option.

       When attempting to verify AH authentication data in a packet that
       contains a Home Address option, the receiving node MUST calculate the
       AH authentication data as if the following were true: the Home
       Address option contains the care-of address, and the source IPv6
       address field of the IPv6 header contains the home address.  This
       conforms with the calculation specified in Section 11.3.2.

    9.3.2.  Sending Packets to a Mobile Node

       Before sending any packet, the sending node SHOULD examine its
       Binding Cache for an entry for the destination address to which the
       packet is being sent.  If the sending node has a Binding Cache entry
       for this address, the sending node SHOULD use a type 2 routing header
       to route the packet to this mobile node (the destination node) by way
       of its care-of address.  However, the sending node MUST NOT do this
       in the following cases:

       o  When sending an IPv6 Neighbor Discovery [18] packet.

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       o  Where otherwise noted in Section 6.1.

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

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

       o  The Destination Address in the packet's IPv6 header is set to the
          mobile node's home address (the original destination address to
          which the packet was being sent).

       o  The routing header is initialized to contain a single route
          segment, containing the mobile node's care-of address copied from
          the Binding Cache entry.  The Segments Left field is, however,
          temporarily set to zero.

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

       Following the definition of a type 2 routing header in Section 6.4,
       this packet will be routed to the mobile node's care-of address,
       where it will be delivered to the mobile node (the mobile node has
       associated the care-of address with its network interface).

       Note that following the above conceptual model in an implementation
       creates some additional requirements for path MTU discovery since the
       layer that determines 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

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       node and processed normally by it.  If, however, the destination node
       is a mobile node that is currently away from home, the packet will be
       intercepted by the mobile node's home agent and tunneled to the
       mobile node's current primary care-of address.

    9.3.3.  Sending Binding Error Messages

       Sections 9.2 and 9.3.1 describe error conditions that lead to a need
       to send a Binding Error message.

       A Binding Error message is sent directly to the address that appeared
       in the IPv6 Source Address field of the offending packet.  If the
       Source Address field does not contain a unicast address, the Binding
       Error message MUST NOT be sent.

       The Home Address field in the Binding Error message MUST be copied
       from the Home Address field in the Home Address destination option of
       the offending packet, or set to the unspecified address if no such
       option appeared in the packet.

       Note that the IPv6 Source Address and Home Address field values
       discussed above are the values from the wire, i.e., before any
       modifications possibly performed as specified in Section 9.3.1.

       Binding Error messages SHOULD be subject to rate limiting in the same
       manner as is done for ICMPv6 messages [17].

    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 [7], 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

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       persistent ICMP Destination Unreachable messages after sending
       packets to a mobile node based on an entry in its Binding Cache, the
       correspondent node SHOULD delete this Binding Cache entry.  Note that
       if the mobile node continues to send packets with the Home Address
       destination option to this correspondent node, they will be dropped
       due to the lack of a binding.  For this reason it is important that
       only persistent ICMP messages lead to the deletion of the Binding
       Cache entry.

    9.4.  Return Routability Procedure

       This subsection specifies actions taken by a correspondent node
       during the return routability procedure.

    9.4.1.  Receiving Home Test Init Messages

       Upon receiving a Home Test Init message, the correspondent node
       verifies the following:

       o  The packet MUST NOT include a Home Address destination option.

       Any packet carrying a Home Test Init message that fails to satisfy
       this test MUST be silently ignored.

       Otherwise, in preparation for sending the corresponding Home Test
       Message, the correspondent node checks that it has the necessary
       material to engage in a return routability procedure, as specified in
       Section 5.2.  The correspondent node MUST have a secret Kcn and a
       nonce.  If it does not have this material yet, it MUST produce it
       before continuing with the return routability procedure.

       Section 9.4.3 specifies further processing.

    9.4.2.  Receiving Care-of Test Init Messages

       Upon receiving a Care-of Test Init message, the correspondent node
       verifies the following:

       o  The packet MUST NOT include a Home Address destination option.

       Any packet carrying a Care-of Test Init message that fails to satisfy
       this test MUST be silently ignored.

       Otherwise, in preparation for sending the corresponding Care-of Test
       Message, the correspondent node checks that it has the necessary
       material to engage in a return routability procedure in the manner
       described in Section 9.4.1.

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       Section 9.4.4 specifies further processing.

    9.4.3.  Sending Home Test Messages

       The correspondent node creates a home keygen token and uses the
       current nonce index as the Home Nonce Index.  It then creates a Home
       Test message (Section 6.1.5) and sends it to the mobile node at the
       latter's home address.

    9.4.4.  Sending Care-of Test Messages

       The correspondent node creates a care-of keygen token and uses the
       current nonce index as the Care-of Nonce Index.  It then creates a
       Care-of Test message (Section 6.1.6) and sends it to the mobile node
       at the latter's care-of address.

    9.5.  Processing Bindings

       This section explains how the correspondent node processes messages
       related to bindings.  These messages are:

       o  Binding Update

       o  Binding Refresh Request

       o  Binding Acknowledgement

       o  Binding Error

    9.5.1.  Receiving Binding Updates

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

       o  The packet MUST contain a unicast routable home address, either in
          the Home Address option or in the Source Address, if the Home
          Address option is not present.

       o  The Sequence Number field in the Binding Update is greater than
          the Sequence Number received in the previous valid Binding Update
          for this home address, if any.

          If the receiving node has no Binding Cache entry for the indicated
          home address, it MUST accept any Sequence Number value in a
          received Binding Update from this mobile node.

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          This Sequence Number comparison MUST be performed modulo 2**16,
          i.e., the number is a free running counter represented modulo
          65536.  A Sequence Number in a received Binding Update is
          considered less than or equal to the last received number if its
          value lies in the range of the last received number and the
          preceding 32768 values, inclusive.  For example, if the last
          received sequence number was 15, then messages with sequence
          numbers 0 through 15, as well as 32783 through 65535, would be
          considered less than or equal.

       When the Home Registration (H) bit is not set, the following are also
       required:

       o  A Nonce Indices mobility option MUST be present, and the Home and
          Care-of Nonce Index values in this option MUST be recent enough to
          be recognized by the correspondent node.  (Care-of Nonce Index
          values are not inspected for requests to delete a binding.)

       o  The correspondent node MUST re-generate the home keygen token and
          the care-of keygen token from the information contained in the
          packet.  It then generates the binding management key Kbm and uses
          it to verify the authenticator field in the Binding Update as
          specified in Section 6.1.7.

       o  The Binding Authorization Data mobility option MUST be present,
          and its contents MUST satisfy rules presented in Section 5.2.6.
          Note that a care-of address different from the Source Address MAY
          have been specified by including an Alternate Care-of Address
          mobility option in the Binding Update.  When such a message is
          received and the return routability procedure is used as an
          authorization method, the correspondent node MUST verify the
          authenticator by using the address within the Alternate Care-of
          Address in the calculations.

       o  The Binding Authorization Data mobility option MUST be the last
          option and MUST NOT have trailing padding.

       If the Home Registration (H) bit is set, the Nonce Indices mobility
       option MUST NOT be present.

       If the mobile node sends a sequence number that is not greater than
       the sequence number from the last valid Binding Update for this home
       address, then the receiving node MUST send back a Binding
       Acknowledgement with status code 135, and the last accepted sequence
       number in the Sequence Number field of the Binding Acknowledgement.

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       If a binding already exists for the given home address and the home
       registration flag has a different value than the Home Registration
       (H) bit in the Binding Update, then the receiving node MUST send back
       a Binding Acknowledgement with status code 139 (registration type
       change disallowed).  The home registration flag stored in the Binding
       Cache entry MUST NOT be changed.

       If the receiving node no longer recognizes the Home Nonce Index
       value, Care-of Nonce Index value, or both values from the Binding
       Update, then the receiving node MUST send back a Binding
       Acknowledgement with status code 136, 137, or 138, respectively.

       Packets carrying Binding Updates that fail to satisfy all of these
       tests for any reason other than insufficiency of the Sequence Number,
       registration type change, or expired nonce index values, MUST be
       silently discarded.

       If the Binding Update is valid according to the tests above, then the
       Binding Update is processed further as follows:

       o  The Sequence Number value received from a mobile node in a Binding
          Update is stored by the receiving node in its Binding Cache entry
          for the given home address.

       o  If the Lifetime specified in the Binding Update is not zero, then
          this is a request to cache a binding for the home address.  If the
          Home Registration (H) bit is set in the Binding Update, the
          Binding Update is processed according to the procedure specified
          in Section 10.3.1; otherwise, it is processed according to the
          procedure specified in Section 9.5.2.

       o  If the Lifetime specified in the Binding Update is zero, then this
          is a request to delete the cached binding for the home address.
          In this case, the Binding Update MUST include a valid home nonce
          index, and the care-of nonce index MUST be ignored by the
          correspondent node.  The generation of the binding management key
          depends then exclusively on the home keygen token (Section 5.2.5).
          If the Home Registration (H) bit is set in the Binding Update, the
          Binding Update is processed according to the procedure specified
          in Section 10.3.2; otherwise, it is processed according to the
          procedure specified in Section 9.5.3.

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

       o  If the Alternate Care-of Address option is present, the care-of
          address is the address in that option.

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       o  Otherwise, the care-of address is the Source Address field in the
          packet's IPv6 header.

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

       o  If the Home Address destination option is present, the home
          address is the address in that option.

       o  Otherwise, the home address is the Source Address field in the
          packet's IPv6 header.

    9.5.2.  Requests to Cache a Binding

       This section describes the processing of a valid Binding Update that
       requests a node to cache a binding, for which the Home Registration
       (H) bit is not set in the Binding Update.

       In this case, the receiving node SHOULD create a new entry in its
       Binding Cache for this home address, or update its existing Binding
       Cache entry for this home address, if such an entry already exists.
       The lifetime for the Binding Cache entry is initialized from the
       Lifetime field specified in the Binding Update, although this
       lifetime MAY be reduced by the node caching the binding; the lifetime
       for the Binding Cache entry MUST NOT be greater than the Lifetime
       value specified in the Binding Update.  Any Binding Cache entry MUST
       be deleted after the expiration of its lifetime.

       Note that if the mobile node did not request a Binding
       Acknowledgement, then it is not aware of the selected shorter
       lifetime.  The mobile node may thus use route optimization and send
       packets with the Home Address destination option.  As discussed in
       Section 9.3.1, such packets will be dropped if there is no binding.
       This situation is recoverable, but can cause temporary packet loss.

       The correspondent node MAY refuse to accept a new Binding Cache entry
       if it does not have sufficient resources.  A new entry MAY also be
       refused if the correspondent node believes its resources are utilized
       more efficiently in some other purpose, such as serving another
       mobile node with higher amount of traffic.  In both cases the
       correspondent node SHOULD return a Binding Acknowledgement with
       status value 130.

    9.5.3.  Requests to Delete a Binding

       This section describes the processing of a valid Binding Update that
       requests a node to delete a binding when the Home Registration (H)
       bit is not set in the Binding Update.

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       Any existing binding for the given home address MUST be deleted.  A
       Binding Cache entry for the home address MUST NOT be created in
       response to receiving the Binding Update.

       If the Binding Cache entry was created by use of return routability
       nonces, the correspondent node MUST ensure that the same nonces are
       not used again with the particular home and care-of address.  If both
       nonces are still valid, the correspondent node has to remember the
       particular combination of nonce indices, 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:

       o  If the Binding Update was discarded as described in Sections 9.2
          or 9.5.1, a Binding Acknowledgement MUST NOT be sent.  Otherwise,
          the treatment depends on the following rules.

       o  If the Acknowledge (A) bit is set in the Binding Update, a Binding
          Acknowledgement MUST be sent.  Otherwise, the treatment depends on
          the next rule.

       o  If the node rejects the Binding Update due to an expired nonce
          index, sequence number being out of window (Section 9.5.1), or
          insufficiency of resources (Section 9.5.2), a Binding
          Acknowledgement MUST be sent.  If the node accepts the Binding
          Update, the Binding Acknowledgement SHOULD NOT be sent.

       If the node accepts the Binding Update and creates or updates an
       entry for this binding, the Status field in the Binding
       Acknowledgement MUST be set to a value less than 128.  Otherwise, the
       Status field MUST be set to a value greater than or equal to 128.
       Values for the Status field are described in Section 6.1.8 and in the
       IANA registry of assigned numbers [30].

       If the Status field in the Binding Acknowledgement contains the value
       136 (expired home nonce index), 137 (expired care-of nonce index), or
       138 (expired nonces), then the message MUST NOT include the Binding
       Authorization Data mobility option.  Otherwise, the Binding
       Authorization Data mobility option MUST be included, and MUST meet
       the specific authentication requirements for Binding Acknowledgements
       as defined in Section 5.2.

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       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 that contains the mobile node's
       home address.

    9.5.5.  Sending Binding Refresh Requests

       If a Binding Cache entry being deleted is still in active use when
       sending packets to a mobile node, then the next packet sent to the
       mobile node will be routed normally to the mobile node's home link.
       Communication with the mobile node continues, but the tunneling from
       the home network creates additional overhead and latency in
       delivering packets to the mobile node.

       If the sender knows that the Binding Cache entry is still in active
       use, it MAY send a Binding Refresh Request message to the mobile node
       in an attempt to avoid this overhead and latency due to deleting and
       recreating the Binding Cache entry.  This message is always sent to
       the home address of the mobile node.

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

    9.6.  Cache Replacement Policy

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

       Each node's Binding Cache will, by necessity, have a finite size.  A
       node MAY use any reasonable local policy for managing the space
       within its Binding Cache.

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       A node MAY choose to drop any entry already in its Binding Cache in
       order to make space for a new entry.  For example, a "least-recently
       used" (LRU) strategy for cache entry replacement among entries should
       work well, unless the size of the Binding Cache is substantially
       insufficient.  When entries are deleted, the correspondent node MUST
       follow the rules in Section 5.2.8 in order to guard the return
       routability procedure against replay attacks.

       If the node sends a packet to a destination for which it has dropped
       the entry from its Binding Cache, the packet will be routed through
       the mobile node's home link.  The mobile node can detect this and
       establish a new binding if necessary.

       However, if the mobile node believes that the binding still exists,
       it may use route optimization and send packets with the Home Address
       destination option.  This can create temporary packet loss, as
       discussed earlier, in the context of binding lifetime reductions
       performed by the correspondent node (Section 9.5.2).

    10.  Home Agent Operation

    10.1.  Conceptual Data Structures

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

       The rules for maintaining a Binding Cache are the same for home
       agents and correspondent nodes and have already been described in
       Section 9.1.

       The Home Agents List is maintained by each home agent, recording
       information about each router on the same link that is acting as a
       home agent.  This list is used by the dynamic home agent address
       discovery mechanism.  A router is known to be acting as a home agent,
       if it sends a Router Advertisement in which the Home Agent (H) bit is
       set.  When the lifetime for a list entry (defined below) expires,
       that entry is removed from the Home Agents List.  The Home Agents
       List is similar to the Default Router List conceptual data structure
       maintained by each host for Neighbor Discovery [18].  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:

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       o  The link-local IP address of a home agent on the link.  This
          address is learned through the Source Address of the Router
          Advertisements [18] received from the router.

       o  One or more global IP addresses for this home agent.  Global
          addresses are learned through Prefix Information options with the
          Router Address (R) bit set and received in Router Advertisements
          from this link-local address.  Global addresses for the router in
          a Home Agents List entry MUST be deleted once the prefix
          associated with that address is no longer valid [18].

       o  The remaining lifetime of this Home Agents List entry.  If a Home
          Agent Information Option is present in a Router Advertisement
          received from a home agent, the lifetime of the Home Agents List
          entry representing that home agent is initialized from the Home
          Agent Lifetime field in the option (if present); otherwise, the
          lifetime is initialized from the Router Lifetime field in the
          received Router Advertisement.  If Home Agents List entry lifetime
          reaches zero, the entry MUST be deleted from the Home Agents List.

       o  The preference for this home agent; higher values indicate a more
          preferable home agent.  The preference value is taken from the
          Home Agent Preference field in the received Router Advertisement,
          if the Router Advertisement contains a Home Agent Information
          Option and is otherwise set to the default value of 0.  A home
          agent uses this preference in ordering the Home Agents List when
          it sends an ICMP Home Agent Address Discovery message.

    10.2.  Processing Mobility Headers

       All IPv6 home agents MUST observe the rules described in Section 9.2
       when processing Mobility Headers.

    10.3.  Processing Bindings

    10.3.1.  Primary Care-of Address Registration

       When a node receives a Binding Update, it MUST validate it and
       determine the type of Binding Update according to the steps described
       in Section 9.5.1.  Furthermore, it MUST authenticate the Binding
       Update as described in Section 5.1.  An authorization step specific
       for the home agent is also needed to ensure that only the right node
       can control a particular home address.  This is provided through the
       home address unequivocally identifying the security association that
       must be used.

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

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

       o  If the node implements only correspondent node functionality, or
          has not been configured to act as a home agent, then the node MUST
          reject the Binding Update.  The node MUST also return a Binding
          Acknowledgement to the mobile node, in which the Status field is
          set to 131 (home registration not supported).

       o  Else, if the home address for the binding (the Home Address field
          in the packet's Home Address option) is not an on-link IPv6
          address with respect to the home agent's current Prefix List, then
          the home agent MUST reject the Binding Update and SHOULD return a
          Binding Acknowledgement to the mobile node, in which the Status
          field is set to 132 (not home subnet).

       o  Else, if the home agent chooses to reject the Binding Update for
          any other reason (e.g., insufficient resources to serve another
          mobile node as a home agent), then the home agent SHOULD return a
          Binding Acknowledgement to the mobile node, in which the Status
          field is set to an appropriate value to indicate the reason for
          the rejection.

       o  A Home Address destination option MUST be present in the message.
          It MUST be validated as described in Section 9.3.1 with the
          following additional rule.  The Binding Cache entry existence test
          MUST NOT be done for IPsec packets when the Home Address option
          contains an address for which the receiving node could act as a
          home agent.

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

       The home agent MUST mark this Binding Cache entry as a home
       registration to indicate that the node is serving as a home agent for
       this binding.  Binding Cache entries marked as a home registration
       MUST be excluded from the normal cache replacement policy used for
       the Binding Cache (Section 9.6) and MUST NOT be removed from the
       Binding Cache until the expiration of the Lifetime period.

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       Unless this home agent already has a binding for the given home
       address, the home agent MUST perform Duplicate Address Detection [19]
       on the mobile node's home link before returning the Binding
       Acknowledgement.  This ensures that no other node on the home link
       was using the mobile node's home address when the Binding Update
       arrived.  If this Duplicate Address Detection fails for the given
       home address or an associated link local address, then the home agent
       MUST reject the complete Binding Update and MUST return a Binding
       Acknowledgement to the mobile node, in which the Status field is set
       to 134 (Duplicate Address Detection failed).  When the home agent
       sends a successful Binding Acknowledgement to the mobile node, the
       home agent assures to the mobile node that its address(es) will be
       kept unique by the home agent for as long as the lifetime was granted
       for the binding.

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

       o  L=0: Defend only the given address.  Do not derive a link-local
          address.

       o  L=1: Defend both the given non link-local unicast (home) address
          and the derived link-local.  The link-local address is derived by
          replacing the subnet prefix in the mobile node's home address with
          the link-local prefix.

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

       o  The lifetime for the Binding Cache entry MUST NOT be greater than
          the Lifetime value specified in the Binding Update.

       o  The lifetime for the Binding Cache entry MUST NOT be greater than
          the remaining valid lifetime for the subnet prefix in the mobile
          node's home address specified with the Binding Update.  The
          remaining valid lifetime for this prefix is determined by the home
          agent based on its own Prefix List entry [18].

          The remaining preferred lifetime SHOULD NOT have any impact on the
          lifetime for the Binding Cache entry.

          The home agent MUST remove a binding when the valid lifetime of
          the prefix associated with it expires.

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       o  The home agent MAY further decrease the specified lifetime for the
          binding, for example, based on a local policy.  The resulting
          lifetime is stored by the home agent in the Binding Cache entry,
          and this Binding Cache entry MUST be deleted by the home agent
          after the expiration of this lifetime.

       Regardless of the setting of the Acknowledge (A) bit in the Binding
       Update, the home agent MUST return a Binding Acknowledgement to the
       mobile node constructed as follows:

       o  The Status field MUST be set to a value indicating success.  The
          value 1 (accepted but prefix discovery necessary) MUST be used if
          the subnet prefix of the specified home address is deprecated, or
          becomes deprecated during the lifetime of the binding, or becomes
          invalid at the end of the lifetime.  The value 0 MUST be used
          otherwise.  For the purposes of comparing the binding and prefix
          lifetimes, the prefix lifetimes are first converted into units of
          four seconds by ignoring the two least significant bits.

       o  The Key Management Mobility Capability (K) bit is set if the
          following conditions are all fulfilled, and cleared otherwise:

          *  The Key Management Mobility Capability (K) bit was set in the
             Binding Update.

          *  The IPsec security associations between the mobile node and the
             home agent have been established dynamically.

          *  The home agent has the capability to update its endpoint in the
             used key management protocol to the new care-of address every
             time it moves.

          Depending on the final value of the bit in the Binding
          Acknowledgement, the home agent SHOULD perform the following
          actions:

          K = 0

             Discard key management connections, if any, to the old care-of
             address.  If the mobile node did not have a binding before
             sending this Binding Update, discard the connections to the
             home address.

          K = 1

             Move the peer endpoint of the key management protocol
             connection, if any, to the new care-of address.

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       o  The Sequence Number field MUST be copied from the Sequence Number
          given in the Binding Update.

       o  The Lifetime field MUST be set to the remaining lifetime for the
          binding as set by the home agent in its home registration Binding
          Cache entry for the mobile node, as described above.

       o  If the home agent stores the Binding Cache entry in nonvolatile
          storage, then the Binding Refresh Advice mobility option MUST be
          omitted.  Otherwise, the home agent MAY include this option to
          suggest that the mobile node refreshes its binding before the
          actual lifetime of the binding ends.

          If the Binding Refresh Advice mobility option is present, the
          Refresh Interval field in the option MUST be set to a value less
          than the Lifetime value being returned in the Binding
          Acknowledgement.  This indicates that the mobile node SHOULD
          attempt to refresh its home registration at the indicated shorter
          interval.  The home agent MUST still retain the registration for
          the Lifetime period, even if the mobile node does not refresh its
          registration within the Refresh period.

       The rules for selecting the Destination IP address (and possibly
       routing header construction) for the Binding Acknowledgement to the
       mobile node are the same as in Section 9.5.4.

       In addition, the home agent MUST follow the procedure defined in
       Section 10.4.1 to intercept packets on the mobile node's home link
       addressed to the mobile node, while the home agent is serving as the
       home agent for this mobile node.  The home agent MUST also be
       prepared to accept reverse-tunneled packets from the new care-of
       address of the mobile node, as described in Section 10.4.5.  Finally,
       the home agent MUST also propagate new home network prefixes, as
       described in Section 10.6.

    10.3.2.  Primary Care-of Address De-Registration

       A binding may need to be de-registered when the mobile node returns
       home or when the mobile node knows that it will not have any care-of
       addresses in the visited network.

       A Binding Update is validated and authorized in the manner described
       in the previous section; note that when the mobile node de-registers
       when it is at home, it MAY choose to omit the Home Address
       destination option, in which case the mobile node's home address is
       the source IP address of the de-registration Binding Update.  This

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       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:

       o  If the receiving node has no entry marked as a home registration
          in its Binding Cache for this mobile node, then this node MUST
          reject the Binding Update and SHOULD return a Binding
          Acknowledgement to the mobile node, in which the Status field is
          set to 133 (not home agent for this mobile node).

       If the home agent does not reject the Binding Update as described
       above, then the home agent MUST return a Binding Acknowledgement to
       the mobile node, constructed as follows:

       o  The Status field MUST be set to a value 0, indicating success.

       o  The Key Management Mobility Capability (K) bit is set or cleared
          and actions based on its value are performed as described in the
          previous section.  The mobile node's home address is used as its
          new care-of address for the purposes of moving the key management
          connection to a new endpoint.

       o  The Sequence Number field MUST be copied from the Sequence Number
          given in the Binding Update.

       o  The Lifetime field MUST be set to zero.

       o  The Binding Refresh Advice mobility option MUST be omitted.

       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, and
       the Binding Update came from a mobile node on the same 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).

       When a mobile node sends a Binding Update to refresh the binding from
       the visited link and soon after moves to the home link and sends a
       de-registration Binding Update, a race condition can happen if the
       first Binding Update gets delayed.  The delayed Binding Update can
       cause the home agent to create a new Binding Cache entry for a mobile

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       node that had just attached to the home link and successfully deleted
       the binding.  This would prevent the mobile node from using its home
       address from the home link.

       In order to prevent this, the home agent SHOULD NOT remove the
       Binding Cache entry immediately after receiving the de-registration
       Binding Update from the mobile node.  It SHOULD mark the Binding
       Cache entry as invalid, and MUST stop intercepting packets on the
       mobile node's home link that are addressed to the mobile node
       (Section 10.4.1).  The home agent should wait for
       MAX_DELETE_BCE_TIMEOUT (Section 12) seconds before removing the
       Binding Cache entry completely.  In the scenario described above, if
       the home agent receives the delayed Binding Update that the mobile
       node sent from the visited link, it would reject the message since
       the sequence number would be less than the last received de-
       registration Binding Update from the home link.  The home agent would
       then send a Binding Acknowledgment with status '135' (Sequence number
       out of window) to the care-of address on the visited link.  The
       mobile node can continue using the home address from the home link.

    10.4.  Packet Processing

    10.4.1.  Intercepting Packets for a Mobile Node

       While a node is serving as the home agent for a mobile node it MUST
       attempt to intercept packets on the mobile node's home link that are
       addressed to the mobile node.

       In order to do this, when a node begins serving as the home agent it
       MUST have performed Duplicate Address Detection (as specified in
       Section 10.3.1), and subsequently it MUST multicast onto the home
       link a Neighbor Advertisement message [18] on behalf of the mobile
       node.  For the home address specified in the Binding Update, the home
       agent sends a Neighbor Advertisement message [18] to the all-nodes
       multicast address on the home link to advertise the home agent's own
       link-layer address for this IP address on behalf of the mobile node.
       If the Link-Layer Address Compatibility (L) flag has been specified
       in the Binding Update, the home agent MUST do the same for the link-
       local address of the mobile node.

       All fields in each Neighbor Advertisement message SHOULD be set in
       the same way they would be set by the mobile node if it was sending
       this Neighbor Advertisement [18] while at home, with the following
       exceptions:

       o  The Target Address in the Neighbor Advertisement MUST be set to
          the specific IP address for the mobile node.

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       o  The Advertisement MUST include a Target Link-layer Address option
          specifying the home agent's link-layer address.

       o  The Router (R) bit in the Advertisement MUST be set to zero.

       o  The Solicited (S) flag in the Advertisement MUST NOT be set, since
          it was not solicited by any Neighbor Solicitation.

       o  The Override (O) flag in the Advertisement MUST be set, indicating
          that the Advertisement SHOULD override any existing Neighbor Cache
          entry at any node receiving it.

       o  The Source Address in the IPv6 header MUST be set to the home
          agent's IP address on the interface used to send the
          advertisement.

       Any node on the home link that receives one of the Neighbor
       Advertisement messages (described above) will update its Neighbor
       Cache to associate the mobile node's address with the home agent's
       link-layer address, causing it to transmit any future packets
       normally destined to the mobile node to the mobile node's home agent.
       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
       [18]) times to increase its reliability.  It is still possible that
       some nodes on the home link will not receive any of the Neighbor
       Advertisements, but these nodes will eventually be able to detect the
       link-layer address change for the mobile node's address through use
       of Neighbor Unreachability Detection [18].

       While a node is serving as a home agent for some mobile node, the
       home agent uses IPv6 Neighbor Discovery [18] to intercept unicast
       packets on the home link addressed to the mobile node.  In order to
       intercept packets in this way, the home agent MUST act as a proxy for
       this mobile node and reply to any received Neighbor Solicitations for
       it.  When a home agent receives a Neighbor Solicitation, it MUST
       check if the Target Address specified in the message matches the
       address of any mobile node for which it has a Binding Cache entry
       marked as a home registration.

       If such an entry exists in the home agent's Binding Cache, the home
       agent MUST reply to the Neighbor Solicitation with a Neighbor
       Advertisement giving the home agent's own link-layer address as the
       link-layer address for the specified Target Address.  In addition,
       the Router (R) bit in the Advertisement MUST be set to zero.  Acting

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       as a proxy in this way allows other nodes on the mobile node's home
       link to resolve the mobile node's address and for the home agent to
       defend these addresses on the home link for Duplicate Address
       Detection [18].

    10.4.2.  Processing Intercepted Packets

       For any packet sent to a mobile node from the mobile node's home
       agent (in which the home agent is the original sender of the packet),
       the home agent is operating as a correspondent node of the mobile
       node for this packet and the procedures described in Section 9.3.2
       apply.  The home agent then uses a routing header to route the packet
       to the mobile node by way of the primary care-of address in the home
       agent's Binding Cache.

       While the mobile node is away from home, the home agent intercepts
       any packets on the home link addressed to the mobile node's home
       address, as described in Section 10.4.1.  In order to forward each
       intercepted packet to the mobile node, the home agent MUST tunnel the
       packet to the mobile node using IPv6 encapsulation [7].  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 [7] will result in decapsulation and processing of the
       original packet by the mobile node.

       However, packets addressed to the mobile node's link-local address
       MUST NOT be tunneled to the mobile node.  Instead, these packets MUST
       be discarded and the home agent SHOULD return an ICMP Destination
       Unreachable, Code 3, message to the packet's Source Address (unless
       this Source Address is a multicast address).

       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 address.
       Multicast packets addressed to a multicast address with link-local
       scope [16], to which the mobile node is subscribed, MUST NOT be
       tunneled to the mobile node.  These packets SHOULD be silently
       discarded (after delivering to other local multicast recipients).
       Multicast packets addressed to a multicast address with a scope
       larger than link-local, but smaller than global (e.g., site-local and
       organization-local [16]), to which the mobile node is subscribed,

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       SHOULD NOT be tunneled to the mobile node.  Multicast packets
       addressed with a global scope, to which the mobile node has
       successfully subscribed, MUST be tunneled to the mobile node.

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

    10.4.3.  Multicast Membership Control

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

       In order to forward multicast data packets from the home network to
       all the proper mobile nodes, the home agent SHOULD be capable of
       receiving tunneled multicast group membership control information
       from the mobile node in order to determine which groups the mobile
       node has subscribed to.  These multicast group membership messages
       are Listener Report messages specified in Multicast Listener
       Discovery (MLD) [9] or in other protocols such as [41].

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

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

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

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       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 that 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 security association (SA)
       was used, or by other distinguishing means.

       This specification puts no requirement on how the functions in this
       section and the multicast forwarding in Section 10.4.2 are to be
       achieved.  At the time of this writing, it was thought that a full
       IPv6 multicast router function would be necessary on the home agent,
       but it may be possible to achieve the same effects through a "proxy
       MLD" application coupled with kernel multicast forwarding.  This may
       be the subject of future specifications.

    10.4.4.  Stateful Address Autoconfiguration

       This section describes how home agents support the use of stateful
       address autoconfiguration mechanisms such as DHCPv6 [31] from the
       mobile nodes.  If this support is not provided, then the M and O bits
       must remain cleared on the Mobile Prefix Advertisement Messages.  Any
       mobile node that sends DHCPv6 messages to the home agent without this
       support will not receive a response.

       If DHCPv6 is used, packets are sent with link-local source addresses
       either to a link-scope multicast address or a link-local address.
       Mobile nodes desiring to locate a DHCPv6 service may reverse tunnel
       standard DHCPv6 packets to the home agent.  Since these link-scope
       packets cannot be forwarded onto the home network, it is necessary
       for the home agent to implement either 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:

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       o  The tunneled traffic arrives to the home agent's address using
          IPv6 encapsulation [7].

       o  Depending on the security policies used by the home agent,
          reverse-tunneled packets MAY be discarded unless accompanied by a
          valid ESP header.  The support for authenticated reverse tunneling
          allows the home agent to protect the home network and
          correspondent nodes from malicious nodes masquerading as a mobile
          node.

       o  Otherwise, when a home agent decapsulates a tunneled packet from
          the mobile node, the home agent MUST verify that the Source
          Address in the tunnel IP header is the mobile node's primary
          care-of address.  Otherwise, any node in the Internet could send
          traffic through the home agent and escape ingress filtering
          limitations.  This simple check forces the attacker to know the
          current location of the real mobile node and be able to defeat
          ingress filtering.  This check is not necessary if the reverse-
          tunneled packet is protected by ESP in tunnel mode.

    10.4.6.  Protecting Return Routability Packets

       The return routability procedure, described in Section 5.2.5, assumes
       that the confidentiality of the Home Test Init and Home Test messages
       is protected as they are tunneled between the home agent and the
       mobile node.  Therefore, the home agent MUST support tunnel mode
       IPsec ESP for the protection of packets belonging to the return
       routability procedure.  Support for a non-null encryption transform
       and authentication algorithm MUST be available.  It is not necessary
       to distinguish between different kinds of packets during the return
       routability procedure.

       Security associations are needed to provide this protection.  When
       the care-of address for the mobile node changes as a result of an
       accepted Binding Update, special treatment is needed for the next
       packets sent using these security associations.  The home agent MUST
       set the new care-of address as the destination address of these
       packets, as if the outer header destination address in the security
       association had changed.

       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

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       the security policy database these messages are indistinguishable.
       When IPsec is used to protect return routability signaling or payload
       packets, this protection MUST only be applied to the return
       routability packets entering the IPv6 encapsulated tunnel interface
       between the mobile node and the home agent.  This can be achieved,
       for instance, by defining the security policy database entries
       specifically for the tunnel interface.  That is, the policy entries
       are not generally applied on all traffic on the physical interface(s)
       of the nodes, but rather only on traffic that enters the tunnel.
       This makes use of per-interface security policy database entries [3]
       specific to the tunnel interface (the node's attachment to the tunnel
       [6]).

    10.5.  Dynamic Home Agent Address Discovery

       This section describes an optional mechanism by which a home agent
       can help mobile nodes to discover the addresses of other home agents
       on the mobile node's home network.  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; the mobile node uses the list
       as described in Section 11.4.1.  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 [18].  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 [18], the home
       agent performs the following steps in addition to any steps already
       required of it by Neighbor Discovery:

       o  If the Home Agent (H) bit in the Router Advertisement is not set,
          delete the sending node's entry in the current Home Agents List
          (if one exists).  Skip all the following steps.

       o  Otherwise, extract the Source Address from the IP header of the
          Router Advertisement.  This is the link-local IP address on this
          link of the home agent sending this Advertisement [18].

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       o  Determine the preference for this home agent.  If the Router
          Advertisement contains a Home Agent Information Option, then the
          preference is taken from the Home Agent Preference field in the
          option; otherwise, the default preference of 0 MUST be used.

       o  Determine the lifetime for this home agent.  If the Router
          Advertisement contains a Home Agent Information Option, then the
          lifetime is taken from the Home Agent Lifetime field in the
          option; otherwise, the lifetime specified by the Router Lifetime
          field in the Router Advertisement SHOULD be used.

       o  If the link-local address of the home agent sending this
          Advertisement is already present in this home agent's Home Agents
          List and the received home agent lifetime value is zero,
          immediately delete this entry in the Home Agents List.

       o  Otherwise, if the link-local address of the home agent sending
          this Advertisement is already present in the receiving home
          agent's Home Agents List, reset its lifetime and preference to the
          values determined above.

       o  If the link-local address of the home agent sending this
          Advertisement is not already present in the Home Agents List
          maintained by the receiving home agent, and the lifetime for the
          sending home agent is non-zero, create a new entry in the list,
          and initialize its lifetime and preference to the values
          determined above.

       o  If the Home Agents List entry for the link-local address of the
          home agent sending this Advertisement was not deleted as described
          above, determine any global address(es) of the home agent based on
          each Prefix Information option received in this Advertisement in
          which the Router Address (R) bit is set (Section 7.2).  Add all
          such global addresses to the list of global addresses in this Home
          Agents List entry.

       A home agent SHOULD maintain an entry in its Home Agents List for
       each valid home agent address until that entry's lifetime expires,
       after which time the entry MUST be deleted.

       As described in Section 11.4.1, a mobile node attempts dynamic home
       agent address discovery by sending an ICMP Home Agent Address
       Discovery Request message to the Mobile IPv6 Home-Agents anycast
       address [8] for its home IP subnet prefix.  A home agent receiving a
       Home Agent Address Discovery Request message that serves this subnet
       SHOULD return an ICMP Home Agent Address Discovery Reply message to

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       the mobile node with the Source Address of the Reply packet set to
       one of the global unicast addresses of the home agent.  The Home
       Agent Addresses field in the Reply message is constructed as follows:

       o  The Home Agent Addresses field SHOULD contain all global IP
          addresses for each home agent currently listed in this home
          agent's own Home Agents List (Section 10.1).

       o  The IP addresses in the Home Agent Addresses field SHOULD be
          listed in order of decreasing preference values, based either on
          the respective advertised preference from a Home Agent Information
          option or on the default preference of 0 if no preference is
          advertised (or on the configured home agent preference for this
          home agent itself).

       o  Among home agents with equal preference, their IP addresses in the
          Home Agent Addresses field SHOULD be listed in an order randomized
          with respect to other home agents with equal preference every time
          a Home Agent Address Discovery Reply message is returned by this
          home agent.

       o  If more than one global IP address is associated with a home
          agent, these addresses SHOULD be listed in a randomized order.

       o  The home agent SHOULD reduce the number of home agent IP addresses
          so that the packet fits within the minimum IPv6 MTU [6].  The home
          agent addresses selected for inclusion in the packet SHOULD be
          those from the complete list with the highest preference.  This
          limitation avoids the danger of the Reply message packet being
          fragmented (or rejected by an intermediate router with an ICMP
          Packet Too Big message [17]).

    10.6.  Sending Prefix Information to the Mobile Node

    10.6.1.  List of Home Network Prefixes

       Mobile IPv6 arranges to propagate relevant prefix information to the
       mobile node when it is away from home, so that it may be used in
       mobile node home address configuration and in network renumbering.
       In this mechanism, mobile nodes away from home receive Mobile Prefix
       Advertisement messages.  These messages include Prefix Information
       Options for the prefixes configured on the home subnet interface(s)
       of the home agent.

       If there are multiple home agents, differences in the advertisements
       sent by different home agents can lead to an inability to use a
       particular home address when changing to another home agent.  In

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       order to ensure that the mobile nodes get the same information from
       different home agents, it is preferred that all of the home agents on
       the same link be configured in the same manner.

       To support this, the home agent monitors prefixes advertised by
       itself and other home agents on the home link.  In Neighbor Discovery
       (RFC 4861 [18]) it is acceptable for two routers to advertise
       different sets of prefixes on the same link.  For home agents, the
       differences should be detected for a given home address because the
       mobile node communicates only with one home agent at a time and the
       mobile node needs to know the full set of prefixes assigned to the
       home link.  All other comparisons of Router Advertisements are as
       specified in Section 6.2.7 of RFC 4861.

    10.6.2.  Scheduling Prefix Deliveries

       A home agent serving a mobile node will schedule the delivery of the
       new prefix information to that mobile node when any of the following
       conditions occur:

       MUST:

       o  The state of the flags changes for the prefix of the mobile node's
          registered home address.

       o  The valid or preferred lifetime is reconfigured or changes for any
          reason other than advancing real time.

       o  The mobile node requests the information with a Mobile Prefix
          Solicitation (see Section 11.4.2).

       SHOULD:

       o  A new prefix is added to the home subnet interface(s) of the home
          agent.

       MAY:

       o  The valid or preferred lifetime or the state of the flags changes
          for a prefix that is not used in any Binding Cache entry for this
          mobile node.

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

       o  If a mobile node sends a solicitation, answer right away.

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       o  If no Mobile Prefix Advertisement has been sent to the mobile node
          in the last MaxMobPfxAdvInterval seconds (see Section 13), then
          ensure that a transmission is scheduled.  The actual transmission
          time is randomized as described below.

       o  If a prefix matching the mobile node's home registration is added
          on the home subnet interface or if its information changes in any
          way that does not deprecate the mobile node's address, ensure that
          a transmission is scheduled.  The actual transmission time is
          randomized as described below.

       o  If a home registration expires, cancel any scheduled
          advertisements to the mobile node.

       The list of prefixes is sent in its entirety in all cases.

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

       Otherwise, the home agent computes a fresh value for RAND_ADV_DELAY
       that offsets from the current time for the scheduled transmission.
       First, calculate the maximum delay for the scheduled Advertisement:

         MaxScheduleDelay = min (MaxMobPfxAdvInterval, Preferred Lifetime),

       where MaxMobPfxAdvInterval is as defined in Section 12.  Then,
       compute the final delay for the advertisement:

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

       Here rand() returns a random integer value in the range of 0 to the
       maximum possible integer value.  This computation is expected to
       alleviate bursts of advertisements when prefix information changes.
       In addition, a home agent MAY further reduce the rate of packet
       transmission by further delaying individual advertisements, when
       necessary to avoid overwhelming local network resources.  The home
       agent SHOULD periodically continue to retransmit an unsolicited
       Advertisement to the mobile node, until it is acknowledged by the
       receipt of a Mobile Prefix Solicitation from the mobile node.

       The home agent MUST wait PREFIX_ADV_TIMEOUT (see Section 12) before
       the first retransmission and double the retransmission wait time for
       every succeeding retransmission until a maximum number of

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       PREFIX_ADV_RETRIES attempts (see Section 12) has been tried.  If the
       mobile node's bindings expire before the matching Binding Update has
       been received, then the home agent MUST NOT attempt any more
       retransmissions, even if not all PREFIX_ADV_RETRIES have been
       retransmitted.  In the meantime, if the mobile node sends another
       Binding Update without returning home, then the home agent SHOULD
       begin transmitting the unsolicited Advertisement again.

       If some condition, as described above, occurs on the home link and
       causes another Prefix Advertisement to be sent to the mobile node,
       before the mobile node acknowledges a previous transmission, the home
       agent SHOULD combine any Prefix Information options in the
       unacknowledged Mobile Prefix Advertisement into a new Advertisement.
       The home agent then discards the old Advertisement.

    10.6.3.  Sending Advertisements

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

       o  The Source Address in the packet's IPv6 header MUST be set to the
          home agent's IP address to which the mobile node addressed its
          current home registration or its default global home agent address
          if no binding exists.

       o  If the advertisement was solicited, it MUST be destined to the
          source address of the solicitation.  If it was triggered by prefix
          changes or renumbering, the advertisement's destination will be
          the mobile node's home address in the binding that triggered the
          rule.

       o  A type 2 routing header MUST be included with the mobile node's
          home address.

       o  IPsec headers MUST be supported and SHOULD be used.

       o  The home agent MUST send the packet as it would any other unicast
          IPv6 packet that it originates.

       o  Set the Managed Address Configuration (M) flag if the
          corresponding flag has been set in any of the Router
          Advertisements from which the prefix information has been learned
          (including the ones sent by this home agent).

       o  Set the Other Stateful Configuration (O) flag if the corresponding
          flag has been set in any of the Router Advertisements from which
          the prefix information has been learned (including the ones sent
          by this home agent).

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    10.6.4.  Lifetimes for Changed Prefixes

       As described in Section 10.3.1, the lifetime returned by the home
       agent in a Binding Acknowledgement MUST NOT be greater than the
       remaining valid lifetime for the subnet prefix in the mobile node's
       home address.  This limit on the binding lifetime serves to prohibit
       use of a mobile node's home address after it becomes invalid.

    11.  Mobile Node Operation

    11.1.  Conceptual Data Structures

       Each mobile node MUST maintain a Binding Update List.

       The Binding Update List records information for each Binding Update
       sent by this mobile node, in which the lifetime of the binding has
       not yet expired.  The Binding Update List includes all bindings sent
       by the mobile node to either its home agent or correspondent nodes.
       It also contains Binding Updates that 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:

       o  The IP address of the node to which a Binding Update was sent.

       o  The home address for which that Binding Update was sent.

       o  The care-of address sent in that Binding Update.  This value is
          necessary for the mobile node to determine if it has sent a
          Binding Update while giving its new care-of address to this
          destination after changing its care-of address.

       o  The initial value of the Lifetime field sent in that Binding
          Update.

       o  The remaining lifetime of that binding.  This lifetime is
          initialized from the Lifetime value sent in the Binding Update and
          is decremented until it reaches zero, at which time this entry
          MUST be deleted from the Binding Update List.

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       o  The maximum value of the Sequence Number field sent in previous
          Binding Updates to this destination.  The Sequence Number field is
          16 bits long and all comparisons between Sequence Number values
          MUST be performed modulo 2**16 (see Section 9.5.1).

       o  The time at which a Binding Update was last sent to this
          destination, as needed to implement the rate limiting restriction
          for sending Binding Updates.

       o  The state of any retransmissions needed for this Binding Update.
          This state includes the time remaining until the next
          retransmission attempt for the Binding Update and the current
          state of the exponential back-off mechanism for retransmissions.

       o  A flag specifying whether or not future Binding Updates should be
          sent to this destination.  The mobile node sets this flag in the
          Binding Update List entry when it receives an ICMP Parameter
          Problem, Code 1, error message in response to a return routability
          message or Binding Update sent to that destination, as described
          in Section 11.3.5.

       The Binding Update List is used to determine whether a particular
       packet is sent directly to the correspondent node or tunneled via the
       home agent (see Section 11.3.1).

       The Binding Update list also conceptually contains the following data
       related to running the return routability procedure.  This data is
       relevant only for Binding Updates sent to correspondent nodes.

       o  The time at which a Home Test Init or Care-of Test Init message
          was last sent to this destination, as needed to implement the rate
          limiting restriction for the return routability procedure.

       o  The state of any retransmissions needed for this return
          routability procedure.  This state includes the time remaining
          until the next retransmission attempt and the current state of the
          exponential back-off mechanism for retransmissions.

       o  Cookie values used in the Home Test Init and Care-of Test Init
          messages.

       o  Home and care-of keygen tokens received from the correspondent
          node.

       o  Home and care-of nonce indices received from the correspondent
          node.

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       o  The time at which each of the tokens and nonces were received from
          the correspondent node, as needed to implement reuse while moving.

    11.2.  Processing Mobility Headers

       All IPv6 mobile nodes MUST observe the rules described in Section 9.2
       when processing Mobility Headers.

    11.3.  Packet Processing

    11.3.1.  Sending Packets While Away from Home

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

       o  Protocols layered over IP will generally treat the mobile node's
          home address as its IP source address for most packets.  For
          packets sent that are part of transport-level connections
          established while the mobile node was at home, the mobile node
          MUST use its home address.  Likewise, for packets sent that are
          part of transport-level connections that the mobile node may still
          be using after moving to a new location, the mobile node SHOULD
          use its home address in this way.  If a binding exists, the mobile
          node SHOULD send the packets directly to the correspondent node.
          Otherwise, if a binding does not exist, the mobile node MUST use
          reverse tunneling.

       o  The mobile node MAY choose to directly use one of its care-of
          addresses as the source of the packet, not requiring the use of a
          Home Address option in the packet.  This is particularly useful
          for short-term communication that may easily be retried if it
          fails.  Using the mobile node's care-of address as the source for
          such queries will generally have a lower overhead than using the
          mobile node's home address, since no extra options need to be used
          in either the query or its reply.  Such packets can be routed
          normally, directly between their source and destination without
          relying on Mobile IPv6.  If application running on the mobile node
          has no particular knowledge that the communication being sent fits
          within this general type of communication, however, the mobile
          node should not use its care-of address as the source of the
          packet in this way.

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          The choice of the most efficient communications method is
          application specific, and outside the scope of this specification.
          The APIs necessary for controlling the choice are also out of
          scope.  One example of such an API is described in the IPv6 Socket
          API for Source Address Selection specification [44].

       o  While not at its home link, the mobile node MUST NOT use the Home
          Address destination option when communicating with link-local
          peers.

          Similarly, the mobile node MUST NOT use the Home Address
          destination option for IPv6 Neighbor Discovery [18] packets.

       Detailed operation of these cases is described later in this section
       and also discussed in [33].

       For packets sent by a mobile node while it is at home, no special
       Mobile IPv6 processing is required.  Likewise, if the mobile node
       uses any address other than one of its home addresses as the source
       of a packet sent while away from home, no special Mobile IPv6
       processing is required.  In either case, the packet is simply
       addressed and transmitted in the same way as any normal IPv6 packet.

       For packets sent by the mobile node sent while away from home using
       the mobile node's home address as the source, special Mobile IPv6
       processing of the packet is required.  This can be done in the
       following two ways:

       Route Optimization

          This manner of delivering packets does not require going through
          the home network, and typically will enable faster and more
          reliable transmission.

          The mobile node needs to ensure that a Binding Cache entry exists
          for its home address so that the correspondent node can process
          the packet (Section 9.3.1 specifies the rules for Home Address
          Destination Option Processing at a correspondent node).  The
          mobile node SHOULD examine its Binding Update List for an entry
          that fulfills the following conditions:

          *  The Source Address field of the packet being sent is equal to
             the home address in the entry.

          *  The Destination Address field of the packet being sent is equal
             to the address of the correspondent node in the entry.

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          *  One of the current care-of addresses of the mobile node appears
             as the care-of address in the entry.

          *  The entry indicates that a binding has been successfully
             created.

          *  The remaining lifetime of the binding is greater than zero.

          When these conditions are met, the mobile node knows that the
          correspondent node has a suitable Binding Cache entry.

          A mobile node SHOULD arrange to supply the home address in a Home
          Address option, and MUST set the IPv6 header's Source Address
          field to the care-of address that the mobile node has registered
          to be used with this correspondent node.  The correspondent node
          will then use the address supplied in the Home Address option to
          serve the function traditionally done by the Source IP address in
          the IPv6 header.  The mobile node's home address is then supplied
          to higher protocol layers and applications.

          Specifically:

          *  Construct the packet using the mobile node's home address as
             the packet's Source Address, in the same way as if the mobile
             node were at home.  This includes the calculation of upper-
             layer checksums using the home address as the value of the
             source.

          *  Insert a Home Address option into the packet with the Home
             Address field copied from the original value of the Source
             Address field in the packet.

          *  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].

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       Reverse Tunneling

          This is the mechanism that tunnels the packets via the home agent.
          It is not as efficient as the above mechanism, but is needed if
          there is no binding yet with the correspondent node.

          This mechanism is used for packets that have the mobile node's
          home address as the Source Address in the IPv6 header, or with
          multicast control protocol packets as described in Section 11.3.4.
          Specifically:

          *  The packet is sent to the home agent using IPv6 encapsulation
             [7].

          *  The Source Address in the tunnel packet is the primary care-of
             address as registered with the home agent.

          *  The Destination Address in the tunnel packet is the home
             agent's address.

          Then, the home agent will pass the encapsulated packet to the
          correspondent node.

    11.3.2.  Interaction with Outbound IPsec Processing

       This section sketches the interaction between outbound Mobile IPv6
       processing and outbound IP Security (IPsec) processing for packets
       sent by a mobile node while away from home.  Any specific
       implementation MAY use algorithms and data structures other than
       those suggested here, but its processing MUST be consistent with the
       effect of the operation described here and with the relevant IPsec
       specifications.  In the steps described below, it is assumed that
       IPsec is being used in transport mode [3] and that the mobile node is
       using its home address as the source for the packet (from the point
       of view of higher protocol layers or applications, as described in
       Section 11.3.1):

       o  The packet is created by higher-layer protocols and applications
          (e.g., by TCP) as if the mobile node were at home and Mobile IPv6
          were not being used.

       o  Determine the outgoing interface for the packet.  (Note that the
          selection between reverse tunneling and route optimization may
          imply different interfaces, particularly if tunnels are considered
          interfaces as well.)

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       o  As part of outbound packet processing in IP, the packet is
          compared against the IPsec security policy database to determine
          what processing is required for the packet [3].

       o  If IPsec processing is required, the packet is either mapped to an
          existing security association (or SA bundle), or a new SA (or SA
          bundle) is created for the packet, according to the procedures
          defined for IPsec.

       o  Since the mobile node is away from home, the mobile is using
          either reverse tunneling or route optimization to reach the
          correspondent node.

          If reverse tunneling is used, the packet is constructed in the
          normal manner and then tunneled through the home agent.

          If route optimization is in use, the mobile node inserts a Home
          Address destination option into the packet, replacing the Source
          Address in the packet's IP header with the care-of address used
          with this correspondent node, as described in Section 11.3.1.  The
          Destination Options header in which the Home Address destination
          option is inserted MUST appear in the packet after the routing
          header, if present, and before the IPsec (AH [4] or ESP [5])
          header, so that the Home Address destination option is processed
          by the destination node before the IPsec header is processed.

          Finally, once the packet is fully assembled, the necessary IPsec
          authentication (and encryption, if required) processing is
          performed on the packet, initializing the Authentication Data in
          the IPsec header.

          The treatment of destination options described in RFC 4302 is
          extended as follows.  The AH authentication data MUST be
          calculated as if the following were true:

          *  the IPv6 source address in the IPv6 header contains the mobile
             node's home address, and

          *  the Home Address field of the Home Address destination option
             (Section 6.3) contains the new care-of address.

       o  This allows, but does not require, the receiver of the packet
          containing a Home Address destination option to exchange the two
          fields of the incoming packet to reach the above situation,
          simplifying processing for all subsequent packet headers.
          However, such an exchange is not required, as long as the result
          of the authentication calculation remains the same.

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       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 IKEv2 [24] as the
       automated key management protocol, such problems can be avoided by
       the following requirements when communicating with its home agent:

       o  When the mobile node is away from home, it MUST use its care-of
          address as the Source Address of all packets it sends as part of
          the key management protocol (without use of Mobile IPv6 for these
          packets, as suggested in Section 11.3.1).

       The Key Management Mobility Capability (K) bit in Binding Updates and
       Acknowledgements can be used to avoid the need to rerun IKEv2 upon
       movements.

    11.3.3.  Receiving Packets While Away from Home

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

       o  Packets sent by a correspondent node that does not have a Binding
          Cache entry for the mobile node will be sent to the home address,
          captured by the home agent and tunneled to the mobile node.

       o  Packets sent by a correspondent node that has a Binding Cache
          entry for the mobile node that contains the mobile node's current
          care-of address will be sent by the correspondent node using a
          type 2 routing header.  The packet will be addressed to the mobile
          node's care-of address, with the final hop in the routing header
          directing the packet to the mobile node's home address; the
          processing of this last hop of the routing header is entirely
          internal to the mobile node, since the care-of address and home
          address are both addresses within the mobile node.

       For packets received by the first method, the mobile node MUST check
       that the IPv6 source address of the tunneled packet is the IP address
       of its home agent.  In this method, the mobile node may also send a
       Binding Update to the original sender of the packet as described in
       Section 11.7.2 and subject to the rate limiting defined in
       Section 11.8.  The mobile node MUST also process the received packet
       in the manner defined for IPv6 encapsulation [7], which will result

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       in the encapsulated (inner) packet being processed normally by upper-
       layer protocols within the mobile node as if it had been addressed
       (only) to the mobile node's home address.

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

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

       o  The length field in the routing header is exactly 2.

       o  The segments left field in the routing header is 1 on the wire.
          (But implementations may process the routing header so that the
          value may become 0 after the routing header has been processed,
          but before the rest of the packet is processed.)

       o  The Home Address field in the routing header is one of the node's
          home addresses, if the segments left field was 1.  Thus, in
          particular the address field is required to be a unicast routable
          address.

       Once the above checks have been performed, the node swaps the IPv6
       destination field with the Home Address field in the routing header,
       decrements segments left by one from the value it had on the wire,
       and resubmits the packet to IP for processing the next header.
       Conceptually, this follows the same model as in RFC 2460.  However,
       in the case of the type 2 routing header, this can be simplified
       since it is known that the packet will not be forwarded to a
       different node.

       The definition of AH requires the sender to calculate the AH
       integrity check value of a routing header in the same way it appears
       in the receiver after it has processed the header.  Since IPsec
       headers follow the routing header, any IPsec processing will operate
       on the packet with the home address in the IP destination field and
       segments left being zero.  Thus, the AH calculations at the sender
       and receiver will have an identical view of the packet.

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    11.3.4.  Routing Multicast Packets

       A mobile node that is connected to its home link functions in the
       same way as any other (stationary) node.  Thus, when it is at home, a
       mobile node functions identically to other multicast senders and
       receivers.  Therefore, this section describes the behavior of a
       mobile node that is not on its home link.

       In order to receive packets sent to some multicast group, a mobile
       node must join that multicast group.  One method, in which a mobile
       node MAY join the group, is via a (local) multicast router on the
       foreign link being visited.  In this case, the mobile node MUST use
       its care-of address and MUST NOT use the Home Address destination
       option when sending MLD packets [9].

       Alternatively, a mobile node MAY join multicast groups via a
       bidirectional tunnel to its home agent.  The mobile node tunnels its
       multicast group membership control packets (such as those defined in
       [9] or in [41]) to its home agent, and the home agent forwards
       multicast packets down the tunnel to the mobile node.  A mobile node
       MUST NOT tunnel multicast group membership control packets until (1)
       the mobile node has a binding in place at the home agent, and (2) the
       latter sends at least one multicast group membership control packet
       via the tunnel.  Once this condition is true, the mobile node SHOULD
       assume it does not change as long as the binding does not expire.

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

       1.  Send directly on the foreign link being visited.

           To do this, the application uses the care-of address as a source
           address for multicast traffic, just as it would use a stationary
           address.  This requires that the application either knows the
           care-of address, or uses an API such as the IPv6 Socket API for
           Source Address Selection specification [44] to request that the
           care-of address be used as the source address in transmitted
           packets.  The mobile node MUST NOT use the Home Address
           destination option in such traffic.

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       2.  Send via a tunnel to its home agent.

           Because multicast routing in general depends upon the Source
           Address used in the IPv6 header of the multicast packet, a mobile
           node that tunnels a multicast packet to its home agent MUST use
           its home address as the IPv6 Source Address of the inner
           multicast packet.

       Note that direct sending from the foreign link is only applicable
       while the mobile node is at that foreign link.  This is because the
       associated multicast tree is specific to that source location and any
       change of location and source address will invalidate the source-
       specific tree or branch and the application context of the other
       multicast group members.

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

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

    11.3.5.  Receiving ICMP Error Messages

       Any node that does not recognize the Mobility header will return an
       ICMP Parameter Problem, Code 1, message to the sender of the packet.
       If the mobile node receives such an ICMP error message in response to
       a return routability procedure or Binding Update, it SHOULD record in
       its Binding Update List that future Binding Updates SHOULD NOT be
       sent to this destination.  Such Binding Update List entries SHOULD be
       removed after a period of time in order to allow for retrying route
       optimization.

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

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       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, e.g., on
       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 three actions:

       o  If the Binding Error Message was sent by the home agent, the
          mobile node SHOULD send a Binding Update to the home agent
          according to Section 11.7.1.

       o  If the mobile node has recent upper-layer progress information,
          which indicates that communications with the correspondent node
          are progressing, it MAY ignore the message.  This can be done in
          order to limit the damage that spoofed Binding Error messages can
          cause to ongoing communications.

       o  If the mobile node has no upper-layer progress information, it
          MUST remove the entry and route further communications through the
          home agent.  It MAY also optionally start a return routability
          procedure (see Section 5.2).

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

       o  If the mobile node is not expecting an acknowledgement or response
          from the correspondent node, the mobile node SHOULD ignore this
          message.

       o  Otherwise, the mobile node SHOULD cease the use of any extensions
          to this specification.  If no extensions had been used, the mobile
          node should cease the attempt to use route optimization.

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    11.4.  Home Agent and Prefix Management

    11.4.1.  Dynamic Home Agent Address Discovery

       Sometimes when the mobile node needs to send a Binding Update to its
       home agent to register its new primary care-of address, as described
       in Section 11.7.1, the mobile node may not know the address of any
       router on its home link that can serve as a home agent for it.  For
       example, some nodes on its home link may have been reconfigured while
       the mobile node has been away from home, such that the router that
       was operating as the mobile node's home agent has been replaced by a
       different router serving this role.

       In this case, the mobile node MAY attempt to discover the address of
       a suitable home agent on its home link.  To do so, the mobile node
       sends an ICMP Home Agent Address Discovery Request message to the
       Mobile IPv6 Home-Agents anycast address [8] for its home subnet
       prefix.  As described in Section 10.5, the home agent on its home
       link that receives this Request message will return an ICMP Home
       Agent Address Discovery Reply message.  This message gives the
       addresses for the home agents operating on the home link.

       The mobile node, upon receiving this Home Agent Address Discovery
       Reply message, MAY then send its home registration Binding Update to
       any of the unicast IP addresses listed in the Home Agent Addresses
       field in the Reply.  For example, the mobile node MAY attempt its
       home registration to each of these addresses, in turn, until its
       registration is accepted.  The mobile node sends a Binding Update to
       an address and waits for the matching Binding Acknowledgement, moving
       on to the next address if there is no response.  The mobile node
       MUST, however, wait at least InitialBindackTimeoutFirstReg seconds
       (see Section 13) before sending a Binding Update to the next home
       agent.  In trying each of the returned home agent addresses, the
       mobile node SHOULD try each of them in the order they appear in the
       Home Agent Addresses field in the received Home Agent Address
       Discovery Reply message.  In order to do this, the mobile node SHOULD
       store the list of home agents for later use in case the home agent
       currently managing the mobile node's care-of address forwarding
       should become unavailable.  The list MAY be stored, along with any
       available lifetime information for the home agent addresses, in
       nonvolatile memory to survive reboots by the mobile node.

       If the mobile node has a current registration with some home agent
       (the Lifetime for that registration has not yet expired), then the
       mobile node MUST attempt any new registration first with that home
       agent.  If that registration attempt fails (e.g., timed out or
       rejected), the mobile node SHOULD then reattempt this registration

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       with another home agent.  If the mobile node knows of no other
       suitable home agent, then it MAY attempt the dynamic home agent
       address discovery mechanism described above.

       If, after a mobile node transmits a Home Agent Address Discovery
       Request message to the Home Agents Anycast address, it does not
       receive a corresponding Home Agent Address Discovery Reply message
       within INITIAL_DHAAD_TIMEOUT (see Section 12) seconds, the mobile
       node MAY retransmit the same Request message to the same anycast
       address.  This retransmission MAY be repeated up to a maximum of
       DHAAD_RETRIES (see Section 12) attempts.  Each retransmission MUST be
       delayed by twice the time interval of the previous retransmission.

    11.4.2.  Sending Mobile Prefix Solicitations

       When a mobile node has a home address that is about to become
       invalid, it SHOULD send a Mobile Prefix Solicitation to its home
       agent in an attempt to acquire fresh routing prefix information.  The
       new information also enables the mobile node to participate in
       renumbering operations affecting the home network, as described in
       Section 10.6.

       The mobile node MUST use the Home Address destination option to carry
       its home address.  The mobile node MUST support and SHOULD use IPsec
       to protect the solicitation.  The mobile node MUST set the Identifier
       field in the ICMP header to a random value.

       As described in Section 11.7.2, Binding Updates sent by the mobile
       node to other nodes MUST use a lifetime no greater than the remaining
       lifetime of its home registration of its primary care-of address.
       The mobile node SHOULD further limit the lifetimes that it sends on
       any Binding Updates to be within the remaining valid lifetime (see
       Section 10.6.2) for the prefix in its home address.

       When the lifetime for a changed prefix decreases, and the change
       would cause cached bindings at correspondent nodes in the Binding
       Update List to be stored past the newly shortened lifetime, the
       mobile node MUST issue a Binding Update to all such correspondent
       nodes.

       These limits on the binding lifetime serve to prohibit use of a
       mobile node's home address after it becomes invalid.

    11.4.3.  Receiving Mobile Prefix Advertisements

       Section 10.6 describes the operation of a home agent to support boot
       time configuration and renumbering a mobile node's home subnet while
       the mobile node is away from home.  The home agent sends Mobile

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       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 [18], except it is routed from the mobile
       node on the visited network to the home agent on the home network by
       usual unicast routing rules.

       When a mobile node receives a Mobile Prefix Advertisement, it MUST
       validate it according to the following test:

       o  The Source Address of the IP packet carrying the Mobile Prefix
          Advertisement is the same as the home agent address to which the
          mobile node last sent an accepted home registration Binding Update
          to register its primary care-of address.  Otherwise, if no such
          registrations have been made, it SHOULD be the mobile node's
          stored home agent address, if one exists.  Otherwise, if the
          mobile node has not yet discovered its home agent's address, it
          MUST NOT accept Mobile Prefix Advertisements.

       o  The packet MUST have a type 2 routing header and SHOULD be
          protected by an IPsec header as described in Sections 5.4 and 6.8.

       o  If the ICMP Identifier value matches the ICMP Identifier value of
          the most recently sent Mobile Prefix Solicitation and no other
          advertisement has yet been received for this value, then the
          advertisement is considered to be solicited and will be processed
          further.

          Otherwise, the advertisement is unsolicited, and MUST be
          discarded.  In this case the mobile node SHOULD send a Mobile
          Prefix Solicitation.

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

       For an accepted Mobile Prefix Advertisement, the mobile node MUST
       process Managed Address Configuration (M), Other Stateful
       Configuration (O), and the Prefix Information Options as if they
       arrived in a Router Advertisement [18] on the mobile node's home
       link.  (This specification does not, however, describe how to acquire
       home addresses through stateful protocols.)  Such processing may
       result in the mobile node configuring a new home address, although
       due to separation between preferred lifetime and valid lifetime, such
       changes should not affect most communications by the mobile node, in
       the same way as for nodes that are at home.

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       This specification assumes that any security associations and
       security policy entries that may be needed for new prefixes have been
       pre-configured in the mobile node.  Note that while dynamic key
       management avoids the need to configure new security associations, it
       is still necessary to add policy entries to protect the
       communications involving the home address(es).  Mechanisms for
       setting up these entries are outside the scope of this specification.

    11.5.  Movement

    11.5.1.  Movement Detection

       The primary goal of movement detection is to detect L3 handovers.
       This section does not attempt to specify a fast movement detection
       algorithm that will function optimally for all types of applications,
       link layers, and deployment scenarios; instead, it describes a
       generic method that uses the facilities of IPv6 Neighbor Discovery,
       including Router Discovery and Neighbor Unreachability Detection.  At
       the time of this writing, this method is considered well enough
       understood to recommend for standardization; however, it is expected
       that future versions of this specification or other specifications
       may contain updated versions of the movement detection algorithm that
       have better performance.

       Generic movement detection uses Neighbor Unreachability Detection to
       detect when the default router is no longer bidirectionally
       reachable, in which case the mobile node must discover a new default
       router (usually on a new link).  However, this detection only occurs
       when the mobile node has packets to send, and in the absence of
       frequent Router Advertisements or indications from the link-layer,
       the mobile node might become unaware of an L3 handover that occurred.
       Therefore, the mobile node should supplement this method with other
       information whenever it is available to the mobile node (e.g., from
       lower protocol layers).

       When the mobile node detects an L3 handover, it performs Duplicate
       Address Detection [19] on its link-local address, selects a new
       default router as a consequence of Router Discovery, and then
       performs prefix discovery with that new router to form new care-of
       address(es) as described in Section 11.5.3.  It then registers its
       new primary care-of address with its home agent as described in
       Section 11.7.1.  After updating its home registration, the mobile
       node then updates associated mobility bindings in correspondent nodes
       that it is performing route optimization with as specified in
       Section 11.7.2.

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       Due to the temporary packet flow disruption and signaling overhead
       involved in updating mobility bindings, the mobile node should avoid
       performing an L3 handover until it is strictly necessary.

       Specifically, when the mobile node receives a Router Advertisement
       from a new router that contains a different set of on-link prefixes,
       if the mobile node detects that the currently selected default router
       on the old link is still bidirectionally reachable, it should
       generally continue to use the old router on the old link rather than
       switch away from it to use a new default router.

       Mobile nodes can use the information in received Router
       Advertisements to detect L3 handovers.  In doing so the mobile node
       needs to consider the following issues:

       o  There might be multiple routers on the same link.  Thus, hearing a
          new router does not necessarily constitute an L3 handover.

       o  When there are multiple routers on the same link they might
          advertise different prefixes.  Thus, even hearing a new router
          with a new prefix might not be a reliable indication of an L3
          handover.

       o  The link-local addresses of routers are not globally unique, hence
          after completing an L3 handover the mobile node might continue to
          receive Router Advertisements with the same link-local source
          address.  This might be common if routers use the same link-local
          address on multiple interfaces.  This issue can be avoided when
          routers use the Router Address (R) bit, since that provides a
          global address of the router.

       In addition, the mobile node should consider the following events as
       indications that an L3 handover may have occurred.  Upon receiving
       such indications, the mobile node needs to perform Router Discovery
       to discover routers and prefixes on the new link, as described in
       Section 6.3.7 of Neighbor Discovery (RFC 4861 [18]).

       o  If Router Advertisements that the mobile node receives include an
          Advertisement Interval option, the mobile node may use its
          Advertisement Interval field as an indication of the frequency
          with which it should expect to continue to receive future
          Advertisements from that router.  This field specifies the minimum
          rate (the maximum amount of time between successive
          Advertisements) that the mobile node should expect.  If this
          amount of time elapses without the mobile node receiving any
          Advertisement from this router, the mobile node can be sure that
          at least one Advertisement sent by the router has been lost.  The

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          mobile node can then implement its own policy to determine how
          many lost Advertisements from its current default router
          constitute an L3 handover indication.

       o  Neighbor Unreachability Detection determines that the default
          router is no longer reachable.

       o  With some types of networks, notification that an L2 handover has
          occurred might be obtained from lower-layer protocols or device
          driver software within the mobile node.  While further details
          around handling L2 indications as movement hints is an item for
          further study, at the time of writing this specification the
          following is considered reasonable:

          An L2 handover indication may or may not imply L2 movement and L2
          movement may or may not imply L3 movement; the correlations might
          be a function of the type of L2 but might also be a function of
          actual deployment of the wireless topology.

          Unless it is well-known that an L2 handover indication is likely
          to imply L3 movement, instead of immediately multicasting a router
          solicitation it may be better to attempt to verify whether the
          default router is still bidirectionally reachable.  This can be
          accomplished by sending a unicast Neighbor Solicitation and
          waiting for a Neighbor Advertisement with the Solicited flag set.
          Note that this is similar to Neighbor Unreachability detection,
          but it does not have the same state machine, such as the STALE
          state.

          If the default router does not respond to the Neighbor
          Solicitation it makes sense to proceed to multicasting a Router
          Solicitation.

    11.5.2.  Home Link Detection

       When an MN detects that it has arrived on a new link using the
       movement detection algorithm in use (Section 11.5.1) or on
       bootstrapping, it performs the following steps to determine if it is
       on the home link.

       o  The MN performs the procedure described in Section 11.5.3 and
          configures an address.  It also keeps track of all the on-link
          prefix(es) received in the RA along with their prefix lengths.

       o  If the home prefix has not been statically configured the MN uses
          some form of bootstrapping procedure (e.g., RFC 5026 [22]) to
          determine the home prefix.

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       o  Given the availability of the home prefix, the MN checks whether
          or not the home prefix matches one of the prefixes received in the
          RA.  If it does, the MN concludes that it is connected to the home
          link.

    11.5.3.  Forming New Care-of Addresses

       After detecting that it has moved a mobile node SHOULD generate a new
       primary care-of address using normal IPv6 mechanisms.  This SHOULD
       also be done when the current primary care-of address becomes
       deprecated.  A mobile node MAY form a new primary care-of address at
       any time, but a mobile node MUST NOT send a Binding Update about a
       new care-of address to its home agent more than MAX_UPDATE_RATE times
       within a second.

       In addition, a mobile node MAY form new non-primary care-of addresses
       even when it has not switched to a new default router.  A mobile node
       can have only one primary care-of address at a time (which is
       registered with its home agent), but it MAY have an additional
       care-of address for any or all of the prefixes on its current link.
       Furthermore, since a wireless network interface may actually allow a
       mobile node to be reachable on more than one link at a time (i.e.,
       within wireless transmitter range of routers on more than one
       separate link), a mobile node MAY have care-of addresses on more than
       one link at a time.  The use of more than one care-of address at a
       time is described in Section 11.5.4.

       As described in Section 4, in order to form a new care-of address, a
       mobile node MAY use either stateless [19] or stateful (e.g., DHCPv6
       [31]) Address Autoconfiguration.  If a mobile node needs to use a
       source address (other than the unspecified address) in packets sent
       as a part of address autoconfiguration, it MUST use an IPv6 link-
       local address rather than its own IPv6 home address.

       RFC 4862 [19] 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 Duplicate Address
       Detection (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 4862 unless the implementation has a behavior that
       desynchronizes the steps that happen before the DAD in the case that
       multiple nodes experience handover at the same time.  Such
       desynchronizing behaviors might be due to random delays in the L2
       protocols or device drivers, or due to the movement detection
       mechanism that is used.

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    11.5.4.  Using Multiple Care-of Addresses

       As described in Section 11.5.3, a mobile node MAY use more than one
       care-of address at a time.  Particularly in the case of many wireless
       networks, a mobile node effectively might be reachable through
       multiple links at the same time (e.g., with overlapping wireless
       cells), on which different on-link subnet prefixes may exist.  The
       mobile node MUST ensure that its primary care-of address always has a
       prefix that is advertised by its current default router.  After
       selecting a new primary care-of address, the mobile node MUST send a
       Binding Update containing that care-of address to its home agent.
       The Binding Update MUST have the Home Registration (H) and
       Acknowledge (A) bits set its home agent, as described on
       Section 11.7.1.

       To assist with smooth handovers, a mobile node SHOULD retain its
       previous primary care-of address as a (non-primary) care-of address,
       and SHOULD still accept packets at this address, even after
       registering its new primary care-of address with its home agent.
       This is reasonable, since the mobile node could only receive packets
       at its previous primary care-of address if it were indeed still
       connected to that link.  If the previous primary care-of address was
       allocated using stateful Address Autoconfiguration [31], 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 that are not in the current set of address
       prefixes advertised by the (possibly new) current default router.

    11.5.5.  Returning Home

       A mobile node detects that it has returned to its home link through
       the movement detection algorithm in use (Section 11.5.2), when the
       mobile node detects that its home subnet prefix is again on-link.  To
       be able to send and receive packets using its home address from the
       home link, the mobile node MUST send a Binding Update to its home
       agent to instruct its home agent to no longer intercept or tunnel
       packets for it.  Until the mobile node sends such a de-registration
       Binding Update, it MUST NOT attempt to send and receive packets using
       its home address from the home link.  The home agent will continue to
       intercept all packets sent to the mobile's home address and tunnel
       them to the previously registered care-of address.

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       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 [18] (if needed)
       to learn the home agent's link-layer address, since the home agent
       will be currently configured to intercept packets to the mobile
       node's home address using Proxy Neighbor Discovery (Proxy ND).  In
       particular, the mobile node is unable to use its home address as the
       Source Address in the Neighbor Solicitation until the home agent
       stops defending the home address.

       Neighbor Solicitation by the mobile node for the home agent's address
       will normally not be necessary, since the mobile node has already
       learned the home agent's link-layer address from a Source Link-Layer
       Address option in a Router Advertisement.  However, if there are
       multiple home agents it may still be necessary to send a
       solicitation.  In this special case of the mobile node returning
       home, the mobile node MUST multicast the packet, and in addition set
       the Source Address of this Neighbor Solicitation to the unspecified
       address (0:0:0:0:0:0:0:0).  The target of the Neighbor Solicitation
       MUST be set to the mobile node's home address.  The destination IP
       address MUST be set to the Solicited-Node multicast address [16].
       The home agent will send a multicast Neighbor Advertisement back to
       the mobile node with the Solicited (S) flag set to zero.  In any
       case, the mobile node SHOULD record the information from the Source
       Link-Layer Address option or from the advertisement, and set the
       state of the Neighbor Cache entry for the home agent to REACHABLE.

       The mobile node then sends its Binding Update to the home agent's
       link-layer address, instructing its home agent to no longer serve as
       a home agent for it.  By processing this Binding Update, the home
       agent will cease defending the mobile node's home address for
       Duplicate Address Detection and will no longer respond to Neighbor
       Solicitations for the mobile node's home address.  The mobile node is
       then the only node on the link receiving packets at the mobile node's
       home address.  In addition, when returning home prior to the
       expiration of a current binding for its home address, and configuring
       its home address on its network interface on its home link, the
       mobile node MUST NOT perform Duplicate Address Detection on its own
       home address, in order to avoid confusion or conflict with its home
       agent's use of the same address.  This rule also applies to the
       derived link-local address of the mobile node, if the Link Local

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       Address Compatibility (L) bit was set when the binding was created.
       If the mobile node returns home after the bindings for all of its
       care-of addresses have expired, then it SHOULD perform DAD.

       After the mobile node sends the Binding Update, it MUST be prepared
       to reply to Neighbor Solicitations for its home address.  Such
       replies MUST be sent using a unicast Neighbor Advertisement to the
       sender's link-layer address.  It is necessary to reply, since sending
       the Binding Acknowledgement from the home agent may require
       performing Neighbor Discovery, and the mobile node may not be able to
       distinguish Neighbor Solicitations coming from the home agent from
       other Neighbor Solicitations.  Note that a race condition exists
       where both the mobile node and the home agent respond to the same
       solicitations sent by other nodes; this will be only temporary,
       however, until the Binding Update is accepted.

       After receiving the Binding Acknowledgement for its Binding Update to
       its home agent, the mobile node MUST multicast onto the home link (to
       the all-nodes multicast address) a Neighbor Advertisement [18], 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.  The
       Solicited (S) flag in these Advertisements MUST NOT be set, since
       they were not solicited by any Neighbor Solicitation.  The Override
       (O) flag 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 [18] 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 [18].

       Note that the tunnel via the home agent typically stops operating at
       the same time that the home registration is deleted.

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    RFC 6275                Mobility Support in IPv6               July 2011

    11.6.  Return Routability Procedure

       This section defines the rules that the mobile node must follow when
       performing the return routability procedure.  Section 11.7.2
       describes the rules when the return routability procedure needs to be
       initiated.

    11.6.1.  Sending Test Init Messages

       A mobile node that initiates a return routability procedure MUST send
       (in parallel) a Home Test Init message and a Care-of Test Init
       message.  However, if the mobile node has recently received (see
       Section 5.2.7) one or both home or care-of keygen tokens, and
       associated nonce indices for the desired addresses, it MAY reuse
       them.  Therefore, the return routability procedure may in some cases
       be completed with only one message pair.  It may even be completed
       without any messages at all, if the mobile node has a recent home
       keygen token and has previously visited the same care-of address so
       that it also has a recent care-of keygen token.  If the mobile node
       intends to send a Binding Update with the Lifetime set to zero and
       the care-of address equal to its home address -- such as when
       returning home -- sending a Home Test Init message is sufficient.  In
       this case, generation of the binding management key depends
       exclusively on the home keygen token (Section 5.2.5).

       A Home Test Init message MUST be created as described in
       Section 6.1.3.

       A Care-of Test Init message MUST be created as described in
       Section 6.1.4.  When sending a Home Test Init or Care-of Test Init
       message, the mobile node MUST record in its Binding Update List the
       following fields from the messages:

       o  The IP address of the node to which the message was sent.

       o  The home address of the mobile node.  This value will appear in
          the Source Address field of the Home Test Init message.  When
          sending the Care-of Test Init message, this address does not
          appear in the message, but represents the home address for which
          the binding is desired.

       o  The time at which each of these messages was sent.

       o  The cookies used in the messages.

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       Note that a single Care-of Test Init message may be sufficient even
       when there are multiple home addresses.  In this case the mobile node
       MAY record the same information in multiple Binding Update List
       entries.

    11.6.2.  Receiving Test Messages

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

       o  The Source Address of the packet belongs to a correspondent node
          for which the mobile node has a Binding Update List entry with a
          state indicating that return routability procedure is in progress.
          Note that there may be multiple such entries.

       o  The Binding Update List indicates that no home keygen token has
          been received yet.

       o  The Destination Address of the packet has the home address of the
          mobile node, and the packet has been received in a tunnel from the
          home agent.

       o  The Home Init Cookie field in the message matches the value stored
          in the Binding Update List.

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

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

       o  The Source Address of the packet belongs to a correspondent node
          for which the mobile node has a Binding Update List entry with a
          state indicating that return routability procedure is in progress.
          Note that there may be multiple such entries.

       o  The Binding Update List indicates that no care-of keygen token has
          been received yet.

       o  The Destination Address of the packet is the current care-of
          address of the mobile node.

       o  The Care-of Init Cookie field in the message matches the value
          stored in the Binding Update List.

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

       If after receiving either the Home Test or the Care-of Test message
       and performing the above actions, the Binding Update List entry has
       both the home and the care-of keygen tokens, the return routability
       procedure is complete.  The mobile node SHOULD then proceed with
       sending a Binding Update as described in Section 11.7.2.

       Correspondent nodes from the time before this specification was
       published may not support the Mobility Header protocol.  These nodes
       will respond to Home Test Init and Care-of Test Init messages with an
       ICMP Parameter Problem code 1.  The mobile node SHOULD take such
       messages as an indication that the correspondent node cannot provide
       route optimization, and revert back to the use of bidirectional
       tunneling.

    11.6.3.  Protecting Return Routability Packets

       The mobile node MUST support the protection of Home Test and Home
       Test Init messages as described in Section 10.4.6.

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

    11.7.  Processing Bindings

    11.7.1.  Sending Binding Updates to the Home Agent

       In order to change its primary care-of address as described in
       Sections 11.5.1 and 11.5.3, a mobile node MUST register this care-of
       address with its home agent in order to make this its primary care-of
       address.

       Also, if the mobile node wants the services of the home agent beyond
       the current registration period, the mobile node should send a new
       Binding Update to it well before the expiration of this period, even
       if it is not changing its primary care-of address.  However, if the
       home agent returned a Binding Acknowledgement for the current
       registration with the Status field set to 1 (accepted but prefix
       discovery necessary), the mobile node should not try to register

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       again before it has learned the validity of its home prefixes through
       mobile prefix discovery.  This is typically necessary every time this
       Status value is received, because information learned earlier may
       have changed.

       To register a care-of address or to extend the lifetime of an
       existing registration, the mobile node sends a packet to its home
       agent containing a Binding Update, with the packet constructed as
       follows:

       o  The Home Registration (H) bit MUST be set in the Binding Update.

       o  The Acknowledge (A) bit MUST be set in the Binding Update.

       o  The packet MUST contain a Home Address destination option, giving
          the mobile node's home address for the binding.

       o  The care-of address for the binding MUST be used as the Source
          Address in the packet's IPv6 header, unless an Alternate Care-of
          Address mobility option is included in the Binding Update.  This
          option MUST be included in all home registrations, as the ESP
          protocol will not be able to protect care-of addresses in the IPv6
          header.  (Mobile IPv6 implementations that know they are using
          IPsec AH to protect a particular message might avoid this option.
          For brevity the usage of AH is not discussed in this document.)

       o  If the mobile node's link-local address has the same interface
          identifier as the home address for which it is supplying a new
          care-of address, then the mobile node SHOULD set the Link-Local
          Address Compatibility (L) bit.

       o  If the home address was generated using RFC 4941 [21], then the
          link local address is unlikely to have a compatible interface
          identifier.  In this case, the mobile node MUST clear the Link-
          Local Address Compatibility (L) bit.

       o  If the IPsec security associations between the mobile node and the
          home agent have been established dynamically, and the mobile node
          has the capability to update its endpoint in the used key
          management protocol to the new care-of address every time it
          moves, the mobile node SHOULD set the Key Management Mobility
          Capability (K) bit in the Binding Update.  Otherwise, the mobile
          node MUST clear the bit.

       o  The value specified in the Lifetime field MUST be non-zero and
          SHOULD be less than or equal to the remaining valid lifetime of
          the home address and the care-of address specified for the
          binding.

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          Mobile nodes that use dynamic home agent address discovery should
          be careful with long lifetimes.  If the mobile node loses the
          knowledge of its binding with a specific home agent, registering a
          new binding with another home agent may be impossible as the
          previous home agent is still defending the existing binding.
          Therefore, to ensure that mobile nodes using home agent address
          discovery do not lose information about their binding, they SHOULD
          de-register before losing this information, or use small
          lifetimes.

       The Acknowledge (A) bit in the Binding Update requests the home agent
       to return a Binding Acknowledgement in response to this Binding
       Update.  As described in Section 6.1.8, the mobile node SHOULD
       retransmit this Binding Update to its home agent until it receives a
       matching Binding Acknowledgement.  Once reaching a retransmission
       timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart
       the process of delivering the Binding Update, but trying instead the
       next home agent returned during dynamic home agent address discovery
       (see Section 11.4.1).  If there was only one home agent, the mobile
       node instead SHOULD continue to periodically retransmit the Binding
       Update at this rate until acknowledged (or until it begins attempting
       to register a different primary care-of address).  See Section 11.8
       for information about retransmitting Binding Updates.

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

       Each Binding Update MUST be authenticated as coming from the right
       mobile node, as defined in Section 5.1.  The mobile node MUST use its
       home address -- either in the Home Address destination option or in
       the Source Address field of the IPv6 header -- in Binding Updates
       sent to the home agent.  This is necessary in order to allow the
       IPsec policies to be matched with the correct home address.

       When sending a Binding Update to its home agent, the mobile node MUST
       also create or update the corresponding Binding Update List entry, as
       specified in Section 11.7.2.

       The last Sequence Number value sent to the home agent in a Binding
       Update is stored by the mobile node.  If the sending mobile node has
       no knowledge of the correct Sequence Number value, it may start at
       any value.  If the home agent rejects the value, it sends back a
       Binding Acknowledgement with a status code 135, and the last accepted
       sequence number in the Sequence Number field of the Binding
       Acknowledgement.  The mobile node MUST store this information and use
       the next Sequence Number value for the next Binding Update it sends.

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       If the mobile node has additional home addresses, then the mobile
       node SHOULD send an additional packet containing a Binding Update to
       its home agent to register the care-of address for each such other
       home address.

       The home agent will only perform DAD for the mobile node's home
       address when the mobile node has supplied a valid binding between its
       home address and a care-of address.  If some time elapses during
       which the mobile node has no binding at the home agent, it might be
       possible for another node to autoconfigure the mobile node's home
       address.  Therefore, the mobile node MUST treat the creation of a new
       binding with the home agent using an existing home address, the same
       as creation of a new home address.  In the unlikely event that the
       mobile node's home address is autoconfigured as the IPv6 address of
       another network node on the home network, the home agent will reply
       to the mobile node's subsequent Binding Update with a Binding
       Acknowledgement containing a Status of 134 (Duplicate Address
       Detection failed).  In this case, the mobile node MUST NOT attempt to
       re-use the same home address.  It SHOULD continue to register the
       care-of addresses for its other home addresses, if any.  Mechanisms
       outlined in "Mobile IPv6 Bootstrapping in Split Scenario" [22] allow
       mobile nodes to acquire new home addresses to replace the one for
       which Status 134 was received.

    11.7.2.  Correspondent Registration

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

       This section defines when the correspondent registration is to be
       initiated and the rules to follow while it is being performed.

       After the mobile node has sent a Binding Update to its home agent,
       registering a new primary care-of address (as described in
       Section 11.7.1), the mobile node SHOULD initiate a correspondent
       registration for each node that already appears in the mobile node's
       Binding Update List.  The initiated procedures can be used to either
       update or delete binding information in the correspondent node.

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

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       In addition, the mobile node MAY initiate the correspondent
       registration in response to receiving a packet that meets all of the
       following tests:

       o  The packet was tunneled using IPv6 encapsulation.

       o  The Destination Address in the tunnel (outer) IPv6 header is equal
          to any of the mobile node's care-of addresses.

       o  The Destination Address in the original (inner) IPv6 header is
          equal to one of the mobile node's home addresses.

       o  The Source Address in the tunnel (outer) IPv6 header differs from
          the Source Address in the original (inner) IPv6 header.

       o  The packet does not contain a Home Test, Home Test Init, Care-of
          Test, or Care-of Test Init message.

       If a mobile node has multiple home addresses, it becomes important to
       select the right home address to use in the correspondent
       registration.  The used home address MUST be the Destination Address
       of the original (inner) packet.

       The peer address used in the procedure MUST be determined as follows:

       o  If a Home Address destination option is present in the original
          (inner) packet, the address from this option is used.

       o  Otherwise, the Source Address in the original (inner) IPv6 header
          of the packet is used.

       Note that the validity of the original packet is checked before
       attempting to initiate a correspondent registration.  For instance,
       if a Home Address destination option appeared in the original packet,
       then rules in Section 9.3.1 are followed.

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

       Upon successfully completing the return routability procedure, and
       after receiving a successful Binding Acknowledgement from the home
       agent, a Binding Update MAY be sent to the correspondent node.

       In any Binding Update sent by a mobile node, the care-of address
       (either the Source Address in the packet's IPv6 header or the Care-of
       Address in the Alternate Care-of Address mobility option of the
       Binding Update) MUST be set to one of the care-of addresses currently

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       in use by the mobile node or to the mobile node's home address.  A
       mobile node MAY set the care-of address differently for sending
       Binding Updates to different correspondent nodes.

       A mobile node MAY also send a Binding Update to such a correspondent
       node, instructing it to delete any existing binding for the mobile
       node from its Binding Cache, as described in Section 6.1.7.  Even in
       this case a successful completion of the return routability procedure
       is required first.

       If the care-of address is not set to the mobile node's home address,
       the Binding Update requests that the correspondent node create or
       update an entry for the mobile node in the correspondent node's
       Binding Cache.  This is done in order to record a care-of address for
       use in sending future packets to the mobile node.  In this case, the
       value specified in the Lifetime field sent in the Binding Update
       SHOULD be less than or equal to the remaining lifetime of the home
       registration and the care-of address specified for the binding.  The
       care-of address given in the Binding Update MAY differ from the
       mobile node's primary care-of address.

       If the Binding Update is sent to the correspondent node, requesting
       the deletion of any existing Binding Cache entry it has for the
       mobile node, the care-of address is set to the mobile node's home
       address and the Lifetime field set to zero.  In this case, generation
       of the binding management key depends exclusively on the home keygen
       token (Section 5.2.5).  The care-of nonce index SHOULD be set to zero
       in this case.  In keeping with the Binding Update creation rules
       below, the care-of address MUST be set to the home address if the
       mobile node is at home, or to the current care-of address if it is
       away from home.

       If the mobile node wants to ensure that its new care-of address has
       been entered into a correspondent node's Binding Cache, the mobile
       node needs to request an acknowledgement by setting the Acknowledge
       (A) bit in the Binding Update.

       A Binding Update is created as follows:

       o  The current care-of address of the mobile node MUST be sent either
          in the Source Address of the IPv6 header or in the Alternate
          Care-of Address mobility option.

       o  The Destination Address of the IPv6 header MUST contain the
          address of the correspondent node.

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       o  The Mobility Header is constructed according to rules in Sections
          6.1.7 and 5.2.6, including the Binding Authorization Data
          (calculated as defined in Section 6.2.7) and possibly the Nonce
          Indices mobility options.

       o  The home address of the mobile node MUST be added to the packet in
          a Home Address destination option, unless the Source Address is
          the home address.

       Each Binding Update MUST have a Sequence Number greater than the
       Sequence Number value sent in the previous Binding Update to the same
       destination address (if any).  The sequence numbers are compared
       modulo 2**16, as described in Section 9.5.1.  There is no
       requirement, however, that the Sequence Number value strictly
       increase by 1 with each new Binding Update sent or received, as long
       as the value stays within the window.  The last Sequence Number value
       sent to a destination in a Binding Update is stored by the mobile
       node in its Binding Update List entry for that destination.  If the
       sending mobile node has no Binding Update List entry, the Sequence
       Number SHOULD start at a random value.  The mobile node MUST NOT use
       the same Sequence Number in two different Binding Updates to the same
       correspondent node, even if the Binding Updates provide different
       care-of addresses.

       The mobile node is responsible for the completion of the
       correspondent registration, as well as any retransmissions that may
       be needed (subject to the rate limitation defined in Section 11.8).

    11.7.3.  Receiving Binding Acknowledgements

       Upon receiving a packet carrying a Binding Acknowledgement, a mobile
       node MUST validate the packet according to the following tests:

       o  The packet meets the authentication requirements for Binding
          Acknowledgements defined in Sections 6.1.8 and 5.  That is, if the
          Binding Update was sent to the home agent, the underlying IPsec
          protection is used.  If the Binding Update was sent to the
          correspondent node, the Binding Authorization Data mobility option
          MUST be present and have a valid value.

       o  The Binding Authorization Data mobility option, if present, MUST
          be the last option and MUST NOT have trailing padding.

       o  The Sequence Number field matches the Sequence Number sent by the
          mobile node to this destination address in an outstanding Binding
          Update, and the Status field is not 135.

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       Any Binding Acknowledgement not satisfying all of these tests MUST be
       silently ignored.

       When a mobile node receives a packet carrying a valid Binding
       Acknowledgement, the mobile node MUST examine the Status field as
       follows:

       o  If the Status field indicates that the Binding Update was accepted
          (the Status field is less than 128), then the mobile node MUST
          update the corresponding entry in its Binding Update List to
          indicate that the Binding Update has been acknowledged; the mobile
          node MUST then stop retransmitting the Binding Update.  In
          addition, if the value specified in the Lifetime field in the
          Binding Acknowledgement is less than the Lifetime value sent in
          the Binding Update being acknowledged, the mobile node MUST
          subtract the difference between these two Lifetime values from the
          remaining lifetime for the binding as maintained in the
          corresponding Binding Update List entry (with a minimum value for
          the Binding Update List entry lifetime of 0).  That is, if the
          Lifetime value sent in the Binding Update was L_update, the
          Lifetime value received in the Binding Acknowledgement was L_ack,
          and the current remaining lifetime of the Binding Update List
          entry is L_remain, then the new value for the remaining lifetime
          of the Binding Update List entry should be

             max((L_remain - (L_update - L_ack)), 0)

          where max(X, Y) is the maximum of X and Y.  The effect of this
          step is to correctly manage the mobile node's view of the
          binding's remaining lifetime (as maintained in the corresponding
          Binding Update List entry) so that it correctly counts down from
          the Lifetime value given in the Binding Acknowledgement, but with
          the timer countdown beginning at the time that the Binding Update
          was sent.

          Mobile nodes SHOULD send a new Binding Update well before the
          expiration of this period in order to extend the lifetime.  This
          helps to avoid disruptions in communications that might otherwise
          be caused by network delays or clock drift.

       o  If the Binding Acknowledgement correctly passes authentication and
          the Status field value is 135 (Sequence Number out of window),
          then the mobile node MUST update its binding sequence number
          appropriately to match the sequence number given in the Binding
          Acknowledgement.  Otherwise, if the Status field value is 135 but
          the Binding Acknowledgement does not pass authentication, the
          message MUST be silently ignored.

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       o  If the Status field value is 1 (accepted but prefix discovery
          necessary), the mobile node SHOULD send a Mobile Prefix
          Solicitation message to update its information about the available
          prefixes.

       o  If the Status field indicates that the Binding Update was rejected
          (the Status field is greater than or equal to 128), then the
          mobile node can take steps to correct the cause of the error and
          retransmit the Binding Update (with a new Sequence Number value),
          subject to the rate limiting restriction specified in
          Section 11.8.  If this is not done or it fails, then the mobile
          node SHOULD record in its Binding Update List that future Binding
          Updates SHOULD NOT be sent to this destination.

       The treatment of a Binding Refresh Advice mobility option within the
       Binding Acknowledgement depends on where the acknowledgement came
       from.  This option MUST be ignored if the acknowledgement came from a
       correspondent node.  If it came from the home agent, the mobile node
       uses the Refresh Interval field in the option as a suggestion that it
       SHOULD attempt to refresh its home registration at the indicated
       shorter interval.

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

       If this bit is set, the mobile node SHOULD move its own endpoint in
       the key management protocol connections to the home agent, if any.
       The mobile node's new endpoint should be the new care-of address.

    11.7.4.  Receiving Binding Refresh Requests

       When a mobile node receives a packet containing a Binding Refresh
       Request message, if the mobile node has a Binding Update List entry
       for the source of the Binding Refresh Request, and the mobile node
       wants to retain its Binding Cache entry at the correspondent node,
       then the mobile node should start a return routability procedure.  If
       the mobile node wants to have its Binding Cache entry removed, it can
       either ignore the Binding Refresh Request and wait for the binding to
       time out, or at any time, it can delete its binding from a
       correspondent node with an explicit Binding Update with a zero
       lifetime and the care-of address set to the home address.  If the
       mobile node does not know if it needs the Binding Cache entry, it can
       make the decision in an implementation-dependent manner, such as
       based on available resources.

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       Note that the mobile node should be careful not to respond to Binding
       Refresh Requests for addresses not in the Binding Update List to
       avoid being subjected to a denial of service attack.

       If the return routability procedure completes successfully, a Binding
       Update message SHOULD be sent, as described in Section 11.7.2.  The
       Lifetime field in this Binding Update SHOULD be set to a new
       lifetime, extending any current lifetime remaining from a previous
       Binding Update sent to this node (as indicated in any existing
       Binding Update List entry for this node), and the lifetime SHOULD
       again be less than or equal to the remaining lifetime of the home
       registration and the care-of address specified for the binding.  When
       sending this Binding Update, the mobile node MUST update its Binding
       Update List in the same way as for any other Binding Update sent by
       the mobile node.

    11.8.  Retransmissions and Rate Limiting

       The mobile node is responsible for retransmissions and rate limiting
       in the return routability procedure, in registrations, and in
       solicited prefix discovery.

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

       o  If the mobile node is sending a Mobile Prefix Solicitation, it
          SHOULD use an initial retransmission interval of
          INITIAL_SOLICIT_TIMER (see Section 12).

       o  If the mobile node is sending a Binding Update and does not have
          an existing binding at the home agent, it SHOULD use
          InitialBindackTimeoutFirstReg (see Section 13) as a value for the
          initial retransmission timer.  This long retransmission interval
          will allow the home agent to complete the Duplicate Address
          Detection procedure mandated in this case, as detailed in
          Section 11.7.1.

       o  Otherwise, the mobile node should use the specified value of
          INITIAL_BINDACK_TIMEOUT for the initial retransmission timer.

       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.

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       The retransmissions by the mobile node MUST use an exponential back-
       off process in which the timeout period is doubled upon each
       retransmission, until either the node receives a response or the
       timeout period reaches the value MAX_BINDACK_TIMEOUT.  The mobile
       node MAY continue to send these messages at this slower rate
       indefinitely.

       The mobile node SHOULD start a separate back-off process for
       different message types, different home addresses, and different
       care-of addresses.  However, in addition an overall rate limitation
       applies for messages sent to a particular correspondent node.  This
       ensures that the correspondent node has a sufficient amount of time
       to respond when bindings for multiple home addresses are registered,
       for instance.  The mobile node MUST NOT send Mobility Header messages
       of a particular type to a particular correspondent node more than
       MAX_UPDATE_RATE times within a second.

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

    12.  Protocol Constants

               DHAAD_RETRIES                   4 retransmissions
               INITIAL_BINDACK_TIMEOUT         1 second
               INITIAL_DHAAD_TIMEOUT           3 seconds
               INITIAL_SOLICIT_TIMER           3 seconds
               MAX_BINDACK_TIMEOUT             32 seconds
               MAX_DELETE_BCE_TIMEOUT          10 seconds
               MAX_NONCE_LIFETIME              240 seconds
               MAX_TOKEN_LIFETIME              210 seconds
               MAX_RO_FAILURE                  3 retries
               MAX_RR_BINDING_LIFETIME         420 seconds
               MAX_UPDATE_RATE                 3 times
               PREFIX_ADV_RETRIES              3 retransmissions
               PREFIX_ADV_TIMEOUT              3 seconds

    13.  Protocol Configuration Variables

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

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

       The default value for InitialBindackTimeoutFirstReg has been
       calculated as 1.5 times the default value of RetransTimer, as
       specified in Neighbor Discovery (RFC 4861 [18]) times the default
       value of DupAddrDetectTransmits, as specified in Stateless Address
       Autoconfiguration (RFC 4862 [19]).

       The value MinDelayBetweenRAs overrides the value of the protocol
       constant MIN_DELAY_BETWEEN_RAS, as specified in Neighbor Discovery
       (RFC 4861 [18]).  This variable SHOULD be set to MinRtrAdvInterval,
       if MinRtrAdvInterval is less than 3 seconds.

    14.  IANA Considerations

       This document defines a new IPv6 protocol, the Mobility Header,
       described in Section 6.1.  This protocol has been assigned protocol
       number 135.

       This document also creates a new name space "Mobility Header Type",
       for the MH Type field in the Mobility Header.  The current message
       types are described starting from Section 6.1.2, and are the
       following:

          0  Binding Refresh Request

          1  Home Test Init

          2  Care-of Test Init

          3  Home Test

          4  Care-of Test

          5  Binding Update

          6  Binding Acknowledgement

          7  Binding Error

       Future values of the MH Type can be allocated using Standards Action
       or IESG Approval [23].

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       Furthermore, each mobility message may contain mobility options as
       described in Section 6.2.  This document defines a new name space
       "Mobility Option" to identify these options.  The current mobility
       options are defined starting from Section 6.2.2 and are the
       following:

          0  Pad1

          1  PadN

          2  Binding Refresh Advice

          3  Alternate Care-of Address

          4  Nonce Indices

          5  Authorization Data

       Future values of the Option Type can be allocated using Standards
       Action or IESG Approval [23].

       Finally, this document creates a third new name space "Status Code"
       for the Status field in the Binding Acknowledgement message.  The
       current values are listed in Section 6.1.8 and are the following:

       0  Binding Update accepted

       1  Accepted but prefix discovery necessary

       128  Reason unspecified

       129  Administratively prohibited

       130  Insufficient resources

       131  Home registration not supported

       132  Not home subnet

       133  Not home agent for this mobile node

       134  Duplicate Address Detection failed

       135  Sequence number out of window

       136  Expired home nonce index

       137  Expired care-of nonce index

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       138  Expired nonces

       139  Registration type change disallowed

       174  Invalid Care-of Address

       Future values of the Status field can be allocated using Standards
       Action or IESG Approval [23].

       All fields labeled "Reserved" are only to be assigned through
       Standards Action or IESG Approval.

       This document also defines a new IPv6 destination option, the Home
       Address option, described in Section 6.3.  This option has been
       assigned the Option Type value 0xC9.

       This document also defines a new IPv6 type 2 routing header,
       described in Section 6.4.  The value 2 has been allocated by IANA.

       In addition, this document defines four ICMP message types, two used
       as part of the dynamic home agent address discovery mechanism, and
       two used in lieu of Router Solicitations and Advertisements when the
       mobile node is away from the home link.  These messages have been
       assigned ICMPv6 type numbers from the informational message range:

       o  The Home Agent Address Discovery Request message, described in
          Section 6.5;

       o  The Home Agent Address Discovery Reply message, described in
          Section 6.6;

       o  The Mobile Prefix Solicitation, described in Section 6.7; and

       o  The Mobile Prefix Advertisement, described in Section 6.8.

       This document also defines two new Neighbor Discovery [18] options,
       which have been assigned Option Type values within the option
       numbering space for Neighbor Discovery messages:

       o  The Advertisement Interval option, described in Section 7.3; and

       o  The Home Agent Information option, described in Section 7.4.

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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:

       o  Threats involving Binding Updates sent to home agents and
          correspondent nodes.  For instance, an attacker might claim that a
          certain mobile node is currently at a different location than it
          really is.  If a home agent accepts such spoofed information sent
          to it, the mobile node might not get traffic destined to it.
          Similarly, a malicious (mobile) node might use the home address of
          a victim node in a forged Binding Update sent to a correspondent
          node.

          These pose threats against confidentiality, integrity, and
          availability.  That is, an attacker might learn the contents of
          packets destined to another node by redirecting the traffic to
          itself.  Furthermore, an attacker might use the redirected packets
          in an attempt to set itself as a man in the middle between a
          mobile and a correspondent node.  This would allow the attacker to
          impersonate the mobile node, leading to integrity and availability
          problems.

          A malicious (mobile) node might also send Binding Updates in which
          the care-of address is set to the address of a victim node.  If
          such Binding Updates were accepted, the malicious node could lure
          the correspondent node into sending potentially large amounts of
          data to the victim; the correspondent node's replies to messages
          sent by the malicious mobile node will be sent to the victim host
          or network.  This could be used to cause a distributed denial-of-
          service attack.  For example, the correspondent node might be a
          site that will send a high-bandwidth stream of video to anyone who
          asks for it.  Note that the use of flow-control protocols such as
          TCP does not necessarily defend against this type of attack,
          because the attacker can fake the acknowledgements.  Even keeping
          TCP initial sequence numbers secret does not help, because the
          attacker can receive the first few segments (including the ISN) at
          its own address, and only then redirect the stream to the victim's
          address.  These types of attacks may also be directed to networks
          instead of nodes.  Further variations of this threat are described
          elsewhere [28] [35].

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          An attacker might also attempt to disrupt a mobile node's
          communications by replaying a Binding Update that the node had
          sent earlier.  If the old Binding Update was accepted, packets
          destined for the mobile node would be sent to its old location as
          opposed to its current location.

          A malicious mobile node associated to multiple home agents could
          create a routing loop amongst them.  This can be achieved when a
          mobile node binds one home address located on a first home agent
          to another home address on a second home agent.  This type of
          binding will force the home agents to route the same packet among
          each other without knowledge that a routing loop has been created.
          Such looping problem is limited to cases where a mobile node has
          multiple home agents and is permitted to be associated with the
          multiple home agents.  For the single home agent case, a policy at
          the home agent would prevent the binding of one home address to
          another home address hosted by the same home agent.

          The potential problems caused by such routing loops in this
          scenario can be substantially reduced by use of the Tunnel-Limit
          Option specified in RFC 2473 [7].

          In conclusion, there are denial-of-service, man-in-the-middle,
          confidentiality, and impersonation threats against the parties
          involved in sending legitimate Binding Updates, the threat of
          routing loops when there are multiple home agents, and denial-of-
          service threats against any other party.

       o  Threats associated with payload packets: Payload packets exchanged
          with mobile nodes are exposed to similar threats as that of
          regular IPv6 traffic.  However, Mobile IPv6 introduces the Home
          Address destination option and a new routing header type (type 2),
          and uses tunneling headers in the payload packets.  The protocol
          must protect against potential new threats involving the use of
          these mechanisms.

          Third parties become exposed to a reflection threat via the Home
          Address destination option, unless appropriate security
          precautions are followed.  The Home Address destination option
          could be used to direct response traffic toward a node whose IP
          address appears in the option.  In this case, ingress filtering
          would not catch the forged "return address" [38] [43].

          A similar threat exists with the tunnels between the mobile node
          and the home agent.  An attacker might forge tunnel packets
          between the mobile node and the home agent, making it appear that
          the traffic is coming from the mobile node when it is not.  Note
          that an attacker who is able to forge tunnel packets would

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          typically also be able to forge packets that appear to come
          directly from the mobile node.  This is not a new threat as such.
          However, it may make it easier for attackers to escape detection
          by avoiding ingress filtering and packet tracing mechanisms.
          Furthermore, spoofed tunnel packets might be used to gain access
          to the home network.

          Finally, a routing header could also be used in reflection
          attacks, and in attacks designed to bypass firewalls.  The
          generality of the regular routing header would allow circumvention
          of IP-address based rules in firewalls.  It would also allow
          reflection of traffic to other nodes.  These threats exist with
          routing headers in general, even if the usage that Mobile IPv6
          requires is safe.

       o  Threats associated with dynamic home agent and mobile prefix
          discovery.

       o  Threats against the Mobile IPv6 security mechanisms themselves: An
          attacker might, for instance, lure the participants into executing
          expensive cryptographic operations or allocating memory for the
          purpose of keeping state.  The victim node would have no resources
          left to handle other tasks.

       As a fundamental service in an IPv6 stack, Mobile IPv6 is expected to
       be deployed in most nodes of the IPv6 Internet.  The above threats
       should therefore be considered as being applicable to the whole
       Internet.

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

    15.2.  Features

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

       o  Reverse tunneling as a mandatory feature.

       o  Protection of Binding Updates sent to home agents.

       o  Protection of Binding Updates sent to correspondent nodes.

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       o  Protection against reflection attacks that use the Home Address
          destination option.

       o  Protection of tunnels between the mobile node and the home agent.

       o  Closing routing header vulnerabilities.

       o  Mitigating denial-of-service threats to the Mobile IPv6 security
          mechanisms themselves.

       The support for encrypted reverse tunneling (see Section 11.3.1)
       allows mobile nodes to defeat certain kinds of traffic analysis.

       Protecting those Binding Updates that are sent to home agents and
       those that are sent to arbitrary correspondent nodes requires very
       different security solutions due to the different situations.  Mobile
       nodes and home agents are naturally expected to be subject to the
       network administration of the home domain.

       Thus, they can and are supposed to have a security association that
       can be used to reliably authenticate the exchanged messages.  See
       Section 5.1 for the description of the protocol mechanisms, and
       Section 15.3 below for a discussion of the resulting level of
       security.

       It is expected that Mobile IPv6 route optimization will be used on a
       global basis between nodes belonging to different administrative
       domains.  It would be a very demanding task to build an
       authentication infrastructure on this scale.  Furthermore, a
       traditional authentication infrastructure cannot be easily used to
       authenticate IP addresses because IP addresses can change often.  It
       is not sufficient to just authenticate the mobile nodes;
       authorization to claim the right to use an address is needed as well.
       Thus, an "infrastructureless" approach is necessary.  The chosen
       infrastructureless method is described in Section 5.2, and
       Section 15.4 discusses the resulting security level and the design
       rationale of this approach.

       Specific rules guide the use of the Home Address destination option,
       the routing header, and the tunneling headers in the payload packets.
       These rules are necessary to remove the vulnerabilities associated
       with their unrestricted use.  The effect of the rules is discussed in
       Sections 15.7, 15.8, and 15.9.

       Denial-of-service threats against Mobile IPv6 security mechanisms
       themselves concern mainly the Binding Update procedures with
       correspondent nodes.  The protocol has been designed to limit the
       effects of such attacks, as will be described in Section 15.4.5.

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    15.3.  Binding Updates to Home Agent

       Signaling between the mobile node and the home agent requires message
       integrity.  This is necessary to assure the home agent that a Binding
       Update is from a legitimate mobile node.  In addition, correct
       ordering and anti-replay protection are optionally needed.

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

       IPsec can provide anti-replay protection only if dynamic keying is
       used (which may not always be the case).  IPsec does not guarantee
       correct ordering of packets, only that they have not been replayed.
       Because of this, sequence numbers within the Mobile IPv6 messages are
       used to ensure correct ordering (see Section 5.1).  However, if the
       16-bit Mobile IPv6 sequence number space is cycled through, or the
       home agent reboots and loses its state regarding the sequence
       numbers, replay and reordering attacks become possible.  The use of
       dynamic keying, IPsec anti-replay protection, and the Mobile IPv6
       sequence numbers can together prevent such attacks.  It is also
       recommended that use of non-volatile storage be considered for home
       agents, to avoid losing their state.

       A sliding window scheme is used for the sequence numbers.  The
       protection against replays and reordering attacks without a key
       management mechanism works when the attacker remembers up to a
       maximum of 2**15 Binding Updates.

       The above mechanisms do not show that the care-of address given in
       the Binding Update is correct.  This opens the possibility for
       denial-of-service attacks against third parties.  However, since the
       mobile node and home agent have a security association, the home
       agent can always identify an ill-behaving mobile node.  This allows
       the home agent operator to discontinue the mobile node's service, and
       possibly take further actions based on the business relationship with
       the mobile node's owner.

       Note that the use of a single pair of manually keyed security
       associations conflicts with the generation of a new home address [21]
       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

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       automatic procedures to establish new certificates.  Therefore, this
       specification restricts the generation of new home addresses (for any
       reason) to those situations where a security association or
       certificate for the new address already exists.

       Support for IKEv2 has been specified as optional.  The following
       should be observed about the use of manual keying:

       o  As discussed above, with manually keyed IPsec, only a limited form
          of protection exists against replay and reordering attacks.  A
          vulnerability exists if either the sequence number space is cycled
          through or the home agent reboots and forgets its sequence numbers
          (and uses volatile memory to store the sequence numbers).

          Assuming the mobile node moves continuously every 10 minutes, it
          takes roughly 455 days before the sequence number space has been
          cycled through.  Typical movement patterns rarely reach this high
          frequency today.

       o  A mobile node and its home agent belong to the same domain.  If
          this were not the case, manual keying would not be possible [42],
          but in Mobile IPv6 only these two parties need to know the
          manually configured keys.  Similarly, we note that Mobile IPv6
          employs standard block ciphers in IPsec, and is not vulnerable to
          problems associated with stream ciphers and manual keying.

       o  It is expected that the owner of the mobile node and the
          administrator of the home agent agree on the used keys and other
          parameters with some off-line mechanism.

       The use of IKEv2 with Mobile IPv6 is documented in more detail in
       [20].  The following should be observed regarding the use of IKEv2:

       o  It is necessary to prevent a mobile node from claiming another
          mobile node's home address.  The home agent must verify that the
          mobile node trying to negotiate the SA for a particular home
          address is authorized for that home address.  This implies that
          even with the use of IKEv2, a policy entry needs to be configured
          for each home address served by the home agent.

          It may be possible to include home addresses in the Subject
          AltName field of certificate to avoid this.  However,
          implementations are not guaranteed to support the use of a
          particular IP address (care-of address) while another address
          (home address) appears in the certificate.  In any case, even this
          approach would require user-specific tasks in the certificate
          authority.

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       o  Due to the problems outlined in Section 11.3.2, the IKEv2 SA
          between the mobile node and its home agent is established using
          the mobile node's current care-of address.  This implies that when
          the mobile node moves to a new location, it may have to
          re-establish an IKEv2 security association.  A Key Management
          Mobility Capability (K) flag is provided for implementations that
          can update the IKEv2 endpoints without re-establishing an IKEv2
          security association, but the support for this behavior is
          optional.

       o  Nevertheless, even if per-mobile node configuration is required
          with IKEv2, an important benefit of IKEv2 is that it automates the
          negotiation of cryptographic parameters, including the Security
          Parameter Indices (SPIs), cryptographic algorithms, and so on.
          Thus, less configuration information is needed.

       o  The frequency of movements in some link layers or deployment
          scenarios may be high enough to make replay and reordering attacks
          possible, if only manual keying is used.  IKEv2 SHOULD be used in
          such cases.  Potentially vulnerable scenarios involve continuous
          movement through small cells, or uncontrolled alternation between
          available network attachment points.

       o  Similarly, in some deployment scenarios the number of mobile nodes
          may be very large.  In these cases, it can be necessary to use
          automatic mechanisms to reduce the management effort in the
          administration of cryptographic parameters, even if some per-
          mobile node configuration is always needed.  IKEv2 SHOULD also be
          used in such cases.

    15.4.  Binding Updates to Correspondent Nodes

       The motivation for designing the return routability procedure was to
       have sufficient support for Mobile IPv6, without creating significant
       new security problems.  The goal for this procedure was not to
       protect against attacks that were already possible before the
       introduction of Mobile IPv6.

       The next sections will describe the security properties of the used
       method, both from the point of view of possible on-path attackers who
       can see those cryptographic values that have been sent in the clear
       (Sections 15.4.2 and 15.4.3) and from the point of view of other
       attackers (Section 15.4.6).

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    15.4.1.  Overview

       The chosen infrastructureless method verifies that the mobile node is
       "live" (that is, it responds to probes) at its home and care-of
       addresses.  Section 5.2 describes the return routability procedure in
       detail.  The procedure uses the following principles:

       o  A message exchange verifies that the mobile node is reachable at
          its addresses, i.e., is at least able to transmit and receive
          traffic at both the home and care-of addresses.

       o  The eventual Binding Update is cryptographically bound to the
          tokens supplied in the exchanged messages.

       o  Symmetric exchanges are employed to avoid the use of this protocol
          in reflection attacks.  In a symmetric exchange, the responses are
          always sent to the same address from which the request was sent.

       o  The correspondent node operates in a stateless manner until it
          receives a fully authorized Binding Update.

       o  Some additional protection is provided by encrypting the tunnels
          between the mobile node and home agent with IPsec ESP.  As the
          tunnel also transports the nonce exchanges, the ability of
          attackers to see these nonces is limited.  For instance, this
          prevents attacks from being launched from the mobile node's
          current foreign link, even when no link-layer confidentiality is
          available.

          The resulting level of security is in theory the same even without
          this additional protection: the return routability tokens are
          still exposed only to one path within the whole Internet.
          However, the mobile nodes are often found on an insecure link,
          such as a public access Wireless LAN.  Thus, in many cases, this
          addition makes a practical difference.

       For further information about the design rationale of the return
       routability procedure, see [28] [35] [34] [43].  The mechanisms used
       have been adopted from these documents.

    15.4.2.  Achieved Security Properties

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

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       it is at its home location.  The possibility of such attacks is not
       an impediment to the deployment of Mobile IPv6 because these attacks
       are possible regardless of whether or not Mobile IPv6 is in use.

       This procedure also protects against denial-of-service attacks in
       which the attacker pretends to be mobile, but uses the victim's
       address as the care-of address.  This would cause the correspondent
       node to send the victim some unexpected traffic.  This procedure
       defends against these attacks by requiring at least the passive
       presence of the attacker at the care-of address or on the path from
       the correspondent to the care-of address.  Normally, this will be the
       mobile node.

    15.4.3.  Comparison to Regular IPv6 Communications

       This section discusses the protection offered by the return
       routability method by comparing it to the security of regular IPv6
       communications.  We will divide vulnerabilities into three classes:
       (1) those related to attackers on the local network of the mobile
       node, home agent, or the correspondent node, (2) those related to
       attackers on the path between the home network and the correspondent
       node, and (3) off-path attackers, i.e., the rest of the Internet.

       We will now discuss the vulnerabilities of regular IPv6
       communications.  The on-link vulnerabilities of IPv6 communications
       include denial-of-service, masquerading, man-in-the-middle,
       eavesdropping, and other attacks.  These attacks can be launched
       through spoofing Router Discovery, Neighbor Discovery, and other IPv6
       mechanisms.  Some of these attacks can be prevented with the use of
       cryptographic protection in the packets.

       A similar situation exists with on-path attackers.  That is, without
       cryptographic protection, the traffic is completely vulnerable.

       Assuming that attackers have not penetrated the security of the
       Internet routing protocols, attacks are much harder to launch from
       off-path locations.  Attacks that can be launched from these
       locations are mainly denial-of-service attacks, such as flooding
       and/or reflection attacks.  It is not possible for an off-path
       attacker to become a man in the middle.

       Next, we will consider the vulnerabilities that exist when IPv6 is
       used together with Mobile IPv6 and the return routability procedure.
       On the local link, the vulnerabilities are the same as those in IPv6,
       but masquerade and man-in-the-middle attacks can now also be launched
       against future communications, and not just against current
       communications.  If a Binding Update was sent while the attacker was
       present on the link, its effects remain for the lifetime of the

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       binding.  This happens even if the attacker moves away from the link.
       In contrast, an attacker who uses only plain IPv6 generally has to
       stay on the link in order to continue the attack.  Note that in order
       to launch these new attacks, the IP address of the victim must be
       known.  This makes this attack feasible, mainly in the context of
       well-known interface IDs, such as those already appearing in the
       traffic on the link or registered in the DNS.

       On-path attackers can exploit similar vulnerabilities as in regular
       IPv6.  There are some minor differences, however.  Masquerade, man-
       in-the-middle, and denial-of-service attacks can be launched with
       just the interception of a few packets, whereas in regular IPv6 it is
       necessary to intercept every packet.  The effect of the attacks is
       the same regardless of the method, however.  In any case, the most
       difficult task an attacker faces in these attacks is getting on the
       right path.

       The vulnerabilities for off-path attackers are the same as in regular
       IPv6.  Those nodes that are not on the path between the home agent
       and the correspondent node will not be able to receive the home
       address probe messages.

       In conclusion, we can state the following main results from this
       comparison:

       o  Return routability prevents any off-path attacks beyond those that
          are already possible in regular IPv6.  This is the most important
          result, preventing attackers on the Internet from exploiting any
          vulnerabilities.

       o  Vulnerabilities to attackers on the home agent link, the
          correspondent node link, and the path between them are roughly the
          same as in regular IPv6.

       o  However, one difference is that in basic IPv6 an on-path attacker
          must be constantly present on the link or the path, whereas with
          Mobile IPv6, an attacker can leave a binding behind after moving
          away.

          For this reason, this specification limits the creation of
          bindings to at most MAX_TOKEN_LIFETIME seconds after the last
          routability check has been performed, and limits the duration of a
          binding to at most MAX_RR_BINDING_LIFETIME seconds.  With these
          limitations, attackers cannot take any practical advantages of
          this vulnerability.

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       o  There are some other minor differences, such as an effect to the
          denial-of-service vulnerabilities.  These can be considered to be
          insignificant.

       o  The path between the home agent and a correspondent node is
          typically easiest to attack on the links at either end, in
          particular if these links are publicly accessible wireless LANs.
          Attacks against the routers or switches on the path are typically
          harder to accomplish.  The security on layer 2 of the links plays
          then a major role in the resulting overall network security.
          Similarly, security of IPv6 Neighbor and Router Discovery on these
          links has a large impact.  If these were secured using some new
          technology in the future, this could change the situation
          regarding the easiest point of attack.

       For a more in-depth discussion of these issues, see [43].

    15.4.4.  Replay Attacks

       The return routability procedure also protects the participants
       against replayed Binding Updates.  The attacker is unable replay the
       same message due to the sequence number that is a part of the Binding
       Update.  It is also unable to modify the Binding Update since the MAC
       verification would fail after such a modification.

       Care must be taken when removing bindings at the correspondent node,
       however.  If a binding is removed while the nonce used in its
       creation is still valid, an attacker could replay the old Binding
       Update.  Rules outlined in Section 5.2.8 ensure that this cannot
       happen.

    15.4.5.  Denial-of-Service Attacks

       The return routability procedure has protection against resource
       exhaustion denial-of-service attacks.  The correspondent nodes do not
       retain any state about individual mobile nodes until an authentic
       Binding Update arrives.  This is achieved through the construct of
       keygen tokens from the nonces and node keys that are not specific to
       individual mobile nodes.  The keygen tokens can be reconstructed by
       the correspondent node, based on the home and care-of address
       information that arrives with the Binding Update.  This means that
       the correspondent nodes are safe against memory exhaustion attacks
       except where on-path attackers are concerned.  Due to the use of
       symmetric cryptography, the correspondent nodes are relatively safe
       against CPU resource exhaustion attacks as well.

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       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 help
       determine whether there is a need to establish a binding with a
       specific peer.  For example, TCP knows if the node has a queue of
       data that it is trying to send to a peer.  An implementation of this
       specification is not required to make use of information from higher
       protocol layers, but some implementations are likely to be able to
       manage resources more effectively by making use of such information.

       We also require that all implementations be capable of
       administratively disabling route optimization.

    15.4.6.  Key Lengths

       Attackers can try to break the return routability procedure in many
       ways.  Section 15.4.2 discusses the situation where the attacker can
       see the cryptographic values sent in the clear, and Section 15.4.3
       discusses the impact this has on IPv6 communications.  This section
       discusses whether attackers can guess the correct values without
       seeing them.

       While the return routability procedure is in progress, 64-bit cookies
       are used to protect spoofed responses.  This is believed to be
       sufficient, given that to blindly spoof a response a very large
       number of messages would have to be sent before success would be
       probable.

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       The tokens used in the return routability procedure provide together
       128 bits of information.  This information is used internally as
       input to a hash function to produce a 160-bit quantity suitable for
       producing the keyed hash in the Binding Update using the HMAC_SHA1
       algorithm.  The final keyed hash length is 96 bits.  The limiting
       factors in this case are the input token lengths and the final keyed
       hash length.  The internal hash function application does not reduce
       the entropy.

       The 96-bit final keyed hash is of typical size and is believed to be
       secure.  The 128-bit input from the tokens is broken in two pieces,
       the home keygen token and the care-of keygen token.  An attacker can
       try to guess the correct cookie value, but again this would require a
       large number of messages (an the average 2**63 messages for one or
       2**127 for two).  Furthermore, given that the cookies are valid only
       for a short period of time, the attack has to keep a high constant
       message rate to achieve a lasting effect.  This does not appear
       practical.

       When the mobile node is returning home, it is allowed to use just the
       home keygen token of 64 bits.  This is less than 128 bits, but
       attacking it blindly would still require a large number of messages
       to be sent.  If the attacker is on the path and capable of seeing the
       Binding Update, it could conceivably break the keyed hash with brute
       force.  However, in this case the attacker has to be on the path,
       which appears to offer easier ways for denial of service than
       preventing route optimization.

    15.5.  Dynamic Home Agent Address Discovery

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

       The ability to learn addresses of nodes may be useful to attackers
       because brute-force scanning of the address space is not practical
       with IPv6.  Thus, they could benefit from any means that make mapping
       the networks easier.  For example, if a security threat targeted at
       routers or even home agents is discovered, having a simple ICMP
       mechanism to easily find out possible targets may prove to be an
       additional (though minor) security risk.

       This document does not define any authentication mechanism for
       dynamic home agent address discovery messages.  Therefore, the home
       agent cannot verify the home address of the mobile node that
       requested the list of home agents.

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       Apart from discovering the address(es) of home agents, attackers will
       not be able to learn much from this information, and mobile nodes
       cannot be tricked into using wrong home agents, as all other
       communication with the home agents is secure.

       In cases where additional security is needed, one may consider
       instead the use of MIPv6 bootstrapping [22], (based on DNS SRV
       Resource Records [10]) in conjunction with security mechanisms
       suggested in these specifications.  In that solution, security is
       provided by the DNS Security (DNSSEC) [13] framework.  The needed
       pre-configured data on the mobile node for this mechanism is the
       domain name of the mobile service provider, which is marginally
       better than the home subnet prefix.  For the security, a trust anchor
       that dominates the domain is needed.

    15.6.  Mobile Prefix Discovery

       The mobile prefix discovery function may leak interesting information
       about network topology and prefix lifetimes to eavesdroppers; for
       this reason, requests for this information 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
       Section 5.5.

       Binding Updates to the home agents are secure.  When receiving
       tunneled traffic, the home agent verifies that the outer IP address
       corresponds to the current location of the mobile node.  This acts as
       a weak form of protection against spoofing packets that appear to
       come from the mobile node.  This is particularly useful, if no end-
       to-end security is being applied between the mobile and correspondent
       nodes.  The outer IP address check prevents attacks where the
       attacker is controlled by ingress filtering.  It also prevents
       attacks when the attacker does not know the current care-of address
       of the mobile node.  Attackers who know the care-of address and are
       not controlled by ingress filtering could still send traffic through
       the home agent.  This includes attackers on the same local link as
       the mobile node is currently on.  But such attackers could send
       packets that appear to come from the mobile node without attacking

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       the tunnel; the attacker could simply send packets with the source
       address set to the mobile node's home address.  However, this attack
       does not work if the final destination of the packet is in the home
       network, and some form of perimeter defense is being applied for
       packets sent to those destinations.  In such cases it is recommended
       that either end-to-end security or additional tunnel protection be
       applied, as is usual in remote access situations.

       Home agents and mobile nodes may use IPsec ESP to protect payload
       packets tunneled between themselves.  This is useful for protecting
       communications against attackers on the path of the tunnel.

       When a unique-local address (ULA, RFC 4193 [15]) is used as a home
       address, reverse tunneling can be used to send 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 these kinds of
       home addresses.

    15.8.  Home Address Option

       When the mobile node sends packets directly to the correspondent
       node, the Source Address field of the packet's IPv6 header is the
       care-of address.  Therefore, ingress filtering [27] works in the
       usual manner even for mobile nodes, as the Source Address is
       topologically correct.  The Home Address option is used to inform the
       correspondent node of the mobile node's home address.

       However, the care-of address in the Source Address field does not
       survive in replies sent by the correspondent node unless it has a
       binding for this mobile node.  Also, not all attacker tracing
       mechanisms work when packets are being reflected through
       correspondent nodes using the Home Address option.  For these
       reasons, this specification restricts the use of the Home Address
       option.  It may only be used when a binding has already been
       established with the participation of the node at the home address,
       as described in Sections 5.5 and 6.3.  This prevents reflection
       attacks through the use of the Home Address option.  It also ensures
       that the correspondent nodes reply to the same address that the
       mobile node sends traffic from.

       No special authentication of the Home Address option is required
       beyond the above, but note that if the IPv6 header of a packet is
       covered by IPsec Authentication Header, then that authentication
       covers the Home Address option as well.  Thus, even when
       authentication is used in the IPv6 header, the security of the Source
       Address field in the IPv6 header is not compromised by the presence

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       of a Home Address option.  Without authentication of the packet, any
       field in the IPv6 header including the Source Address field or any
       other part of the packet and the Home Address option can be forged or
       modified in transit.  In this case, the contents of the Home Address
       option is no more suspect than any other part of the packet.

    15.9.  Type 2 Routing Header

       The definition of the type 2 routing header is described in
       Section 6.4.  This definition and the associated processing rules
       have been chosen so that the header cannot be used for what is
       traditionally viewed as source routing.  In particular, the home
       address in the routing header will always have to be assigned to the
       home address of the receiving node; otherwise, the packet will be
       dropped.

       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 that implements the filtering of packets
       should be able to distinguish between a type 2 routing header and
       other routing headers, as required in Section 8.3.  This is necessary
       in order to allow Mobile IPv6 traffic while still having the option
       of filtering out other uses of routing headers.

    15.10.  SHA-1 Secure Enough for Mobile IPv6 Control Messages

       This document relies on hash-based message authentication codes
       (HMAC) computed using the SHA-1 [11] hash algorithm for the home
       keygen token and care-of keygen token, as well as the authentication
       fields in the binding update and binding authorization data (see
       Section 5.2.4).  While SHA-1 has been deprecated for some
       cryptographic mechanisms, SHA-1 is considered secure for the
       foreseeable future when used as specified here.  For additional
       details, see [39].

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    16.  Contributors

       Work done by Tuomas Aura, Mike Roe, Greg O'Shea, Pekka Nikander, Erik
       Nordmark, and Michael Thomas shaped the return routability protocols
       described in [35].

       Significant contributions were made by members of the Mobile IPv6
       Security Design Team, including (in alphabetical order) Gabriel
       Montenegro, Pekka Nikander, and Erik Nordmark.

    17.  Acknowledgements

       We would like to thank the members of the Mobile IP, Mobility
       Extensions for IPv6, and IPng Working Groups for their comments and
       suggestions on this work.  We would particularly like to thank (in
       alphabetical order) Fred Baker, Josh Broch, Samita Chakrabarti,
       Robert Chalmers, Noel Chiappa, Jean-Michel Combes, Greg Daley, Vijay
       Devarapalli, Rich Draves, Francis Dupont, Ashutosh Dutta, Arnaud
       Ebalard, Wesley Eddy, Thomas Eklund, Jun-Ichiro Itojun Hagino, Brian
       Haley, Marc Hasson, John Ioannidis, James Kempf, Rajeev Koodli,
       Suresh Krishnan, Krishna Kumar, T.J. Kniveton, Joe Lau, Aime Le
       Rouzic, Julien Laganier, Jiwoong Lee, Benjamin Lim, Vesa-Matti
       Mantyla, Kevin Miles, Glenn Morrow, Ahmad Muhanna, Thomas Narten,
       Karen Nielsen, Simon Nybroe, David Oran, Mohan Parthasarathy,
       Basavaraj Patil, Brett Pentland, Lars Henrik Petander, Alexandru
       Petrescu, Mattias Petterson, Ken Powell, Ed Remmell, Phil Roberts,
       Patrice Romand, Luis A. Sanchez, Pekka Savola, Jeff Schiller, Arvind
       Sevalkar, Keiichi Shima, Tom Soderlund, Hesham Soliman, Jim Solomon,
       Tapio Suihko, Dave Thaler, Pascal Thubert, Benny Van Houdt, Jon-Olov
       Vatn, Ryuji Wakikawa, Kilian Weniger, Carl E. Williams, Vladislav
       Yasevich, Alper Yegin, and Xinhua Zhao, for their detailed reviews of
       earlier versions of this document.  Their suggestions have helped to
       improve both the design and presentation of the protocol.

       We would also like to thank the participants of the Mobile IPv6
       testing event (1999), implementers who participated in Mobile IPv6
       interoperability testing at Connectathons (2000, 2001, 2002, and
       2003), and the participants at the ETSI interoperability testing
       (2000, 2002).  Finally, we would like to thank the TAHI project that
       has provided test suites for Mobile IPv6.

    18.  References

    18.1.  Normative References

       [1]   Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
             for Message Authentication", RFC 2104, February 1997.

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       [2]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
             Levels", BCP 14, RFC 2119, March 1997.

       [3]   Kent, S. and K. Seo, "Security Architecture for the Internet
             Protocol", RFC 4301, December 2005.

       [4]   Kent, S., "IP Authentication Header", RFC 4302, December 2005.

       [5]   Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
             December 2005.

       [6]   Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
             Specification", RFC 2460, December 1998.

       [7]   Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6
             Specification", RFC 2473, December 1998.

       [8]   Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
             Addresses", RFC 2526, March 1999.

       [9]   Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
             Discovery (MLD) for IPv6", RFC 2710, October 1999.

       [10]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
             specifying the location of services (DNS SRV)", RFC 2782,
             February 2000.

       [11]  National Institute of Standards and Technology, "Secure Hash
             Standard", FIPS PUB 180-1, April 1995,
             <http://www.itl.nist.gov/fipspubs/fip180-1.htm>.

       [12]  Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to
             Protect Mobile IPv6 Signaling Between Mobile Nodes and Home
             Agents", RFC 3776, June 2004.

       [13]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
             "DNS Security Introduction and Requirements", RFC 4033,
             March 2005.

       [14]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
             Requirements for Security", BCP 106, RFC 4086, June 2005.

       [15]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
             Addresses", RFC 4193, October 2005.

       [16]  Hinden, R. and S. Deering, "IP Version 6 Addressing
             Architecture", RFC 4291, February 2006.

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       [17]  Conta, A., Deering, S., and M. Gupta, "Internet Control Message
             Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)
             Specification", RFC 4443, March 2006.

       [18]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
             "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
             September 2007.

       [19]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address
             Autoconfiguration", RFC 4862, September 2007.

       [20]  Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with
             IKEv2 and the Revised IPsec Architecture", RFC 4877,
             April 2007.

       [21]  Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions
             for Stateless Address Autoconfiguration in IPv6", RFC 4941,
             September 2007.

       [22]  Giaretta, G., Kempf, J., and V. Devarapalli, "Mobile IPv6
             Bootstrapping in Split Scenario", RFC 5026, October 2007.

       [23]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
             Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.

       [24]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, "Internet Key
             Exchange Protocol Version 2 (IKEv2)", RFC 5996, September 2010.

    18.2.  Informative References

       [25]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
             October 1996.

       [26]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
             October 1996.

       [27]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
             Defeating Denial of Service Attacks which employ IP Source
             Address Spoofing", BCP 38, RFC 2827, May 2000.

       [28]  Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses", Work
             in Progress, March 2002.

       [29]  Krishnan, S. and G. Tsirtsis, "MIPv6 Home Link Detection", Work
             in Progress, March 2008.

       [30]  Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On-
             line Database", RFC 3232, January 2002.

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       [31]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
             Carney, "Dynamic Host Configuration Protocol for IPv6
             (DHCPv6)", RFC 3315, July 2003.

       [32]  Perkins, C., "IP Mobility Support for IPv4, Revised", RFC 5944,
             November 2010.

       [33]  Draves, R., "Default Address Selection for Internet Protocol
             version 6 (IPv6)", RFC 3484, February 2003.

       [34]  Nordmark, E., "Securing MIPv6 BUs using return routability
             (BU3WAY)", Work in Progress, November 2001.

       [35]  Roe, M., "Authentication of Mobile IPv6 Binding Updates and
             Acknowledgments", Work in Progress, March 2002.

       [36]  Chowdhury, K. and A. Yegin, "MIP6-bootstrapping for the
             Integrated Scenario", Work in Progress, April 2008.

       [37]  Savola, P., "Use of /127 Prefix Length Between Routers
             Considered Harmful", RFC 3627, September 2003.

       [38]  Savola, P., "Security of IPv6 Routing Header and Home Address
             Options", Work in Progress, March 2002.

       [39]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
             Considerations for the SHA-0 and SHA-1 Message-Digest
             Algorithms", RFC 6194, March 2011.

       [40]  Manner, J. and M. Kojo, "Mobility Related Terminology",
             RFC 3753, June 2004.

       [41]  Vida, R. and L. Costa, "Multicast Listener Discovery Version 2
             (MLDv2) for IPv6", RFC 3810, June 2004.

       [42]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key
             Management", BCP 107, RFC 4107, June 2005.

       [43]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
             Nordmark, "Mobile IP Version 6 Route Optimization Security
             Design Background", RFC 4225, December 2005.

       [44]  Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6 Socket
             API for Source Address Selection", RFC 5014, September 2007.

       [45]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation of
             Type 0 Routing Headers in IPv6", RFC 5095, December 2007.

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    Appendix A.  Future Extensions

    A.1.  Piggybacking

       This document does not specify how to piggyback payload packets on
       the binding-related messages.  However, it is envisioned that this
       can be specified in a separate document when issues such as the
       interaction between piggybacking and IPsec are fully resolved (see
       also Appendix A.3).  The return routability messages can indicate
       support for piggybacking with a new mobility option.

    A.2.  Triangular Routing

       Due to the concerns about opening reflection attacks with the Home
       Address destination option, this specification requires that this
       option be verified against the Binding Cache, i.e., there must be a
       Binding Cache entry for the home address and care-of address.

       Future extensions may be specified that allow the use of unverified
       Home Address destination options in ways that do not introduce
       security issues.

    A.3.  New Authorization Methods

       While the return routability procedure provides a good level of
       security, there exist methods that have even higher levels of
       security.  Second, as discussed in Section 15.4, future enhancements
       of IPv6 security may cause a need to also improve the security of the
       return routability procedure.  Using IPsec as the sole method for
       authorizing Binding Updates to correspondent nodes is also possible.
       The protection of the Mobility Header for this purpose is easy,
       though one must ensure that the IPsec SA was created with appropriate
       authorization to use the home address referenced in the Binding
       Update.  For instance, a certificate used by IKEv2 to create the
       security association might contain the home address.  A future
       specification may specify how this is done.

    A.4.  Neighbor Discovery Extensions

       Future specifications may improve the efficiency of Neighbor
       Discovery tasks, which could be helpful for fast movements.  One
       factor is currently being looked at: the delays caused by the
       Duplicate Address Detection mechanism.  Currently, Duplicate Address
       Detection needs to be performed for every new care-of address as the
       mobile node moves, and for the mobile node's link-local address on
       every new link.  In particular, the need and the trade-offs of
       re-performing Duplicate Address Detection for the link-local address
       every time the mobile node moves on to new links will need to be

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       examined.  Improvements in this area are, however, generally
       applicable and progress independently from the Mobile IPv6
       specification.

       Future functional improvements may also be relevant for Mobile IPv6
       and other applications.  For instance, mechanisms that would allow
       recovery from a Duplicate Address Detection collision would be useful
       for link-local, care-of, and home addresses.

    Appendix B.  Changes since RFC 3775

       The following issues were identified during the evolution of the
       current document.  Discussion about most of the issues can be found
       on the [mext] working group page
       http://trac.tools.ietf.org/wg/mext/trac/report/6

       Issue #1  Last Accepted SQN [Ahmad Muhanna]

          Solution: specify that the mobile node update its binding sequence
          number to match the sequence number given in the Binding
          Acknowledgement (if the Binding Acknowledgement correctly passes
          authentication and the status is 135 (Sequence Number out of
          window).  See Section 11.7.3.

       Issue #4  Remove references to site-local addresses [George
          Tsirtsis].

          Fixed.

       Issue #5  Wrong protocol number (2 instead of 135) used in discussion
          about checksum pseudo-header.

          Fixed.  See Section 6.1.1.

       Issue #8  Application using the care-of address [Julien Laganier]

          Cite IPv6 Socket API for Source Address Selection specification
          [44].  See Section 11.3.4.

       Issue #10  The usage of "HA lifetime" [Ryuji Wakikawa]

          The mobile node SHOULD store the list of home agents for later use
          in case the home agent currently managing the mobile node's
          care-of address forwarding should become unavailable.  See
          Section 11.4.1.

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       Issue #11  De-registration when returning home [Vijay Devarapalli]

          To be able to send and receive packets using its home address from
          the home link, the mobile node MUST send a Binding Update to its
          home agent to instruct its home agent to no longer intercept or
          tunnel packets for it.  Until the mobile node sends such a
          de-registration Binding Update, it MUST NOT attempt to send and
          receive packets using its home address from the home link.  See
          Section 11.5.5.

       Issue #12  BErr sent by HA too, not only by CN [Alexandru Petrescu]

          Fixed.  See Section 4.2.

       Issue #13  Home Link Detection [Suresh Krishnan]

          Proposal: Add Section 11.5.2 for Home Link Detection, drawing on
          "MIPv6 Home Link Detection" [29].

       Issue #14  References to bootstrapping [Vijay Devarapalli]

          Cite "Mobile IPv6 Bootstrapping in Split Scenario" [22] and "MIP6-
          bootstrapping for the Integrated Scenario" [36].  See Section 4.1.

       Issue #17  Multi-homed mobile node can cause routing loop between
          home agents [Benjamin Lim]

          Added security advisory in Section 15.1, to highlight risk of
          routing loop among HAs (e.g., in 3GPP):

          A malicious mobile node associated to multiple home agents could
          create a routing loop amongst them.  This would happen when a
          mobile node binds one home address located on a first home agent
          to another home address on a second home agent.

       Issue #18  Subject: Issues regarding Home Address Option and ICMP /
          Binding Errors [Fabian Mauchle]

          Proposal: Use the value in the Next Header field {50 (ESP), 51
          (AH), 135 (Mobility Header)} to determine, if a Binding Cache
          entry is required.  See Section 9.3.1.

          Proposal: If the Binding Error message was sent by the home agent,
          the mobile node SHOULD send a Binding Update to the home agent
          according to Section 11.7.1.  See Section 11.3.6.

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       Issue #19  BU de-registration race condition [Kilian Weniger]

          Problem arises if de-registration arrives at home agent before an
          immediately preceding Binding Update.

          Solution: Home agent defers BCE removal after sending the Binding
          Acknowledgement.  See Section 10.3.2.

       Issue #6  Minor editorial corrections and updates.

          Update IPsec and IKE references to the revised IPsec architecture
          and IKEv2.

          Update HMAC_SHA1 [1] to Normative instead of Informational.

          Include discussion (see Section 15.10) to inform implementers that
          HMAC_SHA1 is considered to offer sufficient protection for control
          messages as required by Mobile IPv6.

    Authors' Addresses

       Charles E. Perkins (editor)
       Tellabs, Inc.
       4555 Great America Parkway, Suite 150
       Santa Clara  CA 95054
       USA

       EMail: charliep@computer.org

       David B. Johnson
       Rice University
       Dept. of Computer Science, MS 132
       6100 Main Street
       Houston  TX 77005-1892
       USA

       EMail: dbj@cs.rice.edu

       Jari Arkko
       Ericsson
       Jorvas  02420
       Finland

       EMail: jari.arkko@ericsson.com

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