Movatterモバイル変換

Network Working Group                                         D. JohnsonRequest for Comments: 3775                               Rice UniversityCategory: Standards Track                                     C. Perkins                                                   Nokia Research Center                                                                J. Arkko                                                                Ericsson                                                               June 2004Mobility Support in IPv6Status of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2004).Abstract   This document specifies a protocol which allows nodes to remain   reachable while moving around in the IPv6 Internet.  Each mobile node   is always identified by its home address, regardless of its current   point of attachment to the Internet.  While situated away from its   home, a mobile node is also associated with a care-of address, which   provides information about the mobile node's current location.  IPv6   packets addressed to a mobile node's home address are transparently   routed to its care-of address.  The protocol enables IPv6 nodes to   cache the binding of a mobile node's home address with its care-of   address, and to then send any packets destined for the mobile node   directly to it at this care-of address.  To support this operation,   Mobile IPv6 defines a new IPv6 protocol and a new destination option.   All IPv6 nodes, whether mobile or stationary, can communicate with   mobile nodes.Johnson, et al.              Standard Track                     [Page 1]

RFC 3775                Mobility Support in IPv6               June 2004Table of Contents1.     Introduction . . . . . . . . . . . . . . . . . . . . . . .52.     Comparison with Mobile IP for IPv4 . . . . . . . . . . . .63.     Terminology. . . . . . . . . . . . . . . . . . . . . . . .73.1.   General Terms . . . . . . . . . . . . . . . . . . .83.2.   Mobile IPv6 Terms . . . . . . . . . . . . . . . . .104.     Overview of Mobile IPv6. . . . . . . . . . . . . . . . . .134.1.   Basic Operation . . . . . . . . . . . . . . . . . .134.2.   New IPv6 Protocol . . . . . . . . . . . . . . . . .154.3.   New IPv6 Destination Option . . . . . . . . . . . .174.4.   New IPv6 ICMP Messages. . . . . . . . . . . . . . .174.5.   Conceptual Data Structure Terminology . . . . . . .174.6.   Site-Local Addressability . . . . . . . . . . . . .185.     Overview of Mobile IPv6 Security . . . . . . . . . . . . .185.1.   Binding Updates to Home Agents. . . . . . . . . . .185.2.   Binding Updates to Correspondent Nodes. . . . . . .205.2.1.  Node Keys . . . . . . . . . . . . . . . . .205.2.2.  Nonces. . . . . . . . . . . . . . . . . . .205.2.3.  Cookies and Tokens. . . . . . . . . . . . .215.2.4.  Cryptographic Functions . . . . . . . . . .225.2.5.  Return Routability Procedure. . . . . . . .225.2.6.  Authorizing Binding Management Messages . .275.2.7.  Updating Node Keys and Nonces . . . . . . .295.2.8.  Preventing Replay Attacks . . . . . . . . .305.3.   Dynamic Home Agent Address Discovery. . . . . . . .305.4.   Mobile Prefix Discovery . . . . . . . . . . . . . .305.5.   Payload Packets . . . . . . . . . . . . . . . . . .30   6.     New IPv6 Protocol, Message Types, and Destination Option .  316.1.   Mobility Header . . . . . . . . . . . . . . . . . .316.1.1.  Format. . . . . . . . . . . . . . . . . . .326.1.2.  Binding Refresh Request Message . . . . . .346.1.3.  Home Test Init Message. . . . . . . . . . .356.1.4.  Care-of Test Init Message . . . . . . . . .366.1.5.  Home Test Message . . . . . . . . . . . . .376.1.6.  Care-of Test Message. . . . . . . . . . . .386.1.7.  Binding Update Message. . . . . . . . . . .396.1.8.  Binding Acknowledgement Message . . . . . .426.1.9.  Binding Error Message . . . . . . . . . . .446.2.   Mobility Options. . . . . . . . . . . . . . . . . .466.2.1.  Format. . . . . . . . . . . . . . . . . . .466.2.2.  Pad1. . . . . . . . . . . . . . . . . . . .476.2.3.  PadN. . . . . . . . . . . . . . . . . . . .486.2.4.  Binding Refresh Advice. . . . . . . . . . .486.2.5.  Alternate Care-of Address . . . . . . . . .496.2.6.  Nonce Indices . . . . . . . . . . . . . . .496.2.7.  Binding Authorization Data. . . . . . . . .506.3.   Home Address Option . . . . . . . . . . . . . . . .51Johnson, et al.              Standard Track                     [Page 2]

RFC 3775                Mobility Support in IPv6               June 20046.4.   Type 2 Routing Header . . . . . . . . . . . . . . .536.4.1.  Format. . . . . . . . . . . . . . . . . . .54          6.5.   ICMP Home Agent Address Discovery Request Message .  556.6.   ICMP Home Agent Address Discovery Reply Message . .566.7.   ICMP Mobile Prefix Solicitation Message Format. . .576.8.   ICMP Mobile Prefix Advertisement Message Format . .597.     Modifications to IPv6 Neighbor Discovery . . . . . . . . .617.1.   Modified Router Advertisement Message Format. . . .617.2.   Modified Prefix Information Option Format . . . . .627.3.   New Advertisement Interval Option Format. . . . . .647.4.   New Home Agent Information Option Format. . . . . .657.5.   Changes to Sending Router Advertisements. . . . . .678.     Requirements for Types of IPv6 Nodes . . . . . . . . . . .698.1.   All IPv6 Nodes. . . . . . . . . . . . . . . . . . .698.2.   IPv6 Nodes with Support for Route Optimization. . .698.3.   All IPv6 Routers. . . . . . . . . . . . . . . . . .718.4.   IPv6 Home Agents. . . . . . . . . . . . . . . . . .718.5.   IPv6 Mobile Nodes . . . . . . . . . . . . . . . . .739.     Correspondent Node Operation . . . . . . . . . . . . . . .749.1.   Conceptual Data Structures. . . . . . . . . . . . .749.2.   Processing Mobility Headers . . . . . . . . . . . .759.3.   Packet Processing . . . . . . . . . . . . . . . . .76                 9.3.1.  Receiving Packets with Home Address Option.  769.3.2.  Sending Packets to a Mobile Node. . . . . .779.3.3.  Sending Binding Error Messages. . . . . . .789.3.4.  Receiving ICMP Error Messages . . . . . . .799.4.   Return Routability Procedure. . . . . . . . . . . .799.4.1.  Receiving Home Test Init Messages . . . . .809.4.2.  Receiving Care-of Test Init Messages. . . .809.4.3.  Sending Home Test Messages. . . . . . . . .809.4.4.  Sending Care-of Test Messages . . . . . . .819.5    Processing Bindings . . . . . . . . . . . . . . . .819.5.1.  Receiving Binding Updates . . . . . . . . .819.5.2.  Requests to Cache a Binding . . . . . . . .849.5.3.  Requests to Delete a Binding. . . . . . . .849.5.4.  Sending Binding Acknowledgements. . . . . .859.5.5.  Sending Binding Refresh Requests. . . . . .869.6.   Cache Replacement Policy. . . . . . . . . . . . . .8610.    Home Agent Operation . . . . . . . . . . . . . . . . . . .8710.1.  Conceptual Data Structures. . . . . . . . . . . . .8710.2.  Processing Mobility Headers . . . . . . . . . . . .8810.3.  Processing Bindings . . . . . . . . . . . . . . . .8810.3.1. Primary Care-of Address Registration. . . .8810.3.2. Primary Care-of Address De-Registration . .9210.4.  Packet Processing . . . . . . . . . . . . . . . . .9410.4.1. Intercepting Packets for a Mobile Node. . .9410.4.2. Processing Intercepted Packets. . . . . . .9510.4.3. Multicast Membership Control. . . . . . . .96Johnson, et al.              Standard Track                     [Page 3]

RFC 3775                Mobility Support in IPv6               June 200410.4.4. Stateful Address Autoconfiguration. . . . .9810.4.5. Handling Reverse Tunneled Packets . . . . .9810.4.6. Protecting Return Routability Packets . . .9910.5.  Dynamic Home Agent Address Discovery. . . . . . . .9910.5.1. Receiving Router Advertisement Messages . .10010.6.  Sending Prefix Information to the Mobile Node . . .10210.6.1. List of Home Network Prefixes . . . . . . .10210.6.2. Scheduling Prefix Deliveries. . . . . . . .10210.6.3. Sending Advertisements. . . . . . . . . . .10410.6.4. Lifetimes for Changed Prefixes. . . . . . .10511.    Mobile Node Operation. . . . . . . . . . . . . . . . . . .10511.1.  Conceptual Data Structures. . . . . . . . . . . . .10511.2.  Processing Mobility Headers . . . . . . . . . . . .10711.3.  Packet Processing . . . . . . . . . . . . . . . . .10711.3.1. Sending Packets While Away from Home. . . .107                 11.3.2. Interaction with Outbound IPsec Processing. 11011.3.3. Receiving Packets While Away from Home. . .11211.3.4. Routing Multicast Packets . . . . . . . . .11411.3.5. Receiving ICMP Error Messages . . . . . . .11511.3.6. Receiving Binding Error Messages. . . . . .11611.4.  Home Agent and Prefix Management. . . . . . . . . .11711.4.1. Dynamic Home Agent Address Discovery. . . .11711.4.2. Sending Mobile Prefix Solicitations . . . .11811.4.3. Receiving Mobile Prefix Advertisements. . .11811.5.  Movement. . . . . . . . . . . . . . . . . . . . . .12011.5.1. Movement Detection. . . . . . . . . . . . .12011.5.2. Forming New Care-of Addresses . . . . . . .12211.5.3. Using Multiple Care-of Addresses. . . . . .12311.5.4. Returning Home. . . . . . . . . . . . . . .12411.6.  Return Routability Procedure. . . . . . . . . . . .12611.6.1. Sending Test Init Messages. . . . . . . . .12611.6.2. Receiving Test Messages . . . . . . . . . .12711.6.3. Protecting Return Routability Packets . . .12811.7.  Processing Bindings . . . . . . . . . . . . . . . .128                 11.7.1. Sending Binding Updates to the Home Agent . 12811.7.2. Correspondent Registration. . . . . . . . .13111.7.3. Receiving Binding Acknowledgements. . . . .13411.7.4. Receiving Binding Refresh Requests. . . . .13611.8.  Retransmissions and Rate Limiting . . . . . . . . .13712.    Protocol Constants . . . . . . . . . . . . . . . . . . . .13813.    Protocol Configuration Variables . . . . . . . . . . . . .13814.    IANA Considerations. . . . . . . . . . . . . . . . . . . .13915.    Security Considerations. . . . . . . . . . . . . . . . . .14215.1.  Threats . . . . . . . . . . . . . . . . . . . . . .14215.2.  Features. . . . . . . . . . . . . . . . . . . . . .14415.3.  Binding Updates to Home Agent . . . . . . . . . . .14515.4.  Binding Updates to Correspondent Nodes. . . . . . .14815.4.1. Overview. . . . . . . . . . . . . . . . . .149Johnson, et al.              Standard Track                     [Page 4]

RFC 3775                Mobility Support in IPv6               June 200415.4.2. Achieved Security Properties. . . . . . . .149                 15.4.3. Comparison to Regular IPv6 Communications . 15015.4.4. Replay Attacks. . . . . . . . . . . . . . .15215.4.5. Denial-of-Service Attacks . . . . . . . . .15215.4.6. Key Lengths . . . . . . . . . . . . . . . .15315.5.  Dynamic Home Agent Address Discovery. . . . . . . .15415.6.  Mobile Prefix Discovery . . . . . . . . . . . . . .15515.7.  Tunneling via the Home Agent. . . . . . . . . . . .15515.8.  Home Address Option . . . . . . . . . . . . . . . .15615.9.  Type 2 Routing Header . . . . . . . . . . . . . . .15616.    Contributors . . . . . . . . . . . . . . . . . . . . . . .15717.    Acknowledgements . . . . . . . . . . . . . . . . . . . . .15718.    References . . . . . . . . . . . . . . . . . . . . . . . .15818.1.  Normative References. . . . . . . . . . . . . . . .15818.2.  Informative References. . . . . . . . . . . . . . .159Appendix A.   Future Extensions . . . . . . . . . . . . . . . . .161A.1.   Piggybacking. . . . . . . . . . . . . . . . . . . .161A.2.   Triangular Routing. . . . . . . . . . . . . . . . .161A.3.   New Authorization Methods . . . . . . . . . . . . .161A.4.   Dynamically Generated Home Addresses. . . . . . . .161A.5.   Remote Home Address Configuration . . . . . . . . .162A.6.   Neighbor Discovery Extensions . . . . . . . . . . .163   Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . .164   Full Copyright Statement. . . . . . . . . . . . . . . . . . . . .1651.  Introduction   This document specifies a protocol which allows nodes to remain   reachable while moving around in the IPv6 Internet.  Without specific   support for mobility in IPv6 [11], packets destined to a mobile node   would not be able to reach it while the mobile node is away from its   home link.  In order to continue communication in spite of its   movement, a mobile node could change its IP address each time it   moves to a new link, but the mobile node would then not be able to   maintain transport and higher-layer connections when it changes   location.  Mobility support in IPv6 is particularly important, as   mobile computers are likely to account for a majority or at least a   substantial fraction of the population of the Internet during the   lifetime of IPv6.   The protocol defined in this document, known as Mobile IPv6, allows a   mobile node to move from one link to another without changing the   mobile node's "home address".  Packets may be routed to the mobile   node using this address regardless of the mobile node's current point   of attachment to the Internet.  The mobile node may also continue to   communicate with other nodes (stationary or mobile) after moving to aJohnson, et al.              Standard Track                     [Page 5]

RFC 3775                Mobility Support in IPv6               June 2004   new link.  The movement of a mobile node away from its home link is   thus transparent to transport and higher-layer protocols and   applications.   The Mobile IPv6 protocol is just as suitable for mobility across   homogeneous media as for mobility across heterogeneous media.  For   example, Mobile IPv6 facilitates node movement from one Ethernet   segment to another as well as it facilitates node movement from an   Ethernet segment to a wireless LAN cell, with the mobile node's IP   address remaining unchanged in spite of such movement.   One can think of the Mobile IPv6 protocol as solving the network-   layer mobility management problem.  Some mobility management   applications -- for example, handover among wireless transceivers,   each of which covers only a very small geographic area -- have been   solved using link-layer techniques.  For example, in many current   wireless LAN products, link-layer mobility mechanisms allow a   "handover" of a mobile node from one cell to another, re-establishing   link-layer connectivity to the node in each new location.   Mobile IPv6 does not attempt to solve all general problems related to   the use of mobile computers or wireless networks.  In particular,   this protocol does not attempt to solve:   o  Handling links with unidirectional connectivity or partial      reachability, such as the hidden terminal problem where a host is      hidden from only some of the routers on the link.   o  Access control on a link being visited by a mobile node.   o  Local or hierarchical forms of mobility management (similar to      many current link-layer mobility management solutions).   o  Assistance for adaptive applications.   o  Mobile routers.   o  Service Discovery.   o  Distinguishing between packets lost due to bit errors vs.  network      congestion.2.  Comparison with Mobile IP for IPv4   The design of Mobile IP support in IPv6 (Mobile IPv6) benefits both   from the experiences gained from the development of Mobile IP support   in IPv4 (Mobile IPv4) [22,23,24], and from the opportunities   provided by IPv6.  Mobile IPv6 thus shares many features with MobileJohnson, et al.              Standard Track                     [Page 6]

RFC 3775                Mobility Support in IPv6               June 2004   IPv4, but is integrated into IPv6 and offers many other improvements.   This section summarizes the major differences between Mobile IPv4 and   Mobile IPv6:   o  There is no need to deploy special routers as "foreign agents", as      in Mobile IPv4.  Mobile IPv6 operates in any location without any      special support required from the local router.   o  Support for route optimization is a fundamental part of the      protocol, rather than a nonstandard set of extensions.   o  Mobile IPv6 route optimization can operate securely even without      pre-arranged security associations.  It is expected that route      optimization can be deployed on a global scale between all mobile      nodes and correspondent nodes.   o  Support is also integrated into Mobile IPv6 for allowing route      optimization to coexist efficiently with routers that perform      "ingress filtering" [26].   o  The IPv6 Neighbor Unreachability Detection assures symmetric      reachability between the mobile node and its default router in the      current location.   o  Most packets sent to a mobile node while away from home in Mobile      IPv6 are sent using an IPv6 routing header rather than IP      encapsulation, reducing the amount of resulting overhead compared      to Mobile IPv4.   o  Mobile IPv6 is decoupled from any particular link layer, as it      uses IPv6 Neighbor Discovery [12] instead of ARP.  This also      improves the robustness of the protocol.   o  The use of IPv6 encapsulation (and the routing header) removes the      need in Mobile IPv6 to manage "tunnel soft state".   o  The dynamic home agent address discovery mechanism in Mobile IPv6      returns a single reply to the mobile node.  The directed broadcast      approach used in IPv4 returns separate replies from each home      agent.3.  Terminology   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inBCP 14,RFC 2119 [2].Johnson, et al.              Standard Track                     [Page 7]

RFC 3775                Mobility Support in IPv6               June 20043.1.  General Terms   IP      Internet Protocol Version 6 (IPv6).   node      A device that implements IP.   router      A node that forwards IP packets not explicitly addressed to      itself.   unicast routable address      An identifier for a single interface such that a packet sent to it      from another IPv6 subnet is delivered to the interface identified      by that address.  Accordingly, a unicast routable address must      have either a global or site-local scope (but not link-local).   host      Any node that is not a router.   link      A communication facility or medium over which nodes can      communicate at the link layer, such as an Ethernet (simple or      bridged).  A link is the layer immediately below IP.   interface      A node's attachment to a link.   subnet prefix      A bit string that consists of some number of initial bits of an IP      address.   interface identifier      A number used to identify a node's interface on a link.  The      interface identifier is the remaining low-order bits in the node's      IP address after the subnet prefix.Johnson, et al.              Standard Track                     [Page 8]

RFC 3775                Mobility Support in IPv6               June 2004   link-layer address      A link-layer identifier for an interface, such as IEEE 802      addresses on Ethernet links.   packet      An IP header plus payload.   security association      An IPsec security association is a cooperative relationship formed      by the sharing of cryptographic keying material and associated      context.  Security associations are simplex.  That is, two      security associations are needed to protect bidirectional traffic      between two nodes, one for each direction.   security policy database      A database that specifies what security services are to be offered      to IP packets and in what fashion.   destination option      Destination options are carried by the IPv6 Destination Options      extension header.  Destination options include optional      information that need be examined only by the IPv6 node given as      the destination address in the IPv6 header, not by routers in      between.  Mobile IPv6 defines one new destination option, the Home      Address destination option (seeSection 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 inSection 6.4.   "|" (concatenation)      Some formulas in this specification use the symbol "|" to indicate      bytewise concatenation, as in A | B.  This concatenation requires      that all of the octets of the datum A appear first in the result,      followed by all of the octets of the datum B.Johnson, et al.              Standard Track                     [Page 9]

RFC 3775                Mobility Support in IPv6               June 2004   First (size, input)      Some formulas in this specification use a functional form "First      (size, input)" to indicate truncation of the "input" data so that      only the first "size" bits remain to be used.3.2.  Mobile IPv6 Terms   home address      A unicast routable address assigned to a mobile node, used as the      permanent address of the mobile node.  This address is within the      mobile node's home link.  Standard IP routing mechanisms will      deliver packets destined for a mobile node's home address to its      home link.  Mobile nodes can have multiple home addresses, for      instance when there are multiple home prefixes on the home link.   home subnet prefix      The IP subnet prefix corresponding to a mobile node's home      address.   home link      The link on which a mobile node's home subnet prefix is defined.   mobile node      A node that can change its point of attachment from one link to      another, while still being reachable via its home address.   movement      A change in a mobile node's point of attachment to the Internet      such that it is no longer connected to the same link as it was      previously.  If a mobile node is not currently attached to its      home link, the mobile node is said to be "away from home".   L2 handover      A process by which the mobile node changes from one link-layer      connection to another.  For example, a change of wireless access      point is an L2 handover.Johnson, et al.              Standard Track                    [Page 10]

RFC 3775                Mobility Support in IPv6               June 2004   L3 handover      Subsequent to an L2 handover, a mobile node detects a change in an      on-link subnet prefix that would require a change in the primary      care-of address.  For example, a change of access router      subsequent to a change of wireless access point typically results      in an L3 handover.   correspondent node      A peer node with which a mobile node is communicating.  The      correspondent node may be either mobile or stationary.   foreign subnet prefix      Any IP subnet prefix other than the mobile node's home subnet      prefix.   foreign link      Any link other than the mobile node's home link.   care-of address      A unicast routable address associated with a mobile node while      visiting a foreign link; the subnet prefix of this IP address is a      foreign subnet prefix.  Among the multiple care-of addresses that      a mobile node may have at any given time (e.g., with different      subnet prefixes), the one registered with the mobile node's home      agent for a given home address is called its "primary" care-of      address.   home agent      A router on a mobile node's home link with which the mobile node      has registered its current care-of address.  While the mobile node      is away from home, the home agent intercepts packets on the home      link destined to the mobile node's home address, encapsulates      them, and tunnels them to the mobile node's registered care-of      address.   binding      The association of the home address of a mobile node with a care-      of address for that mobile node, along with the remaining lifetime      of that association.Johnson, et al.              Standard Track                    [Page 11]

RFC 3775                Mobility Support in IPv6               June 2004   registration      The process during which a mobile node sends a Binding Update to      its home agent or a correspondent node, causing a binding for the      mobile node to be registered.   mobility message      A message containing a Mobility Header (seeSection 6.1).   binding authorization      Correspondent registration needs to be authorized to allow the      recipient to believe that the sender has the right to specify a      new binding.   return routability procedure      The return routability procedure authorizes registrations by the      use of a cryptographic token exchange.   correspondent registration      A return routability procedure followed by a registration, run      between the mobile node and a correspondent node.   home registration      A registration between the mobile node and its home agent,      authorized by the use of IPsec.   nonce      Nonces are random numbers used internally by the correspondent      node in the creation of keygen tokens related to the return      routability procedure.  The nonces are not specific to a mobile      node, and are kept secret within the correspondent node.   nonce index      A nonce index is used to indicate which nonces have been used when      creating keygen token values, without revealing the nonces      themselves.Johnson, et al.              Standard Track                    [Page 12]

RFC 3775                Mobility Support in IPv6               June 2004   cookie      A cookie is a random number used by a mobile node to prevent      spoofing by a bogus correspondent node in the return routability      procedure.   care-of init cookie      A cookie sent to the correspondent node in the Care-of Test Init      message, to be returned in the Care-of Test message.   home init cookie      A cookie sent to the correspondent node in the Home Test Init      message, to be returned in the Home Test message.   keygen token      A keygen token is a number supplied by a correspondent node in the      return routability procedure to enable the mobile node to compute      the necessary binding management key for authorizing a Binding      Update.   care-of keygen token      A keygen token sent by the correspondent node in the Care-of Test      message.   home keygen token      A keygen token sent by the correspondent node in the Home Test      message.   binding management key (Kbm)      A binding management key (Kbm) is a key used for authorizing a      binding cache management message (e.g., Binding Update or Binding      Acknowledgement).  Return routability provides a way to create a      binding management key.4.  Overview of Mobile IPv64.1.  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 aJohnson, et al.              Standard Track                    [Page 13]

RFC 3775                Mobility Support in IPv6               June 2004   mobile node is at home, packets addressed to its home address are   routed to the mobile node's home link, using conventional Internet   routing mechanisms.   While a mobile node is attached to some foreign link away from home,   it is also addressable at one or more care-of addresses.  A care-of   address is an IP address associated with a mobile node that has the   subnet prefix of a particular foreign link.  The mobile node can   acquire its care-of address through conventional IPv6 mechanisms,   such as stateless or stateful auto-configuration.  As long as the   mobile node stays in this location, packets addressed to this care-of   address will be routed to the mobile node.  The mobile node may also   accept packets from several care-of addresses, such as when it is   moving but still reachable at the previous link.   The association between a mobile node's home address and care-of   address is known as a "binding" for the mobile node.  While away from   home, a mobile node registers its primary care-of address with a   router on its home link, requesting this router to function as the   "home agent" for the mobile node.  The mobile node performs this   binding registration by sending a "Binding Update" message to the   home agent.  The home agent replies to the mobile node by returning a   "Binding Acknowledgement" message.  The operation of the mobile node   is specified inSection 11, and the operation of the home agent is   specified inSection 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 inSection 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 homeJohnson, et al.              Standard Track                    [Page 14]

RFC 3775                Mobility Support in IPv6               June 2004   address (or home addresses) on the home link.  Each intercepted   packet is tunneled to the mobile node's primary care-of address.   This tunneling is performed using IPv6 encapsulation [15].   The second mode, "route optimization", requires the mobile node to   register its current binding at the correspondent node.  Packets from   the correspondent node can be routed directly to the care-of address   of the mobile node.  When sending a packet to any IPv6 destination,   the correspondent node checks its cached bindings for an entry for   the packet's destination address.  If a cached binding for this   destination address is found, the node uses a new type of IPv6   routing header [11] (seeSection 6.4) to route the packet to the   mobile node by way of the care-of address indicated in this binding.   Routing packets directly to the mobile node's care-of address allows   the shortest communications path to be used.  It also eliminates   congestion at the mobile node's home agent and home link.  In   addition, the impact of any possible failure of the home agent or   networks on the path to or from it is reduced.   When routing packets directly to the mobile node, the correspondent   node sets the Destination Address in the IPv6 header to the care-of   address of the mobile node.  A new type of IPv6 routing header (seeSection 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 (seeSection6.3) 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 fromSection 6.5.4.2.  New IPv6 Protocol   Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header   (seeSection 6.1).  This Header is used to carry the following   messages:Johnson, et al.              Standard Track                    [Page 15]

RFC 3775                Mobility Support in IPv6               June 2004   Home Test Init   Home Test   Care-of Test Init   Care-of Test      These four messages are used to perform the return routability      procedure from the mobile node to a correspondent node.  This      ensures authorization of subsequent Binding Updates, as described      inSection 5.2.5.   Binding Update      A Binding Update is used by a mobile node to notify a      correspondent node or the mobile node's home agent of its current      binding.  The Binding Update sent to the mobile node's home agent      to register its primary care-of address is marked as a "home      registration".   Binding Acknowledgement      A Binding Acknowledgement is used to acknowledge receipt of a      Binding Update, if an acknowledgement was requested in the Binding      Update, the binding update was sent to a home agent, or an error      occurred.   Binding Refresh Request      A Binding Refresh Request is used by a correspondent node to      request a mobile node to re-establish its binding with the      correspondent node.  This message is typically used when the      cached binding is in active use but the binding's lifetime is      close to expiration.  The correspondent node may use, for      instance, recent traffic and open transport layer connections as      an indication of active use.   Binding Error      The Binding Error is used by the correspondent node to signal an      error related to mobility, such as an inappropriate attempt to use      the Home Address destination option without an existing binding.Johnson, et al.              Standard Track                    [Page 16]

RFC 3775                Mobility Support in IPv6               June 20044.3.  New IPv6 Destination Option   Mobile IPv6 defines a new IPv6 destination option, the Home Address   destination option.  This option is described in detail inSection6.3.4.4.  New IPv6 ICMP Messages   Mobile IPv6 also introduces four new ICMP message types, two for use   in the dynamic home agent address discovery mechanism, and two for   renumbering and mobile configuration mechanisms.  As described inSection 10.5 andSection 11.4.1, the following two new ICMP message   types are used for home agent address discovery:   o  Home Agent Address Discovery Request, described inSection 6.5.   o  Home Agent Address Discovery Reply, described inSection 6.6.   The next two message types are used for network renumbering and   address configuration on the mobile node, as described inSection10.6:   o  Mobile Prefix Solicitation, described inSection 6.7.   o  Mobile Prefix Advertisement, described inSection 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 (seeSection 9.1) and "home      registration" entries (seeSection 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.  SeeSection11.1.Johnson, et al.              Standard Track                    [Page 17]

RFC 3775                Mobility Support in IPv6               June 2004   Home Agents List      Home agents need to know which other home agents are on the same      link.  This information is stored in the Home Agents List, as      described in more detail inSection 10.1.  The list is used for      informing mobile nodes during dynamic home agent address      discovery.4.6.  Site-Local Addressability   This specification requires that home and care-of addresses MUST be   unicast routable addresses.  Site-local addresses may be usable on   networks that are not connected to the Internet, but this   specification does not define when such usage is safe and when it is   not.  Mobile nodes may not be aware of which site they are currently   in, it is hard to prevent accidental attachment to other sites, and   ambiguity of site-local addresses can cause problems if the home and   visited networks use the same addresses.  Therefore, site-local   addresses SHOULD NOT be used as home or care-of addresses.5.  Overview of Mobile IPv6 Security   This specification provides a number of security features.  These   include the protection of Binding Updates both to home agents and   correspondent nodes, the protection of mobile prefix discovery, and   the protection of the mechanisms that Mobile IPv6 uses for   transporting data packets.   Binding Updates are protected by the use of IPsec extension headers,   or by the use of the Binding Authorization Data option.  This option   employs a binding management key, Kbm, which can be established   through the return routability procedure.  Mobile prefix discovery is   protected through the use of IPsec extension headers.  Mechanisms   related to transporting payload packets - such as the Home Address   destination option and type 2 routing header - have been specified in   a manner which restricts their use in attacks.5.1.  Binding Updates to Home Agents   The mobile node and the home agent MUST use an IPsec security   association to protect the integrity and authenticity of the Binding   Updates and Acknowledgements.  Both the mobile nodes and the home   agents MUST support and SHOULD use the Encapsulating Security Payload   (ESP) [6] header in transport mode and MUST use a non-NULL payload   authentication algorithm to provide data origin authentication,   connectionless integrity and optional anti-replay protection.  Note   that Authentication Header (AH) [5] is also possible but for brevity   not discussed in this specification.Johnson, et al.              Standard Track                    [Page 18]

RFC 3775                Mobility Support in IPv6               June 2004   In order to protect messages exchanged between the mobile node and   the home agent with IPsec, appropriate security policy database   entries must be created.  A mobile node must be prevented from using   its security association to send a Binding Update on behalf of   another mobile node using the same home agent.  This MUST be achieved   by having the home agent check that the given home address has been   used with the right security association.  Such a check is provided   in the IPsec processing, by having the security policy database   entries unequivocally identify a single security association for   protecting Binding Updates between any given home address and home   agent.  In order to make this possible, it is necessary that the home   address of the mobile node is visible in the Binding Updates and   Acknowledgements.  The home address is used in these packets as a   source or destination, or in the Home Address Destination option or   the type 2 routing header.   As with all IPsec security associations in this specification, manual   configuration of security associations MUST be supported.  The used   shared secrets MUST be random and unique for different mobile nodes,   and MUST be distributed off-line to the mobile nodes.   Automatic key management with IKE [9] MAY be supported.  When IKE is   used, either the security policy database entries or the Mobile IPv6   processing MUST unequivocally identify the IKE phase 1 credentials   which can be used to authorize the creation of security associations   for protecting Binding Updates for a particular home address.  How   these mappings are maintained is outside the scope of this   specification, but they may be maintained, for instance, as a locally   administered table in the home agent.  If the phase 1 identity is a   Fully Qualified Domain Name (FQDN), secure forms of DNS may also be   used.Section 11.3.2 discusses how IKE connections to the home agent need a   careful treatment of the addresses used for transporting IKE.  This   is necessary to ensure that a Binding Update is not needed before the   IKE exchange which is needed for securing the Binding Update.   When IKE version 1 is used with preshared secret authentication   between the mobile node and the home agent, aggressive mode MUST be   used.   The ID_IPV6_ADDR Identity Payload MUST NOT be used in IKEv1 phase 1.   Reference [21] contains a more detailed description and examples on   using IPsec to protect the communications between the mobile node and   the home agent.Johnson, et al.              Standard Track                    [Page 19]

RFC 3775                Mobility Support in IPv6               June 20045.2.  Binding Updates to Correspondent Nodes   The protection of Binding Updates sent to correspondent nodes does   not require the configuration of security associations or the   existence of an authentication infrastructure between the mobile   nodes and correspondent nodes.  Instead, a method called the return   routability procedure is used to assure that the right mobile node is   sending the message.  This method does not protect against attackers   who are on the path between the home network and the correspondent   node.  However, attackers in such a location are capable of   performing the same attacks even without Mobile IPv6.  The main   advantage of the return routability procedure is that it limits the   potential attackers to those having an access to one specific path in   the Internet, and avoids forged Binding Updates from anywhere else in   the Internet.  For a more in depth explanation of the security   properties of the return routability procedure, seeSection 15.   The integrity and authenticity of the Binding Updates messages to   correspondent nodes is protected by using a keyed-hash algorithm.   The binding management key, Kbm, is used to key the hash algorithm   for this purpose.  Kbm is established using data exchanged during the   return routability procedure.  The data exchange is accomplished by   use of node keys, nonces, cookies, tokens, and certain cryptographic   functions.Section 5.2.5 outlines the basic return routability   procedure.Section 5.2.6 shows how the results of this procedure are   used to authorize a Binding Update to a correspondent node.5.2.1.  Node Keys   Each correspondent node has a secret key, Kcn, called the "node key",   which it uses to produce the keygen tokens sent to the mobile nodes.   The node key MUST be a random number, 20 octets in length.  The node   key allows the correspondent node to verify that the keygen tokens   used by the mobile node in authorizing a Binding Update are indeed   its own.  This key MUST NOT be shared with any other entity.   A correspondent node MAY generate a fresh node key at any time; this   avoids the need for secure persistent key storage.  Procedures for   optionally updating the node key are discussed later inSection5.2.7.5.2.2.  Nonces   Each correspondent node also generates nonces at regular intervals.   The nonces should be generated by using a random number generator   that is known to have good randomness properties [1].  A   correspondent node may use the same Kcn and nonce with all the   mobiles it is in communication with.Johnson, et al.              Standard Track                    [Page 20]

RFC 3775                Mobility Support in IPv6               June 2004   Each nonce is identified by a nonce index.  When a new nonce is   generated, it must be associated with a new nonce index; this may be   done, for example, by incrementing the value of the previous nonce   index, if the nonce index is used as an array pointer into a linear   array of nonces.  However, there is no requirement that nonces be   stored that way, or that the values of subsequent nonce indices have   any particular relationship to each other.  The index value is   communicated in the protocol, so that if a nonce is replaced by new   nonce during the run of a protocol, the correspondent node can   distinguish messages that should be checked against the old nonce   from messages that should be checked against the new nonce.  Strictly   speaking, indices are not necessary in the authentication, but allow   the correspondent node to efficiently find the nonce value that it   used in creating a keygen token.   Correspondent nodes keep both the current nonce and a small set of   valid previous nonces whose lifetime has not yet expired.  Expired   values MUST be discarded, and messages using stale or unknown indices   will be rejected.   The specific nonce index values cannot be used by mobile nodes to   determine the validity of the nonce.  Expected validity times for the   nonces values and the procedures for updating them are discussed   later inSection 5.2.7.   A nonce is an octet string of any length.  The recommended length is   64 bits.5.2.3.  Cookies and Tokens   The return routability address test procedure uses cookies and keygen   tokens as opaque values within the test init and test messages,   respectively.   o  The "home init cookie" and "care-of init cookie" are 64 bit values      sent to the correspondent node from the mobile node, and later      returned to the mobile node.  The home init cookie is sent in the      Home Test Init message, and returned in the Home Test message.      The care-of init cookie is sent in the Care-of Test Init message,      and returned in the Care-of Test message.   o  The "home keygen token" and "care-of keygen token" are 64-bit      values sent by the correspondent node to the mobile node via the      home agent (via the Home Test message) and the care-of address (by      the Care-of Test message), respectively.Johnson, et al.              Standard Track                    [Page 21]

RFC 3775                Mobility Support in IPv6               June 2004   The mobile node should set the home init or care-of init cookie to a   newly generated random number in every Home or Care-of Test Init   message it sends.  The cookies are used to verify that the Home Test   or Care-of Test message matches the Home Test Init or Care-of Test   Init message, respectively.  These cookies also serve to ensure that   parties who have not seen the request cannot spoof responses.   Home and care-of keygen tokens are produced by the correspondent node   based on its currently active secret key (Kcn) and nonces, as well as   the home or care-of address (respectively).  A keygen token is valid   as long as both the secret key (Kcn) and the nonce used to create it   are valid.5.2.4.  Cryptographic Functions   In this specification, the function used to compute hash values is   SHA1 [20].  Message Authentication Codes (MACs) are computed using   HMAC_SHA1 [25,20].  HMAC_SHA1(K,m) denotes such a MAC computed on   message m with key K.5.2.5.  Return Routability Procedure   The Return Routability Procedure enables the correspondent node to   obtain some reasonable assurance that the mobile node is in fact   addressable at its claimed care-of address as well as at its home   address.  Only with this assurance is the correspondent node able to   accept Binding Updates from the mobile node which would then instruct   the correspondent node to direct that mobile node's data traffic to   its claimed care-of address.   This is done by testing whether packets addressed to the two claimed   addresses are routed to the mobile node.  The mobile node can pass   the test only if it is able to supply proof that it received certain   data (the "keygen tokens") which the correspondent node sends to   those addresses.  These data are combined by the mobile node into a   binding management key, denoted Kbm.   The figure below shows the message flow for the return routability   procedure.Johnson, et al.              Standard Track                    [Page 22]

RFC 3775                Mobility Support in IPv6               June 2004   Mobile node                 Home agent           Correspondent node        |                                                     |        |  Home Test Init (HoTI)   |                          |        |------------------------->|------------------------->|        |                          |                          |        |  Care-of Test Init (CoTI)                           |        |---------------------------------------------------->|        |                                                     |        |                          |  Home Test (HoT)         |        |<-------------------------|<-------------------------|        |                          |                          |        |                             Care-of Test (CoT)      |        |<----------------------------------------------------|        |                                                     |   The Home and Care-of Test Init messages are sent at the same time.   The procedure requires very little processing at the correspondent   node, and the Home and Care-of Test messages can be returned quickly,   perhaps nearly simultaneously.  These four messages form the return   routability procedure.   Home Test Init      A mobile node sends a Home Test Init message to the correspondent      node (via the home agent) to acquire the home keygen token.  The      contents of the message can be summarized as follows:      *  Source Address = home address      *  Destination Address = correspondent      *  Parameters:            +  home init cookie      The Home Test Init message conveys the mobile node's home address      to the correspondent node.  The mobile node also sends along a      home init cookie that the correspondent node must return later.      The Home Test Init message is reverse tunneled through the home      agent.  (The headers and addresses related to  reverse tunneling      have been omitted from the above discussion of the message      contents.)  The mobile node remembers these cookie values to      obtain some assurance that its protocol messages are being      processed by the desired correspondent node.Johnson, et al.              Standard Track                    [Page 23]

RFC 3775                Mobility Support in IPv6               June 2004   Care-of Test Init      The mobile node sends a Care-of Test Init message to the      correspondent node (directly, not via the home agent) to acquire      the care-of keygen token.  The contents of this message can be      summarized as follows:      *  Source Address = care-of address      *  Destination Address = correspondent      *  Parameters:            +  care-of init cookie      The Care-of Test Init message conveys the mobile node's care-of      address to the correspondent node.  The mobile node also sends      along a care-of init cookie that the correspondent node must      return later.  The Care-of Test Init message is sent directly to      the correspondent node.   Home Test      The Home Test message is sent in response to a Home Test Init      message.  It is sent via the home agent.  The contents of the      message are:      *  Source Address = correspondent      *  Destination Address = home address      *  Parameters:         +  home init cookie         +  home keygen token         +  home nonce indexJohnson, et al.              Standard Track                    [Page 24]

RFC 3775                Mobility Support in IPv6               June 2004      When the correspondent node receives the Home Test Init message,      it generates a home keygen token as follows:      home keygen token :=           First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0)))      where | denotes concatenation.  The final "0" inside the HMAC_SHA1      function is a single zero octet, used to distinguish home and      care-of cookies from each other.      The home keygen token is formed from the first 64 bits of the MAC.      The home keygen token tests that the mobile node can receive were      messages sent to its home address.  Kcn is used in the production      of home keygen token in order to allow the correspondent node to      verify that it generated the home and care-of nonces, without      forcing the correspondent node to remember a list of all tokens it      has handed out.      The Home Test message is sent to the mobile node via the home      network, where it is presumed that the home agent will tunnel the      message to the mobile node.  This means that the mobile node needs      to already have sent a Binding Update to the home agent, so that      the home agent will have received and authorized the new care-of      address for the mobile node before the return routability      procedure.  For improved security, the data passed between the      home agent and the mobile node is made immune to inspection and      passive attacks.  Such protection is gained by encrypting the home      keygen token as it is tunneled from the home agent to the mobile      node as specified inSection 10.4.6.  The security properties of      this additional security are discussed inSection 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 addressJohnson, et al.              Standard Track                    [Page 25]

RFC 3775                Mobility Support in IPv6               June 2004      *  Parameters:         +  care-of init cookie         +  care-of keygen token         +  care-of nonce index      When the correspondent node receives the Care-of Test Init      message, it generates a care-of keygen token as follows:      care-of keygen token :=         First (64, HMAC_SHA1 (Kcn, (care-of address | nonce | 1)))      Here, the final "1" inside the HMAC_SHA1 function is a single      octet containing the hex value 0x01, and is used to distinguish      home and care-of cookies from each other.  The keygen token is      formed from the first 64 bits of the MAC, and sent directly to the      mobile node at its care-of address.  The care-of init cookie from      the Care-of Test Init message is returned to ensure that the      message comes from a node on the route to the correspondent node.      The care-of nonce index is provided to identify the nonce used for      the care-of keygen token.  The home and care-of nonce indices MAY      be the same, or different, in the Home and Care-of Test messages.   When the mobile node has received both the Home and Care-of Test   messages, the return routability procedure is complete.  As a result   of the procedure, the mobile node has the data it needs to send a   Binding Update to the correspondent node.  The mobile node hashes the   tokens together to form a 20 octet binding key Kbm:      Kbm = SHA1 (home keygen token | care-of keygen token)   A Binding Update may also be used to delete a previously established   binding (Section 6.1.7).  In this case, the care-of keygen token is   not used.  Instead, the binding management key is generated as   follows:      Kbm = SHA1(home keygen token)   Note that the correspondent node does not create any state specific   to the mobile node, until it receives the Binding Update from that   mobile node.  The correspondent node does not maintain the value for   the binding management key Kbm; it creates Kbm when given the nonce   indices and the mobile node's addresses.Johnson, et al.              Standard Track                    [Page 26]

RFC 3775                Mobility Support in IPv6               June 20045.2.6.  Authorizing Binding Management Messages   After the mobile node has created the binding management key (Kbm),   it can supply a verifiable Binding Update to the correspondent node.   This section provides an overview of this registration.  The below   figure shows the message flow.   Mobile node                                Correspondent node        |                                               |        |             Binding Update (BU)               |        |---------------------------------------------->|        |  (MAC, seq#, nonce indices, care-of address)  |        |                                               |        |                                               |        |    Binding Acknowledgement (BA) (if sent)     |        |<----------------------------------------------|        |              (MAC, seq#, status)              |   Binding Update      To authorize a Binding Update, the mobile node creates a binding      management key Kbm from the keygen tokens as described in the      previous section.  The contents of the Binding Update include the      following:      *  Source Address = care-of address      *  Destination Address = correspondent      *  Parameters:         +  home address (within the Home Address destination option if            different from the Source Address)         +  sequence number (within the Binding Update message header)         +  home nonce index (within the Nonce Indices option)         +  care-of nonce index (within the Nonce Indices option)         +  First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent            | BU)))Johnson, et al.              Standard Track                    [Page 27]

RFC 3775                Mobility Support in IPv6               June 2004      The Binding Update contains a Nonce Indices option, indicating to      the correspondent node which home and care-of nonces to use to      recompute Kbm, the binding management key.  The MAC is computed as      described inSection 6.2.7, using the correspondent node's address      as the destination address and the Binding Update message itself      ("BU" above) as the MH Data.      Once the correspondent node has verified the MAC, it can create a      Binding Cache entry for the mobile.   Binding Acknowledgement      The Binding Update is in some cases acknowledged by the      correspondent node.  The contents of the message are as follows:      *  Source Address = correspondent      *  Destination Address = care-of address      *  Parameters:         +  sequence number (within the Binding Update message header)         +  First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent            | BA)))      The Binding Acknowledgement contains the same sequence number as      the Binding Update.  The MAC is computed as described inSection6.2.7, 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 (seeSection 12).      The value in the Source Address field in the IPv6 header carrying      the Binding Update is normally also the care-of address which is      used in the binding.  However, a different care-of address MAY be      specified by including an Alternate Care-of Address mobility      option in the Binding Update (seeSection 6.2.5).  When such a      message is sent to the correspondent node and the return      routability procedure is used as the authorization method, the      Care-of Test Init and Care-of Test messages MUST have been      performed for the address in the Alternate Care-of Address option      (not the Source Address).  The nonce indices and MAC value MUST be      based on information gained in this test.Johnson, et al.              Standard Track                    [Page 28]

RFC 3775                Mobility Support in IPv6               June 2004      Binding Updates may also be sent to delete a previously      established binding.  In this case, generation of the binding      management key depends exclusively on the home keygen token and      the care-of nonce index is ignored.5.2.7.  Updating Node Keys and Nonces   Correspondent nodes generate nonces at regular intervals.  It is   recommended to keep each nonce (identified by a nonce index)   acceptable for at least MAX_TOKEN_LIFETIME seconds (seeSection 12)   after it has been first used in constructing a return routability   message response.  However, the correspondent node MUST NOT accept   nonces beyond MAX_NONCE_LIFETIME seconds (seeSection 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 inSection6.1.8).  The mobile node can then retry the return routability   procedure.   An update of Kcn SHOULD be done at the same time as an update of a   nonce, so that nonce indices can identify both the nonce and the key.   Old Kcn values have to be therefore remembered as long as old nonce   values.   Given that the tokens are normally expected to be usable for   MAX_TOKEN_LIFETIME seconds, the mobile node MAY use them beyond a   single run of the return routability procedure until   MAX_TOKEN_LIFETIME expires.  After this the mobile node SHOULD NOT   use the tokens.  A fast moving mobile node MAY reuse a recent home   keygen token from a correspondent node when moving to a new location,   and just acquire a new care-of keygen token to show routability in   the new location.Johnson, et al.              Standard Track                    [Page 29]

RFC 3775                Mobility Support in IPv6               June 2004   While this does not save the number of round-trips due to the   simultaneous processing of home and care-of return routability tests,   there are fewer messages being exchanged, and a potentially long   round-trip through the home agent is avoided.  Consequently, this   optimization is often useful.  A mobile node that has multiple home   addresses, MAY also use the same care-of keygen token for Binding   Updates concerning all of these addresses.5.2.8.  Preventing Replay Attacks   The return routability procedure also protects the participants   against replayed Binding Updates through the use of the sequence   number and a MAC.  Care must be taken when removing bindings at the   correspondent node, however.  Correspondent nodes must retain   bindings and the associated sequence number information at least as   long as the nonces used in the authorization of the binding are still   valid.  Alternatively, if memory is very constrained, the   correspondent node MAY invalidate the nonces that were used for the   binding being deleted (or some larger group of nonces that they   belong to).  This may, however, impact the ability to accept Binding   Updates from mobile nodes that have recently received keygen tokens.   This alternative is therefore recommended only as a last measure.5.3.  Dynamic Home Agent Address Discovery   No security is required for dynamic home agent address discovery.5.4.  Mobile Prefix Discovery   The mobile node and the home agent SHOULD use an IPsec security   association to protect the integrity and authenticity of the Mobile   Prefix Solicitations and Advertisements.  Both the mobile nodes and   the home agents MUST support and SHOULD use the Encapsulating   Security Payload (ESP) header in transport mode with a non-NULL   payload authentication algorithm to provide data origin   authentication, connectionless integrity and optional anti-replay   protection.5.5.  Payload Packets   Payload packets exchanged with mobile nodes can be protected in the   usual manner, in the same way as stationary hosts can protect them.   However, Mobile IPv6 introduces the Home Address destination option,   a routing header, and tunneling headers in the payload packets.  In   the following we define the security measures taken to protect these,   and to prevent their use in attacks against other parties.Johnson, et al.              Standard Track                    [Page 30]

RFC 3775                Mobility Support in IPv6               June 2004   This specification limits the use of the Home Address destination   option to the situation where the correspondent node already has a   Binding Cache entry for the given home address.  This avoids the use   of the Home Address option in attacks described inSection 15.1.   Mobile IPv6 uses a Mobile IPv6 specific type of a routing header.   This type provides the necessary functionality but does not open   vulnerabilities discussed inSection 15.1.   Tunnels between the mobile node and the home agent are protected by   ensuring proper use of source addresses, and optional cryptographic   protection.  The mobile node verifies that the outer IP address   corresponds to its home agent.  The home agent verifies that the   outer IP address corresponds to the current location of the mobile   node (Binding Updates sent to the home agents are secure).  The home   agent identifies the mobile node through the source address of the   inner packet.  (Typically, this is the home address of the mobile   node, but it can also be a link-local address, as discussed inSection 10.4.2.  To recognize the latter type of addresses, the home   agent requires that the Link-Local Address Compatibility (L) was set   in the Binding Update.)  These measures protect the tunnels against   vulnerabilities discussed inSection 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 Option6.1.  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 inSection 9.5.4 for Binding   Acknowledgement.  Mobility Header messages also MUST NOT be used with   a Home Address destination option, except as described inSection11.7.1 andSection 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 inSection 9.3.2 and the Binding Update List check described in SectionJohnson, et al.              Standard Track                    [Page 31]

RFC 3775                Mobility Support in IPv6               June 2004   11.3.1 which are normally performed for all packets.  This applies   even to messages sent to or from a correspondent node which is itself   a mobile node.6.1.1.  Format   The Mobility Header is identified by a Next Header value of 135 in   the immediately preceding header, and has the following format:    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    | Payload Proto |  Header Len   |   MH Type     |   Reserved    |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |           Checksum            |                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |    |                                                               |    .                                                               .    .                       Message Data                            .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Payload Proto      8-bit selector.  Identifies the type of header immediately      following the Mobility Header.  Uses the same values as the IPv6      Next Header field [11].      This field is intended to be used by a future extension (seeAppendix B.1).      Implementations conforming to this specification SHOULD set the      payload protocol type to IPPROTO_NONE (59 decimal).   Header Len      8-bit unsigned integer, representing the length of the Mobility      Header in units of 8 octets, excluding the first 8 octets.      The length of the Mobility Header MUST be a multiple of 8 octets.   MH Type      8-bit selector.  Identifies the particular mobility message in      question.  Current values are specified inSection 6.1.2 and      onward.  An unrecognized MH Type field causes an error indication      to be sent.Johnson, et al.              Standard Track                    [Page 32]

RFC 3775                Mobility Support in IPv6               June 2004   Reserved      8-bit field reserved for future use.  The value MUST be      initialized to zero by the sender, and MUST be ignored by the      receiver.   Checksum      16-bit unsigned integer.  This field contains the checksum of the      Mobility Header.  The checksum is calculated from the octet string      consisting of a "pseudo-header" followed by the entire Mobility      Header starting with the Payload Proto field.  The checksum is the      16-bit one's complement of the one's complement sum of this      string.      The pseudo-header contains IPv6 header fields, as specified inSection 8.1 of RFC 2460 [11].  The Next Header value used in the      pseudo-header is 2.  The addresses used in the pseudo-header are      the addresses that appear in the Source and Destination Address      fields in the IPv6 packet carrying the Mobility Header.      Note that the procedures of calculating upper layer checksums      while away from home described inSection 11.3.1 apply even for      the Mobility Header.  If a mobility message has a Home Address      destination option, then the checksum calculation uses the home      address in this option as the value of the IPv6 Source Address      field.  The type 2 routing header is treated as explained in [11].      The Mobility Header is considered as the upper layer protocol for      the purposes of calculating the pseudo-header.  The Upper-Layer      Packet Length field in the pseudo-header MUST be set to the total      length of the Mobility Header.      For computing the checksum, the checksum field is set to zero.   Message Data      A variable length field containing the data specific to the      indicated Mobility Header type.   Mobile IPv6 also defines a number of "mobility options" for use   within these messages; if included, any options MUST appear after the   fixed portion of the message data specified in this document.  The   presence of such options will be indicated by the Header Len field   within the message.  When the Header Len value is greater than the   length required for the message specified here, the remaining octets   are interpreted as mobility options.  These options include padding   options that can be used to ensure that other options are alignedJohnson, et al.              Standard Track                    [Page 33]

RFC 3775                Mobility Support in IPv6               June 2004   properly, and that the total length of the message is divisible by 8.   The encoding and format of defined options are described inSection6.2.   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 inSection 9.5.5.  When a mobile node   receives a packet containing a Binding Refresh Request message it   processes the message according to the rules inSection 11.7.4.   The Binding Refresh Request message uses the MH Type value 0.  When   this value is indicated in the MH Type field, the format of the   Message Data field in the Mobility Header is as follows:                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    |          Reserved             |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    .                                                               .    .                        Mobility options                       .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Reserved      16-bit field reserved for future use.  The value MUST be      initialized to zero by the sender, and MUST be ignored by the      receiver.   Mobility Options      Variable-length field of such length that the complete Mobility      Header is an integer multiple of 8 octets long.  This field      contains zero or more TLV-encoded mobility options.  The encoding      and format of defined options are described inSection 6.2.  The      receiver MUST ignore and skip any options which it does not      understand.Johnson, et al.              Standard Track                    [Page 34]

RFC 3775                Mobility Support in IPv6               June 2004      There MAY be additional information, associated with this Binding      Refresh Request message that need not be present in all Binding      Refresh Request messages sent.  Mobility options allow future      extensions to the format of the Binding Refresh Request message to      be defined.  This specification does not define any options valid      for the Binding Refresh Request message.   If no actual options are present in this message, no padding is   necessary and the Header Len field will be set to 0.6.1.3.  Home Test Init Message   A mobile node uses the Home Test Init (HoTI) message to initiate the   return routability procedure and request a home keygen token from a   correspondent node (seeSection 11.6.1).  The Home Test Init message   uses the MH Type value 1.  When this value is indicated in the MH   Type field, the format of the Message Data field in the Mobility   Header is as follows:                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    |           Reserved            |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    +                       Home Init Cookie                        +    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    .                                                               .    .                       Mobility Options                        .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Reserved      16-bit field reserved for future use.  This value MUST be      initialized to zero by the sender, and MUST be ignored by the      receiver.   Home Init Cookie      64-bit field which contains a random value, the home init cookie.   Mobility Options      Variable-length field of such length that the complete Mobility      Header is an integer multiple of 8 octets long.  This field      contains zero or more TLV-encoded mobility options.  The receiverJohnson, et al.              Standard Track                    [Page 35]

RFC 3775                Mobility Support in IPv6               June 2004      MUST ignore and skip any options which it does not understand.      This specification does not define any options valid for the Home      Test Init message.   If no actual options are present in this message, no padding is   necessary and the Header Len field will be set to 1.   This message is tunneled through the home agent when the mobile node   is away from home.  Such tunneling SHOULD employ IPsec ESP in tunnel   mode between the home agent and the mobile node.  This protection is   indicated by the IPsec security policy database.  The protection of   Home Test Init messages is unrelated to the requirement to protect   regular payload traffic, which MAY use such tunnels as well.6.1.4.  Care-of Test Init Message   A mobile node uses the Care-of Test Init (CoTI) message to initiate   the return routability procedure and request a care-of keygen token   from a correspondent node (seeSection 11.6.1).  The Care-of Test   Init message uses the MH Type value 2.  When this value is indicated   in the MH Type field, the format of the Message Data field in the   Mobility Header is as follows:                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    |           Reserved            |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    +                      Care-of Init Cookie                      +    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    .                                                               .    .                        Mobility Options                       .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Reserved      16-bit field reserved for future use.  The value MUST be      initialized to zero by the sender, and MUST be ignored by the      receiver.   Care-of Init Cookie      64-bit field which contains a random value, the care-of init      cookie.Johnson, et al.              Standard Track                    [Page 36]

RFC 3775                Mobility Support in IPv6               June 2004   Mobility Options      Variable-length field of such length that the complete Mobility      Header is an integer multiple of 8 octets long.  This field      contains zero or more TLV-encoded mobility options.  The receiver      MUST ignore and skip any options which it does not understand.      This specification does not define any options valid for the      Care-of Test Init message.   If no actual options are present in this message, no padding is   necessary and the Header Len field will be set to 1.6.1.5.  Home Test Message   The Home Test (HoT) message is a response to the Home Test Init   message, and is sent from the correspondent node to the mobile node   (seeSection 5.2.5).  The Home Test message uses the MH Type value 3.   When this value is indicated in the MH Type field, the format of the   Message Data field in the Mobility Header is as follows:                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    |       Home Nonce Index        |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    +                        Home Init Cookie                       +    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    +                       Home Keygen Token                       +    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    .                                                               .    .                        Mobility options                       .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Home Nonce Index      This field will be echoed back by the mobile node to the      correspondent node in a subsequent Binding Update.   Home Init Cookie      64-bit field which contains the home init cookie.Johnson, et al.              Standard Track                    [Page 37]

RFC 3775                Mobility Support in IPv6               June 2004   Home Keygen Token      This field contains the 64 bit home keygen token used in the      return routability procedure.   Mobility Options      Variable-length field of such length that the complete Mobility      Header is an integer multiple of 8 octets long.  This field      contains zero or more TLV-encoded mobility options.  The receiver      MUST ignore and skip any options which it does not understand.      This specification does not define any options valid for the Home      Test message.   If no actual options are present in this message, no padding is   necessary and the Header Len field will be set to 2.6.1.6.  Care-of Test Message   The Care-of Test (CoT) message is a response to the Care-of Test Init   message, and is sent from the correspondent node to the mobile node   (seeSection 11.6.2).  The Care-of Test message uses the MH Type   value 4.  When this value is indicated in the MH Type field, the   format of the Message Data field in the Mobility Header is as   follows:                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    |      Care-of Nonce Index      |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    +                      Care-of Init Cookie                      +    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    +                     Care-of Keygen Token                      +    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    .                                                               .    .                        Mobility Options                       .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Care-of Nonce Index      This value will be echoed back by the mobile node to the      correspondent node in a subsequent Binding Update.Johnson, et al.              Standard Track                    [Page 38]

RFC 3775                Mobility Support in IPv6               June 2004   Care-of Init Cookie      64-bit field which contains the care-of init cookie.   Care-of Keygen Token      This field contains the 64 bit care-of keygen token used in the      return routability procedure.   Mobility Options      Variable-length field of such length that the complete Mobility      Header is an integer multiple of 8 octets long.  This field      contains zero or more TLV-encoded mobility options.  The receiver      MUST ignore and skip any options which it does not understand.      This specification does not define any options valid for the      Care-of Test message.   If no actual options are present in this message, no padding is   necessary and the Header Len field will be set to 2.6.1.7.  Binding Update Message   The Binding Update (BU) message is used by a mobile node to notify   other nodes of a new care-of address for itself.  Binding Updates are   sent as described inSection 11.7.1 andSection 11.7.2.   The Binding Update uses the MH Type value 5.  When this value is   indicated in the MH Type field, the format of the Message Data field   in the Mobility Header is as follows:                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    |          Sequence #           |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |A|H|L|K|        Reserved       |           Lifetime            |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    .                                                               .    .                        Mobility options                       .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Acknowledge (A)      The Acknowledge (A) bit is set by the sending mobile node to      request a Binding Acknowledgement (Section 6.1.8) be returned upon      receipt of the Binding Update.Johnson, et al.              Standard Track                    [Page 39]

RFC 3775                Mobility Support in IPv6               June 2004   Home Registration (H)      The Home Registration (H) bit is set by the sending mobile node to      request that the receiving node should act as this node's home      agent.  The destination of the packet carrying this message MUST      be that of a router sharing the same subnet prefix as the home      address of the mobile node in the binding.   Link-Local Address Compatibility (L)      The Link-Local Address Compatibility (L) bit is set when the home      address reported by the mobile node has the same interface      identifier as the mobile node's link-local address.   Key Management Mobility Capability (K)      If this bit is cleared, the protocol used for establishing the      IPsec security associations between the mobile node and the home      agent does not survive movements.  It may then have to be rerun.      (Note that the IPsec security associations themselves are expected      to survive movements.)  If manual IPsec configuration is used, the      bit MUST be cleared.      This bit is valid only in Binding Updates sent to the home agent,      and MUST be cleared in other Binding Updates.  Correspondent nodes      MUST ignore this bit.   Reserved      These fields are unused.  They MUST be initialized to zero by the      sender and MUST be ignored by the receiver.   Sequence #      A 16-bit unsigned integer used by the receiving node to sequence      Binding Updates and by the sending node to match a returned      Binding Acknowledgement with this Binding Update.   Lifetime      16-bit unsigned integer.  The number of time units remaining      before the binding MUST be considered expired.  A value of zero      indicates that the Binding Cache entry for the mobile node MUST be      deleted.  (In this case the specified care-of address MUST also be      set equal to the home address.)  One time unit is 4 seconds.Johnson, et al.              Standard Track                    [Page 40]

RFC 3775                Mobility Support in IPv6               June 2004   Mobility Options      Variable-length field of such length that the complete Mobility      Header is an integer multiple of 8 octets long.  This field      contains zero or more TLV-encoded mobility options.  The encoding      and format of defined options are described inSection 6.2.  The      receiver MUST ignore and skip any options which it does not      understand.      The following options are valid in a Binding Update:      *  Binding Authorization Data option (this option is mandatory in         Binding Updates sent to a correspondent node)      *  Nonce Indices option.      *  Alternate Care-of Address option   If no options are present in this message, 4 octets of padding are   necessary and the Header Len field will be set to 1.   The care-of address is specified either by the Source Address field   in the IPv6 header or by the Alternate Care-of Address option, if   present.  The care-of address MUST be a unicast routable address.   IPv6 Source Address MUST be a topologically correct source address.   Binding Updates for a care-of address which is not a unicast routable   address MUST be silently discarded.  Similarly, the Binding Update   MUST be silently discarded if the care-of address appears as a home   address in an existing Binding Cache entry, with its current location   creating a circular reference back to the home address specified in   the Binding Update (possibly through additional entries).   The deletion of a binding can be indicated by setting the Lifetime   field to 0 and by setting the care-of address equal to the home   address.  In deletion, the generation of the binding management key   depends exclusively on the home keygen token, as explained inSection5.2.5.  (Note that while the senders are required to set both the   Lifetime field to 0 and the care-of address equal to the home   address,Section 9.5.1 rules for receivers are more liberal, and   interpret either condition as a deletion.)   Correspondent nodes SHOULD NOT delete the Binding Cache entry before   the lifetime expires, if any application hosted by the correspondent   node is still likely to require communication with the mobile node.   A Binding Cache entry that is de-allocated prematurely might cause   subsequent packets to be dropped from the mobile node, if they   contain the Home Address destination option.  This situation is   recoverable, since a Binding Error message is sent to the mobile nodeJohnson, et al.              Standard Track                    [Page 41]

RFC 3775                Mobility Support in IPv6               June 2004   (seeSection 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 inSection 9.5.4 andSection 10.3.1.   The Binding Acknowledgement has the MH Type value 6.  When this value   is indicated in the MH Type field, the format of the Message Data   field in the Mobility Header is as follows:                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    |    Status     |K|  Reserved   |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |           Sequence #          |           Lifetime            |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    .                                                               .    .                        Mobility options                       .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Key Management Mobility Capability (K)      If this bit is cleared, the protocol used by the home agent for      establishing the IPsec security associations between the mobile      node and the home agent does not survive movements.  It may then      have to be rerun.  (Note that the IPsec security associations      themselves are expected to survive movements.)      Correspondent nodes MUST set the K bit to 0.   Reserved      These fields are unused.  They MUST be initialized to zero by the      sender and MUST be ignored by the receiver.Johnson, et al.              Standard Track                    [Page 42]

RFC 3775                Mobility Support in IPv6               June 2004   Status      8-bit unsigned integer indicating the disposition of the Binding      Update.  Values of the Status field less than 128 indicate that      the Binding Update was accepted by the receiving node.  Values      greater than or equal to 128 indicate that the Binding Update was      rejected by the receiving node.  The following Status values are      currently defined:        0 Binding Update accepted        1 Accepted but prefix discovery necessary      128 Reason unspecified      129 Administratively prohibited      130 Insufficient resources      131 Home registration not supported      132 Not home subnet      133 Not home agent for this mobile node      134 Duplicate Address Detection failed      135 Sequence number out of window      136 Expired home nonce index      137 Expired care-of nonce index      138 Expired nonces      139 Registration type change disallowed   Up-to-date values of the Status field are to be specified in the IANA   registry of assigned numbers [19].   Sequence #      The Sequence Number in the Binding Acknowledgement is copied from      the Sequence Number field in the Binding Update.  It is used by      the mobile node in matching this Binding Acknowledgement with an      outstanding Binding Update.Johnson, et al.              Standard Track                    [Page 43]

RFC 3775                Mobility Support in IPv6               June 2004   Lifetime      The granted lifetime, in time units of 4 seconds, for which this      node SHOULD retain the entry for this mobile node in its Binding      Cache.      The value of this field is undefined if the Status field indicates      that the Binding Update was rejected.   Mobility Options      Variable-length field of such length that the complete Mobility      Header is an integer multiple of 8 octets long.  This field      contains zero or more TLV-encoded mobility options.  The encoding      and format of defined options are described inSection 6.2.  The      receiver MUST ignore and skip any options which it does not      understand.      There MAY be additional information, associated with this Binding      Acknowledgement that need not be present in all Binding      Acknowledgements sent.  Mobility options allow future extensions      to the format of the Binding Acknowledgement to be defined.  The      following options are valid for the Binding Acknowledgement:      *  Binding Authorization Data option (this option is mandatory in         Binding Acknowledgements sent by a correspondent node, except         where otherwise noted inSection 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; seeSection 9.3.3 for details.Johnson, et al.              Standard Track                    [Page 44]

RFC 3775                Mobility Support in IPv6               June 2004   The Binding Error message uses the MH Type value 7.  When this value   is indicated in the MH Type field, the format of the Message Data   field in the Mobility Header is as follows:                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                    |     Status    |   Reserved    |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    +                                                               +    |                                                               |    +                          Home Address                         +    |                                                               |    +                                                               +    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    .                                                               .    .                        Mobility Options                       .    .                                                               .    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Status      8-bit unsigned integer indicating the reason for this message.      The following values are currently defined:         1 Unknown binding for Home Address destination option         2 Unrecognized MH Type value   Reserved      A 8-bit field reserved for future use.  The value MUST be      initialized to zero by the sender, and MUST be ignored by the      receiver.   Home Address      The home address that was contained in the Home Address      destination option.  The mobile node uses this information to      determine which binding does not exist, in cases where the mobile      node has several home addresses.Johnson, et al.              Standard Track                    [Page 45]

RFC 3775                Mobility Support in IPv6               June 2004   Mobility Options      Variable-length field of such length that the complete Mobility      Header is an integer multiple of 8 octets long.  This field      contains zero or more TLV-encoded mobility options.  The receiver      MUST ignore and skip any options which it does not understand.      There MAY be additional information, associated with this Binding      Error message that need not be present in all Binding Error      messages sent.  Mobility options allow future extensions to the      format of the format of the Binding Error message to be defined.      The encoding and format of defined options are described inSection 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 ofSection 6.1.   The presence of such options will be indicated by the Header Len of   the Mobility Header.  If included, the Binding Authorization Data   option (Section 6.2.7) MUST be the last option and MUST NOT have   trailing padding.  Otherwise, options can be placed in any order.6.2.1.  Format   Mobility options are encoded within the remaining space of the   Message Data field of a mobility message, using a type-length-value   (TLV) format as follows:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Option Type  | Option Length |   Option Data...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Johnson, et al.              Standard Track                    [Page 46]

RFC 3775                Mobility Support in IPv6               June 2004   Option Type      8-bit identifier of the type of mobility option.  When processing      a Mobility Header containing an option for which the Option Type      value is not recognized by the receiver, the receiver MUST quietly      ignore and skip over the option, correctly handling any remaining      options in the message.   Option Length      8-bit unsigned integer, representing the length in octets of the      mobility option, not including the Option Type and Option Length      fields.   Option Data      A variable length field that contains data specific to the option.   The following subsections specify the Option types which are   currently defined for use in the Mobility Header.   Implementations MUST silently ignore any mobility options that they   do not understand.   Mobility options may have alignment requirements.  Following the   convention in IPv6, these options are aligned in a packet so that   multi-octet values within the Option Data field of each option fall   on natural boundaries (i.e., fields of width n octets are placed at   an integer multiple of n octets from the start of the header, for n =   1, 2, 4, or 8) [11].6.2.2.  Pad1   The Pad1 option does not have any alignment requirements.  Its format   is as follows:       0       0 1 2 3 4 5 6 7      +-+-+-+-+-+-+-+-+      |   Type = 0    |      +-+-+-+-+-+-+-+-+   NOTE! the format of the Pad1 option is a special case - it has   neither Option Length nor Option Data fields.Johnson, et al.              Standard Track                    [Page 47]

RFC 3775                Mobility Support in IPv6               June 2004   The Pad1 option is used to insert one octet of padding in the   Mobility Options area of a Mobility Header.  If more than one octet   of padding is required, the PadN option, described next, should be   used rather than multiple Pad1 options.6.2.3.  PadN   The PadN option does not have any alignment requirements.  Its format   is as follows:       0                   1       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -      |   Type = 1    | Option Length | Option Data      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -   The PadN option is used to insert two or more octets of padding in   the Mobility Options area of a mobility message.  For N octets of   padding, the Option Length field contains the value N-2, and the   Option Data consists of N-2 zero-valued octets.  PadN Option data   MUST be ignored by the receiver.6.2.4.  Binding Refresh Advice   The Binding Refresh Advice option has an alignment requirement of 2n.   Its format is as follows:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |   Type = 2    |   Length = 2  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |       Refresh Interval        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   The Binding Refresh Advice option is only valid in the Binding   Acknowledgement, and only on Binding Acknowledgements sent from the   mobile node's home agent in reply to a home registration.  The   Refresh Interval is measured in units of four seconds, and indicates   remaining time until the mobile node SHOULD send a new home   registration to the home agent.  The Refresh Interval MUST be set to   indicate a smaller time interval than the Lifetime value of the   Binding Acknowledgement.Johnson, et al.              Standard Track                    [Page 48]

RFC 3775                Mobility Support in IPv6               June 20046.2.5.  Alternate Care-of Address   The Alternate Care-of Address option has an alignment requirement of   8n+6.  Its format is as follows:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |   Type = 3    |  Length = 16  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   +                                                               +   |                                                               |   +                   Alternate Care-of Address                   +   |                                                               |   +                                                               +   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Normally, a Binding Update specifies the desired care-of address in   the Source Address field of the IPv6 header.  However, this is not   possible in some cases, such as when the mobile node wishes to   indicate a care-of address which it cannot use as a topologically   correct source address (Section 6.1.7 andSection 11.7.2) or when the   used security mechanism does not protect the IPv6 header (Section11.7.1).   The Alternate Care-of Address option is provided for these   situations.  This option is valid only in Binding Update.  The   Alternate Care-of Address field contains an address to use as the   care-of address for the binding, rather than using the Source Address   of the packet as the care-of address.6.2.6.  Nonce Indices   The Nonce Indices option has an alignment requirement of 2n.  Its   format is as follows:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |   Type = 4    |   Length = 4  |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |         Home Nonce Index      |     Care-of Nonce Index       |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Johnson, et al.              Standard Track                    [Page 49]

RFC 3775                Mobility Support in IPv6               June 2004   The Nonce Indices option is valid only in the Binding Update message   sent to a correspondent node, and only when present together with a   Binding Authorization Data option.  When the correspondent node   authorizes the Binding Update, it needs to produce home and care-of   keygen tokens from its stored random nonce values.   The Home Nonce Index field tells the correspondent node which nonce   value to use when producing the home keygen token.   The Care-of Nonce Index field is ignored in requests to delete a   binding.  Otherwise, it tells the correspondent node which nonce   value to use when producing the care-of keygen token.6.2.7.  Binding Authorization Data   The Binding Authorization Data option does not have alignment   requirements as such.  However, since this option must be the last   mobility option, an implicit alignment requirement is 8n + 2.  The   format of this option is as follows:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |   Type = 5    | Option Length |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   +                                                               +   |                         Authenticator                         |   +                                                               +   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   The Binding Authorization Data option is valid in the Binding Update   and Binding Acknowledgement.   The Option Length field contains the length of the authenticator in   octets.   The Authenticator field contains a cryptographic value which can be   used to determine that the message in question comes from the right   authority.  Rules for calculating this value depends on the used   authorization procedure.Johnson, et al.              Standard Track                    [Page 50]

RFC 3775                Mobility Support in IPv6               June 2004   For the return routability procedure, this option can appear in the   Binding Update and Binding Acknowledgements.  Rules for calculating   the Authenticator value are the following:      Mobility Data = care-of address | correspondent | MH Data      Authenticator = First (96, HMAC_SHA1 (Kbm, Mobility Data))   Where | denotes concatenation. "Care-of address" is the care-of   address which will be registered for the mobile node if the Binding   Update succeeds, or the home address of the mobile node if this   option is used in de-registration.  Note also that this address might   be different from the source address of the Binding Update message,   if the Alternative Care-of Address mobility option is used, or when   the lifetime of the binding is set to zero.   The "correspondent" is the IPv6 address of the correspondent node.   Note that, if the message is sent to a destination which is itself   mobile, the "correspondent" address may not be the address found in   the Destination Address field of the IPv6 header; instead the home   address from the type 2 Routing header should be used.   "MH Data" is the content of the Mobility Header, excluding the   Authenticator field itself.  The Authenticator value is calculated as   if the Checksum field in the Mobility Header was zero.  The Checksum   in the transmitted packet is still calculated in the usual manner,   with the calculated Authenticator being a part of the packet   protected by the Checksum.  Kbm is the binding management key, which   is typically created using nonces provided by the correspondent node   (seeSection 9.4).  Note that while the contents of a potential Home   Address destination option are not covered in this formula, the rules   for the calculation of the Kbm do take the home address in account.   This ensures that the MAC will be different for different home   addresses.   The first 96 bits from the MAC result are used as the Authenticator   field.6.3.  Home Address Option   The Home Address option is carried by the Destination Option   extension header (Next Header value = 60).  It is used in a packet   sent by a mobile node while away from home, to inform the recipient   of the mobile node's home address.Johnson, et al.              Standard Track                    [Page 51]

RFC 3775                Mobility Support in IPv6               June 2004   The Home Address option is encoded in type-length-value (TLV) format   as follows:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1                                   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |  Option Type  | Option Length |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   +                                                               +   |                                                               |   +                          Home Address                         +   |                                                               |   +                                                               +   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Option Type      201 = 0xC9   Option Length      8-bit unsigned integer.  Length of the option, in octets,      excluding the Option Type and Option Length fields.  This field      MUST be set to 16.   Home Address      The home address of the mobile node sending the packet.  This      address MUST be a unicast routable address.   The alignment requirement [11] for the Home Address option is 8n+6.   The three highest-order bits of the Option Type field are encoded to   indicate specific processing of the option [11]; for the Home Address   option, these three bits are set to 110.  This indicates the   following processing requirements:   o  Any IPv6 node that does not recognize the Option Type must discard      the packet, and if the packet's Destination Address was not a      multicast address, return an ICMP Parameter Problem, Code 2,      message to the packet's Source Address.  The Pointer field in the      ICMP message SHOULD point at the Option Type field.  Otherwise,      for multicast addresses, the ICMP message MUST NOT be sent.   o  The data within the option cannot change en route to the packet's      final destination.Johnson, et al.              Standard Track                    [Page 52]

RFC 3775                Mobility Support in IPv6               June 2004   The Home Address option MUST be placed as follows:   o  After the routing header, if that header is present   o  Before the Fragment Header, if that header is present   o  Before the AH Header or ESP Header, if either one of those headers      are present   For each IPv6 packet header, the Home Address Option MUST NOT appear   more than once.  However, an encapsulated packet [15] MAY contain a   separate Home Address option associated with each encapsulating IP   header.   The inclusion of a Home Address destination option in a packet   affects the receiving node's processing of only this single packet.   No state is created or modified in the receiving node as a result of   receiving a Home Address option in a packet.  In particular, the   presence of a Home Address option in a received packet MUST NOT alter   the contents of the receiver's Binding Cache and MUST NOT cause any   changes in the routing of subsequent packets sent by this receiving   node.6.4.  Type 2 Routing Header   Mobile IPv6 defines a new routing header variant, the type 2 routing   header, to allow the packet to be routed directly from a   correspondent to the mobile node's care-of address.  The mobile   node's care-of address is inserted into the IPv6 Destination Address   field.  Once the packet arrives at the care-of address, the mobile   node retrieves its home address from the routing header, and this is   used as the final destination address for the packet.   The new routing header uses a different type than defined for   "regular" IPv6 source routing, enabling firewalls to apply different   rules to source routed packets than to Mobile IPv6.  This routing   header type (type 2) is restricted to carry only one IPv6 address.   All IPv6 nodes which process this routing header MUST verify that the   address contained within is the node's own home address in order to   prevent packets from being forwarded outside the node.  The IP   address contained in the routing header, since it is the mobile   node's home address, MUST be a unicast routable address.   Furthermore, if the scope of the home address is smaller than the   scope of the care-of address, the mobile node MUST discard the packet   (seeSection 4.6).Johnson, et al.              Standard Track                    [Page 53]

RFC 3775                Mobility Support in IPv6               June 20046.4.1.  Format   The type 2 routing header has the following format:   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Next Header  | Hdr Ext Len=2 | Routing Type=2|Segments Left=1|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                            Reserved                           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   +                                                               +   |                                                               |   +                         Home Address                          +   |                                                               |   +                                                               +   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Next Header      8-bit selector.  Identifies the type of header immediately      following the routing header.  Uses the same values as the IPv6      Next Header field [11].   Hdr Ext Len      2 (8-bit unsigned integer);  length of the routing header in 8-      octet units, not including the first 8 octets.   Routing Type      2 (8-bit unsigned integer).   Segments Left      1 (8-bit unsigned integer).   Reserved      32-bit reserved field.  The value MUST be initialized to zero by      the sender, and MUST be ignored by the receiver.   Home Address      The Home Address of the destination Mobile Node.Johnson, et al.              Standard Track                    [Page 54]

RFC 3775                Mobility Support in IPv6               June 2004   For a type 2 routing header, the Hdr Ext Len MUST be 2.  The Segments   Left value describes the number of route segments remaining; i.e.,   number of explicitly listed intermediate nodes still to be visited   before reaching the final destination.  Segments Left MUST be 1.  The   ordering rules for extension headers in an IPv6 packet are described   inSection 4.1 of RFC 2460 [11].  The type 2 routing header defined   for Mobile IPv6 follows the same ordering as other routing headers.   If both a type 0 and a type 2 routing header are present, the type 2   routing header should follow the other routing header.  A packet   containing such nested encapsulation should be created as if the   inner (type 2) routing header was constructed first and then treated   as an original packet by the outer (type 0) routing header   construction process.   In addition, the general procedures defined by IPv6 for routing   headers suggest that a received routing header MAY be automatically   "reversed" to construct a routing header for use in any response   packets sent by upper-layer protocols, if the received packet is   authenticated [6].  This MUST NOT be done automatically for type 2   routing headers.6.5.  ICMP Home Agent Address Discovery Request Message   The ICMP Home Agent Address Discovery Request message is used by a   mobile node to initiate the dynamic home agent address discovery   mechanism, as described inSection 11.4.1.  The mobile node sends the   Home Agent Address Discovery Request message to the Mobile IPv6   Home-Agents anycast address [16] for its own home subnet prefix.   (Note that the currently defined anycast addresses may not work with   all prefix lengths other than those defined inRFC 2373 [3,35].)    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |     Code      |            Checksum           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Identifier           |            Reserved           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      144   Code      0Johnson, et al.              Standard Track                    [Page 55]

RFC 3775                Mobility Support in IPv6               June 2004   Checksum      The ICMP checksum [14].   Identifier      An identifier to aid in matching Home Agent Address Discovery      Reply messages to this Home Agent Address Discovery Request      message.   Reserved      This field is unused.  It MUST be initialized to zero by the      sender and MUST be ignored by the receiver.   The Source Address of the Home Agent Address Discovery Request   message packet is typically one of the mobile node's current care-of   addresses.  At the time of performing this dynamic home agent address   discovery procedure, it is likely that the mobile node is not   registered with any home agent.  Therefore, neither the nature of the   address nor the identity of the mobile node can be established at   this time.  The home agent MUST then return the Home Agent Address   Discovery Reply message directly to the Source Address chosen by the   mobile node.6.6.  ICMP Home Agent Address Discovery Reply Message   The ICMP Home Agent Address Discovery Reply message is used by a home   agent to respond to a mobile node that uses the dynamic home agent   address discovery mechanism, as described inSection 10.5.     0                   1                   2                   3     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |     Type      |     Code      |            Checksum           |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |           Identifier          |             Reserved          |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                                                               |    +                                                               +    .                                                               .    .                      Home Agent Addresses                     .    .                                                               .    +                                                               +    |                                                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Johnson, et al.              Standard Track                    [Page 56]

RFC 3775                Mobility Support in IPv6               June 2004   Type      145   Code      0   Checksum      The ICMP checksum [14].   Identifier      The identifier from the invoking Home Agent Address Discovery      Request message.   Reserved      This field is unused.  It MUST be initialized to zero by the      sender and MUST be ignored by the receiver.   Home Agent Addresses      A list of addresses of home agents on the home link for the mobile      node.  The number of addresses presented in the list is indicated      by the remaining length of the IPv6 packet carrying the Home Agent      Address Discovery Reply message.6.7.  ICMP Mobile Prefix Solicitation Message Format   The ICMP Mobile Prefix Solicitation Message is sent by a mobile node   to its home agent while it is away from home.  The purpose of the   message is to solicit a Mobile Prefix Advertisement from the home   agent, which will allow the mobile node to gather prefix information   about its home network.  This information can be used to configure   and update home address(es) according to changes in prefix   information supplied by the home agent.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |     Code      |          Checksum             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Identifier           |            Reserved           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Johnson, et al.              Standard Track                    [Page 57]

RFC 3775                Mobility Support in IPv6               June 2004   IP Fields:   Source Address      The mobile node's care-of address.   Destination Address      The address of the mobile node's home agent.  This home agent must      be on the link that the mobile node wishes to learn prefix      information about.   Hop Limit      Set to an initial hop limit value, similarly to any other unicast      packet sent by the mobile node.   Destination Option:      A Home Address destination option MUST be included.   ESP header:      IPsec headers MUST be supported and SHOULD be used as described inSection 5.4.   ICMP Fields:   Type      146   Code      0   Checksum      The ICMP checksum [14].   Identifier      An identifier to aid in matching a future Mobile Prefix      Advertisement to this Mobile Prefix Solicitation.Johnson, et al.              Standard Track                    [Page 58]

RFC 3775                Mobility Support in IPv6               June 2004   Reserved      This field is unused.  It MUST be initialized to zero by the      sender and MUST be ignored by the receiver.   The Mobile Prefix Solicitation messages may have options.  These   options MUST use the option format defined inRFC 2461 [12].  This   document does not define any option types for the Mobile Prefix   Solicitation message, but future documents may define new options.   Home agents MUST silently ignore any options they do not recognize   and continue processing the message.6.8.  ICMP Mobile Prefix Advertisement Message Format   A home agent will send a Mobile Prefix Advertisement to a mobile node   to distribute prefix information about the home link while the mobile   node is traveling away from the home network.  This will occur in   response to a Mobile Prefix Solicitation with an Advertisement, or by   an unsolicited Advertisement sent according to the rules inSection10.6.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |     Code      |          Checksum             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Identifier           |M|O|        Reserved           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |           Options ...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   IP Fields:   Source Address      The home agent's address as the mobile node would expect to see it      (i.e., same network prefix).   Destination Address      If this message is a response to a Mobile Prefix Solicitation,      this field contains the Source Address field from that packet.      For unsolicited messages, the mobile node's care-of address SHOULD      be used.  Note that unsolicited messages can only be sent if the      mobile node is currently registered with the home agent.Johnson, et al.              Standard Track                    [Page 59]

RFC 3775                Mobility Support in IPv6               June 2004   Routing header:      A type 2 routing header MUST be included.   ESP header:      IPsec headers MUST be supported and SHOULD be used as described inSection 5.4.   ICMP Fields:   Type      147   Code      0   Checksum      The ICMP checksum [14].   Identifier      An identifier to aid in matching this Mobile Prefix Advertisement      to a previous Mobile Prefix Solicitation.   M      1-bit Managed Address Configuration flag.  When set, hosts use the      administered (stateful) protocol for address autoconfiguration in      addition to any addresses autoconfigured using stateless address      autoconfiguration.  The use of this flag is described in [12,13].   O      1-bit Other Stateful Configuration flag.  When set, hosts use the      administered (stateful) protocol for autoconfiguration of other      (non-address) information.  The use of this flag is described in      [12,13].   Reserved      This field is unused.  It MUST be initialized to zero by the      sender and MUST be ignored by the receiver.Johnson, et al.              Standard Track                    [Page 60]

RFC 3775                Mobility Support in IPv6               June 2004   The Mobile Prefix Advertisement messages may have options.  These   options MUST use the option format defined inRFC 2461 [12].  This   document defines one option which may be carried in a Mobile Prefix   Advertisement message, but future documents may define new options.   Mobile nodes MUST silently ignore any options they do not recognize   and continue processing the message.   Prefix Information      Each message contains one or more Prefix Information options.      Each option carries the prefix(es) that the mobile node should use      to configure its home address(es).Section 10.6 describes which      prefixes should be advertised to the mobile node.      The Prefix Information option is defined in Section 4.6.2 ofRFC2461 [12], with modifications defined inSection 7.2 of this      specification.  The home agent MUST use this modified Prefix      Information option to send home network prefixes as defined inSection 10.6.1.   If the Advertisement is sent in response to a Mobile Prefix   Solicitation, the home agent MUST copy the Identifier value from that   message into the Identifier field of the Advertisement.   The home agent MUST NOT send more than one Mobile Prefix   Advertisement message per second to any mobile node.   The M and O bits MUST be cleared if the Home Agent DHCPv6 support is   not provided.  If such support is provided then they are set in   concert with the home network's administrative settings.7.  Modifications to IPv6 Neighbor Discovery7.1.  Modified Router Advertisement Message Format   Mobile IPv6 modifies the format of the Router Advertisement message   [12] by the addition of a single flag bit to indicate that the router   sending the Advertisement message is serving as a home agent on this   link.  The format of the Router Advertisement message is as follows:Johnson, et al.              Standard Track                    [Page 61]

RFC 3775                Mobility Support in IPv6               June 2004    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |     Code      |          Checksum             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Cur Hop Limit |M|O|H| Reserved|       Router Lifetime         |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         Reachable Time                        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                          Retrans Timer                        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   Options ...   +-+-+-+-+-+-+-+-+-+-+-+-   This format represents the following changes over that originally   specified for Neighbor Discovery [12]:   Home Agent (H)      The Home Agent (H) bit is set in a Router Advertisement to      indicate that the router sending this Router Advertisement is also      functioning as a Mobile IPv6 home agent on this link.   Reserved      Reduced from a 6-bit field to a 5-bit field to account for the      addition of the above bit.7.2.  Modified Prefix Information Option Format   Mobile IPv6 requires knowledge of a router's global address in   building a Home Agents List as part of the dynamic home agent address   discovery mechanism.   However, Neighbor Discovery [12] only advertises a router's link-   local address, by requiring this address to be used as the IP Source   Address of each Router Advertisement.   Mobile IPv6 extends Neighbor Discovery to allow a router to advertise   its global address, by the addition of a single flag bit in the   format of a Prefix Information option for use in Router Advertisement   messages.  The format of the Prefix Information option is as follows:Johnson, et al.              Standard Track                    [Page 62]

RFC 3775                Mobility Support in IPv6               June 2004    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     | Prefix Length |L|A|R|Reserved1|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         Valid Lifetime                        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                       Preferred Lifetime                      |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                           Reserved2                           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   +                                                               +   |                                                               |   +                            Prefix                             +   |                                                               |   +                                                               +   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   This format represents the following changes over that originally   specified for Neighbor Discovery [12]:   Router Address (R)      1-bit router address flag.  When set, indicates that the Prefix      field contains a complete IP address assigned to the sending      router.  The indicated prefix is the first Prefix Length bits of      the Prefix field.  The router IP address has the same scope and      conforms to the same lifetime values as the advertised prefix.      This use of the Prefix field is compatible with its use in      advertising the prefix itself, since Prefix Advertisement uses      only the leading bits.  Interpretation of this flag bit is thus      independent of the processing required for the On-Link (L) and      Autonomous Address-Configuration (A) flag bits.   Reserved1      Reduced from a 6-bit field to a 5-bit field to account for the      addition of the above bit.   In a Router Advertisement, a home agent MUST, and all other routers   MAY, include at least one Prefix Information option with the Router   Address (R) bit set.  Neighbor Discovery specifies that, if including   all options in a Router Advertisement causes the size of the   Advertisement to exceed the link MTU, multiple Advertisements can be   sent, each containing a subset of the options [12].  Also, when   sending unsolicited multicast Router Advertisements more frequentlyJohnson, et al.              Standard Track                    [Page 63]

RFC 3775                Mobility Support in IPv6               June 2004   than the limit specified inRFC 2461 [12], the sending router need   not include all options in each of these Advertisements.  However, in   both of these cases the router SHOULD include at least one Prefix   Information option with the Router Address (R) bit set in each such   advertisement, if this bit is set in some advertisement sent by the   router.   In addition, the following requirement can assist mobile nodes in   movement detection.  Barring changes in the prefixes for the link,   routers that send multiple Router Advertisements with the Router   Address (R) bit set in some of the included Prefix Information   options SHOULD provide at least one option and router address which   stays the same in all of the Advertisements.7.3.  New Advertisement Interval Option Format   Mobile IPv6 defines a new Advertisement Interval option, used in   Router Advertisement messages to advertise the interval at which the   sending router sends unsolicited multicast Router Advertisements.   The format of the Advertisement Interval option is as follows:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |           Reserved            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                     Advertisement Interval                    |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      7   Length      8-bit unsigned integer.  The length of the option (including the      type and length fields) is in units of 8 octets.  The value of      this field MUST be 1.   Reserved      This field is unused.  It MUST be initialized to zero by the      sender and MUST be ignored by the receiver.Johnson, et al.              Standard Track                    [Page 64]

RFC 3775                Mobility Support in IPv6               June 2004   Advertisement Interval      32-bit unsigned integer.  The maximum time, in milliseconds,      between successive unsolicited Router Advertisement messages sent      by this router on this network interface.  Using the conceptual      router configuration variables defined by Neighbor Discovery [12],      this field MUST be equal to the value MaxRtrAdvInterval, expressed      in milliseconds.   Routers MAY include this option in their Router Advertisements.  A   mobile node receiving a Router Advertisement containing this option   SHOULD utilize the specified Advertisement Interval for that router   in its movement detection algorithm, as described inSection 11.5.1.   This option MUST be silently ignored for other Neighbor Discovery   messages.7.4.  New Home Agent Information Option Format   Mobile IPv6 defines a new Home Agent Information option, used in   Router Advertisements sent by a home agent to advertise information   specific to this router's functionality as a home agent.  The format   of the Home Agent Information option is as follows:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |           Reserved            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Home Agent Preference     |      Home Agent Lifetime      |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      8   Length      8-bit unsigned integer.  The length of the option (including the      type and length fields) in units of 8 octets.  The value of this      field MUST be 1.   Reserved      This field is unused.  It MUST be initialized to zero by the      sender and MUST be ignored by the receiver.Johnson, et al.              Standard Track                    [Page 65]

RFC 3775                Mobility Support in IPv6               June 2004   Home Agent Preference      16-bit unsigned integer.  The preference for the home agent      sending this Router Advertisement, for use in ordering the      addresses returned to a mobile node in the Home Agent Addresses      field of a Home Agent Address Discovery Reply message.  Higher      values mean more preferable.  If this option is not included in a      Router Advertisement in which the Home Agent (H) bit is set, the      preference value for this home agent MUST be considered to be 0.      Greater values indicate a more preferable home agent than lower      values.      The manual configuration of the Home Agent Preference value is      described inSection 8.4.  In addition, the sending home agent MAY      dynamically set the Home Agent Preference value, for example      basing it on the number of mobile nodes it is currently serving or      on its remaining resources for serving additional mobile nodes;      such dynamic settings are beyond the scope of this document.  Any      such dynamic setting of the Home Agent Preference, however, MUST      set the preference appropriately, relative to the default Home      Agent Preference value of 0 that may be in use by some home agents      on this link (i.e., a home agent not including a Home Agent      Information option in its Router Advertisements will be considered      to have a Home Agent Preference value of 0).   Home Agent Lifetime      16-bit unsigned integer.  The lifetime associated with the home      agent in units of seconds.  The default value is the same as the      Router Lifetime, as specified in the main body of the Router      Advertisement.  The maximum value corresponds to 18.2 hours.  A      value of 0 MUST NOT be used.  The Home Agent Lifetime applies only      to this router's usefulness as a home agent; it does not apply to      information contained in other message fields or options.   Home agents MAY include this option in their Router Advertisements.   This option MUST NOT be included in a Router Advertisement in which   the Home Agent (H) bit (seeSection 7.1) is not set.  If this option   is not included in a Router Advertisement in which the Home Agent (H)   bit is set, the lifetime for this home agent MUST be considered to be   the same as the Router Lifetime in the Router Advertisement.  If   multiple Advertisements are being sent instead of a single larger   unsolicited multicast Advertisement, all of the multiple   Advertisements with the Router Address (R) bit set MUST include this   option with the same contents, otherwise this option MUST be omitted   from all Advertisements.Johnson, et al.              Standard Track                    [Page 66]

RFC 3775                Mobility Support in IPv6               June 2004   This option MUST be silently ignored for other Neighbor Discovery   messages.   If both the Home Agent Preference and Home Agent Lifetime are set to   their default values specified above, this option SHOULD NOT be   included in the Router Advertisement messages sent by this home   agent.7.5.  Changes to Sending Router Advertisements   The Neighbor Discovery protocol specification [12] limits routers to   a minimum interval of 3 seconds between sending unsolicited multicast   Router Advertisement messages from any given network interface   (limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that:      "Routers generate Router Advertisements frequently enough that      hosts will learn of their presence within a few minutes, but not      frequently enough to rely on an absence of advertisements to      detect router failure; a separate Neighbor Unreachability      Detection algorithm provides failure detection."   This limitation, however, is not suitable to providing timely   movement detection for mobile nodes.  Mobile nodes detect their own   movement by learning the presence of new routers as the mobile node   moves into wireless transmission range of them (or physically   connects to a new wired network), and by learning that previous   routers are no longer reachable.  Mobile nodes MUST be able to   quickly detect when they move to a link served by a new router, so   that they can acquire a new care-of address and send Binding Updates   to register this care-of address with their home agent and to notify   correspondent nodes as needed.   One method which can provide for faster movement detection, is to   increase the rate at which unsolicited Router Advertisements are   sent.  Mobile IPv6 relaxes this limit such that routers MAY send   unsolicited multicast Router Advertisements more frequently.  This   method can be applied where the router is expecting to provide   service to visiting mobile nodes (e.g., wireless network interfaces),   or on which it is serving as a home agent to one or more mobile nodes   (who may return home and need to hear its Advertisements).   Routers supporting mobility SHOULD be able to be configured with a   smaller MinRtrAdvInterval value and MaxRtrAdvInterval value to allow   sending of unsolicited multicast Router Advertisements more often.   The minimum allowed values are:Johnson, et al.              Standard Track                    [Page 67]

RFC 3775                Mobility Support in IPv6               June 2004   o  MinRtrAdvInterval 0.03 seconds   o  MaxRtrAdvInterval 0.07 seconds   In the case where the minimum intervals and delays are used, the mean   time between unsolicited multicast router advertisements is 50 ms.   Use of these modified limits MUST be configurable (see also the   configuration variable MinDelayBetweenRas inSection 13 which may   also have to be modified accordingly).  Systems where these values   are available MUST NOT default to them, and SHOULD default to values   specified inRFC 2461.  Knowledge of the type of network interface   and operating environment SHOULD be taken into account in configuring   these limits for each network interface.  This is important with some   wireless links, where increasing the frequency of multicast beacons   can cause considerable overhead.  Routers SHOULD adhere to the   intervals specified inRFC 2461 [12], if this overhead is likely to   cause service degradation.   Additionally, the possible low values of MaxRtrAdvInterval may cause   some problems with movement detection in some mobile nodes.  To   ensure that this is not a problem, Routers SHOULD add 20 ms to any   Advertisement Intervals sent in RAs, which are below 200 ms, in order   to account for scheduling granularities on both the MN and the   Router.   Note that multicast Router Advertisements are not always required in   certain wireless networks that have limited bandwidth.  Mobility   detection or link changes in such networks may be done at lower   layers.  Router advertisements in such networks SHOULD be sent only   when solicited.  In such networks it SHOULD be possible to disable   unsolicited multicast Router Advertisements on specific interfaces.   The MinRtrAdvInterval and MaxRtrAdvInterval in such a case can be set   to some high values.   Home agents MUST include the Source Link-Layer Address option in all   Router Advertisements they send.  This simplifies the process of   returning home, as discussed inSection 11.5.4.   Note that according toRFC 2461 [12], AdvDefaultLifetime is by   default based on the value of MaxRtrAdvInterval.  AdvDefaultLifetime   is used in the Router Lifetime field of Router Advertisements.  Given   that this field is expressed in seconds, a small MaxRtrAdvInterval   value can result in a zero value for this field.  To prevent this,   routers SHOULD keep AdvDefaultLifetime in at least one second, even   if the use of MaxRtrAdvInterval would result in a smaller value.Johnson, et al.              Standard Track                    [Page 68]

RFC 3775                Mobility Support in IPv6               June 20048.  Requirements for Types of IPv6 Nodes   Mobile IPv6 places some special requirements on the functions   provided by different types of IPv6 nodes.  This section summarizes   those requirements, identifying the functionality each requirement is   intended to support.   The requirements are set for the following groups of nodes:   o  All IPv6 nodes.   o  All IPv6 nodes with support for route optimization.   o  All IPv6 routers.   o  All Mobile IPv6 home agents.   o  All Mobile IPv6 mobile nodes.   It is outside the scope of this specification to specify which of   these groups are mandatory in IPv6.  We only describe what is   mandatory for a node that supports, for instance, route optimization.   Other specifications are expected to define the extent of IPv6.8.1.  All IPv6 Nodes   Any IPv6 node may at any time be a correspondent node of a mobile   node, either sending a packet to a mobile node or receiving a packet   from a mobile node.  There are no Mobile IPv6 specific MUST   requirements for such nodes, and basic IPv6 techniques are   sufficient.  If a mobile node attempts to set up route optimization   with a node with only basic IPv6 support, an ICMP error will signal   that the node does not support such optimizations (Section 11.3.5),   and communications will flow through the home agent.   An IPv6 node MUST NOT support the Home Address destination option,   type 2 routing header, or the Mobility Header unless it fully   supports the requirements listed in the next sections for either   route optimization, mobile node, or home agent functionality.8.2.  IPv6 Nodes with Support for Route Optimization   Nodes that implement route optimization are a subset of all IPv6   nodes on the Internet.  The ability of a correspondent node to   participate in route optimization is essential for the efficient   operation of the IPv6 Internet, for the following reasons:Johnson, et al.              Standard Track                    [Page 69]

RFC 3775                Mobility Support in IPv6               June 2004   o  Avoidance of congestion in the home network, and enabling the use      of lower-performance home agent equipment even for supporting      thousands of mobile nodes.   o  Reduced network load across the entire Internet, as mobile devices      begin to predominate.   o  Reduction of jitter and latency for the communications.   o  Greater likelihood of success for QoS signaling as tunneling is      avoided and, again, fewer sources of congestion.   o  Improved robustness against network partitions, congestion, and      other problems, since fewer routing path segments are traversed.   These effects combine to enable much better performance and   robustness for communications between mobile nodes and IPv6   correspondent nodes.  Route optimization introduces a small amount of   additional state for the peers, some additional messaging, and up to   1.5 roundtrip delays before it can be turned on.  However, it is   believed that the benefits far outweigh the costs in most cases.Section 11.3.1 discusses how mobile nodes may avoid route   optimization for some of the remaining cases, such as very short-term   communications.   The following requirements apply to all correspondent nodes that   support route optimization:   o  The node MUST be able to validate a Home Address option using an      existing Binding Cache entry, as described inSection 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 inSection9.3.2.   o  Unless the correspondent node is also acting as a mobile node, it      MUST ignore type 2 routing headers and silently discard all      packets that it has received with such headers.   o  The node SHOULD be able to interpret ICMP messages as described inSection 9.3.4.   o  The node MUST be able to send Binding Error messages as described      inSection 9.3.3.   o  The node MUST be able to process Mobility Headers as described inSection 9.2.Johnson, et al.              Standard Track                    [Page 70]

RFC 3775                Mobility Support in IPv6               June 2004   o  The node MUST be able to participate in a return routability      procedure (Section 9.4).   o  The node MUST be able to process Binding Update messages (Section9.5).   o  The node MUST be able to return a Binding Acknowledgement (Section9.5.4).   o  The node MUST be able to maintain a Binding Cache of the bindings      received in accepted Binding Updates, as described inSection 9.1      andSection 9.6.   o  The node SHOULD allow route optimization to be administratively      enabled or disabled.  The default SHOULD be enabled.8.3.  All IPv6 Routers   All IPv6 routers, even those not serving as a home agent for Mobile   IPv6, have an effect on how well mobile nodes can communicate:   o  Every IPv6 router SHOULD be able to send an Advertisement Interval      option (Section 7.3) in each of its Router Advertisements [12], to      aid movement detection by mobile nodes (as inSection 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 inSection 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 inSection 7.2).   o  Routers supporting filtering packets with routing headers SHOULD      support different rules for type 0 and type 2 routing headers (seeSection 6.4) so that filtering of source routed packets (type 0)      will not necessarily limit Mobile IPv6 traffic which is delivered      via type 2 routing headers.8.4.  IPv6 Home Agents   In order for a mobile node to operate correctly while away from home,   at least one IPv6 router on the mobile node's home link must function   as a home agent for the mobile node.  The following additional   requirements apply to all IPv6 routers that serve as a home agent:Johnson, et al.              Standard Track                    [Page 71]

RFC 3775                Mobility Support in IPv6               June 2004   o  Every home agent MUST be able to maintain an entry in its Binding      Cache for each mobile node for which it is serving as the home      agent (Section 10.1 andSection 10.3.1).   o  Every home agent MUST be able to intercept packets (using proxy      Neighbor Discovery [12]) addressed to a mobile node for which it      is currently serving as the home agent, on that mobile node's home      link, while the mobile node is away from home (Section 10.4.1).   o  Every home agent MUST be able to encapsulate [15] such intercepted      packets in order to tunnel them to the primary care-of address for      the mobile node indicated in its binding in the home agent's      Binding Cache (Section 10.4.2).   o  Every home agent MUST support decapsulating [15] reverse tunneled      packets sent to it from a mobile node's home address.  Every home      agent MUST also check that the source address in the tunneled      packets corresponds to the currently registered location of the      mobile node (Section 10.4.5).   o  The node MUST be able to process Mobility Headers as described inSection 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 inSection 10.1 andSection 10.5.1.   o  Every home agent MUST be able to accept packets addressed to the      Mobile IPv6 Home-Agents anycast address [16] for the subnet on      which it is serving as a home agent, and MUST be able to      participate in dynamic home agent address discovery (Section10.5).   o  Every home agent SHOULD support a configuration mechanism to allow      a system administrator to manually set the value to be sent by      this home agent in the Home Agent Preference field of the Home      Agent Information Option in Router Advertisements that it sends      (Section 7.4).   o  Every home agent SHOULD support sending ICMP Mobile Prefix      Advertisements (Section 6.8), and SHOULD respond to Mobile Prefix      Solicitations (Section 6.7).  If supported, this behavior MUST be      configurable, so that home agents can be configured to avoid      sending such Prefix Advertisements according to the needs of the      network administration in the home domain.Johnson, et al.              Standard Track                    [Page 72]

RFC 3775                Mobility Support in IPv6               June 2004   o  Every home agent MUST support IPsec ESP for protection of packets      belonging to the return routability procedure (Section 10.4.6).   o  Every home agent SHOULD support the multicast group membership      control protocols as described inSection 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 inSection 10.4.4.8.5.  IPv6 Mobile Nodes   Finally, the following requirements apply to all IPv6 nodes capable   of functioning as mobile nodes:   o  The node MUST maintain a Binding Update List (Section 11.1).   o  The node MUST support sending packets containing a Home Address      option (Section 11.3.1), and follow the required IPsec interaction      (Section 11.3.2).   o  The node MUST be able to perform IPv6 encapsulation and      decapsulation [15].   o  The node MUST be able to process type 2 routing header as defined      inSection 6.4 andSection 11.3.3.   o  The node MUST support receiving a Binding Error message (Section11.3.6).   o  The node MUST support receiving ICMP errors (Section 11.3.5).   o  The node MUST support movement detection, care-of address      formation, and returning home (Section 11.5).   o  The node MUST be able to process Mobility Headers as described inSection 11.2.   o  The node MUST support the return routability procedure (Section11.6).   o  The node MUST be able to send Binding Updates, as specified inSection 11.7.1 andSection 11.7.2.   o  The node MUST be able to receive and process Binding      Acknowledgements, as specified inSection 11.7.3.Johnson, et al.              Standard Track                    [Page 73]

RFC 3775                Mobility Support in IPv6               June 2004   o  The node MUST support receiving a Binding Refresh Request (Section6.1.2), 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 inSection 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 inSection11.3.4.  If this support is provided, the mobile node MUST be able      to receive tunneled multicast packets from the home agent.   o  The node MAY support stateful address autoconfiguration mechanisms      such as DHCPv6 [29] on the interface represented by the tunnel to      the home agent.9.  Correspondent Node Operation9.1.  Conceptual Data Structures   IPv6 nodes with route optimization support maintain a Binding Cache   of bindings for other nodes.  A separate Binding Cache SHOULD be   maintained by each IPv6 node for each of its unicast routable   addresses.  The Binding Cache MAY be implemented in any manner   consistent with the external behavior described in this document, for   example by being combined with the node's Destination Cache as   maintained by Neighbor Discovery [12].  When sending a packet, the   Binding Cache is searched before the Neighbor Discovery conceptual   Destination Cache [12].   Each Binding Cache entry conceptually contains the following fields:   o  The home address of the mobile node for which this is the Binding      Cache entry.  This field is used as the key for searching the      Binding Cache for the destination address of a packet being sent.   o  The care-of address for the mobile node indicated by the home      address field in this Binding Cache entry.Johnson, et al.              Standard Track                    [Page 74]

RFC 3775                Mobility Support in IPv6               June 2004   o  A lifetime value, indicating the remaining lifetime for this      Binding Cache entry.  The lifetime value is initialized from the      Lifetime field in the Binding Update that created or last modified      this Binding Cache entry.   o  A flag indicating whether or not this Binding Cache entry is a      home registration entry (applicable only on nodes which support      home agent functionality).   o  The maximum value of the Sequence Number field received in      previous Binding Updates for this home address.  The Sequence      Number field is 16 bits long.  Sequence Number values MUST be      compared modulo 2**16 as explained inSection 9.5.1.   o  Usage information for this Binding Cache entry.  This is needed to      implement the cache replacement policy in use in the Binding      Cache.  Recent use of a cache entry also serves as an indication      that a Binding Refresh Request should be sent when the lifetime of      this entry nears expiration.   Binding Cache entries not marked as home registrations MAY be   replaced at any time by any reasonable local cache replacement policy   but SHOULD NOT be unnecessarily deleted.  The Binding Cache for any   one of a node's IPv6 addresses may contain at most one entry for each   mobile node home address.  The contents of a node's Binding Cache   MUST NOT be changed in response to a Home Address option in a   received packet.9.2.  Processing Mobility Headers   Mobility Header processing MUST observe the following rules:   o  The checksum must be verified as perSection 6.1.  Otherwise, the      node MUST silently discard the message.   o  The MH Type field MUST have a known value (Section 6.1.1).      Otherwise, the node MUST discard the message and issue a Binding      Error message as described inSection 9.3.3, with Status field set      to 2 (unrecognized MH Type value).   o  The Payload Proto field MUST be IPPROTO_NONE (59 decimal).      Otherwise, the node MUST discard the message and SHOULD send ICMP      Parameter Problem, Code 0, directly to the Source Address of the      packet as specified inRFC 2463 [14].  Thus no Binding Cache      information is used in sending the ICMP message.  The Pointer      field in the ICMP message SHOULD point at the Payload Proto field.Johnson, et al.              Standard Track                    [Page 75]

RFC 3775                Mobility Support in IPv6               June 2004   o  The Header Len field in the Mobility Header MUST NOT be less than      the length specified for this particular type of message inSection 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 inRFC 2463 [14].  (The Binding      Cache information is again not used.) The Pointer field in the      ICMP message SHOULD point at the Header Len field.      Subsequent checks depend on the particular Mobility Header.9.3.  Packet Processing   This section describes how the correspondent node sends packets to   the mobile node, and receives packets from it.9.3.1.  Receiving Packets with Home Address Option   Packets containing a Home Address option MUST be dropped if the given   home address is not a unicast routable address.   Mobile nodes can include a Home Address destination option in a   packet if they believe the correspondent node has a Binding Cache   entry for the home address of a mobile node.  Packets containing a   Home Address option MUST be dropped if there is no corresponding   Binding Cache entry.  A corresponding Binding Cache entry MUST have   the same home address as appears in the Home Address destination   option, and the currently registered care-of address MUST be equal to   the source address of the packet.  These tests MUST NOT be done for   packets that contain a Home Address option and a Binding Update.   If the packet is dropped due the above tests, the correspondent node   MUST send the Binding Error message as described inSection 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, sinceJohnson, et al.              Standard Track                    [Page 76]

RFC 3775                Mobility Support in IPv6               June 2004   it indicates that the data within the option cannot change en route   to the packet's final destination, and thus the option is included in   the AH computation.  By requiring that any authentication of the IPv6   header also cover the Home Address option, the security of the Source   Address field in the IPv6 header is not compromised by the presence   of a Home Address option.   When attempting to verify AH authentication data in a packet that   contains a Home Address option, the receiving node MUST calculate the   AH authentication data as if the following were true: The Home   Address option contains the care-of address, and the source IPv6   address field of the IPv6 header contains the home address.  This   conforms with the calculation specified inSection 11.3.2.9.3.2.  Sending Packets to a Mobile Node   Before sending any packet, the sending node SHOULD examine its   Binding Cache for an entry for the destination address to which the   packet is being sent.  If the sending node has a Binding Cache entry   for this address, the sending node SHOULD use a type 2 routing header   to route the packet to this mobile node (the destination node) by way   of its care-of address.  However, the sending node MUST not do this   in the following cases:   o  When sending an IPv6 Neighbor Discovery [12] packet.   o  Where otherwise noted inSection 6.1.   When calculating authentication data in a packet that contains a type   2 routing header, the correspondent node MUST calculate the AH   authentication data as if the following were true: The routing header   contains the care-of address, the destination IPv6 address field of   the IPv6 header contains the home address, and the Segments Left   field is zero.  The IPsec Security Policy Database lookup MUST based   on the mobile node's home address.   For instance, assuming there are no additional routing headers in   this packet beyond those needed by Mobile IPv6, the correspondent   node could set the fields in the packet's IPv6 header and routing   header as follows:   o  The Destination Address in the packet's IPv6 header is set to the      mobile node's home address (the original destination address to      which the packet was being sent).Johnson, et al.              Standard Track                    [Page 77]

RFC 3775                Mobility Support in IPv6               June 2004   o  The routing header is initialized to contain a single route      segment, containing the mobile node's care-of address copied from      the Binding Cache entry.  The Segments Left field is, however,      temporarily set to zero.   The IP layer will insert the routing header before performing any   necessary IPsec processing.  Once all IPsec processing has been   performed, the node swaps the IPv6 destination field with the Home   Address field in the routing header, sets the Segments Left field to   one, and sends the packet.  This ensures the AH calculation is done   on the packet in the form it will have on the receiver after   advancing the routing header.   Following the definition of a type 2 routing header inSection 6.4,   this packet will be routed to the mobile node's care-of address,   where it will be delivered to the mobile node (the mobile node has   associated the care-of address with its network interface).   Note that following the above conceptual model in an implementation   creates some additional requirements for path MTU discovery since the   layer that decides the packet size (e.g., TCP and applications using   UDP) needs to be aware of the size of the headers added by the IP   layer on the sending node.   If, instead, the sending node has no Binding Cache entry for the   destination address to which the packet is being sent, the sending   node simply sends the packet normally, with no routing header.  If   the destination node is not a mobile node (or is a mobile node that   is currently at home), the packet will be delivered directly to this   node and processed normally by it.  If, however, the destination node   is a mobile node that is currently away from home, the packet will be   intercepted by the mobile node's home agent and tunneled to the   mobile node's current primary care-of address.9.3.3.  Sending Binding Error MessagesSection 9.2 andSection 9.3.1 describe error conditions that lead to   a need to send a Binding Error message.   A Binding Error message is sent directly to the address that appeared   in the IPv6 Source Address field of the offending packet.  If the   Source Address field does not contain a unicast address, the Binding   Error message MUST NOT be sent.   The Home Address field in the Binding Error message MUST be copied   from the Home Address field in the Home Address destination option of   the offending packet, or set to the unspecified address if no such   option appeared in the packet.Johnson, et al.              Standard Track                    [Page 78]

RFC 3775                Mobility Support in IPv6               June 2004   Note that the IPv6 Source Address and Home Address field values   discussed above are the values from the wire, i.e., before any   modifications possibly performed as specified inSection 9.3.1.   Binding Error messages SHOULD be subject to rate limiting in the same   manner as is done for ICMPv6 messages [14].9.3.4.  Receiving ICMP Error Messages   When the correspondent node has a Binding Cache entry for a mobile   node, all traffic destined to the mobile node goes directly to the   current care-of address of the mobile node using a routing header.   Any ICMP error message caused by packets on their way to the care-of   address will be returned in the normal manner to the correspondent   node.   On the other hand, if the correspondent node has no Binding Cache   entry for the mobile node, the packet will be routed through the   mobile node's home link.  Any ICMP error message caused by the packet   on its way to the mobile node while in the tunnel, will be   transmitted to the mobile node's home agent.  By the definition of   IPv6 encapsulation [15], the home agent MUST relay certain ICMP error   messages back to the original sender of the packet, which in this   case is the correspondent node.   Thus, in all cases, any meaningful ICMP error messages caused by   packets from a correspondent node to a mobile node will be returned   to the correspondent node.  If the correspondent node receives   persistent ICMP Destination Unreachable messages after sending   packets to a mobile node based on an entry in its Binding Cache, the   correspondent node SHOULD delete this Binding Cache entry.  Note that   if the mobile node continues to send packets with the Home Address   destination option to this correspondent node, they will be dropped   due to the lack of a binding.  For this reason it is important that   only persistent ICMP messages lead to the deletion of the Binding   Cache entry.9.4.  Return Routability Procedure   This subsection specifies actions taken by a correspondent node   during the return routability procedure.Johnson, et al.              Standard Track                    [Page 79]

RFC 3775                Mobility Support in IPv6               June 20049.4.1.  Receiving Home Test Init Messages   Upon receiving a Home Test Init message, the correspondent node   verifies the following:   o  The packet MUST NOT include a Home Address destination option.   Any packet carrying a Home Test Init message which fails to satisfy   all of these tests MUST be silently ignored.   Otherwise, in preparation for sending the corresponding Home Test   Message, the correspondent node checks that it has the necessary   material to engage in a return routability procedure, as specified inSection 5.2.  The correspondent node MUST have a secret Kcn and a   nonce.  If it does not have this material yet, it MUST produce it   before continuing with the return routability procedure.Section 9.4.3 specifies further processing.9.4.2.  Receiving Care-of Test Init Messages   Upon receiving a Care-of Test Init message, the correspondent node   verifies the following:   o  The packet MUST NOT include a Home Address destination option.   Any packet carrying a Care-of Test Init message which fails to   satisfy all of these tests MUST be silently ignored.   Otherwise, in preparation for sending the corresponding Care-of Test   Message, the correspondent node checks that it has the necessary   material to engage in a return routability procedure in the manner   described inSection 9.4.1.Section 9.4.4 specifies further processing.9.4.3.  Sending Home Test Messages   The correspondent node creates a home keygen token and uses the   current nonce index as the Home Nonce Index.  It then creates a Home   Test message (Section 6.1.5) and sends it to the mobile node at the   latter's home address.Johnson, et al.              Standard Track                    [Page 80]

RFC 3775                Mobility Support in IPv6               June 20049.4.4.  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 Error9.5.1.  Receiving Binding Updates   Before accepting a Binding Update, the receiving node MUST validate   the Binding Update according to the following tests:   o  The packet MUST contain a unicast routable home address, either in      the Home Address option or in the Source Address, if the Home      Address option is not present.   o  The Sequence Number field in the Binding Update is greater than      the Sequence Number received in the previous valid Binding Update      for this home address, if any.   If the receiving node has no Binding Cache entry for the indicated   home address, it MUST accept any Sequence Number value in a received   Binding Update from this mobile node.   This Sequence Number comparison MUST be performed modulo 2**16, i.e.,   the number is a free running counter represented modulo 65536.  A   Sequence Number in a received Binding Update is considered less than   or equal to the last received number if its value lies in the range   of the last received number and the preceding 32768 values,   inclusive.  For example, if the last received sequence number was 15,   then messages with sequence numbers 0 through 15, as well as 32783   through 65535, would be considered less than or equal.Johnson, et al.              Standard Track                    [Page 81]

RFC 3775                Mobility Support in IPv6               June 2004   When the Home Registration (H) bit is not set, the following are also   required:   o  A Nonce Indices mobility option MUST be present, and the Home and      Care-of Nonce Index values in this option MUST be recent enough to      be recognized by the correspondent node.  (Care-of Nonce Index      values are not inspected for requests to delete a binding.)   o  The correspondent node MUST re-generate the home keygen token and      the care-of keygen token from the information contained in the      packet.  It then generates the binding management key Kbm and uses      it to verify the authenticator field in the Binding Update as      specified inSection 6.1.7.   o  The Binding Authorization Data mobility option MUST be present,      and its contents MUST satisfy rules presented inSection 5.2.6.      Note that a care-of address different from the Source Address MAY      have been specified by including an Alternate Care-of Address      mobility option in the Binding Update.  When such a message is      received and the return routability procedure is used as an      authorization method, the correspondent node MUST verify the      authenticator by using the address within the Alternate Care-of      Address in the calculations.   o  The Binding Authorization Data mobility option MUST be the last      option and MUST NOT have trailing padding.   If the Home Registration (H) bit is set, the Nonce Indices mobility   option MUST NOT be present.   If the mobile node sends a sequence number which is not greater than   the sequence number from the last valid Binding Update for this home   address, then the receiving node MUST send back a Binding   Acknowledgement with status code 135, and the last accepted sequence   number in the Sequence Number field of the Binding Acknowledgement.   If a binding already exists for the given home address and the home   registration flag has a different value than the Home Registration   (H) bit in the Binding Update, then the receiving node MUST send back   a Binding Acknowledgement with status code 139 (registration type   change disallowed).  The home registration flag stored in the Binding   Cache entry MUST NOT be changed.   If the receiving node no longer recognizes the Home Nonce Index   value, Care-of Nonce Index value, or both values from the Binding   Update, then the receiving node MUST send back a Binding   Acknowledgement with status code 136, 137, or 138, respectively.Johnson, et al.              Standard Track                    [Page 82]

RFC 3775                Mobility Support in IPv6               June 2004   Packets carrying Binding Updates that fail to satisfy all of these   tests for any reason other than insufficiency of the Sequence Number,   registration type change, or expired nonce index values, MUST be   silently discarded.   If the Binding Update is valid according to the tests above, then the   Binding Update is processed further as follows:   o  The Sequence Number value received from a mobile node in a Binding      Update is stored by the receiving node in its Binding Cache entry      for the given home address.   o  If the Lifetime specified in the Binding Update is nonzero and the      specified care-of address is not equal to the home address for the      binding, then this is a request to cache a binding for the home      address.  If the Home Registration (H) bit is set in the Binding      Update, the Binding Update is processed according to the procedure      specified inSection 10.3.1; otherwise, it is processed according      to the procedure specified inSection 9.5.2.   o  If the Lifetime specified in the Binding Update is zero or the      specified care-of address matches the home address for the      binding, then this is a request to delete the cached binding for      the home address.  In this case, the Binding Update MUST include a      valid home nonce index, and the care-of nonce index MUST be      ignored by the correspondent node.  The generation of the binding      management key depends then exclusively on the home keygen token      (Section 5.2.5).  If the Home Registration (H) bit is set in the      Binding Update, the Binding Update is processed according to the      procedure specified inSection 10.3.2; otherwise, it is processed      according to the procedure specified inSection 9.5.3.   The specified care-of address MUST be determined as follows:   o  If the Alternate Care-of Address option is present, the care-of      address is the address in that option.   o  Otherwise, the care-of address is the Source Address field in the      packet's IPv6 header.   The home address for the binding MUST be determined as follows:   o  If the Home Address destination option is present, the home      address is the address in that option.   o  Otherwise, the home address is the Source Address field in the      packet's IPv6 header.Johnson, et al.              Standard Track                    [Page 83]

RFC 3775                Mobility Support in IPv6               June 20049.5.2.  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 inSection 9.3.1, such packets will be dropped if there is no binding.   This situation is recoverable, but can cause temporary packet loss.   The correspondent node MAY refuse to accept a new Binding Cache entry   if it does not have sufficient resources.  A new entry MAY also be   refused if the correspondent node believes its resources are utilized   more efficiently in some other purpose, such as serving another   mobile node with higher amount of traffic.  In both cases the   correspondent node SHOULD return a Binding Acknowledgement with   status value 130.9.5.3 Requests to Delete a Binding   This section describes the processing of a valid Binding Update that   requests a node to delete a binding when the Home Registration (H)   bit is not set in the Binding Update.   Any existing binding for the given home address MUST be deleted.  A   Binding Cache entry for the home address MUST NOT be created in   response to receiving the Binding Update.   If the Binding Cache entry was created by use of return routability   nonces, the correspondent node MUST ensure that the same nonces are   not used again with the particular home and care-of address.  If both   nonces are still valid, the correspondent node has to remember the   particular combination of nonce indexes, addresses, and sequence   number as illegal until at least one of the nonces has become too   old.Johnson, et al.              Standard Track                    [Page 84]

RFC 3775                Mobility Support in IPv6               June 20049.5.4.  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 inSection 9.2 orSection 9.5.1, a Binding Acknowledgement MUST NOT be sent.      Otherwise the treatment depends on the following rules.   o  If the Acknowledge (A) bit set is set in the Binding Update, a      Binding Acknowledgement MUST be sent.  Otherwise, the treatment      depends on the below rule.   o  If the node rejects the Binding Update due to an expired nonce      index, sequence number being out of window (Section 9.5.1), or      insufficiency of resources (Section 9.5.2), a Binding      Acknowledgement MUST be sent.  If the node accepts the Binding      Update, the Binding Acknowledgement SHOULD NOT be sent.   If the node accepts the Binding Update and creates or updates an   entry for this binding, the Status field in the Binding   Acknowledgement MUST be set to a value less than 128.  Otherwise, the   Status field MUST be set to a value greater than or equal to 128.   Values for the Status field are described inSection 6.1.8 and in the   IANA registry of assigned numbers [19].   If the Status field in the Binding Acknowledgement contains the value   136 (expired home nonce index), 137 (expired care-of nonce index), or   138 (expired nonces) then the message MUST NOT include the Binding   Authorization Data mobility option.  Otherwise, the Binding   Authorization Data mobility option MUST be included, and MUST meet   the specific authentication requirements for Binding Acknowledgements   as defined inSection 5.2.   If the Source Address field of the IPv6 header that carried the   Binding Update does not contain a unicast address, the Binding   Acknowledgement MUST NOT be sent and the Binding Update packet MUST   be silently discarded.  Otherwise, the acknowledgement MUST be sent   to the Source Address.  Unlike the treatment of regular packets, this   addressing procedure does not use information from the Binding Cache.   However, a routing header is needed in some cases.  If the Source   Address is the home address of the mobile node, i.e., the Binding   Update did not contain a Home Address destination option, then the   Binding Acknowledgement MUST be sent to that address and the routing   header MUST NOT be used.  Otherwise, the Binding Acknowledgement MUST   be sent using a type 2 routing header which contains the mobile   node's home address.Johnson, et al.              Standard Track                    [Page 85]

RFC 3775                Mobility Support in IPv6               June 20049.5.5.  Sending Binding Refresh Requests   If a Binding Cache entry being deleted is still in active use when   sending packets to a mobile node, then the next packet sent to the   mobile node will be routed normally to the mobile node's home link.   Communication with the mobile node continues, but the tunneling from   the home network creates additional overhead and latency in   delivering packets to the mobile node.   If the sender knows that the Binding Cache entry is still in active   use, it MAY send a Binding Refresh Request message to the mobile node   in an attempt to avoid this overhead and latency due to deleting and   recreating the Binding Cache entry.  This message is always sent to   the home address of the mobile node.   The correspondent node MAY retransmit Binding Refresh Request   messages as long as the rate limitation is applied.  The   correspondent node MUST stop retransmitting when it receives a   Binding Update.9.6.  Cache Replacement Policy   Conceptually, a node maintains a separate timer for each entry in its   Binding Cache.  When creating or updating a Binding Cache entry in   response to a received and accepted Binding Update, the node sets the   timer for this entry to the specified Lifetime period.  Any entry in   a node's Binding Cache MUST be deleted after the expiration of the   Lifetime specified in the Binding Update from which the entry was   created or last updated.   Each node's Binding Cache will, by necessity, have a finite size.  A   node MAY use any reasonable local policy for managing the space   within its Binding Cache.   A node MAY choose to drop any entry already in its Binding Cache in   order to make space for a new entry.  For example, a "least-recently   used" (LRU) strategy for cache entry replacement among entries should   work well, unless the size of the Binding Cache is substantially   insufficient.  When entries are deleted, the correspondent node MUST   follow the rules inSection 5.2.8 in order to guard the return   routability procedure against replay attacks.   If the node sends a packet to a destination for which it has dropped   the entry from its Binding Cache, the packet will be routed through   the mobile node's home link.  The mobile node can detect this and   establish a new binding if necessary.Johnson, et al.              Standard Track                    [Page 86]

RFC 3775                Mobility Support in IPv6               June 2004   However, if the mobile node believes that the binding still exists,   it may use route optimization and send packets with the Home Address   destination option.  This can create temporary packet loss, as   discussed earlier, in the context of binding lifetime reductions   performed by the correspondent node (Section 9.5.2).10.  Home Agent Operation10.1.  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 inSection 9.1.   The Home Agents List is maintained by each home agent, recording   information about each router on the same link that is acting as a   home agent.  This list is used by the dynamic home agent address   discovery mechanism.  A router is known to be acting as a home agent,   if it sends a Router Advertisement in which the Home Agent (H) bit is   set.  When the lifetime for a list entry (defined below) expires,   that entry is removed from the Home Agents List.  The Home Agents   List is similar to the Default Router List conceptual data structure   maintained by each host for Neighbor Discovery [12].  The Home Agents   List MAY be implemented in any manner consistent with the external   behavior described in this document.   Each home agent maintains a separate Home Agents List for each link   on which it is serving as a home agent.  A new entry is created or an   existing entry is updated in response to receipt of a valid Router   Advertisement in which the Home Agent (H) bit is set.  Each Home   Agents List entry conceptually contains the following fields:   o  The link-local IP address of a home agent on the link.  This      address is learned through the Source Address of the Router      Advertisements [12] received from the router.   o  One or more global IP addresses for this home agent.  Global      addresses are learned through Prefix Information options with the      Router Address (R) bit set and received in Router Advertisements      from this link-local address.  Global addresses for the router in      a Home Agents List entry MUST be deleted once the prefix      associated with that address is no longer valid [12].Johnson, et al.              Standard Track                    [Page 87]

RFC 3775                Mobility Support in IPv6               June 2004   o  The remaining lifetime of this Home Agents List entry.  If a Home      Agent Information Option is present in a Router Advertisement      received from a home agent, the lifetime of the Home Agents List      entry representing that home agent is initialized from the Home      Agent Lifetime field in the option (if present); otherwise, the      lifetime is initialized from the Router Lifetime field in the      received Router Advertisement.  If Home Agents List entry lifetime      reaches zero, the entry MUST be deleted from the Home Agents List.   o  The preference for this home agent; higher values indicate a more      preferable home agent.  The preference value is taken from the      Home Agent Preference field in the received Router Advertisement,      if the Router Advertisement contains a Home Agent Information      Option and is otherwise set to the default value of 0.  A home      agent uses this preference in ordering the Home Agents List when      it sends an ICMP Home Agent Address Discovery message.10.2.  Processing Mobility Headers   All IPv6 home agents MUST observe the rules described inSection 9.2   when processing Mobility Headers.10.3.  Processing Bindings10.3.1.  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   inSection 9.5.1.  Furthermore, it MUST authenticate the Binding   Update as described inSection 5.1.  An authorization step specific   for the home agent is also needed to ensure that only the right node   can control a particular home address.  This is provided through the   home address unequivocally identifying the security association that   must be used.   This section describes the processing of a valid and authorized   Binding Update when it requests the registration of the mobile node's   primary care-of address.   To begin processing the Binding Update, the home agent MUST perform   the following sequence of tests:   o  If the node implements only correspondent node functionality, or      has not been configured to act as a home agent, then the node MUST      reject the Binding Update.  The node MUST also return a Binding      Acknowledgement to the mobile node, in which the Status field is      set to 131 (home registration not supported).Johnson, et al.              Standard Track                    [Page 88]

RFC 3775                Mobility Support in IPv6               June 2004   o  Else, if the home address for the binding (the Home Address field      in the packet's Home Address option) is not an on-link IPv6      address with respect to the home agent's current Prefix List, then      the home agent MUST reject the Binding Update and SHOULD return a      Binding Acknowledgement to the mobile node, in which the Status      field is set to 132 (not home subnet).   o  Else, if the home agent chooses to reject the Binding Update for      any other reason (e.g., insufficient resources to serve another      mobile node as a home agent), then the home agent SHOULD return a      Binding Acknowledgement to the mobile node, in which the Status      field is set to an appropriate value to indicate the reason for      the rejection.   o  A Home Address destination option MUST be present in the message.      It MUST be validated as described inSection 9.3.1 with the      following additional rule.  The Binding Cache entry existence test      MUST NOT be done for IPsec packets when the Home Address option      contains an address for which the receiving node could act as a      home agent.   If home agent accepts the Binding Update, it MUST then create a new   entry in its Binding Cache for this mobile node or update its   existing Binding Cache entry, if such an entry already exists.  The   Home Address field as received in the Home Address option provides   the home address of the mobile node.   The home agent MUST mark this Binding Cache entry as a home   registration to indicate that the node is serving as a home agent for   this binding.  Binding Cache entries marked as a home registration   MUST be excluded from the normal cache replacement policy used for   the Binding Cache (Section 9.6) and MUST NOT be removed from the   Binding Cache until the expiration of the Lifetime period.   Unless this home agent already has a binding for the given home   address, the home agent MUST perform Duplicate Address Detection [13]   on the mobile node's home link before returning the Binding   Acknowledgement.  This ensures that no other node on the home link   was using the mobile node's home address when the Binding Update   arrived.  If this Duplicate Address Detection fails for the given   home address or an associated link local address, then the home agent   MUST reject the complete Binding Update and MUST return a Binding   Acknowledgement to the mobile node, in which the Status field is set   to 134 (Duplicate Address Detection failed).  When the home agent   sends a successful Binding Acknowledgement to the mobile node, the   home agent assures to the mobile node that its address(es) will be   kept unique by the home agent for as long as the lifetime was granted   for the binding.Johnson, et al.              Standard Track                    [Page 89]

RFC 3775                Mobility Support in IPv6               June 2004   The specific addresses, which are to be tested before accepting the   Binding Update and later to be defended by performing Duplicate   Address Detection, depend on the setting of the Link-Local Address   Compatibility (L) bit, as follows:   o  L=0: Defend only the given address.  Do not derive a link-local      address.   o  L=1: Defend both the given non link-local unicast (home) address      and the derived link-local.  The link-local address is derived by      replacing the subnet prefix in the mobile node's home address with      the link-local prefix.   The lifetime of the Binding Cache entry depends on a number of   factors:   o  The lifetime for the Binding Cache entry MUST NOT be greater than      the Lifetime value specified in the Binding Update.   o  The lifetime for the Binding Cache entry MUST NOT be greater than      the remaining valid lifetime for the subnet prefix in the mobile      node's home address specified with the Binding Update.  The      remaining valid lifetime for this prefix is determined by the home      agent based on its own Prefix List entry [12].      The remaining preferred lifetime SHOULD NOT have any impact on the      lifetime for the binding cache entry.      The home agent MUST remove a binding when the valid lifetime of      the prefix associated with it expires.   o  The home agent MAY further decrease the specified lifetime for the      binding, for example based on a local policy.  The resulting      lifetime is stored by the home agent in the Binding Cache entry,      and this Binding Cache entry MUST be deleted by the home agent      after the expiration of this lifetime.   Regardless of the setting of the Acknowledge (A) bit in the Binding   Update, the home agent MUST return a Binding Acknowledgement to the   mobile node constructed as follows:   o  The Status field MUST be set to a value indicating success.  The      value 1 (accepted but prefix discovery necessary) MUST be used if      the subnet prefix of the specified home address is deprecated, or      becomes deprecated during the lifetime of the binding, or becomes      invalid at the end of the lifetime.  The value 0 MUST be usedJohnson, et al.              Standard Track                    [Page 90]

RFC 3775                Mobility Support in IPv6               June 2004      otherwise.  For the purposes of comparing the binding and prefix      lifetimes, the prefix lifetimes are first converted into units of      four seconds by ignoring the two least significant bits.   o  The Key Management Mobility Capability (K) bit is set if the      following conditions are all fulfilled, and cleared otherwise:      *  The Key Management Mobility Capability (K) bit was set in the         Binding Update.      *  The IPsec security associations between the mobile node and the         home agent have been established dynamically.      *  The home agent has the capability to update its endpoint in the         used key management protocol to the new care-of address every         time it moves.      Depending on the final value of the bit in the Binding      Acknowledgement, the home agent SHOULD perform the following      actions:      K = 0         Discard key management connections, if any, to the old care-of         address.  If the mobile node did not have a binding before         sending this Binding Update, discard the connections to the         home address.      K = 1         Move the peer endpoint of the key management protocol         connection, if any, to the new care-of address.  For an IKE         phase 1 connection, this means that any IKE packets sent to the         peer are sent to this address, and packets from this address         with the original ISAKMP cookies are accepted.         Note thatRFC 2408 [8]Section 2.5.3 gives specific rules that         ISAKMP cookies must satisfy: they must depend on specific         parties and can only be generated by the entity itself.  Then         it recommends a particular way to do this, namely a hash of IP         addresses.  With the K bit set to 1, the recommended         implementation technique does not work directly.  To satisfy         the two rules, the specific parties must be treated as the         original IP addresses, not the ones in use at the specific         moment.   o  The Sequence Number field MUST be copied from the Sequence Number      given in the Binding Update.Johnson, et al.              Standard Track                    [Page 91]

RFC 3775                Mobility Support in IPv6               June 2004   o  The Lifetime field MUST be set to the remaining lifetime for the      binding as set by the home agent in its home registration Binding      Cache entry for the mobile node, as described above.   o  If the home agent stores the Binding Cache entry in nonvolatile      storage, then the Binding Refresh Advice mobility option MUST be      omitted.  Otherwise, the home agent MAY include this option to      suggest that the mobile node refreshes its binding before the      actual lifetime of the binding ends.      If the Binding Refresh Advice mobility option is present, the      Refresh Interval field in the option MUST be set to a value less      than the Lifetime value being returned in the Binding      Acknowledgement.  This indicates that the mobile node SHOULD      attempt to refresh its home registration at the indicated shorter      interval.  The home agent MUST still retain the registration for      the Lifetime period, even if the mobile node does not refresh its      registration within the Refresh period.   The rules for selecting the Destination IP address (and possibly   routing header construction) for the Binding Acknowledgement to the   mobile node are the same as inSection 9.5.4.   In addition, the home agent MUST follow the procedure defined inSection 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 inSection 10.4.5.  Finally,   the home agent MUST also propagate new home network prefixes, as   described inSection 10.6.10.3.2.  Primary Care-of Address De-Registration   A binding may need to be de-registered when the mobile node returns   home or when the mobile node knows that it will not have any care-of   addresses in the visited network.   A Binding Update is validated and authorized in the manner described   in the previous section; note that when the mobile node de-registers   when it is at home, it may not include the Home Address destination   option, in which case the mobile node's home address is the source IP   address of the de-registration Binding Update.  This section   describes the processing of a valid Binding Update that requests the   receiving node to no longer serve as its home agent, de-registering   its primary care-of address.Johnson, et al.              Standard Track                    [Page 92]

RFC 3775                Mobility Support in IPv6               June 2004   To begin processing the Binding Update, the home agent MUST perform   the following test:   o  If the receiving node has no entry marked as a home registration      in its Binding Cache for this mobile node, then this node MUST      reject the Binding Update and SHOULD return a Binding      Acknowledgement to the mobile node, in which the Status field is      set to 133 (not home agent for this mobile node).   If the home agent does not reject the Binding Update as described   above, then it MUST delete any existing entry in its Binding Cache   for this mobile node.  Then, the home agent MUST return a Binding   Acknowledgement to the mobile node, constructed as follows:   o  The Status field MUST be set to a value 0, indicating success.   o  The Key Management Mobility Capability (K) bit is set or cleared      and actions based on its value are performed as described in the      previous section.  The mobile node's home address is used as its      new care-of address for the purposes of moving the key management      connection to a new endpoint.   o  The Sequence Number field MUST be copied from the Sequence Number      given in the Binding Update.   o  The Lifetime field MUST be set to zero.   o  The Binding Refresh Advice mobility option MUST be omitted.   In addition, the home agent MUST stop intercepting packets on the   mobile node's home link that are addressed to the mobile node   (Section 10.4.1).   The rules for selecting the Destination IP address (and, if required,   routing header construction) for the Binding Acknowledgement to the   mobile node are the same as in the previous section.  When the Status   field in the Binding Acknowledgement is greater than or equal to 128   and the Source Address of the Binding Update is on the home link, the   home agent MUST send it to the mobile node's link layer address   (retrieved either from the Binding Update or through Neighbor   Solicitation).Johnson, et al.              Standard Track                    [Page 93]

RFC 3775                Mobility Support in IPv6               June 200410.4.  Packet Processing10.4.1.  Intercepting Packets for a Mobile Node   While a node is serving as the home agent for mobile node it MUST   attempt to intercept packets on the mobile node's home link that are   addressed to the mobile node.   In order to do this, when a node begins serving as the home agent it   MUST multicast onto the home link a Neighbor Advertisement message   [12] on behalf of the mobile node.  For the home address specified in   the Binding Update, the home agent sends a Neighbor Advertisement   message [12] to the all-nodes multicast address on the home link to   advertise the home agent's own link-layer address for this IP address   on behalf of the mobile node.  If the Link-Layer Address   Compatibility (L) flag has been specified in the Binding Update, the   home agent MUST do the same for the link-local address of the mobile   node.   All fields in each Neighbor Advertisement message SHOULD be set in   the same way they would be set by the mobile node if it was sending   this Neighbor Advertisement [12] while at home, with the following   exceptions:   o  The Target Address in the Neighbor Advertisement MUST be set to      the specific IP address for the mobile node.   o  The Advertisement MUST include a Target Link-layer Address option      specifying the home agent's link-layer address.   o  The Router (R) bit in the Advertisement MUST be set to zero.   o  The Solicited Flag (S) in the Advertisement MUST NOT be set, since      it was not solicited by any Neighbor Solicitation.   o  The Override Flag (O) in the Advertisement MUST be set, indicating      that the Advertisement SHOULD override any existing Neighbor Cache      entry at any node receiving it.   o  The Source Address in the IPv6 header MUST be set to the home      agent's IP address on the interface used to send the      advertisement.   Any node on the home link that receives one of the Neighbor   Advertisement messages (described above) will update its Neighbor   Cache to associate the mobile node's address with the home agent's   link layer address, causing it to transmit any future packets   normally destined to the mobile node to the mobile node's home agent.Johnson, et al.              Standard Track                    [Page 94]

RFC 3775                Mobility Support in IPv6               June 2004   Since multicasting on the local link (such as Ethernet) is typically   not guaranteed to be reliable, the home agent MAY retransmit this   Neighbor Advertisement message up to MAX_NEIGHBOR_ADVERTISEMENT (see   [12]) times to increase its reliability.  It is still possible that   some nodes on the home link will not receive any of the Neighbor   Advertisements, but these nodes will eventually be able to detect the   link-layer address change for the mobile node's address through use   of Neighbor Unreachability Detection [12].   While a node is serving as a home agent for some mobile node, the   home agent uses IPv6 Neighbor Discovery [12] to intercept unicast   packets on the home link addressed to the mobile node.  In order to   intercept packets in this way, the home agent MUST act as a proxy for   this mobile node and reply to any received Neighbor Solicitations for   it.  When a home agent receives a Neighbor Solicitation, it MUST   check if the Target Address specified in the message matches the   address of any mobile node for which it has a Binding Cache entry   marked as a home registration.   If such an entry exists in the home agent's Binding Cache, the home   agent MUST reply to the Neighbor Solicitation with a Neighbor   Advertisement giving the home agent's own link-layer address as the   link-layer address for the specified Target Address.  In addition,   the Router (R) bit in the Advertisement MUST be set to zero.  Acting   as a proxy in this way allows other nodes on the mobile node's home   link to resolve the mobile node's address and for the home agent to   defend these addresses on the home link for Duplicate Address   Detection [12].10.4.2.  Processing Intercepted Packets   For any packet sent to a mobile node from the mobile node's home   agent (in which the home agent is the original sender of the packet),   the home agent is operating as a correspondent node of the mobile   node for this packet and the procedures described inSection 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 inSection 10.4.1.  In order to forward each   intercepted packet to the mobile node, the home agent MUST tunnel the   packet to the mobile node using IPv6 encapsulation [15].  When a home   agent encapsulates an intercepted packet for forwarding to the mobile   node, the home agent sets the Source Address in the new tunnel IP   header to the home agent's own IP address and sets the Destination   Address in the tunnel IP header to the mobile node's primary care-ofJohnson, et al.              Standard Track                    [Page 95]

RFC 3775                Mobility Support in IPv6               June 2004   address.  When received by the mobile node, normal processing of the   tunnel header [15] will result in decapsulation and processing of the   original packet by the mobile node.   However, packets addressed to the mobile node's link-local address   MUST NOT be tunneled to the mobile node.  Instead, these packets MUST   be discarded and the home agent SHOULD return an ICMP Destination   Unreachable, Code 3, message to the packet's Source Address (unless   this Source Address is a multicast address).  Packets addressed to   the mobile node's site-local address SHOULD NOT be tunneled to the   mobile node by default.   Interception and tunneling of the following multicast addressed   packets on the home network are only done if the home agent supports   multicast group membership control messages from the mobile node as   described in the next section.  Tunneling of multicast packets to a   mobile node follows similar limitations to those defined above for   unicast packets addressed to the mobile node's link-local and site-   local addresses.  Multicast packets addressed to a multicast address   with link-local scope [3], to which the mobile node is subscribed,   MUST NOT be tunneled to the mobile node.  These packets SHOULD be   silently discarded (after delivering to other local multicast   recipients).  Multicast packets addressed to a multicast address with   a scope larger than link-local, but smaller than global (e.g., site-   local and organization-local [3], to which the mobile node is   subscribed, SHOULD NOT be tunneled to the mobile node.  Multicast   packets addressed with a global scope, to which the mobile node has   successfully subscribed, MUST be tunneled to the mobile node.   Before tunneling a packet to the mobile node, the home agent MUST   perform any IPsec processing as indicated by the security policy data   base.10.4.3.  Multicast Membership Control   This section is a prerequisite for the multicast data packet   forwarding, described in the previous section.  If this support is   not provided, multicast group membership control messages are   silently ignored.   In order to forward multicast data packets from the home network to   all the proper mobile nodes, the home agent SHOULD be capable of   receiving tunneled multicast group membership control information   from the mobile node in order to determine which groups the mobile   node has subscribed to.  These multicast group membership messages   are Listener Report messages specified in MLD [17] or in other   protocols such as [37].Johnson, et al.              Standard Track                    [Page 96]

RFC 3775                Mobility Support in IPv6               June 2004   The messages are issued by the mobile node, but sent through the   reverse tunnel to the home agent.  These messages are issued whenever   the mobile node decides to enable reception of packets for a   multicast group or in response to an MLD Query from the home agent.   The mobile node will also issue multicast group control messages to   disable reception of multicast packets when it is no longer   interested in receiving multicasts for a particular group.   To obtain the mobile node's current multicast group membership the   home agent must periodically transmit MLD Query messages through the   tunnel to the mobile node.  These MLD periodic transmissions will   ensure the home agent has an accurate record of the groups in which   the mobile node is interested despite packet losses of the mobile   node's MLD group membership messages.   All MLD packets are sent directly between the mobile node and the   home agent.  Since all of these packets are destined to a link-scope   multicast address and have a hop limit of 1, there is no direct   forwarding of such packets between the home network and the mobile   node.  The MLD packets between the mobile node and the home agent are   encapsulated within the same tunnel header used for other packet   flows between the mobile node and home agent.   Note that at this time, even though a link-local source is used on   MLD packets, no functionality depends on these addresses being   unique, nor do they elicit direct responses.  All MLD messages are   sent to multicast destinations.  To avoid ambiguity on the home   agent, due to mobile nodes which may choose identical link-local   source addresses for their MLD function, it is necessary for the home   agent to identify which mobile node was actually the issuer of a   particular MLD message.  This may be accomplished by noting which   tunnel such an MLD arrived by, which IPsec SA was used, or by other   distinguishing means.   This specification puts no requirement on how the functions in this   section and the multicast forwarding inSection 10.4.2 are to be   achieved.  At the time of this writing it was thought that a full   IPv6 multicast router function would be necessary on the home agent,   but it may be possible to achieve the same effects through a "proxy   MLD" application coupled with kernel multicast forwarding.  This may   be the subject of future specifications.Johnson, et al.              Standard Track                    [Page 97]

RFC 3775                Mobility Support in IPv6               June 200410.4.4.  Stateful Address Autoconfiguration   This section describes how home agents support the use of stateful   address autoconfiguration mechanisms such as DHCPv6 [29] from the   mobile nodes.  If this support is not provided, then the M and O bits   must remain cleared on the Mobile Prefix Advertisement Messages.  Any   mobile node which sends DHCPv6 messages to the home agent without   this support will not receive a response.   If DHCPv6 is used, packets are sent with link-local source addresses   either to a link-scope multicast address or a link-local address.   Mobile nodes desiring to locate a DHCPv6 service may reverse tunnel   standard DHCPv6 packets to the home agent.  Since these link-scope   packets cannot be forwarded onto the home network, it is necessary   for the home agent to either implement a DHCPv6 relay agent or a   DHCPv6 server function itself.  The arriving tunnel or IPsec SA of   DHCPv6 link-scope messages from the mobile node must be noted so that   DHCPv6 responses may be sent back to the appropriate mobile node.   DHCPv6 messages sent to the mobile node with a link-local destination   must be tunneled within the same tunnel header used for other packet   flows.10.4.5.  Handling Reverse Tunneled Packets   Unless a binding has been established between the mobile node and a   correspondent node, traffic from the mobile node to the correspondent   node goes through a reverse tunnel.  Home agents MUST support reverse   tunneling as follows:   o  The tunneled traffic arrives to the home agent's address using      IPv6 encapsulation [15].   o  Depending on the security policies used by the home agent, reverse      tunneled packets MAY be discarded unless accompanied by a valid      ESP header.  The support for authenticated reverse tunneling      allows the home agent to protect the home network and      correspondent nodes from malicious nodes masquerading as a mobile      node.   o  Otherwise, when a home agent decapsulates a tunneled packet from      the mobile node, the home agent MUST verify that the Source      Address in the tunnel IP header is the mobile node's primary      care-of address.  Otherwise, any node in the Internet could send      traffic through the home agent and escape ingress filtering      limitations.  This simple check forces the attacker to know the      current location of the real mobile node and be able to defeat      ingress filtering. This check is not necessary if the reverse-      tunneled packet is protected by ESP in tunnel mode.Johnson, et al.              Standard Track                    [Page 98]

RFC 3775                Mobility Support in IPv6               June 200410.4.6.  Protecting Return Routability Packets   The return routability procedure, described inSection 5.2.5, assumes   that the confidentiality of the Home Test Init and Home Test messages   is protected as they are tunneled between the home agent and the   mobile node.  Therefore, the home agent MUST support tunnel mode   IPsec ESP for the protection of packets belonging to the return   routability procedure.  Support for a non-null encryption transform   and authentication algorithm MUST be available.  It is not necessary   to distinguish between different kinds of packets during the return   routability procedure.   Security associations are needed to provide this protection.  When   the care-of address for the mobile node changes as a result of an   accepted Binding Update, special treatment is needed for the next   packets sent using these security associations.  The home agent MUST   set the new care-of address as the destination address of these   packets, as if the outer header destination address in the security   association had changed [21].   The above protection SHOULD be used with all mobile nodes.  The use   is controlled by configuration of the IPsec security policy database   both at the mobile node and at the home agent.   As described earlier, the Binding Update and Binding Acknowledgement   messages require protection between the home agent and the mobile   node.  The Mobility Header protocol carries both these messages as   well as the return routability messages.  From the point of view of   the security policy database these messages are indistinguishable.   When IPsec is used to protect return routability signaling or payload   packets, this protection MUST only be applied to the return   routability packets entering the IPv6 encapsulated tunnel interface   between the mobile node and the home agent.  This can be achieved,   for instance, by defining the security policy database entries   specifically for the tunnel interface.  That is, the policy entries   are not generally applied on all traffic on the physical interface(s)   of the nodes, but rather only on traffic that enters the tunnel.   This makes use of per-interface security policy database entries [4]   specific to the tunnel interface (the node's attachment to the tunnel   [11]).10.5.  Dynamic Home Agent Address Discovery   This section describes how a home agent can help mobile nodes to   discover the addresses of the home agents.  The home agent keeps   track of the other home agents on the same link and responds to   queries sent by the mobile node.Johnson, et al.              Standard Track                    [Page 99]

RFC 3775                Mobility Support in IPv6               June 200410.5.1.  Receiving Router Advertisement Messages   For each link on which a router provides service as a home agent, the   router maintains a Home Agents List recording information about all   other home agents on that link.  This list is used in the dynamic   home agent address discovery mechanism, described inSection 10.5.   The information for the list is learned through receipt of the   periodic unsolicited multicast Router Advertisements, in a manner   similar to the Default Router List conceptual data structure   maintained by each host for Neighbor Discovery [12].  In the   construction of the Home Agents List, the Router Advertisements are   from each (other) home agent on the link and the Home Agent (H) bit   is set in them.   On receipt of a valid Router Advertisement, as defined in the   processing algorithm specified for Neighbor Discovery [12], the home   agent performs the following steps in addition to any steps already   required of it by Neighbor Discovery:   o  If the Home Agent (H) bit in the Router Advertisement is not set,      delete the sending node's entry in the current Home Agents List      (if one exists).  Skip all the following steps.   o  Otherwise, extract the Source Address from the IP header of the      Router Advertisement.  This is the link-local IP address on this      link of the home agent sending this Advertisement [12].   o  Determine the preference for this home agent.  If the Router      Advertisement contains a Home Agent Information Option, then the      preference is taken from the Home Agent Preference field in the      option; otherwise, the default preference of 0 MUST be used.   o  Determine the lifetime for this home agent.  If the Router      Advertisement contains a Home Agent Information Option, then the      lifetime is taken from the Home Agent Lifetime field in the      option; otherwise, the lifetime specified by the Router Lifetime      field in the Router Advertisement SHOULD be used.   o  If the link-local address of the home agent sending this      Advertisement is already present in this home agent's Home Agents      List and the received home agent lifetime value is zero,      immediately delete this entry in the Home Agents List.   o  Otherwise, if the link-local address of the home agent sending      this Advertisement is already present in the receiving home      agent's Home Agents List, reset its lifetime and preference to the      values determined above.Johnson, et al.              Standard Track                   [Page 100]

RFC 3775                Mobility Support in IPv6               June 2004   o  If the link-local address of the home agent sending this      Advertisement is not already present in the Home Agents List      maintained by the receiving home agent, and the lifetime for the      sending home agent is non-zero, create a new entry in the list,      and initialize its lifetime and preference to the values      determined above.   o  If the Home Agents List entry for the link-local address of the      home agent sending this Advertisement was not deleted as described      above, determine any global address(es) of the home agent based on      each Prefix Information option received in this Advertisement in      which the Router Address (R) bit is set (Section 7.2).  Add all      such global addresses to the list of global addresses in this Home      Agents List entry.   A home agent SHOULD maintain an entry in its Home Agents List for   each valid home agent address until that entry's lifetime expires,   after which time the entry MUST be deleted.   As described inSection 11.4.1, a mobile node attempts dynamic home   agent address discovery by sending an ICMP Home Agent Address   Discovery Request message to the Mobile IPv6 Home-Agents anycast   address [16] for its home IP subnet prefix.  A home agent receiving a   Home Agent Address Discovery Request message that serves this subnet   SHOULD return an ICMP Home Agent Address Discovery Reply message to   the mobile node with the Source Address of the Reply packet set to   one of the global unicast addresses of the home agent.  The Home   Agent Addresses field in the Reply message is constructed as follows:   o  The Home Agent Addresses field SHOULD contain all global IP      addresses for each home agent currently listed in this home      agent's own Home Agents List (Section 10.1).   o  The IP addresses in the Home Agent Addresses field SHOULD be      listed in order of decreasing preference values, based either on      the respective advertised preference from a Home Agent Information      option or on the default preference of 0 if no preference is      advertised (or on the configured home agent preference for this      home agent itself).   o  Among home agents with equal preference, their IP addresses in the      Home Agent Addresses field SHOULD be listed in an order randomized      with respect to other home agents with equal preference every time      a Home Agent Address Discovery Reply message is returned by this      home agent.   o  If more than one global IP address is associated with a home      agent, these addresses SHOULD be listed in a randomized order.Johnson, et al.              Standard Track                   [Page 101]

RFC 3775                Mobility Support in IPv6               June 2004   o  The home agent SHOULD reduce the number of home agent IP addresses      so that the packet fits within the minimum IPv6 MTU [11].  The      home agent addresses selected for inclusion in the packet SHOULD      be those from the complete list with the highest preference.  This      limitation avoids the danger of the Reply message packet being      fragmented (or rejected by an intermediate router with an ICMP      Packet Too Big message [14]).10.6.  Sending Prefix Information to the Mobile Node10.6.1.  List of Home Network Prefixes   Mobile IPv6 arranges to propagate relevant prefix information to the   mobile node when it is away from home, so that it may be used in   mobile node home address configuration and in network renumbering.   In this mechanism, mobile nodes away from home receive Mobile Prefix   Advertisements messages.  These messages include Prefix Information   Options for the prefixes configured on the home subnet interface(s)   of the home agent.   If there are multiple home agents, differences in the advertisements   sent by different home agents can lead to an inability to use a   particular home address when changing to another home agent.  In   order to ensure that the mobile nodes get the same information from   different home agents, it is preferred that all of the home agents on   the same link be configured in the same manner.   To support this, the home agent monitors prefixes advertised by   itself and other home agents on the home link.  InRFC 2461 [12] it   is acceptable for two routers to advertise different sets of prefixes   on the same link.  For home agents, the differences should be   detected for a given home address because the mobile node   communicates only with one home agent at a time and the mobile node   needs to know the full set of prefixes assigned to the home link.   All other comparisons of Router Advertisements are as specified inSection 6.2.7 of RFC 2461.10.6.2.  Scheduling Prefix Deliveries   A home agent serving a mobile node will schedule the delivery of the   new prefix information to that mobile node when any of the following   conditions occur:   MUST:   o  The state of the flags changes for the prefix of the mobile node's      registered home address.Johnson, et al.              Standard Track                   [Page 102]

RFC 3775                Mobility Support in IPv6               June 2004   o  The valid or preferred lifetime is reconfigured or changes for any      reason other than advancing real time.   o  The mobile node requests the information with a Mobile Prefix      Solicitation (seeSection 11.4.2).   SHOULD:   o  A new prefix is added to the home subnet interface(s) of the home      agent.   MAY:   o  The valid or preferred lifetime or the state of the flags changes      for a prefix which is not used in any Binding Cache entry for this      mobile node.   The home agent uses the following algorithm to determine when to send   prefix information to the mobile node.   o  If a mobile node sends a solicitation, answer right away.   o  If no Mobile Prefix Advertisement has been sent to the mobile node      in the last MaxMobPfxAdvInterval seconds (seeSection 13), then      ensure that a transmission is scheduled.  The actual transmission      time is randomized as described below.   o  If a prefix matching the mobile node's home registration is added      on the home subnet interface or if its information changes in any      way that does not deprecate the mobile node's address, ensure that      a transmission is scheduled.  The actual transmission time is      randomized as described below.   o  If a home registration expires, cancel any scheduled      advertisements to the mobile node.   The list of prefixes is sent in its entirety in all cases.   If the home agent has already scheduled the transmission of a Mobile   Prefix Advertisement to the mobile node, then the home agent will   replace the advertisement with a new one to be sent at the scheduled   time.   Otherwise, the home agent computes a fresh value for RAND_ADV_DELAY   which offsets from the current time for the scheduled transmission.   First calculate the maximum delay for the scheduled Advertisement:Johnson, et al.              Standard Track                   [Page 103]

RFC 3775                Mobility Support in IPv6               June 2004      MaxScheduleDelay = min (MaxMobPfxAdvInterval, Preferred Lifetime),   where MaxMobPfxAdvInterval is as defined inSection 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 (seeSection 12) before   the first retransmission and double the retransmission wait time for   every succeeding retransmission until a maximum number of   PREFIX_ADV_RETRIES attempts (seeSection 12) has been tried.  If the   mobile node's bindings expire before the matching Binding Update has   been received, then the home agent MUST NOT attempt any more   retransmissions, even if not all PREFIX_ADV_RETRIES have been   retransmitted.  In the meantime, if the mobile node sends another   Binding Update without returning home, then the home agent SHOULD   begin transmitting the unsolicited Advertisement again.   If some condition, as described above, occurs on the home link and   causes another Prefix Advertisement to be sent to the mobile node,   before the mobile node acknowledges a previous transmission, the home   agent SHOULD combine any Prefix Information options in the   unacknowledged Mobile Prefix Advertisement into a new Advertisement.   The home agent then discards the old Advertisement.10.6.3.  Sending Advertisements   When sending a Mobile Prefix Advertisement to the mobile node, the   home agent MUST construct the packet as follows:   o  The Source Address in the packet's IPv6 header MUST be set to the      home agent's IP address to which the mobile node addressed its      current home registration or its default global home agent address      if no binding exists.Johnson, et al.              Standard Track                   [Page 104]

RFC 3775                Mobility Support in IPv6               June 2004   o  If the advertisement was solicited, it MUST be destined to the      source address of the solicitation.  If it was triggered by prefix      changes or renumbering, the advertisement's destination will be      the mobile node's home address in the binding which triggered the      rule.   o  A type 2 routing header MUST be included with the mobile node's      home address.   o  IPsec headers MUST be supported and SHOULD be used.   o  The home agent MUST send the packet as it would any other unicast      IPv6 packet that it originates.   o  Set the Managed Address Configuration (M) flag if the      corresponding flag has been set in any of the Router      Advertisements from which the prefix information has been learned      (including the ones sent by this home agent).   o  Set the Other Stateful Configuration (O) flag if the corresponding      flag has been set in any of the Router Advertisements from which      the prefix information has been learned (including the ones sent      by this home agent).10.6.4.  Lifetimes for Changed Prefixes   As described inSection 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 Operation11.1.  Conceptual Data Structures   Each mobile node MUST maintain a Binding Update List.   The Binding Update List records information for each Binding Update   sent by this mobile node, in which the lifetime of the binding has   not yet expired.  The Binding Update List includes all bindings sent   by the mobile node either to its home agent or correspondent nodes.   It also contains Binding Updates which are waiting for the completion   of the return routability procedure before they can be sent.   However, for multiple Binding Updates sent to the same destination   address, the Binding Update List contains only the most recent   Binding Update (i.e., with the greatest Sequence Number value) sent   to that destination.  The Binding Update List MAY be implemented inJohnson, et al.              Standard Track                   [Page 105]

RFC 3775                Mobility Support in IPv6               June 2004   any manner consistent with the external behavior described in this   document.   Each Binding Update List entry conceptually contains the following   fields:   o  The IP address of the node to which a Binding Update was sent.   o  The home address for which that Binding Update was sent.   o  The care-of address sent in that Binding Update.  This value is      necessary for the mobile node to determine if it has sent a      Binding Update while giving its new care-of address to this      destination after changing its care-of address.   o  The initial value of the Lifetime field sent in that Binding      Update.   o  The remaining lifetime of that binding.  This lifetime is      initialized from the Lifetime value sent in the Binding Update and      is decremented until it reaches zero, at which time this entry      MUST be deleted from the Binding Update List.   o  The maximum value of the Sequence Number field sent in previous      Binding Updates to this destination.  The Sequence Number field is      16 bits long and all comparisons between Sequence Number values      MUST be performed modulo 2**16 (seeSection 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      inSection 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 (seeSection 11.3.1).Johnson, et al.              Standard Track                   [Page 106]

RFC 3775                Mobility Support in IPv6               June 2004   The Binding Update list also conceptually contains the following data   related to running the return routability procedure.  This data is   relevant only for Binding Updates sent to correspondent nodes.   o  The time at which a Home Test Init or Care-of Test Init message      was last sent to this destination, as needed to implement the rate      limiting restriction for the return routability procedure.   o  The state of any retransmissions needed for this return      routability procedure.  This state includes the time remaining      until the next retransmission attempt and the current state of the      exponential back-off mechanism for retransmissions.   o  Cookie values used in the Home Test Init and Care-of Test Init      messages.   o  Home and care-of keygen tokens received from the correspondent      node.   o  Home and care-of nonce indices received from the correspondent      node.   o  The time at which each of the tokens and nonces were received from      the correspondent node, as needed to implement reuse while moving.11.2.  Processing Mobility Headers   All IPv6 mobile nodes MUST observe the rules described inSection 9.2   when processing Mobility Headers.11.3.  Packet Processing11.3.1.  Sending Packets While Away from Home   While a mobile node is away from home, it continues to use its home   address, as well as also using one or more care-of addresses.  When   sending a packet while away from home, a mobile node MAY choose among   these in selecting the address that it will use as the source of the   packet, as follows:   o  Protocols layered over IP will generally treat the mobile node's      home address as its IP address for most packets.  For packets sent      that are part of transport-level connections established while the      mobile node was at home, the mobile node MUST use its home      address.  Likewise, for packets sent that are part of transport-      level connections that the mobile node may still be using after      moving to a new location, the mobile node SHOULD use its home      address in this way.  If a binding exists, the mobile node SHOULDJohnson, et al.              Standard Track                   [Page 107]

RFC 3775                Mobility Support in IPv6               June 2004      send the packets directly to the correspondent node.  Otherwise,      if a binding does not exist, the mobile node MUST use reverse      tunneling.   o  The mobile node MAY choose to directly use one of its care-of      addresses as the source of the packet, not requiring the use of a      Home Address option in the packet.  This is particularly useful      for short-term communication that may easily be retried if it      fails.  Using the mobile node's care-of address as the source for      such queries will generally have a lower overhead than using the      mobile node's home address, since no extra options need be used in      either the query or its reply.  Such packets can be routed      normally, directly between their source and destination without      relying on Mobile IPv6.  If application running on the mobile node      has no particular knowledge that the communication being sent fits      within this general type of communication, however, the mobile      node should not use its care-of address as the source of the      packet in this way.      The choice of the most efficient communications method is      application specific, and outside the scope of this specification.      The APIs necessary for controlling the choice are also out of      scope.   o  While not at its home link, the mobile node MUST NOT use the Home      Address destination option when communicating with link-local or      site-local peers, if the scope of the home address is larger than      the scope of the peer's address.      Similarly, the mobile node MUST NOT use the Home Address      destination option for IPv6 Neighbor Discovery [12] packets.   Detailed operation of these cases is described later in this section   and also discussed in [31].   For packets sent by a mobile node while it is at home, no special   Mobile IPv6 processing is required.  Likewise, if the mobile node   uses any address other than one of its home addresses as the source   of a packet sent while away from home, no special Mobile IPv6   processing is required.  In either case, the packet is simply   addressed and transmitted in the same way as any normal IPv6 packet.   For packets sent by the mobile node sent while away from home using   the mobile node's home address as the source, special Mobile IPv6   processing of the packet is required.  This can be done in the   following two ways:Johnson, et al.              Standard Track                   [Page 108]

RFC 3775                Mobility Support in IPv6               June 2004   Route Optimization   This manner of delivering packets does not require going through the   home network, and typically will enable faster and more reliable   transmission.   The mobile node needs to ensure that a Binding Cache entry exists for   its home address so that the correspondent node can process the   packet (Section 9.3.1 specifies the rules for Home Address   Destination Option Processing at a correspondent node).  The mobile   node SHOULD examine its Binding Update List for an entry which   fulfills the following conditions:   *  The Source Address field of the packet being sent is equal to the      home address in the entry.   *  The Destination Address field of the packet being sent is equal to      the address of the correspondent node in the entry.   *  One of the current care-of addresses of the mobile node appears as      the care-of address in the entry.   *  The entry indicates that a binding has been successfully created.   *  The remaining lifetime of the binding is greater than zero.   When these conditions are met, the mobile node knows that the   correspondent node has a suitable Binding Cache entry.   A mobile node SHOULD arrange to supply the home address in a Home   Address option, and MUST set the IPv6 header's Source Address field   to the care-of address which the mobile node has registered to be   used with this correspondent node.  The correspondent node will then   use the address supplied in the Home Address option to serve the   function traditionally done by the Source IP address in the IPv6   header.  The mobile node's home address is then supplied to higher   protocol layers and applications.   Specifically:   *  Construct the packet using the mobile node's home address as the      packet's Source Address, in the same way as if the mobile node      were at home.  This includes the calculation of upper layer      checksums using the home address as the value of the source.   *  Insert a Home Address option into the packet with the Home Address      field copied from the original value of the Source Address field      in the packet.Johnson, et al.              Standard Track                   [Page 109]

RFC 3775                Mobility Support in IPv6               June 2004   *  Change the Source Address field in the packet's IPv6 header to one      of the mobile node's care-of addresses.  This will typically be      the mobile node's current primary care-of address, but MUST be an      address assigned to the interface on the link being used.   By using the care-of address as the Source Address in the IPv6   header, with the mobile node's home address instead in the Home   Address option, the packet will be able to safely pass through any   router implementing ingress filtering [26].   Reverse Tunneling      This is the mechanism which tunnels the packets via the home      agent.  It is not as efficient as the above mechanism, but is      needed if there is no binding yet with the correspondent node.      This mechanism is used for packets that have the mobile node's      home address as the Source Address in the IPv6 header, or with      multicast control protocol packets as described inSection 11.3.4.      Specifically:      *  The packet is sent to the home agent using IPv6 encapsulation         [15].      *  The Source Address in the tunnel packet is the primary care-of         address as registered with the home agent.      *  The Destination Address in the tunnel packet is the home         agent's address.      Then, the home agent will pass the encapsulated packet to the      correspondent node.11.3.2.  Interaction with Outbound IPsec Processing   This section sketches the interaction between outbound Mobile IPv6   processing and outbound IP Security (IPsec) processing for packets   sent by a mobile node while away from home.  Any specific   implementation MAY use algorithms and data structures other than   those suggested here, but its processing MUST be consistent with the   effect of the operation described here and with the relevant IPsec   specifications.  In the steps described below, it is assumed that   IPsec is being used in transport mode [4] and that the mobile node is   using its home address as the source for the packet (from the point   of view of higher protocol layers or applications, as described inSection 11.3.1):Johnson, et al.              Standard Track                   [Page 110]

RFC 3775                Mobility Support in IPv6               June 2004   o  The packet is created by higher layer protocols and applications      (e.g., by TCP) as if the mobile node were at home and Mobile IPv6      were not being used.   o  Determine the outgoing interface for the packet.  (Note that the      selection between reverse tunneling and route optimization may      imply different interfaces, particularly if tunnels are considered      interfaces as well.)   o  As part of outbound packet processing in IP, the packet is      compared against the IPsec security policy database to determine      what processing is required for the packet [4].   o  If IPsec processing is required, the packet is either mapped to an      existing Security Association (or SA bundle), or a new SA (or SA      bundle) is created for the packet, according to the procedures      defined for IPsec.   o  Since the mobile node is away from home, the mobile is either      using reverse tunneling or route optimization to reach the      correspondent node.      If reverse tunneling is used, the packet is constructed in the      normal manner and then tunneled through the home agent.      If route optimization is in use, the mobile node inserts a Home      Address destination option into the packet, replacing the Source      Address in the packet's IP header with the care-of address used      with this correspondent node, as described inSection 11.3.1.  The      Destination Options header in which the Home Address destination      option is inserted MUST appear in the packet after the routing      header, if present, and before the IPsec (AH [5] or ESP [6])      header, so that the Home Address destination option is processed      by the destination node before the IPsec header is processed.      Finally, once the packet is fully assembled, the necessary IPsec      authentication (and encryption, if required) processing is      performed on the packet, initializing the Authentication Data in      the IPsec header.RFC 2402 treatment of destination options is extended as follows.      The AH authentication data MUST be calculated as if the following      were true:      *  the IPv6 source address in the IPv6 header contains the mobile         node's home address,Johnson, et al.              Standard Track                   [Page 111]

RFC 3775                Mobility Support in IPv6               June 2004      *  the Home Address field of the Home Address destination option         (Section 6.3) contains the new care-of address.   o  This allows, but does not require, the receiver of the packet      containing a Home Address destination option to exchange the two      fields of the incoming packet to reach the above situation,      simplifying processing for all subsequent packet headers.      However, such an exchange is not required, as long as the result      of the authentication calculation remains the same.   When an automated key management protocol is used to create new   security associations for a peer, it is important to ensure that the   peer can send the key management protocol packets to the mobile node.   This may not be possible if the peer is the home agent of the mobile   node and the purpose of the security associations would be to send a   Binding Update to the home agent.  Packets addressed to the home   address of the mobile node cannot be used before the Binding Update   has been processed.  For the default case of using IKE [9] as the   automated key management protocol, such problems can be avoided by   the following requirements when communicating with its home agent:   o  When the mobile node is away from home, it MUST use its care-of      address as the Source Address of all packets it sends as part of      the key management protocol (without use of Mobile IPv6 for these      packets, as suggested inSection 11.3.1).   o  In addition, for all security associations bound to the mobile      node's home address established by IKE, the mobile node MUST      include an ISAKMP Identification Payload [8] in the IKE phase 2      exchange, giving the mobile node's home address as the initiator      of the Security Association [7].   The Key Management Mobility Capability (K) bit in Binding Updates and   Acknowledgements can be used to avoid the need to rerun IKE upon   movements.11.3.3.  Receiving Packets While Away from Home   While away from home, a mobile node will receive packets addressed to   its home address, by one of two methods:   o  Packets sent by a correspondent node, that does not have a Binding      Cache entry for the mobile node, will be sent to the home address,      captured by the home agent and tunneled to the mobile node.   o  Packets sent by a correspondent node that has a Binding Cache      entry for the mobile node that contains the mobile node's current      care-of address, will be sent by the correspondent node using aJohnson, et al.              Standard Track                   [Page 112]

RFC 3775                Mobility Support in IPv6               June 2004      type 2 routing header.  The packet will be addressed to the mobile      node's care-of address, with the final hop in the routing header      directing the packet to the mobile node's home address; the      processing of this last hop of the routing header is entirely      internal to the mobile node, since the care-of address and home      address are both addresses within the mobile node.   For packets received by the first method, the mobile node MUST check   that the IPv6 source address of the tunneled packet is the IP address   of its home agent.  In this method, the mobile node may also send a   Binding Update to the original sender of the packet as described inSection 11.7.2 and subject to the rate limiting defined inSection11.8.  The mobile node MUST also process the received packet in the   manner defined for IPv6 encapsulation [15], which will result in the   encapsulated (inner) packet being processed normally by upper-layer   protocols within the mobile node as if it had been addressed (only)   to the mobile node's home address.   For packets received by the second method, the following rules will   result in the packet being processed normally by upper-layer   protocols within the mobile node as if it had been addressed to the   mobile node's home address.   A node receiving a packet addressed to itself (i.e., one of the   node's addresses is in the IPv6 destination field) follows the next   header chain of headers and processes them.  When it encounters a   type 2 routing header during this processing, it performs the   following checks.  If any of these checks fail, the node MUST   silently discard the packet.   o  The length field in the routing header is exactly 2.   o  The segments left field in the routing header is 1 on the wire.      (But implementations may process the routing header so that the      value may become 0 after the routing header has been processed,      but before the rest of the packet is processed.)   o  The Home Address field in the routing header is one of the node's      home addresses, if the segments left field was 1.  Thus, in      particular the address field is required to be a unicast routable      address.   Once the above checks have been performed, the node swaps the IPv6   destination field with the Home Address field in the routing header,   decrements segments left by one from the value it had on the wire,   and resubmits the packet to IP for processing the next header.Johnson, et al.              Standard Track                   [Page 113]

RFC 3775                Mobility Support in IPv6               June 2004   Conceptually, this follows the same model as inRFC 2460.  However,   in the case of type 2 routing header this can be simplified since it   is known that the packet will not be forwarded to a different node.   The definition of AH requires the sender to calculate the AH   integrity check value of a routing header in the same way it appears   in the receiver after it has processed the header.  Since IPsec   headers follow the routing header, any IPsec processing will operate   on the packet with the home address in the IP destination field and   segments left being zero.  Thus, the AH calculations at the sender   and receiver will have an identical view of the packet.11.3.4.  Routing Multicast Packets   A mobile node that is connected to its home link functions in the   same way as any other (stationary) node.  Thus, when it is at home, a   mobile node functions identically to other multicast senders and   receivers.  Therefore, this section describes the behavior of a   mobile node that is not on its home link.   In order to receive packets sent to some multicast group, a mobile   node must join that multicast group.  One method, in which a mobile   node MAY join the group, is via a (local) multicast router on the   foreign link being visited.  In this case, the mobile node MUST use   its care-of address and MUST NOT use the Home Address destination   option when sending MLD packets [17].   Alternatively, a mobile node MAY join multicast groups via a bi-   directional tunnel to its home agent.  The mobile node tunnels its   multicast group membership control packets (such as those defined in   [17] or in [37]) to its home agent, and the home agent forwards   multicast packets down the tunnel to the mobile node.  A mobile node   MUST NOT tunnel multicast group membership control packets until (1)   the mobile node has a binding in place at the home agent, and (2) the   latter sends at least one multicast group membership control packet   via the tunnel.  Once this condition is true, the mobile node SHOULD   assume it does not change as long as the binding does not expire.   A mobile node that wishes to send packets to a multicast group also   has two options:   1.  Send directly on the foreign link being visited.       The application is aware of the care-of address and uses it as a       source address for multicast traffic, just like it would use a       stationary address.  The mobile node MUST NOT use Home Address       destination option in such traffic.Johnson, et al.              Standard Track                   [Page 114]

RFC 3775                Mobility Support in IPv6               June 2004   2.  Send via a tunnel to its home agent.       Because multicast routing in general depends upon the Source       Address used in the IPv6 header of the multicast packet, a mobile       node that tunnels a multicast packet to its home agent MUST use       its home address as the IPv6 Source Address of the inner       multicast packet.   Note that direct sending from the foreign link is only applicable   while the mobile node is at that foreign link.  This is because the   associated multicast tree is specific to that source location and any   change of location and source address will invalidate the source   specific tree or branch and the application context of the other   multicast group members.   This specification does not provide mechanisms to enable such local   multicast session to survive hand-off and to seamlessly continue from   a new care-of address on each new foreign link.  Any such mechanism,   developed as an extension to this specification, needs to take into   account the impact of fast moving mobile nodes on the Internet   multicast routing protocols and their ability to maintain the   integrity of source specific multicast trees and branches.   While the use of bidirectional tunneling can ensure that multicast   trees are independent of the mobile nodes movement, in some case such   tunneling can have adverse affects.  The latency of specific types of   multicast applications (such as multicast based discovery protocols)   will be affected when the round-trip time between the foreign subnet   and the home agent is significant compared to that of the topology to   be discovered.  In addition, the delivery tree from the home agent in   such circumstances relies on unicast encapsulation from the agent to   the mobile node.  Therefore, bandwidth usage is inefficient compared   to the native multicast forwarding in the foreign multicast system.11.3.5.  Receiving ICMP Error Messages   Any node that does not recognize the Mobility header will return an   ICMP Parameter Problem, Code 1, message to the sender of the packet.   If the mobile node receives such an ICMP error message in response to   a return routability procedure or Binding Update, it SHOULD record in   its Binding Update List that future Binding Updates SHOULD NOT be   sent to this destination.  Such Binding Update List entries SHOULD be   removed after a period of time in order to allow for retrying route   optimization.   New Binding Update List entries MUST NOT be created as a result of   receiving ICMP error messages.Johnson, et al.              Standard Track                   [Page 115]

RFC 3775                Mobility Support in IPv6               June 2004   Correspondent nodes that have participated in the return routability   procedure MUST implement the ability to correctly process received   packets containing a Home Address destination option.  Therefore,   correctly implemented correspondent nodes should always be able to   recognize Home Address options.  If a mobile node receives an ICMP   Parameter Problem, Code 2, message from some node indicating that it   does not support the Home Address option, the mobile node SHOULD log   the error and then discard the ICMP message.11.3.6.  Receiving Binding Error Messages   When a mobile node receives a packet containing a Binding Error   message, it should first check if the mobile node has a Binding   Update List entry for the source of the Binding Error message.  If   the mobile node does not have such an entry, it MUST ignore the   message.  This is necessary to prevent a waste of resources on, e.g.,   return routability procedure due to spoofed Binding Error messages.   Otherwise, if the message Status field was 1 (unknown binding for   Home Address destination option), the mobile node should perform one   of the following two actions:   o  If the mobile node has recent upper layer progress information,      which indicates that communications with the correspondent node      are progressing, it MAY ignore the message.  This can be done in      order to limit the damage that spoofed Binding Error messages can      cause to ongoing communications.   o  If the mobile node has no upper layer progress information, it      MUST remove the entry and route further communications through the      home agent.  It MAY also optionally start a return routability      procedure (seeSection 5.2).   If the message Status field was 2 (unrecognized MH Type value), the   mobile node should perform one of the following two actions:   o  If the mobile node is not expecting an acknowledgement or response      from the correspondent node, the mobile node SHOULD ignore this      message.   o  Otherwise, the mobile node SHOULD cease the use of any extensions      to this specification.  If no extensions had been used, the mobile      node should cease the attempt to use route optimization.Johnson, et al.              Standard Track                   [Page 116]

RFC 3775                Mobility Support in IPv6               June 200411.4.  Home Agent and Prefix Management11.4.1.  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   inSection 11.7.1, the mobile node may not know the address of any   router on its home link that can serve as a home agent for it.  For   example, some nodes on its home link may have been reconfigured while   the mobile node has been away from home, such that the router that   was operating as the mobile node's home agent has been replaced by a   different router serving this role.   In this case, the mobile node MAY attempt to discover the address of   a suitable home agent on its home link.  To do so, the mobile node   sends an ICMP Home Agent Address Discovery Request message to the   Mobile IPv6 Home-Agents anycast address [16] for its home subnet   prefix.  As described inSection 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   (seeSection 13) before sending a Binding Update to the next home   agent.  In trying each of the returned home agent addresses, the   mobile node SHOULD try each of them in the order they appear in the   Home Agent Addresses field in the received Home Agent Address   Discovery Reply message.   If the mobile node has a current registration with some home agent   (the Lifetime for that registration has not yet expired), then the   mobile node MUST attempt any new registration first with that home   agent.  If that registration attempt fails (e.g., timed out or   rejected), the mobile node SHOULD then reattempt this registration   with another home agent.  If the mobile node knows of no other   suitable home agent, then it MAY attempt the dynamic home agent   address discovery mechanism described above.Johnson, et al.              Standard Track                   [Page 117]

RFC 3775                Mobility Support in IPv6               June 2004   If, after a mobile node transmits a Home Agent Address Discovery   Request message to the Home Agents Anycast address, it does not   receive a corresponding Home Agent Address Discovery Reply message   within INITIAL_DHAAD_TIMEOUT (seeSection 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 (seeSection 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 inSection 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 inSection 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 (seeSection 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 AdvertisementsSection 10.6 describes the operation of a home agent to support boot   time configuration and renumbering a mobile node's home subnet while   the mobile node is away from home.  The home agent sends Mobile   Prefix Advertisements to the mobile node while away from home, giving   "important" Prefix Information options that describe changes in the   prefixes in use on the mobile node's home link.Johnson, et al.              Standard Track                   [Page 118]

RFC 3775                Mobility Support in IPv6               June 2004   The Mobile Prefix Solicitation is similar to the Router Solicitation   used in Neighbor Discovery [12], except it is routed from the mobile   node on the visited network to the home agent on the home network by   usual unicast routing rules.   When a mobile node receives a Mobile Prefix Advertisement, it MUST   validate it according to the following test:   o  The Source Address of the IP packet carrying the Mobile Prefix      Advertisement is the same as the home agent address to which the      mobile node last sent an accepted home registration Binding Update      to register its primary care-of address.  Otherwise, if no such      registrations have been made, it SHOULD be the mobile node's      stored home agent address, if one exists.  Otherwise, if the      mobile node has not yet discovered its home agent's address, it      MUST NOT accept Mobile Prefix Advertisements.   o  The packet MUST have a type 2 routing header and SHOULD be      protected by an IPsec header as described inSection 5.4 andSection 6.8.   o  If the ICMP Identifier value matches the ICMP Identifier value of      the most recently sent Mobile Prefix Solicitation and no other      advertisement has yet been received for this value, then the      advertisement is considered to be solicited and will be processed      further.      Otherwise, the advertisement is unsolicited, and MUST be      discarded.  In this case the mobile node SHOULD send a Mobile      Prefix Solicitation.   Any received Mobile Prefix Advertisement not meeting these tests MUST   be silently discarded.   For an accepted Mobile Prefix Advertisement, the mobile node MUST   process Managed Address Configuration (M), Other Stateful   Configuration (O), and the Prefix Information Options as if they   arrived in a Router Advertisement [12] on the mobile node's home   link.  (This specification does not, however, describe how to acquire   home addresses through stateful protocols.)  Such processing may   result in the mobile node configuring a new home address, although   due to separation between preferred lifetime and valid lifetime, such   changes should not affect most communications by the mobile node, in   the same way as for nodes that are at home.   This specification assumes that any security associations and   security policy entries that may be needed for new prefixes have been   pre-configured in the mobile node.  Note that while dynamic keyJohnson, et al.              Standard Track                   [Page 119]

RFC 3775                Mobility Support in IPv6               June 2004   management avoids the need to create new security associations, it is   still necessary to add policy entries to protect the communications   involving the home address(es).  Mechanisms for automatic set-up of   these entries are outside the scope of this specification.11.5.  Movement11.5.1.  Movement Detection   The primary goal of movement detection is to detect L3 handovers.   This section does not attempt to specify a fast movement detection   algorithm which will function optimally for all types of   applications, link-layers and deployment scenarios; instead, it   describes a generic method that uses the facilities of IPv6 Neighbor   Discovery, including Router Discovery and Neighbor Unreachability   Detection.  At the time of this writing, this method is considered   well enough understood to recommend for standardization, however it   is expected that future versions of this specification or other   specifications may contain updated versions of the movement detection   algorithm that have better performance.   Generic movement detection uses Neighbor Unreachability Detection to   detect when the default router is no longer bi-directionally   reachable, in which case the mobile node must discover a new default   router (usually on a new link).  However, this detection only occurs   when the mobile node has packets to send, and in the absence of   frequent Router Advertisements or indications from the link-layer,   the mobile node might become unaware of an L3 handover that occurred.   Therefore, the mobile node should supplement this method with other   information whenever it is available to the mobile node (e.g., from   lower protocol layers).   When the mobile node detects an L3 handover, it performs Duplicate   Address Detection [13] on its link-local address, selects a new   default router as a consequence of Router Discovery, and then   performs Prefix Discovery with that new router to form new care-of   address(es) as described inSection 11.5.2.  It then registers its   new primary care-of address with its home agent as described inSection 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 inSection11.7.2.   Due to the temporary packet flow disruption and signaling overhead   involved in updating mobility bindings, the mobile node should avoid   performing an L3 handover until it is strictly necessary.   Specifically, when the mobile node receives a Router Advertisement   from a new router that contains a different set of on-link prefixes,Johnson, et al.              Standard Track                   [Page 120]

RFC 3775                Mobility Support in IPv6               June 2004   if the mobile node detects that the currently selected default router   on the old link is still bi-directionally reachable, it should   generally continue to use the old router on the old link rather than   switch away from it to use a new default router.   Mobile nodes can use the information in received Router   Advertisements to detect L3 handovers.  In doing so the mobile node   needs to consider the following issues:   o  There might be multiple routers on the same link, thus hearing a      new router does not necessarily constitute an L3 handover.   o  When there are multiple routers on the same link they might      advertise different prefixes.  Thus even hearing a new router with      a new prefix might not be a reliable indication of an L3 handover.   o  The link-local addresses of routers are not globally unique, hence      after completing an L3 handover the mobile node might continue to      receive Router Advertisements with the same link-local source      address.  This might be common if routers use the same link-local      address on multiple interfaces.  This issue can be avoided when      routers use the Router Address (R) bit, since that provides a      global address of the router.   In addition, the mobile node should consider the following events as   indications that an L3 handover may have occurred.  Upon receiving   such indications, the mobile node needs to perform Router Discovery   to discover routers and prefixes on the new link, as described inSection 6.3.7 of RFC 2461 [12].   o  If Router Advertisements that the mobile node receives include an      Advertisement Interval option, the mobile node may use its      Advertisement Interval field as an indication of the frequency      with which it should expect to continue to receive future      Advertisements from that router.  This field specifies the minimum      rate (the maximum amount of time between successive      Advertisements) that the mobile node should expect.  If this      amount of time elapses without the mobile node receiving any      Advertisement from this router, the mobile node can be sure that      at least one Advertisement sent by the router has been lost.  The      mobile node can then implement its own policy to determine how      many lost Advertisements from its current default router      constitute an L3 handover indication.   o  Neighbor Unreachability Detection determines that the default      router is no longer reachable.Johnson, et al.              Standard Track                   [Page 121]

RFC 3775                Mobility Support in IPv6               June 2004   o  With some types of networks, notification that an L2 handover has      occurred might be obtained from lower layer protocols or device      driver software within the mobile node.  While further details      around handling L2 indications as movement hints is an item for      further study, at the time of writing this specification the      following is considered reasonable:      An L2 handover indication may or may not imply L2 movement and L2      movement may or may not imply L3 movement; the correlations might      be a function of the type of L2 but might also be a function of      actual deployment of the wireless topology.      Unless it is well-known that an L2 handover indication is likely      to imply L3 movement, instead of immediately multicasting a router      solicitation it may be better to attempt to verify whether the      default router is still bi-directionally reachable.  This can be      accomplished by sending a unicast Neighbor Solicitation and      waiting for a Neighbor Advertisement with the solicited flag set.      Note that this is similar to Neighbor Unreachability detection but      it does not have the same state machine, such as the STALE state.      If the default router does not respond to the Neighbor      Solicitation it makes sense to proceed to multicasting a Router      Solicitation.11.5.2.  Forming New Care-of Addresses   After detecting that it has moved a mobile node SHOULD generate a new   primary care-of address using normal IPv6 mechanisms.  This SHOULD   also be done when the current primary care-of address becomes   deprecated.  A mobile node MAY form a new primary care-of address at   any time, but a mobile node MUST NOT send a Binding Update about a   new care-of address to its home agent more than MAX_UPDATE_RATE times   within a second.   In addition, a mobile node MAY form new non-primary care-of addresses   even when it has not switched to a new default router.  A mobile node   can have only one primary care-of address at a time (which is   registered with its home agent), but it MAY have an additional care-   of address for any or all of the prefixes on its current link.   Furthermore, since a wireless network interface may actually allow a   mobile node to be reachable on more than one link at a time (i.e.,   within wireless transmitter range of routers on more than one   separate link), a mobile node MAY have care-of addresses on more than   one link at a time.  The use of more than one care-of address at a   time is described inSection 11.5.3.Johnson, et al.              Standard Track                   [Page 122]

RFC 3775                Mobility Support in IPv6               June 2004   As described inSection 4, in order to form a new care-of address, a   mobile node MAY use either stateless [13] or stateful (e.g., DHCPv6   [29]) Address Autoconfiguration.  If a mobile node needs to use a   source address (other than the unspecified address) in packets sent   as a part of address autoconfiguration, it MUST use an IPv6 link-   local address rather than its own IPv6 home address.RFC 2462 [13] specifies that in normal processing for Duplicate   Address Detection, the node SHOULD delay sending the initial Neighbor   Solicitation message by a random delay between 0 and   MAX_RTR_SOLICITATION_DELAY.  Since delaying DAD can result in   significant delays in configuring a new care-of address when the   Mobile Node moves to a new link, the Mobile Node preferably SHOULD   NOT delay DAD when configuring a new care-of address.  The Mobile   Node SHOULD delay according to the mechanisms specified inRFC 2462   unless the implementation has a behavior that desynchronizes the   steps that happen before the DAD in the case that multiple nodes   experience handover at the same time.  Such desynchronizing behaviors   might be due to random delays in the L2 protocols or device drivers,   or due to the movement detection mechanism that is used.11.5.3.  Using Multiple Care-of Addresses   As described inSection 11.5.2, a mobile node MAY use more than one   care-of address at a time.  Particularly in the case of many wireless   networks, a mobile node effectively might be reachable through   multiple links at the same time (e.g., with overlapping wireless   cells), on which different on-link subnet prefixes may exist.  The   mobile node MUST ensure that its primary care-of address always has a   prefix that is advertised by its current default router.  After   selecting a new primary care-of address, the mobile node MUST send a   Binding Update containing that care-of address to its home agent.   The Binding Update MUST have the Home Registration (H) and   Acknowledge (A) bits set its home agent, as described onSection11.7.1.   To assist with smooth handovers, a mobile node SHOULD retain its   previous primary care-of address as a (non-primary) care-of address,   and SHOULD still accept packets at this address, even after   registering its new primary care-of address with its home agent.   This is reasonable, since the mobile node could only receive packets   at its previous primary care-of address if it were indeed still   connected to that link.  If the previous primary care-of address was   allocated using stateful Address Autoconfiguration [29], the mobile   node may not wish to release the address immediately upon switching   to a new primary care-of address.Johnson, et al.              Standard Track                   [Page 123]

RFC 3775                Mobility Support in IPv6               June 2004   Whenever a mobile node determines that it is no longer reachable   through a given link, it SHOULD invalidate all care-of addresses   associated with address prefixes that it discovered from routers on   the unreachable link which are not in the current set of address   prefixes advertised by the (possibly new) current default router.11.5.4.  Returning Home   A mobile node detects that it has returned to its home link through   the movement detection algorithm in use (Section 11.5.1), when the   mobile node detects that its home subnet prefix is again on-link.   The mobile node SHOULD then send a Binding Update to its home agent,   to instruct its home agent to no longer intercept or tunnel packets   for it.  In this home registration, the mobile node MUST set the   Acknowledge (A) and Home Registration (H) bits, set the Lifetime   field to zero, and set the care-of address for the binding to the   mobile node's own home address.  The mobile node MUST use its home   address as the source address in the Binding Update.   When sending this Binding Update to its home agent, the mobile node   must be careful in how it uses Neighbor Solicitation [12] (if needed)   to learn the home agent's link-layer address, since the home agent   will be currently configured to intercept packets to the mobile   node's home address using Duplicate Address Detection (DAD).  In   particular, the mobile node is unable to use its home address as the   Source Address in the Neighbor Solicitation until the home agent   stops defending the home address.   Neighbor Solicitation by the mobile node for the home agent's address   will normally not be necessary, since the mobile node has already   learned the home agent's link-layer address from a Source Link-Layer   Address option in a Router Advertisement.  However, if there are   multiple home agents it may still be necessary to send a   solicitation.  In this special case of the mobile node returning   home, the mobile node MUST multicast the packet, and in addition set   the Source Address of this Neighbor Solicitation to the unspecified   address (0:0:0:0:0:0:0:0).  The target of the Neighbor Solicitation   MUST be set to the mobile node's home address.  The destination IP   address MUST be set to the Solicited-Node multicast address [3].  The   home agent will send a multicast Neighbor Advertisement back to the   mobile node with the Solicited flag (S) set to zero.  In any case,   the mobile node SHOULD record the information from the Source Link-   Layer Address option or from the advertisement, and set the state of   the Neighbor Cache entry for the home agent to REACHABLE.   The mobile node then sends its Binding Update to the home agent's   link-layer address, instructing its home agent to no longer serve as   a home agent for it.  By processing this Binding Update, the homeJohnson, et al.              Standard Track                   [Page 124]

RFC 3775                Mobility Support in IPv6               June 2004   agent will cease defending the mobile node's home address for   Duplicate Address Detection and will no longer respond to Neighbor   Solicitations for the mobile node's home address.  The mobile node is   then the only node on the link receiving packets at the mobile node's   home address.  In addition, when returning home prior to the   expiration of a current binding for its home address, and configuring   its home address on its network interface on its home link, the   mobile node MUST NOT perform Duplicate Address Detection on its own   home address, in order to avoid confusion or conflict with its home   agent's use of the same address.  This rule also applies to the   derived link-local address of the mobile node, if the Link Local   Address Compatibility (L) bit was set when the binding was created.   If the mobile node returns home after the bindings for all of its   care-of addresses have expired, then it SHOULD perform DAD.   After the Mobile Node sends the Binding Update, it MUST be prepared   to reply to Neighbor Solicitations for its home address.  Such   replies MUST be sent using a unicast Neighbor Advertisement to the   sender's link-layer address.  It is necessary to reply, since sending   the Binding Acknowledgement from the home agent may require   performing Neighbor Discovery, and the mobile node may not be able to   distinguish Neighbor Solicitations coming from the home agent from   other Neighbor Solicitations.  Note that a race condition exists   where both the mobile node and the home agent respond to the same   solicitations sent by other nodes; this will be only temporary,   however, until the Binding Update is accepted.   After receiving the Binding Acknowledgement for its Binding Update to   its home agent, the mobile node MUST multicast onto the home link (to   the all-nodes multicast address) a Neighbor Advertisement [12], to   advertise the mobile node's own link-layer address for its own home   address.  The Target Address in this Neighbor Advertisement MUST be   set to the mobile node's home address, and the Advertisement MUST   include a Target Link-layer Address option specifying the mobile   node's link-layer address.  The mobile node MUST multicast such a   Neighbor Advertisement for each of its home addresses, as defined by   the current on-link prefixes, including its link-local address and   site-local address.  The Solicited Flag (S) in these Advertisements   MUST NOT be set, since they were not solicited by any Neighbor   Solicitation.  The Override Flag (O) in these Advertisements MUST be   set, indicating that the Advertisements SHOULD override any existing   Neighbor Cache entries at any node receiving them.   Since multicasting on the local link (such as Ethernet) is typically   not guaranteed to be reliable, the mobile node MAY retransmit these   Neighbor Advertisements [12] up to MAX_NEIGHBOR_ADVERTISEMENT times   to increase their reliability.  It is still possible that some nodesJohnson, et al.              Standard Track                   [Page 125]

RFC 3775                Mobility Support in IPv6               June 2004   on the home link will not receive any of these Neighbor   Advertisements, but these nodes will eventually be able to recover   through use of Neighbor Unreachability Detection [12].   Note that the tunnel via the home agent typically stops operating at   the same time that the home registration is deleted.11.6.  Return Routability Procedure   This section defines the rules that the mobile node must follow when   performing the return routability procedure.Section 11.7.2   describes the rules when the return routability procedure needs to be   initiated.11.6.1.  Sending Test Init Messages   A mobile node that initiates a return routability procedure MUST send   (in parallel) a Home Test Init message and a Care-of Test Init   messages.  However, if the mobile node has recently received (seeSection 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 inSection6.1.3.   A Care-of Test Init message MUST be created as described inSection6.1.4.  When sending a Home Test Init or Care-of Test Init message   the mobile node MUST record in its Binding Update List the following   fields from the messages:   o  The IP address of the node to which the message was sent.   o  The home address of the mobile node.  This value will appear in      the Source Address field of the Home Test Init message.  When      sending the Care-of Test Init message, this address does not      appear in the message, but represents the home address for which      the binding is desired.Johnson, et al.              Standard Track                   [Page 126]

RFC 3775                Mobility Support in IPv6               June 2004   o  The time at which each of these messages was sent.   o  The cookies used in the messages.   Note that a single Care-of Test Init message may be sufficient even   when there are multiple home addresses.  In this case the mobile node   MAY record the same information in multiple Binding Update List   entries.11.6.2.  Receiving Test Messages   Upon receiving a packet carrying a Home Test message, a mobile node   MUST validate the packet according to the following tests:   o  The Source Address of the packet belongs to a correspondent node      for which the mobile node has a Binding Update List entry with a      state indicating that return routability procedure is in progress.      Note that there may be multiple such entries.   o  The Binding Update List indicates that no home keygen token has      been received yet.   o  The Destination Address of the packet has the home address of the      mobile node, and the packet has been received in a tunnel from the      home agent.   o  The Home Init Cookie field in the message matches the value stored      in the Binding Update List.   Any Home Test message not satisfying all of these tests MUST be   silently ignored.  Otherwise, the mobile node MUST record the Home   Nonce Index and home keygen token in the Binding Update List.  If the   Binding Update List entry does not have a care-of keygen token, the   mobile node SHOULD continue waiting for the Care-of Test message.   Upon receiving a packet carrying a Care-of Test message, a mobile   node MUST validate the packet according to the following tests:   o  The Source Address of the packet belongs to a correspondent node      for which the mobile node has a Binding Update List entry with a      state indicating that return routability procedure is in progress.      Note that there may be multiple such entries.   o  The Binding Update List indicates that no care-of keygen token has      been received yet.   o  The Destination Address of the packet is the current care-of      address of the mobile node.Johnson, et al.              Standard Track                   [Page 127]

RFC 3775                Mobility Support in IPv6               June 2004   o  The Care-of Init Cookie field in the message matches the value      stored in the Binding Update List.   Any Care-of Test message not satisfying all of these tests MUST be   silently ignored.  Otherwise, the mobile node MUST record the Care-of   Nonce Index and care-of keygen token in the Binding Update List.  If   the Binding Update List entry does not have a home keygen token, the   mobile node SHOULD continue waiting for the Home Test message.   If after receiving either the Home Test or the Care-of Test message   and performing the above actions, the Binding Update List entry has   both the home and the care-of keygen tokens, the return routability   procedure is complete.  The mobile node SHOULD then proceed with   sending a Binding Update as described inSection 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 inSection 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 Bindings11.7.1.  Sending Binding Updates to the Home Agent   After deciding to change its primary care-of address as described inSection 11.5.1 andSection 11.5.2, a mobile node MUST register this   care-of address with its home agent in order to make this its primary   care-of address.   Also, if the mobile node wants the services of the home agent beyond   the current registration period, the mobile node should send a new   Binding Update to it well before the expiration of this period, even   if it is not changing its primary care-of address.  However, if theJohnson, et al.              Standard Track                   [Page 128]

RFC 3775                Mobility Support in IPv6               June 2004   home agent returned a Binding Acknowledgement for the current   registration with Status field set to 1 (accepted but prefix   discovery necessary), the mobile node should not try to register   again before it has learned the validity of its home prefixes through   mobile prefix discovery.  This is typically necessary every time this   Status value is received, because information learned earlier may   have changed.   To register a care-of address or to extend the lifetime of an   existing registration, the mobile node sends a packet to its home   agent containing a Binding Update, with the packet constructed as   follows:   o  The Home Registration (H) bit MUST be set in the Binding Update.   o  The Acknowledge (A) bit MUST be set in the Binding Update.   o  The packet MUST contain a Home Address destination option, giving      the mobile node's home address for the binding.   o  The care-of address for the binding MUST be used as the Source      Address in the packet's IPv6 header, unless an Alternate Care-of      Address mobility option is included in the Binding Update.  This      option MUST be included in all home registrations, as the ESP      protocol will not be able to protect care-of addresses in the IPv6      header.  (Mobile IPv6 implementations that know they are using      IPsec AH to protect a particular message might avoid this option.      For brevity the usage of AH is not discussed in this document.)   o  If the mobile node's link-local address has the same interface      identifier as the home address for which it is supplying a new      care-of address, then the mobile node SHOULD set the Link-Local      Address Compatibility (L) bit.   o  If the home address was generated usingRFC 3041 [18], then the      link local address is unlikely to have a compatible interface      identifier.  In this case, the mobile node MUST clear the Link-      Local Address Compatibility (L) bit.   o  If the IPsec security associations between the mobile node and the      home agent have been established dynamically, and the mobile node      has the capability to update its endpoint in the used key      management protocol to the new care-of address every time it      moves, the mobile node SHOULD set the Key Management Mobility      Capability (K) bit in the Binding Update.  Otherwise, the mobile      node MUST clear the bit.Johnson, et al.              Standard Track                   [Page 129]

RFC 3775                Mobility Support in IPv6               June 2004   o  The value specified in the Lifetime field MUST be non-zero and      SHOULD be less than or equal to the remaining valid lifetime of      the home address and the care-of address specified for the      binding.      Mobile nodes that use dynamic home agent address discovery should      be careful with long lifetimes.  If the mobile node loses the      knowledge of its binding with a specific home agent, registering a      new binding with another home agent may be impossible as the      previous home agent is still defending the existing binding.      Therefore, to ensure that mobile nodes using home agent address      discovery do not lose information about their binding, they SHOULD      de-register before losing this information, or use small      lifetimes.   The Acknowledge (A) bit in the Binding Update requests the home agent   to return a Binding Acknowledgement in response to this Binding   Update.  As described inSection 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   (seeSection 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).  SeeSection 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 inSection 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 inSection 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 atJohnson, et al.              Standard Track                   [Page 130]

RFC 3775                Mobility Support in IPv6               June 2004   any value.  If the home agent rejects the value, it sends back a   Binding Acknowledgement with a status code 135, and the last accepted   sequence number in the Sequence Number field of the Binding   Acknowledgement.  The mobile node MUST store this information and use   the next Sequence Number value for the next Binding Update it sends.   If the mobile node has additional home addresses, then the mobile   node SHOULD send an additional packet containing a Binding Update to   its home agent to register the care-of address for each such other   home address.   The home agent will only perform DAD for the mobile node's home   address when the mobile node has supplied a valid binding between its   home address and a care-of address.  If some time elapses during   which the mobile node has no binding at the home agent, it might be   possible for another node to autoconfigure the mobile node's home   address.  Therefore, the mobile node MUST treat the creation of a new   binding with the home agent using an existing home address, the same   as creation of a new home address.  In the unlikely event that the   mobile node's home address is autoconfigured as the IPv6 address of   another network node on the home network, the home agent will reply   to the mobile node's subsequent Binding Update with a Binding   Acknowledgement containing a Status of 134 (Duplicate Address   Detection failed).  In this case, the mobile node MUST NOT attempt to   re-use the same home address.  It SHOULD continue to register the   care-of addresses for its other home addresses, if any.  (Mechanisms   outlined inAppendix B.5 may in the future allow mobile nodes to   acquire new home addresses to replace the one for which Status 134   was received.)11.7.2.  Correspondent Registration   When the mobile node is assured that its home address is valid, it   can initiate a correspondent registration with the purpose of   allowing the correspondent node to cache the mobile node's current   care-of address.  This procedure consists of the return routability   procedure followed by a registration.   This section defines when the correspondent registration is to be   initiated and the rules to follow while it is being performed.   After the mobile node has sent a Binding Update to its home agent,   registering a new primary care-of address (as described inSection11.7.1), the mobile node SHOULD initiate a correspondent registration   for each node that already appears in the mobile node's Binding   Update List.  The initiated procedures can be used to either update   or delete binding information in the correspondent node.Johnson, et al.              Standard Track                   [Page 131]

RFC 3775                Mobility Support in IPv6               June 2004   For nodes that do not appear in the mobile node's Binding Update   List, the mobile node MAY initiate a correspondent registration at   any time after sending the Binding Update to its home agent.   Considerations regarding when (and if) to initiate the procedure   depend on the specific movement and traffic patterns of the mobile   node and are outside the scope of this document.   In addition, the mobile node MAY initiate the correspondent   registration in response to receiving a packet that meets all of the   following tests:   o  The packet was tunneled using IPv6 encapsulation.   o  The Destination Address in the tunnel (outer) IPv6 header is equal      to any of the mobile node's care-of addresses.   o  The Destination Address in the original (inner) IPv6 header is      equal to one of the mobile node's home addresses.   o  The Source Address in the tunnel (outer) IPv6 header differs from      the Source Address in the original (inner) IPv6 header.   o  The packet does not contain a Home Test, Home Test Init, Care-of      Test, or Care-of Test Init message.   If a mobile node has multiple home addresses, it becomes important to   select the right home address to use in the correspondent   registration.  The used home address MUST be the Destination Address   of the original (inner) packet.   The peer address used in the procedure MUST be determined as follows:   o  If a Home Address destination option is present in the original      (inner) packet, the address from this option is used.   o  Otherwise, the Source Address in the original (inner) IPv6 header      of the packet is used.   Note that the validity of the original packet is checked before   attempting to initiate a correspondent registration.  For instance,   if a Home Address destination option appeared in the original packet,   then rules inSection 9.3.1 are followed.   A mobile node MAY also choose to keep its topological location   private from certain correspondent nodes, and thus need not initiate   the correspondent registration.Johnson, et al.              Standard Track                   [Page 132]

RFC 3775                Mobility Support in IPv6               June 2004   Upon successfully completing the return routability procedure, and   after receiving a successful Binding Acknowledgement from the Home   Agent, a Binding Update MAY be sent to the correspondent node.   In any Binding Update sent by a mobile node, the care-of address   (either the Source Address in the packet's IPv6 header or the Care-of   Address in the Alternate Care-of Address mobility option of the   Binding Update) MUST be set to one of the care-of addresses currently   in use by the mobile node or to the mobile node's home address.  A   mobile node MAY set the care-of address differently for sending   Binding Updates to different correspondent nodes.   A mobile node MAY also send a Binding Update to such a correspondent   node, instructing it to delete any existing binding for the mobile   node from its Binding Cache, as described inSection 6.1.7.  Even in   this case a successful completion of the return routability procedure   is required first.   If the care-of address is not set to the mobile node's home address,   the Binding Update requests that the correspondent node create or   update an entry for the mobile node in the correspondent node's   Binding Cache.  This is done in order to record a care-of address for   use in sending future packets to the mobile node.  In this case, the   value specified in the Lifetime field sent in the Binding Update   SHOULD be less than or equal to the remaining lifetime of the home   registration and the care-of address specified for the binding.  The   care-of address given in the Binding Update MAY differ from the   mobile node's primary care-of address.   If the Binding Update is sent to the correspondent node, requesting   the deletion of any existing Binding Cache entry it has for the   mobile node, the care-of address is set to the mobile node's home   address and the Lifetime field set to zero.  In this case, generation   of the binding management key depends exclusively on the home keygen   token (Section 5.2.5).  The care-of nonce index SHOULD be set to zero   in this case.  In keeping with the Binding Update creation rules   below, the care-of address MUST be set to the home address if the   mobile node is at home, or to the current care-of address if it is   away from home.   If the mobile node wants to ensure that its new care-of address has   been entered into a correspondent node's Binding Cache, the mobile   node needs to request an acknowledgement by setting the Acknowledge   (A) bit in the Binding Update.Johnson, et al.              Standard Track                   [Page 133]

RFC 3775                Mobility Support in IPv6               June 2004   A Binding Update is created as follows:   o  The current care-of address of the mobile node MUST be sent either      in the Source Address of the IPv6 header, or in the Alternate      Care-of Address mobility option.   o  The Destination Address of the IPv6 header MUST contain the      address of the correspondent node.   o  The Mobility Header is constructed according to rules inSection6.1.7 andSection 5.2.6, including the Binding Authorization Data      (calculated as defined inSection 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 inSection 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 inSection 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 inSection 6.1.8 andSection 5.  That is,      if the Binding Update was sent to the home agent, underlying IPsec      protection is used.  If the Binding Update was sent to the      correspondent node, the Binding Authorization Data mobility option      MUST be present and have a valid value.Johnson, et al.              Standard Track                   [Page 134]

RFC 3775                Mobility Support in IPv6               June 2004   o  The Binding Authorization Data mobility option, if present, MUST      be the last option and MUST not have trailing padding.   o  The Sequence Number field matches the Sequence Number sent by the      mobile node to this destination address in an outstanding Binding      Update.   Any Binding Acknowledgement not satisfying all of these tests MUST be   silently ignored.   When a mobile node receives a packet carrying a valid Binding   Acknowledgement, the mobile node MUST examine the Status field as   follows:   o  If the Status field indicates that the Binding Update was accepted      (the Status field is less than 128), then the mobile node MUST      update the corresponding entry in its Binding Update List to      indicate that the Binding Update has been acknowledged; the mobile      node MUST then stop retransmitting the Binding Update.  In      addition, if the value specified in the Lifetime field in the      Binding Acknowledgement is less than the Lifetime value sent in      the Binding Update being acknowledged, the mobile node MUST      subtract the difference between these two Lifetime values from the      remaining lifetime for the binding as maintained in the      corresponding Binding Update List entry (with a minimum value for      the Binding Update List entry lifetime of 0).  That is, if the      Lifetime value sent in the Binding Update was L_update, the      Lifetime value received in the Binding Acknowledgement was L_ack,      and the current remaining lifetime of the Binding Update List      entry is L_remain, then the new value for the remaining lifetime      of the Binding Update List entry should be         max((L_remain - (L_update - L_ack)), 0)      where max(X, Y) is the maximum of X and Y.  The effect of this      step is to correctly manage the mobile node's view of the      binding's remaining lifetime (as maintained in the corresponding      Binding Update List entry) so that it correctly counts down from      the Lifetime value given in the Binding Acknowledgement, but with      the timer countdown beginning at the time that the Binding Update      was sent.      Mobile nodes SHOULD send a new Binding Update well before the      expiration of this period in order to extend the lifetime.  This      helps to avoid disruptions in communications which might otherwise      be caused by network delays or clock drift.Johnson, et al.              Standard Track                   [Page 135]

RFC 3775                Mobility Support in IPv6               June 2004   o  Additionally, if the Status field value is 1 (accepted but prefix      discovery necessary), the mobile node SHOULD send a Mobile Prefix      Solicitation message to update its information about the available      prefixes.   o  If the Status field indicates that the Binding Update was rejected      (the Status field is greater than or equal to 128), then the      mobile node can take steps to correct the cause of the error and      retransmit the Binding Update (with a new Sequence Number value),      subject to the rate limiting restriction specified inSection11.8.  If this is not done or it fails, then the mobile node      SHOULD record in its Binding Update List that future Binding      Updates SHOULD NOT be sent to this destination.   The treatment of a Binding Refresh Advice mobility option within the   Binding Acknowledgement depends on where the acknowledgement came   from.  This option MUST be ignored if the acknowledgement came from a   correspondent node.  If it came from the home agent, the mobile node   uses the Refresh Interval field in the option as a suggestion that it   SHOULD attempt to refresh its home registration at the indicated   shorter interval.   If the acknowledgement came from the home agent, the mobile node   examines the value of the Key Management Mobility Capability (K) bit.   If this bit is not set, the mobile node SHOULD discard key management   protocol connections, if any, to the home agent.  The mobile node MAY   also initiate a new key management connection.   If this bit is set, the mobile node SHOULD move its own endpoint in   the key management protocol connections to the home agent, if any.   The mobile node's new endpoint should be the new care-of address.   For an IKE phase 1 connection, this means that packets sent to this   address with the original ISAKMP cookies are accepted.11.7.4.  Receiving Binding Refresh Requests   When a mobile node receives a packet containing a Binding Refresh   Request message, the mobile node has a Binding Update List entry for   the source of the Binding Refresh Request, and the mobile node wants   to retain its binding cache entry at the correspondent node, then the   mobile node should start a return routability procedure.  If the   mobile node wants to have its binding cache entry removed, it can   either ignore the Binding Refresh Request and wait for the binding to   time out, or at any time, it can delete its binding from a   correspondent node with an explicit binding update with a zero   lifetime and the care-of address set to the home address.  If theJohnson, et al.              Standard Track                   [Page 136]

RFC 3775                Mobility Support in IPv6               June 2004   mobile node does not know if it needs the binding cache entry, it can   make the decision in an implementation dependent manner, such as   based on available resources.   Note that the mobile node should be careful to not respond to Binding   Refresh Requests for addresses not in the Binding Update List to   avoid being subjected to a denial of service attack.   If the return routability procedure completes successfully, a Binding   Update message SHOULD be sent, as described inSection 11.7.2.  The   Lifetime field in this Binding Update SHOULD be set to a new   lifetime, extending any current lifetime remaining from a previous   Binding Update sent to this node (as indicated in any existing   Binding Update List entry for this node), and the lifetime SHOULD   again be less than or equal to the remaining lifetime of the home   registration and the care-of address specified for the binding.  When   sending this Binding Update, the mobile node MUST update its Binding   Update List in the same way as for any other Binding Update sent by   the mobile node.11.8.  Retransmissions and Rate Limiting   The mobile node is responsible for retransmissions and rate limiting   in the return routability procedure, registrations, and in solicited   prefix discovery.   When the mobile node sends a Mobile Prefix Solicitation, Home Test   Init, Care-of Test Init or Binding Update for which it expects a   response, the mobile node has to determine a value for the initial   retransmission timer:   o  If the mobile node is sending a Mobile Prefix Solicitation, it      SHOULD use an initial retransmission interval of      INITIAL_SOLICIT_TIMER (seeSection 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 (seeSection 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 inSection11.7.1.   o  Otherwise, the mobile node should use the specified value of      INITIAL_BINDACK_TIMEOUT for the initial retransmission timer.Johnson, et al.              Standard Track                   [Page 137]

RFC 3775                Mobility Support in IPv6               June 2004   If the mobile node fails to receive a valid matching response within   the selected initial retransmission interval, the mobile node SHOULD   retransmit the message until a response is received.   The retransmissions by the mobile node MUST use an exponential back-   off process in which the timeout period is doubled upon each   retransmission, until either the node receives a response or the   timeout period reaches the value MAX_BINDACK_TIMEOUT.  The mobile   node MAY continue to send these messages at this slower rate   indefinitely.   The mobile node SHOULD start a separate back-off process for   different message types, different home addresses and different   care-of addresses.  However, in addition an overall rate limitation   applies for messages sent to a particular correspondent node.  This   ensures that the correspondent node has a sufficient amount of time   to respond when bindings for multiple home addresses are registered,   for instance.  The mobile node MUST NOT send Mobility Header messages   of a particular type to a particular correspondent node more than   MAX_UPDATE_RATE times within a second.   Retransmitted Binding Updates MUST use a Sequence Number value   greater than that used for the previous transmission of this Binding   Update.  Retransmitted Home Test Init and Care-of Test Init messages   MUST use new cookie values.12.  Protocol Constants   DHAAD_RETRIES                   4 retransmissions   INITIAL_BINDACK_TIMEOUT         1 second   INITIAL_DHAAD_TIMEOUT           3 seconds   INITIAL_SOLICIT_TIMER           3 seconds   MAX_BINDACK_TIMEOUT             32 seconds   MAX_NONCE_LIFETIME              240 seconds   MAX_TOKEN_LIFETIME              210 seconds   MAX_RR_BINDING_LIFETIME         420 seconds   MAX_UPDATE_RATE                 3 times   PREFIX_ADV_RETRIES              3 retransmissions   PREFIX_ADV_TIMEOUT              3 seconds13.  Protocol Configuration Variables   MaxMobPfxAdvInterval            Default: 86,400 seconds   MinDelayBetweenRAs              Default: 3 seconds,                                   Min: 0.03 seconds   MinMobPfxAdvInterval            Default: 600 seconds   InitialBindackTimeoutFirstReg   Default: 1.5 secondsJohnson, et al.              Standard Track                   [Page 138]

RFC 3775                Mobility Support in IPv6               June 2004   Home agents MUST allow the first three variables to be configured by   system management, and mobile nodes MUST allow the last variable to   be configured by system management.   The default value for InitialBindackTimeoutFirstReg has been   calculated as 1.5 times the default value of RetransTimer [12] times   the default value of DupAddrDetectTransmits [13].   The value MinDelayBetweenRAs overrides the value of the protocol   constant MIN_DELAY_BETWEEN_RAS, as specified inRFC 2461 [12].  This   variable SHOULD be set to MinRtrAdvInterval, if MinRtrAdvInterval is   less than 3 seconds.14.  IANA Considerations   This document defines a new IPv6 protocol, the Mobility Header,   described inSection 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 fromSection 6.1.2, and are the   following:      0  Binding Refresh Request      1  Home Test Init      2  Care-of Test Init      3  Home Test      4  Care-of Test      5  Binding Update      6  Binding Acknowledgement      7  Binding Error   Future values of the MH Type can be allocated using Standards Action   or IESG Approval [10].   Furthermore, each mobility message may contain mobility options as   described inSection 6.2.  This document defines a new name space   "Mobility Option" to identify these options.  The current mobility   options are defined starting fromSection 6.2.2 and are the   following:Johnson, et al.              Standard Track                   [Page 139]

RFC 3775                Mobility Support in IPv6               June 2004      0  Pad1      1  PadN      2  Binding Refresh Advice      3  Alternate Care-of Address      4  Nonce Indices      5  Authorization Data   Future values of the Option Type can be allocated using Standards   Action or IESG Approval [10].   Finally, this document creates a third new name space "Status Code"   for the Status field in the Binding Acknowledgement message. The   current values are described inSection 6.1.8, and are the following:        0 Binding Update accepted        1 Accepted but prefix discovery necessary      128 Reason unspecified      129 Administratively prohibited      130 Insufficient resources      131 Home registration not supported      132 Not home subnet      133 Not home agent for this mobile node      134 Duplicate Address Detection failed      135 Sequence number out of window      136 Expired home nonce index      137 Expired care-of nonce index      138 Expired nonces      139 Registration type change disallowedJohnson, et al.              Standard Track                   [Page 140]

RFC 3775                Mobility Support in IPv6               June 2004   Future values of the Status field can be allocated using Standards   Action or IESG Approval [10].   All fields labeled "Reserved" are only to be assigned through   Standards Action or IESG Approval.   This document also defines a new IPv6 destination option, the Home   Address option, described inSection 6.3.  This option has been   assigned the Option Type value 0xC9.   This document also defines a new IPv6 type 2 routing header,   described inSection 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 inSection 6.5;   o  The Home Agent Address Discovery Reply message, described inSection 6.6;   o  The Mobile Prefix Solicitation, described inSection 6.7; and   o  The Mobile Prefix Advertisement, described inSection 6.8.   This document also defines two new Neighbor Discovery [12] options,   which have been assigned Option Type values within the option   numbering space for Neighbor Discovery messages:   o  The Advertisement Interval option, described inSection 7.3; and   o  The Home Agent Information option, described inSection 7.4.Johnson, et al.              Standard Track                   [Page 141]

RFC 3775                Mobility Support in IPv6               June 200415.  Security Considerations15.1.  Threats   Any mobility solution must protect itself against misuses of the   mobility features and mechanisms.  In Mobile IPv6, most of the   potential threats are concerned with false Bindings, usually   resulting in Denial-of-Service attacks.  Some of the threats also   pose potential for Man-in-the-Middle, Hijacking, Confidentiality, and   Impersonation attacks.  The main threats this protocol protects   against are the following:   o  Threats involving Binding Updates sent to home agents and      correspondent nodes.  For instance, an attacker might claim that a      certain mobile node is currently at a different location than it      really is.  If a home agent accepts such spoofed information sent      to it, the mobile node might not get traffic destined to it.      Similarly, a malicious (mobile) node might use the home address of      a victim node in a forged Binding Update sent to a correspondent      node.      These pose threats against confidentiality, integrity, and      availability.  That is, an attacker might learn the contents of      packets destined to another node by redirecting the traffic to      itself.  Furthermore, an attacker might use the redirected packets      in an attempt to set itself as a Man-in-the-Middle between a      mobile and a correspondent node.  This would allow the attacker to      impersonate the mobile node, leading to integrity and availability      problems.      A malicious (mobile) node might also send Binding Updates in which      the care-of address is set to the address of a victim node.  If      such Binding Updates were accepted, the malicious node could lure      the correspondent node into sending potentially large amounts of      data to the victim; the correspondent node's replies to messages      sent by the malicious mobile node will be sent to the victim host      or network.  This could be used to cause a Distributed Denial-of-      Service attack.  For example, the correspondent node might be a      site that will send a high-bandwidth stream of video to anyone who      asks for it.  Note that the use of flow-control protocols such as      TCP does not necessarily defend against this type of attack,      because the attacker can fake the acknowledgements.  Even keeping      TCP initial sequence numbers secret does not help, because the      attacker can receive the first few segments (including the ISN) at      its own address, and only then redirect the stream to the victim's      address.  These types of attacks may also be directed to networks      instead of nodes.  Further variations of this threat are described      elsewhere [27,34].Johnson, et al.              Standard Track                   [Page 142]

RFC 3775                Mobility Support in IPv6               June 2004      An attacker might also attempt to disrupt a mobile node's      communications by replaying a Binding Update that the node had      sent earlier.  If the old Binding Update was accepted, packets      destined for the mobile node would be sent to its old location as      opposed to its current location.      In conclusion, there are Denial-of-Service, Man-in-the-Middle,      Confidentiality, and Impersonation threats against the parties      involved in sending legitimate Binding Updates, and Denial-of-      Service threats against any other party.   o  Threats associated with payload packets: Payload packets exchanged      with mobile nodes are exposed to similar threats as that of      regular IPv6 traffic.  However, Mobile IPv6 introduces the Home      Address destination option, a new routing header type (type 2),      and uses tunneling headers in the payload packets.  The protocol      must protect against potential new threats involving the use of      these mechanisms.      Third parties become exposed to a reflection threat via the Home      Address destination option, unless appropriate security      precautions are followed.  The Home Address destination option      could be used to direct response traffic toward a node whose IP      address appears in the option.  In this case, ingress filtering      would not catch the forged "return address" [36,32].      A similar threat exists with the tunnels between the mobile node      and the home agent.  An attacker might forge tunnel packets      between the mobile node and the home agent, making it appear that      the traffic is coming from the mobile node when it is not.  Note      that an attacker who is able to forge tunnel packets would      typically also be able to forge packets that appear to come      directly from the mobile node.  This is not a new threat as such.      However, it may make it easier for attackers to escape detection      by avoiding ingress filtering and packet tracing mechanisms.      Furthermore, spoofed tunnel packets might be used to gain access      to the home network.      Finally, a routing header could also be used in reflection      attacks, and in attacks designed to bypass firewalls.  The      generality of the regular routing header would allow circumvention      of IP-address based rules in firewalls.  It would also allow      reflection of traffic to other nodes.  These threats exist with      routing headers in general, even if the usage that Mobile IPv6      requires is safe.   o  Threats associated with dynamic home agent and mobile prefix      discovery.Johnson, et al.              Standard Track                   [Page 143]

RFC 3775                Mobility Support in IPv6               June 2004   o  Threats against the Mobile IPv6 security mechanisms themselves: An      attacker might, for instance, lure the participants into executing      expensive cryptographic operations or allocating memory for the      purpose of keeping state.  The victim node would have no resources      left to handle other tasks.   As a fundamental service in an IPv6 stack, Mobile IPv6 is expected to   be deployed in most nodes of the IPv6 Internet.  The above threats   should therefore be considered as being applicable to the whole   Internet.   It should also be noted that some additional threats result from   movements as such, even without the involvement of mobility   protocols.  Mobile nodes must be capable to defend themselves in the   networks that they visit, as typical perimeter defenses applied in   the home network no longer protect them.15.2.  Features   This specification provides a series of features designed to mitigate   the risk introduced by the threats listed above.  The main security   features are the following:   o  Reverse Tunneling as a mandatory feature.   o  Protection of Binding Updates sent to home agents.   o  Protection of Binding Updates sent to correspondent nodes.   o  Protection against reflection attacks that use the Home Address      destination option.   o  Protection of tunnels between the mobile node and the home agent.   o  Closing routing header vulnerabilities.   o  Mitigating Denial-of-Service threats to the Mobile IPv6 security      mechanisms themselves.   The support for encrypted reverse tunneling (seeSection 11.3.1)   allows mobile nodes to defeat certain kinds of traffic analysis.   Protecting those Binding Updates that are sent to home agents and   those that are sent to arbitrary correspondent nodes requires very   different security solutions due to the different situations.  Mobile   nodes and home agents are naturally expected to be subject to the   network administration of the home domain.Johnson, et al.              Standard Track                   [Page 144]

RFC 3775                Mobility Support in IPv6               June 2004   Thus, they can and are supposed to have a security association that   can be used to reliably authenticate the exchanged messages.  SeeSection 5.1 for the description of the protocol mechanisms, andSection 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 inSection 5.2, andSection15.4 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 inSection 15.7,Section 15.8, andSection 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 inSection 15.4.5.15.3.  Binding Updates to Home Agent   Signaling between the mobile node and the home agent requires message   integrity.  This is necessary to assure the home agent that a Binding   Update is from a legitimate mobile node.  In addition, correct   ordering and anti-replay protection are optionally needed.   IPsec ESP protects the integrity of the Binding Updates and Binding   Acknowledgements by securing mobility messages between the mobile   node and the home agent.   IPsec can provide anti-replay protection only if dynamic keying is   used (which may not always be the case).  IPsec does not guarantee   correct ordering of packets, only that they have not been replayed.   Because of this, sequence numbers within the Mobile IPv6 messages are   used to ensure correct ordering (seeSection 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 sequenceJohnson, et al.              Standard Track                   [Page 145]

RFC 3775                Mobility Support in IPv6               June 2004   numbers, replay and reordering attacks become possible.  The use of   dynamic keying, IPsec anti-replay protection, and the Mobile IPv6   sequence numbers can together prevent such attacks.  It is also   recommended that use of non-volatile storage be considered for home   agents, to avoid losing their state.   A sliding window scheme is used for the sequence numbers.  The   protection against replays and reordering attacks without a key   management mechanism works when the attacker remembers up to a   maximum of 2**15 Binding Updates.   The above mechanisms do not show that the care-of address given in   the Binding Update is correct.  This opens the possibility for   Denial-of-Service attacks against third parties.  However, since the   mobile node and home agent have a security association, the home   agent can always identify an ill-behaving mobile node.  This allows   the home agent operator to discontinue the mobile node's service, and   possibly take further actions based on the business relationship with   the mobile node's owner.   Note that the use of a single pair of manually keyed security   associations conflicts with the generation of a new home address [18]   for the mobile node, or with the adoption of a new home subnet   prefix.  This is because IPsec security associations are bound to the   used addresses.  While certificate-based automatic keying alleviates   this problem to an extent, it is still necessary to ensure that a   given mobile node cannot send Binding Updates for the address of   another mobile node.  In general, this leads to the inclusion of home   addresses in certificates in the Subject AltName field.  This again   limits the introduction of new addresses without either manual or   automatic procedures to establish new certificates.  Therefore, this   specification restricts the generation of new home addresses (for any   reason) to those situations where a security association or   certificate for the new address already exists.  (Appendix B.4 lists   the improvement of security for new addresses as one of the future   developments for Mobile IPv6.)   Support for IKE has been specified as optional.  The following should   be observed about the use of manual keying:   o  As discussed above, with manually keyed IPsec, only a limited form      of protection exists against replay and reordering attacks.  A      vulnerability exists if either the sequence number space is cycled      through, or if the home agent reboots and forgets its sequence      numbers (and uses volatile memory to store the sequence numbers).      Assuming the mobile node moves continuously every 10 minutes, itJohnson, et al.              Standard Track                   [Page 146]

RFC 3775                Mobility Support in IPv6               June 2004      takes roughly 455 days before the sequence number space has been      cycled through.  Typical movement patterns rarely reach this high      frequency today.   o  A mobile node and its home agent belong to the same domain.  If      this were not the case, manual keying would not be possible [28],      but in Mobile IPv6 only these two parties need to know the      manually configured keys.  Similarly, we note that Mobile IPv6      employs standard block ciphers in IPsec, and is not vulnerable to      problems associated with stream ciphers and manual keying.   o  It is expected that the owner of the mobile node and the      administrator of the home agent agree on the used keys and other      parameters with some off-line mechanism.   The use of IKEv1 with Mobile IPv6 is documented in more detail in   [21].  The following should be observed from the use of IKEv1:   o  It is necessary to prevent a mobile node from claiming another      mobile node's home address.  The home agent must verify that the      mobile node trying to negotiate the SA for a particular home      address is authorized for that home address.  This implies that      even with the use of IKE, a policy entry needs to be configured      for each home address served by the home agent.      It may be possible to include home addresses in the Subject      AltName field of certificate to avoid this.  However,      implementations are not guaranteed to support the use of a      particular IP address (care-of address) while another address      (home address) appears in the certificate.  In any case, even this      approach would require user-specific tasks in the certificate      authority.   o  If preshared secret authentication is used, IKEv1 main mode cannot      be used.  Aggressive mode or group preshared secrets need to be      used with corresponding security implications instead.      Note that, like many other issues, this is a general IKEv1 issue      related to the ability to use different IP addresses, and not      specifically related to Mobile IPv6.  For further information, see      Section 4.4 in [21].   o  Due to the problems outlined inSection 11.3.2, IKE phase 1      between the mobile node and its home agent is established using      the mobile node's current care-of address.  This implies that when      the mobile node moves to a new location, it may have to re-      establish phase 1.  A Key Management Mobility Capability (K) flagJohnson, et al.              Standard Track                   [Page 147]

RFC 3775                Mobility Support in IPv6               June 2004      is provided for implementations that can update the IKE phase 1      endpoints without re-establishing phase 1, but the support for      this behavior is optional.   o  When certificates are used, IKE fragmentation can occur as      discussed in Section 7 in [21].   o  Nevertheless, even if per-mobile node configuration is required      with IKE, an important benefit of IKE is that it automates the      negotiation of cryptographic parameters, including the SPIs,      cryptographic algorithms, and so on.  Thus, less configuration      information is needed.   o  The frequency of movements in some link layers or deployment      scenarios may be high enough to make replay and reordering attacks      possible, if only manual keying is used.  IKE SHOULD be used in      such cases.  Potentially vulnerable scenarios involve continuous      movement through small cells, or uncontrolled alternation between      available network attachment points.   o  Similarly, in some deployment scenarios the number of mobile nodes      may be very large.  In these cases, it can be necessary to use      automatic mechanisms to reduce the management effort in the      administration of cryptographic parameters, even if some per-      mobile node configuration is always needed.  IKE SHOULD also be      used in such cases.   o  Other automatic key management mechanisms exist beyond IKEv1, but      this document does not address the issues related to them.  We      note, however, that most of the above discussion applies to IKEv2      [30] as well, at least as it is currently specified.15.4.  Binding Updates to Correspondent Nodes   The motivation for designing the return routability procedure was to   have sufficient support for Mobile IPv6, without creating significant   new security problems.  The goal for this procedure was not to   protect against attacks that were already possible before the   introduction of Mobile IPv6.   The next sections will describe the security properties of the used   method, both from the point of view of possible on-path attackers who   can see those cryptographic values that have been sent in the clear   (Section 15.4.2 andSection 15.4.3) and from the point of view of   other attackers (Section 15.4.6).Johnson, et al.              Standard Track                   [Page 148]

RFC 3775                Mobility Support in IPv6               June 200415.4.1.  Overview   The chosen infrastructureless method verifies that the mobile node is   "live" (that is, it responds to probes) at its home and care-of   addresses.Section 5.2 describes the return routability procedure in   detail.  The procedure uses the following principles:   o  A message exchange verifies that the mobile node is reachable at      its addresses, i.e., is at least able to transmit and receive      traffic at both the home and care-of addresses.   o  The eventual Binding Update is cryptographically bound to the      tokens supplied in the exchanged messages.   o  Symmetric exchanges are employed to avoid the use of this protocol      in reflection attacks.  In a symmetric exchange, the responses are      always sent to the same address the request was sent from.   o  The correspondent node operates in a stateless manner until it      receives a fully authorized Binding Update.   o  Some additional protection is provided by encrypting the tunnels      between the mobile node and home agent with IPsec ESP.  As the      tunnel also transports the nonce exchanges, the ability of      attackers to see these nonces is limited.  For instance, this      prevents attacks from being launched from the mobile node's      current foreign link, even when no link-layer confidentiality is      available.      The resulting level of security is in theory the same even without      this additional protection: the return routability tokens are      still exposed only to one path within the whole Internet.      However, the mobile nodes are often found on an insecure link,      such as a public access Wireless LAN.  Thus, in many cases, this      addition makes a practical difference.   For further information about the design rationale of the return   routability procedure, see [27,34,33,32].  The mechanisms used   have been adopted from these documents.15.4.2.  Achieved Security Properties   The return routability procedure protects Binding Updates against all   attackers who are unable to monitor the path between the home agent   and the correspondent node.  The procedure does not defend against   attackers who can monitor this path.  Note that such attackers are in   any case able to mount an active attack against the mobile node whenJohnson, et al.              Standard Track                   [Page 149]

RFC 3775                Mobility Support in IPv6               June 2004   it is at its home location.  The possibility of such attacks is not   an impediment to the deployment of Mobile IPv6 because these attacks   are possible regardless of whether or not Mobile IPv6 is in use.   This procedure also protects against Denial-of-Service attacks in   which the attacker pretends to be mobile, but uses the victim's   address as the care-of address.  This would cause the correspondent   node to send the victim some unexpected traffic.  This procedure   defends against these attacks by requiring at least the passive   presence of the attacker at the care-of address or on the path from   the correspondent to the care-of address.  Normally, this will be the   mobile node.15.4.3.  Comparison to Regular IPv6 Communications   This section discusses the protection offered by the return   routability method by comparing it to the security of regular IPv6   communications.  We will divide vulnerabilities into three classes:   (1) those related to attackers on the local network of the mobile   node, home agent, or the correspondent node, (2) those related to   attackers on the path between the home network and the correspondent   node, and (3) off-path attackers, i.e., the rest of the Internet.   We will now discuss the vulnerabilities of regular IPv6   communications.  The on-link vulnerabilities of IPv6 communications   include Denial-of-Service, Masquerading, Man-in-the-Middle,   Eavesdropping, and other attacks.  These attacks can be launched   through spoofing Router Discovery, Neighbor Discovery and other IPv6   mechanisms.  Some of these attacks can be prevented with the use of   cryptographic protection in the packets.   A similar situation exists with on-path attackers.  That is, without   cryptographic protection, the traffic is completely vulnerable.   Assuming that attackers have not penetrated the security of the   Internet routing protocols, attacks are much harder to launch from   off-path locations.  Attacks that can be launched from these   locations are mainly Denial-of-Service attacks, such as flooding and/   or reflection attacks.  It is not possible for an off-path attacker   to become a Man-in-the-Middle.   Next, we will consider the vulnerabilities that exist when IPv6 is   used together with Mobile IPv6 and the return routability procedure.   On the local link, the vulnerabilities are the same as those in IPv6,   but Masquerade and Man-in-the-Middle attacks can now also be launched   against future communications, and not just against current   communications.  If a Binding Update was sent while the attacker was   present on the link, its effects remain for the lifetime of theJohnson, et al.              Standard Track                   [Page 150]

RFC 3775                Mobility Support in IPv6               June 2004   binding.  This happens even if the attacker moves away from the link.   In contrast, an attacker who uses only plain IPv6 generally has to   stay on the link in order to continue the attack.  Note that in order   to launch these new attacks, the IP address of the victim must be   known.  This makes this attack feasible, mainly in the context of   well-known interface IDs, such as those already appearing in the   traffic on the link or registered in the DNS.   On-path attackers can exploit similar vulnerabilities as in regular   IPv6.  There are some minor differences, however.  Masquerade, Man-   in-the-Middle, and Denial-of-Service attacks can be launched with   just the interception of a few packets, whereas in regular IPv6 it is   necessary to intercept every packet.  The effect of the attacks is   the same regardless of the method, however.  In any case, the most   difficult task an attacker faces in these attacks is getting on the   right path.   The vulnerabilities for off-path attackers are the same as in regular   IPv6.  Those nodes that are not on the path between the home agent   and the correspondent node will not be able to receive the home   address probe messages.   In conclusion, we can state the following main results from this   comparison:   o  Return routability prevents any off-path attacks beyond those that      are already possible in regular IPv6.  This is the most important      result, preventing attackers on the Internet from exploiting any      vulnerabilities.   o  Vulnerabilities to attackers on the home agent link, the      correspondent node link, and the path between them are roughly the      same as in regular IPv6.   o  However, one difference is that in basic IPv6 an on-path attacker      must be constantly present on the link or the path, whereas with      Mobile IPv6, an attacker can leave a binding behind after moving      away.      For this reason, this specification limits the creation of      bindings to at most MAX_TOKEN_LIFETIME seconds after the last      routability check has been performed, and limits the duration of a      binding to at most MAX_RR_BINDING_LIFETIME seconds.  With these      limitations, attackers cannot take any practical advantages of      this vulnerability.Johnson, et al.              Standard Track                   [Page 151]

RFC 3775                Mobility Support in IPv6               June 2004   o  There are some other minor differences, such as an effect to the      Denial-of-Service vulnerabilities.  These can be considered to be      insignificant.   o  The path between the home agent and a correspondent node is      typically easiest to attack on the links at either end, in      particular if these links are publicly accessible wireless LANs.      Attacks against the routers or switches on the path are typically      harder to accomplish.  The security on layer 2 of the links plays      then a major role in the resulting overall network security.      Similarly, security of IPv6 Neighbor and Router Discovery on these      links has a large impact.  If these were secured using some new      technology in the future, this could change the situation      regarding the easiest point of attack.   For a more in-depth discussion of these issues, see [32].15.4.4.  Replay Attacks   The return routability procedure also protects the participants   against replayed Binding Updates.  The attacker is unable replay the   same message due to the sequence number which is a part of the   Binding Update.  It is also unable to modify the Binding Update since   the MAC verification would fail after such a modification.   Care must be taken when removing bindings at the correspondent node,   however.  If a binding is removed while the nonce used in its   creation is still valid, an attacker could replay the old Binding   Update.  Rules outlined inSection 5.2.8 ensure that this cannot   happen.15.4.5.  Denial-of-Service Attacks   The return routability procedure has protection against resource   exhaustion Denial-of-Service attacks.  The correspondent nodes do not   retain any state about individual mobile nodes until an authentic   Binding Update arrives.  This is achieved through the construct of   keygen tokens from the nonces and node keys that are not specific to   individual mobile nodes.  The keygen tokens can be reconstructed by   the correspondent node, based on the home and care-of address   information that arrives with the Binding Update.  This means that   the correspondent nodes are safe against memory exhaustion attacks   except where on-path attackers are concerned.  Due to the use of   symmetric cryptography, the correspondent nodes are relatively safe   against CPU resource exhaustion attacks as well.Johnson, et al.              Standard Track                   [Page 152]

RFC 3775                Mobility Support in IPv6               June 2004   Nevertheless, as [27] describes, there are situations in which it is   impossible for the mobile and correspondent nodes to determine if   they actually need a binding or whether they just have been fooled   into believing so by an attacker.  Therefore, it is necessary to   consider situations where such attacks are being made.   Even if route optimization is a very important optimization, it is   still only an optimization.  A mobile node can communicate with a   correspondent node even if the correspondent refuses to accept any   Binding Updates.  However, performance will suffer because packets   from the correspondent node to the mobile node will be routed via the   mobile's home agent rather than a more direct route.  A correspondent   node can protect itself against some of these resource exhaustion   attacks as follows.  If the correspondent node is flooded with a   large number of Binding Updates that fail the cryptographic integrity   checks, it can stop processing Binding Updates.  If a correspondent   node finds that it is spending more resources on checking bogus   Binding Updates than it is likely to save by accepting genuine   Binding Updates, then it may silently discard some or all Binding   Updates without performing any cryptographic operations.   Layers above IP can usually provide additional information to help   decide if there is a need to establish a binding with a specific   peer.  For example, TCP knows if the node has a queue of data that it   is trying to send to a peer.  An implementation of this specification   is not required to make use of information from higher protocol   layers, but some implementations are likely to be able to manage   resources more effectively by making use of such information.   We also require that all implementations be capable of   administratively disabling route optimization.15.4.6.  Key Lengths   Attackers can try to break the return routability procedure in many   ways.Section 15.4.2 discusses the situation where the attacker can   see the cryptographic values sent in the clear, andSection 15.4.3   discusses the impact this has on IPv6 communications.  This section   discusses whether attackers can guess the correct values without   seeing them.   While the return routability procedure is in progress, 64 bit cookies   are used to protect spoofed responses.  This is believed to be   sufficient, given that to blindly spoof a response a very large   number of messages would have to be sent before success would be   probable.Johnson, et al.              Standard Track                   [Page 153]

RFC 3775                Mobility Support in IPv6               June 2004   The tokens used in the return routability procedure provide together   128 bits of information.  This information is used internally as   input to a hash function to produce a 160 bit quantity suitable for   producing the keyed hash in the Binding Update using the HMAC_SHA1   algorithm.  The final keyed hash length is 96 bits.  The limiting   factors in this case are the input token lengths and the final keyed   hash length.  The internal hash function application does not reduce   the entropy.   The 96 bit final keyed hash is of typical size and is believed to be   secure.  The 128 bit input from the tokens is broken in two pieces,   the home keygen token and the care-of keygen token.  An attacker can   try to guess the correct cookie value, but again this would require a   large number of messages (an the average 2**63 messages for one or   2**127 for two).  Furthermore, given that the cookies are valid only   for a short period of time, the attack has to keep a high constant   message rate to achieve a lasting effect.  This does not appear   practical.   When the mobile node is returning home, it is allowed to use just the   home keygen token of 64 bits.  This is less than 128 bits, but   attacking it blindly would still require a large number of messages   to be sent.  If the attacker is on the path and capable of seeing the   Binding Update, it could conceivably break the keyed hash with brute   force.  However, in this case the attacker has to be on the path,   which appears to offer easier ways for denial-of-service than   preventing route optimization.15.5.  Dynamic Home Agent Address Discovery   The dynamic home agent address discovery function could be used to   learn the addresses of home agents in the home network.   The ability to learn addresses of nodes may be useful to attackers   because brute-force scanning of the address space is not practical   with IPv6.  Thus, they could benefit from any means which make   mapping the networks easier.  For example, if a security threat   targeted at routers or even home agents is discovered, having a   simple ICMP mechanism to easily find out possible targets may prove   to be an additional (though minor) security risk.   Apart from discovering the address(es) of home agents, attackers will   not be able to learn much from this information, and mobile nodes   cannot be tricked into using wrong home agents, as all other   communication with the home agents is secure.Johnson, et al.              Standard Track                   [Page 154]

RFC 3775                Mobility Support in IPv6               June 200415.6.  Mobile Prefix Discovery   The mobile prefix discovery function may leak interesting information   about network topology and prefix lifetimes to eavesdroppers; for   this reason, requests for this information has to be authenticated.   Responses and unsolicited prefix information needs to be   authenticated to prevent the mobile nodes from being tricked into   believing false information about the prefixes and possibly   preventing communications with the existing addresses.  Optionally,   encryption may be applied to prevent leakage of the prefix   information.15.7.  Tunneling via the Home Agent   Tunnels between the mobile node and the home agent can be protected   by ensuring proper use of source addresses, and optional   cryptographic protection.  These procedures are discussed inSection5.5.   Binding Updates to the home agents are secure.  When receiving   tunneled traffic, the home agent verifies that the outer IP address   corresponds to the current location of the mobile node.  This acts as   a weak form of protection against spoofing packets that appear to   come from the mobile node.  This is particularly useful, if no end-   to-end security is being applied between the mobile and correspondent   nodes.  The outer IP address check prevents attacks where the   attacker is controlled by ingress filtering.  It also prevents   attacks when the attacker does not know the current care-of address   of the mobile node.  Attackers who know the care-of address and are   not controlled by ingress filtering could still send traffic through   the home agent.  This includes attackers on the same local link as   the mobile node is currently on.  But such attackers could send   packets that appear to come from the mobile node without attacking   the tunnel; the attacker could simply send packets with the source   address set to the mobile node's home address.  However, this attack   does not work if the final destination of the packet is in the home   network, and some form of perimeter defense is being applied for   packets sent to those destinations.  In such cases it is recommended   that either end-to-end security or additional tunnel protection be   applied, as is usual in remote access situations.   Home agents and mobile nodes may use IPsec ESP to protect payload   packets tunneled between themselves.  This is useful for protecting   communications against attackers on the path of the tunnel.   When site local home addresses are used, reverse tunneling can be   used to send site local traffic from another location.   Administrators should be aware of this when allowing such homeJohnson, et al.              Standard Track                   [Page 155]

RFC 3775                Mobility Support in IPv6               June 2004   addresses.  In particular, the outer IP address check described above   is not sufficient against all attackers.  The use of encrypted   tunnels is particularly useful for these kinds of home addresses.15.8.  Home Address Option   When the mobile node sends packets directly to the correspondent   node, the Source Address field of the packet's IPv6 header is the   care-of address.  Therefore, ingress filtering [26] works in the   usual manner even for mobile nodes, as the Source Address is   topologically correct.  The Home Address option is used to inform the   correspondent node of the mobile node's home address.   However, the care-of address in the Source Address field does not   survive in replies sent by the correspondent node unless it has a   binding for this mobile node.  Also, not all attacker tracing   mechanisms work when packets are being reflected through   correspondent nodes using the Home Address option.  For these   reasons, this specification restricts the use of the Home Address   option.  It may only be used when a binding has already been   established with the participation of the node at the home address,   as described inSection 5.5 andSection 6.3.  This prevents   reflection attacks through the use of the Home Address option.  It   also ensures that the correspondent nodes reply to the same address   that the mobile node sends traffic from.   No special authentication of the Home Address option is required   beyond the above, but note that if the IPv6 header of a packet is   covered by IPsec Authentication Header, then that authentication   covers the Home Address option as well.  Thus, even when   authentication is used in the IPv6 header, the security of the Source   Address field in the IPv6 header is not compromised by the presence   of a Home Address option.  Without authentication of the packet, any   field in the IPv6 header, including the Source Address field or any   other part of the packet and the Home Address option can be forged or   modified in transit.  In this case, the contents of the Home Address   option is no more suspect than any other part of the packet.15.9.  Type 2 Routing Header   The definition of the type 2 routing header is described inSection6.4.  This definition and the associated processing rules have been   chosen so that the header cannot be used for what is traditionally   viewed as source routing.  In particular, the Home Address in the   routing header will always have to be assigned to the home address of   the receiving node; otherwise the packet will be dropped.Johnson, et al.              Standard Track                   [Page 156]

RFC 3775                Mobility Support in IPv6               June 2004   Generally, source routing has a number of security concerns.  These   include the automatic reversal of unauthenticated source routes   (which is an issue for IPv4, but not for IPv6).  Another concern is   the ability to use source routing to "jump" between nodes inside, as   well as outside a firewall.  These security concerns are not issues   in Mobile IPv6, due to the rules mentioned above.   In essence the semantics of the type 2 routing header is the same as   a special form of IP-in-IP tunneling where the inner and outer source   addresses are the same.   This implies that a device which implements the filtering of packets   should be able to distinguish between a type 2 routing header and   other routing headers, as required inSection 8.3.  This is necessary   in order to allow Mobile IPv6 traffic while still having the option   of filtering out other uses of routing headers.16.  Contributors   Tuomas Aura, Mike Roe, Greg O'Shea, Pekka Nikander, Erik Nordmark,   and Michael Thomas worked on the return routability protocols   eventually led to the procedures used in this protocol.  The   procedures described in [34] were adopted in the protocol.   Significant contributions were made by members of the Mobile IPv6   Security Design Team, including (in alphabetical order) Gabriel   Montenegro, Erik Nordmark and Pekka Nikander.17.  Acknowledgements   We would like to thank the members of the Mobile IP and IPng Working   Groups for their comments and suggestions on this work.  We would   particularly like to thank (in alphabetical order) Fred Baker, Josh   Broch, Samita Chakrabarti, Robert Chalmers, Noel Chiappa, Greg Daley,   Vijay Devarapalli, Rich Draves, Francis Dupont, Thomas Eklund, Jun-   Ichiro Itojun Hagino, Brian Haley, Marc Hasson, John Ioannidis, James   Kempf, Rajeev Koodli, Krishna Kumar, T.J. Kniveton, Joe Lau, Jiwoong   Lee, Aime Le Rouzic, Vesa-Matti Mantyla, Kevin Miles, Glenn Morrow,   Thomas Narten, Karen Nielsen, Simon Nybroe, David Oran, Brett   Pentland, Lars Henrik Petander, Basavaraj Patil, Mohan Parthasarathy,   Alexandru Petrescu, Mattias Petterson, Ken Powell, Phil Roberts, Ed   Remmell, Patrice Romand, Luis A. Sanchez, Jeff Schiller, Pekka   Savola, Arvind Sevalkar, Keiichi Shima, Tom Soderlund, Hesham   Soliman, Jim Solomon, Tapio Suihko, Dave Thaler, Benny Van Houdt,   Jon-Olov Vatn, Carl E. Williams, Vladislav Yasevich, Alper Yegin, andJohnson, et al.              Standard Track                   [Page 157]

RFC 3775                Mobility Support in IPv6               June 2004   Xinhua Zhao, for their detailed reviews of earlier versions of this   document.  Their suggestions have helped to improve both the design   and presentation of the protocol.   We would also like to thank the participants of the Mobile IPv6   testing event (1999), implementors who participated in Mobile IPv6   interoperability testing at Connectathons (2000, 2001, 2002, and   2003), and the participants at the ETSI interoperability testing   (2000, 2002).  Finally, we would like to thank the TAHI project who   has provided test suites for Mobile IPv6.18.  References18.1.  Normative References   [1]   Eastlake 3rd., D., Crocker, S. and J. Schiller, "Randomness         Recommendations for Security",RFC 1750, December 1994.   [2]   Bradner, S., "Key words for use in RFCs to Indicate Requirement         Levels",BCP 14,RFC 2119, March 1997.   [3]   Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)         Addressing Architecture",RFC 3513, April 2003.   [4]   Kent, S. and R. Atkinson, "Security Architecture for the         Internet Protocol",RFC 2401, November 1998.   [5]   Kent, S. and R. Atkinson, "IP Authentication Header",RFC 2402,         November 1998.   [6]   Kent, S. and R. Atkinson, "IP Encapsulating Security Payload         (ESP)",RFC 2406, November 1998.   [7]   Piper, D., "The Internet IP Security Domain of Interpretation         for ISAKMP",RFC 2407, November 1998.   [8]   Maughan, D., Schertler, M., Schneider, M. and J. Turner,         "Internet Security Association and Key Management Protocol         (ISAKMP)",RFC 2408, November 1998.   [9]   Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",RFC 2409, November 1998.   [10]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA         Considerations Section in RFCs",BCP 26,RFC 2434, October         1998.Johnson, et al.              Standard Track                   [Page 158]

RFC 3775                Mobility Support in IPv6               June 2004   [11]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)         Specification",RFC 2460, December 1998.   [12]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery         for IP Version 6 (IPv6)",RFC 2461, December 1998.   [13]  Thomson, S. and T. Narten, "IPv6 Stateless Address         Autoconfiguration",RFC 2462, December 1998.   [14]  Conta, A. and S. Deering, "Internet Control Message Protocol         (ICMPv6) for the Internet Protocol Version 6 (IPv6)         Specification",RFC 2463, December 1998.   [15]  Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6         Specification",RFC 2473, December 1998.   [16]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast         Addresses",RFC 2526, March 1999.   [17]  Deering, S., Fenner, W. and B. Haberman, "Multicast Listener         Discovery (MLD) for IPv6",RFC 2710, October 1999.   [18]  Narten, T. and R. Draves, "Privacy Extensions for Stateless         Address Autoconfiguration in IPv6",RFC 3041, January 2001.   [19]  Reynolds, J., Ed., "Assigned Numbers:RFC 1700 is Replaced by         an On-line Database",RFC 3232, January 2002.   [20]  National Institute of Standards and Technology, "Secure Hash         Standard", FIPS PUB 180-1, April 1995, <http://www.itl.nist.gov/fipspubs/fip180-1.htm>.   [21]  Arkko, J., Devarapalli, V. and F. Dupont, "Using IPsec to         Protect Mobile IPv6 Signaling Between Mobile Nodes and Home         Agents",RFC 3776, June 2004.18.2.  Informative References   [22]  Perkins, C., Ed., "IP Mobility Support for IPv4",RFC 3344,         August 2002.   [23]  Perkins, C., "IP Encapsulation within IP",RFC 2003, October         1996.   [24]  Perkins, C., "Minimal Encapsulation within IP",RFC 2004,         October 1996.Johnson, et al.              Standard Track                   [Page 159]

RFC 3775                Mobility Support in IPv6               June 2004   [25]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing         for Message Authentication",RFC 2104, February 1997.   [26]  Ferguson, P. and D. Senie, "Network Ingress Filtering:         Defeating Denial of Service Attacks which employ IP Source         Address Spoofing",BCP 38,RFC 2827, May 2000.   [27]  Aura, T. and J. Arkko,"MIPv6 BU Attacks and Defenses", Work in         Progress, March 2002.   [28]  Bellovin, S., "Guidelines for Mandating Automated Key         Management", Work in Progress, August 2003.   [29]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C. and         M. Carney, "Dynamic Host Configuration Protocol for IPv6         (DHCPv6)",RFC 3315, July 2003.   [30]  Kaufman, C.,"Internet Key Exchange (IKEv2) Protocol", Work in         Progress, April 2003.   [31]  Draves, R., "Default Address Selection for Internet Protocol         version 6 (IPv6)",RFC 3484, February 2003.   [32]  Nikander, P., Aura, T., Arkko, J., Montenegro, G. and E.         Nordmark, "Mobile IP version 6 Route Optimization Security         Design Background", Work in Progress, April 2003.   [33]  Nordmark, E., "Securing MIPv6 BUs using return routability         (BU3WAY)", Work in Progress, November 2001.   [34]  Roe, M., Aura, T., O'Shea, G. and J. Arkko, "Authentication of         Mobile IPv6 Binding Updates and Acknowledgments", Work in         Progress, March 2002.   [35]  Savola, P., "Use of /127 Prefix Length Between Routers         Considered Harmful",RFC 3627, September 2003.   [36]  Savola, P., "Security of IPv6 Routing Header and Home Address         Options", Work in Progress, December 2002.   [37]  Vida, R. and L. Costa, Eds., "Multicast Listener Discovery         Version 2 (MLDv2) for IPv6",RFC 3810, June 2004.Johnson, et al.              Standard Track                   [Page 160]

RFC 3775                Mobility Support in IPv6               June 2004Appendix A.  Future ExtensionsA.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   alsoAppendix A.3).  The return routability messages can indicate   support for piggybacking with a new mobility option.A.2.  Triangular Routing   Due to the concerns about opening reflection attacks with the Home   Address destination option, this specification requires that this   option be verified against the Binding Cache, i.e., there must be a   Binding Cache entry for the Home Address and Care-of Address.   Future extensions may be specified that allow the use of unverified   Home Address destination options in ways that do not introduce   security issues.A.3.  New Authorization Methods   While the return routability procedure provides a good level of   security, there exist methods that have even higher levels of   security.  Secondly, as discussed inSection 15.4, future   enhancements of IPv6 security may cause a need to also improve the   security of the return routability procedure.  Using IPsec as the   sole method for authorizing Binding Updates to correspondent nodes is   also possible.  The protection of the Mobility Header for this   purpose is easy, though one must ensure that the IPsec SA was created   with appropriate authorization to use the home address referenced in   the Binding Update.  For instance, a certificate used by IKE to   create the security association might contain the home address.  A   future specification may specify how this is done.A.4.  Dynamically Generated Home Addresses   A future version of this specification may include functionality that   allows the generation of new home addresses without requiring pre-   arranged security associations or certificates even for the new   addresses.Johnson, et al.              Standard Track                   [Page 161]

RFC 3775                Mobility Support in IPv6               June 2004A.5.  Remote Home Address Configuration   The method for initializing a mobile node's home address upon power-   up or after an extended period of being disconnected from the network   is beyond the scope of this specification.  Whatever procedure is   used should result in the mobile node having the same stateless or   stateful (e.g., DHCPv6) home address autoconfiguration information it   would have if it were attached to the home network.  Due to the   possibility that the home network could be renumbered while the   mobile node is disconnected, a robust mobile node would not rely   solely on storing these addresses locally.   Such a mobile node could be initialized by using the following   procedure:   1.  Generate a care-of address.   2.  Query DNS for an anycast address associated with the FQDN of the       home agent(s).   3.  Perform home agent address discovery, and select a home agent.   4.  Configure one home address based on the selected home agent's       subnet prefix and the interface identifier of the mobile node.   5.  Create security associations and security policy database entries       for protecting the traffic between the selected home address and       home agent.   6.  Perform a home registration on the selected home agent.   7.  Perform mobile prefix discovery.   8.  Make a decision if further home addresses need to be configured.   This procedure is restricted to those situations where the home   prefix is 64 bits and the mobile node knows its own interface   identifier, which is also 64 bits.Johnson, et al.              Standard Track                   [Page 162]

RFC 3775                Mobility Support in IPv6               June 2004A.6.  Neighbor Discovery Extensions   Future specifications may improve the efficiency of Neighbor   Discovery tasks, which could be helpful for fast movements.  One   factor is currently being looked at: the delays caused by the   Duplicate Address Detection mechanism.  Currently, Duplicate Address   Detection needs to be performed for every new care-of address as the   mobile node moves, and for the mobile node's link-local address on   every new link.  In particular, the need and the trade-offs of re-   performing Duplicate Address Detection for the link-local address   every time the mobile node moves on to new links will need to be   examined.  Improvements in this area are, however, generally   applicable and progress independently from the Mobile IPv6   specification.   Future functional improvements may also be relevant for Mobile IPv6   and other applications.  For instance, mechanisms that would allow   recovery from a Duplicate Address Detection collision would be useful   for link-local, care-of, and home addresses.Johnson, et al.              Standard Track                   [Page 163]

RFC 3775                Mobility Support in IPv6               June 2004Authors' Addresses   David B. Johnson   Rice University   Dept. of Computer Science, MS 132   6100 Main Street   Houston  TX 77005-1892   USA   EMail: dbj@cs.rice.edu   Charles E. Perkins   Nokia Research Center   313 Fairchild Drive   Mountain View  CA 94043   USA   EMail: charliep@iprg.nokia.com   Jari Arkko   Ericsson   02420  Jorvas   Finland   EMail: jari.arkko@ericsson.comJohnson, et al.              Standard Track                   [Page 164]

RFC 3775                Mobility Support in IPv6               June 2004Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 78, and   except as set forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Johnson, et al.              Standard Track                   [Page 165]