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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 2401 (ref. '2') (Obsoleted by RFC 4301) ** Obsolete normative reference: RFC 2402 (ref. '3') (Obsoleted by RFC 4302, RFC 4305) ** Obsolete normative reference: RFC 2406 (ref. '4') (Obsoleted by RFC 4303, RFC 4305) ** Obsolete normative reference: RFC 2460 (ref. '5') (Obsoleted by RFC 8200) ** Downref: Normative reference to an Informational RFC: RFC 3232 (ref. '10') -- Possible downref: Non-RFC (?) normative reference: ref. '11' ** Obsolete normative reference: RFC 4941 (ref. '20') (Obsoleted by RFC 8981) ** Downref: Normative reference to an Informational RFC: RFC 5014 (ref. '21') ** Obsolete normative reference: RFC 5226 (ref. '23') (Obsoleted by RFC 8126) -- Obsolete informational reference (is this intentional?): RFC 3315 (ref. '29') (Obsoleted by RFC 8415) -- Obsolete informational reference (is this intentional?): RFC 3344 (ref. '30') (Obsoleted by RFC 5944) -- Obsolete informational reference (is this intentional?): RFC 3484 (ref. '31') (Obsoleted by RFC 6724) -- Obsolete informational reference (is this intentional?): RFC 3627 (ref. '35') (Obsoleted by RFC 6547) == Outdated reference: A later version (-03) exists of draft-savola-ipv6-rh-ha-security-02 -- Obsolete informational reference (is this intentional?): RFC 4306 (ref. '41') (Obsoleted by RFC 5996) Summary: 8 errors (**), 0 flaws (~~), 4 warnings (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IETF Mobile IP Working Group C. Perkins (Ed.) 3 Internet-Draft Tellabs Inc. 4 Obsoletes: 3775 (if approved) D. Johnson 5 Expires: January 13, 2011 Rice University 6 J. Arkko 7 Ericsson 8 July 12, 2010 10 Mobility Support in IPv6 11 draft-ietf-mext-rfc3775bis-06.txt 13 Abstract 15 This document specifies a protocol which allows nodes to remain 16 reachable while moving around in the IPv6 Internet. Each mobile node 17 is always identified by its home address, regardless of its current 18 point of attachment to the Internet. While situated away from its 19 home, a mobile node is also associated with a care-of address, which 20 provides information about the mobile node's current location. IPv6 21 packets addressed to a mobile node's home address are transparently 22 routed to its care-of address. The protocol enables IPv6 nodes to 23 cache the binding of a mobile node's home address with its care-of 24 address, and to then send any packets destined for the mobile node 25 directly to it at this care-of address. To support this operation, 26 Mobile IPv6 defines a new IPv6 protocol and a new destination option. 27 All IPv6 nodes, whether mobile or stationary, can communicate with 28 mobile nodes. 30 Status of this Memo 32 This Internet-Draft is submitted to IETF in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on January 13, 2011. 47 Copyright Notice 48 Copyright (c) 2010 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6 64 2. Comparison with Mobile IP for IPv4 . . . . . . . . . . . . . 8 65 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9 66 3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 9 67 3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 11 68 4. Overview of Mobile IPv6 . . . . . . . . . . . . . . . . . . . 15 69 4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 15 70 4.2. New IPv6 Protocol . . . . . . . . . . . . . . . . . . . . 17 71 4.3. New IPv6 Destination Option . . . . . . . . . . . . . . . 18 72 4.4. New IPv6 ICMP Messages . . . . . . . . . . . . . . . . . 18 73 4.5. Conceptual Data Structure Terminology . . . . . . . . . . 19 74 4.6. Unique-Local Addressability . . . . . . . . . . . . . . . 19 75 5. Overview of Mobile IPv6 Security . . . . . . . . . . . . . . 21 76 5.1. Binding Updates to Home Agents . . . . . . . . . . . . . 21 77 5.2. Binding Updates to Correspondent Nodes . . . . . . . . . 22 78 5.2.1. Node Keys . . . . . . . . . . . . . . . . . . . . . . 22 79 5.2.2. Nonces . . . . . . . . . . . . . . . . . . . . . . . 23 80 5.2.3. Cookies and Tokens . . . . . . . . . . . . . . . . . 23 81 5.2.4. Cryptographic Functions . . . . . . . . . . . . . . . 24 82 5.2.5. Return Routability Procedure . . . . . . . . . . . . 24 83 5.2.6. Authorizing Binding Management Messages . . . . . . . 29 84 5.2.7. Updating Node Keys and Nonces . . . . . . . . . . . . 31 85 5.2.8. Preventing Replay Attacks . . . . . . . . . . . . . . 32 86 5.2.9. Handling Interruptions to Return Routability . . . . 32 87 5.3. Dynamic Home Agent Address Discovery . . . . . . . . . . 33 88 5.4. Mobile Prefix Discovery . . . . . . . . . . . . . . . . . 33 89 5.5. Payload Packets . . . . . . . . . . . . . . . . . . . . . 33 90 6. New IPv6 Protocol, Message Types, and Destination Option . . 35 91 6.1. Mobility Header . . . . . . . . . . . . . . . . . . . . . 35 92 6.1.1. Format . . . . . . . . . . . . . . . . . . . . . . . 35 93 6.1.2. Binding Refresh Request Message . . . . . . . . . . . 37 94 6.1.3. Home Test Init Message . . . . . . . . . . . . . . . 38 95 6.1.4. Care-of Test Init Message . . . . . . . . . . . . . . 39 96 6.1.5. Home Test Message . . . . . . . . . . . . . . . . . . 40 97 6.1.6. Care-of Test Message . . . . . . . . . . . . . . . . 41 98 6.1.7. Binding Update Message . . . . . . . . . . . . . . . 43 99 6.1.8. Binding Acknowledgement Message . . . . . . . . . . . 45 100 6.1.9. Binding Error Message . . . . . . . . . . . . . . . . 48 101 6.2. Mobility Options . . . . . . . . . . . . . . . . . . . . 49 102 6.2.1. Format . . . . . . . . . . . . . . . . . . . . . . . 49 103 6.2.2. Pad1 . . . . . . . . . . . . . . . . . . . . . . . . 50 104 6.2.3. PadN . . . . . . . . . . . . . . . . . . . . . . . . 51 105 6.2.4. Binding Refresh Advice . . . . . . . . . . . . . . . 51 106 6.2.5. Alternate Care-of Address . . . . . . . . . . . . . . 51 107 6.2.6. Nonce Indices . . . . . . . . . . . . . . . . . . . . 52 108 6.2.7. Binding Authorization Data . . . . . . . . . . . . . 53 109 6.3. Home Address Option . . . . . . . . . . . . . . . . . . . 54 110 6.4. Type 2 Routing Header . . . . . . . . . . . . . . . . . . 56 111 6.4.1. Format . . . . . . . . . . . . . . . . . . . . . . . 56 112 6.5. ICMP Home Agent Address Discovery Request Message . . . . 58 113 6.6. ICMP Home Agent Address Discovery Reply Message . . . . . 59 114 6.7. ICMP Mobile Prefix Solicitation Message Format . . . . . 60 115 6.8. ICMP Mobile Prefix Advertisement Message Format . . . . . 61 116 7. Modifications to IPv6 Neighbor Discovery . . . . . . . . . . 65 117 7.1. Modified Router Advertisement Message Format . . . . . . 65 118 7.2. Modified Prefix Information Option Format . . . . . . . . 65 119 7.3. New Advertisement Interval Option Format . . . . . . . . 67 120 7.4. New Home Agent Information Option Format . . . . . . . . 68 121 7.5. Changes to Sending Router Advertisements . . . . . . . . 70 122 8. Requirements for Types of IPv6 Nodes . . . . . . . . . . . . 72 123 8.1. All IPv6 Nodes . . . . . . . . . . . . . . . . . . . . . 72 124 8.2. IPv6 Nodes with Support for Route Optimization . . . . . 72 125 8.3. All IPv6 Routers . . . . . . . . . . . . . . . . . . . . 74 126 8.4. IPv6 Home Agents . . . . . . . . . . . . . . . . . . . . 74 127 8.5. IPv6 Mobile Nodes . . . . . . . . . . . . . . . . . . . . 76 128 9. Correspondent Node Operation . . . . . . . . . . . . . . . . 78 129 9.1. Conceptual Data Structures . . . . . . . . . . . . . . . 78 130 9.2. Processing Mobility Headers . . . . . . . . . . . . . . . 79 131 9.3. Packet Processing . . . . . . . . . . . . . . . . . . . . 79 132 9.3.1. Receiving Packets with Home Address Option . . . . . 79 133 9.3.2. Sending Packets to a Mobile Node . . . . . . . . . . 80 134 9.3.3. Sending Binding Error Messages . . . . . . . . . . . 82 135 9.3.4. Receiving ICMP Error Messages . . . . . . . . . . . . 82 136 9.4. Return Routability Procedure . . . . . . . . . . . . . . 83 137 9.4.1. Receiving Home Test Init Messages . . . . . . . . . . 83 138 9.4.2. Receiving Care-of Test Init Messages . . . . . . . . 83 139 9.4.3. Sending Home Test Messages . . . . . . . . . . . . . 84 140 9.4.4. Sending Care-of Test Messages . . . . . . . . . . . . 84 141 9.5. Processing Bindings . . . . . . . . . . . . . . . . . . . 84 142 9.5.1. Receiving Binding Updates . . . . . . . . . . . . . . 84 143 9.5.2. Requests to Cache a Binding . . . . . . . . . . . . . 87 144 9.5.3. Requests to Delete a Binding . . . . . . . . . . . . 87 145 9.5.4. Sending Binding Acknowledgements . . . . . . . . . . 88 146 9.5.5. Sending Binding Refresh Requests . . . . . . . . . . 89 147 9.6. Cache Replacement Policy . . . . . . . . . . . . . . . . 89 148 10. Home Agent Operation . . . . . . . . . . . . . . . . . . . . 91 149 10.1. Conceptual Data Structures . . . . . . . . . . . . . . . 91 150 10.2. Processing Mobility Headers . . . . . . . . . . . . . . . 92 151 10.3. Processing Bindings . . . . . . . . . . . . . . . . . . . 92 152 10.3.1. Primary Care-of Address Registration . . . . . . . . 92 153 10.3.2. Primary Care-of Address De-Registration . . . . . . . 96 154 10.4. Packet Processing . . . . . . . . . . . . . . . . . . . . 97 155 10.4.1. Intercepting Packets for a Mobile Node . . . . . . . 97 156 10.4.2. Processing Intercepted Packets . . . . . . . . . . . 99 157 10.4.3. Multicast Membership Control . . . . . . . . . . . . 100 158 10.4.4. Stateful Address Autoconfiguration . . . . . . . . . 101 159 10.4.5. Handling Reverse Tunneled Packets . . . . . . . . . . 102 160 10.4.6. Protecting Return Routability Packets . . . . . . . . 102 161 10.5. Dynamic Home Agent Address Discovery . . . . . . . . . . 103 162 10.5.1. Receiving Router Advertisement Messages . . . . . . . 103 163 10.6. Sending Prefix Information to the Mobile Node . . . . . . 106 164 10.6.1. List of Home Network Prefixes . . . . . . . . . . . . 106 165 10.6.2. Scheduling Prefix Deliveries . . . . . . . . . . . . 106 166 10.6.3. Sending Advertisements . . . . . . . . . . . . . . . 108 167 10.6.4. Lifetimes for Changed Prefixes . . . . . . . . . . . 109 168 11. Mobile Node Operation . . . . . . . . . . . . . . . . . . . . 110 169 11.1. Conceptual Data Structures . . . . . . . . . . . . . . . 110 170 11.2. Processing Mobility Headers . . . . . . . . . . . . . . . 111 171 11.3. Packet Processing . . . . . . . . . . . . . . . . . . . . 112 172 11.3.1. Sending Packets While Away from Home . . . . . . . . 112 173 11.3.2. Interaction with Outbound IPsec Processing . . . . . 115 174 11.3.3. Receiving Packets While Away from Home . . . . . . . 117 175 11.3.4. Routing Multicast Packets . . . . . . . . . . . . . . 118 176 11.3.5. Receiving ICMP Error Messages . . . . . . . . . . . . 120 177 11.3.6. Receiving Binding Error Messages . . . . . . . . . . 120 178 11.4. Home Agent and Prefix Management . . . . . . . . . . . . 121 179 11.4.1. Dynamic Home Agent Address Discovery . . . . . . . . 121 180 11.4.2. Sending Mobile Prefix Solicitations . . . . . . . . . 122 181 11.4.3. Receiving Mobile Prefix Advertisements . . . . . . . 123 182 11.5. Movement . . . . . . . . . . . . . . . . . . . . . . . . 124 183 11.5.1. Movement Detection . . . . . . . . . . . . . . . . . 124 184 11.5.2. Home Link Detection . . . . . . . . . . . . . . . . . 127 185 11.5.3. Forming New Care-of Addresses . . . . . . . . . . . . 127 186 11.5.4. Using Multiple Care-of Addresses . . . . . . . . . . 128 187 11.5.5. Returning Home . . . . . . . . . . . . . . . . . . . 129 188 11.6. Return Routability Procedure . . . . . . . . . . . . . . 131 189 11.6.1. Sending Test Init Messages . . . . . . . . . . . . . 131 190 11.6.2. Receiving Test Messages . . . . . . . . . . . . . . . 132 191 11.6.3. Protecting Return Routability Packets . . . . . . . . 133 192 11.7. Processing Bindings . . . . . . . . . . . . . . . . . . . 133 193 11.7.1. Sending Binding Updates to the Home Agent . . . . . . 133 194 11.7.2. Correspondent Registration . . . . . . . . . . . . . 136 195 11.7.3. Receiving Binding Acknowledgements . . . . . . . . . 139 196 11.7.4. Receiving Binding Refresh Requests . . . . . . . . . 141 197 11.8. Retransmissions and Rate Limiting . . . . . . . . . . . . 142 198 12. Protocol Constants . . . . . . . . . . . . . . . . . . . . . 144 199 13. Protocol Configuration Variables . . . . . . . . . . . . . . 145 200 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 146 201 15. Security Considerations . . . . . . . . . . . . . . . . . . . 149 202 15.1. Threats . . . . . . . . . . . . . . . . . . . . . . . . . 149 203 15.2. Features . . . . . . . . . . . . . . . . . . . . . . . . 151 204 15.3. Binding Updates to Home Agent . . . . . . . . . . . . . . 153 205 15.4. Binding Updates to Correspondent Nodes . . . . . . . . . 155 206 15.4.1. Overview . . . . . . . . . . . . . . . . . . . . . . 155 207 15.4.2. Achieved Security Properties . . . . . . . . . . . . 156 208 15.4.3. Comparison to Regular IPv6 Communications . . . . . . 157 209 15.4.4. Replay Attacks . . . . . . . . . . . . . . . . . . . 159 210 15.4.5. Denial-of-Service Attacks . . . . . . . . . . . . . . 159 211 15.4.6. Key Lengths . . . . . . . . . . . . . . . . . . . . . 160 212 15.5. Dynamic Home Agent Address Discovery . . . . . . . . . . 161 213 15.6. Mobile Prefix Discovery . . . . . . . . . . . . . . . . . 162 214 15.7. Tunneling via the Home Agent . . . . . . . . . . . . . . 162 215 15.8. Home Address Option . . . . . . . . . . . . . . . . . . . 163 216 15.9. Type 2 Routing Header . . . . . . . . . . . . . . . . . . 163 217 16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 165 218 17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 166 219 18. References . . . . . . . . . . . . . . . . . . . . . . . . . 167 220 18.1. Normative References . . . . . . . . . . . . . . . . . . 167 221 18.2. Informative References . . . . . . . . . . . . . . . . . 168 222 Appendix A. Future Extensions . . . . . . . . . . . . . . . . . 171 223 A.1. Piggybacking . . . . . . . . . . . . . . . . . . . . . . 171 224 A.2. Triangular Routing . . . . . . . . . . . . . . . . . . . 171 225 A.3. New Authorization Methods . . . . . . . . . . . . . . . . 171 226 A.4. Neighbor Discovery Extensions . . . . . . . . . . . . . . 171 227 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 173 229 1. Introduction 231 This document specifies a protocol which allows nodes to remain 232 reachable while moving around in the IPv6 Internet. Without specific 233 support for mobility in IPv6 [5], packets destined to a mobile node 234 would not be able to reach it while the mobile node is away from its 235 home link. In order to continue communication in spite of its 236 movement, a mobile node could change its IP address each time it 237 moves to a new link, but the mobile node would then not be able to 238 maintain transport and higher-layer connections when it changes 239 location. Mobility support in IPv6 is particularly important, as 240 mobile computers are likely to account for a majority or at least a 241 substantial fraction of the population of the Internet during the 242 lifetime of IPv6. 244 The protocol defined in this document, known as Mobile IPv6, allows a 245 mobile node to move from one link to another without changing the 246 mobile node's "home address". Packets may be routed to the mobile 247 node using this address regardless of the mobile node's current point 248 of attachment to the Internet. The mobile node may also continue to 249 communicate with other nodes (stationary or mobile) after moving to a 250 new link. The movement of a mobile node away from its home link is 251 thus transparent to transport and higher-layer protocols and 252 applications. 254 The Mobile IPv6 protocol is just as suitable for mobility across 255 homogeneous media as for mobility across heterogeneous media. For 256 example, Mobile IPv6 facilitates node movement from one Ethernet 257 segment to another as well as it facilitates node movement from an 258 Ethernet segment to a wireless LAN cell, with the mobile node's IP 259 address remaining unchanged in spite of such movement. 261 One can think of the Mobile IPv6 protocol as solving the network- 262 layer mobility management problem. Some mobility management 263 applications -- for example, handover among wireless transceivers, 264 each of which covers only a very small geographic area -- have been 265 solved using link-layer techniques. For example, in many current 266 wireless LAN products, link-layer mobility mechanisms allow a 267 "handover" of a mobile node from one cell to another, re-establishing 268 link-layer connectivity to the node in each new location. 270 Mobile IPv6 does not attempt to solve all general problems related to 271 the use of mobile computers or wireless networks. In particular, 272 this protocol does not attempt to solve: 274 o Handling links with unidirectional connectivity or partial 275 reachability, such as the hidden terminal problem where a host is 276 hidden from only some of the routers on the link. 278 o Access control on a link being visited by a mobile node. 280 o Local or hierarchical forms of mobility management (similar to 281 many current link-layer mobility management solutions). 283 o Assistance for adaptive applications. 285 o Mobile routers. 287 o Service Discovery. 289 o Distinguishing between packets lost due to bit errors vs. network 290 congestion. 292 2. Comparison with Mobile IP for IPv4 294 The design of Mobile IP support in IPv6 (Mobile IPv6) benefits both 295 from the experiences gained from the development of Mobile IP support 296 in IPv4 (Mobile IPv4) [30] [24] [25], and from the opportunities 297 provided by IPv6. Mobile IPv6 thus shares many features with Mobile 298 IPv4, but is integrated into IPv6 and offers many other improvements. 299 This section summarizes the major differences between Mobile IPv4 and 300 Mobile IPv6: 302 o There is no need to deploy special routers as "foreign agents", as 303 in Mobile IPv4. Mobile IPv6 operates in any location without any 304 special support required from the local router. 306 o Support for route optimization is a fundamental part of the 307 protocol, rather than a nonstandard set of extensions. 309 o Mobile IPv6 route optimization can operate securely even without 310 pre-arranged security associations. It is expected that route 311 optimization can be deployed on a global scale between all mobile 312 nodes and correspondent nodes. 314 o Support is also integrated into Mobile IPv6 for allowing route 315 optimization to coexist efficiently with routers that perform 316 "ingress filtering" [27]. 318 o The IPv6 Neighbor Unreachability Detection assures symmetric 319 reachability between the mobile node and its default router in the 320 current location. 322 o Most packets sent to a mobile node while away from home in Mobile 323 IPv6 are sent using an IPv6 routing header rather than IP 324 encapsulation, reducing the amount of resulting overhead compared 325 to Mobile IPv4. 327 o Mobile IPv6 is decoupled from any particular link layer, as it 328 uses IPv6 Neighbor Discovery [17] instead of ARP. This also 329 improves the robustness of the protocol. 331 o The use of IPv6 encapsulation (and the routing header) removes the 332 need in Mobile IPv6 to manage "tunnel soft state". 334 o The dynamic home agent address discovery mechanism in Mobile IPv6 335 returns a single reply to the mobile node. The directed broadcast 336 approach used in IPv4 returns separate replies from each home 337 agent. 339 3. Terminology 341 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 342 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 343 document are to be interpreted as described in RFC 2119 [1]. 345 3.1. General Terms 347 IP 349 Internet Protocol Version 6 (IPv6). 351 node 353 A device that implements IP. 355 router 357 A node that forwards IP packets not explicitly addressed to 358 itself. 360 unicast routable address 362 An identifier for a single interface such that a packet sent to it 363 from another IPv6 subnet is delivered to the interface identified 364 by that address. Accordingly, a unicast routable address must 365 either be global IPv6 address or a unique local IPv6 address. 367 host 369 Any node that is not a router. 371 link 373 A communication facility or medium over which nodes can 374 communicate at the link layer, such as an Ethernet (simple or 375 bridged). A link is the layer immediately below IP. 377 interface 379 A node's attachment to a link. 381 subnet prefix 383 A bit string that consists of some number of initial bits of an IP 384 address. 386 interface identifier 388 A number used to identify a node's interface on a link. The 389 interface identifier is the remaining low-order bits in the node's 390 IP address after the subnet prefix. 392 link-layer address 394 A link-layer identifier for an interface, such as IEEE 802 395 addresses on Ethernet links. 397 packet 399 An IP header plus payload. 401 security association 403 An IPsec security association is a cooperative relationship formed 404 by the sharing of cryptographic keying material and associated 405 context. Security associations are simplex. That is, two 406 security associations are needed to protect bidirectional traffic 407 between two nodes, one for each direction. 409 security policy database 411 A database that specifies what security services are to be offered 412 to IP packets and in what fashion. 414 destination option 416 Destination options are carried by the IPv6 Destination Options 417 extension header. Destination options include optional 418 information that need be examined only by the IPv6 node given as 419 the destination address in the IPv6 header, not by routers in 420 between. Mobile IPv6 defines one new destination option, the Home 421 Address destination option (see Section 6.3). 423 routing header 425 A routing header may be present as an IPv6 header extension, and 426 indicates that the payload has to be delivered to a destination 427 IPv6 address in some way that is different from what would be 428 carried out by standard Internet routing. In this document, use 429 of the term "routing header" typically refers to use of a type 2 430 routing header, as specified in Section 6.4. 432 "|" (concatenation) 434 Some formulas in this specification use the symbol "|" to indicate 435 bytewise concatenation, as in A | B. This concatenation requires 436 that all of the octets of the datum A appear first in the result, 437 followed by all of the octets of the datum B. 439 First (size, input) 441 Some formulas in this specification use a functional form "First 442 (size, input)" to indicate truncation of the "input" data so that 443 only the first "size" bits remain to be used. 445 3.2. Mobile IPv6 Terms 447 These terms are intended to be compatible with the definitions given 448 in RFC 3753[37]. However, if there is any conflict, the definitions 449 given here should be considered to supersede those in RFC 3753. 451 home address 453 A unicast routable address assigned to a mobile node, used as the 454 permanent address of the mobile node. This address is within the 455 mobile node's home link. Standard IP routing mechanisms will 456 deliver packets destined for a mobile node's home address to its 457 home link. Mobile nodes can have multiple home addresses, for 458 instance when there are multiple home prefixes on the home link. 460 home subnet prefix 462 The IP subnet prefix corresponding to a mobile node's home 463 address. 465 home link 467 The link on which a mobile node's home subnet prefix is defined. 469 mobile node 471 A node that can change its point of attachment from one link to 472 another, while still being reachable via its home address. 474 movement 476 A change in a mobile node's point of attachment to the Internet 477 such that it is no longer connected to the same link as it was 478 previously. If a mobile node is not currently attached to its 479 home link, the mobile node is said to be "away from home". 481 L2 handover 483 A process by which the mobile node changes from one link-layer 484 connection to another. For example, a change of wireless access 485 point is a L2 handover. 487 L3 handover 489 Subsequent to a L2 handover, a mobile node detects a change in an 490 on-link subnet prefix that would require a change in the primary 491 care-of address. For example, a change of access router 492 subsequent to a change of wireless access point typically results 493 in an L3 handover. 495 correspondent node 497 A peer node with which a mobile node is communicating. The 498 correspondent node may be either mobile or stationary. 500 foreign subnet prefix 502 Any IP subnet prefix other than the mobile node's home subnet 503 prefix. 505 foreign link 507 Any link other than the mobile node's home link. 509 care-of address 511 A unicast routable address associated with a mobile node while 512 visiting a foreign link; the subnet prefix of this IP address is a 513 foreign subnet prefix. Among the multiple care-of addresses that 514 a mobile node may have at any given time (e.g., with different 515 subnet prefixes), the one registered with the mobile node's home 516 agent for a given home address is called its "primary" care-of 517 address. 519 home agent 521 A router on a mobile node's home link with which the mobile node 522 has registered its current care-of address. While the mobile node 523 is away from home, the home agent intercepts packets on the home 524 link destined to the mobile node's home address, encapsulates 525 them, and tunnels them to the mobile node's registered care-of 526 address. 528 binding 530 The association of the home address of a mobile node with a 531 care-of address for that mobile node, along with the remaining 532 lifetime of that association. 534 registration 536 The process during which a mobile node sends a Binding Update to 537 its home agent or a correspondent node, causing a binding for the 538 mobile node to be registered. 540 mobility message 542 A message containing a Mobility Header (see Section 6.1). 544 binding authorization 546 Correspondent registration needs to be authorized to allow the 547 recipient to believe that the sender has the right to specify a 548 new binding. 550 return routability procedure 552 The return routability procedure authorizes registrations by the 553 use of a cryptographic token exchange. 555 correspondent registration 557 A return routability procedure followed by a registration, run 558 between the mobile node and a correspondent node. 560 home registration 562 A registration between the mobile node and its home agent, 563 authorized by the use of IPsec. 565 nonce 567 Nonces are random numbers used internally by the correspondent 568 node in the creation of keygen tokens related to the return 569 routability procedure. The nonces are not specific to a mobile 570 node, and are kept secret within the correspondent node. 572 nonce index 574 A nonce index is used to indicate which nonces have been used when 575 creating keygen token values, without revealing the nonces 576 themselves. 578 cookie 580 A cookie is a random number used by a mobile node to prevent 581 spoofing by a bogus correspondent node in the return routability 582 procedure. 584 care-of init cookie 586 A cookie sent to the correspondent node in the Care-of Test Init 587 message, to be returned in the Care-of Test message. 589 home init cookie 591 A cookie sent to the correspondent node in the Home Test Init 592 message, to be returned in the Home Test message. 594 keygen token 596 A keygen token is a number supplied by a correspondent node in the 597 return routability procedure to enable the mobile node to compute 598 the necessary binding management key for authorizing a Binding 599 Update. 601 care-of keygen token 603 A keygen token sent by the correspondent node in the Care-of Test 604 message. 606 home keygen token 608 A keygen token sent by the correspondent node in the Home Test 609 message. 611 binding management key (Kbm) 613 A binding management key (Kbm) is a key used for authorizing a 614 binding cache management message (e.g., Binding Update or Binding 615 Acknowledgement). Return routability provides a way to create a 616 binding management key. 618 4. Overview of Mobile IPv6 620 4.1. Basic Operation 622 A mobile node is always expected to be addressable at its home 623 address, whether it is currently attached to its home link or is away 624 from home. The "home address" is an IP address assigned to the 625 mobile node within its home subnet prefix on its home link. While a 626 mobile node is at home, packets addressed to its home address are 627 routed to the mobile node's home link, using conventional Internet 628 routing mechanisms. 630 While a mobile node is attached to some foreign link away from home, 631 it is also addressable at one or more care-of addresses. A care-of 632 address is an IP address associated with a mobile node that has the 633 subnet prefix of a particular foreign link. The mobile node can 634 acquire its care-of address through conventional IPv6 mechanisms, 635 such as stateless or stateful auto-configuration. As long as the 636 mobile node stays in this location, packets addressed to this care-of 637 address will be routed to the mobile node. The mobile node may also 638 accept packets from several care-of addresses, such as when it is 639 moving but still reachable at the previous link. 641 The association between a mobile node's home address and care-of 642 address is known as a "binding" for the mobile node. While away from 643 home, a mobile node registers its primary care-of address with a 644 router on its home link, requesting this router to function as the 645 "home agent" for the mobile node. The mobile node performs this 646 binding registration by sending a "Binding Update" message to the 647 home agent. The home agent replies to the mobile node by returning a 648 "Binding Acknowledgement" message. The operation of the mobile node 649 is specified in Section 11, and the operation of the home agent is 650 specified in Section 10. 652 Any node communicating with a mobile node is referred to in this 653 document as a "correspondent node" of the mobile node, and may itself 654 be either a stationary node or a mobile node. Mobile nodes can 655 provide information about their current location to correspondent 656 nodes. This happens through the correspondent registration. As a 657 part of this procedure, a return routability test is performed in 658 order to authorize the establishment of the binding. The operation 659 of the correspondent node is specified in Section 9. 661 There are two possible modes for communications between the mobile 662 node and a correspondent node. The first mode, bidirectional 663 tunneling, does not require Mobile IPv6 support from the 664 correspondent node and is available even if the mobile node has not 665 registered its current binding with the correspondent node. Packets 666 from the correspondent node are routed to the home agent and then 667 tunneled to the mobile node. Packets to the correspondent node are 668 tunneled from the mobile node to the home agent ("reverse tunneled") 669 and then routed normally from the home network to the correspondent 670 node. In this mode, the home agent uses proxy Neighbor Discovery to 671 intercept any IPv6 packets addressed to the mobile node's home 672 address (or home addresses) on the home link. Each intercepted 673 packet is tunneled to the mobile node's primary care-of address. 674 This tunneling is performed using IPv6 encapsulation [6]. 676 The second mode, "route optimization", requires the mobile node to 677 register its current binding at the correspondent node. Packets from 678 the correspondent node can be routed directly to the care-of address 679 of the mobile node. When sending a packet to any IPv6 destination, 680 the correspondent node checks its cached bindings for an entry for 681 the packet's destination address. If a cached binding for this 682 destination address is found, the node uses a new type of IPv6 683 routing header [5] (see Section 6.4) to route the packet to the 684 mobile node by way of the care-of address indicated in this binding. 686 Routing packets directly to the mobile node's care-of address allows 687 the shortest communications path to be used. It also eliminates 688 congestion at the mobile node's home agent and home link. In 689 addition, the impact of temporary failures of the home agent or 690 networks on the path to or from the home agent is reduced. 692 When routing packets directly to the mobile node, the correspondent 693 node sets the Destination Address in the IPv6 header to the care-of 694 address of the mobile node. A new type of IPv6 routing header (see 695 Section 6.4) is also added to the packet to carry the desired home 696 address. Similarly, the mobile node sets the Source Address in the 697 packet's IPv6 header to its current care-of addresses. The mobile 698 node adds a new IPv6 "Home Address" destination option (see 699 Section 6.3) to carry its home address. The inclusion of home 700 addresses in these packets makes the use of the care-of address 701 transparent above the network layer (e.g., at the transport layer). 703 Mobile IPv6 also provides support for multiple home agents, and a 704 limited support for the reconfiguration of the home network. In 705 these cases, the mobile node may not know the IP address of its own 706 home agent, and even the home subnet prefixes may change over time. 707 A mechanism, known as "dynamic home agent address discovery" allows a 708 mobile node to dynamically discover the IP address of a home agent on 709 its home link, even when the mobile node is away from home. Mobile 710 nodes can also learn new information about home subnet prefixes 711 through the "mobile prefix discovery" mechanism. These mechanisms 712 are described starting from Section 6.5. 714 This document is written under the assumption that the mobile node is 715 configured with the home prefix for the mobile node to be able to 716 discover a home agent and configure a home address. This might be 717 limiting in deployments where the home agent and the home address for 718 the mobile node needs to be assigned dynamically. Additional 719 mechanisms have been specified for the mobile node to dynamically 720 configure a home agent, a home address and the home prefix. These 721 mechanisms are described in "Mobile IPv6 Bootstrapping in Split 722 Scenario" [22] and "MIP6 bootstrapping for the Integrated Scenario" 723 [34]. 725 4.2. New IPv6 Protocol 727 Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header 728 (see Section 6.1). This Header is used to carry the following 729 messages: 731 Home Test Init 733 Home Test 735 Care-of Test Init 737 Care-of Test 739 These four messages are used to perform the return routability 740 procedure from the mobile node to a correspondent node. This 741 ensures authorization of subsequent Binding Updates, as described 742 in Section 5.2.5. 744 Binding Update 746 A Binding Update is used by a mobile node to notify a 747 correspondent node or the mobile node's home agent of its current 748 binding. The Binding Update sent to the mobile node's home agent 749 to register its primary care-of address is marked as a "home 750 registration". 752 Binding Acknowledgement 754 A Binding Acknowledgement is used to acknowledge receipt of a 755 Binding Update, if an acknowledgement was requested in the Binding 756 Update (e.g., the binding update was sent to a home agent), or an 757 error occurred. 759 Binding Refresh Request 761 A Binding Refresh Request is used by a correspondent node to 762 request that a mobile node re-establish its binding with the 763 correspondent node. This message is typically used when the 764 cached binding is in active use but the binding's lifetime is 765 close to expiration. The correspondent node may use, for 766 instance, recent traffic and open transport layer connections as 767 an indication of active use. 769 Binding Error 771 The Binding Error is used by the correspondent node to signal an 772 error related to mobility, such as an inappropriate attempt to use 773 the Home Address destination option without an existing binding. 774 The Binding Error message is also used by the Home Agent to signal 775 an error to the mobile node, if it receives an unrecognized 776 Mobility Header Message Type from the mobile node. 778 4.3. New IPv6 Destination Option 780 Mobile IPv6 defines a new IPv6 destination option, the Home Address 781 destination option. This option is described in detail in 782 Section 6.3. 784 4.4. New IPv6 ICMP Messages 786 Mobile IPv6 also introduces four new ICMP message types, two for use 787 in the dynamic home agent address discovery mechanism, and two for 788 renumbering and mobile configuration mechanisms. As described in 789 Section 10.5 and Section 11.4.1, the following two new ICMP message 790 types are used for home agent address discovery: 792 o Home Agent Address Discovery Request, described in Section 6.5. 794 o Home Agent Address Discovery Reply, described in Section 6.6. 796 The next two message types are used for network renumbering and 797 address configuration on the mobile node, as described in 798 Section 10.6: 800 o Mobile Prefix Solicitation, described in Section 6.7. 802 o Mobile Prefix Advertisement, described in Section 6.8. 804 4.5. Conceptual Data Structure Terminology 806 This document describes the Mobile IPv6 protocol in terms of the 807 following conceptual data structures: 809 Binding Cache 811 A cache of bindings for other nodes. This cache is maintained by 812 home agents and correspondent nodes. The cache contains both 813 "correspondent registration" entries (see Section 9.1) and "home 814 registration" entries (see Section 10.1). 816 Binding Update List 818 This list is maintained by each mobile node. The list has an item 819 for every binding that the mobile node has or is trying to 820 establish with a specific other node. Both correspondent and home 821 registrations are included in this list. Entries from the list 822 are deleted as the lifetime of the binding expires. See 823 Section 11.1. 825 Home Agents List 827 Home agents need to know which other home agents are on the same 828 link. This information is stored in the Home Agents List, as 829 described in more detail in Section 10.1. The list is used for 830 informing mobile nodes during dynamic home agent address 831 discovery. 833 4.6. Unique-Local Addressability 835 This specification requires that home and care-of addresses MUST be 836 unicast routable addresses. Unique-local IPv6 unicast addresses 837 (ULAs) RFC4193 [19] may be usable on networks that use such non- 838 globally routable addresses but this specification does not define 839 when such usage is safe and when it is not. Mobile nodes may not be 840 able to distinguish between their home site and the site at which 841 they are currently located. This can make it hard to prevent 842 accidental attachment to other sites, because the mobile node might 843 use the ULA at another site, which could not be used to successfully 844 send packets to the mobile node's HA. This would result in 845 unreachability between the MN and the HA, when unique-local IPv6 846 routable addresses are used as care-of addresses. Similarly, CNs 847 outside the MN's own site will not be reachable when ULAs are used as 848 home addresses. Therefore, unique-local IPv6 unicast addresses 849 SHOULD NOT be used as home or care-of addresses when other address 850 choices are available. If such addresses are used, however, 851 according to RFC4193 [19], they are treated as any global unicast 852 IPv6 address so, for the remainder of this specification, use of 853 unique-local IPv6 unicast addresses is not differentiated from other 854 globally unique IPv6 addresses. 856 5. Overview of Mobile IPv6 Security 858 This specification provides a number of security features. These 859 include the protection of Binding Updates both to home agents and 860 correspondent nodes, the protection of mobile prefix discovery, and 861 the protection of the mechanisms that Mobile IPv6 uses for 862 transporting data packets. 864 Binding Updates are protected by the use of IPsec extension headers, 865 or by the use of the Binding Authorization Data option. This option 866 employs a binding management key, Kbm, which can be established 867 through the return routability procedure. Mobile prefix discovery is 868 protected through the use of IPsec extension headers. Mechanisms 869 related to transporting payload packets - such as the Home Address 870 destination option and type 2 routing header - have been specified in 871 a manner which restricts their use in attacks. 873 5.1. Binding Updates to Home Agents 875 The mobile node and the home agent MUST use an IPsec security 876 association to protect the integrity and authenticity of the Binding 877 Updates and Acknowledgements. Both the mobile nodes and the home 878 agents MUST support and SHOULD use the Encapsulating Security Payload 879 (ESP) [4] header in transport mode and MUST use a non-NULL payload 880 authentication algorithm to provide data origin authentication, 881 connectionless integrity and optional anti-replay protection. Note 882 that Authentication Header (AH) [3] is also possible but for brevity 883 not discussed in this specification. 885 In order to protect messages exchanged between the mobile node and 886 the home agent with IPsec, appropriate security policy database 887 entries must be created. A mobile node must be prevented from using 888 its security association to send a Binding Update on behalf of 889 another mobile node using the same home agent. This MUST be achieved 890 by having the home agent check that the given home address has been 891 used with the right security association. Such a check is provided 892 in the IPsec processing, by having the security policy database 893 entries unequivocally identify a single security association for 894 protecting Binding Updates between any given home address and home 895 agent. In order to make this possible, it is necessary that the home 896 address of the mobile node is visible in the Binding Updates and 897 Acknowledgements. The home address is used in these packets as a 898 source or destination, or in the Home Address Destination option or 899 the type 2 routing header. 901 As with all IPsec security associations in this specification, manual 902 configuration of security associations MUST be supported. The shared 903 secrets used MUST be random and unique for different mobile nodes, 904 and MUST be distributed off-line to the mobile nodes. Automatic key 905 management with IKEv2 [41] MAY be supported as described in [42]. 907 Section 11.3.2 discusses how IKEv2 connections to the home agent need 908 a careful treatment of the addresses used for transporting IKEv2. 909 This is necessary to ensure that a Binding Update is not needed 910 before the IKEv2 exchange which is needed for securing the Binding 911 Update. 913 More detailed descriptions and examples using IPsec to protect 914 communications between the mobile node and the home agent have been 915 published [42]. 917 5.2. Binding Updates to Correspondent Nodes 919 The protection of Binding Updates sent to correspondent nodes does 920 not require the configuration of security associations or the 921 existence of an authentication infrastructure between the mobile 922 nodes and correspondent nodes. Instead, a method called the return 923 routability procedure is used to assure that the right mobile node is 924 sending the message. This method does not protect against attackers 925 who are on the path between the home network and the correspondent 926 node. However, attackers in such a location are capable of 927 performing the same attacks even without Mobile IPv6. The main 928 advantage of the return routability procedure is that it limits the 929 potential attackers to those having an access to one specific path in 930 the Internet, and avoids forged Binding Updates from anywhere else in 931 the Internet. For a more in depth explanation of the security 932 properties of the return routability procedure, see Section 15. 933 Also, consult [40] 935 The integrity and authenticity of the Binding Updates messages to 936 correspondent nodes is protected by using a keyed-hash algorithm. 937 The binding management key, Kbm, is used to key the hash algorithm 938 for this purpose. Kbm is established using data exchanged during the 939 return routability procedure. The data exchange is accomplished by 940 use of node keys, nonces, cookies, tokens, and certain cryptographic 941 functions. Section 5.2.5 outlines the basic return routability 942 procedure. Section 5.2.6 shows how the results of this procedure are 943 used to authorize a Binding Update to a correspondent node. 945 5.2.1. Node Keys 947 Each correspondent node has a secret key, Kcn, called the "node key", 948 which it uses to produce the keygen tokens sent to the mobile nodes. 949 The node key MUST be a random number, 20 octets in length. The node 950 key allows the correspondent node to verify that the keygen tokens 951 used by the mobile node in authorizing a Binding Update are indeed 952 its own. This key MUST NOT be shared with any other entity. 954 A correspondent node MAY generate a fresh node key at any time; this 955 avoids the need for secure persistent key storage. Procedures for 956 optionally updating the node key are discussed later in 957 Section 5.2.7. 959 5.2.2. Nonces 961 Each correspondent node also generates nonces at regular intervals. 962 The nonces should be generated by using a random number generator 963 that is known to have good randomness properties [14]. A 964 correspondent node may use the same Kcn and nonce with all the 965 mobiles it is in communication with. 967 Each nonce is identified by a nonce index. When a new nonce is 968 generated, it must be associated with a new nonce index; this may be 969 done, for example, by incrementing the value of the previous nonce 970 index, if the nonce index is used as an array pointer into a linear 971 array of nonces. However, there is no requirement that nonces be 972 stored that way, or that the values of subsequent nonce indices have 973 any particular relationship to each other. The index value is 974 communicated in the protocol, so that if a nonce is replaced by new 975 nonce during the run of a protocol, the correspondent node can 976 distinguish messages that should be checked against the old nonce 977 from messages that should be checked against the new nonce. Strictly 978 speaking, indices are not necessary in the authentication, but allow 979 the correspondent node to efficiently find the nonce value that it 980 used in creating a keygen token. 982 Correspondent nodes keep both the current nonce and a small set of 983 valid previous nonces whose lifetime has not yet expired. Expired 984 values MUST be discarded, and messages using stale or unknown indices 985 will be rejected. 987 The specific nonce index values cannot be used by mobile nodes to 988 determine the validity of the nonce. Expected validity times for the 989 nonces values and the procedures for updating them are discussed 990 later in Section 5.2.7. 992 A nonce is an octet string of any length. The recommended length is 993 64 bits. 995 5.2.3. Cookies and Tokens 997 The return routability address test procedure uses cookies and keygen 998 tokens as opaque values within the test init and test messages, 999 respectively. 1001 o The "home init cookie" and "care-of init cookie" are 64 bit values 1002 sent to the correspondent node from the mobile node, and later 1003 returned to the mobile node. The home init cookie is sent in the 1004 Home Test Init message, and returned in the Home Test message. 1005 The care-of init cookie is sent in the Care-of Test Init message, 1006 and returned in the Care-of Test message. 1008 o The "home keygen token" and "care-of keygen token" are 64-bit 1009 values sent by the correspondent node to the mobile node via the 1010 home agent (via the Home Test message) and the care-of address (by 1011 the Care-of Test message), respectively. 1013 The mobile node should set the home init or care-of init cookie to a 1014 newly generated random number in every Home or Care-of Test Init 1015 message it sends. The cookies are used to verify that the Home Test 1016 or Care-of Test message matches the Home Test Init or Care-of Test 1017 Init message, respectively. These cookies also serve to ensure that 1018 parties who have not seen the request cannot spoof responses. 1020 Home and care-of keygen tokens are produced by the correspondent node 1021 based on its currently active secret key (Kcn) and nonces, as well as 1022 the home or care-of address (respectively). A keygen token is valid 1023 as long as both the secret key (Kcn) and the nonce used to create it 1024 are valid. 1026 5.2.4. Cryptographic Functions 1028 By default in this specification, the function used to compute hash 1029 values is SHA1 [11]. Message Authentication Codes (MACs) are then 1030 computed using HMAC_SHA1 [26] [11]. HMAC_SHA1(K,m) denotes such a 1031 MAC computed on message m with key K. 1033 5.2.5. Return Routability Procedure 1035 The Return Routability Procedure enables the correspondent node to 1036 obtain some reasonable assurance that the mobile node is in fact 1037 addressable at its claimed care-of address as well as at its home 1038 address. Only with this assurance is the correspondent node able to 1039 accept Binding Updates from the mobile node which would then instruct 1040 the correspondent node to direct that mobile node's data traffic to 1041 its claimed care-of address. 1043 This is done by testing whether packets addressed to the two claimed 1044 addresses are routed to the mobile node. The mobile node can pass 1045 the test only if it is able to supply proof that it received certain 1046 data (the "keygen tokens") which the correspondent node sends to 1047 those addresses. These data are combined by the mobile node into a 1048 binding management key, denoted Kbm. 1050 The figure below shows the message flow for the return routability 1051 procedure. 1053 Mobile node Home agent Correspondent node 1054 | | 1055 | Home Test Init (HoTI) | | 1056 |------------------------->|------------------------->| 1057 | | | 1058 | Care-of Test Init (CoTI) | 1059 |---------------------------------------------------->| 1060 | | 1061 | | Home Test (HoT) | 1062 |<-------------------------|<-------------------------| 1063 | | | 1064 | Care-of Test (CoT) | 1065 |<----------------------------------------------------| 1066 | | 1068 The Home and Care-of Test Init messages are sent at the same time. 1069 The procedure requires very little processing at the correspondent 1070 node, and the Home and Care-of Test messages can be returned quickly, 1071 perhaps nearly simultaneously. These four messages form the return 1072 routability procedure. 1074 Home Test Init 1076 A mobile node sends a Home Test Init message to the correspondent 1077 node (via the home agent) to acquire the home keygen token. The 1078 contents of the message can be summarized as follows: 1080 * Source Address = home address 1082 * Destination Address = correspondent 1084 * Parameters: 1086 + home init cookie 1088 The Home Test Init message conveys the mobile node's home address 1089 to the correspondent node. The mobile node also sends along a 1090 home init cookie that the correspondent node must return later. 1091 The Home Test Init message is reverse tunneled through the home 1092 agent. (The headers and addresses related to reverse tunneling 1093 have been omitted from the above discussion of the message 1094 contents.) The mobile node remembers these cookie values to 1095 obtain some assurance that its protocol messages are being 1096 processed by the desired correspondent node. 1098 Care-of Test Init 1100 The mobile node sends a Care-of Test Init message to the 1101 correspondent node (directly, not via the home agent) to acquire 1102 the care-of keygen token. The contents of this message can be 1103 summarized as follows: 1105 * Source Address = care-of address 1107 * Destination Address = correspondent 1109 * Parameters: 1111 + care-of init cookie 1113 The Care-of Test Init message conveys the mobile node's care-of 1114 address to the correspondent node. The mobile node also sends 1115 along a care-of init cookie that the correspondent node must 1116 return later. The Care-of Test Init message is sent directly to 1117 the correspondent node. 1119 Home Test 1121 The Home Test message is sent in response to a Home Test Init 1122 message. It is sent via the home agent. The contents of the 1123 message are: 1125 * Source Address = correspondent 1127 * Destination Address = home address 1129 * Parameters: 1131 + home init cookie 1133 + home keygen token 1135 + home nonce index 1137 When the correspondent node receives the Home Test Init message, 1138 it generates a home keygen token as follows: 1140 home keygen token := 1141 First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0))) 1143 where | denotes concatenation. The final "0" inside the HMAC_SHA1 1144 function is a single zero octet, used to distinguish home and 1145 care-of cookies from each other. 1147 The home keygen token is formed from the first 64 bits of the MAC. 1148 The home keygen token tests that the mobile node can receive 1149 messages sent to its home address. Kcn is used in the production 1150 of home keygen token in order to allow the correspondent node to 1151 verify that it generated the home and care-of nonces, without 1152 forcing the correspondent node to remember a list of all tokens it 1153 has handed out. 1155 The Home Test message is sent to the mobile node via the home 1156 network, where it is presumed that the home agent will tunnel the 1157 message to the mobile node. This means that the mobile node needs 1158 to already have sent a Binding Update to the home agent, so that 1159 the home agent will have received and authorized the new care-of 1160 address for the mobile node before the return routability 1161 procedure. For improved security, the data passed between the 1162 home agent and the mobile node is made immune to inspection and 1163 passive attacks. Such protection is gained by encrypting the home 1164 keygen token as it is tunneled from the home agent to the mobile 1165 node as specified in Section 10.4.6. The security properties of 1166 this additional security are discussed in Section 15.4.1. 1168 The home init cookie from the mobile node is returned in the Home 1169 Test message, to ensure that the message comes from a node on the 1170 route between the home agent and the correspondent node. 1172 The home nonce index is delivered to the mobile node to later 1173 allow the correspondent node to efficiently find the nonce value 1174 that it used in creating the home keygen token. 1176 Care-of Test 1178 This message is sent in response to a Care-of Test Init message. 1179 This message is not sent via the home agent, it is sent directly 1180 to the mobile node. The contents of the message are: 1182 * Source Address = correspondent 1184 * Destination Address = care-of address 1185 * Parameters: 1187 + care-of init cookie 1189 + care-of keygen token 1191 + care-of nonce index 1193 When the correspondent node receives the Care-of Test Init 1194 message, it generates a care-of keygen token as follows: 1196 care-of keygen token := 1197 First (64, HMAC_SHA1 (Kcn, (care-of address | nonce | 1))) 1199 Here, the final "1" inside the HMAC_SHA1 function is a single 1200 octet containing the hex value 0x01, and is used to distinguish 1201 home and care-of cookies from each other. The keygen token is 1202 formed from the first 64 bits of the MAC, and sent directly to the 1203 mobile node at its care-of address. The care-of init cookie from 1204 the Care-of Test Init message is returned to ensure that the 1205 message comes from a node on the route to the correspondent node. 1207 The care-of nonce index is provided to identify the nonce used for 1208 the care-of keygen token. The home and care-of nonce indices MAY 1209 be the same, or different, in the Home and Care-of Test messages. 1211 When the mobile node has received both the Home and Care-of Test 1212 messages, the return routability procedure is complete. As a result 1213 of the procedure, the mobile node has the data it needs to send a 1214 Binding Update to the correspondent node. The mobile node hashes the 1215 tokens together to form a 20 octet binding key Kbm: 1217 Kbm = SHA1 (home keygen token | care-of keygen token) 1219 A Binding Update may also be used to delete a previously established 1220 binding (Section 6.1.7). In this case, the care-of keygen token is 1221 not used. Instead, the binding management key is generated as 1222 follows: 1224 Kbm = SHA1(home keygen token) 1226 Note that the correspondent node does not create any state specific 1227 to the mobile node, until it receives the Binding Update from that 1228 mobile node. The correspondent node does not maintain the value for 1229 the binding management key Kbm; it creates Kbm when given the nonce 1230 indices and the mobile node's addresses. 1232 5.2.6. Authorizing Binding Management Messages 1234 After the mobile node has created the binding management key (Kbm), 1235 it can supply a verifiable Binding Update to the correspondent node. 1236 This section provides an overview of this registration. The below 1237 figure shows the message flow. 1239 Mobile node Correspondent node 1240 | | 1241 | Binding Update (BU) | 1242 |---------------------------------------------->| 1243 | (MAC, seq#, nonce indices, care-of address) | 1244 | | 1245 | | 1246 | Binding Acknowledgement (BA) (if sent) | 1247 |<----------------------------------------------| 1248 | (MAC, seq#, status) | 1250 Binding Update 1252 To authorize a Binding Update, the mobile node creates a binding 1253 management key Kbm from the keygen tokens as described in the 1254 previous section. The contents of the Binding Update include the 1255 following: 1257 * Source Address = care-of address 1259 * Destination Address = correspondent 1261 * Parameters: 1263 + home address (within the Home Address destination option if 1264 different from the Source Address) 1266 + sequence number (within the Binding Update message header) 1268 + home nonce index (within the Nonce Indices option) 1270 + care-of nonce index (within the Nonce Indices option) 1272 + First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent 1273 | BU))) 1275 The Binding Update contains a Nonce Indices option, indicating to 1276 the correspondent node which home and care-of nonces to use to 1277 recompute Kbm, the binding management key. The MAC is computed as 1278 described in Section 6.2.7, using the correspondent node's address 1279 as the destination address and the Binding Update message itself 1280 ("BU" above) as the MH Data. 1282 Once the correspondent node has verified the MAC, it can create a 1283 Binding Cache entry for the mobile. 1285 Binding Acknowledgement 1287 The Binding Update is in some cases acknowledged by the 1288 correspondent node. The contents of the message are as follows: 1290 * Source Address = correspondent 1292 * Destination Address = care-of address 1294 * Parameters: 1296 + sequence number (within the Binding Update message header) 1298 + First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent 1299 | BA))) 1301 The Binding Acknowledgement contains the same sequence number as 1302 the Binding Update. The MAC is computed as described in 1303 Section 6.2.7, using the correspondent node's address as the 1304 destination address and the message itself ("BA" above) as the MH 1305 Data. 1307 Bindings established with correspondent nodes using keys created by 1308 way of the return routability procedure MUST NOT exceed 1309 MAX_RR_BINDING_LIFETIME seconds (see Section 12). 1311 The value in the Source Address field in the IPv6 header carrying the 1312 Binding Update is normally also the care-of address which is used in 1313 the binding. However, a different care-of address MAY be specified 1314 by including an Alternate Care-of Address mobility option in the 1315 Binding Update (see Section 6.2.5). When such a message is sent to 1316 the correspondent node and the return routability procedure is used 1317 as the authorization method, the Care-of Test Init and Care-of Test 1318 messages MUST have been performed for the address in the Alternate 1319 Care-of Address option (not the Source Address). The nonce indices 1320 and MAC value MUST be based on information gained in this test. 1322 Binding Updates may also be sent to delete a previously established 1323 binding. In this case, generation of the binding management key 1324 depends exclusively on the home keygen token and the care-of nonce 1325 index is ignored. 1327 5.2.7. Updating Node Keys and Nonces 1329 Correspondent nodes generate nonces at regular intervals. It is 1330 recommended to keep each nonce (identified by a nonce index) 1331 acceptable for at least MAX_TOKEN_LIFETIME seconds (see Section 12) 1332 after it has been first used in constructing a return routability 1333 message response. However, the correspondent node MUST NOT accept 1334 nonces beyond MAX_NONCE_LIFETIME seconds (see Section 12) after the 1335 first use. As the difference between these two constants is 30 1336 seconds, a convenient way to enforce the above lifetimes is to 1337 generate a new nonce every 30 seconds. The node can then continue to 1338 accept tokens that have been based on the last 8 (MAX_NONCE_LIFETIME 1339 / 30) nonces. This results in tokens being acceptable 1340 MAX_TOKEN_LIFETIME to MAX_NONCE_LIFETIME seconds after they have been 1341 sent to the mobile node, depending on whether the token was sent at 1342 the beginning or end of the first 30 second period. Note that the 1343 correspondent node may also attempt to generate new nonces on demand, 1344 or only if the old nonces have been used. This is possible, as long 1345 as the correspondent node keeps track of how long a time ago the 1346 nonces were used for the first time, and does not generate new nonces 1347 on every return routability request. 1349 Due to resource limitations, rapid deletion of bindings, or reboots 1350 the correspondent node may not in all cases recognize the nonces that 1351 the tokens were based on. If a nonce index is unrecognized, the 1352 correspondent node replies with an error code in the Binding 1353 Acknowledgement (either 136, 137, or 138 as discussed in 1354 Section 6.1.8). The mobile node can then retry the return 1355 routability procedure. 1357 An update of Kcn SHOULD be done at the same time as an update of a 1358 nonce, so that nonce indices can identify both the nonce and the key. 1359 Old Kcn values have to be therefore remembered as long as old nonce 1360 values. 1362 Given that the tokens are normally expected to be usable for 1363 MAX_TOKEN_LIFETIME seconds, the mobile node MAY use them beyond a 1364 single run of the return routability procedure until 1365 MAX_TOKEN_LIFETIME expires. After this the mobile node SHOULD NOT 1366 use the tokens. A fast moving mobile node MAY reuse a recent home 1367 keygen token from a correspondent node when moving to a new location, 1368 and just acquire a new care-of keygen token to show routability in 1369 the new location. 1371 While this does not save the number of round-trips due to the 1372 simultaneous processing of home and care-of return routability tests, 1373 there are fewer messages being exchanged, and a potentially long 1374 round-trip through the home agent is avoided. Consequently, this 1375 optimization is often useful. A mobile node that has multiple home 1376 addresses, MAY also use the same care-of keygen token for Binding 1377 Updates concerning all of these addresses. 1379 5.2.8. Preventing Replay Attacks 1381 The return routability procedure also protects the participants 1382 against replayed Binding Updates through the use of the sequence 1383 number and a MAC. Care must be taken when removing bindings at the 1384 correspondent node, however. Correspondent nodes must retain 1385 bindings and the associated sequence number information at least as 1386 long as the nonces used in the authorization of the binding are still 1387 valid. Alternatively, if memory is very constrained, the 1388 correspondent node MAY invalidate the nonces that were used for the 1389 binding being deleted (or some larger group of nonces that they 1390 belong to). This may, however, impact the ability to accept Binding 1391 Updates from mobile nodes that have recently received keygen tokens. 1392 This alternative is therefore recommended only as a last measure. 1394 5.2.9. Handling Interruptions to Return Routability 1396 In some scenarios, such as simultaneous mobility, where both 1397 correspondent host and mobile host move at the same time, or in the 1398 case where the correspondent node reboots and loses data, route 1399 optimization may not complete, or relevant data in the binding cache 1400 might be lost. 1402 o Return Routability signalling MUST be sent to the correspondent 1403 node's home address if it has one (i.e. not to the correspondent 1404 nodes care-of address if the correspondent node is also mobile). 1406 o If Return Routability signalling timed out after MAX_RO_FAILURE 1407 attempts, the mobile node MUST revert to sending packets to the 1408 correspondent node's home address through its home agent. 1410 The mobile node may run the bidirectional tunnelling in parallel with 1411 the return routability procedure until it is successful. Exponential 1412 backoff SHOULD be used for retransmission of return routability 1413 messages. 1415 The return routability procedure may be triggered by movement of the 1416 mobile node or by sustained loss of end-to-end communication with a 1417 correspondent node (e.g. based on indications from upper-layers) that 1418 has been using a route optimised connection to the mobile node. If 1419 such indications are received, the mobile node MAY revert to bi- 1420 directional tunnelling while re-starting the return routability 1421 procedure. 1423 5.3. Dynamic Home Agent Address Discovery 1425 Dynamic home agent address discovery has been designed for use in 1426 deployments where security is not needed. For this reason, no 1427 security solution is provided in this document for dynamic home agent 1428 address discovery. 1430 5.4. Mobile Prefix Discovery 1432 The mobile node and the home agent SHOULD use an IPsec security 1433 association to protect the integrity and authenticity of the Mobile 1434 Prefix Solicitations and Advertisements. Both the mobile nodes and 1435 the home agents MUST support and SHOULD use the Encapsulating 1436 Security Payload (ESP) header in transport mode with a non-NULL 1437 payload authentication algorithm to provide data origin 1438 authentication, connectionless integrity and optional anti-replay 1439 protection. 1441 5.5. Payload Packets 1443 Payload packets exchanged with mobile nodes can be protected in the 1444 usual manner, in the same way as stationary hosts can protect them. 1445 However, Mobile IPv6 introduces the Home Address destination option, 1446 a routing header, and tunneling headers in the payload packets. In 1447 the following we define the security measures taken to protect these, 1448 and to prevent their use in attacks against other parties. 1450 This specification limits the use of the Home Address destination 1451 option to the situation where the correspondent node already has a 1452 Binding Cache entry for the given home address. This avoids the use 1453 of the Home Address option in attacks described in Section 15.1. 1455 Mobile IPv6 uses a type of routing header specific to Mobile IPv6. 1456 This type provides the necessary functionality but does not open 1457 vulnerabilities discussed in Section 15.1 and RFC 5095 [43]. 1459 Tunnels between the mobile node and the home agent are protected by 1460 ensuring proper use of source addresses, and optional cryptographic 1461 protection. The mobile node verifies that the outer IP address 1462 corresponds to its home agent. The home agent verifies that the 1463 outer IP address corresponds to the current location of the mobile 1464 node (Binding Updates sent to the home agents are secure). The home 1465 agent identifies the mobile node through the source address of the 1466 inner packet. (Typically, this is the home address of the mobile 1467 node, but it can also be a link-local address, as discussed in 1468 Section 10.4.2. To recognize the latter type of addresses, the home 1469 agent requires that the Link-Local Address Compatibility (L) was set 1470 in the Binding Update.) These measures protect the tunnels against 1471 vulnerabilities discussed in Section 15.1. 1473 For traffic tunneled via the home agent, additional IPsec ESP 1474 encapsulation MAY be supported and used. If multicast group 1475 membership control protocols or stateful address autoconfiguration 1476 protocols are supported, payload data protection MUST be supported. 1478 6. New IPv6 Protocol, Message Types, and Destination Option 1480 6.1. Mobility Header 1482 The Mobility Header is an extension header used by mobile nodes, 1483 correspondent nodes, and home agents in all messaging related to the 1484 creation and management of bindings. The subsections within this 1485 section describe the message types that may be sent using the 1486 Mobility Header. 1488 Mobility Header messages MUST NOT be sent with a type 2 routing 1489 header, except as described in Section 9.5.4 for Binding 1490 Acknowledgement. Mobility Header messages also MUST NOT be used with 1491 a Home Address destination option, except as described in 1492 Section 11.7.1 and Section 11.7.2 for Binding Update. Binding Update 1493 List or Binding Cache information (when present) for the destination 1494 MUST NOT be used in sending Mobility Header messages. That is, 1495 Mobility Header messages bypass both the Binding Cache check 1496 described in Section 9.3.2 and the Binding Update List check 1497 described in Section 11.3.1 which are normally performed for all 1498 packets. This applies even to messages sent to or from a 1499 correspondent node which is itself a mobile node. 1501 6.1.1. Format 1503 The Mobility Header is identified by a Next Header value of 135 in 1504 the immediately preceding header, and has the following format: 1506 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1507 | Payload Proto | Header Len | MH Type | Reserved | 1508 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1509 | Checksum | | 1510 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1511 | | 1512 . . 1513 . Message Data . 1514 . . 1515 | | 1516 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1518 Payload Proto 1520 8-bit selector. Identifies the type of header immediately 1521 following the Mobility Header. Uses the same values as the IPv6 1522 Next Header field [5]. 1524 This field is intended to be used by a future extension (see 1525 Appendix A.1). 1527 Implementations conforming to this specification SHOULD set the 1528 payload protocol type to IPPROTO_NONE (59 decimal). 1530 Header Len 1532 8-bit unsigned integer, representing the length of the Mobility 1533 Header in units of 8 octets, excluding the first 8 octets. 1535 The length of the Mobility Header MUST be a multiple of 8 octets. 1537 MH Type 1539 8-bit selector. Identifies the particular mobility message in 1540 question. Current values are specified in Section 6.1.2 and 1541 onward. An unrecognized MH Type field causes an error indication 1542 to be sent. 1544 Reserved 1546 8-bit field reserved for future use. The value MUST be 1547 initialized to zero by the sender, and MUST be ignored by the 1548 receiver. 1550 Checksum 1552 16-bit unsigned integer. This field contains the checksum of the 1553 Mobility Header. The checksum is calculated from the octet string 1554 consisting of a "pseudo-header" followed by the entire Mobility 1555 Header starting with the Payload Proto field. The checksum is the 1556 16-bit one's complement of the one's complement sum of this 1557 string. 1559 The pseudo-header contains IPv6 header fields, as specified in 1560 Section 8.1 of RFC 2460 [5]. The Next Header value used in the 1561 pseudo-header is 135. The addresses used in the pseudo-header are 1562 the addresses that appear in the Source and Destination Address 1563 fields in the IPv6 packet carrying the Mobility Header. 1565 Note that the procedures of calculating upper layer checksums 1566 while away from home described in Section 11.3.1 apply even for 1567 the Mobility Header. If a mobility message has a Home Address 1568 destination option, then the checksum calculation uses the home 1569 address in this option as the value of the IPv6 Source Address 1570 field. The type 2 routing header is treated as explained in [5]. 1572 The Mobility Header is considered as the upper layer protocol for 1573 the purposes of calculating the pseudo-header. The Upper-Layer 1574 Packet Length field in the pseudo-header MUST be set to the total 1575 length of the Mobility Header. 1577 For computing the checksum, the checksum field is set to zero. 1579 Message Data 1581 A variable length field containing the data specific to the 1582 indicated Mobility Header type. 1584 Mobile IPv6 also defines a number of "mobility options" for use 1585 within these messages; if included, any options MUST appear after the 1586 fixed portion of the message data specified in this document. The 1587 presence of such options will be indicated by the Header Len field 1588 within the message. When the Header Len value is greater than the 1589 length required for the message specified here, the remaining octets 1590 are interpreted as mobility options. These options include padding 1591 options that can be used to ensure that other options are aligned 1592 properly, and that the total length of the message is divisible by 8. 1593 The encoding and format of defined options are described in 1594 Section 6.2. 1596 Alignment requirements for the Mobility Header are the same as for 1597 any IPv6 protocol Header. That is, they MUST be aligned on an 1598 8-octet boundary. 1600 6.1.2. Binding Refresh Request Message 1602 The Binding Refresh Request (BRR) message requests a mobile node to 1603 update its mobility binding. This message is sent by correspondent 1604 nodes according to the rules in Section 9.5.5. When a mobile node 1605 receives a packet containing a Binding Refresh Request message it 1606 processes the message according to the rules in Section 11.7.4. 1608 The Binding Refresh Request message uses the MH Type value 0. When 1609 this value is indicated in the MH Type field, the format of the 1610 Message Data field in the Mobility Header is as follows: 1612 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1613 | Reserved | 1614 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1615 | | 1616 . . 1617 . Mobility options . 1618 . . 1619 | | 1620 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1622 Reserved 1624 16-bit field reserved for future use. The value MUST be 1625 initialized to zero by the sender, and MUST be ignored by the 1626 receiver. 1628 Mobility Options 1630 Variable-length field of such length that the complete Mobility 1631 Header is an integer multiple of 8 octets long. This field 1632 contains zero or more TLV-encoded mobility options. The encoding 1633 and format of defined options are described in Section 6.2. The 1634 receiver MUST ignore and skip any options which it does not 1635 understand. 1637 There MAY be additional information, associated with this Binding 1638 Refresh Request message that need not be present in all Binding 1639 Refresh Request messages sent. Mobility options allow future 1640 extensions to the format of the Binding Refresh Request message to 1641 be defined. This specification does not define any options valid 1642 for the Binding Refresh Request message. 1644 If no actual options are present in this message, no padding is 1645 necessary and the Header Len field will be set to 0. 1647 6.1.3. Home Test Init Message 1649 A mobile node uses the Home Test Init (HoTI) message to initiate the 1650 return routability procedure and request a home keygen token from a 1651 correspondent node (see Section 11.6.1). The Home Test Init message 1652 uses the MH Type value 1. When this value is indicated in the MH 1653 Type field, the format of the Message Data field in the Mobility 1654 Header is as follows: 1656 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1657 | Reserved | 1658 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1659 | | 1660 + Home Init Cookie + 1661 | | 1662 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1663 | | 1664 . . 1665 . Mobility Options . 1666 . . 1667 | | 1668 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1670 Reserved 1672 16-bit field reserved for future use. This value MUST be 1673 initialized to zero by the sender, and MUST be ignored by the 1674 receiver. 1676 Home Init Cookie 1678 64-bit field which contains a random value, the home init cookie. 1680 Mobility Options 1682 Variable-length field of such length that the complete Mobility 1683 Header is an integer multiple of 8 octets long. This field 1684 contains zero or more TLV-encoded mobility options. The receiver 1685 MUST ignore and skip any options which it does not understand. 1686 This specification does not define any options valid for the Home 1687 Test Init message. 1689 If no actual options are present in this message, no padding is 1690 necessary and the Header Len field will be set to 1. 1692 This message is tunneled through the home agent when the mobile node 1693 is away from home. Such tunneling SHOULD employ IPsec ESP in tunnel 1694 mode between the home agent and the mobile node. This protection is 1695 indicated by the IPsec security policy database. The protection of 1696 Home Test Init messages is unrelated to the requirement to protect 1697 regular payload traffic, which MAY use such tunnels as well. 1699 6.1.4. Care-of Test Init Message 1701 A mobile node uses the Care-of Test Init (CoTI) message to initiate 1702 the return routability procedure and request a care-of keygen token 1703 from a correspondent node (see Section 11.6.1). The Care-of Test 1704 Init message uses the MH Type value 2. When this value is indicated 1705 in the MH Type field, the format of the Message Data field in the 1706 Mobility Header is as follows: 1708 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1709 | Reserved | 1710 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1711 | | 1712 + Care-of Init Cookie + 1713 | | 1714 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1715 | | 1716 . . 1717 . Mobility Options . 1718 . . 1719 | | 1720 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1722 Reserved 1724 16-bit field reserved for future use. The value MUST be 1725 initialized to zero by the sender, and MUST be ignored by the 1726 receiver. 1728 Care-of Init Cookie 1730 64-bit field which contains a random value, the care-of init 1731 cookie. 1733 Mobility Options 1735 Variable-length field of such length that the complete Mobility 1736 Header is an integer multiple of 8 octets long. This field 1737 contains zero or more TLV-encoded mobility options. The receiver 1738 MUST ignore and skip any options which it does not understand. 1739 This specification does not define any options valid for the 1740 Care-of Test Init message. 1742 If no actual options are present in this message, no padding is 1743 necessary and the Header Len field will be set to 1. 1745 6.1.5. Home Test Message 1747 The Home Test (HoT) message is a response to the Home Test Init 1748 message, and is sent from the correspondent node to the mobile node 1749 (see Section 5.2.5). The Home Test message uses the MH Type value 3. 1750 When this value is indicated in the MH Type field, the format of the 1751 Message Data field in the Mobility Header is as follows: 1753 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1754 | Home Nonce Index | 1755 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1756 | | 1757 + Home Init Cookie + 1758 | | 1759 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1760 | | 1761 + Home Keygen Token + 1762 | | 1763 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1764 | | 1765 . . 1766 . Mobility options . 1767 . . 1768 | | 1769 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1771 Home Nonce Index 1773 This field will be echoed back by the mobile node to the 1774 correspondent node in a subsequent Binding Update. 1776 Home Init Cookie 1778 64-bit field which contains the home init cookie. 1780 Home Keygen Token 1782 This field contains the 64 bit home keygen token used in the 1783 return routability procedure. 1785 Mobility Options 1787 Variable-length field of such length that the complete Mobility 1788 Header is an integer multiple of 8 octets long. This field 1789 contains zero or more TLV-encoded mobility options. The receiver 1790 MUST ignore and skip any options which it does not understand. 1791 This specification does not define any options valid for the Home 1792 Test message. 1794 If no actual options are present in this message, no padding is 1795 necessary and the Header Len field will be set to 2. 1797 6.1.6. Care-of Test Message 1799 The Care-of Test (CoT) message is a response to the Care-of Test Init 1800 message, and is sent from the correspondent node to the mobile node 1801 (see Section 11.6.2). The Care-of Test message uses the MH Type 1802 value 4. When this value is indicated in the MH Type field, the 1803 format of the Message Data field in the Mobility Header is as 1804 follows: 1806 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1807 | Care-of Nonce Index | 1808 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1809 | | 1810 + Care-of Init Cookie + 1811 | | 1812 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1813 | | 1814 + Care-of Keygen Token + 1815 | | 1816 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1817 | | 1818 . . 1819 . Mobility Options . 1820 . . 1821 | | 1822 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1824 Care-of Nonce Index 1826 This value will be echoed back by the mobile node to the 1827 correspondent node in a subsequent Binding Update. 1829 Care-of Init Cookie 1831 64-bit field which contains the care-of init cookie. 1833 Care-of Keygen Token 1835 This field contains the 64 bit care-of keygen token used in the 1836 return routability procedure. 1838 Mobility Options 1840 Variable-length field of such length that the complete Mobility 1841 Header is an integer multiple of 8 octets long. This field 1842 contains zero or more TLV-encoded mobility options. The receiver 1843 MUST ignore and skip any options which it does not understand. 1844 This specification does not define any options valid for the 1845 Care-of Test message. 1847 If no actual options are present in this message, no padding is 1848 necessary and the Header Len field will be set to 2. 1850 6.1.7. Binding Update Message 1852 The Binding Update (BU) message is used by a mobile node to notify 1853 other nodes of a new care-of address for itself. Binding Updates are 1854 sent as described in Section 11.7.1 and Section 11.7.2. 1856 The Binding Update uses the MH Type value 5. When this value is 1857 indicated in the MH Type field, the format of the Message Data field 1858 in the Mobility Header is as follows: 1860 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1861 | Sequence # | 1862 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1863 |A|H|L|K| Reserved | Lifetime | 1864 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1865 | | 1866 . . 1867 . Mobility options . 1868 . . 1869 | | 1870 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1872 Acknowledge (A) 1874 The Acknowledge (A) bit is set by the sending mobile node to 1875 request a Binding Acknowledgement (Section 6.1.8) be returned upon 1876 receipt of the Binding Update. 1878 Home Registration (H) 1880 The Home Registration (H) bit is set by the sending mobile node to 1881 request that the receiving node should act as this node's home 1882 agent. The destination of the packet carrying this message MUST 1883 be that of a router sharing the same subnet prefix as the home 1884 address of the mobile node in the binding. 1886 Link-Local Address Compatibility (L) 1888 The Link-Local Address Compatibility (L) bit is set when the home 1889 address reported by the mobile node has the same interface 1890 identifier as the mobile node's link-local address. 1892 Key Management Mobility Capability (K) 1894 If this bit is cleared, the protocol used for establishing the 1895 IPsec security associations between the mobile node and the home 1896 agent does not survive movements. It may then have to be rerun. 1897 (Note that the IPsec security associations themselves are expected 1898 to survive movements.) If manual IPsec configuration is used, the 1899 bit MUST be cleared. 1901 This bit is valid only in Binding Updates sent to the home agent, 1902 and MUST be cleared in other Binding Updates. Correspondent nodes 1903 MUST ignore this bit. 1905 Reserved 1907 These fields are unused. They MUST be initialized to zero by the 1908 sender and MUST be ignored by the receiver. 1910 Sequence # 1912 A 16-bit unsigned integer used by the receiving node to sequence 1913 Binding Updates and by the sending node to match a returned 1914 Binding Acknowledgement with this Binding Update. 1916 Lifetime 1918 16-bit unsigned integer. The number of time units remaining 1919 before the binding MUST be considered expired. A value of zero 1920 indicates that the Binding Cache entry for the mobile node MUST be 1921 deleted. One time unit is 4 seconds. 1923 Mobility Options 1925 Variable-length field of such length that the complete Mobility 1926 Header is an integer multiple of 8 octets long. This field 1927 contains zero or more TLV-encoded mobility options. The encoding 1928 and format of defined options are described in Section 6.2. The 1929 receiver MUST ignore and skip any options which it does not 1930 understand. 1932 The following options are valid in a Binding Update: 1934 * Binding Authorization Data option (this option is mandatory in 1935 Binding Updates sent to a correspondent node) 1937 * Nonce Indices option. 1939 * Alternate Care-of Address option 1941 If no options are present in this message, 4 octets of padding are 1942 necessary and the Header Len field will be set to 1. 1944 The care-of address is specified either by the Source Address field 1945 in the IPv6 header or by the Alternate Care-of Address option, if 1946 present. The care-of address MUST be a unicast routable address. 1947 IPv6 Source Address MUST be a topologically correct source address. 1948 Binding Updates for a care-of address which is not a unicast routable 1949 address MUST be silently discarded. 1951 The deletion of a binding MUST be indicated by setting the Lifetime 1952 field to 0. In deletion, the generation of the binding management 1953 key depends exclusively on the home keygen token, as explained in 1954 Section 5.2.5. 1956 Correspondent nodes SHOULD NOT delete the Binding Cache entry before 1957 the lifetime expires, if any application hosted by the correspondent 1958 node is still likely to require communication with the mobile node. 1959 A Binding Cache entry that is de-allocated prematurely might cause 1960 subsequent packets to be dropped from the mobile node, if they 1961 contain the Home Address destination option. This situation is 1962 recoverable, since a Binding Error message is sent to the mobile node 1963 (see Section 6.1.9); however, it causes unnecessary delay in the 1964 communications. 1966 6.1.8. Binding Acknowledgement Message 1968 The Binding Acknowledgement is used to acknowledge receipt of a 1969 Binding Update (Section 6.1.7). This packet is sent as described in 1970 Section 9.5.4 and Section 10.3.1. 1972 The Binding Acknowledgement has the MH Type value 6. When this value 1973 is indicated in the MH Type field, the format of the Message Data 1974 field in the Mobility Header is as follows: 1976 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1977 | Status |K| Reserved | 1978 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1979 | Sequence # | Lifetime | 1980 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1981 | | 1982 . . 1983 . Mobility options . 1984 . . 1985 | | 1986 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1988 Status 1990 8-bit unsigned integer indicating the disposition of the Binding 1991 Update. Values of the Status field less than 128 indicate that 1992 the Binding Update was accepted by the receiving node. Values 1993 greater than or equal to 128 indicate that the Binding Update was 1994 rejected by the receiving node. The following Status values are 1995 currently defined: 1997 0 Binding Update accepted 1999 1 Accepted but prefix discovery necessary 2001 128 Reason unspecified 2003 129 Administratively prohibited 2005 130 Insufficient resources 2007 131 Home registration not supported 2009 132 Not home subnet 2011 133 Not home agent for this mobile node 2013 134 Duplicate Address Detection failed 2015 135 Sequence number out of window 2017 136 Expired home nonce index 2019 137 Expired care-of nonce index 2021 138 Expired nonces 2023 139 Registration type change disallowed 2025 TBD Invalid Care-of Address 2027 Up-to-date values of the Status field are to be specified in the 2028 IANA registry of assigned numbers [10]. 2030 Key Management Mobility Capability (K) 2032 If this bit is cleared, the protocol used by the home agent for 2033 establishing the IPsec security associations between the mobile 2034 node and the home agent does not survive movements. It may then 2035 have to be rerun. (Note that the IPsec security associations 2036 themselves are expected to survive movements.) 2038 Correspondent nodes MUST set the K bit to 0. 2040 Reserved 2042 This field is unused. It MUST be initialized to zero by the 2043 sender and MUST be ignored by the receiver. 2045 Sequence # 2047 The Sequence Number in the Binding Acknowledgement is copied from 2048 the Sequence Number field in the Binding Update. It is used by 2049 the mobile node in matching this Binding Acknowledgement with an 2050 outstanding Binding Update. 2052 Lifetime 2054 The granted lifetime, in time units of 4 seconds, for which this 2055 node SHOULD retain the entry for this mobile node in its Binding 2056 Cache. 2058 The value of this field is undefined if the Status field indicates 2059 that the Binding Update was rejected. 2061 Mobility Options 2063 Variable-length field of such length that the complete Mobility 2064 Header is an integer multiple of 8 octets long. This field 2065 contains zero or more TLV-encoded mobility options. The encoding 2066 and format of defined options are described in Section 6.2. The 2067 receiver MUST ignore and skip any options which it does not 2068 understand. 2070 There MAY be additional information, associated with this Binding 2071 Acknowledgement that need not be present in all Binding 2072 Acknowledgements sent. Mobility options allow future extensions 2073 to the format of the Binding Acknowledgement to be defined. The 2074 following options are valid for the Binding Acknowledgement: 2076 * Binding Authorization Data option (this option is mandatory in 2077 Binding Acknowledgements sent by a correspondent node, except 2078 where otherwise noted in Section 9.5.4) 2080 * Binding Refresh Advice option 2082 If no options are present in this message, 4 octets of padding are 2083 necessary and the Header Len field will be set to 1. 2085 6.1.9. Binding Error Message 2087 The Binding Error (BE) message is used by the correspondent node to 2088 signal an error related to mobility, such as an inappropriate attempt 2089 to use the Home Address destination option without an existing 2090 binding; see Section 9.3.3 for details. 2092 The Binding Error message uses the MH Type value 7. When this value 2093 is indicated in the MH Type field, the format of the Message Data 2094 field in the Mobility Header is as follows: 2096 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2097 | Status | Reserved | 2098 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2099 | | 2100 + + 2101 | | 2102 + Home Address + 2103 | | 2104 + + 2105 | | 2106 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2107 . . 2108 . Mobility Options . 2109 . . 2110 | | 2111 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2113 Status 2115 8-bit unsigned integer indicating the reason for this message. 2116 The following values are currently defined: 2118 1 Unknown binding for Home Address destination option 2120 2 Unrecognized MH Type value 2122 Reserved 2124 A 8-bit field reserved for future use. The value MUST be 2125 initialized to zero by the sender, and MUST be ignored by the 2126 receiver. 2128 Home Address 2130 The home address that was contained in the Home Address 2131 destination option. The mobile node uses this information to 2132 determine which binding does not exist, in cases where the mobile 2133 node has several home addresses. 2135 Mobility Options 2137 Variable-length field of such length that the complete Mobility 2138 Header is an integer multiple of 8 octets long. This field 2139 contains zero or more TLV-encoded mobility options. The receiver 2140 MUST ignore and skip any options which it does not understand. 2142 There MAY be additional information, associated with this Binding 2143 Error message that need not be present in all Binding Error 2144 messages sent. Mobility options allow future extensions to the 2145 format of the Binding Error message to be defined. The encoding 2146 and format of defined options are described in Section 6.2. This 2147 specification does not define any options valid for the Binding 2148 Error message. 2150 If no actual options are present in this message, no padding is 2151 necessary and the Header Len field will be set to 2. 2153 6.2. Mobility Options 2155 Mobility messages can include zero or more mobility options. This 2156 allows optional fields that may not be needed in every use of a 2157 particular Mobility Header, as well as future extensions to the 2158 format of the messages. Such options are included in the Message 2159 Data field of the message itself, after the fixed portion of the 2160 message data specified in the message subsections of Section 6.1. 2162 The presence of such options will be indicated by the Header Len of 2163 the Mobility Header. If included, the Binding Authorization Data 2164 option (Section 6.2.7) MUST be the last option and MUST NOT have 2165 trailing padding. Otherwise, options can be placed in any order. 2167 6.2.1. Format 2169 Mobility options are encoded within the remaining space of the 2170 Message Data field of a mobility message, using a type-length-value 2171 (TLV) format as follows: 2173 0 1 2 3 2174 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 2175 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2176 | Option Type | Option Length | Option Data... 2177 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2179 Option Type 2181 8-bit identifier of the type of mobility option. When processing 2182 a Mobility Header containing an option for which the Option Type 2183 value is not recognized by the receiver, the receiver MUST quietly 2184 ignore and skip over the option, correctly handling any remaining 2185 options in the message. 2187 Option Length 2189 8-bit unsigned integer, representing the length in octets of the 2190 mobility option, not including the Option Type and Option Length 2191 fields. 2193 Option Data 2195 A variable length field that contains data specific to the option. 2197 The following subsections specify the Option types which are 2198 currently defined for use in the Mobility Header. 2200 Implementations MUST silently ignore any mobility options that they 2201 do not understand. 2203 Mobility options may have alignment requirements. Following the 2204 convention in IPv6, these options are aligned in a packet so that 2205 multi-octet values within the Option Data field of each option fall 2206 on natural boundaries (i.e., fields of width n octets are placed at 2207 an integer multiple of n octets from the start of the header, for n = 2208 1, 2, 4, or 8) [5]. 2210 6.2.2. Pad1 2212 The Pad1 option does not have any alignment requirements. Its format 2213 is as follows: 2215 0 2216 0 1 2 3 4 5 6 7 2217 +-+-+-+-+-+-+-+-+ 2218 | Type = 0 | 2219 +-+-+-+-+-+-+-+-+ 2221 NOTE! the format of the Pad1 option is a special case - it has 2222 neither Option Length nor Option Data fields. 2224 The Pad1 option is used to insert one octet of padding in the 2225 Mobility Options area of a Mobility Header. If more than one octet 2226 of padding is required, the PadN option, described next, should be 2227 used rather than multiple Pad1 options. 2229 6.2.3. PadN 2231 The PadN option does not have any alignment requirements. Its format 2232 is as follows: 2234 0 1 2235 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 2236 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 2237 | Type = 1 | Option Length | Option Data 2238 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 2240 The PadN option is used to insert two or more octets of padding in 2241 the Mobility Options area of a mobility message. For N octets of 2242 padding, the Option Length field contains the value N-2, and the 2243 Option Data consists of N-2 zero-valued octets. PadN Option data 2244 MUST be ignored by the receiver. 2246 6.2.4. Binding Refresh Advice 2248 The Binding Refresh Advice option has an alignment requirement of 2n. 2249 Its format is as follows: 2251 0 1 2 3 2252 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 2253 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2254 | Type = 2 | Length = 2 | 2255 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2256 | Refresh Interval | 2257 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2259 The Binding Refresh Advice option is only valid in the Binding 2260 Acknowledgement, and only on Binding Acknowledgements sent from the 2261 mobile node's home agent in reply to a home registration. The 2262 Refresh Interval is measured in units of four seconds, and indicates 2263 remaining time until the mobile node SHOULD send a new home 2264 registration to the home agent. The Refresh Interval MUST be set to 2265 indicate a smaller time interval than the Lifetime value of the 2266 Binding Acknowledgement. 2268 6.2.5. Alternate Care-of Address 2270 The Alternate Care-of Address option has an alignment requirement of 2271 8n+6. Its format is as follows: 2273 0 1 2 3 2274 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 2275 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2276 | Type = 3 | Length = 16 | 2277 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2278 | | 2279 + + 2280 | | 2281 + Alternate Care-of Address + 2282 | | 2283 + + 2284 | | 2285 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2287 Normally, a Binding Update specifies the desired care-of address in 2288 the Source Address field of the IPv6 header. However, this is not 2289 possible in some cases, such as when the mobile node wishes to 2290 indicate a care-of address which it cannot use as a topologically 2291 correct source address (Section 6.1.7 and Section 11.7.2) or when the 2292 used security mechanism does not protect the IPv6 header 2293 (Section 11.7.1). 2295 The Alternate Care-of Address option is provided for these 2296 situations. This option is valid only in Binding Update. The 2297 Alternate Care-of Address field contains an address to use as the 2298 care-of address for the binding, rather than using the Source Address 2299 of the packet as the care-of address. 2301 6.2.6. Nonce Indices 2303 The Nonce Indices option has an alignment requirement of 2n. Its 2304 format is as follows: 2306 0 1 2 3 2307 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 2308 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2309 | Type = 4 | Length = 4 | 2310 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2311 | Home Nonce Index | Care-of Nonce Index | 2312 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2314 The Nonce Indices option is valid only in the Binding Update message 2315 sent to a correspondent node, and only when present together with a 2316 Binding Authorization Data option. When the correspondent node 2317 authorizes the Binding Update, it needs to produce home and care-of 2318 keygen tokens from its stored random nonce values. 2320 The Home Nonce Index field tells the correspondent node which nonce 2321 value to use when producing the home keygen token. 2323 The Care-of Nonce Index field is ignored in requests to delete a 2324 binding. Otherwise, it tells the correspondent node which nonce 2325 value to use when producing the care-of keygen token. 2327 6.2.7. Binding Authorization Data 2329 The Binding Authorization Data option does not have alignment 2330 requirements as such. However, since this option must be the last 2331 mobility option, an implicit alignment requirement is 8n + 2. The 2332 format of this option is as follows: 2334 0 1 2 3 2335 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 2336 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2337 | Type = 5 | Option Length | 2338 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2339 | | 2340 + + 2341 | Authenticator | 2342 + + 2343 | | 2344 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2346 The Binding Authorization Data option is valid in the Binding Update 2347 and Binding Acknowledgement. 2349 The Option Length field contains the length of the authenticator in 2350 octets. 2352 The Authenticator field contains a cryptographic value which can be 2353 used to determine that the message in question comes from the right 2354 authority. Rules for calculating this value depends on the used 2355 authorization procedure. 2357 For the return routability procedure, this option can appear in the 2358 Binding Update and Binding Acknowledgements. Rules for calculating 2359 the Authenticator value are the following: 2361 Mobility Data = care-of address | correspondent | MH Data 2362 Authenticator = First (96, HMAC_SHA1 (Kbm, Mobility Data)) 2364 Where | denotes concatenation. "Care-of address" is the care-of 2365 address which will be registered for the mobile node if the Binding 2366 Update succeeds, or the home address of the mobile node if this 2367 option is used in de-registration. Note also that this address might 2368 be different from the source address of the Binding Update message, 2369 if the Alternative Care-of Address mobility option is used, or when 2370 the lifetime of the binding is set to zero. 2372 The "correspondent" is the IPv6 address of the correspondent node. 2373 Note that, if the message is sent to a destination which is itself 2374 mobile, the "correspondent" address may not be the address found in 2375 the Destination Address field of the IPv6 header; instead the home 2376 address from the type 2 Routing header should be used. 2378 "MH Data" is the content of the Mobility Header, excluding the 2379 Authenticator field itself. The Authenticator value is calculated as 2380 if the Checksum field in the Mobility Header was zero. The Checksum 2381 in the transmitted packet is still calculated in the usual manner, 2382 with the calculated Authenticator being a part of the packet 2383 protected by the Checksum. Kbm is the binding management key, which 2384 is typically created using nonces provided by the correspondent node 2385 (see Section 9.4). Note that while the contents of a potential Home 2386 Address destination option are not covered in this formula, the rules 2387 for the calculation of the Kbm do take the home address in account. 2388 This ensures that the MAC will be different for different home 2389 addresses. 2391 The first 96 bits from the MAC result are used as the Authenticator 2392 field. 2394 6.3. Home Address Option 2396 The Home Address option is carried by the Destination Option 2397 extension header (Next Header value = 60). It is used in a packet 2398 sent by a mobile node while away from home, to inform the recipient 2399 of the mobile node's home address. 2401 The Home Address option is encoded in type-length-value (TLV) format 2402 as follows: 2404 0 1 2 3 2405 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 2406 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2407 | Option Type | Option Length | 2408 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2409 | | 2410 + + 2411 | | 2412 + Home Address + 2413 | | 2414 + + 2415 | | 2416 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2418 Option Type 2420 201 = 0xC9 2422 Option Length 2424 8-bit unsigned integer. Length of the option, in octets, 2425 excluding the Option Type and Option Length fields. This field 2426 MUST be set to 16. 2428 Home Address 2430 The home address of the mobile node sending the packet. This 2431 address MUST be a unicast routable address. 2433 The alignment requirement [5] for the Home Address option is 8n+6. 2435 The three highest-order bits of the Option Type field are encoded to 2436 indicate specific processing of the option [5]; for the Home Address 2437 option, these three bits are set to 110. This indicates the 2438 following processing requirements: 2440 o Any IPv6 node that does not recognize the Option Type must discard 2441 the packet, and if the packet's Destination Address was not a 2442 multicast address, return an ICMP Parameter Problem, Code 2, 2443 message to the packet's Source Address. The Pointer field in the 2444 ICMP message SHOULD point at the Option Type field. Otherwise, 2445 for multicast addresses, the ICMP message MUST NOT be sent. 2447 o The data within the option cannot change en route to the packet's 2448 final destination. 2450 The Home Address option MUST be placed as follows: 2452 o After the routing header, if that header is present 2454 o Before the Fragment Header, if that header is present 2456 o Before the AH Header or ESP Header, if either one of those headers 2457 are present 2459 For each IPv6 packet header, the Home Address Option MUST NOT appear 2460 more than once. However, an encapsulated packet [6] MAY contain a 2461 separate Home Address option associated with each encapsulating IP 2462 header. 2464 The inclusion of a Home Address destination option in a packet 2465 affects the receiving node's processing of only this single packet. 2467 No state is created or modified in the receiving node as a result of 2468 receiving a Home Address option in a packet. In particular, the 2469 presence of a Home Address option in a received packet MUST NOT alter 2470 the contents of the receiver's Binding Cache and MUST NOT cause any 2471 changes in the routing of subsequent packets sent by this receiving 2472 node. 2474 6.4. Type 2 Routing Header 2476 Mobile IPv6 defines a new routing header variant, the type 2 routing 2477 header, to allow the packet to be routed directly from a 2478 correspondent to the mobile node's care-of address. The mobile 2479 node's care-of address is inserted into the IPv6 Destination Address 2480 field. Once the packet arrives at the care-of address, the mobile 2481 node retrieves its home address from the routing header, and this is 2482 used as the final destination address for the packet. 2484 The new routing header uses a different type than defined for 2485 "regular" IPv6 source routing, enabling firewalls to apply different 2486 rules to source routed packets than to Mobile IPv6. This routing 2487 header type (type 2) is restricted to carry only one IPv6 address. 2488 All IPv6 nodes which process this routing header MUST verify that the 2489 address contained within is the node's own home address in order to 2490 prevent packets from being forwarded outside the node. The IP 2491 address contained in the routing header, since it is the mobile 2492 node's home address, MUST be a unicast routable address. 2493 Furthermore, if the scope of the home address is smaller than the 2494 scope of the care-of address, the mobile node MUST discard the packet 2495 (see Section 4.6). 2497 6.4.1. Format 2499 The type 2 routing header has the following format: 2501 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2502 | Next Header | Hdr Ext Len=2 | Routing Type=2|Segments Left=1| 2503 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2504 | Reserved | 2505 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2506 | | 2507 + + 2508 | | 2509 + Home Address + 2510 | | 2511 + + 2512 | | 2513 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2515 Next Header 2517 8-bit selector. Identifies the type of header immediately 2518 following the routing header. Uses the same values as the IPv6 2519 Next Header field [5]. 2521 Hdr Ext Len 2523 2 (8-bit unsigned integer); length of the routing header in 2524 8-octet units, not including the first 8 octets. 2526 Routing Type 2528 2 (8-bit unsigned integer). 2530 Segments Left 2532 1 (8-bit unsigned integer). 2534 Reserved 2536 32-bit reserved field. The value MUST be initialized to zero by 2537 the sender, and MUST be ignored by the receiver. 2539 Home Address 2541 The Home Address of the destination Mobile Node. 2543 For a type 2 routing header, the Hdr Ext Len MUST be 2. The Segments 2544 Left value describes the number of route segments remaining; i.e., 2545 number of explicitly listed intermediate nodes still to be visited 2546 before reaching the final destination. Segments Left MUST be 1. The 2547 ordering rules for extension headers in an IPv6 packet are described 2548 in Section 4.1 of RFC 2460 [5]. The type 2 routing header defined 2549 for Mobile IPv6 follows the same ordering as other routing headers. 2550 If another routing header is present along with a type 2 routing 2551 header, the type 2 routing header should follow the other routing 2552 header. A packet containing such nested encapsulation should be 2553 created as if the inner (type 2) routing header was constructed first 2554 and then treated as an original packet by header construction process 2555 for the other routing header. 2557 In addition, the general procedures defined by IPv6 for routing 2558 headers suggest that a received routing header MAY be automatically 2559 "reversed" to construct a routing header for use in any response 2560 packets sent by upper-layer protocols, if the received packet is 2561 authenticated [6]. This MUST NOT be done automatically for type 2 2562 routing headers. 2564 6.5. ICMP Home Agent Address Discovery Request Message 2566 The ICMP Home Agent Address Discovery Request message is used by a 2567 mobile node to initiate the dynamic home agent address discovery 2568 mechanism, as described in Section 11.4.1. The mobile node sends the 2569 Home Agent Address Discovery Request message to the Mobile IPv6 Home- 2570 Agents anycast address [7] for its own home subnet prefix. (Note 2571 that the currently defined anycast addresses may not work with all 2572 prefix lengths other than those defined in RFC 4291 [15] [35].) 2574 0 1 2 3 2575 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 2576 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2577 | Type | Code | Checksum | 2578 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2579 | Identifier | Reserved | 2580 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2582 Type 2584 144 2586 Code 2588 0 2590 Checksum 2592 The ICMP checksum [16]. 2594 Identifier 2596 An identifier to aid in matching Home Agent Address Discovery 2597 Reply messages to this Home Agent Address Discovery Request 2598 message. 2600 Reserved 2602 This field is unused. It MUST be initialized to zero by the 2603 sender and MUST be ignored by the receiver. 2605 The Source Address of the Home Agent Address Discovery Request 2606 message packet is typically one of the mobile node's current care-of 2607 addresses. At the time of performing this dynamic home agent address 2608 discovery procedure, it is likely that the mobile node is not 2609 registered with any home agent. Therefore, neither the nature of the 2610 address nor the identity of the mobile node can be established at 2611 this time. The home agent MUST then return the Home Agent Address 2612 Discovery Reply message directly to the Source Address chosen by the 2613 mobile node. 2615 6.6. ICMP Home Agent Address Discovery Reply Message 2617 The ICMP Home Agent Address Discovery Reply message is used by a home 2618 agent to respond to a mobile node that uses the dynamic home agent 2619 address discovery mechanism, as described in Section 10.5. 2621 0 1 2 3 2622 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 2623 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2624 | Type | Code | Checksum | 2625 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2626 | Identifier | Reserved | 2627 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2628 | | 2629 + + 2630 . . 2631 . Home Agent Addresses . 2632 . . 2633 + + 2634 | | 2635 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2637 Type 2639 145 2641 Code 2643 0 2645 Checksum 2647 The ICMP checksum [16]. 2649 Identifier 2651 The identifier from the invoking Home Agent Address Discovery 2652 Request message. 2654 Reserved 2656 This field is unused. It MUST be initialized to zero by the 2657 sender and MUST be ignored by the receiver. 2659 Home Agent Addresses 2661 A list of addresses of home agents on the home link for the mobile 2662 node. The number of addresses presented in the list is indicated 2663 by the remaining length of the IPv6 packet carrying the Home Agent 2664 Address Discovery Reply message. 2666 6.7. ICMP Mobile Prefix Solicitation Message Format 2668 The ICMP Mobile Prefix Solicitation Message is sent by a mobile node 2669 to its home agent while it is away from home. The purpose of the 2670 message is to solicit a Mobile Prefix Advertisement from the home 2671 agent, which will allow the mobile node to gather prefix information 2672 about its home network. This information can be used to configure 2673 and update home address(es) according to changes in prefix 2674 information supplied by the home agent. 2676 0 1 2 3 2677 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 2678 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2679 | Type | Code | Checksum | 2680 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2681 | Identifier | Reserved | 2682 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2684 IP Fields: 2686 Source Address 2688 The mobile node's care-of address. 2690 Destination Address 2692 The address of the mobile node's home agent. This home agent must 2693 be on the link that the mobile node wishes to learn prefix 2694 information about. 2696 Hop Limit 2698 Set to an initial hop limit value, similarly to any other unicast 2699 packet sent by the mobile node. 2701 Destination Option: 2703 A Home Address destination option MUST be included. 2705 ESP header: 2707 IPsec headers MUST be supported and SHOULD be used as described in 2708 Section 5.4. 2710 ICMP Fields: 2712 Type 2714 146 2716 Code 2718 0 2720 Checksum 2722 The ICMP checksum [16]. 2724 Identifier 2726 An identifier to aid in matching a future Mobile Prefix 2727 Advertisement to this Mobile Prefix Solicitation. 2729 Reserved 2731 This field is unused. It MUST be initialized to zero by the 2732 sender and MUST be ignored by the receiver. 2734 The Mobile Prefix Solicitation messages may have options. These 2735 options MUST use the option format defined in Neighbor Discovery (RFC 2736 4861 [17]). This document does not define any option types for the 2737 Mobile Prefix Solicitation message, but future documents may define 2738 new options. Home agents MUST silently ignore any options they do 2739 not recognize and continue processing the message. 2741 6.8. ICMP Mobile Prefix Advertisement Message Format 2743 A home agent will send a Mobile Prefix Advertisement to a mobile node 2744 to distribute prefix information about the home link while the mobile 2745 node is traveling away from the home network. This will occur in 2746 response to a Mobile Prefix Solicitation with an Advertisement, or by 2747 an unsolicited Advertisement sent according to the rules in 2748 Section 10.6. 2750 0 1 2 3 2751 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 2752 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2753 | Type | Code | Checksum | 2754 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2755 | Identifier |M|O| Reserved | 2756 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2757 | Options ... 2758 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2760 IP Fields: 2762 Source Address 2764 The home agent's address as the mobile node would expect to see it 2765 (i.e., same network prefix). 2767 Destination Address 2769 If this message is a response to a Mobile Prefix Solicitation, 2770 this field contains the Source Address field from that packet. 2771 For unsolicited messages, the mobile node's care-of address SHOULD 2772 be used. Note that unsolicited messages can only be sent if the 2773 mobile node is currently registered with the home agent. 2775 Routing header: 2777 A type 2 routing header MUST be included. 2779 ESP header: 2781 IPsec headers MUST be supported and SHOULD be used as described in 2782 Section 5.4. 2784 ICMP Fields: 2786 Type 2788 147 2790 Code 2792 0 2794 Checksum 2796 The ICMP checksum [16]. 2798 Identifier 2800 An identifier to aid in matching this Mobile Prefix Advertisement 2801 to a previous Mobile Prefix Solicitation. 2803 M 2805 1-bit Managed Address Configuration flag. When set, hosts use the 2806 administered (stateful) protocol for address autoconfiguration in 2807 addition to any addresses autoconfigured using stateless address 2808 autoconfiguration. The use of this flag is described in [17] 2809 [18]. 2811 O 2813 1-bit Other Stateful Configuration flag. When set, hosts use the 2814 administered (stateful) protocol for autoconfiguration of other 2815 (non-address) information. The use of this flag is described in 2816 [17] [18]. 2818 Reserved 2820 This field is unused. It MUST be initialized to zero by the 2821 sender and MUST be ignored by the receiver. 2823 The Mobile Prefix Advertisement messages may have options. These 2824 options MUST use the option format defined in Neighbor Discovery (RFC 2825 4861 [17]). This document defines one option which may be carried in 2826 a Mobile Prefix Advertisement message, but future documents may 2827 define new options. Mobile nodes MUST silently ignore any options 2828 they do not recognize and continue processing the message. 2830 Prefix Information 2832 Each message contains one or more Prefix Information options. 2833 Each option carries the prefix(es) that the mobile node should use 2834 to configure its home address(es). Section 10.6 describes which 2835 prefixes should be advertised to the mobile node. 2837 The Prefix Information option is defined in Section 4.6.2 of 2838 Neighbor Discovery (RFC 4861 [17]), with modifications defined in 2839 Section 7.2 of this specification. The home agent MUST use this 2840 modified Prefix Information option to send home network prefixes 2841 as defined in Section 10.6.1. 2843 If the Advertisement is sent in response to a Mobile Prefix 2844 Solicitation, the home agent MUST copy the Identifier value from that 2845 message into the Identifier field of the Advertisement. 2847 The home agent MUST NOT send more than one Mobile Prefix 2848 Advertisement message per second to any mobile node. 2850 The M and O bits MUST be cleared if the Home Agent DHCPv6 support is 2851 not provided. If such support is provided then they are set in 2852 concert with the home network's administrative settings. 2854 7. Modifications to IPv6 Neighbor Discovery 2856 7.1. Modified Router Advertisement Message Format 2858 Mobile IPv6 modifies the format of the Router Advertisement message 2859 [17] by the addition of a single flag bit to indicate that the router 2860 sending the Advertisement message is serving as a home agent on this 2861 link. The format of the Router Advertisement message is as follows: 2863 0 1 2 3 2864 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 2865 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2866 | Type | Code | Checksum | 2867 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2868 | Cur Hop Limit |M|O|H| Reserved| Router Lifetime | 2869 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2870 | Reachable Time | 2871 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2872 | Retrans Timer | 2873 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2874 | Options ... 2875 +-+-+-+-+-+-+-+-+-+-+-+- 2877 This format represents the following changes over that originally 2878 specified for Neighbor Discovery [17]: 2880 Home Agent (H) 2882 The Home Agent (H) bit is set in a Router Advertisement to 2883 indicate that the router sending this Router Advertisement is also 2884 functioning as a Mobile IPv6 home agent on this link. 2886 Reserved 2888 Reduced from a 6-bit field to a 5-bit field to account for the 2889 addition of the above bit. 2891 7.2. Modified Prefix Information Option Format 2893 Mobile IPv6 requires knowledge of a router's global address in 2894 building a Home Agents List as part of the dynamic home agent address 2895 discovery mechanism. 2897 However, Neighbor Discovery [17] only advertises a router's link- 2898 local address, by requiring this address to be used as the IP Source 2899 Address of each Router Advertisement. 2901 Mobile IPv6 extends Neighbor Discovery to allow a router to advertise 2902 its global address, by the addition of a single flag bit in the 2903 format of a Prefix Information option for use in Router Advertisement 2904 messages. The format of the Prefix Information option is as follows: 2906 0 1 2 3 2907 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 2908 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2909 | Type | Length | Prefix Length |L|A|R|Reserved1| 2910 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2911 | Valid Lifetime | 2912 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2913 | Preferred Lifetime | 2914 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2915 | Reserved2 | 2916 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2917 | | 2918 + + 2919 | | 2920 + Prefix + 2921 | | 2922 + + 2923 | | 2924 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2926 This format represents the following changes over that originally 2927 specified for Neighbor Discovery [17]: 2929 Router Address (R) 2931 1-bit router address flag. When set, indicates that the Prefix 2932 field contains a complete IP address assigned to the sending 2933 router. The indicated prefix is the first Prefix Length bits of 2934 the Prefix field. The router IP address has the same scope and 2935 conforms to the same lifetime values as the advertised prefix. 2936 This use of the Prefix field is compatible with its use in 2937 advertising the prefix itself, since Prefix Advertisement uses 2938 only the leading bits. Interpretation of this flag bit is thus 2939 independent of the processing required for the On-Link (L) and 2940 Autonomous Address-Configuration (A) flag bits. 2942 Reserved1 2944 Reduced from a 6-bit field to a 5-bit field to account for the 2945 addition of the above bit. 2947 In a Router Advertisement, a home agent MUST, and all other routers 2948 MAY, include at least one Prefix Information option with the Router 2949 Address (R) bit set. Neighbor Discovery (RFC 4861 [17]) specifies 2950 that, when including all options in a Router Advertisement causes the 2951 size of the Advertisement to exceed the link MTU, multiple 2952 Advertisements can be sent, each containing a subset of the Neighbor 2953 Discovery options. Also, when sending unsolicited multicast Router 2954 Advertisements more frequently than the limit specified in RFC 4861, 2955 the sending router need not include all options in each of these 2956 Advertisements. However, in both of these cases the router SHOULD 2957 include at least one Prefix Information option with the Router 2958 Address (R) bit set in each such advertisement, if this bit is set in 2959 some advertisement sent by the router. 2961 In addition, the following requirement can assist mobile nodes in 2962 movement detection. Barring changes in the prefixes for the link, 2963 routers that send multiple Router Advertisements with the Router 2964 Address (R) bit set in some of the included Prefix Information 2965 options SHOULD provide at least one option and router address which 2966 stays the same in all of the Advertisements. 2968 7.3. New Advertisement Interval Option Format 2970 Mobile IPv6 defines a new Advertisement Interval option, used in 2971 Router Advertisement messages to advertise the interval at which the 2972 sending router sends unsolicited multicast Router Advertisements. 2973 The format of the Advertisement Interval option is as follows: 2975 0 1 2 3 2976 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 2977 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2978 | Type | Length | Reserved | 2979 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2980 | Advertisement Interval | 2981 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2983 Type 2985 7 2987 Length 2989 8-bit unsigned integer. The length of the option (including the 2990 type and length fields) is in units of 8 octets. The value of 2991 this field MUST be 1. 2993 Reserved 2995 This field is unused. It MUST be initialized to zero by the 2996 sender and MUST be ignored by the receiver. 2998 Advertisement Interval 3000 32-bit unsigned integer. The maximum time, in milliseconds, 3001 between successive unsolicited Router Advertisement messages sent 3002 by this router on this network interface. Using the conceptual 3003 router configuration variables defined by Neighbor Discovery [17], 3004 this field MUST be equal to the value MaxRtrAdvInterval, expressed 3005 in milliseconds. 3007 Routers MAY include this option in their Router Advertisements. A 3008 mobile node receiving a Router Advertisement containing this option 3009 SHOULD utilize the specified Advertisement Interval for that router 3010 in its movement detection algorithm, as described in Section 11.5.1. 3012 This option MUST be silently ignored for other Neighbor Discovery 3013 messages. 3015 7.4. New Home Agent Information Option Format 3017 Mobile IPv6 defines a new Home Agent Information option, used in 3018 Router Advertisements sent by a home agent to advertise information 3019 specific to this router's functionality as a home agent. The format 3020 of the Home Agent Information option is as follows: 3022 0 1 2 3 3023 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 3024 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3025 | Type | Length | Reserved | 3026 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3027 | Home Agent Preference | Home Agent Lifetime | 3028 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3030 Type 3032 8 3034 Length 3036 8-bit unsigned integer. The length of the option (including the 3037 type and length fields) in units of 8 octets. The value of this 3038 field MUST be 1. 3040 Reserved 3042 This field is unused. It MUST be initialized to zero by the 3043 sender and MUST be ignored by the receiver. 3045 Home Agent Preference 3047 16-bit unsigned integer. The preference for the home agent 3048 sending this Router Advertisement, for use in ordering the 3049 addresses returned to a mobile node in the Home Agent Addresses 3050 field of a Home Agent Address Discovery Reply message. Higher 3051 values mean more preferable. If this option is not included in a 3052 Router Advertisement in which the Home Agent (H) bit is set, the 3053 preference value for this home agent MUST be considered to be 0. 3054 Greater values indicate a more preferable home agent than lower 3055 values. 3057 The manual configuration of the Home Agent Preference value is 3058 described in Section 8.4. In addition, the sending home agent MAY 3059 dynamically set the Home Agent Preference value, for example 3060 basing it on the number of mobile nodes it is currently serving or 3061 on its remaining resources for serving additional mobile nodes; 3062 such dynamic settings are beyond the scope of this document. Any 3063 such dynamic setting of the Home Agent Preference, however, MUST 3064 set the preference appropriately, relative to the default Home 3065 Agent Preference value of 0 that may be in use by some home agents 3066 on this link (i.e., a home agent not including a Home Agent 3067 Information option in its Router Advertisements will be considered 3068 to have a Home Agent Preference value of 0). 3070 Home Agent Lifetime 3072 16-bit unsigned integer. The lifetime associated with the home 3073 agent in units of seconds. The default value is the same as the 3074 Router Lifetime, as specified in the main body of the Router 3075 Advertisement. The maximum value corresponds to 18.2 hours. A 3076 value of 0 MUST NOT be used. The Home Agent Lifetime applies only 3077 to this router's usefulness as a home agent; it does not apply to 3078 information contained in other message fields or options. 3080 Home agents MAY include this option in their Router Advertisements. 3081 This option MUST NOT be included in a Router Advertisement in which 3082 the Home Agent (H) bit (see Section 7.1) is not set. If this option 3083 is not included in a Router Advertisement in which the Home Agent (H) 3084 bit is set, the lifetime for this home agent MUST be considered to be 3085 the same as the Router Lifetime in the Router Advertisement. If 3086 multiple Advertisements are being sent instead of a single larger 3087 unsolicited multicast Advertisement, all of the multiple 3088 Advertisements with the Router Address (R) bit set MUST include this 3089 option with the same contents, otherwise this option MUST be omitted 3090 from all Advertisements. 3092 This option MUST be silently ignored for other Neighbor Discovery 3093 messages. 3095 If both the Home Agent Preference and Home Agent Lifetime are set to 3096 their default values specified above, this option SHOULD NOT be 3097 included in the Router Advertisement messages sent by this home 3098 agent. 3100 7.5. Changes to Sending Router Advertisements 3102 The Neighbor Discovery protocol specification [17] limits routers to 3103 a minimum interval of 3 seconds between sending unsolicited multicast 3104 Router Advertisement messages from any given network interface 3105 (limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that: 3107 "Routers generate Router Advertisements frequently enough that 3108 hosts will learn of their presence within a few minutes, but not 3109 frequently enough to rely on an absence of advertisements to 3110 detect router failure; a separate Neighbor Unreachability 3111 Detection algorithm provides failure detection." 3113 This limitation, however, is not suitable to providing timely 3114 movement detection for mobile nodes. Mobile nodes detect their own 3115 movement by learning the presence of new routers as the mobile node 3116 moves into wireless transmission range of them (or physically 3117 connects to a new wired network), and by learning that previous 3118 routers are no longer reachable. Mobile nodes MUST be able to 3119 quickly detect when they move to a link served by a new router, so 3120 that they can acquire a new care-of address and send Binding Updates 3121 to register this care-of address with their home agent and to notify 3122 correspondent nodes as needed. 3124 One method which can provide for faster movement detection, is to 3125 increase the rate at which unsolicited Router Advertisements are 3126 sent. Mobile IPv6 relaxes this limit such that routers MAY send 3127 unsolicited multicast Router Advertisements more frequently. This 3128 method can be applied where the router is expecting to provide 3129 service to visiting mobile nodes (e.g., wireless network interfaces), 3130 or on which it is serving as a home agent to one or more mobile nodes 3131 (who may return home and need to hear its Advertisements). 3133 Routers supporting mobility SHOULD be able to be configured with a 3134 smaller MinRtrAdvInterval value and MaxRtrAdvInterval value to allow 3135 sending of unsolicited multicast Router Advertisements more often. 3136 The minimum allowed values are: 3138 o MinRtrAdvInterval 0.03 seconds 3139 o MaxRtrAdvInterval 0.07 seconds 3141 In the case where the minimum intervals and delays are used, the mean 3142 time between unsolicited multicast router advertisements is 50ms. 3143 Use of these modified limits MUST be configurable (see also the 3144 configuration variable MinDelayBetweenRas in Section 13 which may 3145 also have to be modified accordingly). Systems where these values 3146 are available MUST NOT default to them, and SHOULD default to values 3147 specified in Neighbor Discovery (RFC 4861 [17]). Knowledge of the 3148 type of network interface and operating environment SHOULD be taken 3149 into account in configuring these limits for each network interface. 3150 This is important with some wireless links, where increasing the 3151 frequency of multicast beacons can cause considerable overhead. 3152 Routers SHOULD adhere to the intervals specified in RFC 4861 [17], if 3153 this overhead is likely to cause service degradation. 3155 Additionally, the possible low values of MaxRtrAdvInterval may cause 3156 some problems with movement detection in some mobile nodes. To 3157 ensure that this is not a problem, Routers SHOULD add 20ms to any 3158 Advertisement Intervals sent in RAs, which are below 200 ms, in order 3159 to account for scheduling granularities on both the MN and the 3160 Router. 3162 Note that multicast Router Advertisements are not always required in 3163 certain wireless networks that have limited bandwidth. Mobility 3164 detection or link changes in such networks may be done at lower 3165 layers. Router advertisements in such networks SHOULD be sent only 3166 when solicited. In such networks it SHOULD be possible to disable 3167 unsolicited multicast Router Advertisements on specific interfaces. 3168 The MinRtrAdvInterval and MaxRtrAdvInterval in such a case can be set 3169 to some high values. 3171 Home agents MUST include the Source Link-Layer Address option in all 3172 Router Advertisements they send. This simplifies the process of 3173 returning home, as discussed in Section 11.5.5. 3175 Note that according to Neighbor Discovery (RFC 4861 [17]), 3176 AdvDefaultLifetime is by default based on the value of 3177 MaxRtrAdvInterval. AdvDefaultLifetime is used in the Router Lifetime 3178 field of Router Advertisements. Given that this field is expressed 3179 in seconds, a small MaxRtrAdvInterval value can result in a zero 3180 value for this field. To prevent this, routers SHOULD keep 3181 AdvDefaultLifetime in at least one second, even if the use of 3182 MaxRtrAdvInterval would result in a smaller value. 3184 8. Requirements for Types of IPv6 Nodes 3186 Mobile IPv6 places some special requirements on the functions 3187 provided by different types of IPv6 nodes. This section summarizes 3188 those requirements, identifying the functionality each requirement is 3189 intended to support. 3191 The requirements are set for the following groups of nodes: 3193 o All IPv6 nodes. 3195 o All IPv6 nodes with support for route optimization. 3197 o All IPv6 routers. 3199 o All Mobile IPv6 home agents. 3201 o All Mobile IPv6 mobile nodes. 3203 It is outside the scope of this specification to specify which of 3204 these groups are mandatory in IPv6. We only describe what is 3205 mandatory for a node that supports, for instance, route optimization. 3206 Other specifications are expected to define the extent of IPv6. 3208 8.1. All IPv6 Nodes 3210 Any IPv6 node may at any time be a correspondent node of a mobile 3211 node, either sending a packet to a mobile node or receiving a packet 3212 from a mobile node. There are no Mobile IPv6 specific MUST 3213 requirements for such nodes, and basic IPv6 techniques are 3214 sufficient. If a mobile node attempts to set up route optimization 3215 with a node with only basic IPv6 support, an ICMP error will signal 3216 that the node does not support such optimizations (Section 11.3.5), 3217 and communications will flow through the home agent . 3219 An IPv6 node MUST NOT support the Home Address destination option, 3220 type 2 routing header, or the Mobility Header unless it fully 3221 supports the requirements listed in the next sections for either 3222 route optimization, mobile node, or home agent functionality. 3224 8.2. IPv6 Nodes with Support for Route Optimization 3226 Nodes that implement route optimization are a subset of all IPv6 3227 nodes on the Internet. The ability of a correspondent node to 3228 participate in route optimization is essential for the efficient 3229 operation of the IPv6 Internet, for the following reasons: 3231 o Avoidance of congestion in the home network, and enabling the use 3232 of lower-performance home agent equipment even for supporting 3233 thousands of mobile nodes. 3235 o Reduced network load across the entire Internet, as mobile devices 3236 begin to predominate. 3238 o Reduction of jitter and latency for the communications. 3240 o Greater likelihood of success for QoS signaling as tunneling is 3241 avoided and, again, fewer sources of congestion. 3243 o Improved robustness against network partitions, congestion, and 3244 other problems, since fewer routing path segments are traversed. 3246 These effects combine to enable much better performance and 3247 robustness for communications between mobile nodes and IPv6 3248 correspondent nodes. Route optimization introduces a small amount of 3249 additional state for the peers, some additional messaging, and up to 3250 1.5 roundtrip delays before it can be turned on. However, it is 3251 believed that the benefits far outweigh the costs in most cases. 3252 Section 11.3.1 discusses how mobile nodes may avoid route 3253 optimization for some of the remaining cases, such as very short-term 3254 communications. 3256 The following requirements apply to all correspondent nodes that 3257 support route optimization: 3259 o The node MUST be able to validate a Home Address option using an 3260 existing Binding Cache entry, as described in Section 9.3.1. 3262 o The node MUST be able to insert a type 2 routing header into 3263 packets to be sent to a mobile node, as described in 3264 Section 9.3.2. 3266 o Unless the correspondent node is also acting as a mobile node, it 3267 MUST ignore type 2 routing headers and silently discard all 3268 packets that it has received with such headers. 3270 o The node SHOULD be able to interpret ICMP messages as described in 3271 Section 9.3.4. 3273 o The node MUST be able to send Binding Error messages as described 3274 in Section 9.3.3. 3276 o The node MUST be able to process Mobility Headers as described in 3277 Section 9.2. 3279 o The node MUST be able to participate in a return routability 3280 procedure (Section 9.4). 3282 o The node MUST be able to process Binding Update messages 3283 (Section 9.5). 3285 o The node MUST be able to return a Binding Acknowledgement 3286 (Section 9.5.4). 3288 o The node MUST be able to maintain a Binding Cache of the bindings 3289 received in accepted Binding Updates, as described in Section 9.1 3290 and Section 9.6. 3292 o The node SHOULD allow route optimization to be administratively 3293 enabled or disabled. The default SHOULD be enabled. 3295 8.3. All IPv6 Routers 3297 All IPv6 routers, even those not serving as a home agent for Mobile 3298 IPv6, have an effect on how well mobile nodes can communicate: 3300 o Every IPv6 router SHOULD be able to send an Advertisement Interval 3301 option (Section 7.3) in each of its Router Advertisements [17], to 3302 aid movement detection by mobile nodes (as in Section 11.5.1). 3303 The use of this option in Router Advertisements SHOULD be 3304 configurable. 3306 o Every IPv6 router SHOULD be able to support sending unsolicited 3307 multicast Router Advertisements at the faster rate described in 3308 Section 7.5. If the router supports a faster rate, the used rate 3309 MUST be configurable. 3311 o Each router SHOULD include at least one prefix with the Router 3312 Address (R) bit set and with its full IP address in its Router 3313 Advertisements (as described in Section 7.2). 3315 o Routers supporting filtering packets with routing headers SHOULD 3316 support different rules for type 0 and type 2 routing headers (see 3317 Section 6.4) so that filtering of source routed packets (type 0) 3318 will not necessarily limit Mobile IPv6 traffic which is delivered 3319 via type 2 routing headers. 3321 8.4. IPv6 Home Agents 3323 In order for a mobile node to operate correctly while away from home, 3324 at least one IPv6 router on the mobile node's home link must function 3325 as a home agent for the mobile node. The following additional 3326 requirements apply to all IPv6 routers that serve as a home agent: 3328 o Every home agent MUST be able to maintain an entry in its Binding 3329 Cache for each mobile node for which it is serving as the home 3330 agent (Section 10.1 and Section 10.3.1). 3332 o Every home agent MUST be able to intercept packets (using proxy 3333 Neighbor Discovery [17]) addressed to a mobile node for which it 3334 is currently serving as the home agent, on that mobile node's home 3335 link, while the mobile node is away from home (Section 10.4.1). 3337 o Every home agent MUST be able to encapsulate [6] such intercepted 3338 packets in order to tunnel them to the primary care-of address for 3339 the mobile node indicated in its binding in the home agent's 3340 Binding Cache (Section 10.4.2). 3342 o Every home agent MUST support decapsulating [6] reverse tunneled 3343 packets sent to it from a mobile node's home address. Every home 3344 agent MUST also check that the source address in the tunneled 3345 packets corresponds to the currently registered location of the 3346 mobile node (Section 10.4.5). 3348 o The node MUST be able to process Mobility Headers as described in 3349 Section 10.2. 3351 o Every home agent MUST be able to return a Binding Acknowledgement 3352 in response to a Binding Update (Section 10.3.1). 3354 o Every home agent MUST maintain a separate Home Agents List for 3355 each link on which it is serving as a home agent, as described in 3356 Section 10.1 and Section 10.5.1. 3358 o Every home agent MUST be able to accept packets addressed to the 3359 Mobile IPv6 Home-Agents anycast address [7] for the subnet on 3360 which it is serving as a home agent, and MUST be able to 3361 participate in dynamic home agent address discovery 3362 (Section 10.5). 3364 o Every home agent SHOULD support a configuration mechanism to allow 3365 a system administrator to manually set the value to be sent by 3366 this home agent in the Home Agent Preference field of the Home 3367 Agent Information Option in Router Advertisements that it sends 3368 (Section 7.4). 3370 o Every home agent SHOULD support sending ICMP Mobile Prefix 3371 Advertisements (Section 6.8), and SHOULD respond to Mobile Prefix 3372 Solicitations (Section 6.7). If supported, this behavior MUST be 3373 configurable, so that home agents can be configured to avoid 3374 sending such Prefix Advertisements according to the needs of the 3375 network administration in the home domain. 3377 o Every home agent MUST support IPsec ESP for protection of packets 3378 belonging to the return routability procedure (Section 10.4.6). 3380 o Every home agent SHOULD support the multicast group membership 3381 control protocols as described in Section 10.4.3. If this support 3382 is provided, the home agent MUST be capable of using it to 3383 determine which multicast data packets to forward via the tunnel 3384 to the mobile node. 3386 o Home agents MAY support stateful address autoconfiguration for 3387 mobile nodes as described in Section 10.4.4. 3389 8.5. IPv6 Mobile Nodes 3391 Finally, the following requirements apply to all IPv6 nodes capable 3392 of functioning as mobile nodes: 3394 o The node MUST maintain a Binding Update List (Section 11.1). 3396 o The node MUST support sending packets containing a Home Address 3397 option (Section 11.3.1), and follow the required IPsec interaction 3398 (Section 11.3.2). 3400 o The node MUST be able to perform IPv6 encapsulation and 3401 decapsulation [6]. 3403 o The node MUST be able to process type 2 routing header as defined 3404 in Section 6.4 and Section 11.3.3. 3406 o The node MUST support receiving a Binding Error message 3407 (Section 11.3.6). 3409 o The node MUST support receiving ICMP errors (Section 11.3.5). 3411 o The node MUST support movement detection, care-of address 3412 formation, and returning home (Section 11.5). 3414 o The node MUST be able to process Mobility Headers as described in 3415 Section 11.2. 3417 o The node MUST support the return routability procedure 3418 (Section 11.6). 3420 o The node MUST be able to send Binding Updates, as specified in 3421 Section 11.7.1 and Section 11.7.2. 3423 o The node MUST be able to receive and process Binding 3424 Acknowledgements, as specified in Section 11.7.3. 3426 o The node MUST support receiving a Binding Refresh Request 3427 (Section 6.1.2), by responding with a Binding Update. 3429 o The node MUST support receiving Mobile Prefix Advertisements 3430 (Section 11.4.3) and reconfiguring its home address based on the 3431 prefix information contained therein. 3433 o The node SHOULD support use of the dynamic home agent address 3434 discovery mechanism, as described in Section 11.4.1. 3436 o The node MUST allow route optimization to be administratively 3437 enabled or disabled. The default SHOULD be enabled. 3439 o The node MAY support the multicast address listener part of a 3440 multicast group membership protocol as described in 3441 Section 11.3.4. If this support is provided, the mobile node MUST 3442 be able to receive tunneled multicast packets from the home agent. 3444 o The node MAY support stateful address autoconfiguration mechanisms 3445 such as DHCPv6 [29] on the interface represented by the tunnel to 3446 the home agent. 3448 9. Correspondent Node Operation 3450 9.1. Conceptual Data Structures 3452 IPv6 nodes with route optimization support maintain a Binding Cache 3453 of bindings for other nodes. A separate Binding Cache SHOULD be 3454 maintained by each IPv6 node for each of its unicast routable 3455 addresses. The Binding Cache MAY be implemented in any manner 3456 consistent with the external behavior described in this document, for 3457 example by being combined with the node's Destination Cache as 3458 maintained by Neighbor Discovery [17]. When sending a packet, the 3459 Binding Cache is searched before the Neighbor Discovery conceptual 3460 Destination Cache [17]. 3462 Each Binding Cache entry conceptually contains the following fields: 3464 o The home address of the mobile node for which this is the Binding 3465 Cache entry. This field is used as the key for searching the 3466 Binding Cache for the destination address of a packet being sent. 3468 o The care-of address for the mobile node indicated by the home 3469 address field in this Binding Cache entry. 3471 o A lifetime value, indicating the remaining lifetime for this 3472 Binding Cache entry. The lifetime value is initialized from the 3473 Lifetime field in the Binding Update that created or last modified 3474 this Binding Cache entry. A correspondent node MAY select a 3475 smaller lifetime for the Binding Cache entry, and supply that 3476 value to the mobile node in the Binding Acknowledgment message. 3478 o A flag indicating whether or not this Binding Cache entry is a 3479 home registration entry (applicable only on nodes which support 3480 home agent functionality). 3482 o The maximum value of the Sequence Number field received in 3483 previous Binding Updates for this home address. The Sequence 3484 Number field is 16 bits long. Sequence Number values MUST be 3485 compared modulo 2**16 as explained in Section 9.5.1. 3487 o Usage information for this Binding Cache entry. This is needed to 3488 implement the cache replacement policy in use in the Binding 3489 Cache. Recent use of a cache entry also serves as an indication 3490 that a Binding Refresh Request should be sent when the lifetime of 3491 this entry nears expiration. 3493 Binding Cache entries not marked as home registrations MAY be 3494 replaced at any time by any reasonable local cache replacement policy 3495 but SHOULD NOT be unnecessarily deleted. The Binding Cache for any 3496 one of a node's IPv6 addresses may contain at most one entry for each 3497 mobile node home address. The contents of a node's Binding Cache 3498 MUST NOT be changed in response to a Home Address option in a 3499 received packet. 3501 9.2. Processing Mobility Headers 3503 Mobility Header processing MUST observe the following rules: 3505 o The checksum must be verified as per Section 6.1. If invalid, the 3506 node MUST silently discard the message. 3508 o The MH Type field MUST have a known value (Section 6.1.1). 3509 Otherwise, the node MUST discard the message and issue a Binding 3510 Error message as described in Section 9.3.3, with Status field set 3511 to 2 (unrecognized MH Type value). 3513 o The Payload Proto field MUST be IPPROTO_NONE (59 decimal). 3514 Otherwise, the node MUST discard the message and SHOULD send ICMP 3515 Parameter Problem, Code 0, directly to the Source Address of the 3516 packet as specified in RFC 4443 [16]. Thus no Binding Cache 3517 information is used in sending the ICMP message. The Pointer 3518 field in the ICMP message SHOULD point at the Payload Proto field. 3520 o The Header Len field in the Mobility Header MUST NOT be less than 3521 the length specified for this particular type of message in 3522 Section 6.1. Otherwise, the node MUST discard the message and 3523 SHOULD send ICMP Parameter Problem, Code 0, directly to the Source 3524 Address of the packet as specified in RFC 4443 [16]. (The Binding 3525 Cache information is again not used.) The Pointer field in the 3526 ICMP message SHOULD point at the Header Len field. 3528 Subsequent checks depend on the particular Mobility Header. 3530 9.3. Packet Processing 3532 This section describes how the correspondent node sends packets to 3533 the mobile node, and receives packets from it. 3535 9.3.1. Receiving Packets with Home Address Option 3537 Packets containing a Home Address option MUST be dropped if the given 3538 home address is not a unicast routable address. 3540 Mobile nodes can include a Home Address destination option in a 3541 packet if they believe the correspondent node has a Binding Cache 3542 entry for the home address of a mobile node. If the Next Header 3543 value of the Destination Option is one of the following: {50 (ESP), 3544 51 (AH), 135 (Mobility Header)}, the packet SHOULD be processed 3545 normally. Otherwise, the packet MUST be dropped if there is no 3546 corresponding Binding Cache entry. A corresponding Binding Cache 3547 entry MUST have the same home address as appears in the Home Address 3548 destination option, and the currently registered care-of address MUST 3549 be equal to the source address of the packet. 3551 If the packet is dropped due to the above tests, the correspondent 3552 node MUST send the Binding Error message as described in 3553 Section 9.3.3. The Status field in this message should be set to 1 3554 (unknown binding for Home Address destination option). 3556 The correspondent node MUST process the option in a manner consistent 3557 with exchanging the Home Address field from the Home Address option 3558 into the IPv6 header and replacing the original value of the Source 3559 Address field there. After all IPv6 options have been processed, it 3560 MUST be possible for upper layers to process the packet without the 3561 knowledge that it came originally from a care-of address or that a 3562 Home Address option was used. 3564 The use of IPsec Authentication Header (AH) for the Home Address 3565 option is not required, except that if the IPv6 header of a packet is 3566 covered by AH, then the authentication MUST also cover the Home 3567 Address option; this coverage is achieved automatically by the 3568 definition of the Option Type code for the Home Address option, since 3569 it indicates that the data within the option cannot change en route 3570 to the packet's final destination, and thus the option is included in 3571 the AH computation. By requiring that any authentication of the IPv6 3572 header also cover the Home Address option, the security of the Source 3573 Address field in the IPv6 header is not compromised by the presence 3574 of a Home Address option. 3576 When attempting to verify AH authentication data in a packet that 3577 contains a Home Address option, the receiving node MUST calculate the 3578 AH authentication data as if the following were true: The Home 3579 Address option contains the care-of address, and the source IPv6 3580 address field of the IPv6 header contains the home address. This 3581 conforms with the calculation specified in Section 11.3.2. 3583 9.3.2. Sending Packets to a Mobile Node 3585 Before sending any packet, the sending node SHOULD examine its 3586 Binding Cache for an entry for the destination address to which the 3587 packet is being sent. If the sending node has a Binding Cache entry 3588 for this address, the sending node SHOULD use a type 2 routing header 3589 to route the packet to this mobile node (the destination node) by way 3590 of its care-of address. However, the sending node MUST NOT do this 3591 in the following cases: 3593 o When sending an IPv6 Neighbor Discovery [17] packet. 3595 o Where otherwise noted in Section 6.1. 3597 When calculating authentication data in a packet that contains a type 3598 2 routing header, the correspondent node MUST calculate the AH 3599 authentication data as if the following were true: The routing header 3600 contains the care-of address, the destination IPv6 address field of 3601 the IPv6 header contains the home address, and the Segments Left 3602 field is zero. The IPsec Security Policy Database lookup MUST based 3603 on the mobile node's home address. 3605 For instance, assuming there are no additional routing headers in 3606 this packet beyond those needed by Mobile IPv6, the correspondent 3607 node could set the fields in the packet's IPv6 header and routing 3608 header as follows: 3610 o The Destination Address in the packet's IPv6 header is set to the 3611 mobile node's home address (the original destination address to 3612 which the packet was being sent). 3614 o The routing header is initialized to contain a single route 3615 segment, containing the mobile node's care-of address copied from 3616 the Binding Cache entry. The Segments Left field is, however, 3617 temporarily set to zero. 3619 The IP layer will insert the routing header before performing any 3620 necessary IPsec processing. Once all IPsec processing has been 3621 performed, the node swaps the IPv6 destination field with the Home 3622 Address field in the routing header, sets the Segments Left field to 3623 one, and sends the packet. This ensures the AH calculation is done 3624 on the packet in the form it will have on the receiver after 3625 advancing the routing header. 3627 Following the definition of a type 2 routing header in Section 6.4, 3628 this packet will be routed to the mobile node's care-of address, 3629 where it will be delivered to the mobile node (the mobile node has 3630 associated the care-of address with its network interface). 3632 Note that following the above conceptual model in an implementation 3633 creates some additional requirements for path MTU discovery since the 3634 layer that determines the packet size (e.g., TCP and applications 3635 using UDP) needs to be aware of the size of the headers added by the 3636 IP layer on the sending node. 3638 If, instead, the sending node has no Binding Cache entry for the 3639 destination address to which the packet is being sent, the sending 3640 node simply sends the packet normally, with no routing header. If 3641 the destination node is not a mobile node (or is a mobile node that 3642 is currently at home), the packet will be delivered directly to this 3643 node and processed normally by it. If, however, the destination node 3644 is a mobile node that is currently away from home, the packet will be 3645 intercepted by the mobile node's home agent and tunneled to the 3646 mobile node's current primary care-of address. 3648 9.3.3. Sending Binding Error Messages 3650 Section 9.2 and Section 9.3.1 describe error conditions that lead to 3651 a need to send a Binding Error message. 3653 A Binding Error message is sent directly to the address that appeared 3654 in the IPv6 Source Address field of the offending packet. If the 3655 Source Address field does not contain a unicast address, the Binding 3656 Error message MUST NOT be sent. 3658 The Home Address field in the Binding Error message MUST be copied 3659 from the Home Address field in the Home Address destination option of 3660 the offending packet, or set to the unspecified address if no such 3661 option appeared in the packet. 3663 Note that the IPv6 Source Address and Home Address field values 3664 discussed above are the values from the wire, i.e., before any 3665 modifications possibly performed as specified in Section 9.3.1. 3667 Binding Error messages SHOULD be subject to rate limiting in the same 3668 manner as is done for ICMPv6 messages [16]. 3670 9.3.4. Receiving ICMP Error Messages 3672 When the correspondent node has a Binding Cache entry for a mobile 3673 node, all traffic destined to the mobile node goes directly to the 3674 current care-of address of the mobile node using a routing header. 3675 Any ICMP error message caused by packets on their way to the care-of 3676 address will be returned in the normal manner to the correspondent 3677 node. 3679 On the other hand, if the correspondent node has no Binding Cache 3680 entry for the mobile node, the packet will be routed through the 3681 mobile node's home link. Any ICMP error message caused by the packet 3682 on its way to the mobile node while in the tunnel, will be 3683 transmitted to the mobile node's home agent. By the definition of 3684 IPv6 encapsulation [6], the home agent MUST relay certain ICMP error 3685 messages back to the original sender of the packet, which in this 3686 case is the correspondent node. 3688 Thus, in all cases, any meaningful ICMP error messages caused by 3689 packets from a correspondent node to a mobile node will be returned 3690 to the correspondent node. If the correspondent node receives 3691 persistent ICMP Destination Unreachable messages after sending 3692 packets to a mobile node based on an entry in its Binding Cache, the 3693 correspondent node SHOULD delete this Binding Cache entry. Note that 3694 if the mobile node continues to send packets with the Home Address 3695 destination option to this correspondent node, they will be dropped 3696 due to the lack of a binding. For this reason it is important that 3697 only persistent ICMP messages lead to the deletion of the Binding 3698 Cache entry. 3700 9.4. Return Routability Procedure 3702 This subsection specifies actions taken by a correspondent node 3703 during the return routability procedure. 3705 9.4.1. Receiving Home Test Init Messages 3707 Upon receiving a Home Test Init message, the correspondent node 3708 verifies the following: 3710 o The packet MUST NOT include a Home Address destination option. 3712 Any packet carrying a Home Test Init message which fails to satisfy 3713 this test MUST be silently ignored. 3715 Otherwise, in preparation for sending the corresponding Home Test 3716 Message, the correspondent node checks that it has the necessary 3717 material to engage in a return routability procedure, as specified in 3718 Section 5.2. The correspondent node MUST have a secret Kcn and a 3719 nonce. If it does not have this material yet, it MUST produce it 3720 before continuing with the return routability procedure. 3722 Section 9.4.3 specifies further processing. 3724 9.4.2. Receiving Care-of Test Init Messages 3726 Upon receiving a Care-of Test Init message, the correspondent node 3727 verifies the following: 3729 o The packet MUST NOT include a Home Address destination option. 3731 Any packet carrying a Care-of Test Init message which fails to 3732 satisfy this test MUST be silently ignored. 3734 Otherwise, in preparation for sending the corresponding Care-of Test 3735 Message, the correspondent node checks that it has the necessary 3736 material to engage in a return routability procedure in the manner 3737 described in Section 9.4.1. 3739 Section 9.4.4 specifies further processing. 3741 9.4.3. Sending Home Test Messages 3743 The correspondent node creates a home keygen token and uses the 3744 current nonce index as the Home Nonce Index. It then creates a Home 3745 Test message (Section 6.1.5) and sends it to the mobile node at the 3746 latter's home address. 3748 9.4.4. Sending Care-of Test Messages 3750 The correspondent node creates a care-of keygen token and uses the 3751 current nonce index as the Care-of Nonce Index. It then creates a 3752 Care-of Test message (Section 6.1.6) and sends it to the mobile node 3753 at the latter's care-of address. 3755 9.5. Processing Bindings 3757 This section explains how the correspondent node processes messages 3758 related to bindings. These messages are: 3760 o Binding Update 3762 o Binding Refresh Request 3764 o Binding Acknowledgement 3766 o Binding Error 3768 9.5.1. Receiving Binding Updates 3770 Before accepting a Binding Update, the receiving node MUST validate 3771 the Binding Update according to the following tests: 3773 o The packet MUST contain a unicast routable home address, either in 3774 the Home Address option or in the Source Address, if the Home 3775 Address option is not present. 3777 o The Sequence Number field in the Binding Update is greater than 3778 the Sequence Number received in the previous valid Binding Update 3779 for this home address, if any. 3781 If the receiving node has no Binding Cache entry for the indicated 3782 home address, it MUST accept any Sequence Number value in a 3783 received Binding Update from this mobile node. 3785 This Sequence Number comparison MUST be performed modulo 2**16, 3786 i.e., the number is a free running counter represented modulo 3787 65536. A Sequence Number in a received Binding Update is 3788 considered less than or equal to the last received number if its 3789 value lies in the range of the last received number and the 3790 preceding 32768 values, inclusive. For example, if the last 3791 received sequence number was 15, then messages with sequence 3792 numbers 0 through 15, as well as 32783 through 65535, would be 3793 considered less than or equal. 3795 When the Home Registration (H) bit is not set, the following are also 3796 required: 3798 o A Nonce Indices mobility option MUST be present, and the Home and 3799 Care-of Nonce Index values in this option MUST be recent enough to 3800 be recognized by the correspondent node. (Care-of Nonce Index 3801 values are not inspected for requests to delete a binding.) 3803 o The correspondent node MUST re-generate the home keygen token and 3804 the care-of keygen token from the information contained in the 3805 packet. It then generates the binding management key Kbm and uses 3806 it to verify the authenticator field in the Binding Update as 3807 specified in Section 6.1.7. 3809 o The Binding Authorization Data mobility option MUST be present, 3810 and its contents MUST satisfy rules presented in Section 5.2.6. 3811 Note that a care-of address different from the Source Address MAY 3812 have been specified by including an Alternate Care-of Address 3813 mobility option in the Binding Update. When such a message is 3814 received and the return routability procedure is used as an 3815 authorization method, the correspondent node MUST verify the 3816 authenticator by using the address within the Alternate Care-of 3817 Address in the calculations. 3819 o The Binding Authorization Data mobility option MUST be the last 3820 option and MUST NOT have trailing padding. 3822 If the Home Registration (H) bit is set, the Nonce Indices mobility 3823 option MUST NOT be present. 3825 If the mobile node sends a sequence number which is not greater than 3826 the sequence number from the last valid Binding Update for this home 3827 address, then the receiving node MUST send back a Binding 3828 Acknowledgement with status code 135, and the last accepted sequence 3829 number in the Sequence Number field of the Binding Acknowledgement. 3831 If a binding already exists for the given home address and the home 3832 registration flag has a different value than the Home Registration 3833 (H) bit in the Binding Update, then the receiving node MUST send back 3834 a Binding Acknowledgement with status code 139 (registration type 3835 change disallowed). The home registration flag stored in the Binding 3836 Cache entry MUST NOT be changed. 3838 If the receiving node no longer recognizes the Home Nonce Index 3839 value, Care-of Nonce Index value, or both values from the Binding 3840 Update, then the receiving node MUST send back a Binding 3841 Acknowledgement with status code 136, 137, or 138, respectively. 3843 Packets carrying Binding Updates that fail to satisfy all of these 3844 tests for any reason other than insufficiency of the Sequence Number, 3845 registration type change, or expired nonce index values, MUST be 3846 silently discarded. 3848 If the Binding Update is valid according to the tests above, then the 3849 Binding Update is processed further as follows: 3851 o The Sequence Number value received from a mobile node in a Binding 3852 Update is stored by the receiving node in its Binding Cache entry 3853 for the given home address. 3855 o If the Lifetime specified in the Binding Update is not zero, then 3856 this is a request to cache a binding for the home address. If the 3857 Home Registration (H) bit is set in the Binding Update, the 3858 Binding Update is processed according to the procedure specified 3859 in Section 10.3.1; otherwise, it is processed according to the 3860 procedure specified in Section 9.5.2. 3862 o If the Lifetime specified in the Binding Update is zero, then this 3863 is a request to delete the cached binding for the home address. 3864 In this case, the Binding Update MUST include a valid home nonce 3865 index, and the care-of nonce index MUST be ignored by the 3866 correspondent node. The generation of the binding management key 3867 depends then exclusively on the home keygen token (Section 5.2.5). 3868 If the Home Registration (H) bit is set in the Binding Update, the 3869 Binding Update is processed according to the procedure specified 3870 in Section 10.3.2; otherwise, it is processed according to the 3871 procedure specified in Section 9.5.3. 3873 The specified care-of address MUST be determined as follows: 3875 o If the Alternate Care-of Address option is present, the care-of 3876 address is the address in that option. 3878 o Otherwise, the care-of address is the Source Address field in the 3879 packet's IPv6 header. 3881 The home address for the binding MUST be determined as follows: 3883 o If the Home Address destination option is present, the home 3884 address is the address in that option. 3886 o Otherwise, the home address is the Source Address field in the 3887 packet's IPv6 header. 3889 9.5.2. Requests to Cache a Binding 3891 This section describes the processing of a valid Binding Update that 3892 requests a node to cache a binding, for which the Home Registration 3893 (H) bit is not set in the Binding Update. 3895 In this case, the receiving node SHOULD create a new entry in its 3896 Binding Cache for this home address, or update its existing Binding 3897 Cache entry for this home address, if such an entry already exists. 3898 The lifetime for the Binding Cache entry is initialized from the 3899 Lifetime field specified in the Binding Update, although this 3900 lifetime MAY be reduced by the node caching the binding; the lifetime 3901 for the Binding Cache entry MUST NOT be greater than the Lifetime 3902 value specified in the Binding Update. Any Binding Cache entry MUST 3903 be deleted after the expiration of its lifetime. 3905 Note that if the mobile node did not request a Binding 3906 Acknowledgement, then it is not aware of the selected shorter 3907 lifetime. The mobile node may thus use route optimization and send 3908 packets with the Home Address destination option. As discussed in 3909 Section 9.3.1, such packets will be dropped if there is no binding. 3910 This situation is recoverable, but can cause temporary packet loss. 3912 The correspondent node MAY refuse to accept a new Binding Cache entry 3913 if it does not have sufficient resources. A new entry MAY also be 3914 refused if the correspondent node believes its resources are utilized 3915 more efficiently in some other purpose, such as serving another 3916 mobile node with higher amount of traffic. In both cases the 3917 correspondent node SHOULD return a Binding Acknowledgement with 3918 status value 130. 3920 9.5.3. Requests to Delete a Binding 3922 This section describes the processing of a valid Binding Update that 3923 requests a node to delete a binding when the Home Registration (H) 3924 bit is not set in the Binding Update. 3926 Any existing binding for the given home address MUST be deleted. A 3927 Binding Cache entry for the home address MUST NOT be created in 3928 response to receiving the Binding Update. 3930 If the Binding Cache entry was created by use of return routability 3931 nonces, the correspondent node MUST ensure that the same nonces are 3932 not used again with the particular home and care-of address. If both 3933 nonces are still valid, the correspondent node has to remember the 3934 particular combination of nonce indexes, addresses, and sequence 3935 number as illegal until at least one of the nonces has become too 3936 old. 3938 9.5.4. Sending Binding Acknowledgements 3940 A Binding Acknowledgement may be sent to indicate receipt of a 3941 Binding Update as follows: 3943 o If the Binding Update was discarded as described in Section 9.2 or 3944 Section 9.5.1, a Binding Acknowledgement MUST NOT be sent. 3945 Otherwise the treatment depends on the following rules. 3947 o If the Acknowledge (A) bit is set in the Binding Update, a Binding 3948 Acknowledgement MUST be sent. Otherwise, the treatment depends on 3949 the next rule. 3951 o If the node rejects the Binding Update due to an expired nonce 3952 index, sequence number being out of window (Section 9.5.1), or 3953 insufficiency of resources (Section 9.5.2), a Binding 3954 Acknowledgement MUST be sent. If the node accepts the Binding 3955 Update, the Binding Acknowledgement SHOULD NOT be sent. 3957 If the node accepts the Binding Update and creates or updates an 3958 entry for this binding, the Status field in the Binding 3959 Acknowledgement MUST be set to a value less than 128. Otherwise, the 3960 Status field MUST be set to a value greater than or equal to 128. 3961 Values for the Status field are described in Section 6.1.8 and in the 3962 IANA registry of assigned numbers [10]. 3964 If the Status field in the Binding Acknowledgement contains the value 3965 136 (expired home nonce index), 137 (expired care-of nonce index), or 3966 138 (expired nonces) then the message MUST NOT include the Binding 3967 Authorization Data mobility option. Otherwise, the Binding 3968 Authorization Data mobility option MUST be included, and MUST meet 3969 the specific authentication requirements for Binding Acknowledgements 3970 as defined in Section 5.2. 3972 If the Source Address field of the IPv6 header that carried the 3973 Binding Update does not contain a unicast address, the Binding 3974 Acknowledgement MUST NOT be sent and the Binding Update packet MUST 3975 be silently discarded. Otherwise, the acknowledgement MUST be sent 3976 to the Source Address. Unlike the treatment of regular packets, this 3977 addressing procedure does not use information from the Binding Cache. 3979 However, a routing header is needed in some cases. If the Source 3980 Address is the home address of the mobile node, i.e., the Binding 3981 Update did not contain a Home Address destination option, then the 3982 Binding Acknowledgement MUST be sent to that address and the routing 3983 header MUST NOT be used. Otherwise, the Binding Acknowledgement MUST 3984 be sent using a type 2 routing header which contains the mobile 3985 node's home address. 3987 9.5.5. Sending Binding Refresh Requests 3989 If a Binding Cache entry being deleted is still in active use when 3990 sending packets to a mobile node, then the next packet sent to the 3991 mobile node will be routed normally to the mobile node's home link. 3992 Communication with the mobile node continues, but the tunneling from 3993 the home network creates additional overhead and latency in 3994 delivering packets to the mobile node. 3996 If the sender knows that the Binding Cache entry is still in active 3997 use, it MAY send a Binding Refresh Request message to the mobile node 3998 in an attempt to avoid this overhead and latency due to deleting and 3999 recreating the Binding Cache entry. This message is always sent to 4000 the home address of the mobile node. 4002 The correspondent node MAY retransmit Binding Refresh Request 4003 messages as long as the rate limitation is applied. The 4004 correspondent node MUST stop retransmitting when it receives a 4005 Binding Update. 4007 9.6. Cache Replacement Policy 4009 Conceptually, a node maintains a separate timer for each entry in its 4010 Binding Cache. When creating or updating a Binding Cache entry in 4011 response to a received and accepted Binding Update, the node sets the 4012 timer for this entry to the specified Lifetime period. Any entry in 4013 a node's Binding Cache MUST be deleted after the expiration of the 4014 Lifetime specified in the Binding Update from which the entry was 4015 created or last updated. 4017 Each node's Binding Cache will, by necessity, have a finite size. A 4018 node MAY use any reasonable local policy for managing the space 4019 within its Binding Cache. 4021 A node MAY choose to drop any entry already in its Binding Cache in 4022 order to make space for a new entry. For example, a "least-recently 4023 used" (LRU) strategy for cache entry replacement among entries should 4024 work well, unless the size of the Binding Cache is substantially 4025 insufficient. When entries are deleted, the correspondent node MUST 4026 follow the rules in Section 5.2.8 in order to guard the return 4027 routability procedure against replay attacks. 4029 If the node sends a packet to a destination for which it has dropped 4030 the entry from its Binding Cache, the packet will be routed through 4031 the mobile node's home link. The mobile node can detect this and 4032 establish a new binding if necessary. 4034 However, if the mobile node believes that the binding still exists, 4035 it may use route optimization and send packets with the Home Address 4036 destination option. This can create temporary packet loss, as 4037 discussed earlier, in the context of binding lifetime reductions 4038 performed by the correspondent node (Section 9.5.2). 4040 10. Home Agent Operation 4042 10.1. Conceptual Data Structures 4044 Each home agent MUST maintain a Binding Cache and Home Agents List. 4046 The rules for maintaining a Binding Cache are the same for home 4047 agents and correspondent nodes and have already been described in 4048 Section 9.1. 4050 The Home Agents List is maintained by each home agent, recording 4051 information about each router on the same link that is acting as a 4052 home agent. This list is used by the dynamic home agent address 4053 discovery mechanism. A router is known to be acting as a home agent, 4054 if it sends a Router Advertisement in which the Home Agent (H) bit is 4055 set. When the lifetime for a list entry (defined below) expires, 4056 that entry is removed from the Home Agents List. The Home Agents 4057 List is similar to the Default Router List conceptual data structure 4058 maintained by each host for Neighbor Discovery [17]. The Home Agents 4059 List MAY be implemented in any manner consistent with the external 4060 behavior described in this document. 4062 Each home agent maintains a separate Home Agents List for each link 4063 on which it is serving as a home agent. A new entry is created or an 4064 existing entry is updated in response to receipt of a valid Router 4065 Advertisement in which the Home Agent (H) bit is set. Each Home 4066 Agents List entry conceptually contains the following fields: 4068 o The link-local IP address of a home agent on the link. This 4069 address is learned through the Source Address of the Router 4070 Advertisements [17] received from the router. 4072 o One or more global IP addresses for this home agent. Global 4073 addresses are learned through Prefix Information options with the 4074 Router Address (R) bit set and received in Router Advertisements 4075 from this link-local address. Global addresses for the router in 4076 a Home Agents List entry MUST be deleted once the prefix 4077 associated with that address is no longer valid [17]. 4079 o The remaining lifetime of this Home Agents List entry. If a Home 4080 Agent Information Option is present in a Router Advertisement 4081 received from a home agent, the lifetime of the Home Agents List 4082 entry representing that home agent is initialized from the Home 4083 Agent Lifetime field in the option (if present); otherwise, the 4084 lifetime is initialized from the Router Lifetime field in the 4085 received Router Advertisement. If Home Agents List entry lifetime 4086 reaches zero, the entry MUST be deleted from the Home Agents List. 4088 o The preference for this home agent; higher values indicate a more 4089 preferable home agent. The preference value is taken from the 4090 Home Agent Preference field in the received Router Advertisement, 4091 if the Router Advertisement contains a Home Agent Information 4092 Option and is otherwise set to the default value of 0. A home 4093 agent uses this preference in ordering the Home Agents List when 4094 it sends an ICMP Home Agent Address Discovery message. 4096 10.2. Processing Mobility Headers 4098 All IPv6 home agents MUST observe the rules described in Section 9.2 4099 when processing Mobility Headers. 4101 10.3. Processing Bindings 4103 10.3.1. Primary Care-of Address Registration 4105 When a node receives a Binding Update, it MUST validate it and 4106 determine the type of Binding Update according to the steps described 4107 in Section 9.5.1. Furthermore, it MUST authenticate the Binding 4108 Update as described in Section 5.1. An authorization step specific 4109 for the home agent is also needed to ensure that only the right node 4110 can control a particular home address. This is provided through the 4111 home address unequivocally identifying the security association that 4112 must be used. 4114 This section describes the processing of a valid and authorized 4115 Binding Update when it requests the registration of the mobile node's 4116 primary care-of address. 4118 To begin processing the Binding Update, the home agent MUST perform 4119 the following sequence of tests: 4121 o If the node implements only correspondent node functionality, or 4122 has not been configured to act as a home agent, then the node MUST 4123 reject the Binding Update. The node MUST also return a Binding 4124 Acknowledgement to the mobile node, in which the Status field is 4125 set to 131 (home registration not supported). 4127 o Else, if the home address for the binding (the Home Address field 4128 in the packet's Home Address option) is not an on-link IPv6 4129 address with respect to the home agent's current Prefix List, then 4130 the home agent MUST reject the Binding Update and SHOULD return a 4131 Binding Acknowledgement to the mobile node, in which the Status 4132 field is set to 132 (not home subnet). 4134 o Else, if the home agent chooses to reject the Binding Update for 4135 any other reason (e.g., insufficient resources to serve another 4136 mobile node as a home agent), then the home agent SHOULD return a 4137 Binding Acknowledgement to the mobile node, in which the Status 4138 field is set to an appropriate value to indicate the reason for 4139 the rejection. 4141 o A Home Address destination option MUST be present in the message. 4142 It MUST be validated as described in Section 9.3.1 with the 4143 following additional rule. The Binding Cache entry existence test 4144 MUST NOT be done for IPsec packets when the Home Address option 4145 contains an address for which the receiving node could act as a 4146 home agent. 4148 If home agent accepts the Binding Update, it MUST then create a new 4149 entry in its Binding Cache for this mobile node or update its 4150 existing Binding Cache entry, if such an entry already exists. The 4151 Home Address field as received in the Home Address option provides 4152 the home address of the mobile node. 4154 The home agent MUST mark this Binding Cache entry as a home 4155 registration to indicate that the node is serving as a home agent for 4156 this binding. Binding Cache entries marked as a home registration 4157 MUST be excluded from the normal cache replacement policy used for 4158 the Binding Cache (Section 9.6) and MUST NOT be removed from the 4159 Binding Cache until the expiration of the Lifetime period. 4161 Unless this home agent already has a binding for the given home 4162 address, the home agent MUST perform Duplicate Address Detection [18] 4163 on the mobile node's home link before returning the Binding 4164 Acknowledgement. This ensures that no other node on the home link 4165 was using the mobile node's home address when the Binding Update 4166 arrived. If this Duplicate Address Detection fails for the given 4167 home address or an associated link local address, then the home agent 4168 MUST reject the complete Binding Update and MUST return a Binding 4169 Acknowledgement to the mobile node, in which the Status field is set 4170 to 134 (Duplicate Address Detection failed). When the home agent 4171 sends a successful Binding Acknowledgement to the mobile node, the 4172 home agent assures to the mobile node that its address(es) will be 4173 kept unique by the home agent for as long as the lifetime was granted 4174 for the binding. 4176 The specific addresses, which are to be tested before accepting the 4177 Binding Update and later to be defended by performing Duplicate 4178 Address Detection, depend on the setting of the Link-Local Address 4179 Compatibility (L) bit, as follows: 4181 o L=0: Defend only the given address. Do not derive a link-local 4182 address. 4184 o L=1: Defend both the given non link-local unicast (home) address 4185 and the derived link-local. The link-local address is derived by 4186 replacing the subnet prefix in the mobile node's home address with 4187 the link-local prefix. 4189 The lifetime of the Binding Cache entry depends on a number of 4190 factors: 4192 o The lifetime for the Binding Cache entry MUST NOT be greater than 4193 the Lifetime value specified in the Binding Update. 4195 o The lifetime for the Binding Cache entry MUST NOT be greater than 4196 the remaining valid lifetime for the subnet prefix in the mobile 4197 node's home address specified with the Binding Update. The 4198 remaining valid lifetime for this prefix is determined by the home 4199 agent based on its own Prefix List entry [17]. 4201 The remaining preferred lifetime SHOULD NOT have any impact on the 4202 lifetime for the binding cache entry. 4204 The home agent MUST remove a binding when the valid lifetime of 4205 the prefix associated with it expires. 4207 o The home agent MAY further decrease the specified lifetime for the 4208 binding, for example based on a local policy. The resulting 4209 lifetime is stored by the home agent in the Binding Cache entry, 4210 and this Binding Cache entry MUST be deleted by the home agent 4211 after the expiration of this lifetime. 4213 Regardless of the setting of the Acknowledge (A) bit in the Binding 4214 Update, the home agent MUST return a Binding Acknowledgement to the 4215 mobile node constructed as follows: 4217 o The Status field MUST be set to a value indicating success. The 4218 value 1 (accepted but prefix discovery necessary) MUST be used if 4219 the subnet prefix of the specified home address is deprecated, or 4220 becomes deprecated during the lifetime of the binding, or becomes 4221 invalid at the end of the lifetime. The value 0 MUST be used 4222 otherwise. For the purposes of comparing the binding and prefix 4223 lifetimes, the prefix lifetimes are first converted into units of 4224 four seconds by ignoring the two least significant bits. 4226 o The Key Management Mobility Capability (K) bit is set if the 4227 following conditions are all fulfilled, and cleared otherwise: 4229 * The Key Management Mobility Capability (K) bit was set in the 4230 Binding Update. 4232 * The IPsec security associations between the mobile node and the 4233 home agent have been established dynamically. 4235 * The home agent has the capability to update its endpoint in the 4236 used key management protocol to the new care-of address every 4237 time it moves. 4239 Depending on the final value of the bit in the Binding 4240 Acknowledgement, the home agent SHOULD perform the following 4241 actions: 4243 K = 0 4245 Discard key management connections, if any, to the old care-of 4246 address. If the mobile node did not have a binding before 4247 sending this Binding Update, discard the connections to the 4248 home address. 4250 K = 1 4252 Move the peer endpoint of the key management protocol 4253 connection, if any, to the new care-of address. 4255 o The Sequence Number field MUST be copied from the Sequence Number 4256 given in the Binding Update. 4258 o The Lifetime field MUST be set to the remaining lifetime for the 4259 binding as set by the home agent in its home registration Binding 4260 Cache entry for the mobile node, as described above. 4262 o If the home agent stores the Binding Cache entry in nonvolatile 4263 storage, then the Binding Refresh Advice mobility option MUST be 4264 omitted. Otherwise, the home agent MAY include this option to 4265 suggest that the mobile node refreshes its binding before the 4266 actual lifetime of the binding ends. 4268 If the Binding Refresh Advice mobility option is present, the 4269 Refresh Interval field in the option MUST be set to a value less 4270 than the Lifetime value being returned in the Binding 4271 Acknowledgement. This indicates that the mobile node SHOULD 4272 attempt to refresh its home registration at the indicated shorter 4273 interval. The home agent MUST still retain the registration for 4274 the Lifetime period, even if the mobile node does not refresh its 4275 registration within the Refresh period. 4277 The rules for selecting the Destination IP address (and possibly 4278 routing header construction) for the Binding Acknowledgement to the 4279 mobile node are the same as in Section 9.5.4. 4281 In addition, the home agent MUST follow the procedure defined in 4282 Section 10.4.1 to intercept packets on the mobile node's home link 4283 addressed to the mobile node, while the home agent is serving as the 4284 home agent for this mobile node. The home agent MUST also be 4285 prepared to accept reverse tunneled packets from the new care-of 4286 address of the mobile node, as described in Section 10.4.5. Finally, 4287 the home agent MUST also propagate new home network prefixes, as 4288 described in Section 10.6. 4290 10.3.2. Primary Care-of Address De-Registration 4292 A binding may need to be de-registered when the mobile node returns 4293 home or when the mobile node knows that it will not have any care-of 4294 addresses in the visited network. 4296 A Binding Update is validated and authorized in the manner described 4297 in the previous section; note that when the mobile node de-registers 4298 when it is at home, it MAY choose to omit the Home Address 4299 destination option, in which case the mobile node's home address is 4300 the source IP address of the de-registration Binding Update. This 4301 section describes the processing of a valid Binding Update that 4302 requests the receiving node to no longer serve as its home agent, de- 4303 registering its primary care-of address. 4305 To begin processing the Binding Update, the home agent MUST perform 4306 the following test: 4308 o If the receiving node has no entry marked as a home registration 4309 in its Binding Cache for this mobile node, then this node MUST 4310 reject the Binding Update and SHOULD return a Binding 4311 Acknowledgement to the mobile node, in which the Status field is 4312 set to 133 (not home agent for this mobile node). 4314 If the home agent does not reject the Binding Update as described 4315 above, then the home agent MUST return a Binding Acknowledgement to 4316 the mobile node, constructed as follows: 4318 o The Status field MUST be set to a value 0, indicating success. 4320 o The Key Management Mobility Capability (K) bit is set or cleared 4321 and actions based on its value are performed as described in the 4322 previous section. The mobile node's home address is used as its 4323 new care-of address for the purposes of moving the key management 4324 connection to a new endpoint. 4326 o The Sequence Number field MUST be copied from the Sequence Number 4327 given in the Binding Update. 4329 o The Lifetime field MUST be set to zero. 4331 o The Binding Refresh Advice mobility option MUST be omitted. 4333 The rules for selecting the Destination IP address (and, if required, 4334 routing header construction) for the Binding Acknowledgement to the 4335 mobile node are the same as in the previous section. When the Status 4336 field in the Binding Acknowledgement is greater than or equal to 128 4337 and the Source Address of the Binding Update is on the home link, and 4338 the Binding Update came from a mobile node on the same link, the home 4339 agent MUST send it to the mobile node's link layer address (retrieved 4340 either from the Binding Update or through Neighbor Solicitation). 4342 When a mobile node sends a binding update to refresh the binding from 4343 the visited link and soon after moves to the home link and sends a 4344 de-registration binding update, a race condition can happen if the 4345 first binding update gets delayed. The delayed binding update can 4346 cause the home agent to create a new binding cache entry for a mobile 4347 node that had just attached to the home link and successfully deleted 4348 the binding. This would prevent the mobile node from using its home 4349 address from the home link. 4351 In order to prevent this, the home agent SHOULD NOT remove the 4352 binding cache entry immediately after receiving the deregistration 4353 binding update from the mobile node. It SHOULD mark the binding 4354 cache entry as invalid, and MUST stop intercepting packets on the 4355 mobile node's home link that are addressed to the mobile node 4356 (Section 10.4.1). The home agent should wait for 4357 MAX_DELETE_BCE_TIMEOUT (Section 12) seconds before removing the 4358 binding cache entry completely. In the scenario described above, if 4359 the home agent receives the delayed binding update that the mobile 4360 node sent from the visited link, it would reject the message since 4361 the sequence number would be less than the last received 4362 deregistration binding update from the home link. The home agent 4363 would then send a Binding Acknowledgment with status '135' (Sequence 4364 number out of window) to the care of address on the visited link. 4365 The mobile node can continue using the home address from the home 4366 link. 4368 10.4. Packet Processing 4370 10.4.1. Intercepting Packets for a Mobile Node 4372 While a node is serving as the home agent for a mobile node it MUST 4373 attempt to intercept packets on the mobile node's home link that are 4374 addressed to the mobile node. 4376 In order to do this, when a node begins serving as the home agent it 4377 MUST have performed Duplicate Address Detection (as specified in 4378 Section 10.3.1), and subsequently it MUST multicast onto the home 4379 link a Neighbor Advertisement message [17] on behalf of the mobile 4380 node. For the home address specified in the Binding Update, the home 4381 agent sends a Neighbor Advertisement message [17] to the all-nodes 4382 multicast address on the home link to advertise the home agent's own 4383 link-layer address for this IP address on behalf of the mobile node. 4384 If the Link-Layer Address Compatibility (L) flag has been specified 4385 in the Binding Update, the home agent MUST do the same for the link- 4386 local address of the mobile node. 4388 All fields in each Neighbor Advertisement message SHOULD be set in 4389 the same way they would be set by the mobile node if it was sending 4390 this Neighbor Advertisement [17] while at home, with the following 4391 exceptions: 4393 o The Target Address in the Neighbor Advertisement MUST be set to 4394 the specific IP address for the mobile node. 4396 o The Advertisement MUST include a Target Link-layer Address option 4397 specifying the home agent's link-layer address. 4399 o The Router (R) bit in the Advertisement MUST be set to zero. 4401 o The Solicited Flag (S) in the Advertisement MUST NOT be set, since 4402 it was not solicited by any Neighbor Solicitation. 4404 o The Override Flag (O) in the Advertisement MUST be set, indicating 4405 that the Advertisement SHOULD override any existing Neighbor Cache 4406 entry at any node receiving it. 4408 o The Source Address in the IPv6 header MUST be set to the home 4409 agent's IP address on the interface used to send the 4410 advertisement. 4412 Any node on the home link that receives one of the Neighbor 4413 Advertisement messages (described above) will update its Neighbor 4414 Cache to associate the mobile node's address with the home agent's 4415 link layer address, causing it to transmit any future packets 4416 normally destined to the mobile node to the mobile node's home agent. 4417 Since multicasting on the local link (such as Ethernet) is typically 4418 not guaranteed to be reliable, the home agent MAY retransmit this 4419 Neighbor Advertisement message up to MAX_NEIGHBOR_ADVERTISEMENT (see 4420 [17]) times to increase its reliability. It is still possible that 4421 some nodes on the home link will not receive any of the Neighbor 4422 Advertisements, but these nodes will eventually be able to detect the 4423 link-layer address change for the mobile node's address through use 4424 of Neighbor Unreachability Detection [17]. 4426 While a node is serving as a home agent for some mobile node, the 4427 home agent uses IPv6 Neighbor Discovery [17] to intercept unicast 4428 packets on the home link addressed to the mobile node. In order to 4429 intercept packets in this way, the home agent MUST act as a proxy for 4430 this mobile node and reply to any received Neighbor Solicitations for 4431 it. When a home agent receives a Neighbor Solicitation, it MUST 4432 check if the Target Address specified in the message matches the 4433 address of any mobile node for which it has a Binding Cache entry 4434 marked as a home registration. 4436 If such an entry exists in the home agent's Binding Cache, the home 4437 agent MUST reply to the Neighbor Solicitation with a Neighbor 4438 Advertisement giving the home agent's own link-layer address as the 4439 link-layer address for the specified Target Address. In addition, 4440 the Router (R) bit in the Advertisement MUST be set to zero. Acting 4441 as a proxy in this way allows other nodes on the mobile node's home 4442 link to resolve the mobile node's address and for the home agent to 4443 defend these addresses on the home link for Duplicate Address 4444 Detection [17]. 4446 10.4.2. Processing Intercepted Packets 4448 For any packet sent to a mobile node from the mobile node's home 4449 agent (in which the home agent is the original sender of the packet), 4450 the home agent is operating as a correspondent node of the mobile 4451 node for this packet and the procedures described in Section 9.3.2 4452 apply. The home agent then uses a routing header to route the packet 4453 to the mobile node by way of the primary care-of address in the home 4454 agent's Binding Cache. 4456 While the mobile node is away from home, the home agent intercepts 4457 any packets on the home link addressed to the mobile node's home 4458 address, as described in Section 10.4.1. In order to forward each 4459 intercepted packet to the mobile node, the home agent MUST tunnel the 4460 packet to the mobile node using IPv6 encapsulation [6]. When a home 4461 agent encapsulates an intercepted packet for forwarding to the mobile 4462 node, the home agent sets the Source Address in the new tunnel IP 4463 header to the home agent's own IP address and sets the Destination 4464 Address in the tunnel IP header to the mobile node's primary care-of 4465 address. When received by the mobile node, normal processing of the 4466 tunnel header [6] will result in decapsulation and processing of the 4467 original packet by the mobile node. 4469 However, packets addressed to the mobile node's link-local address 4470 MUST NOT be tunneled to the mobile node. Instead, these packets MUST 4471 be discarded and the home agent SHOULD return an ICMP Destination 4472 Unreachable, Code 3, message to the packet's Source Address (unless 4473 this Source Address is a multicast address). 4475 Interception and tunneling of the following multicast addressed 4476 packets on the home network are only done if the home agent supports 4477 multicast group membership control messages from the mobile node as 4478 described in the next section. Tunneling of multicast packets to a 4479 mobile node follows similar limitations to those defined above for 4480 unicast packets addressed to the mobile node's link-local address. 4481 Multicast packets addressed to a multicast address with link-local 4482 scope [15], to which the mobile node is subscribed, MUST NOT be 4483 tunneled to the mobile node. These packets SHOULD be silently 4484 discarded (after delivering to other local multicast recipients). 4485 Multicast packets addressed to a multicast address with a scope 4486 larger than link-local, but smaller than global (e.g., site-local and 4487 organization-local [15]), to which the mobile node is subscribed, 4488 SHOULD NOT be tunneled to the mobile node. Multicast packets 4489 addressed with a global scope, to which the mobile node has 4490 successfully subscribed, MUST be tunneled to the mobile node. 4492 Before tunneling a packet to the mobile node, the home agent MUST 4493 perform any IPsec processing as indicated by the security policy data 4494 base. 4496 10.4.3. Multicast Membership Control 4498 This section is a prerequisite for the multicast data packet 4499 forwarding, described in the previous section. If this support is 4500 not provided, multicast group membership control messages are 4501 silently ignored. 4503 In order to forward multicast data packets from the home network to 4504 all the proper mobile nodes, the home agent SHOULD be capable of 4505 receiving tunneled multicast group membership control information 4506 from the mobile node in order to determine which groups the mobile 4507 node has subscribed to. These multicast group membership messages 4508 are Listener Report messages specified in MLD [8] or in other 4509 protocols such as [38]. 4511 The messages are issued by the mobile node, but sent through the 4512 reverse tunnel to the home agent. These messages are issued whenever 4513 the mobile node decides to enable reception of packets for a 4514 multicast group or in response to an MLD Query from the home agent. 4515 The mobile node will also issue multicast group control messages to 4516 disable reception of multicast packets when it is no longer 4517 interested in receiving multicasts for a particular group. 4519 To obtain the mobile node's current multicast group membership the 4520 home agent must periodically transmit MLD Query messages through the 4521 tunnel to the mobile node. These MLD periodic transmissions will 4522 ensure the home agent has an accurate record of the groups in which 4523 the mobile node is interested despite packet losses of the mobile 4524 node's MLD group membership messages. 4526 All MLD packets are sent directly between the mobile node and the 4527 home agent. Since all of these packets are destined to a link-scope 4528 multicast address and have a hop limit of 1, there is no direct 4529 forwarding of such packets between the home network and the mobile 4530 node. The MLD packets between the mobile node and the home agent are 4531 encapsulated within the same tunnel header used for other packet 4532 flows between the mobile node and home agent. 4534 Note that at this time, even though a link-local source is used on 4535 MLD packets, no functionality depends on these addresses being 4536 unique, nor do they elicit direct responses. All MLD messages are 4537 sent to multicast destinations. To avoid ambiguity on the home 4538 agent, due to mobile nodes which may choose identical link-local 4539 source addresses for their MLD function, it is necessary for the home 4540 agent to identify which mobile node was actually the issuer of a 4541 particular MLD message. This may be accomplished by noting which 4542 tunnel such an MLD arrived by, which IPsec SA was used, or by other 4543 distinguishing means. 4545 This specification puts no requirement on how the functions in this 4546 section and the multicast forwarding in Section 10.4.2 are to be 4547 achieved. At the time of this writing it was thought that a full 4548 IPv6 multicast router function would be necessary on the home agent, 4549 but it may be possible to achieve the same effects through a "proxy 4550 MLD" application coupled with kernel multicast forwarding. This may 4551 be the subject of future specifications. 4553 10.4.4. Stateful Address Autoconfiguration 4555 This section describes how home agents support the use of stateful 4556 address autoconfiguration mechanisms such as DHCPv6 [29] from the 4557 mobile nodes. If this support is not provided, then the M and O bits 4558 must remain cleared on the Mobile Prefix Advertisement Messages. Any 4559 mobile node which sends DHCPv6 messages to the home agent without 4560 this support will not receive a response. 4562 If DHCPv6 is used, packets are sent with link-local source addresses 4563 either to a link-scope multicast address or a link-local address. 4564 Mobile nodes desiring to locate a DHCPv6 service may reverse tunnel 4565 standard DHCPv6 packets to the home agent. Since these link-scope 4566 packets cannot be forwarded onto the home network, it is necessary 4567 for the home agent to either implement a DHCPv6 relay agent or a 4568 DHCPv6 server function itself. The arriving tunnel or IPsec SA of 4569 DHCPv6 link-scope messages from the mobile node must be noted so that 4570 DHCPv6 responses may be sent back to the appropriate mobile node. 4571 DHCPv6 messages sent to the mobile node with a link-local destination 4572 must be tunneled within the same tunnel header used for other packet 4573 flows. 4575 10.4.5. Handling Reverse Tunneled Packets 4577 Unless a binding has been established between the mobile node and a 4578 correspondent node, traffic from the mobile node to the correspondent 4579 node goes through a reverse tunnel. Home agents MUST support reverse 4580 tunneling as follows: 4582 o The tunneled traffic arrives to the home agent's address using 4583 IPv6 encapsulation [6]. 4585 o Depending on the security policies used by the home agent, reverse 4586 tunneled packets MAY be discarded unless accompanied by a valid 4587 ESP header. The support for authenticated reverse tunneling 4588 allows the home agent to protect the home network and 4589 correspondent nodes from malicious nodes masquerading as a mobile 4590 node. 4592 o Otherwise, when a home agent decapsulates a tunneled packet from 4593 the mobile node, the home agent MUST verify that the Source 4594 Address in the tunnel IP header is the mobile node's primary 4595 care-of address. Otherwise, any node in the Internet could send 4596 traffic through the home agent and escape ingress filtering 4597 limitations. This simple check forces the attacker to know the 4598 current location of the real mobile node and be able to defeat 4599 ingress filtering. This check is not necessary if the reverse- 4600 tunneled packet is protected by ESP in tunnel mode. 4602 10.4.6. Protecting Return Routability Packets 4604 The return routability procedure, described in Section 5.2.5, assumes 4605 that the confidentiality of the Home Test Init and Home Test messages 4606 is protected as they are tunneled between the home agent and the 4607 mobile node. Therefore, the home agent MUST support tunnel mode 4608 IPsec ESP for the protection of packets belonging to the return 4609 routability procedure. Support for a non-null encryption transform 4610 and authentication algorithm MUST be available. It is not necessary 4611 to distinguish between different kinds of packets during the return 4612 routability procedure. 4614 Security associations are needed to provide this protection. When 4615 the care-of address for the mobile node changes as a result of an 4616 accepted Binding Update, special treatment is needed for the next 4617 packets sent using these security associations. The home agent MUST 4618 set the new care-of address as the destination address of these 4619 packets, as if the outer header destination address in the security 4620 association had changed [12]. 4622 The above protection SHOULD be used with all mobile nodes. The use 4623 is controlled by configuration of the IPsec security policy database 4624 both at the mobile node and at the home agent. 4626 As described earlier, the Binding Update and Binding Acknowledgement 4627 messages require protection between the home agent and the mobile 4628 node. The Mobility Header protocol carries both these messages as 4629 well as the return routability messages. From the point of view of 4630 the security policy database these messages are indistinguishable. 4631 When IPsec is used to protect return routability signaling or payload 4632 packets, this protection MUST only be applied to the return 4633 routability packets entering the IPv6 encapsulated tunnel interface 4634 between the mobile node and the home agent. This can be achieved, 4635 for instance, by defining the security policy database entries 4636 specifically for the tunnel interface. That is, the policy entries 4637 are not generally applied on all traffic on the physical interface(s) 4638 of the nodes, but rather only on traffic that enters the tunnel. 4639 This makes use of per-interface security policy database entries [2] 4640 specific to the tunnel interface (the node's attachment to the tunnel 4641 [5]). 4643 10.5. Dynamic Home Agent Address Discovery 4645 This section describes an optional mechanism by which a home agent 4646 can help mobile nodes to discover the addresses of other home agents 4647 on the mobile node's home network. The home agent keeps track of the 4648 other home agents on the same link and responds to queries sent by 4649 the mobile node. 4651 10.5.1. Receiving Router Advertisement Messages 4653 For each link on which a router provides service as a home agent, the 4654 router maintains a Home Agents List recording information about all 4655 other home agents on that link. This list is used in the dynamic 4656 home agent address discovery mechanism; the mobile node uses the list 4657 as described in Section 11.4.1. The information for the list is 4658 learned through receipt of the periodic unsolicited multicast Router 4659 Advertisements, in a manner similar to the Default Router List 4660 conceptual data structure maintained by each host for Neighbor 4661 Discovery [17]. In the construction of the Home Agents List, the 4662 Router Advertisements are from each (other) home agent on the link 4663 and the Home Agent (H) bit is set in them. 4665 On receipt of a valid Router Advertisement, as defined in the 4666 processing algorithm specified for Neighbor Discovery [17], the home 4667 agent performs the following steps in addition to any steps already 4668 required of it by Neighbor Discovery: 4670 o If the Home Agent (H) bit in the Router Advertisement is not set, 4671 delete the sending node's entry in the current Home Agents List 4672 (if one exists). Skip all the following steps. 4674 o Otherwise, extract the Source Address from the IP header of the 4675 Router Advertisement. This is the link-local IP address on this 4676 link of the home agent sending this Advertisement [17]. 4678 o Determine the preference for this home agent. If the Router 4679 Advertisement contains a Home Agent Information Option, then the 4680 preference is taken from the Home Agent Preference field in the 4681 option; otherwise, the default preference of 0 MUST be used. 4683 o Determine the lifetime for this home agent. If the Router 4684 Advertisement contains a Home Agent Information Option, then the 4685 lifetime is taken from the Home Agent Lifetime field in the 4686 option; otherwise, the lifetime specified by the Router Lifetime 4687 field in the Router Advertisement SHOULD be used. 4689 o If the link-local address of the home agent sending this 4690 Advertisement is already present in this home agent's Home Agents 4691 List and the received home agent lifetime value is zero, 4692 immediately delete this entry in the Home Agents List. 4694 o Otherwise, if the link-local address of the home agent sending 4695 this Advertisement is already present in the receiving home 4696 agent's Home Agents List, reset its lifetime and preference to the 4697 values determined above. 4699 o If the link-local address of the home agent sending this 4700 Advertisement is not already present in the Home Agents List 4701 maintained by the receiving home agent, and the lifetime for the 4702 sending home agent is non-zero, create a new entry in the list, 4703 and initialize its lifetime and preference to the values 4704 determined above. 4706 o If the Home Agents List entry for the link-local address of the 4707 home agent sending this Advertisement was not deleted as described 4708 above, determine any global address(es) of the home agent based on 4709 each Prefix Information option received in this Advertisement in 4710 which the Router Address (R) bit is set (Section 7.2). Add all 4711 such global addresses to the list of global addresses in this Home 4712 Agents List entry. 4714 A home agent SHOULD maintain an entry in its Home Agents List for 4715 each valid home agent address until that entry's lifetime expires, 4716 after which time the entry MUST be deleted. 4718 As described in Section 11.4.1, a mobile node attempts dynamic home 4719 agent address discovery by sending an ICMP Home Agent Address 4720 Discovery Request message to the Mobile IPv6 Home-Agents anycast 4721 address [7] for its home IP subnet prefix. A home agent receiving a 4722 Home Agent Address Discovery Request message that serves this subnet 4723 SHOULD return an ICMP Home Agent Address Discovery Reply message to 4724 the mobile node with the Source Address of the Reply packet set to 4725 one of the global unicast addresses of the home agent. The Home 4726 Agent Addresses field in the Reply message is constructed as follows: 4728 o The Home Agent Addresses field SHOULD contain all global IP 4729 addresses for each home agent currently listed in this home 4730 agent's own Home Agents List (Section 10.1). 4732 o The IP addresses in the Home Agent Addresses field SHOULD be 4733 listed in order of decreasing preference values, based either on 4734 the respective advertised preference from a Home Agent Information 4735 option or on the default preference of 0 if no preference is 4736 advertised (or on the configured home agent preference for this 4737 home agent itself). 4739 o Among home agents with equal preference, their IP addresses in the 4740 Home Agent Addresses field SHOULD be listed in an order randomized 4741 with respect to other home agents with equal preference every time 4742 a Home Agent Address Discovery Reply message is returned by this 4743 home agent. 4745 o If more than one global IP address is associated with a home 4746 agent, these addresses SHOULD be listed in a randomized order. 4748 o The home agent SHOULD reduce the number of home agent IP addresses 4749 so that the packet fits within the minimum IPv6 MTU [5]. The home 4750 agent addresses selected for inclusion in the packet SHOULD be 4751 those from the complete list with the highest preference. This 4752 limitation avoids the danger of the Reply message packet being 4753 fragmented (or rejected by an intermediate router with an ICMP 4754 Packet Too Big message [16]). 4756 10.6. Sending Prefix Information to the Mobile Node 4758 10.6.1. List of Home Network Prefixes 4760 Mobile IPv6 arranges to propagate relevant prefix information to the 4761 mobile node when it is away from home, so that it may be used in 4762 mobile node home address configuration and in network renumbering. 4763 In this mechanism, mobile nodes away from home receive Mobile Prefix 4764 Advertisements messages. These messages include Prefix Information 4765 Options for the prefixes configured on the home subnet interface(s) 4766 of the home agent. 4768 If there are multiple home agents, differences in the advertisements 4769 sent by different home agents can lead to an inability to use a 4770 particular home address when changing to another home agent. In 4771 order to ensure that the mobile nodes get the same information from 4772 different home agents, it is preferred that all of the home agents on 4773 the same link be configured in the same manner. 4775 To support this, the home agent monitors prefixes advertised by 4776 itself and other home agents on the home link. In Neighbor Discovery 4777 (RFC 4861 [17]) it is acceptable for two routers to advertise 4778 different sets of prefixes on the same link. For home agents, the 4779 differences should be detected for a given home address because the 4780 mobile node communicates only with one home agent at a time and the 4781 mobile node needs to know the full set of prefixes assigned to the 4782 home link. All other comparisons of Router Advertisements are as 4783 specified in Section 6.2.7 of RFC 4861. 4785 10.6.2. Scheduling Prefix Deliveries 4787 A home agent serving a mobile node will schedule the delivery of the 4788 new prefix information to that mobile node when any of the following 4789 conditions occur: 4791 MUST: 4793 o The state of the flags changes for the prefix of the mobile node's 4794 registered home address. 4796 o The valid or preferred lifetime is reconfigured or changes for any 4797 reason other than advancing real time. 4799 o The mobile node requests the information with a Mobile Prefix 4800 Solicitation (see Section 11.4.2). 4802 SHOULD: 4804 o A new prefix is added to the home subnet interface(s) of the home 4805 agent. 4807 MAY: 4809 o The valid or preferred lifetime or the state of the flags changes 4810 for a prefix which is not used in any Binding Cache entry for this 4811 mobile node. 4813 The home agent uses the following algorithm to determine when to send 4814 prefix information to the mobile node. 4816 o If a mobile node sends a solicitation, answer right away. 4818 o If no Mobile Prefix Advertisement has been sent to the mobile node 4819 in the last MaxMobPfxAdvInterval seconds (see Section 13), then 4820 ensure that a transmission is scheduled. The actual transmission 4821 time is randomized as described below. 4823 o If a prefix matching the mobile node's home registration is added 4824 on the home subnet interface or if its information changes in any 4825 way that does not deprecate the mobile node's address, ensure that 4826 a transmission is scheduled. The actual transmission time is 4827 randomized as described below. 4829 o If a home registration expires, cancel any scheduled 4830 advertisements to the mobile node. 4832 The list of prefixes is sent in its entirety in all cases. 4834 If the home agent has already scheduled the transmission of a Mobile 4835 Prefix Advertisement to the mobile node, then the home agent will 4836 replace the advertisement with a new one to be sent at the scheduled 4837 time. 4839 Otherwise, the home agent computes a fresh value for RAND_ADV_DELAY 4840 which offsets from the current time for the scheduled transmission. 4841 First calculate the maximum delay for the scheduled Advertisement: 4843 MaxScheduleDelay = min (MaxMobPfxAdvInterval, Preferred Lifetime), 4845 where MaxMobPfxAdvInterval is as defined in Section 12. Then compute 4846 the final delay for the advertisement: 4848 RAND_ADV_DELAY = MinMobPfxAdvInterval + 4849 (rand() % abs(MaxScheduleDelay - MinMobPfxAdvInterval)) 4851 Here rand() returns a random integer value in the range of 0 to the 4852 maximum possible integer value. This computation is expected to 4853 alleviate bursts of advertisements when prefix information changes. 4854 In addition, a home agent MAY further reduce the rate of packet 4855 transmission by further delaying individual advertisements, when 4856 necessary to avoid overwhelming local network resources. The home 4857 agent SHOULD periodically continue to retransmit an unsolicited 4858 Advertisement to the mobile node, until it is acknowledged by the 4859 receipt of a Mobile Prefix Solicitation from the mobile node. 4861 The home agent MUST wait PREFIX_ADV_TIMEOUT (see Section 12) before 4862 the first retransmission and double the retransmission wait time for 4863 every succeeding retransmission until a maximum number of 4864 PREFIX_ADV_RETRIES attempts (see Section 12) has been tried. If the 4865 mobile node's bindings expire before the matching Binding Update has 4866 been received, then the home agent MUST NOT attempt any more 4867 retransmissions, even if not all PREFIX_ADV_RETRIES have been 4868 retransmitted. In the mean time, if the mobile node sends another 4869 Binding Update without returning home, then the home agent SHOULD 4870 begin transmitting the unsolicited Advertisement again. 4872 If some condition, as described above, occurs on the home link and 4873 causes another Prefix Advertisement to be sent to the mobile node, 4874 before the mobile node acknowledges a previous transmission, the home 4875 agent SHOULD combine any Prefix Information options in the 4876 unacknowledged Mobile Prefix Advertisement into a new Advertisement. 4877 The home agent then discards the old Advertisement. 4879 10.6.3. Sending Advertisements 4881 When sending a Mobile Prefix Advertisement to the mobile node, the 4882 home agent MUST construct the packet as follows: 4884 o The Source Address in the packet's IPv6 header MUST be set to the 4885 home agent's IP address to which the mobile node addressed its 4886 current home registration or its default global home agent address 4887 if no binding exists. 4889 o If the advertisement was solicited, it MUST be destined to the 4890 source address of the solicitation. If it was triggered by prefix 4891 changes or renumbering, the advertisement's destination will be 4892 the mobile node's home address in the binding which triggered the 4893 rule. 4895 o A type 2 routing header MUST be included with the mobile node's 4896 home address. 4898 o IPsec headers MUST be supported and SHOULD be used. 4900 o The home agent MUST send the packet as it would any other unicast 4901 IPv6 packet that it originates. 4903 o Set the Managed Address Configuration (M) flag if the 4904 corresponding flag has been set in any of the Router 4905 Advertisements from which the prefix information has been learned 4906 (including the ones sent by this home agent). 4908 o Set the Other Stateful Configuration (O) flag if the corresponding 4909 flag has been set in any of the Router Advertisements from which 4910 the prefix information has been learned (including the ones sent 4911 by this home agent). 4913 10.6.4. Lifetimes for Changed Prefixes 4915 As described in Section 10.3.1, the lifetime returned by the home 4916 agent in a Binding Acknowledgement MUST NOT be greater than the 4917 remaining valid lifetime for the subnet prefix in the mobile node's 4918 home address. This limit on the binding lifetime serves to prohibit 4919 use of a mobile node's home address after it becomes invalid. 4921 11. Mobile Node Operation 4923 11.1. Conceptual Data Structures 4925 Each mobile node MUST maintain a Binding Update List. 4927 The Binding Update List records information for each Binding Update 4928 sent by this mobile node, in which the lifetime of the binding has 4929 not yet expired. The Binding Update List includes all bindings sent 4930 by the mobile node either to its home agent or correspondent nodes. 4931 It also contains Binding Updates which are waiting for the completion 4932 of the return routability procedure before they can be sent. 4933 However, for multiple Binding Updates sent to the same destination 4934 address, the Binding Update List contains only the most recent 4935 Binding Update (i.e., with the greatest Sequence Number value) sent 4936 to that destination. The Binding Update List MAY be implemented in 4937 any manner consistent with the external behavior described in this 4938 document. 4940 Each Binding Update List entry conceptually contains the following 4941 fields: 4943 o The IP address of the node to which a Binding Update was sent. 4945 o The home address for which that Binding Update was sent. 4947 o The care-of address sent in that Binding Update. This value is 4948 necessary for the mobile node to determine if it has sent a 4949 Binding Update while giving its new care-of address to this 4950 destination after changing its care-of address. 4952 o The initial value of the Lifetime field sent in that Binding 4953 Update. 4955 o The remaining lifetime of that binding. This lifetime is 4956 initialized from the Lifetime value sent in the Binding Update and 4957 is decremented until it reaches zero, at which time this entry 4958 MUST be deleted from the Binding Update List. 4960 o The maximum value of the Sequence Number field sent in previous 4961 Binding Updates to this destination. The Sequence Number field is 4962 16 bits long and all comparisons between Sequence Number values 4963 MUST be performed modulo 2**16 (see Section 9.5.1). 4965 o The time at which a Binding Update was last sent to this 4966 destination, as needed to implement the rate limiting restriction 4967 for sending Binding Updates. 4969 o The state of any retransmissions needed for this Binding Update. 4970 This state includes the time remaining until the next 4971 retransmission attempt for the Binding Update and the current 4972 state of the exponential back-off mechanism for retransmissions. 4974 o A flag specifying whether or not future Binding Updates should be 4975 sent to this destination. The mobile node sets this flag in the 4976 Binding Update List entry when it receives an ICMP Parameter 4977 Problem, Code 1, error message in response to a return routability 4978 message or Binding Update sent to that destination, as described 4979 in Section 11.3.5. 4981 The Binding Update List is used to determine whether a particular 4982 packet is sent directly to the correspondent node or tunneled via the 4983 home agent (see Section 11.3.1). 4985 The Binding Update list also conceptually contains the following data 4986 related to running the return routability procedure. This data is 4987 relevant only for Binding Updates sent to correspondent nodes. 4989 o The time at which a Home Test Init or Care-of Test Init message 4990 was last sent to this destination, as needed to implement the rate 4991 limiting restriction for the return routability procedure. 4993 o The state of any retransmissions needed for this return 4994 routability procedure. This state includes the time remaining 4995 until the next retransmission attempt and the current state of the 4996 exponential back-off mechanism for retransmissions. 4998 o Cookie values used in the Home Test Init and Care-of Test Init 4999 messages. 5001 o Home and care-of keygen tokens received from the correspondent 5002 node. 5004 o Home and care-of nonce indices received from the correspondent 5005 node. 5007 o The time at which each of the tokens and nonces were received from 5008 the correspondent node, as needed to implement reuse while moving. 5010 11.2. Processing Mobility Headers 5012 All IPv6 mobile nodes MUST observe the rules described in Section 9.2 5013 when processing Mobility Headers. 5015 11.3. Packet Processing 5017 11.3.1. Sending Packets While Away from Home 5019 While a mobile node is away from home, it continues to use its home 5020 address, as well as also using one or more care-of addresses. When 5021 sending a packet while away from home, a mobile node MAY choose among 5022 these in selecting the address that it will use as the source of the 5023 packet, as follows: 5025 o Protocols layered over IP will generally treat the mobile node's 5026 home address as its IP source address for most packets. For 5027 packets sent that are part of transport-level connections 5028 established while the mobile node was at home, the mobile node 5029 MUST use its home address. Likewise, for packets sent that are 5030 part of transport-level connections that the mobile node may still 5031 be using after moving to a new location, the mobile node SHOULD 5032 use its home address in this way. If a binding exists, the mobile 5033 node SHOULD send the packets directly to the correspondent node. 5034 Otherwise, if a binding does not exist, the mobile node MUST use 5035 reverse tunneling. 5037 o The mobile node MAY choose to directly use one of its care-of 5038 addresses as the source of the packet, not requiring the use of a 5039 Home Address option in the packet. This is particularly useful 5040 for short-term communication that may easily be retried if it 5041 fails. Using the mobile node's care-of address as the source for 5042 such queries will generally have a lower overhead than using the 5043 mobile node's home address, since no extra options need to be used 5044 in either the query or its reply. Such packets can be routed 5045 normally, directly between their source and destination without 5046 relying on Mobile IPv6. If application running on the mobile node 5047 has no particular knowledge that the communication being sent fits 5048 within this general type of communication, however, the mobile 5049 node should not use its care-of address as the source of the 5050 packet in this way. 5052 The choice of the most efficient communications method is 5053 application specific, and outside the scope of this specification. 5054 The APIs necessary for controlling the choice are also out of 5055 scope. One example of such an API is described in the IPv6 Socket 5056 API for Source Address Selection specification [21]. 5058 o While not at its home link, the mobile node MUST NOT use the Home 5059 Address destination option when communicating with link-local 5060 peers. 5062 Similarly, the mobile node MUST NOT use the Home Address 5063 destination option for IPv6 Neighbor Discovery [17] packets. 5065 Detailed operation of these cases is described later in this section 5066 and also discussed in [31]. 5068 For packets sent by a mobile node while it is at home, no special 5069 Mobile IPv6 processing is required. Likewise, if the mobile node 5070 uses any address other than one of its home addresses as the source 5071 of a packet sent while away from home, no special Mobile IPv6 5072 processing is required. In either case, the packet is simply 5073 addressed and transmitted in the same way as any normal IPv6 packet. 5075 For packets sent by the mobile node sent while away from home using 5076 the mobile node's home address as the source, special Mobile IPv6 5077 processing of the packet is required. This can be done in the 5078 following two ways: 5080 Route Optimization 5082 This manner of delivering packets does not require going through 5083 the home network, and typically will enable faster and more 5084 reliable transmission. 5086 The mobile node needs to ensure that a Binding Cache entry exists 5087 for its home address so that the correspondent node can process 5088 the packet (Section 9.3.1 specifies the rules for Home Address 5089 Destination Option Processing at a correspondent node). The 5090 mobile node SHOULD examine its Binding Update List for an entry 5091 which fulfills the following conditions: 5093 * The Source Address field of the packet being sent is equal to 5094 the home address in the entry. 5096 * The Destination Address field of the packet being sent is equal 5097 to the address of the correspondent node in the entry. 5099 * One of the current care-of addresses of the mobile node appears 5100 as the care-of address in the entry. 5102 * The entry indicates that a binding has been successfully 5103 created. 5105 * The remaining lifetime of the binding is greater than zero. 5107 When these conditions are met, the mobile node knows that the 5108 correspondent node has a suitable Binding Cache entry. 5110 A mobile node SHOULD arrange to supply the home address in a Home 5111 Address option, and MUST set the IPv6 header's Source Address 5112 field to the care-of address which the mobile node has registered 5113 to be used with this correspondent node. The correspondent node 5114 will then use the address supplied in the Home Address option to 5115 serve the function traditionally done by the Source IP address in 5116 the IPv6 header. The mobile node's home address is then supplied 5117 to higher protocol layers and applications. 5119 Specifically: 5121 * Construct the packet using the mobile node's home address as 5122 the packet's Source Address, in the same way as if the mobile 5123 node were at home. This includes the calculation of upper 5124 layer checksums using the home address as the value of the 5125 source. 5127 * Insert a Home Address option into the packet with the Home 5128 Address field copied from the original value of the Source 5129 Address field in the packet. 5131 * Change the Source Address field in the packet's IPv6 header to 5132 one of the mobile node's care-of addresses. This will 5133 typically be the mobile node's current primary care-of address, 5134 but MUST be an address assigned to the interface on the link 5135 being used. 5137 By using the care-of address as the Source Address in the IPv6 5138 header, with the mobile node's home address instead in the Home 5139 Address option, the packet will be able to safely pass through any 5140 router implementing ingress filtering [27]. 5142 Reverse Tunneling 5144 This is the mechanism which tunnels the packets via the home 5145 agent. It is not as efficient as the above mechanism, but is 5146 needed if there is no binding yet with the correspondent node. 5148 This mechanism is used for packets that have the mobile node's 5149 home address as the Source Address in the IPv6 header, or with 5150 multicast control protocol packets as described in Section 11.3.4. 5151 Specifically: 5153 * The packet is sent to the home agent using IPv6 encapsulation 5154 [6]. 5156 * The Source Address in the tunnel packet is the primary care-of 5157 address as registered with the home agent. 5159 * The Destination Address in the tunnel packet is the home 5160 agent's address. 5162 Then, the home agent will pass the encapsulated packet to the 5163 correspondent node. 5165 11.3.2. Interaction with Outbound IPsec Processing 5167 This section sketches the interaction between outbound Mobile IPv6 5168 processing and outbound IP Security (IPsec) processing for packets 5169 sent by a mobile node while away from home. Any specific 5170 implementation MAY use algorithms and data structures other than 5171 those suggested here, but its processing MUST be consistent with the 5172 effect of the operation described here and with the relevant IPsec 5173 specifications. In the steps described below, it is assumed that 5174 IPsec is being used in transport mode [2] and that the mobile node is 5175 using its home address as the source for the packet (from the point 5176 of view of higher protocol layers or applications, as described in 5177 Section 11.3.1): 5179 o The packet is created by higher layer protocols and applications 5180 (e.g., by TCP) as if the mobile node were at home and Mobile IPv6 5181 were not being used. 5183 o Determine the outgoing interface for the packet. (Note that the 5184 selection between reverse tunneling and route optimization may 5185 imply different interfaces, particularly if tunnels are considered 5186 interfaces as well.) 5188 o As part of outbound packet processing in IP, the packet is 5189 compared against the IPsec security policy database to determine 5190 what processing is required for the packet [2]. 5192 o If IPsec processing is required, the packet is either mapped to an 5193 existing Security Association (or SA bundle), or a new SA (or SA 5194 bundle) is created for the packet, according to the procedures 5195 defined for IPsec. 5197 o Since the mobile node is away from home, the mobile is either 5198 using reverse tunneling or route optimization to reach the 5199 correspondent node. 5201 If reverse tunneling is used, the packet is constructed in the 5202 normal manner and then tunneled through the home agent. 5204 If route optimization is in use, the mobile node inserts a Home 5205 Address destination option into the packet, replacing the Source 5206 Address in the packet's IP header with the care-of address used 5207 with this correspondent node, as described in Section 11.3.1. The 5208 Destination Options header in which the Home Address destination 5209 option is inserted MUST appear in the packet after the routing 5210 header, if present, and before the IPsec (AH [3] or ESP [4]) 5211 header, so that the Home Address destination option is processed 5212 by the destination node before the IPsec header is processed. 5214 Finally, once the packet is fully assembled, the necessary IPsec 5215 authentication (and encryption, if required) processing is 5216 performed on the packet, initializing the Authentication Data in 5217 the IPsec header. 5219 RFC 2402 treatment of destination options is extended as follows. 5220 The AH authentication data MUST be calculated as if the following 5221 were true: 5223 * the IPv6 source address in the IPv6 header contains the mobile 5224 node's home address, 5226 * the Home Address field of the Home Address destination option 5227 (Section 6.3) contains the new care-of address. 5229 o This allows, but does not require, the receiver of the packet 5230 containing a Home Address destination option to exchange the two 5231 fields of the incoming packet to reach the above situation, 5232 simplifying processing for all subsequent packet headers. 5233 However, such an exchange is not required, as long as the result 5234 of the authentication calculation remains the same. 5236 When an automated key management protocol is used to create new 5237 security associations for a peer, it is important to ensure that the 5238 peer can send the key management protocol packets to the mobile node. 5239 This may not be possible if the peer is the home agent of the mobile 5240 node and the purpose of the security associations would be to send a 5241 Binding Update to the home agent. Packets addressed to the home 5242 address of the mobile node cannot be used before the Binding Update 5243 has been processed. For the default case of using IKEv2 [41] as the 5244 automated key management protocol, such problems can be avoided by 5245 the following requirements when communicating with its home agent: 5247 o When the mobile node is away from home, it MUST use its care-of 5248 address as the Source Address of all packets it sends as part of 5249 the key management protocol (without use of Mobile IPv6 for these 5250 packets, as suggested in Section 11.3.1). 5252 The Key Management Mobility Capability (K) bit in Binding Updates and 5253 Acknowledgements can be used to avoid the need to rerun IKEv2 upon 5254 movements. 5256 11.3.3. Receiving Packets While Away from Home 5258 While away from home, a mobile node will receive packets addressed to 5259 its home address, by one of two methods: 5261 o Packets sent by a correspondent node, that does not have a Binding 5262 Cache entry for the mobile node, will be sent to the home address, 5263 captured by the home agent and tunneled to the mobile node. 5265 o Packets sent by a correspondent node that has a Binding Cache 5266 entry for the mobile node that contains the mobile node's current 5267 care-of address, will be sent by the correspondent node using a 5268 type 2 routing header. The packet will be addressed to the mobile 5269 node's care-of address, with the final hop in the routing header 5270 directing the packet to the mobile node's home address; the 5271 processing of this last hop of the routing header is entirely 5272 internal to the mobile node, since the care-of address and home 5273 address are both addresses within the mobile node. 5275 For packets received by the first method, the mobile node MUST check 5276 that the IPv6 source address of the tunneled packet is the IP address 5277 of its home agent. In this method, the mobile node may also send a 5278 Binding Update to the original sender of the packet as described in 5279 Section 11.7.2 and subject to the rate limiting defined in 5280 Section 11.8. The mobile node MUST also process the received packet 5281 in the manner defined for IPv6 encapsulation [6], which will result 5282 in the encapsulated (inner) packet being processed normally by upper- 5283 layer protocols within the mobile node as if it had been addressed 5284 (only) to the mobile node's home address. 5286 For packets received by the second method, the following rules will 5287 result in the packet being processed normally by upper-layer 5288 protocols within the mobile node as if it had been addressed to the 5289 mobile node's home address. 5291 A node receiving a packet addressed to itself (i.e., one of the 5292 node's addresses is in the IPv6 destination field) follows the next 5293 header chain of headers and processes them. When it encounters a 5294 type 2 routing header during this processing, it performs the 5295 following checks. If any of these checks fail, the node MUST 5296 silently discard the packet. 5298 o The length field in the routing header is exactly 2. 5300 o The segments left field in the routing header is 1 on the wire. 5301 (But implementations may process the routing header so that the 5302 value may become 0 after the routing header has been processed, 5303 but before the rest of the packet is processed.) 5305 o The Home Address field in the routing header is one of the node's 5306 home addresses, if the segments left field was 1. Thus, in 5307 particular the address field is required to be a unicast routable 5308 address. 5310 Once the above checks have been performed, the node swaps the IPv6 5311 destination field with the Home Address field in the routing header, 5312 decrements segments left by one from the value it had on the wire, 5313 and resubmits the packet to IP for processing the next header. 5314 Conceptually, this follows the same model as in RFC 2460. However, 5315 in the case of type 2 routing header this can be simplified since it 5316 is known that the packet will not be forwarded to a different node. 5318 The definition of AH requires the sender to calculate the AH 5319 integrity check value of a routing header in the same way it appears 5320 in the receiver after it has processed the header. Since IPsec 5321 headers follow the routing header, any IPsec processing will operate 5322 on the packet with the home address in the IP destination field and 5323 segments left being zero. Thus, the AH calculations at the sender 5324 and receiver will have an identical view of the packet. 5326 11.3.4. Routing Multicast Packets 5328 A mobile node that is connected to its home link functions in the 5329 same way as any other (stationary) node. Thus, when it is at home, a 5330 mobile node functions identically to other multicast senders and 5331 receivers. Therefore, this section describes the behavior of a 5332 mobile node that is not on its home link. 5334 In order to receive packets sent to some multicast group, a mobile 5335 node must join that multicast group. One method, in which a mobile 5336 node MAY join the group, is via a (local) multicast router on the 5337 foreign link being visited. In this case, the mobile node MUST use 5338 its care-of address and MUST NOT use the Home Address destination 5339 option when sending MLD packets [8]. 5341 Alternatively, a mobile node MAY join multicast groups via a bi- 5342 directional tunnel to its home agent. The mobile node tunnels its 5343 multicast group membership control packets (such as those defined in 5344 [8] or in [38]) to its home agent, and the home agent forwards 5345 multicast packets down the tunnel to the mobile node. A mobile node 5346 MUST NOT tunnel multicast group membership control packets until (1) 5347 the mobile node has a binding in place at the home agent, and (2) the 5348 latter sends at least one multicast group membership control packet 5349 via the tunnel. Once this condition is true, the mobile node SHOULD 5350 assume it does not change as long as the binding does not expire. 5352 A mobile node that wishes to send packets to a multicast group also 5353 has two options: 5355 1. Send directly on the foreign link being visited. 5357 To do this, the application uses the care-of address as a source 5358 address for multicast traffic, just as it would use a stationary 5359 address. This requires that the application either knows the 5360 care-of address, or uses an API such as the IPv6 Socket API for 5361 Source Address Selection specification [21] to request that the 5362 care-of address be used as the source address in transmitted 5363 packets. The mobile node MUST NOT use Home Address destination 5364 option in such traffic. 5366 2. Send via a tunnel to its home agent. 5368 Because multicast routing in general depends upon the Source 5369 Address used in the IPv6 header of the multicast packet, a mobile 5370 node that tunnels a multicast packet to its home agent MUST use 5371 its home address as the IPv6 Source Address of the inner 5372 multicast packet. 5374 Note that direct sending from the foreign link is only applicable 5375 while the mobile node is at that foreign link. This is because the 5376 associated multicast tree is specific to that source location and any 5377 change of location and source address will invalidate the source 5378 specific tree or branch and the application context of the other 5379 multicast group members. 5381 This specification does not provide mechanisms to enable such local 5382 multicast session to survive hand-off and to seamlessly continue from 5383 a new care-of address on each new foreign link. Any such mechanism, 5384 developed as an extension to this specification, needs to take into 5385 account the impact of fast moving mobile nodes on the Internet 5386 multicast routing protocols and their ability to maintain the 5387 integrity of source specific multicast trees and branches. 5389 While the use of bidirectional tunneling can ensure that multicast 5390 trees are independent of the mobile nodes movement, in some case such 5391 tunneling can have adverse affects. The latency of specific types of 5392 multicast applications (such as multicast based discovery protocols) 5393 will be affected when the round-trip time between the foreign subnet 5394 and the home agent is significant compared to that of the topology to 5395 be discovered. In addition, the delivery tree from the home agent in 5396 such circumstances relies on unicast encapsulation from the agent to 5397 the mobile node. Therefore, bandwidth usage is inefficient compared 5398 to the native multicast forwarding in the foreign multicast system. 5400 11.3.5. Receiving ICMP Error Messages 5402 Any node that does not recognize the Mobility header will return an 5403 ICMP Parameter Problem, Code 1, message to the sender of the packet. 5404 If the mobile node receives such an ICMP error message in response to 5405 a return routability procedure or Binding Update, it SHOULD record in 5406 its Binding Update List that future Binding Updates SHOULD NOT be 5407 sent to this destination. Such Binding Update List entries SHOULD be 5408 removed after a period of time in order to allow for retrying route 5409 optimization. 5411 New Binding Update List entries MUST NOT be created as a result of 5412 receiving ICMP error messages. 5414 Correspondent nodes that have participated in the return routability 5415 procedure MUST implement the ability to correctly process received 5416 packets containing a Home Address destination option. Therefore, 5417 correctly implemented correspondent nodes should always be able to 5418 recognize Home Address options. If a mobile node receives an ICMP 5419 Parameter Problem, Code 2, message from some node indicating that it 5420 does not support the Home Address option, the mobile node SHOULD log 5421 the error and then discard the ICMP message. 5423 11.3.6. Receiving Binding Error Messages 5425 When a mobile node receives a packet containing a Binding Error 5426 message, it should first check if the mobile node has a Binding 5427 Update List entry for the source of the Binding Error message. If 5428 the mobile node does not have such an entry, it MUST ignore the 5429 message. This is necessary to prevent a waste of resources on, e.g., 5430 return routability procedure due to spoofed Binding Error messages. 5432 Otherwise, if the message Status field was 1 (unknown binding for 5433 Home Address destination option), the mobile node should perform one 5434 of the following three actions: 5436 o If the Binding Error Message was sent by the Home Agent, the 5437 Mobile Node SHOULD send a Binding Update to the Home Agent 5438 according to Section 11.7.1. 5440 o If the mobile node has recent upper layer progress information, 5441 which indicates that communications with the correspondent node 5442 are progressing, it MAY ignore the message. This can be done in 5443 order to limit the damage that spoofed Binding Error messages can 5444 cause to ongoing communications. 5446 o If the mobile node has no upper layer progress information, it 5447 MUST remove the entry and route further communications through the 5448 home agent. It MAY also optionally start a return routability 5449 procedure (see Section 5.2). 5451 If the message Status field was 2 (unrecognized MH Type value), the 5452 mobile node should perform one of the following two actions: 5454 o If the mobile node is not expecting an acknowledgement or response 5455 from the correspondent node, the mobile node SHOULD ignore this 5456 message. 5458 o Otherwise, the mobile node SHOULD cease the use of any extensions 5459 to this specification. If no extensions had been used, the mobile 5460 node should cease the attempt to use route optimization. 5462 11.4. Home Agent and Prefix Management 5464 11.4.1. Dynamic Home Agent Address Discovery 5466 Sometimes when the mobile node needs to send a Binding Update to its 5467 home agent to register its new primary care-of address, as described 5468 in Section 11.7.1, the mobile node may not know the address of any 5469 router on its home link that can serve as a home agent for it. For 5470 example, some nodes on its home link may have been reconfigured while 5471 the mobile node has been away from home, such that the router that 5472 was operating as the mobile node's home agent has been replaced by a 5473 different router serving this role. 5475 In this case, the mobile node MAY attempt to discover the address of 5476 a suitable home agent on its home link. To do so, the mobile node 5477 sends an ICMP Home Agent Address Discovery Request message to the 5478 Mobile IPv6 Home-Agents anycast address [7] for its home subnet 5479 prefix. As described in Section 10.5, the home agent on its home 5480 link that receives this Request message will return an ICMP Home 5481 Agent Address Discovery Reply message. This message gives the 5482 addresses for the home agents operating on the home link. 5484 The mobile node, upon receiving this Home Agent Address Discovery 5485 Reply message, MAY then send its home registration Binding Update to 5486 any of the unicast IP addresses listed in the Home Agent Addresses 5487 field in the Reply. For example, the mobile node MAY attempt its 5488 home registration to each of these addresses, in turn, until its 5489 registration is accepted. The mobile node sends a Binding Update to 5490 an address and waits for the matching Binding Acknowledgement, moving 5491 on to the next address if there is no response. The mobile node 5492 MUST, however, wait at least InitialBindackTimeoutFirstReg seconds 5493 (see Section 13) before sending a Binding Update to the next home 5494 agent. In trying each of the returned home agent addresses, the 5495 mobile node SHOULD try each of them in the order they appear in the 5496 Home Agent Addresses field in the received Home Agent Address 5497 Discovery Reply message. In order to do this, the mobile node SHOULD 5498 store the list of home agents for later use in case the home agent 5499 currently managing the mobile node's care-of address forwarding 5500 should become unavailable. The list MAY be stored, along with any 5501 available lifetime information for the home agent addresses, in 5502 nonvolatile memory to survive reboots by the mobile node. 5504 If the mobile node has a current registration with some home agent 5505 (the Lifetime for that registration has not yet expired), then the 5506 mobile node MUST attempt any new registration first with that home 5507 agent. If that registration attempt fails (e.g., timed out or 5508 rejected), the mobile node SHOULD then reattempt this registration 5509 with another home agent. If the mobile node knows of no other 5510 suitable home agent, then it MAY attempt the dynamic home agent 5511 address discovery mechanism described above. 5513 If, after a mobile node transmits a Home Agent Address Discovery 5514 Request message to the Home Agents Anycast address, it does not 5515 receive a corresponding Home Agent Address Discovery Reply message 5516 within INITIAL_DHAAD_TIMEOUT (see Section 12) seconds, the mobile 5517 node MAY retransmit the same Request message to the same anycast 5518 address. This retransmission MAY be repeated up to a maximum of 5519 DHAAD_RETRIES (see Section 12) attempts. Each retransmission MUST be 5520 delayed by twice the time interval of the previous retransmission. 5522 11.4.2. Sending Mobile Prefix Solicitations 5524 When a mobile node has a home address that is about to become 5525 invalid, it SHOULD send a Mobile Prefix Solicitation to its home 5526 agent in an attempt to acquire fresh routing prefix information. The 5527 new information also enables the mobile node to participate in 5528 renumbering operations affecting the home network, as described in 5529 Section 10.6. 5531 The mobile node MUST use the Home Address destination option to carry 5532 its home address. The mobile node MUST support and SHOULD use IPsec 5533 to protect the solicitation. The mobile node MUST set the Identifier 5534 field in the ICMP header to a random value. 5536 As described in Section 11.7.2, Binding Updates sent by the mobile 5537 node to other nodes MUST use a lifetime no greater than the remaining 5538 lifetime of its home registration of its primary care-of address. 5539 The mobile node SHOULD further limit the lifetimes that it sends on 5540 any Binding Updates to be within the remaining valid lifetime (see 5541 Section 10.6.2) for the prefix in its home address. 5543 When the lifetime for a changed prefix decreases, and the change 5544 would cause cached bindings at correspondent nodes in the Binding 5545 Update List to be stored past the newly shortened lifetime, the 5546 mobile node MUST issue a Binding Update to all such correspondent 5547 nodes. 5549 These limits on the binding lifetime serve to prohibit use of a 5550 mobile node's home address after it becomes invalid. 5552 11.4.3. Receiving Mobile Prefix Advertisements 5554 Section 10.6 describes the operation of a home agent to support boot 5555 time configuration and renumbering a mobile node's home subnet while 5556 the mobile node is away from home. The home agent sends Mobile 5557 Prefix Advertisements to the mobile node while away from home, giving 5558 "important" Prefix Information options that describe changes in the 5559 prefixes in use on the mobile node's home link. 5561 The Mobile Prefix Solicitation is similar to the Router Solicitation 5562 used in Neighbor Discovery [17], except it is routed from the mobile 5563 node on the visited network to the home agent on the home network by 5564 usual unicast routing rules. 5566 When a mobile node receives a Mobile Prefix Advertisement, it MUST 5567 validate it according to the following test: 5569 o The Source Address of the IP packet carrying the Mobile Prefix 5570 Advertisement is the same as the home agent address to which the 5571 mobile node last sent an accepted home registration Binding Update 5572 to register its primary care-of address. Otherwise, if no such 5573 registrations have been made, it SHOULD be the mobile node's 5574 stored home agent address, if one exists. Otherwise, if the 5575 mobile node has not yet discovered its home agent's address, it 5576 MUST NOT accept Mobile Prefix Advertisements. 5578 o The packet MUST have a type 2 routing header and SHOULD be 5579 protected by an IPsec header as described in Section 5.4 and 5580 Section 6.8. 5582 o If the ICMP Identifier value matches the ICMP Identifier value of 5583 the most recently sent Mobile Prefix Solicitation and no other 5584 advertisement has yet been received for this value, then the 5585 advertisement is considered to be solicited and will be processed 5586 further. 5588 Otherwise, the advertisement is unsolicited, and MUST be 5589 discarded. In this case the mobile node SHOULD send a Mobile 5590 Prefix Solicitation. 5592 Any received Mobile Prefix Advertisement not meeting these tests MUST 5593 be silently discarded. 5595 For an accepted Mobile Prefix Advertisement, the mobile node MUST 5596 process Managed Address Configuration (M), Other Stateful 5597 Configuration (O), and the Prefix Information Options as if they 5598 arrived in a Router Advertisement [17] on the mobile node's home 5599 link. (This specification does not, however, describe how to acquire 5600 home addresses through stateful protocols.) Such processing may 5601 result in the mobile node configuring a new home address, although 5602 due to separation between preferred lifetime and valid lifetime, such 5603 changes should not affect most communications by the mobile node, in 5604 the same way as for nodes that are at home. 5606 This specification assumes that any security associations and 5607 security policy entries that may be needed for new prefixes have been 5608 pre-configured in the mobile node. Note that while dynamic key 5609 management avoids the need to configure new security associations, it 5610 is still necessary to add policy entries to protect the 5611 communications involving the home address(es). Mechanisms for 5612 setting up these entries are outside the scope of this specification. 5614 11.5. Movement 5616 11.5.1. Movement Detection 5618 The primary goal of movement detection is to detect L3 handovers. 5619 This section does not attempt to specify a fast movement detection 5620 algorithm which will function optimally for all types of 5621 applications, link-layers and deployment scenarios; instead, it 5622 describes a generic method that uses the facilities of IPv6 Neighbor 5623 Discovery, including Router Discovery and Neighbor Unreachability 5624 Detection. At the time of this writing, this method is considered 5625 well enough understood to recommend for standardization, however it 5626 is expected that future versions of this specification or other 5627 specifications may contain updated versions of the movement detection 5628 algorithm that have better performance. 5630 Generic movement detection uses Neighbor Unreachability Detection to 5631 detect when the default router is no longer bi-directionally 5632 reachable, in which case the mobile node must discover a new default 5633 router (usually on a new link). However, this detection only occurs 5634 when the mobile node has packets to send, and in the absence of 5635 frequent Router Advertisements or indications from the link-layer, 5636 the mobile node might become unaware of an L3 handover that occurred. 5637 Therefore, the mobile node should supplement this method with other 5638 information whenever it is available to the mobile node (e.g., from 5639 lower protocol layers). 5641 When the mobile node detects an L3 handover, it performs Duplicate 5642 Address Detection [18] on its link-local address, selects a new 5643 default router as a consequence of Router Discovery, and then 5644 performs Prefix Discovery with that new router to form new care-of 5645 address(es) as described in Section 11.5.3. It then registers its 5646 new primary care-of address with its home agent as described in 5647 Section 11.7.1. After updating its home registration, the mobile 5648 node then updates associated mobility bindings in correspondent nodes 5649 that it is performing route optimization with as specified in 5650 Section 11.7.2. 5652 Due to the temporary packet flow disruption and signaling overhead 5653 involved in updating mobility bindings, the mobile node should avoid 5654 performing an L3 handover until it is strictly necessary. 5655 Specifically, when the mobile node receives a Router Advertisement 5656 from a new router that contains a different set of on-link prefixes, 5657 if the mobile node detects that the currently selected default router 5658 on the old link is still bi-directionally reachable, it should 5659 generally continue to use the old router on the old link rather than 5660 switch away from it to use a new default router. 5662 Mobile nodes can use the information in received Router 5663 Advertisements to detect L3 handovers. In doing so the mobile node 5664 needs to consider the following issues: 5666 o There might be multiple routers on the same link, thus hearing a 5667 new router does not necessarily constitute an L3 handover. 5669 o When there are multiple routers on the same link they might 5670 advertise different prefixes. Thus even hearing a new router with 5671 a new prefix might not be a reliable indication of an L3 handover. 5673 o The link-local addresses of routers are not globally unique, hence 5674 after completing an L3 handover the mobile node might continue to 5675 receive Router Advertisements with the same link-local source 5676 address. This might be common if routers use the same link-local 5677 address on multiple interfaces. This issue can be avoided when 5678 routers use the Router Address (R) bit, since that provides a 5679 global address of the router. 5681 In addition, the mobile node should consider the following events as 5682 indications that an L3 handover may have occurred. Upon receiving 5683 such indications, the mobile node needs to perform Router Discovery 5684 to discover routers and prefixes on the new link, as described in 5685 Section 6.3.7 of Neighbor Discovery (RFC 4861 [17]). 5687 o If Router Advertisements that the mobile node receives include an 5688 Advertisement Interval option, the mobile node may use its 5689 Advertisement Interval field as an indication of the frequency 5690 with which it should expect to continue to receive future 5691 Advertisements from that router. This field specifies the minimum 5692 rate (the maximum amount of time between successive 5693 Advertisements) that the mobile node should expect. If this 5694 amount of time elapses without the mobile node receiving any 5695 Advertisement from this router, the mobile node can be sure that 5696 at least one Advertisement sent by the router has been lost. The 5697 mobile node can then implement its own policy to determine how 5698 many lost Advertisements from its current default router 5699 constitute an L3 handover indication. 5701 o Neighbor Unreachability Detection determines that the default 5702 router is no longer reachable. 5704 o With some types of networks, notification that a L2 handover has 5705 occurred might be obtained from lower layer protocols or device 5706 driver software within the mobile node. While further details 5707 around handling L2 indications as movement hints is an item for 5708 further study, at the time of writing this specification the 5709 following is considered reasonable: 5711 A L2 handover indication may or may not imply L2 movement and L2 5712 movement may or may not imply L3 movement; the correlations might 5713 be a function of the type of L2 but might also be a function of 5714 actual deployment of the wireless topology. 5716 Unless it is well-known that a L2 handover indication is likely to 5717 imply L3 movement, instead of immediately multicasting a router 5718 solicitation it may be better to attempt to verify whether the 5719 default router is still bi-directionally reachable. This can be 5720 accomplished by sending a unicast Neighbor Solicitation and 5721 waiting for a Neighbor Advertisement with the solicited flag set. 5722 Note that this is similar to Neighbor Unreachability detection but 5723 it does not have the same state machine, such as the STALE state. 5725 If the default router does not respond to the Neighbor 5726 Solicitation it makes sense to proceed to multicasting a Router 5727 Solicitation. 5729 11.5.2. Home Link Detection 5731 When an MN detects that it has arrived on a new link using the 5732 movement detection algorithm in use (Section 11.5.1,) or on 5733 bootstrapping, it performs the following steps to determine if it is 5734 on the home link. 5736 o The MN performs the procedure described in Section 11.5.3 and 5737 configures an address. It also keeps track of all the on-link 5738 prefix(es) received in the RA along with their prefix lengths. 5740 o If the home prefix has not been statically configured the MN uses 5741 some form of bootstrapping procedure (e.g. RFC5026 [22]) to 5742 determine the home prefix. 5744 o Given the availability of the home prefix, the MN checks whether 5745 or not the home prefix matches one of the prefixes received in the 5746 RA. If it does, the MN concludes that it is connected to the home 5747 link. 5749 11.5.3. Forming New Care-of Addresses 5751 After detecting that it has moved a mobile node SHOULD generate a new 5752 primary care-of address using normal IPv6 mechanisms. This SHOULD 5753 also be done when the current primary care-of address becomes 5754 deprecated. A mobile node MAY form a new primary care-of address at 5755 any time, but a mobile node MUST NOT send a Binding Update about a 5756 new care-of address to its home agent more than MAX_UPDATE_RATE times 5757 within a second. 5759 In addition, a mobile node MAY form new non-primary care-of addresses 5760 even when it has not switched to a new default router. A mobile node 5761 can have only one primary care-of address at a time (which is 5762 registered with its home agent), but it MAY have an additional 5763 care-of address for any or all of the prefixes on its current link. 5764 Furthermore, since a wireless network interface may actually allow a 5765 mobile node to be reachable on more than one link at a time (i.e., 5766 within wireless transmitter range of routers on more than one 5767 separate link), a mobile node MAY have care-of addresses on more than 5768 one link at a time. The use of more than one care-of address at a 5769 time is described in Section 11.5.4. 5771 As described in Section 4, in order to form a new care-of address, a 5772 mobile node MAY use either stateless [18] or stateful (e.g., DHCPv6 5773 [29]) Address Autoconfiguration. If a mobile node needs to use a 5774 source address (other than the unspecified address) in packets sent 5775 as a part of address autoconfiguration, it MUST use an IPv6 link- 5776 local address rather than its own IPv6 home address. 5778 RFC 4862 [18] specifies that in normal processing for Duplicate 5779 Address Detection, the node SHOULD delay sending the initial Neighbor 5780 Solicitation message by a random delay between 0 and 5781 MAX_RTR_SOLICITATION_DELAY. Since delaying DAD can result in 5782 significant delays in configuring a new care-of address when the 5783 Mobile Node moves to a new link, the Mobile Node preferably SHOULD 5784 NOT delay DAD when configuring a new care-of address. The Mobile 5785 Node SHOULD delay according to the mechanisms specified in RFC 4862 5786 unless the implementation has a behavior that desynchronizes the 5787 steps that happen before the DAD in the case that multiple nodes 5788 experience handover at the same time. Such desynchronizing behaviors 5789 might be due to random delays in the L2 protocols or device drivers, 5790 or due to the movement detection mechanism that is used. 5792 11.5.4. Using Multiple Care-of Addresses 5794 As described in Section 11.5.3, a mobile node MAY use more than one 5795 care-of address at a time. Particularly in the case of many wireless 5796 networks, a mobile node effectively might be reachable through 5797 multiple links at the same time (e.g., with overlapping wireless 5798 cells), on which different on-link subnet prefixes may exist. The 5799 mobile node MUST ensure that its primary care-of address always has a 5800 prefix that is advertised by its current default router. After 5801 selecting a new primary care-of address, the mobile node MUST send a 5802 Binding Update containing that care-of address to its home agent. 5803 The Binding Update MUST have the Home Registration (H) and 5804 Acknowledge (A) bits set its home agent, as described on 5805 Section 11.7.1. 5807 To assist with smooth handovers, a mobile node SHOULD retain its 5808 previous primary care-of address as a (non-primary) care-of address, 5809 and SHOULD still accept packets at this address, even after 5810 registering its new primary care-of address with its home agent. 5811 This is reasonable, since the mobile node could only receive packets 5812 at its previous primary care-of address if it were indeed still 5813 connected to that link. If the previous primary care-of address was 5814 allocated using stateful Address Autoconfiguration [29], the mobile 5815 node may not wish to release the address immediately upon switching 5816 to a new primary care-of address. 5818 Whenever a mobile node determines that it is no longer reachable 5819 through a given link, it SHOULD invalidate all care-of addresses 5820 associated with address prefixes that it discovered from routers on 5821 the unreachable link which are not in the current set of address 5822 prefixes advertised by the (possibly new) current default router. 5824 11.5.5. Returning Home 5826 A mobile node detects that it has returned to its home link through 5827 the movement detection algorithm in use (Section 11.5.2), when the 5828 mobile node detects that its home subnet prefix is again on-link. To 5829 be able to send and receive packets using its home address from the 5830 home link, the mobile node MUST send a Binding Update to its home 5831 agent to instruct its home agent to no longer intercept or tunnel 5832 packets for it. Until the mobile node sends such a de-registration 5833 Binding Update, it MUST NOT attempt to send and receive packets using 5834 its home address from the home link. The home agent will continue to 5835 intercept all packets sent to the mobile's home address and tunnel 5836 them to the previously registered care-of address. 5838 In this home registration, the mobile node MUST set the Acknowledge 5839 (A) and Home Registration (H) bits, set the Lifetime field to zero, 5840 and set the care-of address for the binding to the mobile node's own 5841 home address. The mobile node MUST use its home address as the 5842 source address in the Binding Update. 5844 When sending this Binding Update to its home agent, the mobile node 5845 must be careful in how it uses Neighbor Solicitation [17] (if needed) 5846 to learn the home agent's link-layer address, since the home agent 5847 will be currently configured to intercept packets to the mobile 5848 node's home address using Proxy Neighbor Discovery (Proxy ND). In 5849 particular, the mobile node is unable to use its home address as the 5850 Source Address in the Neighbor Solicitation until the home agent 5851 stops defending the home address. 5853 Neighbor Solicitation by the mobile node for the home agent's address 5854 will normally not be necessary, since the mobile node has already 5855 learned the home agent's link-layer address from a Source Link-Layer 5856 Address option in a Router Advertisement. However, if there are 5857 multiple home agents it may still be necessary to send a 5858 solicitation. In this special case of the mobile node returning 5859 home, the mobile node MUST multicast the packet, and in addition set 5860 the Source Address of this Neighbor Solicitation to the unspecified 5861 address (0:0:0:0:0:0:0:0). The target of the Neighbor Solicitation 5862 MUST be set to the mobile node's home address. The destination IP 5863 address MUST be set to the Solicited-Node multicast address [15]. 5864 The home agent will send a multicast Neighbor Advertisement back to 5865 the mobile node with the Solicited flag (S) set to zero. In any 5866 case, the mobile node SHOULD record the information from the Source 5867 Link-Layer Address option or from the advertisement, and set the 5868 state of the Neighbor Cache entry for the home agent to REACHABLE. 5870 The mobile node then sends its Binding Update to the home agent's 5871 link-layer address, instructing its home agent to no longer serve as 5872 a home agent for it. By processing this Binding Update, the home 5873 agent will cease defending the mobile node's home address for 5874 Duplicate Address Detection and will no longer respond to Neighbor 5875 Solicitations for the mobile node's home address. The mobile node is 5876 then the only node on the link receiving packets at the mobile node's 5877 home address. In addition, when returning home prior to the 5878 expiration of a current binding for its home address, and configuring 5879 its home address on its network interface on its home link, the 5880 mobile node MUST NOT perform Duplicate Address Detection on its own 5881 home address, in order to avoid confusion or conflict with its home 5882 agent's use of the same address. This rule also applies to the 5883 derived link-local address of the mobile node, if the Link Local 5884 Address Compatibility (L) bit was set when the binding was created. 5885 If the mobile node returns home after the bindings for all of its 5886 care-of addresses have expired, then it SHOULD perform DAD. 5888 After the Mobile Node sends the Binding Update, it MUST be prepared 5889 to reply to Neighbor Solicitations for its home address. Such 5890 replies MUST be sent using a unicast Neighbor Advertisement to the 5891 sender's link-layer address. It is necessary to reply, since sending 5892 the Binding Acknowledgement from the home agent may require 5893 performing Neighbor Discovery, and the mobile node may not be able to 5894 distinguish Neighbor Solicitations coming from the home agent from 5895 other Neighbor Solicitations. Note that a race condition exists 5896 where both the mobile node and the home agent respond to the same 5897 solicitations sent by other nodes; this will be only temporary, 5898 however, until the Binding Update is accepted. 5900 After receiving the Binding Acknowledgement for its Binding Update to 5901 its home agent, the mobile node MUST multicast onto the home link (to 5902 the all-nodes multicast address) a Neighbor Advertisement [17], to 5903 advertise the mobile node's own link-layer address for its own home 5904 address. The Target Address in this Neighbor Advertisement MUST be 5905 set to the mobile node's home address, and the Advertisement MUST 5906 include a Target Link-layer Address option specifying the mobile 5907 node's link-layer address. The mobile node MUST multicast such a 5908 Neighbor Advertisement for each of its home addresses, as defined by 5909 the current on-link prefixes, including its link-local address. The 5910 Solicited Flag (S) in these Advertisements MUST NOT be set, since 5911 they were not solicited by any Neighbor Solicitation. The Override 5912 Flag (O) in these Advertisements MUST be set, indicating that the 5913 Advertisements SHOULD override any existing Neighbor Cache entries at 5914 any node receiving them. 5916 Since multicasting on the local link (such as Ethernet) is typically 5917 not guaranteed to be reliable, the mobile node MAY retransmit these 5918 Neighbor Advertisements [17] up to MAX_NEIGHBOR_ADVERTISEMENT times 5919 to increase their reliability. It is still possible that some nodes 5920 on the home link will not receive any of these Neighbor 5921 Advertisements, but these nodes will eventually be able to recover 5922 through use of Neighbor Unreachability Detection [17]. 5924 Note that the tunnel via the home agent typically stops operating at 5925 the same time that the home registration is deleted. 5927 11.6. Return Routability Procedure 5929 This section defines the rules that the mobile node must follow when 5930 performing the return routability procedure. Section 11.7.2 5931 describes the rules when the return routability procedure needs to be 5932 initiated. 5934 11.6.1. Sending Test Init Messages 5936 A mobile node that initiates a return routability procedure MUST send 5937 (in parallel) a Home Test Init message and a Care-of Test Init 5938 messages. However, if the mobile node has recently received (see 5939 Section 5.2.7) one or both home or care-of keygen tokens, and 5940 associated nonce indices for the desired addresses, it MAY reuse 5941 them. Therefore, the return routability procedure may in some cases 5942 be completed with only one message pair. It may even be completed 5943 without any messages at all, if the mobile node has a recent home 5944 keygen token and has previously visited the same care-of address so 5945 that it also has a recent care-of keygen token. If the mobile node 5946 intends to send a Binding Update with the Lifetime set to zero and 5947 the care-of address equal to its home address - such as when 5948 returning home - sending a Home Test Init message is sufficient. In 5949 this case, generation of the binding management key depends 5950 exclusively on the home keygen token (Section 5.2.5). 5952 A Home Test Init message MUST be created as described in 5953 Section 6.1.3. 5955 A Care-of Test Init message MUST be created as described in 5956 Section 6.1.4. When sending a Home Test Init or Care-of Test Init 5957 message the mobile node MUST record in its Binding Update List the 5958 following fields from the messages: 5960 o The IP address of the node to which the message was sent. 5962 o The home address of the mobile node. This value will appear in 5963 the Source Address field of the Home Test Init message. When 5964 sending the Care-of Test Init message, this address does not 5965 appear in the message, but represents the home address for which 5966 the binding is desired. 5968 o The time at which each of these messages was sent. 5970 o The cookies used in the messages. 5972 Note that a single Care-of Test Init message may be sufficient even 5973 when there are multiple home addresses. In this case the mobile node 5974 MAY record the same information in multiple Binding Update List 5975 entries. 5977 11.6.2. Receiving Test Messages 5979 Upon receiving a packet carrying a Home Test message, a mobile node 5980 MUST validate the packet according to the following tests: 5982 o The Source Address of the packet belongs to a correspondent node 5983 for which the mobile node has a Binding Update List entry with a 5984 state indicating that return routability procedure is in progress. 5985 Note that there may be multiple such entries. 5987 o The Binding Update List indicates that no home keygen token has 5988 been received yet. 5990 o The Destination Address of the packet has the home address of the 5991 mobile node, and the packet has been received in a tunnel from the 5992 home agent. 5994 o The Home Init Cookie field in the message matches the value stored 5995 in the Binding Update List. 5997 Any Home Test message not satisfying all of these tests MUST be 5998 silently ignored. Otherwise, the mobile node MUST record the Home 5999 Nonce Index and home keygen token in the Binding Update List. If the 6000 Binding Update List entry does not have a care-of keygen token, the 6001 mobile node SHOULD continue waiting for the Care-of Test message. 6003 Upon receiving a packet carrying a Care-of Test message, a mobile 6004 node MUST validate the packet according to the following tests: 6006 o The Source Address of the packet belongs to a correspondent node 6007 for which the mobile node has a Binding Update List entry with a 6008 state indicating that return routability procedure is in progress. 6009 Note that there may be multiple such entries. 6011 o The Binding Update List indicates that no care-of keygen token has 6012 been received yet. 6014 o The Destination Address of the packet is the current care-of 6015 address of the mobile node. 6017 o The Care-of Init Cookie field in the message matches the value 6018 stored in the Binding Update List. 6020 Any Care-of Test message not satisfying all of these tests MUST be 6021 silently ignored. Otherwise, the mobile node MUST record the Care-of 6022 Nonce Index and care-of keygen token in the Binding Update List. If 6023 the Binding Update List entry does not have a home keygen token, the 6024 mobile node SHOULD continue waiting for the Home Test message. 6026 If after receiving either the Home Test or the Care-of Test message 6027 and performing the above actions, the Binding Update List entry has 6028 both the home and the care-of keygen tokens, the return routability 6029 procedure is complete. The mobile node SHOULD then proceed with 6030 sending a Binding Update as described in Section 11.7.2. 6032 Correspondent nodes from the time before this specification was 6033 published may not support the Mobility Header protocol. These nodes 6034 will respond to Home Test Init and Care-of Test Init messages with an 6035 ICMP Parameter Problem code 1. The mobile node SHOULD take such 6036 messages as an indication that the correspondent node cannot provide 6037 route optimization, and revert back to the use of bidirectional 6038 tunneling. 6040 11.6.3. Protecting Return Routability Packets 6042 The mobile node MUST support the protection of Home Test and Home 6043 Test Init messages as described in Section 10.4.6. 6045 When IPsec is used to protect return routability signaling or payload 6046 packets, the mobile node MUST set the source address it uses for the 6047 outgoing tunnel packets to the current primary care-of address. The 6048 mobile node starts to use a new primary care-of address immediately 6049 after sending a Binding Update to the home agent to register this new 6050 address. 6052 11.7. Processing Bindings 6054 11.7.1. Sending Binding Updates to the Home Agent 6056 In order to change its primary care-of address as described in 6057 Section 11.5.1 and Section 11.5.3, a mobile node MUST register this 6058 care-of address with its home agent in order to make this its primary 6059 care-of address. 6061 Also, if the mobile node wants the services of the home agent beyond 6062 the current registration period, the mobile node should send a new 6063 Binding Update to it well before the expiration of this period, even 6064 if it is not changing its primary care-of address. However, if the 6065 home agent returned a Binding Acknowledgement for the current 6066 registration with Status field set to 1 (accepted but prefix 6067 discovery necessary), the mobile node should not try to register 6068 again before it has learned the validity of its home prefixes through 6069 mobile prefix discovery. This is typically necessary every time this 6070 Status value is received, because information learned earlier may 6071 have changed. 6073 To register a care-of address or to extend the lifetime of an 6074 existing registration, the mobile node sends a packet to its home 6075 agent containing a Binding Update, with the packet constructed as 6076 follows: 6078 o The Home Registration (H) bit MUST be set in the Binding Update. 6080 o The Acknowledge (A) bit MUST be set in the Binding Update. 6082 o The packet MUST contain a Home Address destination option, giving 6083 the mobile node's home address for the binding. 6085 o The care-of address for the binding MUST be used as the Source 6086 Address in the packet's IPv6 header, unless an Alternate Care-of 6087 Address mobility option is included in the Binding Update. This 6088 option MUST be included in all home registrations, as the ESP 6089 protocol will not be able to protect care-of addresses in the IPv6 6090 header. (Mobile IPv6 implementations that know they are using 6091 IPsec AH to protect a particular message might avoid this option. 6092 For brevity the usage of AH is not discussed in this document.) 6094 o If the mobile node's link-local address has the same interface 6095 identifier as the home address for which it is supplying a new 6096 care-of address, then the mobile node SHOULD set the Link-Local 6097 Address Compatibility (L) bit. 6099 o If the home address was generated using RFC 4941 [20], then the 6100 link local address is unlikely to have a compatible interface 6101 identifier. In this case, the mobile node MUST clear the Link- 6102 Local Address Compatibility (L) bit. 6104 o If the IPsec security associations between the mobile node and the 6105 home agent have been established dynamically, and the mobile node 6106 has the capability to update its endpoint in the used key 6107 management protocol to the new care-of address every time it 6108 moves, the mobile node SHOULD set the Key Management Mobility 6109 Capability (K) bit in the Binding Update. Otherwise, the mobile 6110 node MUST clear the bit. 6112 o The value specified in the Lifetime field MUST be non-zero and 6113 SHOULD be less than or equal to the remaining valid lifetime of 6114 the home address and the care-of address specified for the 6115 binding. 6117 Mobile nodes that use dynamic home agent address discovery should 6118 be careful with long lifetimes. If the mobile node loses the 6119 knowledge of its binding with a specific home agent, registering a 6120 new binding with another home agent may be impossible as the 6121 previous home agent is still defending the existing binding. 6122 Therefore, to ensure that mobile nodes using home agent address 6123 discovery do not lose information about their binding, they SHOULD 6124 de-register before losing this information, or use small 6125 lifetimes. 6127 The Acknowledge (A) bit in the Binding Update requests the home agent 6128 to return a Binding Acknowledgement in response to this Binding 6129 Update. As described in Section 6.1.8, the mobile node SHOULD 6130 retransmit this Binding Update to its home agent until it receives a 6131 matching Binding Acknowledgement. Once reaching a retransmission 6132 timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart 6133 the process of delivering the Binding Update, but trying instead the 6134 next home agent returned during dynamic home agent address discovery 6135 (see Section 11.4.1). If there was only one home agent, the mobile 6136 node instead SHOULD continue to periodically retransmit the Binding 6137 Update at this rate until acknowledged (or until it begins attempting 6138 to register a different primary care-of address). See Section 11.8 6139 for information about retransmitting Binding Updates. 6141 With the Binding Update, the mobile node requests the home agent to 6142 serve as the home agent for the given home address. Until the 6143 lifetime of this registration expires, the home agent considers 6144 itself the home agent for this home address. 6146 Each Binding Update MUST be authenticated as coming from the right 6147 mobile node, as defined in Section 5.1. The mobile node MUST use its 6148 home address - either in the Home Address destination option or in 6149 the Source Address field of the IPv6 header - in Binding Updates sent 6150 to the home agent. This is necessary in order to allow the IPsec 6151 policies to be matched with the correct home address. 6153 When sending a Binding Update to its home agent, the mobile node MUST 6154 also create or update the corresponding Binding Update List entry, as 6155 specified in Section 11.7.2. 6157 The last Sequence Number value sent to the home agent in a Binding 6158 Update is stored by the mobile node. If the sending mobile node has 6159 no knowledge of the correct Sequence Number value, it may start at 6160 any value. If the home agent rejects the value, it sends back a 6161 Binding Acknowledgement with a status code 135, and the last accepted 6162 sequence number in the Sequence Number field of the Binding 6163 Acknowledgement. The mobile node MUST store this information and use 6164 the next Sequence Number value for the next Binding Update it sends. 6166 If the mobile node has additional home addresses, then the mobile 6167 node SHOULD send an additional packet containing a Binding Update to 6168 its home agent to register the care-of address for each such other 6169 home address. 6171 The home agent will only perform DAD for the mobile node's home 6172 address when the mobile node has supplied a valid binding between its 6173 home address and a care-of address. If some time elapses during 6174 which the mobile node has no binding at the home agent, it might be 6175 possible for another node to autoconfigure the mobile node's home 6176 address. Therefore, the mobile node MUST treat the creation of a new 6177 binding with the home agent using an existing home address, the same 6178 as creation of a new home address. In the unlikely event that the 6179 mobile node's home address is autoconfigured as the IPv6 address of 6180 another network node on the home network, the home agent will reply 6181 to the mobile node's subsequent Binding Update with a Binding 6182 Acknowledgement containing a Status of 134 (Duplicate Address 6183 Detection failed). In this case, the mobile node MUST NOT attempt to 6184 re-use the same home address. It SHOULD continue to register the 6185 care-of addresses for its other home addresses, if any. Mechanisms 6186 outlined in "Mobile IPv6 Bootstrapping in Split Scenario" [22] allow 6187 mobile nodes to acquire new home addresses to replace the one for 6188 which Status 134 was received. 6190 11.7.2. Correspondent Registration 6192 When the mobile node is assured that its home address is valid, it 6193 can initiate a correspondent registration with the purpose of 6194 allowing the correspondent node to cache the mobile node's current 6195 care-of address. This procedure consists of the return routability 6196 procedure followed by a registration. 6198 This section defines when the correspondent registration is to be 6199 initiated and the rules to follow while it is being performed. 6201 After the mobile node has sent a Binding Update to its home agent, 6202 registering a new primary care-of address (as described in 6203 Section 11.7.1), the mobile node SHOULD initiate a correspondent 6204 registration for each node that already appears in the mobile node's 6205 Binding Update List. The initiated procedures can be used to either 6206 update or delete binding information in the correspondent node. 6208 For nodes that do not appear in the mobile node's Binding Update 6209 List, the mobile node MAY initiate a correspondent registration at 6210 any time after sending the Binding Update to its home agent. 6211 Considerations regarding when (and if) to initiate the procedure 6212 depend on the specific movement and traffic patterns of the mobile 6213 node and are outside the scope of this document. 6215 In addition, the mobile node MAY initiate the correspondent 6216 registration in response to receiving a packet that meets all of the 6217 following tests: 6219 o The packet was tunneled using IPv6 encapsulation. 6221 o The Destination Address in the tunnel (outer) IPv6 header is equal 6222 to any of the mobile node's care-of addresses. 6224 o The Destination Address in the original (inner) IPv6 header is 6225 equal to one of the mobile node's home addresses. 6227 o The Source Address in the tunnel (outer) IPv6 header differs from 6228 the Source Address in the original (inner) IPv6 header. 6230 o The packet does not contain a Home Test, Home Test Init, Care-of 6231 Test, or Care-of Test Init message. 6233 If a mobile node has multiple home addresses, it becomes important to 6234 select the right home address to use in the correspondent 6235 registration. The used home address MUST be the Destination Address 6236 of the original (inner) packet. 6238 The peer address used in the procedure MUST be determined as follows: 6240 o If a Home Address destination option is present in the original 6241 (inner) packet, the address from this option is used. 6243 o Otherwise, the Source Address in the original (inner) IPv6 header 6244 of the packet is used. 6246 Note that the validity of the original packet is checked before 6247 attempting to initiate a correspondent registration. For instance, 6248 if a Home Address destination option appeared in the original packet, 6249 then rules in Section 9.3.1 are followed. 6251 A mobile node MAY also choose to keep its topological location 6252 private from certain correspondent nodes, and thus need not initiate 6253 the correspondent registration. 6255 Upon successfully completing the return routability procedure, and 6256 after receiving a successful Binding Acknowledgement from the Home 6257 Agent, a Binding Update MAY be sent to the correspondent node. 6259 In any Binding Update sent by a mobile node, the care-of address 6260 (either the Source Address in the packet's IPv6 header or the Care-of 6261 Address in the Alternate Care-of Address mobility option of the 6262 Binding Update) MUST be set to one of the care-of addresses currently 6263 in use by the mobile node or to the mobile node's home address. A 6264 mobile node MAY set the care-of address differently for sending 6265 Binding Updates to different correspondent nodes. 6267 A mobile node MAY also send a Binding Update to such a correspondent 6268 node, instructing it to delete any existing binding for the mobile 6269 node from its Binding Cache, as described in Section 6.1.7. Even in 6270 this case a successful completion of the return routability procedure 6271 is required first. 6273 If the care-of address is not set to the mobile node's home address, 6274 the Binding Update requests that the correspondent node create or 6275 update an entry for the mobile node in the correspondent node's 6276 Binding Cache. This is done in order to record a care-of address for 6277 use in sending future packets to the mobile node. In this case, the 6278 value specified in the Lifetime field sent in the Binding Update 6279 SHOULD be less than or equal to the remaining lifetime of the home 6280 registration and the care-of address specified for the binding. The 6281 care-of address given in the Binding Update MAY differ from the 6282 mobile node's primary care-of address. 6284 If the Binding Update is sent to the correspondent node, requesting 6285 the deletion of any existing Binding Cache entry it has for the 6286 mobile node, the care-of address is set to the mobile node's home 6287 address and the Lifetime field set to zero. In this case, generation 6288 of the binding management key depends exclusively on the home keygen 6289 token (Section 5.2.5). The care-of nonce index SHOULD be set to zero 6290 in this case. In keeping with the Binding Update creation rules 6291 below, the care-of address MUST be set to the home address if the 6292 mobile node is at home, or to the current care-of address if it is 6293 away from home. 6295 If the mobile node wants to ensure that its new care-of address has 6296 been entered into a correspondent node's Binding Cache, the mobile 6297 node needs to request an acknowledgement by setting the Acknowledge 6298 (A) bit in the Binding Update. 6300 A Binding Update is created as follows: 6302 o The current care-of address of the mobile node MUST be sent either 6303 in the Source Address of the IPv6 header, or in the Alternate 6304 Care-of Address mobility option. 6306 o The Destination Address of the IPv6 header MUST contain the 6307 address of the correspondent node. 6309 o The Mobility Header is constructed according to rules in 6310 Section 6.1.7 and Section 5.2.6, including the Binding 6311 Authorization Data (calculated as defined in Section 6.2.7) and 6312 possibly the Nonce Indices mobility options. 6314 o The home address of the mobile node MUST be added to the packet in 6315 a Home Address destination option, unless the Source Address is 6316 the home address. 6318 Each Binding Update MUST have a Sequence Number greater than the 6319 Sequence Number value sent in the previous Binding Update to the same 6320 destination address (if any). The sequence numbers are compared 6321 modulo 2**16, as described in Section 9.5.1. There is no 6322 requirement, however, that the Sequence Number value strictly 6323 increase by 1 with each new Binding Update sent or received, as long 6324 as the value stays within the window. The last Sequence Number value 6325 sent to a destination in a Binding Update is stored by the mobile 6326 node in its Binding Update List entry for that destination. If the 6327 sending mobile node has no Binding Update List entry, the Sequence 6328 Number SHOULD start at a random value. The mobile node MUST NOT use 6329 the same Sequence Number in two different Binding Updates to the same 6330 correspondent node, even if the Binding Updates provide different 6331 care-of addresses. 6333 The mobile node is responsible for the completion of the 6334 correspondent registration, as well as any retransmissions that may 6335 be needed (subject to the rate limitation defined in Section 11.8). 6337 11.7.3. Receiving Binding Acknowledgements 6339 Upon receiving a packet carrying a Binding Acknowledgement, a mobile 6340 node MUST validate the packet according to the following tests: 6342 o The packet meets the authentication requirements for Binding 6343 Acknowledgements defined in Section 6.1.8 and Section 5. That is, 6344 if the Binding Update was sent to the home agent, underlying IPsec 6345 protection is used. If the Binding Update was sent to the 6346 correspondent node, the Binding Authorization Data mobility option 6347 MUST be present and have a valid value. 6349 o The Binding Authorization Data mobility option, if present, MUST 6350 be the last option and MUST NOT have trailing padding. 6352 o The Sequence Number field matches the Sequence Number sent by the 6353 mobile node to this destination address in an outstanding Binding 6354 Update, and the Status field is not 135. 6356 Any Binding Acknowledgement not satisfying all of these tests MUST be 6357 silently ignored. 6359 When a mobile node receives a packet carrying a valid Binding 6360 Acknowledgement, the mobile node MUST examine the Status field as 6361 follows: 6363 o If the Status field indicates that the Binding Update was accepted 6364 (the Status field is less than 128), then the mobile node MUST 6365 update the corresponding entry in its Binding Update List to 6366 indicate that the Binding Update has been acknowledged; the mobile 6367 node MUST then stop retransmitting the Binding Update. In 6368 addition, if the value specified in the Lifetime field in the 6369 Binding Acknowledgement is less than the Lifetime value sent in 6370 the Binding Update being acknowledged, the mobile node MUST 6371 subtract the difference between these two Lifetime values from the 6372 remaining lifetime for the binding as maintained in the 6373 corresponding Binding Update List entry (with a minimum value for 6374 the Binding Update List entry lifetime of 0). That is, if the 6375 Lifetime value sent in the Binding Update was L_update, the 6376 Lifetime value received in the Binding Acknowledgement was L_ack, 6377 and the current remaining lifetime of the Binding Update List 6378 entry is L_remain, then the new value for the remaining lifetime 6379 of the Binding Update List entry should be 6381 max((L_remain - (L_update - L_ack)), 0) 6383 where max(X, Y) is the maximum of X and Y. The effect of this step 6384 is to correctly manage the mobile node's view of the binding's 6385 remaining lifetime (as maintained in the corresponding Binding 6386 Update List entry) so that it correctly counts down from the 6387 Lifetime value given in the Binding Acknowledgement, but with the 6388 timer countdown beginning at the time that the Binding Update was 6389 sent. 6391 Mobile nodes SHOULD send a new Binding Update well before the 6392 expiration of this period in order to extend the lifetime. This 6393 helps to avoid disruptions in communications which might otherwise 6394 be caused by network delays or clock drift. 6396 o If the Binding Acknowledgement correctly passes authentication and 6397 the Status field value is 135 (Sequence Number out of window), 6398 then the mobile node MUST update its binding sequence number 6399 appropriately to match the sequence number given in the Binding 6400 Acknowledgement. Otherwise, if the Status field value is 135 but 6401 the Binding Acknowledgement does not pass authentication, the 6402 message MUST be silently ignored. 6404 o If the Status field value is 1 (accepted but prefix discovery 6405 necessary), the mobile node SHOULD send a Mobile Prefix 6406 Solicitation message to update its information about the available 6407 prefixes. 6409 o If the Status field indicates that the Binding Update was rejected 6410 (the Status field is greater than or equal to 128), then the 6411 mobile node can take steps to correct the cause of the error and 6412 retransmit the Binding Update (with a new Sequence Number value), 6413 subject to the rate limiting restriction specified in 6414 Section 11.8. If this is not done or it fails, then the mobile 6415 node SHOULD record in its Binding Update List that future Binding 6416 Updates SHOULD NOT be sent to this destination. 6418 The treatment of a Binding Refresh Advice mobility option within the 6419 Binding Acknowledgement depends on where the acknowledgement came 6420 from. This option MUST be ignored if the acknowledgement came from a 6421 correspondent node. If it came from the home agent, the mobile node 6422 uses the Refresh Interval field in the option as a suggestion that it 6423 SHOULD attempt to refresh its home registration at the indicated 6424 shorter interval. 6426 If the acknowledgement came from the home agent, the mobile node 6427 examines the value of the Key Management Mobility Capability (K) bit. 6428 If this bit is not set, the mobile node SHOULD discard key management 6429 protocol connections, if any, to the home agent. The mobile node MAY 6430 also initiate a new key management connection. 6432 If this bit is set, the mobile node SHOULD move its own endpoint in 6433 the key management protocol connections to the home agent, if any. 6434 The mobile node's new endpoint should be the new care-of address. 6436 11.7.4. Receiving Binding Refresh Requests 6438 When a mobile node receives a packet containing a Binding Refresh 6439 Request message, if the mobile node has a Binding Update List entry 6440 for the source of the Binding Refresh Request, and the mobile node 6441 wants to retain its binding cache entry at the correspondent node, 6442 then the mobile node should start a return routability procedure. If 6443 the mobile node wants to have its binding cache entry removed, it can 6444 either ignore the Binding Refresh Request and wait for the binding to 6445 time out, or at any time, it can delete its binding from a 6446 correspondent node with an explicit binding update with a zero 6447 lifetime and the care-of address set to the home address. If the 6448 mobile node does not know if it needs the binding cache entry, it can 6449 make the decision in an implementation dependent manner, such as 6450 based on available resources. 6452 Note that the mobile node should be careful to not respond to Binding 6453 Refresh Requests for addresses not in the Binding Update List to 6454 avoid being subjected to a denial of service attack. 6456 If the return routability procedure completes successfully, a Binding 6457 Update message SHOULD be sent, as described in Section 11.7.2. The 6458 Lifetime field in this Binding Update SHOULD be set to a new 6459 lifetime, extending any current lifetime remaining from a previous 6460 Binding Update sent to this node (as indicated in any existing 6461 Binding Update List entry for this node), and the lifetime SHOULD 6462 again be less than or equal to the remaining lifetime of the home 6463 registration and the care-of address specified for the binding. When 6464 sending this Binding Update, the mobile node MUST update its Binding 6465 Update List in the same way as for any other Binding Update sent by 6466 the mobile node. 6468 11.8. Retransmissions and Rate Limiting 6470 The mobile node is responsible for retransmissions and rate limiting 6471 in the return routability procedure, registrations, and in solicited 6472 prefix discovery. 6474 When the mobile node sends a Mobile Prefix Solicitation, Home Test 6475 Init, Care-of Test Init or Binding Update for which it expects a 6476 response, the mobile node has to determine a value for the initial 6477 retransmission timer: 6479 o If the mobile node is sending a Mobile Prefix Solicitation, it 6480 SHOULD use an initial retransmission interval of 6481 INITIAL_SOLICIT_TIMER (see Section 12). 6483 o If the mobile node is sending a Binding Update and does not have 6484 an existing binding at the home agent, it SHOULD use 6485 InitialBindackTimeoutFirstReg (see Section 13) as a value for the 6486 initial retransmission timer. This long retransmission interval 6487 will allow the home agent to complete the Duplicate Address 6488 Detection procedure mandated in this case, as detailed in 6489 Section 11.7.1. 6491 o Otherwise, the mobile node should use the specified value of 6492 INITIAL_BINDACK_TIMEOUT for the initial retransmission timer. 6494 If the mobile node fails to receive a valid matching response within 6495 the selected initial retransmission interval, the mobile node SHOULD 6496 retransmit the message until a response is received. 6498 The retransmissions by the mobile node MUST use an exponential back- 6499 off process in which the timeout period is doubled upon each 6500 retransmission, until either the node receives a response or the 6501 timeout period reaches the value MAX_BINDACK_TIMEOUT. The mobile 6502 node MAY continue to send these messages at this slower rate 6503 indefinitely. 6505 The mobile node SHOULD start a separate back-off process for 6506 different message types, different home addresses and different 6507 care-of addresses. However, in addition an overall rate limitation 6508 applies for messages sent to a particular correspondent node. This 6509 ensures that the correspondent node has a sufficient amount of time 6510 to respond when bindings for multiple home addresses are registered, 6511 for instance. The mobile node MUST NOT send Mobility Header messages 6512 of a particular type to a particular correspondent node more than 6513 MAX_UPDATE_RATE times within a second. 6515 Retransmitted Binding Updates MUST use a Sequence Number value 6516 greater than that used for the previous transmission of this Binding 6517 Update. Retransmitted Home Test Init and Care-of Test Init messages 6518 MUST use new cookie values. 6520 12. Protocol Constants 6522 DHAAD_RETRIES 4 retransmissions 6523 INITIAL_BINDACK_TIMEOUT 1 second 6524 INITIAL_DHAAD_TIMEOUT 3 seconds 6525 INITIAL_SOLICIT_TIMER 3 seconds 6526 MAX_BINDACK_TIMEOUT 32 seconds 6527 MAX_DELETE_BCE_TIMEOUT 10 seconds 6528 MAX_NONCE_LIFETIME 240 seconds 6529 MAX_TOKEN_LIFETIME 210 seconds 6530 MAX_RO_FAILURE 3 retries 6531 MAX_RR_BINDING_LIFETIME 420 seconds 6532 MAX_UPDATE_RATE 3 times 6533 PREFIX_ADV_RETRIES 3 retransmissions 6534 PREFIX_ADV_TIMEOUT 3 seconds 6536 13. Protocol Configuration Variables 6538 MaxMobPfxAdvInterval Default: 86,400 seconds 6539 MinDelayBetweenRAs Default: 3 seconds, 6540 Min: 0.03 seconds 6541 MinMobPfxAdvInterval Default: 600 seconds 6542 InitialBindackTimeoutFirstReg Default: 1.5 seconds 6544 Home agents MUST allow the first three variables to be configured by 6545 system management, and mobile nodes MUST allow the last variable to 6546 be configured by system management. 6548 The default value for InitialBindackTimeoutFirstReg has been 6549 calculated as 1.5 times the default value of RetransTimer, as 6550 specified in Neighbor Discovery (RFC 4861 [17]) times the default 6551 value of DupAddrDetectTransmits, as specified in Stateless Address 6552 Autoconfiguration (RFC 4862 [18]) 6554 The value MinDelayBetweenRAs overrides the value of the protocol 6555 constant MIN_DELAY_BETWEEN_RAS, as specified in Neighbor Discovery 6556 (RFC 4861 [17]). This variable SHOULD be set to MinRtrAdvInterval, 6557 if MinRtrAdvInterval is less than 3 seconds. 6559 14. IANA Considerations 6561 This document defines a new IPv6 protocol, the Mobility Header, 6562 described in Section 6.1. This protocol has been assigned protocol 6563 number 135. 6565 This document also creates a new name space "Mobility Header Type", 6566 for the MH Type field in the Mobility Header. The current message 6567 types are described starting from Section 6.1.2, and are the 6568 following: 6570 0 Binding Refresh Request 6572 1 Home Test Init 6574 2 Care-of Test Init 6576 3 Home Test 6578 4 Care-of Test 6580 5 Binding Update 6582 6 Binding Acknowledgement 6584 7 Binding Error 6586 Future values of the MH Type can be allocated using Standards Action 6587 or IESG Approval [23]. 6589 Furthermore, each mobility message may contain mobility options as 6590 described in Section 6.2. This document defines a new name space 6591 "Mobility Option" to identify these options. The current mobility 6592 options are defined starting from Section 6.2.2 and are the 6593 following: 6595 0 Pad1 6597 1 PadN 6599 2 Binding Refresh Advice 6601 3 Alternate Care-of Address 6603 4 Nonce Indices 6604 5 Authorization Data 6606 Future values of the Option Type can be allocated using Standards 6607 Action or IESG Approval [23]. 6609 Finally, this document creates a third new name space "Status Code" 6610 for the Status field in the Binding Acknowledgement message. The 6611 current values are listed in Section 6.1.8 and are the following: 6613 0 Binding Update accepted 6615 1 Accepted but prefix discovery necessary 6617 128 Reason unspecified 6619 129 Administratively prohibited 6621 130 Insufficient resources 6623 131 Home registration not supported 6625 132 Not home subnet 6627 133 Not home agent for this mobile node 6629 134 Duplicate Address Detection failed 6631 135 Sequence number out of window 6633 136 Expired home nonce index 6635 137 Expired care-of nonce index 6637 138 Expired nonces 6639 139 Registration type change disallowed 6641 TBD Invalid Care-of Address 6643 Future values of the Status field can be allocated using Standards 6644 Action or IESG Approval [23]. 6646 All fields labeled "Reserved" are only to be assigned through 6647 Standards Action or IESG Approval. 6649 This document also defines a new IPv6 destination option, the Home 6650 Address option, described in Section 6.3. This option has been 6651 assigned the Option Type value 0xC9. 6653 This document also defines a new IPv6 type 2 routing header, 6654 described in Section 6.4. The value 2 has been allocated by IANA. 6656 In addition, this document defines four ICMP message types, two used 6657 as part of the dynamic home agent address discovery mechanism, and 6658 two used in lieu of Router Solicitations and Advertisements when the 6659 mobile node is away from the home link. These messages have been 6660 assigned ICMPv6 type numbers from the informational message range: 6662 o The Home Agent Address Discovery Request message, described in 6663 Section 6.5; 6665 o The Home Agent Address Discovery Reply message, described in 6666 Section 6.6; 6668 o The Mobile Prefix Solicitation, described in Section 6.7; and 6670 o The Mobile Prefix Advertisement, described in Section 6.8. 6672 This document also defines two new Neighbor Discovery [17] options, 6673 which have been assigned Option Type values within the option 6674 numbering space for Neighbor Discovery messages: 6676 o The Advertisement Interval option, described in Section 7.3; and 6678 o The Home Agent Information option, described in Section 7.4. 6680 15. Security Considerations 6682 15.1. Threats 6684 Any mobility solution must protect itself against misuses of the 6685 mobility features and mechanisms. In Mobile IPv6, most of the 6686 potential threats are concerned with false Bindings, usually 6687 resulting in Denial-of-Service attacks. Some of the threats also 6688 pose potential for Man-in-the-Middle, Hijacking, Confidentiality, and 6689 Impersonation attacks. The main threats this protocol protects 6690 against are the following: 6692 o Threats involving Binding Updates sent to home agents and 6693 correspondent nodes. For instance, an attacker might claim that a 6694 certain mobile node is currently at a different location than it 6695 really is. If a home agent accepts such spoofed information sent 6696 to it, the mobile node might not get traffic destined to it. 6697 Similarly, a malicious (mobile) node might use the home address of 6698 a victim node in a forged Binding Update sent to a correspondent 6699 node. 6701 These pose threats against confidentiality, integrity, and 6702 availability. That is, an attacker might learn the contents of 6703 packets destined to another node by redirecting the traffic to 6704 itself. Furthermore, an attacker might use the redirected packets 6705 in an attempt to set itself as a Man-in-the-Middle between a 6706 mobile and a correspondent node. This would allow the attacker to 6707 impersonate the mobile node, leading to integrity and availability 6708 problems. 6710 A malicious (mobile) node might also send Binding Updates in which 6711 the care-of address is set to the address of a victim node. If 6712 such Binding Updates were accepted, the malicious node could lure 6713 the correspondent node into sending potentially large amounts of 6714 data to the victim; the correspondent node's replies to messages 6715 sent by the malicious mobile node will be sent to the victim host 6716 or network. This could be used to cause a Distributed Denial-of- 6717 Service attack. For example, the correspondent node might be a 6718 site that will send a high-bandwidth stream of video to anyone who 6719 asks for it. Note that the use of flow-control protocols such as 6720 TCP does not necessarily defend against this type of attack, 6721 because the attacker can fake the acknowledgements. Even keeping 6722 TCP initial sequence numbers secret does not help, because the 6723 attacker can receive the first few segments (including the ISN) at 6724 its own address, and only then redirect the stream to the victim's 6725 address. These types of attacks may also be directed to networks 6726 instead of nodes. Further variations of this threat are described 6727 elsewhere [28] [33]. 6729 An attacker might also attempt to disrupt a mobile node's 6730 communications by replaying a Binding Update that the node had 6731 sent earlier. If the old Binding Update was accepted, packets 6732 destined for the mobile node would be sent to its old location as 6733 opposed to its current location. 6735 A malicious mobile node associated to multiple home agents could 6736 create a routing loop amongst them. This can be achieved when a 6737 mobile node binds one home address located on a first home agent 6738 to another home address on a second home agent. This type of 6739 binding will force the home agents to route the same packet among 6740 each other without knowledge that a routing loop has been created. 6741 Such looping problem is limited to cases where a mobile node has 6742 multiple home agents and is permitted to be associated with the 6743 multiple home agents. For the single home agent case, a policy at 6744 the home agent would prevent the binding of one home address to 6745 another home address hosted by the same home agent. 6747 The potential problems caused by such routing loops in this 6748 scenario can be substantially reduced by use of the Tunnel-Limit 6749 Option specified in RFC 2473 [6]. 6751 In conclusion, there are Denial-of-Service, Man-in-the-Middle, 6752 Confidentiality, and Impersonation threats against the parties 6753 involved in sending legitimate Binding Updates, the threat of 6754 routing loops when there are multiple home agents, and Denial-of- 6755 Service threats against any other party. 6757 o Threats associated with payload packets: Payload packets exchanged 6758 with mobile nodes are exposed to similar threats as that of 6759 regular IPv6 traffic. However, Mobile IPv6 introduces the Home 6760 Address destination option, a new routing header type (type 2), 6761 and uses tunneling headers in the payload packets. The protocol 6762 must protect against potential new threats involving the use of 6763 these mechanisms. 6765 Third parties become exposed to a reflection threat via the Home 6766 Address destination option, unless appropriate security 6767 precautions are followed. The Home Address destination option 6768 could be used to direct response traffic toward a node whose IP 6769 address appears in the option. In this case, ingress filtering 6770 would not catch the forged "return address" [36] [40]. 6772 A similar threat exists with the tunnels between the mobile node 6773 and the home agent. An attacker might forge tunnel packets 6774 between the mobile node and the home agent, making it appear that 6775 the traffic is coming from the mobile node when it is not. Note 6776 that an attacker who is able to forge tunnel packets would 6777 typically also be able to forge packets that appear to come 6778 directly from the mobile node. This is not a new threat as such. 6779 However, it may make it easier for attackers to escape detection 6780 by avoiding ingress filtering and packet tracing mechanisms. 6781 Furthermore, spoofed tunnel packets might be used to gain access 6782 to the home network. 6784 Finally, a routing header could also be used in reflection 6785 attacks, and in attacks designed to bypass firewalls. The 6786 generality of the regular routing header would allow circumvention 6787 of IP-address based rules in firewalls. It would also allow 6788 reflection of traffic to other nodes. These threats exist with 6789 routing headers in general, even if the usage that Mobile IPv6 6790 requires is safe. 6792 o Threats associated with dynamic home agent and mobile prefix 6793 discovery. 6795 o Threats against the Mobile IPv6 security mechanisms themselves: An 6796 attacker might, for instance, lure the participants into executing 6797 expensive cryptographic operations or allocating memory for the 6798 purpose of keeping state. The victim node would have no resources 6799 left to handle other tasks. 6801 As a fundamental service in an IPv6 stack, Mobile IPv6 is expected to 6802 be deployed in most nodes of the IPv6 Internet. The above threats 6803 should therefore be considered as being applicable to the whole 6804 Internet. 6806 It should also be noted that some additional threats result from 6807 movements as such, even without the involvement of mobility 6808 protocols. Mobile nodes must be capable to defend themselves in the 6809 networks that they visit, as typical perimeter defenses applied in 6810 the home network no longer protect them. 6812 15.2. Features 6814 This specification provides a series of features designed to mitigate 6815 the risk introduced by the threats listed above. The main security 6816 features are the following: 6818 o Reverse Tunneling as a mandatory feature. 6820 o Protection of Binding Updates sent to home agents. 6822 o Protection of Binding Updates sent to correspondent nodes. 6824 o Protection against reflection attacks that use the Home Address 6825 destination option. 6827 o Protection of tunnels between the mobile node and the home agent. 6829 o Closing routing header vulnerabilities. 6831 o Mitigating Denial-of-Service threats to the Mobile IPv6 security 6832 mechanisms themselves. 6834 The support for encrypted reverse tunneling (see Section 11.3.1) 6835 allows mobile nodes to defeat certain kinds of traffic analysis. 6837 Protecting those Binding Updates that are sent to home agents and 6838 those that are sent to arbitrary correspondent nodes requires very 6839 different security solutions due to the different situations. Mobile 6840 nodes and home agents are naturally expected to be subject to the 6841 network administration of the home domain. 6843 Thus, they can and are supposed to have a security association that 6844 can be used to reliably authenticate the exchanged messages. See 6845 Section 5.1 for the description of the protocol mechanisms, and 6846 Section 15.3 below for a discussion of the resulting level of 6847 security. 6849 It is expected that Mobile IPv6 route optimization will be used on a 6850 global basis between nodes belonging to different administrative 6851 domains. It would be a very demanding task to build an 6852 authentication infrastructure on this scale. Furthermore, a 6853 traditional authentication infrastructure cannot be easily used to 6854 authenticate IP addresses because IP addresses can change often. It 6855 is not sufficient to just authenticate the mobile nodes; 6856 Authorization to claim the right to use an address is needed as well. 6857 Thus, an "infrastructureless" approach is necessary. The chosen 6858 infrastructureless method is described in Section 5.2, and 6859 Section 15.4 discusses the resulting security level and the design 6860 rationale of this approach. 6862 Specific rules guide the use of the Home Address destination option, 6863 the routing header, and the tunneling headers in the payload packets. 6864 These rules are necessary to remove the vulnerabilities associated 6865 with their unrestricted use. The effect of the rules is discussed in 6866 Section 15.7, Section 15.8, and Section 15.9. 6868 Denial-of-Service threats against Mobile IPv6 security mechanisms 6869 themselves concern mainly the Binding Update procedures with 6870 correspondent nodes. The protocol has been designed to limit the 6871 effects of such attacks, as will be described in Section 15.4.5. 6873 15.3. Binding Updates to Home Agent 6875 Signaling between the mobile node and the home agent requires message 6876 integrity. This is necessary to assure the home agent that a Binding 6877 Update is from a legitimate mobile node. In addition, correct 6878 ordering and anti-replay protection are optionally needed. 6880 IPsec ESP protects the integrity of the Binding Updates and Binding 6881 Acknowledgements by securing mobility messages between the mobile 6882 node and the home agent. 6884 IPsec can provide anti-replay protection only if dynamic keying is 6885 used (which may not always be the case). IPsec does not guarantee 6886 correct ordering of packets, only that they have not been replayed. 6887 Because of this, sequence numbers within the Mobile IPv6 messages are 6888 used to ensure correct ordering (see Section 5.1). However, if the 6889 16 bit Mobile IPv6 sequence number space is cycled through, or the 6890 home agent reboots and loses its state regarding the sequence 6891 numbers, replay and reordering attacks become possible. The use of 6892 dynamic keying, IPsec anti-replay protection, and the Mobile IPv6 6893 sequence numbers can together prevent such attacks. It is also 6894 recommended that use of non-volatile storage be considered for home 6895 agents, to avoid losing their state. 6897 A sliding window scheme is used for the sequence numbers. The 6898 protection against replays and reordering attacks without a key 6899 management mechanism works when the attacker remembers up to a 6900 maximum of 2**15 Binding Updates. 6902 The above mechanisms do not show that the care-of address given in 6903 the Binding Update is correct. This opens the possibility for 6904 Denial-of-Service attacks against third parties. However, since the 6905 mobile node and home agent have a security association, the home 6906 agent can always identify an ill-behaving mobile node. This allows 6907 the home agent operator to discontinue the mobile node's service, and 6908 possibly take further actions based on the business relationship with 6909 the mobile node's owner. 6911 Note that the use of a single pair of manually keyed security 6912 associations conflicts with the generation of a new home address [20] 6913 for the mobile node, or with the adoption of a new home subnet 6914 prefix. This is because IPsec security associations are bound to the 6915 used addresses. While certificate-based automatic keying alleviates 6916 this problem to an extent, it is still necessary to ensure that a 6917 given mobile node cannot send Binding Updates for the address of 6918 another mobile node. In general, this leads to the inclusion of home 6919 addresses in certificates in the Subject AltName field. This again 6920 limits the introduction of new addresses without either manual or 6921 automatic procedures to establish new certificates. Therefore, this 6922 specification restricts the generation of new home addresses (for any 6923 reason) to those situations where a security association or 6924 certificate for the new address already exists. 6926 Support for IKEv2 has been specified as optional. The following 6927 should be observed about the use of manual keying: 6929 o As discussed above, with manually keyed IPsec, only a limited form 6930 of protection exists against replay and reordering attacks. A 6931 vulnerability exists if either the sequence number space is cycled 6932 through, or if the home agent reboots and forgets its sequence 6933 numbers (and uses volatile memory to store the sequence numbers). 6935 Assuming the mobile node moves continuously every 10 minutes, it 6936 takes roughly 455 days before the sequence number space has been 6937 cycled through. Typical movement patterns rarely reach this high 6938 frequency today. 6940 o A mobile node and its home agent belong to the same domain. If 6941 this were not the case, manual keying would not be possible [39], 6942 but in Mobile IPv6 only these two parties need to know the 6943 manually configured keys. Similarly, we note that Mobile IPv6 6944 employs standard block ciphers in IPsec, and is not vulnerable to 6945 problems associated with stream ciphers and manual keying. 6947 o It is expected that the owner of the mobile node and the 6948 administrator of the home agent agree on the used keys and other 6949 parameters with some off-line mechanism. 6951 The use of IKEv2 with Mobile IPv6 is documented in more detail in 6952 [42]. The following should be observed regarding the use of IKEv2: 6954 o It is necessary to prevent a mobile node from claiming another 6955 mobile node's home address. The home agent must verify that the 6956 mobile node trying to negotiate the SA for a particular home 6957 address is authorized for that home address. This implies that 6958 even with the use of IKEv2, a policy entry needs to be configured 6959 for each home address served by the home agent. 6961 It may be possible to include home addresses in the Subject 6962 AltName field of certificate to avoid this. However, 6963 implementations are not guaranteed to support the use of a 6964 particular IP address (care-of address) while another address 6965 (home address) appears in the certificate. In any case, even this 6966 approach would require user-specific tasks in the certificate 6967 authority. 6969 o Due to the problems outlined in Section 11.3.2, the IKEv2 SA 6970 between the mobile node and its home agent is established using 6971 the mobile node's current care-of address. This implies that when 6972 the mobile node moves to a new location, it may have to re- 6973 establish an IKEv2 Security Association. A Key Management 6974 Mobility Capability (K) flag is provided for implementations that 6975 can update the IKEv2 endpoints without re-establishing an IKEv2 6976 Security Association, but the support for this behavior is 6977 optional. 6979 o Nevertheless, even if per-mobile node configuration is required 6980 with IKEv2, an important benefit of IKEv2 is that it automates the 6981 negotiation of cryptographic parameters, including the SPIs, 6982 cryptographic algorithms, and so on. Thus, less configuration 6983 information is needed. 6985 o The frequency of movements in some link layers or deployment 6986 scenarios may be high enough to make replay and reordering attacks 6987 possible, if only manual keying is used. IKEv2 SHOULD be used in 6988 such cases. Potentially vulnerable scenarios involve continuous 6989 movement through small cells, or uncontrolled alternation between 6990 available network attachment points. 6992 o Similarly, in some deployment scenarios the number of mobile nodes 6993 may be very large. In these cases, it can be necessary to use 6994 automatic mechanisms to reduce the management effort in the 6995 administration of cryptographic parameters, even if some per- 6996 mobile node configuration is always needed. IKEv2 SHOULD also be 6997 used in such cases. 6999 15.4. Binding Updates to Correspondent Nodes 7001 The motivation for designing the return routability procedure was to 7002 have sufficient support for Mobile IPv6, without creating significant 7003 new security problems. The goal for this procedure was not to 7004 protect against attacks that were already possible before the 7005 introduction of Mobile IPv6. 7007 The next sections will describe the security properties of the used 7008 method, both from the point of view of possible on-path attackers who 7009 can see those cryptographic values that have been sent in the clear 7010 (Section 15.4.2 and Section 15.4.3) and from the point of view of 7011 other attackers (Section 15.4.6). 7013 15.4.1. Overview 7015 The chosen infrastructureless method verifies that the mobile node is 7016 "live" (that is, it responds to probes) at its home and care-of 7017 addresses. Section 5.2 describes the return routability procedure in 7018 detail. The procedure uses the following principles: 7020 o A message exchange verifies that the mobile node is reachable at 7021 its addresses, i.e., is at least able to transmit and receive 7022 traffic at both the home and care-of addresses. 7024 o The eventual Binding Update is cryptographically bound to the 7025 tokens supplied in the exchanged messages. 7027 o Symmetric exchanges are employed to avoid the use of this protocol 7028 in reflection attacks. In a symmetric exchange, the responses are 7029 always sent to the same address the request was sent from. 7031 o The correspondent node operates in a stateless manner until it 7032 receives a fully authorized Binding Update. 7034 o Some additional protection is provided by encrypting the tunnels 7035 between the mobile node and home agent with IPsec ESP. As the 7036 tunnel also transports the nonce exchanges, the ability of 7037 attackers to see these nonces is limited. For instance, this 7038 prevents attacks from being launched from the mobile node's 7039 current foreign link, even when no link-layer confidentiality is 7040 available. 7042 The resulting level of security is in theory the same even without 7043 this additional protection: the return routability tokens are 7044 still exposed only to one path within the whole Internet. 7045 However, the mobile nodes are often found on an insecure link, 7046 such as a public access Wireless LAN. Thus, in many cases, this 7047 addition makes a practical difference. 7049 For further information about the design rationale of the return 7050 routability procedure, see [28] [33] [32] [40]. The mechanisms used 7051 have been adopted from these documents. 7053 15.4.2. Achieved Security Properties 7055 The return routability procedure protects Binding Updates against all 7056 attackers who are unable to monitor the path between the home agent 7057 and the correspondent node. The procedure does not defend against 7058 attackers who can monitor this path. Note that such attackers are in 7059 any case able to mount an active attack against the mobile node when 7060 it is at its home location. The possibility of such attacks is not 7061 an impediment to the deployment of Mobile IPv6 because these attacks 7062 are possible regardless of whether or not Mobile IPv6 is in use. 7064 This procedure also protects against Denial-of-Service attacks in 7065 which the attacker pretends to be mobile, but uses the victim's 7066 address as the care-of address. This would cause the correspondent 7067 node to send the victim some unexpected traffic. This procedure 7068 defends against these attacks by requiring at least the passive 7069 presence of the attacker at the care-of address or on the path from 7070 the correspondent to the care-of address. Normally, this will be the 7071 mobile node. 7073 15.4.3. Comparison to Regular IPv6 Communications 7075 This section discusses the protection offered by the return 7076 routability method by comparing it to the security of regular IPv6 7077 communications. We will divide vulnerabilities into three classes: 7078 (1) those related to attackers on the local network of the mobile 7079 node, home agent, or the correspondent node, (2) those related to 7080 attackers on the path between the home network and the correspondent 7081 node, and (3) off-path attackers, i.e., the rest of the Internet. 7083 We will now discuss the vulnerabilities of regular IPv6 7084 communications. The on-link vulnerabilities of IPv6 communications 7085 include Denial-of-Service, Masquerading, Man-in-the-Middle, 7086 Eavesdropping, and other attacks. These attacks can be launched 7087 through spoofing Router Discovery, Neighbor Discovery and other IPv6 7088 mechanisms. Some of these attacks can be prevented with the use of 7089 cryptographic protection in the packets. 7091 A similar situation exists with on-path attackers. That is, without 7092 cryptographic protection, the traffic is completely vulnerable. 7094 Assuming that attackers have not penetrated the security of the 7095 Internet routing protocols, attacks are much harder to launch from 7096 off-path locations. Attacks that can be launched from these 7097 locations are mainly Denial-of-Service attacks, such as flooding 7098 and/or reflection attacks. It is not possible for an off-path 7099 attacker to become a Man-in-the-Middle. 7101 Next, we will consider the vulnerabilities that exist when IPv6 is 7102 used together with Mobile IPv6 and the return routability procedure. 7103 On the local link, the vulnerabilities are the same as those in IPv6, 7104 but Masquerade and Man-in-the-Middle attacks can now also be launched 7105 against future communications, and not just against current 7106 communications. If a Binding Update was sent while the attacker was 7107 present on the link, its effects remain for the lifetime of the 7108 binding. This happens even if the attacker moves away from the link. 7109 In contrast, an attacker who uses only plain IPv6 generally has to 7110 stay on the link in order to continue the attack. Note that in order 7111 to launch these new attacks, the IP address of the victim must be 7112 known. This makes this attack feasible, mainly in the context of 7113 well-known interface IDs, such as those already appearing in the 7114 traffic on the link or registered in the DNS. 7116 On-path attackers can exploit similar vulnerabilities as in regular 7117 IPv6. There are some minor differences, however. Masquerade, Man- 7118 in-the-Middle, and Denial-of-Service attacks can be launched with 7119 just the interception of a few packets, whereas in regular IPv6 it is 7120 necessary to intercept every packet. The effect of the attacks is 7121 the same regardless of the method, however. In any case, the most 7122 difficult task an attacker faces in these attacks is getting on the 7123 right path. 7125 The vulnerabilities for off-path attackers are the same as in regular 7126 IPv6. Those nodes that are not on the path between the home agent 7127 and the correspondent node will not be able to receive the home 7128 address probe messages. 7130 In conclusion, we can state the following main results from this 7131 comparison: 7133 o Return routability prevents any off-path attacks beyond those that 7134 are already possible in regular IPv6. This is the most important 7135 result, preventing attackers on the Internet from exploiting any 7136 vulnerabilities. 7138 o Vulnerabilities to attackers on the home agent link, the 7139 correspondent node link, and the path between them are roughly the 7140 same as in regular IPv6. 7142 o However, one difference is that in basic IPv6 an on-path attacker 7143 must be constantly present on the link or the path, whereas with 7144 Mobile IPv6, an attacker can leave a binding behind after moving 7145 away. 7147 For this reason, this specification limits the creation of 7148 bindings to at most MAX_TOKEN_LIFETIME seconds after the last 7149 routability check has been performed, and limits the duration of a 7150 binding to at most MAX_RR_BINDING_LIFETIME seconds. With these 7151 limitations, attackers cannot take any practical advantages of 7152 this vulnerability. 7154 o There are some other minor differences, such as an effect to the 7155 Denial-of-Service vulnerabilities. These can be considered to be 7156 insignificant. 7158 o The path between the home agent and a correspondent node is 7159 typically easiest to attack on the links at either end, in 7160 particular if these links are publicly accessible wireless LANs. 7162 Attacks against the routers or switches on the path are typically 7163 harder to accomplish. The security on layer 2 of the links plays 7164 then a major role in the resulting overall network security. 7165 Similarly, security of IPv6 Neighbor and Router Discovery on these 7166 links has a large impact. If these were secured using some new 7167 technology in the future, this could change the situation 7168 regarding the easiest point of attack. 7170 For a more in-depth discussion of these issues, see [40]. 7172 15.4.4. Replay Attacks 7174 The return routability procedure also protects the participants 7175 against replayed Binding Updates. The attacker is unable replay the 7176 same message due to the sequence number which is a part of the 7177 Binding Update. It is also unable to modify the Binding Update since 7178 the MAC verification would fail after such a modification. 7180 Care must be taken when removing bindings at the correspondent node, 7181 however. If a binding is removed while the nonce used in its 7182 creation is still valid, an attacker could replay the old Binding 7183 Update. Rules outlined in Section 5.2.8 ensure that this cannot 7184 happen. 7186 15.4.5. Denial-of-Service Attacks 7188 The return routability procedure has protection against resource 7189 exhaustion Denial-of-Service attacks. The correspondent nodes do not 7190 retain any state about individual mobile nodes until an authentic 7191 Binding Update arrives. This is achieved through the construct of 7192 keygen tokens from the nonces and node keys that are not specific to 7193 individual mobile nodes. The keygen tokens can be reconstructed by 7194 the correspondent node, based on the home and care-of address 7195 information that arrives with the Binding Update. This means that 7196 the correspondent nodes are safe against memory exhaustion attacks 7197 except where on-path attackers are concerned. Due to the use of 7198 symmetric cryptography, the correspondent nodes are relatively safe 7199 against CPU resource exhaustion attacks as well. 7201 Nevertheless, as [28] describes, there are situations in which it is 7202 impossible for the mobile and correspondent nodes to determine if 7203 they actually need a binding or whether they just have been fooled 7204 into believing so by an attacker. Therefore, it is necessary to 7205 consider situations where such attacks are being made. 7207 Even if route optimization is a very important optimization, it is 7208 still only an optimization. A mobile node can communicate with a 7209 correspondent node even if the correspondent refuses to accept any 7210 Binding Updates. However, performance will suffer because packets 7211 from the correspondent node to the mobile node will be routed via the 7212 mobile's home agent rather than a more direct route. A correspondent 7213 node can protect itself against some of these resource exhaustion 7214 attacks as follows. If the correspondent node is flooded with a 7215 large number of Binding Updates that fail the cryptographic integrity 7216 checks, it can stop processing Binding Updates. If a correspondent 7217 node finds that it is spending more resources on checking bogus 7218 Binding Updates than it is likely to save by accepting genuine 7219 Binding Updates, then it may silently discard some or all Binding 7220 Updates without performing any cryptographic operations. 7222 Layers above IP can usually provide additional information to help 7223 determine whether there is a need to establish a binding with a 7224 specific peer. For example, TCP knows if the node has a queue of 7225 data that it is trying to send to a peer. An implementation of this 7226 specification is not required to make use of information from higher 7227 protocol layers, but some implementations are likely to be able to 7228 manage resources more effectively by making use of such information. 7230 We also require that all implementations be capable of 7231 administratively disabling route optimization. 7233 15.4.6. Key Lengths 7235 Attackers can try to break the return routability procedure in many 7236 ways. Section 15.4.2 discusses the situation where the attacker can 7237 see the cryptographic values sent in the clear, and Section 15.4.3 7238 discusses the impact this has on IPv6 communications. This section 7239 discusses whether attackers can guess the correct values without 7240 seeing them. 7242 While the return routability procedure is in progress, 64 bit cookies 7243 are used to protect spoofed responses. This is believed to be 7244 sufficient, given that to blindly spoof a response a very large 7245 number of messages would have to be sent before success would be 7246 probable. 7248 The tokens used in the return routability procedure provide together 7249 128 bits of information. This information is used internally as 7250 input to a hash function to produce a 160 bit quantity suitable for 7251 producing the keyed hash in the Binding Update using the HMAC_SHA1 7252 algorithm. The final keyed hash length is 96 bits. The limiting 7253 factors in this case are the input token lengths and the final keyed 7254 hash length. The internal hash function application does not reduce 7255 the entropy. 7257 The 96 bit final keyed hash is of typical size and is believed to be 7258 secure. The 128 bit input from the tokens is broken in two pieces, 7259 the home keygen token and the care-of keygen token. An attacker can 7260 try to guess the correct cookie value, but again this would require a 7261 large number of messages (an the average 2**63 messages for one or 7262 2**127 for two). Furthermore, given that the cookies are valid only 7263 for a short period of time, the attack has to keep a high constant 7264 message rate to achieve a lasting effect. This does not appear 7265 practical. 7267 When the mobile node is returning home, it is allowed to use just the 7268 home keygen token of 64 bits. This is less than 128 bits, but 7269 attacking it blindly would still require a large number of messages 7270 to be sent. If the attacker is on the path and capable of seeing the 7271 Binding Update, it could conceivably break the keyed hash with brute 7272 force. However, in this case the attacker has to be on the path, 7273 which appears to offer easier ways for denial-of-service than 7274 preventing route optimization. 7276 15.5. Dynamic Home Agent Address Discovery 7278 The dynamic home agent address discovery function could be used to 7279 learn the addresses of home agents in the home network. 7281 The ability to learn addresses of nodes may be useful to attackers 7282 because brute-force scanning of the address space is not practical 7283 with IPv6. Thus, they could benefit from any means which make 7284 mapping the networks easier. For example, if a security threat 7285 targeted at routers or even home agents is discovered, having a 7286 simple ICMP mechanism to easily find out possible targets may prove 7287 to be an additional (though minor) security risk. 7289 This document does not define any authentication mechanism for 7290 dynamic home agent address discovery messages. Therefore the home 7291 agent cannot verify the home address of the mobile node that 7292 requested the list of home agents. 7294 Apart from discovering the address(es) of home agents, attackers will 7295 not be able to learn much from this information, and mobile nodes 7296 cannot be tricked into using wrong home agents, as all other 7297 communication with the home agents is secure. 7299 In cases where additional security is needed, one may consider 7300 instead the use of MIPv6 bootstrapping [22], (based on DNS SRV 7301 Resource Records [9]) in conjunction with security mechanisms 7302 suggested in these specifications. In that solution, security is 7303 provided by the DNSSEC [13] framework. The needed pre-configured 7304 data on the mobile node for this mechanism is the domain name of the 7305 mobile service provider, which is marginally better than the home 7306 subnet prefix. For the security, a trust anchor which dominates the 7307 domain is needed. 7309 15.6. Mobile Prefix Discovery 7311 The mobile prefix discovery function may leak interesting information 7312 about network topology and prefix lifetimes to eavesdroppers; for 7313 this reason, requests for this information have to be authenticated. 7314 Responses and unsolicited prefix information needs to be 7315 authenticated to prevent the mobile nodes from being tricked into 7316 believing false information about the prefixes and possibly 7317 preventing communications with the existing addresses. Optionally, 7318 encryption may be applied to prevent leakage of the prefix 7319 information. 7321 15.7. Tunneling via the Home Agent 7323 Tunnels between the mobile node and the home agent can be protected 7324 by ensuring proper use of source addresses, and optional 7325 cryptographic protection. These procedures are discussed in 7326 Section 5.5. 7328 Binding Updates to the home agents are secure. When receiving 7329 tunneled traffic, the home agent verifies that the outer IP address 7330 corresponds to the current location of the mobile node. This acts as 7331 a weak form of protection against spoofing packets that appear to 7332 come from the mobile node. This is particularly useful, if no end- 7333 to-end security is being applied between the mobile and correspondent 7334 nodes. The outer IP address check prevents attacks where the 7335 attacker is controlled by ingress filtering. It also prevents 7336 attacks when the attacker does not know the current care-of address 7337 of the mobile node. Attackers who know the care-of address and are 7338 not controlled by ingress filtering could still send traffic through 7339 the home agent. This includes attackers on the same local link as 7340 the mobile node is currently on. But such attackers could send 7341 packets that appear to come from the mobile node without attacking 7342 the tunnel; the attacker could simply send packets with the source 7343 address set to the mobile node's home address. However, this attack 7344 does not work if the final destination of the packet is in the home 7345 network, and some form of perimeter defense is being applied for 7346 packets sent to those destinations. In such cases it is recommended 7347 that either end-to-end security or additional tunnel protection be 7348 applied, as is usual in remote access situations. 7350 Home agents and mobile nodes may use IPsec ESP to protect payload 7351 packets tunneled between themselves. This is useful for protecting 7352 communications against attackers on the path of the tunnel. 7354 When a Unique-Local Address (ULA) RFC4193 [19] is used as a home 7355 address, reverse tunneling can be used to send local traffic from 7356 another location. Administrators should be aware of this when 7357 allowing such home addresses. In particular, the outer IP address 7358 check described above is not sufficient against all attackers. The 7359 use of encrypted tunnels is particularly useful for these kinds of 7360 home addresses. 7362 15.8. Home Address Option 7364 When the mobile node sends packets directly to the correspondent 7365 node, the Source Address field of the packet's IPv6 header is the 7366 care-of address. Therefore, ingress filtering [27] works in the 7367 usual manner even for mobile nodes, as the Source Address is 7368 topologically correct. The Home Address option is used to inform the 7369 correspondent node of the mobile node's home address. 7371 However, the care-of address in the Source Address field does not 7372 survive in replies sent by the correspondent node unless it has a 7373 binding for this mobile node. Also, not all attacker tracing 7374 mechanisms work when packets are being reflected through 7375 correspondent nodes using the Home Address option. For these 7376 reasons, this specification restricts the use of the Home Address 7377 option. It may only be used when a binding has already been 7378 established with the participation of the node at the home address, 7379 as described in Section 5.5 and Section 6.3. This prevents 7380 reflection attacks through the use of the Home Address option. It 7381 also ensures that the correspondent nodes reply to the same address 7382 that the mobile node sends traffic from. 7384 No special authentication of the Home Address option is required 7385 beyond the above, but note that if the IPv6 header of a packet is 7386 covered by IPsec Authentication Header, then that authentication 7387 covers the Home Address option as well. Thus, even when 7388 authentication is used in the IPv6 header, the security of the Source 7389 Address field in the IPv6 header is not compromised by the presence 7390 of a Home Address option. Without authentication of the packet, any 7391 field in the IPv6 header, including the Source Address field or any 7392 other part of the packet and the Home Address option can be forged or 7393 modified in transit. In this case, the contents of the Home Address 7394 option is no more suspect than any other part of the packet. 7396 15.9. Type 2 Routing Header 7398 The definition of the type 2 routing header is described in 7399 Section 6.4. This definition and the associated processing rules 7400 have been chosen so that the header cannot be used for what is 7401 traditionally viewed as source routing. In particular, the Home 7402 Address in the routing header will always have to be assigned to the 7403 home address of the receiving node; otherwise the packet will be 7404 dropped. 7406 Generally, source routing has a number of security concerns. These 7407 include the automatic reversal of unauthenticated source routes 7408 (which is an issue for IPv4, but not for IPv6). Another concern is 7409 the ability to use source routing to "jump" between nodes inside, as 7410 well as outside a firewall. These security concerns are not issues 7411 in Mobile IPv6, due to the rules mentioned above. 7413 In essence the semantics of the type 2 routing header is the same as 7414 a special form of IP-in-IP tunneling where the inner and outer source 7415 addresses are the same. 7417 This implies that a device which implements the filtering of packets 7418 should be able to distinguish between a type 2 routing header and 7419 other routing headers, as required in Section 8.3. This is necessary 7420 in order to allow Mobile IPv6 traffic while still having the option 7421 of filtering out other uses of routing headers. 7423 16. Contributors 7425 Work done by Tuomas Aura, Mike Roe, Greg O'Shea, Pekka Nikander, Erik 7426 Nordmark, and Michael Thomas shaped the return routability protocols 7427 described in [33]. 7429 Significant contributions were made by members of the Mobile IPv6 7430 Security Design Team, including (in alphabetical order) Gabriel 7431 Montenegro, Erik Nordmark and Pekka Nikander. 7433 17. Acknowledgements 7435 We would like to thank the members of the Mobile IP, Mobility 7436 Extensions for IPv6, and IPng Working Groups for their comments and 7437 suggestions on this work. We would particularly like to thank (in 7438 alphabetical order) Fred Baker, Josh Broch, Samita Chakrabarti, 7439 Robert Chalmers, Noel Chiappa, Jean-Michel Combes, Greg Daley, Vijay 7440 Devarapalli, Rich Draves, Francis Dupont, Ashutosh Dutta, Arnaud 7441 Ebalard, Wesley Eddy, Thomas Eklund, Jun-Ichiro Itojun Hagino, Brian 7442 Haley, Marc Hasson, John Ioannidis, James Kempf, Rajeev Koodli, 7443 Suresh Krishnan, Krishna Kumar, T.J. Kniveton, Joe Lau, Aime Le 7444 Rouzic, Julien Laganier, Jiwoong Lee, Benjamin Lim, Vesa-Matti 7445 Mantyla, Kevin Miles, Glenn Morrow, Ahmad Muhanna, Thomas Narten, 7446 Karen Nielsen, Simon Nybroe, David Oran, Mohan Parthasarathy, 7447 Basavaraj Patil, Brett Pentland, Lars Henrik Petander, Alexandru 7448 Petrescu, Mattias Petterson, Ken Powell, Ed Remmell, Phil Roberts, 7449 Patrice Romand, Luis A. Sanchez, Pekka Savola, Jeff Schiller, Arvind 7450 Sevalkar, Keiichi Shima, Tom Soderlund, Hesham Soliman, Jim Solomon, 7451 Tapio Suihko, Dave Thaler, Pascal Thubert, Benny Van Houdt, Jon-Olov 7452 Vatn, Ryuji Wakikawa, Kilian Weniger, Carl E. Williams, Vladislav 7453 Yasevich, Alper Yegin, and Xinhua Zhao, for their detailed reviews of 7454 earlier versions of this document. Their suggestions have helped to 7455 improve both the design and presentation of the protocol. 7457 We would also like to thank the participants of the Mobile IPv6 7458 testing event (1999), implementors who participated in Mobile IPv6 7459 interoperability testing at Connectathons (2000, 2001, 2002, and 7460 2003), and the participants at the ETSI interoperability testing 7461 (2000, 2002). Finally, we would like to thank the TAHI project who 7462 has provided test suites for Mobile IPv6. 7464 18. References 7466 18.1. Normative References 7468 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 7469 Levels", BCP 14, RFC 2119, March 1997. 7471 [2] Kent, S. and R. Atkinson, "Security Architecture for the 7472 Internet Protocol", RFC 2401, November 1998. 7474 [3] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, 7475 November 1998. 7477 [4] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload 7478 (ESP)", RFC 2406, November 1998. 7480 [5] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) 7481 Specification", RFC 2460, December 1998. 7483 [6] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 7484 Specification", RFC 2473, December 1998. 7486 [7] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast 7487 Addresses", RFC 2526, March 1999. 7489 [8] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener 7490 Discovery (MLD) for IPv6", RFC 2710, October 1999. 7492 [9] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 7493 specifying the location of services (DNS SRV)", RFC 2782, 7494 February 2000. 7496 [10] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On- 7497 line Database", RFC 3232, January 2002. 7499 [11] National Institute of Standards and Technology, "Secure Hash 7500 Standard", FIPS PUB 180-1, April 1995, 7501 . 7503 [12] Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to 7504 Protect Mobile IPv6 Signaling Between Mobile Nodes and Home 7505 Agents", RFC 3776, June 2004. 7507 [13] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, 7508 "DNS Security Introduction and Requirements", RFC 4033, 7509 March 2005. 7511 [14] Eastlake, D., Schiller, J., and S. Crocker, "Randomness 7512 Requirements for Security", BCP 106, RFC 4086, June 2005. 7514 [15] Hinden, R. and S. Deering, "IP Version 6 Addressing 7515 Architecture", RFC 4291, February 2006. 7517 [16] Conta, A., Deering, S., and M. Gupta, "Internet Control Message 7518 Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) 7519 Specification", RFC 4443, March 2006. 7521 [17] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 7522 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 7523 September 2007. 7525 [18] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address 7526 Autoconfiguration", RFC 4862, September 2007. 7528 [19] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast 7529 Addresses", RFC 4193, October 2005. 7531 [20] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions 7532 for Stateless Address Autoconfiguration in IPv6", RFC 4941, 7533 September 2007. 7535 [21] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6 Socket 7536 API for Source Address Selection", RFC 5014, September 2007. 7538 [22] Giaretta, G., Kempf, J., and V. Devarapalli, "Mobile IPv6 7539 Bootstrapping in Split Scenario", RFC 5026, October 2007. 7541 [23] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA 7542 Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 7544 18.2. Informative References 7546 [24] Perkins, C., "IP Encapsulation within IP", RFC 2003, 7547 October 1996. 7549 [25] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, 7550 October 1996. 7552 [26] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing 7553 for Message Authentication", RFC 2104, February 1997. 7555 [27] Ferguson, P. and D. Senie, "Network Ingress Filtering: 7556 Defeating Denial of Service Attacks which employ IP Source 7557 Address Spoofing", BCP 38, RFC 2827, May 2000. 7559 [28] Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses", 7560 draft-aura-mipv6-bu-attacks-01 (work in progress), March 2002. 7562 [29] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. 7563 Carney, "Dynamic Host Configuration Protocol for IPv6 7564 (DHCPv6)", RFC 3315, July 2003. 7566 [30] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, 7567 August 2002. 7569 [31] Draves, R., "Default Address Selection for Internet Protocol 7570 version 6 (IPv6)", RFC 3484, February 2003. 7572 [32] Nordmark, E., "Securing MIPv6 BUs using return routability 7573 (BU3WAY)", draft-nordmark-mobileip-bu3way-00 (work in 7574 progress), November 2001. 7576 [33] Roe, M., "Authentication of Mobile IPv6 Binding Updates and 7577 Acknowledgments", draft-roe-mobileip-updateauth-02 (work in 7578 progress), March 2002. 7580 [34] Chowdhury, K. and A. Yegin, "MIP6-bootstrapping for the 7581 Integrated Scenario", 7582 draft-ietf-mip6-bootstrapping-integrated-dhc-06 (work in 7583 progress), April 2008. 7585 [35] Savola, P., "Use of /127 Prefix Length Between Routers 7586 Considered Harmful", RFC 3627, September 2003. 7588 [36] Savola, P., "Security of IPv6 Routing Header and Home Address 7589 Options", draft-savola-ipv6-rh-ha-security-02 (work in 7590 progress), March 2002. 7592 [37] Manner, J. and M. Kojo, "Mobility Related Terminology", 7593 RFC 3753, June 2004. 7595 [38] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 7596 (MLDv2) for IPv6", RFC 3810, June 2004. 7598 [39] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key 7599 Management", BCP 107, RFC 4107, June 2005. 7601 [40] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E. 7602 Nordmark, "Mobile IP Version 6 Route Optimization Security 7603 Design Background", RFC 4225, December 2005. 7605 [41] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", 7606 RFC 4306, December 2005. 7608 [42] Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with 7609 IKEv2 and the Revised IPsec Architecture", RFC 4877, 7610 April 2007. 7612 [43] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation of 7613 Type 0 Routing Headers in IPv6", RFC 5095, December 2007. 7615 Appendix A. Future Extensions 7617 A.1. Piggybacking 7619 This document does not specify how to piggyback payload packets on 7620 the binding related messages. However, it is envisioned that this 7621 can be specified in a separate document when issues such as the 7622 interaction between piggybacking and IPsec are fully resolved (see 7623 also Appendix A.3). The return routability messages can indicate 7624 support for piggybacking with a new mobility option. 7626 A.2. Triangular Routing 7628 Due to the concerns about opening reflection attacks with the Home 7629 Address destination option, this specification requires that this 7630 option be verified against the Binding Cache, i.e., there must be a 7631 Binding Cache entry for the Home Address and Care-of Address. 7633 Future extensions may be specified that allow the use of unverified 7634 Home Address destination options in ways that do not introduce 7635 security issues. 7637 A.3. New Authorization Methods 7639 While the return routability procedure provides a good level of 7640 security, there exist methods that have even higher levels of 7641 security. Secondly, as discussed in Section 15.4, future 7642 enhancements of IPv6 security may cause a need to also improve the 7643 security of the return routability procedure. Using IPsec as the 7644 sole method for authorizing Binding Updates to correspondent nodes is 7645 also possible. The protection of the Mobility Header for this 7646 purpose is easy, though one must ensure that the IPsec SA was created 7647 with appropriate authorization to use the home address referenced in 7648 the Binding Update. For instance, a certificate used by IKEv2 to 7649 create the security association might contain the home address. A 7650 future specification may specify how this is done. 7652 A.4. Neighbor Discovery Extensions 7654 Future specifications may improve the efficiency of Neighbor 7655 Discovery tasks, which could be helpful for fast movements. One 7656 factor is currently being looked at: the delays caused by the 7657 Duplicate Address Detection mechanism. Currently, Duplicate Address 7658 Detection needs to be performed for every new care-of address as the 7659 mobile node moves, and for the mobile node's link-local address on 7660 every new link. In particular, the need and the trade-offs of re- 7661 performing Duplicate Address Detection for the link-local address 7662 every time the mobile node moves on to new links will need to be 7663 examined. Improvements in this area are, however, generally 7664 applicable and progress independently from the Mobile IPv6 7665 specification. 7667 Future functional improvements may also be relevant for Mobile IPv6 7668 and other applications. For instance, mechanisms that would allow 7669 recovery from a Duplicate Address Detection collision would be useful 7670 for link-local, care-of, and home addresses. 7672 Authors' Addresses 7674 Charles E. Perkins 7675 Tellabs Inc. 7676 3590 N. 1st Street, Suite 300 7677 San Jose CA 95134 7678 USA 7680 Email: charliep@computer.org 7682 David B. Johnson 7683 Rice University 7684 Dept. of Computer Science, MS 132 7685 6100 Main Street 7686 Houston TX 77005-1892 7687 USA 7689 Email: dbj@cs.rice.edu 7691 Jari Arkko 7692 Ericsson 7693 Jorvas 02420 7694 Finland 7696 Email: jari.arkko@ericsson.com