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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 IETF Mobile IP Working Group David B. Johnson 2 INTERNET-DRAFT Rice University 3 Charles Perkins 4 Nokia Research Center 5 2 July 2001 7 Mobility Support in IPv6 8 10 Status of This Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC 2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note 17 that other groups may also distribute working documents as 18 Internet-Drafts. 20 Internet-Drafts are draft documents, valid for a maximum of six 21 months, and may be updated, replaced, or obsoleted by other documents 22 at any time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt. 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 Abstract 33 This document specifies the operation of mobile computers using IPv6. 34 Each mobile node is always identified by its home address, regardless 35 of its current point of attachment to the Internet. While situated 36 away from its home, a mobile node is also associated with a care-of 37 address, which provides information about the mobile node's current 38 location. IPv6 packets addressed to a mobile node's home address are 39 transparently routed to its care-of address. The protocol enables 40 IPv6 nodes to cache the binding of a mobile node's home address with 41 its care-of address, and to then send any packets destined for the 42 mobile node directly to it at this care-of address. To support this 43 operation, Mobile IPv6 defines four new IPv6 destination options, 44 including one that MUST be supported in packets received by any node, 45 whether mobile or stationary. 47 Contents 49 Status of This Memo i 51 Abstract i 53 1. Introduction 1 55 2. Comparison with Mobile IP for IPv4 3 57 3. Terminology 6 58 3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 6 59 3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 7 61 4. Overview of Mobile IPv6 8 62 4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 8 63 4.2. New IPv6 Destination Options . . . . . . . . . . . . . . 11 64 4.3. Alignment Requirements for New Destination Options . . . 12 65 4.4. Authentication Requirements for Binding Update and 66 Acknowledgement . . . . . . . . . . . . . . . . . . . 13 67 4.5. New IPv6 ICMP Messages . . . . . . . . . . . . . . . . . 14 68 4.6. Conceptual Data Structures . . . . . . . . . . . . . . . 15 69 4.7. Binding Management . . . . . . . . . . . . . . . . . . . 20 71 5. New IPv6 Destination Options and Message Types 22 72 5.1. Binding Update Option . . . . . . . . . . . . . . . . . . 22 73 5.2. Binding Acknowledgement Option . . . . . . . . . . . . . 26 74 5.3. Binding Request Option . . . . . . . . . . . . . . . . . 30 75 5.4. Home Address Option . . . . . . . . . . . . . . . . . . . 32 76 5.5. Mobile IPv6 Destination Option Sub-Options . . . . . . . 35 77 5.5.1. Pad1 . . . . . . . . . . . . . . . . . . . . . . 36 78 5.5.2. PadN . . . . . . . . . . . . . . . . . . . . . . 36 79 5.5.3. Unique Identifier . . . . . . . . . . . . . . . . 37 80 5.5.4. Alternate Care-of Address . . . . . . . . . . . . 37 81 5.6. Authentication Data . . . . . . . . . . . . . . . . . . . 38 82 5.7. ICMP Home Agent Address Discovery Request Message . . . . 40 83 5.8. ICMP Home Agent Address Discovery Reply Message . . . . . 41 84 5.9. ICMP Mobile Prefix Solicitation Message Format . . . . . 43 85 5.10. ICMP Mobile Prefix Advertisement Message Format . . . . . 45 87 6. Modifications to IPv6 Neighbor Discovery 47 88 6.1. Modified Router Advertisement Message Format . . . . . . 47 89 6.2. Modified Prefix Information Option Format . . . . . . . . 48 90 6.3. New Advertisement Interval Option Format . . . . . . . . 50 91 6.4. New Home Agent Information Option Format . . . . . . . . 51 92 6.5. Changes to Sending Router Advertisements . . . . . . . . 53 93 6.6. Changes to Sending Router Solicitations . . . . . . . . . 54 95 7. Requirements for Types of IPv6 Nodes 56 96 7.1. Requirements for All IPv6 Hosts and Routers . . . . . . . 56 97 7.2. Requirements for All IPv6 Routers . . . . . . . . . . . . 56 98 7.3. Requirements for IPv6 Home Agents . . . . . . . . . . . . 56 99 7.4. Requirements for IPv6 Mobile Nodes . . . . . . . . . . . 57 101 8. Correspondent Node Operation 59 102 8.1. Receiving Packets from a Mobile Node . . . . . . . . . . 59 103 8.2. Receiving Binding Updates . . . . . . . . . . . . . . . . 59 104 8.3. Requests to Cache a Binding . . . . . . . . . . . . . . . 60 105 8.4. Requests to Delete a Binding . . . . . . . . . . . . . . 61 106 8.5. Sending Binding Acknowledgements . . . . . . . . . . . . 61 107 8.6. Sending Binding Requests . . . . . . . . . . . . . . . . 62 108 8.7. Cache Replacement Policy . . . . . . . . . . . . . . . . 62 109 8.8. Receiving ICMP Error Messages . . . . . . . . . . . . . . 63 110 8.9. Sending Packets to a Mobile Node . . . . . . . . . . . . 64 112 9. Home Agent Operation 67 113 9.1. Primary Care-of Address Registration . . . . . . . . . . 67 114 9.2. Primary Care-of Address De-registration . . . . . . . . . 69 115 9.3. Intercepting Packets for a Mobile Node . . . . . . . . . 70 116 9.4. Tunneling Intercepted Packets to a Mobile Node . . . . . 72 117 9.5. Handling Reverse Tunneled Packets from a Mobile Node . . 73 118 9.6. Home Prefix Propagation . . . . . . . . . . . . . . . . . 74 119 9.7. Receiving Router Advertisement Messages . . . . . . . . . 74 120 9.8. Dynamic Home Agent Address Discovery . . . . . . . . . . 76 121 9.8.1. Aggregate List of Home Network Prefixes . . . . . 77 122 9.8.2. Scheduling Prefix Deliveries to the Mobile Node . 79 123 9.8.3. Sending Advertisements to the Mobile Node . . . . 80 124 9.8.4. Lifetimes for Changed Prefixes . . . . . . . . . 82 126 10. Mobile Node Operation 83 127 10.1. Sending Packets While Away from Home . . . . . . . . . . 83 128 10.2. Interaction with Outbound IPsec Processing . . . . . . . 84 129 10.3. Receiving Packets While Away from Home . . . . . . . . . 86 130 10.4. Movement Detection . . . . . . . . . . . . . . . . . . . 87 131 10.5. Receiving Local Router Advertisement Messages . . . . . . 90 132 10.6. Forming New Care-of Addresses . . . . . . . . . . . . . . 91 133 10.7. Sending Binding Updates to the Home Agent . . . . . . . . 93 134 10.8. Dynamic Home Agent Address Discovery . . . . . . . . . . 95 135 10.9. Sending Binding Updates to Correspondent Nodes . . . . . 96 136 10.10. Establishing Forwarding from a Previous Care-of Address . 99 137 10.11. Retransmitting Binding Updates . . . . . . . . . . . . . 100 138 10.12. Rate Limiting for Sending Binding Updates . . . . . . . . 100 139 10.13. Receiving Binding Acknowledgements . . . . . . . . . . . 101 140 10.14. Receiving Binding Requests . . . . . . . . . . . . . . . 102 141 10.15. Receiving ICMP Error Messages . . . . . . . . . . . . . . 102 142 10.16. Sending Mobile Prefix Solicitations . . . . . . . . . . . 103 143 10.17. Receiving Mobile Prefix Advertisements . . . . . . . . . 104 144 10.18. Using Multiple Care-of Addresses . . . . . . . . . . . . 105 145 10.19. Routing Multicast Packets . . . . . . . . . . . . . . . . 105 146 10.20. Returning Home . . . . . . . . . . . . . . . . . . . . . 106 148 11. Protocol Constants 108 150 12. IANA Considerations 109 151 13. Security Considerations 111 152 13.1. Binding Updates, Acknowledgements, and Requests . . . . . 111 153 13.2. Security for the Home Address Option . . . . . . . . . . 111 154 13.3. General Mobile Computing Issues . . . . . . . . . . . . . 112 156 Acknowledgements 113 158 References 114 160 A. Changes from Previous Version of the Draft 115 161 A.1. Changes from Draft Version ...-14 . . . . . . . . . . . . 115 162 A.2. Changes from Previous Versions of the Draft . . . . . . . 117 164 B. Remote Home Address Configuration 118 166 Chairs' Addresses 119 168 Authors' Addresses 120 169 1. Introduction 171 This document specifies the operation of mobile computers using 172 Internet Protocol Version 6 (IPv6) [6]. Without specific support 173 for mobility in IPv6, packets destined to a mobile node (host or 174 router) would not be able to reach it while the mobile node is away 175 from its home link (the link on which its home IPv6 subnet prefix is 176 in use), since routing is based on the subnet prefix in a packet's 177 destination IP address. In order to continue communication in spite 178 of its movement, a mobile node could change its IP address each time 179 it moves to a new link, but the mobile node would then not be able 180 to maintain transport and higher-layer connections when it changes 181 location. Mobility support in IPv6 is particularly important, as 182 mobile computers are likely to account for a majority or at least a 183 substantial fraction of the population of the Internet during the 184 lifetime of IPv6. 186 The protocol operation defined here, known as Mobile IPv6, allows a 187 mobile node to move from one link to another without changing the 188 mobile node's IP address. A mobile node is always addressable by 189 its "home address", an IP address assigned to the mobile node within 190 its home subnet prefix on its home link. Packets may be routed to 191 the mobile node using this address regardless of the mobile node's 192 current point of attachment to the Internet, and the mobile node may 193 continue to communicate with other nodes (stationary or mobile) after 194 moving to a new link. The movement of a mobile node away from its 195 home link is thus transparent to transport and higher-layer protocols 196 and applications. 198 The Mobile IPv6 protocol is just as suitable for mobility across 199 homogeneous media as for mobility across heterogeneous media. For 200 example, Mobile IPv6 facilitates node movement from one Ethernet 201 segment to another as well as it facilitates node movement from an 202 Ethernet segment to a wireless LAN cell, with the mobile node's IP 203 address remaining unchanged in spite of such movement. 205 One can think of the Mobile IPv6 protocol as solving the 206 network-layer mobility management problem. Some mobility management 207 applications -- for example, handover among wireless transceivers, 208 each of which covers only a very small geographic area -- have been 209 solved using link-layer techniques. For example, in many current 210 wireless LAN products, link-layer mobility mechanisms allow a 211 "handover" of a mobile node from one cell to another, reestablishing 212 link-layer connectivity to the node in each new location. Within 213 the natural limitations imposed by link-management solutions, and as 214 long as such handover occurs only within cells of the mobile node's 215 home link, such link-layer mobility mechanisms MAY offer faster 216 convergence and lower overhead than Mobile IPv6. Extensions to the 217 Mobile IPv6 protocol have been proposed to support a more local, 218 hierarchical form of mobility management, but such extensions are 219 beyond the scope of this document. 221 The protocol specified in this document solves the problem of 222 transparently routing packets to and from mobile nodes while away 223 from home. However, it does not attempt to solve all general 224 problems related to the use of mobile computers or wireless networks. 225 In particular, this protocol does not attempt to solve: 227 - Handling links with partial reachability, or unidirectional 228 connectivity, such as are often found in wireless networks. Some 229 aspects of this problem are addressed by the movement detection 230 procedure described in Section 10.4, but no attempt has been made 231 to fully solve this problem in its general form. Most aspects of 232 this problem can be solved by the workaround of restricting such 233 networks to only one router per link, although there are still 234 possible hidden terminal problems when two nodes on the same 235 link (on opposite sides of the router) attempt to communicate 236 directly. 238 - Access control on a link being visited by a mobile node. This 239 is a general problem any time an unauthenticated node is allowed 240 to connect to any link layer. It is independent of whether the 241 connecting node uses Mobile IP, DHCP [2], or just "borrows" an IP 242 address on the link. 244 2. Comparison with Mobile IP for IPv4 246 The design of Mobile IP support in IPv6 (Mobile IPv6) represents a 247 natural combination of the experiences gained from the development 248 of Mobile IP support in IPv4 (Mobile IPv4) [19, 18, 20], together 249 with the opportunities provided by the design and deployment of a new 250 version of IP itself (IPv6) and the new protocol features offered 251 by IPv6. Mobile IPv6 thus shares many features with Mobile IPv4, 252 but the protocol is now fully integrated into IP and provides many 253 improvements over Mobile IPv4. This section summarizes the major 254 differences between Mobile IPv4 and Mobile IPv6: 256 - Support for what is known in Mobile IPv4 as "Route 257 Optimization" [21] is now built in as a fundamental part 258 of the protocol, rather than being added on as an optional 259 set of extensions that may not be supported by all nodes 260 as in Mobile IPv4. This integration of Route Optimization 261 functionality allows direct routing from any correspondent 262 node to any mobile node, without needing to pass through 263 the mobile node's home network and be forwarded by its home 264 agent, and thus eliminates the problem of "triangle routing" 265 present in the base Mobile IPv4 protocol [19]. The Mobile IPv4 266 "registration" functionality and the Mobile IPv4 Route 267 Optimization functionality are performed by a single protocol 268 rather than two separate (and different) protocols. 270 - Support is also integrated into Mobile IPv6 -- and into IPv6 271 itself -- for allowing mobile nodes and Mobile IP to coexist 272 efficiently with routers that perform "ingress filtering" [7]. A 273 mobile node now uses its care-of address as the Source Address in 274 the IP header of packets it sends, allowing the packets to pass 275 normally through ingress filtering routers. The home address 276 of the mobile node is carried in the packet in a Home Address 277 destination option, allowing the use of the care-of address in 278 the packet to be transparent above the IP layer. The ability 279 to correctly process a Home Address option in a received packet 280 is required in all IPv6 nodes, whether mobile nor stationary, 281 whether host or router. 283 - The use of IPv6 destination options allows all Mobile IPv6 284 control traffic to be piggybacked on any existing IPv6 packets, 285 whereas in Mobile IPv4 and its Route Optimization extensions, 286 separate UDP packets were required for each control message. 288 - The use of the care-of address as the Source Address in each 289 packet's IP header also simplifies routing of multicast packets 290 sent by a mobile node. With Mobile IPv4, the mobile node 291 had to tunnel multicast packets to its home agent in order to 292 transparently use its home address as the source of the multicast 293 packets. With Mobile IPv6, the use of the Home Address option 294 allows the home address to be used but still be compatible with 295 multicast routing that is based in part on the packet's Source 296 Address. 298 - There is no longer any need to deploy special routers as 299 "foreign agents" as are used in Mobile IPv4. In Mobile IPv6, 300 mobile nodes make use of IPv6 features, such as Neighbor 301 Discovery [17] and Address Autoconfiguration [27], to operate in 302 any location away from home without any special support required 303 from its local router. 305 - The movement detection mechanism in Mobile IPv6 provides 306 bidirectional confirmation of a mobile node's ability to 307 communicate with its default router in its current location 308 (packets that the router sends are reaching the mobile node, and 309 packets that the mobile node sends are reaching the router). 310 This confirmation provides a detection of the "black hole" 311 situation that may exist in some wireless environments where the 312 link to the router does not work equally well in both directions, 313 such as when the mobile node has moved out of good wireless 314 transmission range from the router. The mobile node may then 315 attempt to find a new router and begin using a new care-of 316 address if its link to its current router is not working well. 317 In contrast, in Mobile IPv4, only the forward direction (packets 318 from the router are reaching the mobile node) is confirmed, 319 allowing the black hole condition to persist. 321 - Most packets sent to a mobile node while away from home in 322 Mobile IPv6 are sent using an IPv6 Routing header rather than IP 323 encapsulation, whereas Mobile IPv4 must use encapsulation for all 324 packets. The use of a Routing header requires less additional 325 header bytes to be added to the packet, reducing the overhead 326 of Mobile IP packet delivery. To avoid modifying the packet in 327 flight, however, packets intercepted and tunneled by a mobile 328 node's home agent in Mobile IPv6 must still use encapsulation for 329 delivery to the mobile node. 331 - While a mobile node is away from home, its home agent intercepts 332 any packets for the mobile node that arrive at the home network, 333 using IPv6 Neighbor Discovery [17] rather than ARP [23] as is 334 used in Mobile IPv4. The use of Neighbor Discovery improves 335 the robustness of the protocol (e.g., due to the Neighbor 336 Advertisement "override" bit) and simplifies implementation 337 of Mobile IP due to the ability to not be concerned with any 338 particular link layer as is required in ARP. 340 - The use of IPv6 encapsulation (and the Routing header) removes 341 the need in Mobile IPv6 to manage "tunnel soft state", which was 342 required in Mobile IPv4 due to limitations in ICMP for IPv4. Due 343 to the definition of ICMP for IPv6, the use of tunnel soft state 344 is no longer required in IPv6 for correctly relaying ICMP error 345 messages from within the tunnel back to the original sender of 346 the packet. 348 - The dynamic home agent address discovery mechanism in Mobile IPv6 349 uses IPv6 anycast [10] and returns a single reply to the mobile 350 node, rather than the corresponding Mobile IPv4 mechanism that 351 used IPv4 directed broadcast and returned a separate reply from 352 each home agent on the mobile node's home link. The Mobile IPv6 353 mechanism is more efficient and more reliable, since only one 354 packet need be sent back to the mobile node. The mobile node is 355 less likely to lose one of the replies because no "implosion" of 356 replies is required by the protocol. 358 - Mobile IPv6 defines an Advertisement Interval option on 359 Router Advertisements (equivalent to Agent Advertisements in 360 Mobile IPv4), allowing a mobile node to decide for itself how 361 many Router Advertisements (Agent Advertisements) it is willing 362 to miss before declaring its current router unreachable. 364 3. Terminology 366 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 367 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 368 document are to be interpreted as described in RFC 2119 [3]. 370 3.1. General Terms 372 IP Internet Protocol Version 6 (IPv6). 374 node A device that implements IP. 376 router A node that forwards IP packets not explicitly 377 addressed to itself. 379 host Any node that is not a router. 381 link A communication facility or medium over which nodes 382 can communicate at the link layer, such as an 383 Ethernet (simple or bridged). A link is the layer 384 immediately below IP. 386 interface A node's attachment to a link. 388 subnet prefix 389 A bit string that consists of some number of initial 390 bits of an IP address. 392 interface identifier 393 A number used to identify a node's interface on a 394 link. The interface identifier is the remaining 395 low-order bits in the node's IP address after the 396 subnet prefix. 398 link-layer address 399 A link-layer identifier for an interface, such as 400 IEEE 802 addresses on Ethernet links. 402 packet An IP header plus payload. 404 Security Association 405 a security object shared between two nodes which 406 includes the data mutually agreed on for operation 407 of some cryptographic algorithm (typically including 408 a key, as defined above). 410 Security Policy Database (SPD) 411 A database of security associations selectable by 412 rulesets (policies) that determine the packets for 413 which each security association is to be applied. 415 3.2. Mobile IPv6 Terms 417 home address An IP address assigned to a mobile node within its 418 home link. 420 home subnet prefix 421 The IP subnet prefix corresponding to a mobile 422 node's home address. 424 home link The link on which a mobile node's home subnet prefix 425 is defined. Standard IP routing mechanisms will 426 deliver packets destined for a mobile node's home 427 address to its home link. 429 mobile node A node that can change its point of attachment from 430 one link to another, while still being reachable via 431 its home address. 433 movement A change in a mobile node's point of attachment to 434 the Internet such that it is no longer connected to 435 the same link as it was previously. If a mobile 436 node is not currently attached to its home link, the 437 mobile node is said to be "away from home". 439 correspondent node 440 A peer node with which a mobile node is 441 communicating. The correspondent node may be either 442 mobile or stationary. 444 foreign subnet prefix 445 Any IP subnet prefix other than the mobile node's 446 home subnet prefix. 448 foreign link Any link other than the mobile node's home link. 450 home agent A router on a mobile node's home link with which 451 the mobile node has registered its current care-of 452 address. While the mobile node is away from home, 453 the home agent intercepts packets on the home 454 link destined to the mobile node's home address, 455 encapsulates them, and tunnels them to the mobile 456 node's registered care-of address. 458 care-of address 459 An IP address associated with a mobile node while 460 visiting a foreign link; the subnet prefix of this 461 IP address is a foreign subnet prefix. Among the 462 multiple care-of addresses that a mobile node 463 may have at a time (e.g., with different subnet 464 prefixes), the one registered with the mobile node's 465 home agent is called its "primary" care-of address. 467 binding The association of the home address of a mobile node 468 with a care-of address for that mobile node, along 469 with the remaining lifetime of that association. 471 Binding Key 472 a key used for authenticating Binding Update 473 messages. 475 Binding Security Association (BSA) 476 a security association established specifically 477 for the purpose of producing and verifying 478 authentication data passed with a Binding Update 479 destination option. 481 4. Overview of Mobile IPv6 483 4.1. Basic Operation 485 A mobile node is always addressable by its home address, whether it 486 is currently attached to its home link or is away from home. While 487 a mobile node is at home, packets addressed to its home address are 488 routed to it using conventional Internet routing mechanisms in the 489 same way as if the node were never mobile. Since the subnet prefix 490 of a mobile node's home address is the subnet prefix (or one of the 491 subnet prefixes) on the mobile node's home link (it is the mobile 492 node's home subnet prefix), packets addressed to it will be routed to 493 its home link. 495 While a mobile node is attached to some foreign link away from home, 496 it is also addressable by one or more care-of addresses, in addition 497 to its home address. A care-of address is an IP address associated 498 with a mobile node while visiting a particular foreign link. The 499 subnet prefix of a mobile node's care-of address is the subnet prefix 500 (or one of the subnet prefixes) on the foreign link being visited by 501 the mobile node; if the mobile node is connected to this foreign link 502 while using that care-of address, packets addressed to this care-of 503 address will be routed to the mobile node in its location away from 504 home. 506 The association between a mobile node's home address and care-of 507 address is known as a "binding" for the mobile node. A mobile node 508 typically acquires its care-of address through stateless [27] or 509 stateful (e.g., DHCPv6 [2]) Address Autoconfiguration, according 510 to the methods of IPv6 Neighbor Discovery [17]. Other methods 511 of acquiring a care-of address are also possible, such as static 512 pre-assignment by the owner or manager of a particular foreign link, 513 but details of such other methods are beyond the scope of this 514 document. 516 While away from home, a mobile node registers one of its care-of 517 addresses with a router on its home link, requesting this router 518 to function as the "home agent" for the mobile node. This binding 519 registration is done by the mobile node sending to the home agent 520 a packet containing a "Binding Update" destination option; the 521 home agent then replies to the mobile node by returning a packet 522 containing a "Binding Acknowledgement" destination option. The 523 care-of address in this binding registered with its home agent is 524 known as the mobile node's "primary care-of address". The mobile 525 node's home agent thereafter uses proxy Neighbor Discovery to 526 intercept any IPv6 packets addressed to the mobile node's home 527 address (or home addresses) on the home link, and tunnels each 528 intercepted packet to the mobile node's primary care-of address. 529 To tunnel each intercepted packet, the home agent encapsulates the 530 packet using IPv6 encapsulation [4], with the outer IPv6 header 531 addressed to the mobile node's primary care-of address. 533 When a mobile node moves from one care-of address to a new care-of 534 address on a new link, it is desirable for packets arriving at the 535 previous care-of address to be tunneled to the mobile node's care-of 536 address. Since the purpose of a Binding Update is to establish 537 exactly this kind of tunneling, it is specified to be used (at 538 least temporarily) for tunnels originating at the mobile node's 539 previous care-of address, in exactly the same way that it is used 540 for establishing tunnels from the mobile node's home address to the 541 mobile node's current care-of address. Section 10.10 describes the 542 use of the Binding Update for this purpose. 544 Section 10.18 discusses the reasons why it may be desirable for 545 a mobile node to use more than one care-of address at the same 546 time. However, a mobile node's primary care-of address is distinct 547 among these in that the home agent maintains only a single care-of 548 address registered for each mobile node, and always tunnels a mobile 549 node's packets intercepted from its home link to this mobile node's 550 registered primary care-of address. The home agent thus need not 551 implement any policy to determine the particular care-of address to 552 which it will tunnel each intercepted packet. The mobile node alone 553 controls the policy by which it selects the care-of addresses to 554 register with its home agent. 556 It is possible that while a mobile node is away from home, some nodes 557 on its home link may be reconfigured, such that the router that was 558 operating as the mobile node's home agent is replaced by a different 559 router serving this role. In this case, the mobile node may not 560 know the IP address of its own home agent. Mobile IPv6 provides a 561 mechanism, known as "dynamic home agent address discovery", that 562 allows a mobile node to dynamically discover the IP address of a 563 home agent on its home link with which it may register its (primary) 564 care-of address while away from home. The mobile node sends an ICMP 565 "Home Agent Address Discovery Request" message to the "Mobile IPv6 566 Home-Agents" anycast address for its own home subnet prefix [10] and 567 thus reaches one of the (possibly many) routers on its home link 568 currently operating as a home agent. This home agent then returns an 569 ICMP "Home Agent Address Discovery Reply" message to the mobile node, 570 including a list of home agents on the home link. This list of home 571 agents is maintained by each home agent on the home link through use 572 of the Home Agent (H) bit in each home agent's periodic unsolicited 573 multicast Router Advertisements. 575 The Binding Update and Binding Acknowledgement destination options, 576 together with a "Binding Request" destination option, are also used 577 to allow IPv6 nodes communicating with a mobile node, to dynamically 578 learn and cache the mobile node's binding. When sending a packet 579 to any IPv6 destination, a node checks its cached bindings for an 580 entry for the packet's destination address. If a cached binding for 581 this destination address is found, the node uses an IPv6 Routing 582 header [6] (instead of IPv6 encapsulation) to route the packet to 583 the mobile node by way of the care-of address indicated in this 584 binding. If, instead, the sending node has no cached binding for 585 this destination address, the node sends the packet normally (with 586 no Routing header), and the packet is subsequently intercepted and 587 tunneled by the mobile node's home agent as described above. Any 588 node communicating with a mobile node is referred to in this document 589 as a "correspondent node" of the mobile node, and may itself be 590 either a stationary node or a mobile node. 592 Since a Binding Update, Binding Acknowledgement, and Binding Request 593 are each represented in a packet as an IPv6 destination option [6], 594 they may be included in any IPv6 packet. Any of these options can be 595 sent in either of two ways: 597 - the messages can be included within any IPv6 packet carrying any 598 payload such as TCP [25] or UDP [24]. 600 - the messages can be sent as a separate IPv6 packet containing 601 no payload. In this case, the Next Header field in the last 602 extension header in the packet is set to the value 59, to 603 indicate "No Next Header" [6]. 605 Mobile IPv6 also defines one additional IPv6 destination option. 606 When a mobile node sends a packet while away from home, it will 607 generally set the Source Address in the packet's IPv6 header to one 608 of its current care-of addresses, and will also include a "Home 609 Address" destination option in the packet, giving the mobile node's 610 home address. Many routers implement security policies such as 611 "ingress filtering" [7] that do not allow forwarding of packets 612 that have a Source Address which appears topologically incorrect. 613 By using the care-of address as the IPv6 header Source Address, 614 the packet will be able to pass normally through such routers, 615 yet ingress filtering rules will still be able to locate the true 616 topological source of the packet in the same way as packets from 617 non-mobile nodes. By also including the Home Address option in each 618 packet, the sending mobile node can communicate its home address to 619 the correspondent node receiving this packet, allowing the use of 620 the care-of address to be transparent above the Mobile IPv6 support 621 level (e.g., at the transport layer). The inclusion of a Home 622 Address option in a packet affects only the correspondent node's 623 receipt of this single packet; no state is created or modified in the 624 correspondent node as a result of receiving a Home Address option in 625 a packet. 627 4.2. New IPv6 Destination Options 629 As mentioned in Section 4.1, the following four new IPv6 destination 630 options are defined for Mobile IPv6: 632 Binding Update 634 A Binding Update option is used by a mobile node to notify 635 a correspondent node or the mobile node's home agent of its 636 current binding. The Binding Update sent to the mobile node's 637 home agent to register its primary care-of address is marked 638 as a "home registration". Any packet that includes a Binding 639 Update option MUST be protected by some authentication data 640 (see section 5.1), as defined in Section 4.4, to guard against 641 malicious Binding Updates. The Binding Update option and its 642 specific authentication requirements are described in detail in 643 Section 5.1. 645 Binding Acknowledgement 647 A Binding Acknowledgement option is used to acknowledge receipt 648 of a Binding Update, if an acknowledgement was requested 649 in the Binding Update. Any packet that includes a Binding 650 Acknowledgement option MUST be protected by some authentication 651 data (see section 5.2), as defined in Section 4.4, to guard 652 against malicious Binding Acknowledgements. The Binding 653 Acknowledgement option and its specific authentication 654 requirements are described in detail in Section 5.2. 656 Binding Request 658 A Binding Request option is used to request a mobile node to 659 send to the requesting node a Binding Update containing the 660 mobile node's current binding. This option is typically used 661 by a correspondent node to refresh a cached binding for a 662 mobile node, when the cached binding is in active use but the 663 binding's lifetime is close to expiration. No authentication 664 is required for the Binding Request option. The Binding 665 Request option is described in detail in Section 5.3. 667 Home Address 669 A Home Address option is used in a packet sent by a mobile 670 node to inform the recipient of that packet of the mobile 671 node's home address. For packets sent by a mobile node while 672 away from home, the mobile node generally uses one of its 673 care-of addresses as the Source Address in the packet's IPv6 674 header. By including a Home Address option in the packet, the 675 correspondent node receiving the packet is able to substitute 676 the mobile node's home address for this care-of address when 677 processing the packet, thus making the use of the care-of 678 address transparent to the correspondent node. If the IP 679 header of a packet carrying a Home Address option is covered 680 by authentication, then the Home Address option MUST also be 681 covered by this authentication, but no other authentication 682 is required for the Home Address option. See sections 10.2 683 and 5.4 for additional details about requirements for the 684 calculation and verification of the authentication data. The 685 Home Address option is described in detail in Section 5.4. 687 Mobile IPv6 also defines a number of "sub-options" for use within 688 these destination options; if included, any sub-options MUST 689 appear after the fixed portion of the option data specified in this 690 document. The presence of such sub-options will be indicated by the 691 Option Length field within the option. When the Option Length is 692 greater than the length required for the option specified here, the 693 remaining octets are interpreted as sub-options. The encoding and 694 format of defined sub-options are described in Section 5.5. 696 4.3. Alignment Requirements for New Destination Options 698 IPv6 requires that options appearing in a Hop-by-Hop Options 699 header or Destination Options header be aligned in a packet so that 700 multi-octet values within the Option Data field of each option fall 701 on natural boundaries (i.e., fields of width n octets are placed 702 at an integer multiple of n octets from the start of the header, 703 for n = 1, 2, 4, or 8) [6]. Mobile IPv6 sub-options have similar 704 alignment requirements, so that multi-octet values within the 705 Sub-Option Data field of each sub-option fall on natural boundaries. 706 The alignment requirement of an option or sub-option is specified in 707 this document using the standard notation used elsewhere for IPv6 708 alignment requirements [6]. Specifically, the notation xn+y means 709 that the Option Type or Sub-Option Type field must fall at an integer 710 multiple of x octets from the start of the header, plus y octets. 711 For example: 713 2n means any 2-octet offset from the start of the header. 715 8n+2 means any 8-octet offset from the start of the header, 716 plus 2 octets. 718 4.4. Authentication Requirements for Binding Update and Acknowledgement 720 Any packet that includes a Binding Update or Binding Acknowledgement 721 option has to contain some Authentication Data to guard against 722 malicious Binding Updates or Acknowledgements. The way that 723 the Authentication Data is computed has to provide sender 724 authentication, data integrity protection, and replay protection. 725 This section specifies requirements for which data is to be protected 726 when the Authentication Data Sub-Option is used to supply the 727 necessary authentication data for the Binding Update or Binding 728 Acknowledgement. Every IPv6 node which supports transmission or 729 reception of these destination options MUST support the computation 730 of the Authentication Data Sub-Option according to the rules in 731 this section. The Authentication Data Sub-Option MUST be the last 732 Sub-Option contained within any destination option in which the 733 Authentication Data Sub-Option occurs. 735 The Authentication Data covering a Binding Update or MUST be computed 736 over a bitstring containing the following fields of the IPv6 header 737 and destination options, in order: 739 - The IPv6 address of the destination of the packet, as seen by the 740 recipient 741 - Care-of Address, in the Source IP Address of the IPv6 header 742 - The Home Address of the mobile node, from the Home Address 743 destination option 744 - Option Type of the Binding Update destination option 745 - Option Length of the Binding Update destination option 746 - All flags of the Binding Update destination option 747 - Reserved Field of the Binding Update destination option 748 - Sequence Number Field of the Binding Update 749 - Lifetime of the Binding Update destination option 750 - The entire data from all Binding Update Sub-Options (except the 751 Authentication Data suboption), if any others exist 752 - The Type (i.e., the 8-bit value 0x04) and the Length of the 753 Authentication Data Sub-Option 754 - Security Parameters Index (SPI) of the Authentication Data 755 Sub-Option 757 When a Routing Header is used to deliver the Binding Update to 758 the receiving node, the address to be used for the Destination IP 759 Address part of the computation will initially be located as the last 760 component of the Routing Header. Otherwise, the address to be used 761 will be the Destination IP Address in the IPv6 header of the outgoing 762 packet. 764 When the Authentication Data Sub-Option is used with a Binding 765 Acknowledgement, the Authentication Data MUST be computed over a 766 bitstring containing the following data, in order: 768 - The IPv6 address of the destination of the packet, as seen by the 769 recipient 770 - The Home Address of the sender 771 - Option Type of the Binding Acknowledgement destination option 772 - Option Length of the Binding Acknowledgement 773 - Status of the Binding Acknowledgement 774 - Sequence Number field of the Binding Acknowledgement 775 - Lifetime field of the Binding Acknowledgement destination option 776 - Refresh field of the Binding Acknowledgement destination option 777 - The entire data from all Binding Acknowledgement Sub-Options 778 (except the Authentication Data suboption), if any others exist 779 - The Type (i.e., the 8-bit value 0x04) and the Length of the 780 Authentication Data Sub-Option 781 - Security Parameters Index (SPI) of the Authentication Data 782 Sub-Option 784 When a Home Address destination option is used to deliver the Binding 785 Acknowledgement to the receiving node, the Home Address of the sender 786 will be located in that destination option. Otherwise, the Home 787 Address of the sender will be located in the Source IP Address field 788 of the IPv6 header. 790 If a Security Association applied to the packet for other reasons 791 requires use of ESP [12], for example to encrypt the transport layer 792 data carried in the packet, this use of ESP is not sufficient to 793 satisfy the authentication requirements of Mobile IPv6. 795 Authentication Data assuring the integrity of Binding Updates and 796 Binding Acknowledgement MAY, in some cases, instead be supplied by 797 other authentication mechanisms outside the scope of this document 798 (e.g., IPsec [13]). When alternative mechanisms are used, the same 799 data as indicated above MUST be included as part of the input data 800 stream for the authentication algorithm; however (according to the 801 requirements of the alternative authentication algorithm) the order 802 of the data elements in the input data stream MAY be changed from the 803 order specified within this section for use with the Authentication 804 Data Sub-Option (see section 5.5). 806 4.5. New IPv6 ICMP Messages 808 Mobile IPv6 also introduces four new ICMP message types, two for use 809 in the dynamic home agent address discovery mechanism, and two for 810 renumbering and mobile configuration mechanisms. As discussed in 811 general in Section 4.1, the following two new ICMP message types are 812 used for home agent address discovery: 814 Home Agent Address Discovery Request 816 The ICMP Home Agent Address Discovery Request message is used 817 by a mobile node to initiate the dynamic home agent address 818 discovery mechanism. When attempting a home registration, the 819 mobile node may use this mechanism to discover the address of 820 one or more routers currently operating as home agents on its 821 home link, with which it may register while away from home. 822 The Home Agent Address Discovery Request message is described 823 in detail in Section 5.7. 825 Home Agent Address Discovery Reply 827 The ICMP Home Agent Address Discovery Reply message is used by 828 a home agent to respond to a mobile node using the dynamic home 829 agent address discovery mechanism. When a home agent receives 830 a Home Agent Address Discovery Request message, it replies with 831 a Home Agent Address Discovery Reply message, giving a list 832 of the routers on the mobile node's home link serving as home 833 agents. The Home Agent Address Discovery Reply message is 834 described in detail in Section 5.8. 836 The next two message types are used for network renumbering 837 and address configuration on the mobile node, as described in 838 Section 9.6: 840 Mobile Prefix Solicitation 842 The ICMP Mobile Prefix Solicitation message is used by a mobile 843 node to request prefix information about the home subnet, in 844 order to retrieve prefixes that are served by home agents and 845 can be used to configure one or more home addresses, or to 846 refresh home addresses before the expiration of their validity. 847 This message is specified in Section 5.9. 849 Mobile Prefix Advertisement 851 The ICMP Mobile Prefix Advertisement is used by a home agent to 852 distribute information to a mobile node about prefixes on the 853 home link which are available for use by the mobile node while 854 away from home. This message may be sent as a response to a 855 Mobile Prefix Solicitation, or due to network renumbering or 856 other prefix changes as specified in Section 5.10 858 4.6. Conceptual Data Structures 860 This document describes the Mobile IPv6 protocol in terms of the 861 following three conceptual data structures: 863 Binding Cache 865 A cache, maintained by each IPv6 node, of bindings for other 866 nodes. A separate Binding Cache SHOULD be maintained by each 867 IPv6 node for each of its IPv6 addresses. The Binding Cache 868 MAY be implemented in any manner consistent with the external 869 behavior described in this document, for example by being 870 combined with the node's Destination Cache as maintained by 871 Neighbor Discovery [17]. When sending a packet, the Binding 872 Cache is searched before the Neighbor Discovery conceptual 873 Destination Cache [17] (i.e., any Binding Cache entry for this 874 destination SHOULD take precedence over any Destination Cache 875 entry for the same destination). Each Binding Cache entry 876 conceptually contains the following fields: 878 - The home address of the mobile node for which this is the 879 Binding Cache entry. This field is used as the key for 880 searching the Binding Cache for the destination address of 881 a packet being sent. If the destination address of the 882 packet matches the home address in the Binding Cache entry, 883 this entry SHOULD be used in routing that packet. 885 - The care-of address for the mobile node indicated by 886 the home address field in this Binding Cache entry. If 887 the destination address of a packet being routed by a 888 node matches the home address in this entry, the packet 889 SHOULD be routed to this care-of address, as described in 890 Section 8.9, for packets originated by this node, or in 891 Section 9.4, if this node is the mobile node's home agent 892 and the packet was intercepted by it on the home link. 894 - A lifetime value, indicating the remaining lifetime 895 for this Binding Cache entry. The lifetime value is 896 initialized from the Lifetime field in the Binding Update 897 that created or last modified this Binding Cache entry. 898 Once the lifetime on this entry expires, the entry MUST be 899 deleted from the Binding Cache. 901 - A flag indicating whether or not this Binding Cache entry 902 is a "home registration" entry. 904 - A flag indicating whether or not this Binding Cache entry 905 represents a mobile node that should be advertised as a 906 router in proxy Neighbor Advertisements sent by this node 907 on its behalf. This flag is only valid if the Binding 908 Cache entry indicates that this is a "home registration" 909 entry. 911 - The length of the routing prefix for the home address. 912 This field is only valid if the "home registration" flag is 913 set on this Binding Cache entry. 915 - The maximum value of the Sequence Number field received 916 in previous Binding Updates for this mobile node home 917 address. The Sequence Number field is 8 bits long, 918 and all comparisons between Sequence Number values 919 MUST be performed modulo 2**8. For example, using an 920 implementation in the C programming language, a Sequence 921 Number value A is greater than another Sequence Number 922 value B if ((char)((a) - (b)) > 0), if the "int" data type 923 is a 8-bit signed integer. 925 - Recent usage information for this Binding Cache entry, as 926 needed to implement the cache replacement policy in use in 927 the Binding Cache and to assist in determining whether a 928 Binding Request should be sent when the lifetime on this 929 entry nears expiration. 931 - The Binding Security Association (BSA) to be be used when 932 authenticating Binding Updates that are received for this 933 Binding Cache entry. 935 - The Binding Security Association (BSA) to be be used when 936 calculating authentication data for inclusion in Binding 937 Acknowledgements in response to Binding Updates that are 938 received for this Binding Cache entry. 940 An entry in a node's Binding Cache for which the node is 941 serving as a home agent is marked as a "home registration" 942 entry and SHOULD NOT be deleted by the home agent until the 943 expiration of its binding lifetime. Other Binding Cache 944 entries MAY be replaced at any time by any reasonable local 945 cache replacement policy but SHOULD NOT be unnecessarily 946 deleted. The Binding Cache for any one of a node's IPv6 947 addresses may contain at most one entry for each mobile node 948 home address. The contents of a node's Binding Cache MUST NOT 949 be changed in response to a Home Address option in a received 950 packet. The contents of all of a node's Binding Cache entries, 951 for each of its IPv6 addresses, must be cleared when the node 952 reboots. 954 Binding Update List 956 A list, maintained by each mobile node, recording information 957 for each Binding Update sent by this mobile node, for which the 958 Lifetime sent in that Binding Update has not yet expired. The 959 Binding Update List includes all bindings sent by the mobile 960 node: those to correspondent nodes, those to the mobile node's 961 home agent, and those to a home agent on the link on which the 962 mobile node's previous care-of address is located. However, 963 for multiple Binding Updates sent to the same destination 964 address, the Binding Update List contains only the most recent 965 Binding Update (i.e., with the greatest Sequence Number value) 966 sent to that destination. The Binding Update List MAY be 967 implemented in any manner consistent with the external behavior 968 described in this document. Each Binding Update List entry 969 conceptually contains the following fields: 971 - The IP address of the node to which a Binding Update was 972 sent. That node might still have a Binding Cache entry 973 created or updated from this Binding Update, if the Binding 974 Update was successfully received by that node (e.g., not 975 lost by the network) and if that node has not deleted the 976 entry before its expiration (e.g., to reclaim space in its 977 Binding Cache for other entries). 979 - The home address for which that Binding Update was sent. 980 This will be one of the following: 982 * the mobile node's home addresses for typical Binding 983 Updates (Sections 10.7 and 10.9), or 985 * the mobile node's previous care-of address for Binding 986 Updates sent to establish forwarding from the mobile 987 node's previous care-of address by a home agent from 988 this previous care-of address (Section 10.10). 990 - The care-of address sent in that Binding Update. This 991 value is necessary for the mobile node to determine if it 992 has sent a Binding Update giving its new care-of address to 993 this destination after changing its care-of address. 995 - The initial value of the Lifetime field sent in that 996 Binding Update. 998 - The remaining lifetime of that binding. This lifetime is 999 initialized from the Lifetime value sent in the Binding 1000 Update and is decremented until it reaches zero, at which 1001 time this entry MUST be deleted from the Binding Update 1002 List. 1004 - The maximum value of the Sequence Number field sent in 1005 previous Binding Updates to this destination. The Sequence 1006 Number field is 8 bits long, and all comparisons between 1007 Sequence Number values MUST be performed modulo 2**8. For 1008 example, using an implementation in the C programming 1009 language, a Sequence Number value A is greater than another 1010 Sequence Number value B if ((char)((a) - (b)) > 0), if the 1011 "char" data type is a 8-bit signed integer. 1013 - The time at which a Binding Update was last sent to this 1014 destination, as needed to implement the rate limiting 1015 restriction for sending Binding Updates. 1017 - The state of any retransmissions needed for this Binding 1018 Update, if the Acknowledge (A) bit was set in this Binding 1019 Update. This state includes the time remaining until the 1020 next retransmission attempt for the Binding Update, and the 1021 current state of the exponential back-off mechanism for 1022 retransmissions. 1024 - A flag that, when set, indicates that future Binding 1025 Updates should not be sent to this destination. The 1026 mobile node sets this flag in the Binding Update List 1027 entry when it receives an ICMP Parameter Problem, Code 2, 1028 error message in response to a Binding Update sent to that 1029 destination, as described in Section 10.15. 1031 Home Agents List 1033 A list, maintained by each home agent and each mobile node, 1034 recording information about each home agent from which this 1035 node has received a Router Advertisement in which the Home 1036 Agent (H) bit is set, for which the remaining lifetime for 1037 this list entry (defined below) has not yet expired. The 1038 home agents list is thus similar to the Default Router 1039 List conceptual data structure maintained by each host for 1040 Neighbor Discovery [17], although the Home Agents List MAY be 1041 implemented in any manner consistent with the external behavior 1042 described in this document. 1044 Each home agent maintains a separate Home Agents List for 1045 each link on which it is serving as a home agent; this list 1046 is used by a home agent in the dynamic home agent address 1047 discovery mechanism. Each mobile node, while away from home, 1048 also maintains a Home Agents List, to enable it to notify a 1049 home agent on its previous link when it moves to a new link; a 1050 mobile node MAY maintain a separate Home Agents List for each 1051 link to which it is (or has recently) connected, or it MAY 1052 maintain a single list for all links. Each Home Agents List 1053 entry conceptually contains the following fields: 1055 - The link-local IP address of a router on the link, that 1056 this node currently believes is operating as a home agent 1057 for that link. A new entry is created or an existing 1058 entry is updated in the Home Agents List in response to 1059 receipt of a valid Router Advertisement in which the Home 1060 Agent (H) bit is set. The link-local address of the home 1061 agent is learned through the Source Address of the Router 1062 Advertisements received from it [17]. 1064 - One or more global IP addresses for this home agent, 1065 learned through Prefix Information options with 1066 the Router Address (R) bit set, received in Router 1067 Advertisements from this link-local address. Global 1068 addresses for the router in a Home Agents List entry MUST 1069 be deleted once the prefix associated with that address is 1070 no longer valid [17]. 1072 Are there interactions with the new Router 1073 Advertisement stuff? 1075 - The remaining lifetime of this Home Agents List entry. If 1076 a Home Agent Information Option is present in a Router 1077 Advertisement received from a home agent, the lifetime of 1078 the Home Agents List entry representing that home agent 1079 is initialized from the Home Agent Lifetime field in the 1080 option; otherwise, the lifetime is initialized from the 1081 Router Lifetime field in the received Router Advertisement. 1082 The Home Agents List entry lifetime is decremented until it 1083 reaches zero, at which time this entry MUST be deleted from 1084 the Home Agents List. 1086 - The preference for this home agent; higher values 1087 indicate a more preferable home agent. The preference 1088 value is taken from the Home Agent Preference field (a 1089 signed, twos-complement integer) in the received Router 1090 Advertisement, if the Router Advertisement contains a Home 1091 Agent Information Option, and is otherwise set to the 1092 default value of 0. A home agent uses this preference in 1093 ordering the Home Agents List returned in an ICMP Home 1094 Agent Address Discovery message in response to a mobile 1095 node's initiation of dynamic home agent address discovery. 1096 A mobile node uses this preference in determining which 1097 of the home agents on its previous link to notify when it 1098 moves to a new link. 1100 Can we delete the preference stuff? Is anyone using 1101 it? 1103 4.7. Binding Management 1105 When a mobile node configures a new care-of address and decides to 1106 use this new address as its primary care-of address, the mobile 1107 node registers this new binding with its home agent by sending 1108 the home agent a Binding Update. The mobile node indicates 1109 that an acknowledgement is needed for this Binding Update and 1110 continues to periodically retransmit it until acknowledged. The 1111 home agent acknowledges the Binding Update by returning a Binding 1112 Acknowledgement to the mobile node. 1114 When a mobile node receives a packet tunneled to it from its home 1115 agent, the mobile node uses that as an indication that the original 1116 sending correspondent node has no Binding Cache entry for the mobile 1117 node, since the correspondent node would otherwise have sent the 1118 packet directly to the mobile node using a Routing header. If the 1119 mobile node has a Binding Security Association (BSA) with that 1120 correspondent node, the mobile node thus returns a Binding Update 1121 to the correspondent node, allowing it to cache the mobile node's 1122 binding for routing future packets to it. Although the mobile node 1123 may request an acknowledgement for this Binding Update, it need not, 1124 since subsequent packets from the correspondent node will continue 1125 to be intercepted and tunneled by the mobile node's home agent, 1126 effectively causing any needed Binding Update retransmission. 1128 If the mobile node receives such a tunneled packet but does not have 1129 a BSA with the correspondent node, the mobile node SHOULD initiate 1130 the process of establishing the necessary security association by 1131 following the procedures outlined in [?]. 1133 A correspondent node with a Binding Cache entry for a mobile node 1134 may refresh this binding, for example if the binding's lifetime 1135 is near expiration, by sending a Binding Request to the mobile 1136 node. Normally, a correspondent node will only refresh a Binding 1137 Cache entry in this way if it is actively communicating with the 1138 mobile node and has indications, such as an open TCP connection to 1139 the mobile node, that it will continue this communication in the 1140 future. When a mobile node receives a Binding Request, it replies by 1141 returning a Binding Update to the node sending the Binding Request. 1143 A mobile node may use more than one care-of address at the same time, 1144 although only one care-of address may be registered for it at its 1145 home agent as its primary care-of address. The mobile node's home 1146 agent will tunnel all intercepted packets for the mobile node to its 1147 (single) registered primary care-of address, but the mobile node 1148 will accept packets that it receives at any of its current care-of 1149 addresses. Use of more than one care-of address by a mobile node 1150 may be useful, for example, to improve smooth handover when the 1151 mobile node moves from one wireless link to another. If each of 1152 these wireless links is connected to the Internet through a separate 1153 base station, such that the wireless transmission range from the 1154 two base stations overlap, the mobile node may be able to remain 1155 connected to both links while in the area of overlap. In this case, 1156 the mobile node could acquire a new care-of address on the new link 1157 before moving out of transmission range and disconnecting from the 1158 old link. The mobile node may thus still accept packets at its 1159 old care-of address while it works to update its home agent and 1160 correspondent nodes, notifying them of its new care-of address on the 1161 new link. 1163 Since correspondent nodes cache bindings, it is expected that 1164 correspondent nodes usually will route packets directly to the mobile 1165 node's care-of address, so that the home agent is rarely involved 1166 with packet transmission to the mobile node. This is essential for 1167 scalability and reliability, and for minimizing overall network load. 1168 By caching the care-of address of a mobile node, optimal routing of 1169 packets can be achieved from the correspondent node to the mobile 1170 node. Routing packets directly to the mobile node's care-of address 1171 also eliminates congestion at the mobile node's home agent and home 1172 link. In addition, the impact of any possible failure of the home 1173 agent, the home link, or intervening networks leading to or from the 1174 home link is reduced, since these nodes and links are not involved in 1175 the delivery of most packets to the mobile node. 1177 5. New IPv6 Destination Options and Message Types 1179 5.1. Binding Update Option 1181 The Binding Update destination option is used by a mobile node 1182 to notify other nodes of a new care-of address for itself. As a 1183 destination option, it MAY be included in any existing packet being 1184 sent to this same destination or MAY be sent in a packet by itself; 1185 a packet containing a Binding Update is sent in the same way as any 1186 packet sent by a mobile node (Section 10.1). 1188 The Binding Update option is encoded in type-length-value (TLV) 1189 format as follows: 1191 0 1 2 3 1192 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 1193 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1194 | Option Type | Option Length | 1195 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1196 |A|H|S|D| Reserved | Sequence # | 1197 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1198 | Lifetime | 1199 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1200 | Sub-Options... 1201 +-+-+-+-+-+-+-+-+-+-+-+- 1203 Option Type 1205 198 = 0xC6 1207 Option Length 1209 8-bit unsigned integer. Length of the option, in octets, 1210 excluding the Option Type and Option Length fields. This field 1211 MUST be set to 8 plus the total length of all sub-options 1212 present, including their Sub-Option Type and Sub-Option Len 1213 fields. 1215 Acknowledge (A) 1217 The Acknowledge (A) bit is set by the sending mobile node to 1218 request a Binding Acknowledgement (Section 5.2) be returned 1219 upon receipt of the Binding Update. 1221 Home Registration (H) 1223 The Home Registration (H) bit is set by the sending mobile node 1224 to request the receiving node to act as this node's home agent. 1225 The destination of the packet carrying this option MUST be that 1226 of a router sharing the same subnet prefix as the home address 1227 of the mobile node in the binding (given by the Home Address 1228 field in the Home Address option in the packet). 1230 Single Address Only (S) 1232 If the `S' bit is set, the mobile node requests that the home 1233 agent make no changes to any other Binding Cache entry except 1234 for the particular one containing the home address specified in 1235 the Home Address option. This disables home agent processing 1236 for other related addresses, as is described in section 9.1. 1238 Duplicate Address Detection (D) 1240 The Duplicate Address Detection (D) bit is set by the sending 1241 mobile node to request the receiving node (the mobile node's 1242 home agent) to perform Duplicate Address Detection [27] on 1243 the mobile node's home link for the home address in this 1244 binding. This bit is only valid when the Home Registration (H) 1245 and Acknowledge (A) bits are also set, and MUST NOT be set 1246 otherwise. If the Duplicate Address Detection performed by 1247 the home agent fails, the Status field in the returned Binding 1248 Acknowledgement will be set to 138 (Duplicate Address Detection 1249 failed). 1251 Reserved 1253 This field is unused. It MUST be initialized to zero by the 1254 sender and MUST be ignored by the receiver. 1256 Sequence # 1258 An 8-bit number used by the receiving node to sequence Binding 1259 Updates and by the sending node to match a returned Binding 1260 Acknowledgement with this Binding Update. Each Binding Update 1261 sent by a mobile node MUST use a Sequence Number greater than 1262 the Sequence Number value sent in the previous Binding Update 1263 (if any) to the same destination address (modulo 2**8, as 1264 defined in Section 4.6). There is no requirement, however, 1265 that the Sequence Number value strictly increase by 1 with each 1266 new Binding Update sent or received. 1268 Lifetime 1270 32-bit unsigned integer. The number of seconds remaining 1271 before the binding MUST be considered expired. A value of all 1272 one bits (0xffffffff) indicates infinity. A value of zero 1273 indicates that the Binding Cache entry for the mobile node MUST 1274 be deleted. 1276 Sub-Options 1278 Additional information, associated with this Binding Update 1279 option. This use of sub-options also allows for future 1280 extensions to the format of the Binding Update option to be 1281 defined. The encoding and format of defined sub-options are 1282 described in Section 5.5. The following sub-options are valid 1283 in a Binding Update option: 1285 - Unique Identifier Sub-Option 1287 - Alternate Care-of Address Sub-Option 1289 - Authentication Data Sub-Option (see section 4.4 for 1290 authentication requirements). 1292 The alignment requirement [6] for the Binding Update option is 4n+2. 1294 Any packet that includes a Binding Update option MUST also include 1295 a Home Address option. The home address of the mobile node in the 1296 binding given in the Binding Update option is that which was received 1297 as the value of the Home Address field in the Home Address option in 1298 the packet. 1300 Any packet that includes a Binding Update option MUST contain 1301 authentication data to guard against malicious Binding Updates. The 1302 computation for this authentication data MUST follow the rules in 1303 Section 4.4. 1305 The care-of address for the binding given in the Binding Update 1306 option is normally that which was received as the value in the Source 1307 Address field in the IPv6 header of the packet carrying the Binding 1308 Update option. However, a care-of address different from the Source 1309 Address MAY be specified by including an Alternate Care-of Address 1310 sub-option in the Binding Update option. In any case, the care-of 1311 address MUST NOT be any IPv6 address which is prohibited for use 1312 within a Routing Header; thus multicast addresses, the unspecified 1313 address, loop-back address, and link-local addresses are excluded. 1314 Binding Updates indicating any such excluded care-of address MUST be 1315 silently discarded. 1317 If the care-of address for the binding (specified either in an 1318 Alternate Care-of Address sub-option in the Binding Update option, if 1319 present, or in the Source Address field in the packet's IPv6 header) 1320 is equal to the home address of the mobile node, the Binding Update 1321 option indicates that any existing binding for the mobile node MUST 1322 be deleted. Likewise, if the Lifetime field in the Binding Update 1323 option is equal to 0, the Binding Update option indicates that any 1324 existing binding for the mobile node MUST be deleted. In each of 1325 these cases, a Binding Cache entry for the mobile node MUST NOT be 1326 created in response to receiving the Binding Update. 1328 When the care-of address is NOT equal to the home address, 1329 what if we just delete that particular care-of address? 1331 The last Sequence Number value sent to a destination in a Binding 1332 Update is stored by the mobile node in its Binding Update List entry 1333 for that destination; the last Sequence Number value received from 1334 a mobile node in a Binding Update is stored by a correspondent node 1335 in its Binding Cache entry for that mobile node. Thus, the mobile 1336 node's and the correspondent node's knowledge of the last sequence 1337 number expire at the same time. If the sending mobile node has no 1338 Binding Update List entry, the Sequence Number may start at any 1339 value; if the receiving correspondent node has no Binding Cache entry 1340 for the sending mobile node, it MUST accept any Sequence Number value 1341 in a received Binding Update from this mobile node. The mobile 1342 node MUST NOT use the same Sequence Number in two different Binding 1343 Updates to the same correspondent node, even if the Binding Updates 1344 provide care-of addresses for two different home addresses of the 1345 mobile node. 1347 The three highest-order bits of the Option Type are encoded to 1348 indicate specific processing of the option [6]. For the Binding 1349 Update option, these three bits are set to 110, indicating that any 1350 IPv6 node processing this option that does not recognize the Option 1351 Type must discard the packet and, only if the packet's Destination 1352 Address was not a multicast address, return an ICMP Parameter 1353 Problem, Code 2, message to the packet's Source Address; and that the 1354 data within the option cannot change en-route to the packet's final 1355 destination. 1357 5.2. Binding Acknowledgement Option 1359 The Binding Acknowledgement destination option is used to acknowledge 1360 receipt of a Binding Update option (Section 5.1). When a node 1361 receives a packet containing a Binding Update option, with this 1362 node being the destination of the packet (only the destination node 1363 processes the option since it is a destination option), this node 1364 MUST return a Binding Acknowledgement to the source of the packet, 1365 if the Acknowledge (A) bit is set in the Binding Update. As a 1366 destination option, this node MAY include the Binding Acknowledgement 1367 in any existing packet being sent to the mobile node or MAY send it 1368 in a packet by itself. A packet containing a Binding Acknowledgement 1369 is sent according to the rules in section 8.5. 1371 The Binding Acknowledgement option is encoded in type-length-value 1372 (TLV) format as follows: 1374 0 1 2 3 1375 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 1376 +-+-+-+-+-+-+-+-+ 1377 | Option Type | 1378 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1379 | Option Length | Status | Reserved | Sequence # | 1380 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1381 | Lifetime | 1382 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1383 | Refresh | 1384 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1385 | Sub-Options... 1386 +-+-+-+-+-+-+-+-+-+-+-+- 1388 Option Type 1390 7 1392 Option Length 1394 8-bit unsigned integer. Length of the option, in octets, 1395 excluding the Option Type and Option Length fields. This field 1396 MUST be set to 11 plus the total length of all sub-options 1397 present, including their Sub-Option Type and Sub-Option Len 1398 fields. 1400 Status 1402 8-bit unsigned integer indicating the disposition of the 1403 Binding Update. Values of the Status field less than 128 1404 indicate that the Binding Update was accepted by the receiving 1405 node. The following such Status values are currently defined: 1407 0 Binding Update accepted 1409 Values of the Status field greater than or equal to 128 1410 indicate that the Binding Update was rejected by the receiving 1411 node. The following such Status values are currently defined: 1413 128 Reason unspecified 1414 130 Administratively prohibited 1415 131 Insufficient resources 1416 132 Home registration not supported 1417 133 Not home subnet 1418 137 Not home agent for this mobile node 1419 138 Duplicate Address Detection failed 1420 139 No security association 1421 141 Sequence number too small 1423 Up-to-date values of the Status field are to be specified in 1424 the most recent "Assigned Numbers" [26]. 1426 Sequence # 1428 The Sequence Number in the Binding Acknowledgement is copied 1429 from the Sequence Number field in the Binding Update being 1430 acknowledged, for use by the mobile node in matching this 1431 Acknowledgement with an outstanding Binding Update. 1433 Lifetime 1435 The granted lifetime, in seconds, for which this node will 1436 attempt to retain the entry for this mobile node in its Binding 1437 Cache. If the node sending the Binding Acknowledgement is 1438 serving as the mobile node's home agent, the Lifetime period 1439 also indicates the period for which this node will continue 1440 this service; if the mobile node requires home agent service 1441 from this node beyond this period, the mobile node MUST send a 1442 new Binding Update to it before the expiration of this period 1443 (even if it is not changing its primary care-of address), in 1444 order to extend the lifetime. The value of this field is 1445 undefined if the Status field indicates that the Binding Update 1446 was rejected. 1448 Refresh 1450 The recommended interval, in seconds, at which the mobile 1451 node SHOULD send a new Binding Update to this node in order 1452 to "refresh" the mobile node's binding in this node's Binding 1453 Cache. This refreshing of the binding is useful in case the 1454 node fails and loses its cache state. The Refresh period is 1455 determined by the node sending the Binding Acknowledgement 1456 (the node caching the binding). If this node is serving as 1457 the mobile node's home agent, the Refresh value may be set, 1458 for example, based on whether the node stores its Binding 1459 Cache in volatile storage or in nonvolatile storage. If the 1460 node sending the Binding Acknowledgement is not serving as the 1461 mobile node's home agent, the Refresh period SHOULD be set 1462 equal to the Lifetime period in the Binding Acknowledgement; 1463 even if this node loses this cache entry due to a failure of 1464 the node, packets from it can still reach the mobile node 1465 through the mobile node's home agent, causing a new Binding 1466 Update to this node to allow it to recreate this cache entry. 1467 The value of this field is undefined if the Status field 1468 indicates that the Binding Update was rejected. 1470 Sub-Options 1472 Additional information, associated with this Binding 1473 Acknowledgement option. The Authentication Data sub-option MAY 1474 be present, and other sub-options (not currently defined) MAY 1475 be optionally included. See section 4.4 for authentication 1476 requirements. This use of sub-options also allows for future 1477 extensions to the format of the Binding Acknowledgement option 1478 to be defined. 1480 The alignment requirement [6] for the Binding Acknowledgement option 1481 is 4n+3. 1483 Any packet that includes a Binding Acknowledgement option MUST 1484 contain authentication data to guard against malicious Binding 1485 Acknowledgements. The computation for this authentication data MUST 1486 follow the rules in Section 4.4. 1488 If the node returning the Binding Acknowledgement accepted the 1489 Binding Update for which the Acknowledgement is being returned (the 1490 value of the Status field in the Acknowledgement is less than 128), 1491 this node will have an entry for the mobile node in its Binding 1492 Cache and MUST use this entry (which includes the care-of address 1493 received in the Binding Update) in sending the packet containing the 1494 Binding Acknowledgement to the mobile node. The details of sending 1495 this packet (using a Routing header) to the mobile node are the same 1496 as for sending any packet to a mobile node using a binding, as are 1497 described in Section 8.9. 1499 If the node returning the Binding Acknowledgement instead 1500 rejected the Binding Update (the value of the Status field in the 1501 Acknowledgement is greater than or equal to 128), this node MUST 1502 similarly use a Routing header in sending the packet containing the 1503 Binding Acknowledgement, as described in Section 8.9, but MUST NOT 1504 use its Binding Cache in forming the IP header or Routing header 1505 in this packet. Rather, the care-of address used by this node in 1506 sending the packet containing the Binding Acknowledgement MUST be 1507 copied from the care-of address received in the rejected Binding 1508 Update; this node MUST NOT modify its Binding Cache in response 1509 to receiving this rejected Binding Update and MUST ignore its 1510 Binding Cache in sending the packet in which it returns this Binding 1511 Acknowledgement. The packet is sent using a Routing header, routing 1512 the packet to the home address of the rejected Binding Update by 1513 way of the care-of address indicated in the packet containing the 1514 Binding Update. When sending a Binding Acknowledgement to reject a 1515 Binding Update, the Binding Acknowledgement MUST be sent in an IPv6 1516 packet containing no payload (with the Next Header field in the last 1517 extension header in the packet set to indicate "No Next Header" [6]). 1519 The three highest-order bits of the Option Type are encoded to 1520 indicate specific processing of the option [6]. For the Binding 1521 Acknowledgement option, these three bits are set to 000, indicating 1522 that any IPv6 node processing this option that does not recognize the 1523 Option Type must skip over this option and continue processing the 1524 header, and that the data within the option cannot change en-route to 1525 the packet's final destination. 1527 If the mobile node sends a sequence number which is not greater than 1528 the sequence number from the last successful Binding Update, then the 1529 home agent MUST send back a Binding Acknowledgement with status code 1530 141, and the last accepted sequence number in the Sequence Number 1531 field of the Binding Acknowledgement. 1533 5.3. Binding Request Option 1535 The Binding Request destination option is used to request a mobile 1536 node's binding from the mobile node. As a destination option, it 1537 MAY be included in any existing packet being sent to the mobile 1538 node or MAY be sent in a packet by itself; a packet containing a 1539 Binding Request option is sent in the same way as any packet to a 1540 mobile node (Section 8.9). When a mobile node receives a packet 1541 containing a Binding Request option, it SHOULD return a Binding 1542 Update (Section 5.1) to the source of the Binding Request. 1544 The Binding Request option is encoded in type-length-value (TLV) 1545 format as follows: 1547 0 1 1548 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1549 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 1550 | Option Type | Option Length | Sub-Options... 1551 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 1553 Option Type 1555 8 1557 Option Length 1559 8-bit unsigned integer. Length of the option, in octets, 1560 excluding the Option Type and Option Length fields. This field 1561 MUST be set to 0 plus the total length of all sub-options 1562 present, including their Sub-Option Type and Sub-Option Len 1563 fields. 1565 Sub-Options 1567 Additional information, associated with this Binding Request 1568 option, that need not be present in all Binding Requests sent. 1569 This use of sub-options also allows for future extensions to 1570 the format of the Binding Request option to be defined. The 1571 encoding and format of defined sub-options are described in 1572 Section 5.5. The following sub-options are valid in a Binding 1573 Request option: 1575 - Unique Identifier Sub-Option 1577 There is no requirement for alignment [6] of the Binding Request 1578 option. 1580 The three highest-order bits of the Option Type are encoded to 1581 indicate specific processing of the option [6]. For the Binding 1582 Request option, these three bits are set to 000, indicating that any 1583 IPv6 node processing this option that does not recognize the Option 1584 Type must skip over this option and continue processing the header, 1585 and that the data within the option cannot change en-route to the 1586 packet's final destination. 1588 5.4. Home Address Option 1590 The Home Address destination option is used in a packet sent by a 1591 mobile node while away from home, to inform the recipient of that 1592 packet of the mobile node's home address. For packets sent by a 1593 mobile node while away from home, the mobile node generally uses 1594 one of its care-of addresses as the Source Address in the packet's 1595 IPv6 header. By including a Home Address option in the packet, the 1596 correspondent node receiving the packet is able to substitute the 1597 mobile node's home address for this care-of address when processing 1598 the packet, thus making the use of the care-of address transparent to 1599 the correspondent node. Note that multicast addresses, link-local 1600 addresses, loopback addresses, IPv4 mapped addresses, and the 1601 unspecified address, MUST NOT be used within a Home Address option. 1603 The Home Address option is encoded in type-length-value (TLV) format 1604 as follows: 1606 0 1 2 3 1607 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 1608 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1609 | Option Type | Option Length | 1610 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1611 | | 1612 + + 1613 | | 1614 + Home Address + 1615 | | 1616 + + 1617 | | 1618 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1619 | Sub-Options... 1620 +-+-+-+-+-+-+-+-+-+-+-+- 1622 Option Type 1624 201 = 0xC9 1626 Option Length 1628 8-bit unsigned integer. Length of the option, in octets, 1629 excluding the Option Type and Option Length fields. This field 1630 MUST be set to 16 plus the total length of all sub-options 1631 present, including their Sub-Option Type and Sub-Option Len 1632 fields. 1634 Home Address 1636 The home address of the mobile node sending the packet. 1638 Sub-Options 1640 Additional information, associated with this Home Address 1641 option, that need not be present in all Home Address options 1642 sent. This use of sub-options also allows for future 1643 extensions to the format of the Home Address option to be 1644 defined. Currently, no valid sub-options are defined for use 1645 in a Home Address option. 1647 The alignment requirement [6] for the Home Address option is 8n+6. 1649 The inclusion of a Home Address option in a packet affects the 1650 receiving node's processing of only this single packet; no state is 1651 created or modified in the receiving node as a result of receiving a 1652 Home Address option in a packet. In particular, the presence of a 1653 Home Address option in a received packet MUST NOT alter the contents 1654 of the receiver's Binding Cache and MUST NOT cause any changes in the 1655 routing of subsequent packets sent by this receiving node. 1657 The Home Address option MUST be placed as follows: 1659 - After the Routing Header, if that header is present 1661 - Before the Fragment Header, if that header is present 1663 - Before the AH Header or ESP Header, if either one of those 1664 headers is present 1666 No authentication of the Home Address option is required, except 1667 that if the IPv6 header of a packet is covered by authentication, 1668 then that authentication MUST also cover the Home Address option; 1669 this coverage is achieved automatically by the definition of the 1670 Option Type code for the Home Address option, since it indicates 1671 that the data within the option cannot change en-route to the 1672 packet's final destination, and thus the option is included in the 1673 authentication computation. By requiring that any authentication of 1674 the IPv6 header also cover the Home Address option, the security of 1675 the Source Address field in the IPv6 header is not compromised by 1676 the presence of a Home Address option. Security issues related to 1677 the Home Address option are discussed further in Section 13. When 1678 attempting to verify authentication data in a packet that contains 1679 a Home Address option, the receiving node MUST make the calculation 1680 as if the care-of address were present in the Home Address option, 1681 and the home address were present in the source IPv6 address field 1682 of the IPv6 header. This conforms with the calculation specified in 1683 section 10.2. 1685 A packet MUST NOT contain more than one Home Address option, except 1686 that an encapsulated packet [4] MAY contain a separate Home Address 1687 option associated with each encapsulating IP header. 1689 The three highest-order bits of the Option Type are encoded to 1690 indicate specific processing of the option [6]. For the Home Address 1691 option, these three bits are set to 110, indicating that any IPv6 1692 node processing this option that does not recognize the Option Type 1693 must discard the packet and, only if the packet's Destination Address 1694 was not a multicast address, return an ICMP Parameter Problem, 1695 Code 2, message to the packet's Source Address; and that the data 1696 within the option cannot change en-route to the packet's final 1697 destination. 1699 5.5. Mobile IPv6 Destination Option Sub-Options 1701 In order to allow optional fields that may not be needed in every 1702 use of any given Mobile IPv6 destination option, and to allow future 1703 extensions to the format of these destination options to be defined, 1704 any of the Mobile IPv6 destination options defined in this document 1705 MAY include one or more sub-options. 1707 Such sub-options are included in the data portion of the destination 1708 option itself, after the fixed portion of the option data specified 1709 for that particular destination option (Sections 5.1 through 5.4). 1710 The presence of such sub-options will be indicated by the Option 1711 Length field. When the Option Length is greater than the standard 1712 length defined for that destination option, the remaining octets are 1713 interpreted as sub-options. 1715 These sub-options are encoded within the remaining space of the 1716 option data for that option, using a type-length-value (TLV) format 1717 as follows: 1719 0 1 2 3 1720 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 1721 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1722 |Sub-Option Type| Sub-Option Len| Sub-Option Data... 1723 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1725 Sub-Option Type 1727 8-bit identifier of the type of sub-option. When processing 1728 a Mobile IPv6 destination option containing a sub-option for 1729 which the Sub-Option Type value is not recognized by the 1730 receiver, the receiver SHOULD quietly ignore and skip over the 1731 sub-option, correctly handling any remaining sub-options in the 1732 option. 1734 Sub-Option Length 1736 8-bit unsigned integer. Length of the Sub-Option Data field 1737 of this sub-option, in octets. The Sub-Option Len does not 1738 include the length of the Sub-Option Type and Sub-Option Len 1739 fields. 1741 Sub-Option Data 1743 Variable-length field. Sub-Option-Type-specific data. 1745 As with IPv6 options appearing in a Hop-by-Hop Options header 1746 or Destination Options header [6], individual sub-options within 1747 a Mobile IPv6 destination option may have specific alignment 1748 requirements, to ensure that multi-octet values within Sub-Option 1749 Data fields fall on natural boundaries. The alignment requirement 1750 of each sub-option is specified as part of the definition of each 1751 sub-option below. 1753 Each section above defining the Mobile IPv6 destination options 1754 specifies which of the defined sub-options is valid for that 1755 destination option. In addition, there are two padding sub-options, 1756 Pad1 and PadN (defined below), which are used when necessary to align 1757 subsequent sub-options. The Pad1 and PadN sub-options are valid for 1758 all Mobile IPv6 destination options. Unlike the padding options 1759 used in Hop-by-Hop Options header or Destination Options header [6], 1760 there is no requirement for padding the total size of any Mobile IPv6 1761 destination option to a multiple of 8 octets in length, and the 1762 Pad1 and PadN sub-options SHOULD NOT be used for this purpose. All 1763 Mobile IPv6 sub-options defined in this document MUST be recognized 1764 by all Mobile IPv6 implementations. 1766 The following subsections specify the sub-option types which are 1767 currently defined for use in Mobile IPv6 destination options. 1769 5.5.1. Pad1 1771 Pad1 Sub-Option (alignment requirement: none) 1773 0 1774 0 1 2 3 4 5 6 7 1775 +-+-+-+-+-+-+-+-+ 1776 | 0 | 1777 +-+-+-+-+-+-+-+-+ 1779 NOTE! the format of the Pad1 sub-option is a special 1780 case -- it has neither Sub-Option Len nor Sub-Option Data 1781 fields. 1783 The Pad1 sub-option is used to insert one octet of padding 1784 into the Sub-Options area of a Mobile IPv6 option. If more 1785 than one octet of padding is required, the PadN sub-option, 1786 described next, should be used, rather than multiple Pad1 1787 sub-options. 1789 5.5.2. PadN 1791 PadN Sub-Option (alignment requirement: none) 1792 0 1 1793 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1794 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 1795 | 1 | Sub-Option Len| Sub-Option Data 1796 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 1798 The PadN sub-option is used to insert two or more octets of 1799 padding into the Sub-Options area of a Mobile IPv6 option. 1800 For N octets of padding, the Sub-Option Len field contains 1801 the value N-2, and the Sub-Option Data consists of N-2 1802 zero-valued octets. 1804 5.5.3. Unique Identifier 1806 Unique Identifier Sub-Option (alignment requirement: 2n) 1808 0 1 2 3 1809 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 1810 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1811 | 2 | 2 | Unique Identifier | 1812 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1814 The Unique Identifier sub-option is valid only in Binding 1815 Request and Binding Update destination options. The Unique 1816 Identifier field contains a 16-bit value that serves to 1817 uniquely identify a Binding Request among those sent by this 1818 Source Address, and to allow the Binding Update to identify 1819 the specific Binding Request to which it responds. This 1820 matching of Binding Updates to Binding Requests is required 1821 in the procedure for renumbering the home subnet while a 1822 mobile node is away from home (Section 9.6). 1824 5.5.4. Alternate Care-of Address 1826 Alternate Care-of Address Sub-Option (alignment requirement: 8n+6) 1827 0 1 2 3 1828 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 1829 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1830 | 3 | 16 | 1831 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1832 | | 1833 + + 1834 | | 1835 + Alternate Care-of Address + 1836 | | 1837 + + 1838 | | 1839 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1841 The Alternate Care-of Address sub-option is valid only in 1842 Binding Update destination options. The Alternate Care-of 1843 Address field contains an address to use as the care-of 1844 address for the binding, rather than using the Source 1845 Address of the packet as the care-of address. 1847 5.6. Authentication Data 1849 Authentication Data Sub-Option (alignment requirement: 8n+6) 1851 0 1 2 3 1852 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 1853 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1854 | 4 | Length | 1855 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1856 | Security Parameters Index (SPI) | 1857 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1858 | | 1859 = Authentication Data (variable length) = 1860 | | 1861 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1863 The Authentication Data sub-option is valid in Binding 1864 Update and Binding Acknowledgement destination options. 1865 The Length of the sub-option is 4 plus the number of bytes 1866 contained in the Authentication Data field. 1868 The SPI is an arbitrary 32-bit value that, in combination 1869 with the destination IP address, uniquely identifies the 1870 BSA for this datagram. The set of SPI values in the range 1871 1 through 255 are reserved by the Internet Assigned Numbers 1872 Authority (IANA) for future use; a reserved SPI value will 1873 not normally be assigned by IANA unless the use of the 1874 assigned SPI value is specified in an RFC. It is ordinarily 1875 selected by the destination system upon establishment of 1876 an SA (see the Security Architecture document for more 1877 details). 1879 The SPI value of zero (0) is reserved for local, 1880 implementation- specific use and MUST NOT be sent on the 1881 wire. 1883 The Authentication Data field is a variable-length 1884 field that contains data necessary to secure a Binding 1885 Update or Binding Acknowledgment. The field must be 1886 an integral multiple of 32 bits in length. The details 1887 of the authentication data computation are described in 1888 Section 4.4. This field may include explicit padding, 1889 depending upon the requirements of the algorithm selected by 1890 the SPI. 1892 5.7. ICMP Home Agent Address Discovery Request Message 1894 The ICMP Home Agent Address Discovery Request message is used by a 1895 mobile node to initiate the dynamic home agent address discovery 1896 mechanism, as described in Sections 9.8 and 10.8. The mobile 1897 node sends a Home Agent Address Discovery Request message to the 1898 "Mobile IPv6 Home-Agents" anycast address for its own home subnet 1899 prefix [10], and one of the home agents there responds to the mobile 1900 node with a Home Agent Address Discovery Reply message giving a list 1901 of the routers on the mobile node's home link serving as home agents. 1903 0 1 2 3 1904 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 1905 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1906 | Type | Code | Checksum | 1907 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1908 | Identifier | Reserved | 1909 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1911 Type 1913 150 1915 Code 1917 0 1919 Checksum 1921 The ICMP checksum [5]. 1923 Identifier 1925 An identifier to aid in matching Home Agent Address Discovery 1926 Reply messages to this Home Agent Address Discovery Request 1927 message. 1929 Reserved 1931 This field is unused. It MUST be initialized to zero by the 1932 sender and MUST be ignored by the receiver. 1934 The Source Address of the Home Agent Address Discovery Request 1935 message packet MUST be one of the mobile node's current care-of 1936 addresses. The home agent then MUST return the Home Agent Address 1937 Discovery Reply message directly to the Source Address chosen by 1938 the mobile node Note that, at the time of performing this dynamic 1939 home agent address discovery, it is likely that the mobile node not 1940 registered with any home agent within the specified anycast group. 1942 5.8. ICMP Home Agent Address Discovery Reply Message 1944 The ICMP Home Agent Address Discovery Reply message is used by a 1945 home agent to respond to a mobile node using the dynamic home agent 1946 address discovery mechanism, as described in Sections 9.8 and 10.8. 1947 The mobile node sends a Home Agent Address Discovery Request message 1948 to the "Mobile IPv6 Home-Agents" anycast address for its own home 1949 subnet prefix [10], and one of the home agents there responds to the 1950 mobile node with a Home Agent Address Discovery Reply message giving 1951 a list of the routers on the mobile node's home link serving as home 1952 agents. 1954 0 1 2 3 1955 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 1956 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1957 | Type | Code | Checksum | 1958 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1959 | Identifier | | 1960 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 1961 | | 1962 + Reserved + 1963 | | 1964 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1965 | | 1966 + + 1967 . . 1968 . Home Agent Addresses . 1969 . . 1970 + + 1971 | | 1972 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1974 Type 1976 151 1978 Code 1980 0 1982 Checksum 1984 The ICMP checksum [5]. 1986 Identifier 1988 The identifier from the invoking Home Agent Address Discovery 1989 Request message. 1991 Reserved 1993 This field is unused. It MUST be initialized to zero by the 1994 sender and MUST be ignored by the receiver. 1996 Home Agent Addresses 1998 A list of addresses of home agents on the home link for the 1999 mobile node. The number of addresses present in the list is 2000 indicated by the remaining length of the IPv6 packet carrying 2001 the Home Agent Address Discovery Reply message. 2003 5.9. ICMP Mobile Prefix Solicitation Message Format 2005 The ICMP Mobile Prefix Solicitation Message is sent by a mobile node 2006 to its home agent while it is away from home. The purpose of the 2007 message is to solicit a Mobile Prefix Advertisement from the home 2008 agent, which will allow the mobile node to gather prefix information 2009 about its home network. This information can be used to configure 2010 home address(es) by stateless address autoconfiguration [27], 2011 or update address(es) according to changes in prefix information 2012 supplied by the home agent. 2014 The Mobile Prefix Solicitation is similar to the Router Solicitation 2015 used in Neighbor Discovery [17], except it is routed from the mobile 2016 node on the visited network to the home agent on the home network by 2017 usual unicast routing rules. 2019 0 1 2 3 2020 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 2021 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2022 | Type | Code | Checksum | 2023 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2024 | Reserved | 2025 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2027 IP Fields: 2029 Source Address 2031 The mobile node's care-of address. 2033 Destination Address 2035 The address of the mobile node's home agent. This home agent 2036 must be on the link which the mobile node wishes to learn 2037 prefix information about. 2039 Hop Limit 2041 Set to an initial hop limit value, and this message is routed 2042 according to the rules of a typical unicast packet. A hop 2043 limit of 64 is currently suggested [26]. 2045 Authentication Header 2047 If a Security Association for the IP Authentication Header 2048 exists between the sender and the destination address, then the 2049 sender SHOULD include this header. [subject to change] 2051 ICMP Fields: 2053 Type 2055 152 2057 Code 2059 0 2061 Checksum 2063 The ICMP checksum [5]. 2065 Reserved 2067 This field is unused. It MUST be initialized to zero by the 2068 sender and MUST be ignored by the receiver. 2070 5.10. ICMP Mobile Prefix Advertisement Message Format 2072 A home agent will send a Mobile Prefix Advertisement message to a 2073 mobile node to distribute prefix information about the home link 2074 while the mobile node is traveling away from the home network. This 2075 will occur in response to a Mobile Prefix Solicitation with an 2076 Advertisement, or by an unsolicited Advertisement sent according to 2077 the rules in Section 5.10. 2079 0 1 2 3 2080 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 2081 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2082 | Type | Code | Checksum | 2083 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2084 | Options ... 2085 +-+-+-+-+-+-+-+-+-+-+-+- 2087 IP Fields: 2089 Source Address 2090 The home agent's address as the mobile node would 2091 expect to see it (i.e. same prefix) 2093 Destination Address 2094 If this message is a response to a Mobile Prefix 2095 Solicitation, the Source Address field from that 2096 packet. For unsolicited messages, the mobile node's 2097 care-of address SHOULD be used, if it is currently 2098 registered with the home agent. Otherwise, the 2099 mobile node's home address SHOULD be used. 2101 Authentication Header 2102 This header MUST be sent, unless the mobile node 2103 has not yet configured, and is using its care-of 2104 address. [subject to change] 2106 ICMP Fields: 2108 Type 2110 153 2112 Code 2114 0 2116 Checksum 2118 The ICMP checksum [5]. 2120 Options: 2122 Prefix Information 2124 Each message contains one or more Prefix Information options, 2125 which contain the prefix(es) the mobile node should configure 2126 its home address(es) with. Section 9.6 describes which 2127 prefixes should be advertised to the mobile node. 2129 The Prefix Information option is defined in Section 4.6.2 2130 of [17], with modifications defined in Section 6.2 of this 2131 specification. The home agent MUST use this modified Prefix 2132 Information option to send the aggregate list of home network 2133 prefixes as defined in Section 9.8.1. 2135 The Mobile Prefix Advertisement sent by the home agent MAY include 2136 the Source Link-layer Address option defined in RFC 2461 [17], or the 2137 Advertisement Interval option specified in Section 6.3. 2139 Future versions of this protocol may define new option types. Home 2140 Agents MUST silently ignore any options they do not recognize and 2141 continue processing the message. 2143 6. Modifications to IPv6 Neighbor Discovery 2145 6.1. Modified Router Advertisement Message Format 2147 Mobile IPv6 modifies the format of the Router Advertisement 2148 message [17] by the addition of a single flag bit to indicate that 2149 the router sending the Advertisement message is serving as a home 2150 agent on this link. The format of the Router Advertisement message 2151 is as follows: 2153 0 1 2 3 2154 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 2155 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2156 | Type | Code | Checksum | 2157 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2158 | Cur Hop Limit |M|O|H| Reserved| Router Lifetime | 2159 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2160 | Reachable Time | 2161 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2162 | Retrans Timer | 2163 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2164 | Options ... 2165 +-+-+-+-+-+-+-+-+-+-+-+- 2167 This format represents the following changes over that originally 2168 specified for Neighbor Discovery [17]: 2170 Home Agent (H) 2172 The Home Agent (H) bit is set in a Router Advertisement to 2173 indicate that the router sending this Router Advertisement is 2174 also functioning as a Mobile IP home agent on this link. 2176 Reserved 2178 Reduced from a 6-bit field to a 5-bit field to account for the 2179 addition of the Home Agent (H) bit. 2181 6.2. Modified Prefix Information Option Format 2183 Mobile IPv6 requires knowledge of a router's global address for two 2184 reasons: 2186 - To allow a home agent (a router) to learn the address of all 2187 other home agents on the link for which it is providing home 2188 agent service, for use in building its Home Agents List as 2189 part of the dynamic home agent address discovery mechanism 2190 (Sections 9.8 and 10.8). 2192 - To allow a mobile node to send a Binding Update to a router on 2193 the link on which its previous care-of address is located, for 2194 purposes of establishing forwarding from this previous care-of 2195 address to its new care-of address (Section 10.10). 2197 However, Neighbor Discovery [17] only advertises a router's 2198 link-local address, by requiring this address to be used as the IP 2199 Source Address of each Router Advertisement. 2201 Mobile IPv6 extends Neighbor Discovery to allow a router to easily 2202 and efficiently advertise its global address, by the addition of a 2203 single flag bit in the format of a Prefix Information option for 2204 use in Router Advertisement messages. The format of the Prefix 2205 Information option is as follows: 2207 0 1 2 3 2208 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 2209 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2210 | Type | Length | Prefix Length |L|A|R|Reserved1| 2211 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2212 | Valid Lifetime | 2213 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2214 | Preferred Lifetime | 2215 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2216 | Reserved2 | 2217 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2218 | | 2219 + + 2220 | | 2221 + Prefix + 2222 | | 2223 + + 2224 | | 2225 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2227 This format represents the following changes over that originally 2228 specified for Neighbor Discovery [17]: 2230 Router Address (R) 2232 1-bit router address flag. When set, indicates that the 2233 Prefix field, in addition to advertising the indicated prefix, 2234 contains a complete IP address assigned to the sending router. 2235 This router IP address has the same scope and conforms to the 2236 same lifetime values as the advertised prefix. This use of 2237 the Prefix field is compatible with its use in advertising 2238 the prefix itself, since prefix advertisement uses only the 2239 leading number Prefix bits specified by the Prefix Length 2240 field. Interpretation of this flag bit is thus independent 2241 of the processing required for the On-Link (L) and Autonomous 2242 Address-Configuration (A) flag bits. 2244 Reserved1 2246 Reduced from a 6-bit field to a 5-bit field to account for the 2247 addition of the Router Address (R) bit. 2249 In a solicited Router Advertisement, a home agent MUST, and all other 2250 routers SHOULD, include at least one Prefix Information option with 2251 the Router Address (R) bit set. Neighbor Discovery specifies that, 2252 if including all options in a Router Advertisement causes the size of 2253 the Advertisement to exceed the link MTU, multiple Advertisements can 2254 be sent, each containing a subset of the options [17]. In this case, 2255 at least one of these multiple Advertisements being sent instead 2256 of a single larger solicited Advertisement, MUST include a Prefix 2257 Information option with the Router Address (R) bit set. 2259 All routers SHOULD include at least one Prefix Information option 2260 with the Router Address (R) bit set, in each unsolicited multicast 2261 Router Advertisement that they send. If multiple Advertisements 2262 are being sent instead of a single larger unsolicited multicast 2263 Advertisement, at least one of these multiple Advertisements SHOULD 2264 include a Prefix Information option with the Router Address (R) bit 2265 set. 2267 6.3. New Advertisement Interval Option Format 2269 Mobile IPv6 defines a new Advertisement Interval option, used in 2270 Router Advertisement messages to advertise the interval at which the 2271 sending router sends unsolicited multicast Router Advertisements. 2272 The format of the Advertisement Interval option is as follows: 2274 0 1 2 3 2275 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 2276 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2277 | Type | Length | Reserved | 2278 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2279 | Advertisement Interval | 2280 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2282 Type 2284 7 2286 Length 2288 8-bit unsigned integer. The length of the option (including 2289 the type and length fields) in units of 8 octets. The value of 2290 this field MUST be 1. 2292 Reserved 2294 This field is unused. It MUST be initialized to zero by the 2295 sender and MUST be ignored by the receiver. 2297 Advertisement Interval 2299 32-bit unsigned integer. The maximum time, in milliseconds, 2300 between successive unsolicited router Router Advertisement 2301 messages sent by this router on this network interface. Using 2302 the conceptual router configuration variables defined by 2303 Neighbor Discovery [17], this field MUST be equal to the value 2304 MaxRtrAdvInterval, expressed in milliseconds. 2306 Routers MAY include this option in their Router Advertisements. A 2307 mobile node receiving a Router Advertisement containing this option 2308 SHOULD utilize the specified Advertisement Interval for that router 2309 in its movement detection algorithm, as described in Section 10.4. 2311 This option MUST be silently ignored for other Neighbor Discovery 2312 messages. 2314 6.4. New Home Agent Information Option Format 2316 Mobile IPv6 defines a new Home Agent Information option, used in 2317 Router Advertisement messages sent by a home agent to advertise 2318 information specific to this router's functionality as a home agent. 2319 The format of the Home Agent Information option is as follows: 2321 0 1 2 3 2322 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 2323 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2324 | Type | Length | Reserved | 2325 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2326 | Home Agent Preference | Home Agent Lifetime | 2327 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2329 Type 2331 8 2333 Length 2335 8-bit unsigned integer. The length of the option (including 2336 the type and length fields) in units of 8 octets. The value of 2337 this field MUST be 1. 2339 Reserved 2341 This field is unused. It MUST be initialized to zero by the 2342 sender and MUST be ignored by the receiver. 2344 Home Agent Preference 2346 16-bit signed, twos-complement integer. The preference for 2347 the home agent sending this Router Advertisement, for use in 2348 ordering the addresses returned to a mobile node in the Home 2349 Agent Addresses field of a Home Agent Address Discovery Reply 2350 message. Higher values mean more preferable. If this option 2351 is not included in a Router Advertisement in which the Home 2352 Agent (H) bit is set, the preference value for this home agent 2353 SHOULD be considered to be 0. Values greater than 0 indicate a 2354 home agent more preferable than this default value, and values 2355 less than 0 indicate a less preferable home agent. 2357 The manual configuration of the Home Agent Preference value 2358 is described in Section 7.3. In addition, the sending home 2359 agent MAY dynamically set the Home Agent Preference value, for 2360 example basing it on the number of mobile nodes it is currently 2361 serving or on its remaining resources for serving additional 2362 mobile nodes; such dynamic settings are beyond the scope of 2363 this document. Any such dynamic setting of the Home Agent 2364 Preference, however, MUST set the preference appropriately, 2365 relative to the default Home Agent Preference value of 0 that 2366 may be in use by some home agents on this link (i.e., a home 2367 agent not including a Home Agent Information option in its 2368 Router Advertisements will be considered to have a Home Agent 2369 Preference value of 0). 2371 Home Agent Lifetime 2373 16-bit unsigned integer. The lifetime associated with the 2374 home agent in units of seconds. The default value is the same 2375 as the Router Lifetime, as specified in the main body of the 2376 Router Advertisement message. The maximum value corresponds 2377 to 18.2 hours. A value of 0 MUST NOT be used. The Home Agent 2378 Lifetime applies only to this router's usefulness as a home 2379 agent; it does not apply to information contained in other 2380 message fields or options. 2382 Home agents MAY include this option in their Router Advertisements. 2383 This option MUST NOT be included in a Router Advertisement in which 2384 the Home Agent (H) bit (see Section 6.1) is not set. If this option 2385 is not included in a Router Advertisement in which the Home Agent (H) 2386 bit is set, the lifetime for this home agent MUST be considered to be 2387 the same as the Router Lifetime in the Router Advertisement. 2389 This option MUST be silently ignored for other Neighbor Discovery 2390 messages. 2392 If both the Home Agent Preference and Home Agent Lifetime are set 2393 to their default values specified above, this option SHOULD NOT be 2394 included in the Router Advertisement messages sent by this home 2395 agent. 2397 6.5. Changes to Sending Router Advertisements 2399 The Neighbor Discovery protocol specification [17] limits routers to 2400 a minimum interval of 3 seconds between sending unsolicited multicast 2401 Router Advertisement messages from any given network interface 2402 (limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that: 2404 "Routers generate Router Advertisements frequently enough 2405 that hosts will learn of their presence within a few 2406 minutes, but not frequently enough to rely on an absence 2407 of advertisements to detect router failure; a separate 2408 Neighbor Unreachability Detection algorithm provides failure 2409 detection." 2411 This limitation, however, is not suitable to providing timely 2412 movement detection for mobile nodes. Mobile nodes detect their 2413 own movement by learning the presence of new routers as the mobile 2414 node moves into wireless transmission range of them (or physically 2415 connects to a new wired network), and by learning that previous 2416 routers are no longer reachable. Mobile nodes MUST be able to 2417 quickly detect when they move to a link served by a new router, so 2418 that they can acquire a new care-of address and send Binding Updates 2419 to register this care-of address with their home agent and to notify 2420 correspondent nodes as needed. 2422 Thus, to provide good support for mobile nodes, Mobile IPv6 relaxes 2423 this limit such that routers MAY send unsolicited multicast Router 2424 Advertisements more frequently. In particular, on network interfaces 2425 where the router is expecting to provide service to visiting mobile 2426 nodes (e.g., wireless network interfaces), or on which it is serving 2427 as a home agent to one or more mobile nodes (who may return home and 2428 need to hear its Advertisements), the router SHOULD be configured 2429 with a smaller MinRtrAdvInterval value and MaxRtrAdvInterval value, 2430 to allow sending of unsolicited multicast Router Advertisements more 2431 often. Recommended values for these limits are: 2433 - MinRtrAdvInterval 0.05 seconds 2435 - MaxRtrAdvInterval 1.5 seconds 2437 Use of these modified limits MUST be configurable, and specific 2438 knowledge of the type of network interface in use SHOULD be taken 2439 into account in configuring these limits for each network interface. 2441 When sending unsolicited multicast Router Advertisements more 2442 frequently than the standard limit on unsolicited multicast 2443 Advertisement frequency, the sending router need not include all 2444 options in each of these Advertisements, but it SHOULD include at 2445 least one Prefix Information option with the Router Address (R) bit 2446 set (Section 6.2) in each. 2448 6.6. Changes to Sending Router Solicitations 2450 In addition to the limit on routers sending unsolicited multicast 2451 Router Advertisement messages (Section 6.5), Neighbor Discovery 2452 defines limits on nodes sending Router Solicitation messages, such 2453 that a node SHOULD send no more than 3 Router Solicitations, and that 2454 these 3 transmissions SHOULD be spaced at least 4 seconds apart. 2455 However, these limits prevent a mobile node from finding a new 2456 default router (and thus a new care-of address) quickly as it moves 2457 about. 2459 Mobile IPv6 relaxes this limit such that, while a mobile node is away 2460 from home, it MAY send Router Solicitations more frequently. The 2461 following limits for sending Router Solicitations are recommended for 2462 mobile nodes while away from home: 2464 - A mobile node that is not configured with any current care-of 2465 address (e.g., the mobile node has moved since its previous 2466 care-of address was configured), MAY send more than the defined 2467 Neighbor Discovery limit of MAX_RTR_SOLICITATIONS Router 2468 Solicitations. 2470 - The rate at which a mobile node sends Router Solicitations MUST 2471 be limited, although a mobile node MAY send Router Solicitations 2472 more frequently than the defined Neighbor Discovery limit of 2473 RTR_SOLICITATION_INTERVAL seconds. The minimum interval MUST 2474 be configurable, and specific knowledge of the type of network 2475 interface in use SHOULD be taken into account in configuring this 2476 limit for each network interface. A recommended minimum interval 2477 is 1 second. 2479 - After sending at most MAX_RTR_SOLICITATIONS Router Solicitations, 2480 a mobile node MUST reduce the rate at which it sends subsequent 2481 Router Solicitations. Subsequent Router Solicitations SHOULD 2482 be sent using a binary exponential backoff mechanism, doubling 2483 the interval between consecutive Router Solicitations, up to a 2484 maximum interval. The maximum interval MUST be configurable and 2485 SHOULD be chosen appropriately based on the characteristics of 2486 the type of network interface in use. 2488 - While still searching for a new default router and care-of 2489 address, a mobile node MUST NOT increase the rate at which it 2490 sends Router Solicitations unless it has received a positive 2491 indication (such as from lower network layers) that it has moved 2492 to a new link. After successfully acquiring a new care-of 2493 address, the mobile node SHOULD also increase the rate at which 2494 it will send Router Solicitations when it next begins searching 2495 for a new default router and care-of address. 2497 - A mobile node that is currently configured with a care-of address 2498 SHOULD NOT send Router Solicitations to the default router 2499 on it current link, until its movement detection algorithm 2500 (Section 10.4) determines that it has moved and that its current 2501 care-of address might no longer be valid. 2503 7. Requirements for Types of IPv6 Nodes 2505 Mobile IPv6 places some special requirements on the functions 2506 provided by different types of IPv6 nodes. This section summarizes 2507 those requirements, identifying the functionality each requirement 2508 is intended to support. Further details on this functionality is 2509 provided in the following sections. 2511 7.1. Requirements for All IPv6 Hosts and Routers 2513 Since any IPv6 node may at any time be a correspondent node of a 2514 mobile node, either sending a packet to a mobile node or receiving a 2515 packet from a mobile node, the following requirements apply to ALL 2516 IPv6 nodes (whether host or router, whether mobile or stationary): 2518 - Every IPv6 node MUST be able to process a Home Address option 2519 received in any IPv6 packet. 2521 - Every IPv6 node SHOULD be able to process a Binding Update option 2522 received in a packet, and to return a Binding Acknowledgement 2523 option if the Acknowledge (A) bit is set in the received Binding 2524 Update. 2526 - Every IPv6 node SHOULD be able to maintain a Binding Cache of the 2527 bindings received in accepted Binding Updates. 2529 7.2. Requirements for All IPv6 Routers 2531 The following requirements apply to all IPv6 routers, even those not 2532 serving as a home agent for Mobile IPv6: 2534 - Every IPv6 router SHOULD be able to send an Advertisement 2535 Interval option in its Router Advertisements, to aid movement 2536 detection by mobile nodes. The use of this option in Router 2537 Advertisements MUST be configurable. 2539 - Every IPv6 router SHOULD be able to support sending unsolicited 2540 multicast Router Advertisements at the faster rate described in 2541 Section 6.5. The use of this faster rate MUST be configurable. 2543 - each router SHOULD include at least one prefix with the 'R' bit 2544 set and with its full IP address in its router advertisements. 2546 7.3. Requirements for IPv6 Home Agents 2548 In order for a mobile node to operate correctly while away from home, 2549 at least one IPv6 router on the mobile node's home link must function 2550 as a home agent for the mobile node. The following additional 2551 requirements apply to all IPv6 routers capable of serving as a home 2552 agent: 2554 - Every home agent MUST be able to maintain an entry in its Binding 2555 Cache for each mobile node for which it is serving as the home 2556 agent. Each such Binding Cache entry records the mobile node's 2557 binding with its primary care-of address and is marked as a "home 2558 registration". 2560 - Every home agent MUST be able to intercept packets (using proxy 2561 Neighbor Discovery) addressed to a mobile node for which it is 2562 currently serving as the home agent, on that mobile node's home 2563 link, while the mobile node is away from home. 2565 - Every home agent MUST be able to encapsulate such intercepted 2566 packets in order to tunnel them to the primary care-of address 2567 for the mobile node indicated in its binding in the home agent's 2568 Binding Cache. 2570 - Every home agent MUST be able to return a Binding Acknowledgement 2571 option in response to a Binding Update option received with the 2572 Acknowledge (A) bit set. 2574 - Every home agent MUST maintain a separate Home Agents List for 2575 each link on which it is serving as a home agent, as described in 2576 Section 4.6. 2578 - Every home agent MUST be able to accept packets addressed to 2579 the "Mobile IPv6 Home-Agents" anycast address for the subnet 2580 on which it is serving as a home agent [10], and MUST be 2581 able to participate in dynamic home agent address discovery 2582 (Section 9.8). 2584 - Every home agent SHOULD support a configuration mechanism to 2585 allow a system administrator to manually set the value to be sent 2586 by this home agent in the Home Agent Preference field of the Home 2587 Agent Information Option in Router Advertisements that it sends. 2589 - Every home agent SHOULD support sending ICMP Mobile 2590 Prefix Advertisements, and SHOULD respond to Mobile Prefix 2591 Solicitations. 2593 7.4. Requirements for IPv6 Mobile Nodes 2595 Finally, the following requirements apply to all IPv6 nodes capable 2596 of functioning as mobile nodes: 2598 - Every IPv6 mobile node MUST be able to perform IPv6 2599 decapsulation [4]. 2601 - Every IPv6 mobile node MUST support sending Binding Update 2602 options, as specified in Sections 10.7, 10.9, and 10.10; and MUST 2603 be able to receive and process Binding Acknowledgement options, 2604 as specified in Section 10.13. 2606 - Every IPv6 mobile node MUST support use of the dynamic home agent 2607 address discovery mechanism, as described in Section 10.8. 2609 - Every IPv6 mobile node MUST maintain a Binding Update List in 2610 which it records the IP address of each other node to which it 2611 has sent a Binding Update, for which the Lifetime sent in that 2612 binding has not yet expired. 2614 - Every IPv6 mobile node MUST support receiving a Binding Request 2615 option, by responding with a Binding Update option. 2617 - Every IPv6 mobile node MUST support sending packets containing a 2618 Home Address option; this option MUST be included in all packets 2619 sent while away from home, if the packet would otherwise have 2620 been sent with the mobile node's home address as the IP Source 2621 Address. 2623 - Every IPv6 mobile node MUST maintain a Home Agents List, as 2624 described in Section 4.6. 2626 - Every mobile node MUST support receiving Mobile Prefix 2627 Advertisements and reconfiguring its home address based on the 2628 prefix information contained therein. 2630 8. Correspondent Node Operation 2632 A correspondent node is any node communicating with a mobile node. 2633 The correspondent node, itself, may be stationary or mobile, and may 2634 possibly also be functioning as a home agent for Mobile IPv6. The 2635 procedures in this section thus apply to all IPv6 nodes. 2637 8.1. Receiving Packets from a Mobile Node 2639 Packets sent by a mobile node while away from home generally include 2640 a Home Address option. When any node receives a packet containing 2641 a Home Address option, it MUST process the option in a manner 2642 consistent with exchanging the Home Address field from the Home 2643 Address option into the IPv6 header, replacing the original value of 2644 the Source Address field there. However, any actual modifications to 2645 the Source Address field in the packet's IPv6 header MUST be carried 2646 out in such a fashion that further processing of such a packet after 2647 all IPv6 options processing (e.g., at the transport layer) does not 2648 need to know that the original Source Address was a care-of address, 2649 or that the Home Address option was used in the packet. 2651 Since the sending mobile node uses its home address at the transport 2652 layer when sending such a packet, the use of the care-of address 2653 and Home Address option is transparent to both the mobile node and 2654 the correspondent node above the level of the Home Address option 2655 generation and processing. 2657 8.2. Receiving Binding Updates 2659 Before accepting a Binding Update option received in any packet, the 2660 receiving node MUST validate the Binding Update according to the 2661 following tests: 2663 - The packet meets the specific authentication requirements for 2664 Binding Updates, defined in Section 4.4. 2666 - The packet MUST contain a Home Address option. 2668 - The Option Length field in the Binding Update option is greater 2669 than or equal to the length specified in Section 5.1. 2671 - The Sequence Number field in the Binding Update option is greater 2672 than the Sequence Number received in the previous Binding Update 2673 for this home address, if any. As noted in Section 4.6, this 2674 Sequence Number comparison MUST be performed modulo 2**8. 2676 If the mobile node sends a sequence number which is not greater than 2677 the sequence number from the last successful Binding Update, then the 2678 receiving node MUST send back a Binding Acknowledgement with status 2679 code 141, and the last accepted sequence number in the Sequence 2680 Number field of the Binding Acknowledgement. 2682 Any Binding Update which fails to satisfy all of these tests for any 2683 other reason (than insufficiency of the Sequence Number) MUST be 2684 silently ignored, and the packet carrying the Binding Update MUST be 2685 discarded. 2687 In this section, the care-of address refers to the IPv6 address, 2688 which was originally located in the IPv6 header when the packet was 2689 transmitted by the mobile node. 2691 If the Binding Update is valid according to the tests above, then the 2692 Binding Update is processed further as follows: 2694 - If the Lifetime specified in the Binding Update is nonzero and 2695 the specified Care-of Address is not equal to the home address 2696 for the binding, then this is a request to cache a binding for 2697 the mobile node. If the Home Registration (H) bit is set in the 2698 Binding Update, the Binding Update is processed according to the 2699 procedure specified in Section 9.1; otherwise, it is processed 2700 according to the procedure specified in Section 8.3. 2702 - If the Lifetime specified in the Binding Update is zero or the 2703 specified Care-of Address matches the home address for the 2704 binding, then this is a request to delete the mobile node's 2705 cached binding. If the Home Registration (H) bit is set in the 2706 Binding Update, the Binding Update is processed according to the 2707 procedure specified in Section 9.2; otherwise, it is processed 2708 according to the procedure specified in Section 8.4. 2710 8.3. Requests to Cache a Binding 2712 When a node receives a Binding Update, it MUST validate it and 2713 determine the type of Binding Update according to the steps described 2714 in Section 8.2. This section describes the processing of a valid 2715 Binding Update that requests a node to cache a mobile node's binding, 2716 for which the Home Registration (H) bit is not set in the Binding 2717 Update. 2719 In this case, the receiving node SHOULD create a new entry in its 2720 Binding Cache for this mobile node (or update its existing Binding 2721 Cache entry for this mobile node, if such an entry already exists). 2722 The new Binding Cache entry records the association between this 2723 home address and the care-of address for the binding. The lifetime 2724 for the Binding Cache entry is initialized from the Lifetime field 2725 specified in the Binding Update, although this lifetime MAY be 2726 reduced by the node caching the binding; the lifetime for the Binding 2727 Cache entry MUST NOT be greater than the Lifetime value specified in 2728 the Binding Update. Any Binding Cache entry MUST be deleted after 2729 the expiration of this lifetime in the Binding Cache entry. 2731 8.4. Requests to Delete a Binding 2733 When a node receives a Binding Update, it MUST validate it and 2734 determine the type of Binding Update according to the steps described 2735 in Section 8.2. This section describes the processing of a valid 2736 Binding Update that requests a node to delete a mobile node's binding 2737 from its Binding Cache, for which the Home Registration (H) bit is 2738 not set in the Binding Update. In this case, the receiving node MUST 2739 delete any existing entry in its Binding Cache for this mobile node. 2741 8.5. Sending Binding Acknowledgements 2743 When any node receives a packet containing a Binding Update option 2744 in which the Acknowledge (A) bit is set, it MUST return a Binding 2745 Acknowledgement option acknowledging receipt of the Binding Update. 2746 If the node accepts the Binding Update and creates or updates 2747 an entry in its Binding Cache for this binding, and the `A' bit 2748 was set in the Binding Update, the Status field in the Binding 2749 Acknowledgement MUST be set to a value less than 128; if, on the 2750 other hand the Binding Update is accepted and the `A' bit is not set, 2751 the node SHOULD NOT send a Binding Acknowledgement. If the node 2752 rejects the Binding Update and does not create or update an entry for 2753 this binding, a Binding Acknowledgement MUST be sent even if the `A' 2754 bit was not sent, and the Status field in the Binding Acknowledgement 2755 MUST be set to a value greater than or equal to 128. Specific values 2756 for the Status field are described in Section 5.2 and in the most 2757 recent "Assigned Numbers" [26]. 2759 The packet in which the Binding Acknowledgement is returned 2760 MUST meet the specific authentication requirements for Binding 2761 Acknowledgements, defined in Section 4.4. Furthermore, if the packet 2762 is to be sent to the mobile node at any address other than the mobile 2763 node's home address, it MUST be sent using a Routing header (even if 2764 the binding was rejected). The intermediate IP address, to which 2765 the packet will be delivered immediately before the home address, is 2766 determined as follows: 2768 - Whenever the Binding Update is accepted with a nonzero lifetime, 2769 the routing header will be constructed using the care-of address 2770 as described in Section 8.9. 2772 - Otherwise, if the Source IP Address of the packet containing 2773 the Binding Update, is legal for inclusion in a Routing Header, 2774 the routing header will be constructed using that IP address. 2775 Note that multicast addresses, link-local addresses, loopback 2776 addresses, IPv4 mapped addresses, and the unspecified address, 2777 MUST NOT be used within a Routing Header for the Binding 2778 Acknowledgement. 2780 - Otherwise, if the Binding Update has a zero lifetime but the 2781 Source IP address is not allowable for use within the Routing 2782 Header, the Binding Acknowledgment MUST be sent to the mobile 2783 node's home address. 2785 In response to a Binding Update, a node MAY send a Binding 2786 Acknowledgement even when the 'A' bit is not set in the Binding 2787 Update. This would happen, for instance, if a mobile node attempted 2788 to send a Binding Update with the 'H' bit set to a correspondent 2789 node. 2791 8.6. Sending Binding Requests 2793 Entries in a node's Binding Cache MUST be deleted when their lifetime 2794 expires. If such an entry is still in active use in sending packets 2795 to a mobile node, the next packet sent to the mobile node will be 2796 routed normally to the mobile node's home link, where it will be 2797 intercepted and tunneled to the mobile node. The mobile node will 2798 then return a Binding Update to the sender, allowing it to create 2799 a new Binding Cache entry for sending future packets to the mobile 2800 node. Communication with the mobile node continues uninterrupted, 2801 but the forwarding of this packet through the mobile node's home 2802 agent creates additional overhead and latency in delivering packets 2803 to the mobile node. Such routing paths could, for instance, 2804 temporarily or permanently disrupt any negotiated Quality of Service 2805 reservations which had been made by the mobile node on its home 2806 network. 2808 If the sender knows that the Binding Cache entry is still in active 2809 use, it MAY send a Binding Request option to the mobile node in 2810 an attempt to avoid this overhead and latency due to deleting and 2811 recreating the Binding Cache entry. Since a Binding Request is a 2812 destination option, it may, for example, be included in any packet 2813 already being sent to the mobile node, such as a packet that is part 2814 of ongoing TCP communication with the mobile node. When the mobile 2815 node receives a packet from some sender containing a Binding Request 2816 option, it returns a Binding Update option to that sender, giving its 2817 current binding and a new lifetime. 2819 8.7. Cache Replacement Policy 2821 Conceptually, a node maintains a separate timer for each entry in its 2822 Binding Cache. When creating or updating a Binding Cache entry in 2823 response to a received and accepted Binding Update, the node sets the 2824 timer for this entry to the specified Lifetime period. Any entry in 2825 a node's Binding Cache MUST be deleted after the expiration of the 2826 Lifetime specified in the Binding Update from which the entry was 2827 created or last updated. 2829 Each node's Binding Cache will, by necessity, have a finite size. 2830 A node MAY use any reasonable local policy for managing the space 2831 within its Binding Cache, except that any entry marked as a "home 2832 registration" (Section 9.1) MUST NOT be deleted from the cache 2833 until the expiration of its lifetime period. When such a "home 2834 registration" entry is deleted, in addition the home agent MUST also 2835 cease intercepting packets on the mobile node's home link addressed 2836 to the mobile node (Section 9.3), just as if the mobile node had 2837 deregistered its primary care-of address (see Section 9.2). 2839 When attempting to add a new "home registration" entry in response 2840 to a Binding Update with the Home Registration (H) bit set, if 2841 insufficient space exists (and sufficient space cannot be reclaimed) 2842 in the node's Binding Cache, the node MUST reject the Binding 2843 Update and SHOULD return a Binding Acknowledgement to the sending 2844 mobile node, in which the Status field is set to 131 (insufficient 2845 resources). When otherwise attempting to add a new entry to its 2846 Binding Cache, a node MAY, if needed, choose to drop any entry 2847 already in its Binding Cache, other than a "home registration" 2848 entry, in order to make space for the new entry. For example, a 2849 "least-recently used" (LRU) strategy for cache entry replacement 2850 among entries not marked as a "home registration" is likely to 2851 work well unless the size of the Binding Cache is substantially 2852 insufficient. 2854 Any binding dropped from a node's Binding Cache due to lack of cache 2855 space will be rediscovered and a new cache entry created, if the 2856 binding is still in active use by the node for sending packets. If 2857 the node sends a packet to a destination for which it has dropped the 2858 entry from its Binding Cache, the packet will be routed normally, 2859 leading to the mobile node's home link. There, the packet will be 2860 intercepted by the mobile node's home agent and tunneled to the 2861 mobile node's current primary care-of address. As when a Binding 2862 Cache entry is initially created, this indirect routing to the mobile 2863 node through its home agent will result in the mobile node sending 2864 a Binding Update to this sending node when it receives the tunneled 2865 packet, allowing it to add an entry again for this destination mobile 2866 node to its Binding Cache. 2868 8.8. Receiving ICMP Error Messages 2870 When a correspondent node sends a packet to a mobile node, if the 2871 correspondent node has a Binding Cache entry for the destination 2872 address of the packet, then the correspondent node uses a Routing 2873 header to deliver the packet to the mobile node through the care-of 2874 address in the binding recorded in the Binding Cache entry. Any ICMP 2875 error message caused by the packet on its way to the mobile node will 2876 be returned normally to the correspondent node. 2878 On the other hand, if the correspondent node has no Binding Cache 2879 entry for the mobile node, the packet will be routed to the mobile 2880 node's home link. There, it will be intercepted by the mobile node's 2881 home agent, encapsulated, and tunneled to the mobile node's primary 2882 care-of address. Any ICMP error message caused by the packet on 2883 its way to the mobile node while in the tunnel, will be transmitted 2884 to the mobile node's home agent (the source of the tunnel). By 2885 the definition of IPv6 encapsulation [4], the home agent (as the 2886 encapsulating node) MUST relay certain ICMP error messages back 2887 to the original sender of the packet, which in this case is the 2888 correspondent node. 2890 Likewise, if a packet for a mobile node arrives at the mobile node's 2891 previous link and is intercepted there by a home agent for the mobile 2892 node's previous care-of address as described in Section 10.10 (e.g., 2893 the mobile node moved after the packet was sent), that home agent 2894 will encapsulate and tunnel the packet to the mobile node's new 2895 care-of address. As above, any ICMP error message caused by the 2896 packet while in this tunnel will be returned to that home agent (the 2897 source of the tunnel), which MUST relay certain ICMP error messages 2898 back to the correspondent node [4]. The relayed packet MUST NOT 2899 contain a routing header entry with the care-of address of the mobile 2900 node. 2902 Thus, in all cases, any meaningful ICMP error messages caused 2903 by packets from a correspondent node to a mobile node will be 2904 returned to the correspondent node. If the correspondent node 2905 receives persistent ICMP Destination Unreachable messages after 2906 sending packets to a mobile node based on an entry in its Binding 2907 Cache, the correspondent node MUST delete this Binding Cache 2908 entry. If the correspondent node subsequently transmits another 2909 packet to the mobile node, the packet will be routed to the mobile 2910 node's home link, intercepted by the mobile node's home agent, and 2911 tunneled to the mobile node's primary care-of address using IPv6 2912 encapsulation. The mobile node will then return a Binding Update to 2913 the correspondent node, allowing it to recreate a (correct) Binding 2914 Cache entry for the mobile node. 2916 8.9. Sending Packets to a Mobile Node 2918 Before sending any packet, the sending node SHOULD examine its 2919 Binding Cache for an entry for the destination address to which the 2920 packet is being sent. If the sending node has a Binding Cache entry 2921 for this address, the sending node SHOULD use a Routing header to 2922 route the packet to this mobile node (the destination node) by way 2923 of the care-of address in the binding recorded in that Binding Cache 2924 entry. For example, assuming use of a Type 0 Routing header [6], if 2925 no other use of a Routing header is involved in the routing of this 2926 packet, the mobile node sets the fields in the packet's IPv6 header 2927 and Routing header as follows: 2929 - The Destination Address in the packet's IPv6 header is set to 2930 the mobile node's care-of address copied from the Binding Cache 2931 entry. 2933 - The Routing header is initialized to contain a single route 2934 segment, with an Address of the mobile node's home address (the 2935 original destination address to which the packet was being sent). 2937 Following the definition of a Type 0 Routing header [6], this packet 2938 will be routed to the mobile node's care-of address, where it will 2939 be delivered to the mobile node (the mobile node has associated the 2940 care-of address with its network interface). Normal processing of 2941 the Routing header by the mobile node will then proceed as follows: 2943 - The mobile node swaps the Destination Address in the packet's 2944 IPv6 header and the Address specified in the Routing header. 2945 This results in the packet's IP Destination Address being set to 2946 the mobile node's home address. 2948 - The mobile node then resubmits the packet to its IPv6 module for 2949 further processing, "looping back" the packet inside the mobile 2950 node. Since the mobile node recognizes its own home address as 2951 one of its current IP addresses, the packet is processed further 2952 within the mobile node, in the same way then as if the mobile 2953 node was at home. 2955 If, instead, the sending node has no Binding Cache entry for the 2956 destination address to which the packet is being sent, the sending 2957 node simply sends the packet normally, with no Routing header. If 2958 the destination node is not a mobile node (or is a mobile node that 2959 is currently at home), the packet will be delivered directly to this 2960 node and processed normally by it. If, however, the destination node 2961 is a mobile node that is currently away from home, the packet will 2962 be intercepted by the mobile node's home agent and tunneled (using 2963 IPv6 encapsulation [4]) to the mobile node's current primary care-of 2964 address, as described in Section 9.4. The mobile node will then send 2965 a Binding Update to the sending node, as described in Section 10.9, 2966 allowing the sending node to create a Binding Cache entry for its use 2967 in sending subsequent packets to this mobile node. 2969 It is possible that a correspondent node, having no knowledge 2970 of the mobile node's care-of address, would still (for reasons 2971 unspecified here but not necessarily related to mobility) attempt 2972 to deliver a packet, either to or by way of the mobile node's home 2973 address, by using a routing header. If the correspondent node 2974 subsequently accepts a Binding Update and creates a Binding Cache 2975 entry for the mobile node, then afterwards, the routing header used 2976 by the corresponding node which includes the mobile node's home 2977 address SHOULD also include the mobile node's care-of address. The 2978 correspondent node SHOULD put the mobile node's care-of address as 2979 the intermediate node address immediately preceding the mobile node's 2980 home address. When the care-of address is the first intermediate 2981 node address, this implies that the care-of address is to be placed 2982 in the Destination Address of the IPv6 header, and the mobile 2983 node's home address is the first entry in the type 0 routing header. 2984 Otherwise, the correspondent node MUST insert the mobile node's 2985 care-of address immediately before the home address entry in the 2986 routing header. 2988 9. Home Agent Operation 2990 9.1. Primary Care-of Address Registration 2992 When a node receives a Binding Update, it MUST validate it and 2993 determine the type of Binding Update according to the steps described 2994 in Section 8.2. This section describes the processing of a valid 2995 Binding Update that requests the receiving node to serve as its home 2996 agent, registering its primary care-of address. 2998 To begin processing the Binding Update, the home agent MUST perform 2999 the following sequence of tests: 3001 - If the node is not a router that implements home agent 3002 functionality, then the node MUST reject the Binding Update and 3003 SHOULD return a Binding Acknowledgement to the mobile node, in 3004 which the Status field is set to 132 (home registration not 3005 supported). 3007 - Else, if the home address for the binding (the Home Address field 3008 in the packet's Home Address option) is not an on-link IPv6 3009 address with respect to the home agent's current Prefix List, 3010 then the home agent MUST reject the Binding Update and SHOULD 3011 return a Binding Acknowledgement to the mobile node, in which the 3012 Status field is set to 133 (not home subnet). 3014 - Else, if the home agent chooses to reject the Binding Update for 3015 any other reason (e.g., insufficient resources to serve another 3016 mobile node as a home agent), then the home agent SHOULD return a 3017 Binding Acknowledgement to the mobile node, in which the Status 3018 field is set to an appropriate value to indicate the reason for 3019 the rejection. 3021 - Finally, if the Duplicate Address Detection (D) bit is set in 3022 the Binding Update, this home agent MUST perform Duplicate 3023 Address Detection [27] on the mobile node's home link for the 3024 home address in this binding (before returning the Binding 3025 Acknowledgement). The address used for Duplicate Address 3026 Detection SHOULD be the mobile node's link-local address. Normal 3027 processing for Duplicate Address Detection specifies that, in 3028 certain cases, the node SHOULD delay sending the initial Neighbor 3029 Solicitation message of Duplicate Address Detection by a random 3030 delay between 0 and MAX_RTR_SOLICITATION_DELAY [17, 27]; however, 3031 in this case, the home agent SHOULD NOT perform such a delay. 3032 If this Duplicate Address Detection fails, then the home agent 3033 MUST reject the Binding Update and SHOULD return a Binding 3034 Acknowledgement to the mobile node, in which the Status field is 3035 set to 138 (Duplicate Address Detection failed). When the home 3036 agent sends a successful Binding Acknowledgement to the mobile 3037 node, in response to a Binding Update with the `D' bit set, the 3038 home agent assures to the mobile node that its home address 3039 will continue to be valid at least as long as the mobile node 3040 transmits Binding Updates with new care-of addresses for that 3041 home address. 3043 If the home agent does not reject the Binding Update, then it becomes 3044 or remains the home agent for the mobile node. The home agent MUST 3045 then create a new entry in its Binding Cache for this mobile node (or 3046 update its existing Binding Cache entry for this mobile node, if such 3047 an entry already exists). The home address of the mobile node is 3048 taken to be the value which, when the packet was originally received, 3049 was located in the Home Address field in the packet's Home Address 3050 option. The care-of address for this Binding Cache entry is taken 3051 to be the value which, when the packet was originally received, was 3052 located either in the Alternate Care-of Address sub-option in the 3053 Binding Update option, if present, or from the Source Address field 3054 in the packet's IPv6 header, otherwise. 3056 The home agent MUST mark this Binding Cache entry as a "home 3057 registration" to indicate that the node is serving as a home 3058 agent for this binding. Binding Cache entries marked as a "home 3059 registration" MUST be excluded from the normal cache replacement 3060 policy used for the Binding Cache (Section 8.7) and MUST NOT be 3061 removed from the Binding Cache until the expiration of the Lifetime 3062 period. 3064 The lifetime for the Binding Cache entry MUST NOT be greater than the 3065 remaining valid lifetime for the subnet prefix in the mobile node's 3066 home address specified with the Binding Update, and MUST NOT be 3067 greater than the Lifetime value specified in the Binding Update. The 3068 remaining valid lifetime for this prefix is determined by the home 3069 agent based on its own Prefix List entry for this prefix [17]. 3071 If the `S' bit field in the Binding Update is zero, The Home Agent 3072 creates or updates Binding Cache entries for each of possible 3073 several home addresses. The set of such home addresses is formed 3074 by replacing the routing prefix for the given home address with 3075 all other routing prefixes that are supported by the home agent 3076 processing the Binding Update. The Home Agent creates such a 3077 separate primary care-of address registration for each such home 3078 address. Note that the same considerations for Duplicate Address 3079 Detection apply for each affected home address. The lifetime for 3080 the each Binding Cache entry MUST NOT be greater than the minimum 3081 remaining valid lifetime for all subnet prefixes on the mobile 3082 node's home link. If the value of the Lifetime field specified by 3083 the mobile node in its Binding Update is greater than this prefix 3084 lifetime, the home agent MUST decrease the binding lifetime to less 3085 than or equal to the prefix valid lifetime. The home agent MAY 3086 further decrease the specified lifetime for the binding, for example 3087 based on a local policy. The resulting lifetime is stored by the 3088 home agent in the Binding Cache entry, and this Binding Cache entry 3089 MUST be deleted by the home agent after the expiration of this 3090 lifetime. 3092 Regardless of the setting of the 'A' bit in the Binding Update, the 3093 home agent MUST return a Binding Acknowledgement to the mobile node, 3094 constructed as follows: 3096 - The Status field MUST be set to a value indicating success; this 3097 value MUST be less than 128. The only currently defined success 3098 Status value is 0, indicating simply that the Binding Update was 3099 accepted. 3101 - The Sequence Number field MUST be copied from the Sequence Number 3102 given in the Binding Update. 3104 - The Lifetime field MUST be set to the remaining lifetime for 3105 the binding as set by the home agent in its "home registration" 3106 Binding Cache entry for the mobile node, as described above. 3108 - The Refresh field MUST be set to a value less than or equal to 3109 the Lifetime value being returned in the Binding Update. If the 3110 home agent stores the Binding Cache entry in nonvolatile storage 3111 (that survives the crash or other failure of the home agent), 3112 then the Refresh field SHOULD be set to the same value as the 3113 Lifetime field; otherwise, the home agent MAY set the Refresh 3114 field to a value less than the Lifetime field, to indicate that 3115 the mobile node SHOULD attempt to refresh its home registration 3116 at the indicated shorter interval (although the home agent will 3117 still retain the registration for the Lifetime period, even if 3118 the mobile node does not refresh its registration within the 3119 Refresh period). 3121 In addition, the home agent MUST follow the procedure defined in 3122 Section 9.3 to intercept packets on the mobile node's home link 3123 addressed to the mobile node, while the home agent is serving as the 3124 home agent for this mobile node. 3126 9.2. Primary Care-of Address De-registration 3128 When a node receives a Binding Update, it MUST validate it and 3129 determine the type of Binding Update according to the steps described 3130 in Section 8.2. This section describes the processing of a valid 3131 Binding Update that requests the receiving node to no longer serve as 3132 its home agent, de-registering its primary care-of address. 3134 To begin processing the Binding Update, the home agent MUST perform 3135 the following test: 3137 - If the receiving node has no entry in its Binding Cache for this 3138 mobile node that is marked as a "home registration", then this 3139 node MUST reject the Binding Update and SHOULD return a Binding 3140 Acknowledgement to the mobile node, in which the Status field is 3141 set to 137 (not home agent for this mobile node). 3143 If the home agent does not reject the Binding Update as described 3144 above, then it MUST delete any existing entry in its Binding Cache 3145 for this mobile node, and proceed as follows. 3147 The home agent MUST return a Binding Acknowledgement to the mobile 3148 node, constructed as follows: 3150 - The Status field MUST be set to a value indicating success (the 3151 value MUST be less than 128). The only currently defined success 3152 Status value is 0, indicating simply that the Binding Update was 3153 accepted. 3155 - The Sequence Number field MUST be copied from the Sequence Number 3156 given in the Binding Update. 3158 - The Lifetime field MUST be set to zero. 3160 - The Refresh field MUST be set to zero. 3162 In addition, the home agent MUST stop intercepting packets on the 3163 mobile node's home link addressed to the mobile node (Section 9.3). 3165 The rules for selecting the Destination IP address (and possibly 3166 Routing Header construction) for the Binding Acknowledgement to the 3167 mobile node are the same as in section 8.5. 3169 9.3. Intercepting Packets for a Mobile Node 3171 While a node is serving as the home agent for mobile node (while the 3172 node has an entry in its Binding Cache for this mobile node that is 3173 marked as a "home registration"), this node MUST attempt to intercept 3174 packets on the mobile node's home link addressed to the mobile node, 3175 and MUST tunnel each intercepted packet to the mobile node using 3176 using IPv6 encapsulation [4]. 3178 In order to intercept such packets on the home link, when a node 3179 begins serving as the home agent for some mobile node (it did not 3180 already have a Binding Cache entry for this mobile node marked as a 3181 "home registration"), then the home agent MUST multicast onto the 3182 home link a "gratuitous" Neighbor Advertisement message [17] on 3183 behalf of the mobile node. Specifically, the home agent performs the 3184 following steps: 3186 - The home agent examines the value of the `S' bit in the new "home 3187 registration" Binding Cache entry. If this bit is nonzero, 3188 the following step is carried out only for the individual home 3189 address specified for this binding. If, instead, this bit is 3190 zero, then the following step is carried out for each address 3191 for the mobile node formed from the interface identifier in 3192 the mobile node's home address in this binding (the remaining 3193 low-order bits in the address after the configured subnet 3194 prefix), together with each one of the subnet prefixes currently 3195 considered by the home agent to be on-link (including both the 3196 link-local and site-local prefix). 3198 - For each specific IP address for the mobile node determined 3199 in the first step above, the home agent multicasts onto the 3200 home link (to the all-nodes multicast address) a Neighbor 3201 Advertisement message [17] on behalf of the mobile node, to 3202 advertise the home agent's own link-layer address for this IP 3203 address. 3205 All fields in each such Neighbor Advertisement message SHOULD be 3206 set in the same way they would be set by the mobile node itself 3207 if sending this Neighbor Advertisement while at home [17], with 3208 the following exceptions: 3210 * The Target Address in the Neighbor Advertisement message MUST 3211 be set to the specific IP address for the mobile node. 3213 * The Advertisement MUST include a Target Link-layer Address 3214 option specifying the home agent's link-layer address. 3216 * The Router (R) bit in the Advertisement MUST be copied from 3217 the corresponding bit in the home agent's Binding Cache entry 3218 for the mobile node. 3220 * The Solicited Flag (S) in the Advertisement MUST NOT be set, 3221 since it was not solicited by any Neighbor Solicitation 3222 message. 3224 * The Override Flag (O) in the Advertisement MUST be set, 3225 indicating that the Advertisement SHOULD override any 3226 existing Neighbor Cache entry at any node receiving it. 3228 Any node on the home link receiving one of the Neighbor Advertisement 3229 messages described above will thus update its Neighbor Cache to 3230 associate the mobile node's address with the home agent's link 3231 layer address, causing it to transmit any future packets for the 3232 mobile node normally destined to this address instead to the mobile 3233 node's home agent. Since multicasts on the local link (such as 3234 Ethernet) are typically not guaranteed to be reliable, the home 3235 agent MAY retransmit this Neighbor Advertisement message up to 3236 MAX_ADVERT_REXMIT times to increase its reliability. It is still 3237 possible that some nodes on the home link will not receive any of 3238 these Neighbor Advertisements, but these nodes will eventually be 3239 able to detect the link-layer address change for the mobile node's 3240 home address, through use of Neighbor Unreachability Detection [17]. 3242 While a node is serving as a home agent for some mobile node (it 3243 still has a "home registration" entry for this mobile node in its 3244 Binding Cache), the home agent uses IPv6 Neighbor Discovery [17] to 3245 intercept unicast packets on the home link addressed to the mobile 3246 node's home address. In order to intercept packets in this way, 3247 the home agent MUST act as a proxy for this mobile node, and reply 3248 to any received Neighbor Solicitation messages for it. When a home 3249 agent receives a Neighbor Solicitation message, it MUST check if the 3250 Target Address specified in the message matches the home address 3251 of any mobile node for which it has a Binding Cache entry marked 3252 as a "home registration". This check MUST include all possible 3253 home addresses for the mobile node, based on the subnet prefixes 3254 currently considered to be on-link by the home agent (including the 3255 corresponding link-local address and site-local address), if the 3256 Prefix Length in the Binding Cache entry for this mobile node (from 3257 the Binding Update that created this Cache entry) is nonzero. 3259 If such an entry exists in the home agent's Binding Cache, the home 3260 agent MUST reply to the Neighbor Solicitation message with a Neighbor 3261 Advertisement message, giving the home agent's own link-layer address 3262 as the link-layer address for the specified Target Address. In 3263 addition, the Router (R) bit in the Advertisement MUST be copied from 3264 the corresponding bit in the home agent's Binding Cache entry for the 3265 mobile node. Acting as a proxy in this way allows other nodes on 3266 the mobile node's home link to resolve the mobile node's IPv6 home 3267 address, and allows the home agent to defend these addresses on the 3268 home link for Duplicate Address Detection [17]. 3270 9.4. Tunneling Intercepted Packets to a Mobile Node 3272 For any packet sent to a mobile node from the mobile node's home 3273 agent (for which the home agent is the original sender of the 3274 packet), the home agent is operating as a correspondent node of 3275 the mobile node for this packet and the procedures described in 3276 Section 8.9 apply. The home agent (as a correspondent node) uses a 3277 Routing header to route the packet to the mobile node by way of the 3278 care-of address in the home agent's Binding Cache (the mobile node's 3279 primary care-of address, in this case). 3281 While the mobile node is away from home and this node is acting 3282 as the mobile node's home agent, the home agent intercepts any 3283 packets on the home link addressed to the mobile node's home address 3284 (including addresses formed from other on-link prefixes, if the 3285 Prefix Length field was nonzero in the Binding Update), as described 3286 in Section 9.3. The home agent cannot use a Routing header to 3287 forward these intercepted packets to the mobile node, since it cannot 3288 modify the packet in flight without invalidating any existing IPv6 3289 AH [11] or ESP [12] header present in the packet. 3291 In order to forward each intercepted packet to the mobile node, the 3292 home agent MUST tunnel the packet to the mobile node using IPv6 3293 encapsulation [4]; the tunnel entry point node is the home agent, 3294 and the tunnel exit point node is the primary care-of address as 3295 registered with the home agent. When a home agent encapsulates 3296 an intercepted packet for forwarding to the mobile node, the home 3297 agent sets the Source Address in the prepended tunnel IP header to 3298 the home agent's own IP address, and sets the Destination Address 3299 in the tunnel IP header to the mobile node's primary care-of 3300 address. When received by the mobile node (using its primary care-of 3301 address), normal processing of the tunnel header [4] will result in 3302 decapsulation and processing of the original packet by the mobile 3303 node. 3305 However, packets addressed to the mobile node's link-local address 3306 MUST NOT be tunneled to the mobile node. Instead, such a packet MUST 3307 be discarded, and the home agent SHOULD return an ICMP Destination 3308 Unreachable, Code 3, message to the packet's Source Address (unless 3309 this Source Address is a multicast address). Packets addressed to 3310 the mobile node's site-local address SHOULD be tunneled to the mobile 3311 node by default, but this behavior MUST be configurable to disable 3312 it; currently, the exact definition and semantics of a "site" and a 3313 site-local address are incompletely defined in IPv6, and this default 3314 behavior might change at some point in the future. 3316 Tunneling of multicast packets to a mobile node follows similar 3317 limitations to those defined above for unicast packets addressed to 3318 the mobile node's link-local and site-local addresses. Multicast 3319 packets addressed to a multicast address with link-local scope [9], 3320 to which the mobile node is subscribed, MUST NOT be tunneled 3321 to the mobile node; such packets SHOULD be silently discarded 3322 (after delivering to other local multicast recipients). Multicast 3323 packets addressed to a multicast address with scope larger 3324 than link-local but smaller than global (e.g., site-local and 3325 organization-local) [9], to which the mobile node is subscribed, 3326 SHOULD be tunneled to the mobile node by default, but this behavior 3327 MUST be configurable to disable it; this default behavior might 3328 change at some point in the future as the definition of these scopes 3329 become more completely defined in IPv6. 3331 9.5. Handling Reverse Tunneled Packets from a Mobile Node 3333 A home agent MUST support decapsulating reverse tunneled packets 3334 sent to it from a mobile node's home address. Such reverse tunneled 3335 packets MAY be discarded unless accompanied by a valid AH or ESP 3336 header. This support for reverse tunneling allows mobile nodes to 3337 defeat certain kinds of traffic analysis. Requiring IPsec headers 3338 on reverse tunneled packets allows the home agent to protect the 3339 home network against unwarranted intrusions by malicious nodes 3340 masquerading as a mobile node with a home address on the network 3341 served by the home agent. 3343 9.6. Home Prefix Propagation 3345 IPv6 provides mechanisms as part of Neighbor Discovery [17] and 3346 Address Autoconfiguration [27] to aid in mobile node configuration 3347 when a mobile node turns on, and in renumbering a subnet, such as 3348 when a site switches to a new network service provider. 3350 In renumbering, new prefixes and addresses can be introduced for the 3351 subnet and old ones can be deprecated and removed. These mechanisms 3352 are defined to work while all nodes using the old prefixes are at 3353 home, connected to the link using these prefixes. Mobile IPv6 3354 extends these mechanisms for the case in which one or more mobile 3355 nodes using the old prefixes are away from home and registered at the 3356 home agent while the renumbering takes place. 3358 In the IPv6 renumbering mechanism, nodes on the visited link receive 3359 Mobile Prefix Advertisements messages with Prefix Information 3360 Options, which give the valid lifetime and preferred lifetime for 3361 available prefixes on the link [17]. 3363 Mobile IPv6 arranges to propagate relevant prefix information 3364 to the mobile node when it is away from home, so that it may be 3365 used in mobile node home address configuration, and in network 3366 renumbering. To avoid possible security attacks from forged Mobile 3367 Prefix Advertisements all such Advertisements must be authenticated 3368 to the mobile node by its home agent using IPsec [13, 11, 12] if a 3369 security associate exists (i.e. unless the mobile node does not yet 3370 have a home address configured). 3372 9.7. Receiving Router Advertisement Messages 3374 For each link on which a router provides service as a home agent, the 3375 router maintains a Home Agents List recording information about all 3376 other home agents on that link. This list is used in the dynamic 3377 home agent address discovery mechanism, described in Section 9.8. 3378 The information for the list is learned through receipt of the 3379 periodic unsolicited multicast Router Advertisements from each other 3380 home agent on the link, in which the Home Agent (H) bit is set, in a 3381 manner similar to the Default Router List conceptual data structure 3382 maintained by each host for Neighbor Discovery [17]. 3384 On receipt of a valid Router Advertisement, as defined in the 3385 processing algorithm specified for Neighbor Discovery [17], the home 3386 agent performs the following steps, in addition to any steps already 3387 required of it by Neighbor Discovery: 3389 - If the Home Agent (H) bit in the Router Advertisement is not set, 3390 skip all of the following steps. 3392 - If the Home Agent (H) bit in the Router Advertisement is not set, 3393 and the sending node has an entry in the current Home Agents 3394 List, delete the corresponding entry. Subsequently, skip all of 3395 the following steps. 3397 - Otherwise, extract the Source Address from the IP header of the 3398 Router Advertisement. This is the link-local IP address on this 3399 link of the home agent sending this Advertisement [17]. 3401 - Determine from the Router Advertisement the preference for this 3402 home agent. If the Router Advertisement contains a Home Agent 3403 Information Option, then the preference is taken from the Home 3404 Agent Preference field in the option; otherwise, the default 3405 preference of 0 MUST be used. 3407 - Determine from the Router Advertisement the lifetime for 3408 this home agent. If the Router Advertisement contains a Home 3409 Agent Information Option, then the lifetime is taken from 3410 the Home Agent Lifetime field in the option; otherwise, the 3411 lifetime specified by the Router Lifetime field in the Router 3412 Advertisement SHOULD be used. 3414 - If the link-local address of the home agent sending this 3415 Advertisement is already present in this home agent's Home 3416 Agents List and the received home agent lifetime value is zero, 3417 immediately delete this entry in the Home Agents List. 3419 - Otherwise, if the link-local address of the home agent sending 3420 this Advertisement is already present in the receiving home 3421 agent's Home Agents List, reset its lifetime and preference to 3422 the values determined above. 3424 - If the link-local address of the home agent sending this 3425 Advertisement, as determined above, is not already present in 3426 the Home Agents List maintained by the receiving home agent, and 3427 the lifetime for the sending home agent, as determined above, 3428 is non-zero, create a new entry in the list, and initialize its 3429 lifetime and preference to the values determined above. 3431 - If the Home Agents List entry for the link-local address of 3432 the home agent sending this Advertisement was not deleted as 3433 described above, determine any global address(es) of the home 3434 agent based on each Prefix Information option received in 3435 this Advertisement in which the Router Address (R) bit is set 3436 (Section 6.2). For each such global address determined from this 3437 Advertisement, add this global address to the list of global 3438 addresses for this home agent in this Home Agents List entry. 3440 A home agent SHOULD maintain an entry in its Home Agents List for 3441 each such valid home agent address until that entry's lifetime 3442 expires, after which time the entry MUST be deleted. 3444 9.8. Dynamic Home Agent Address Discovery 3446 A mobile node, while away from home, MAY use the dynamic home agent 3447 address discovery mechanism in section 10.8 to attempt to discover 3448 the address of one or more routers serving as home agents on its home 3449 link. This discovery might become necessary, for example, if some 3450 nodes on its home link have been reconfigured while the mobile node 3451 has been away from home, such that the router that was operating as 3452 the mobile node's home agent has been replaced by a different router 3453 serving this role. 3455 As described in Section 10.8, a mobile node attempts dynamic home 3456 agent address discovery by sending an ICMP Home Agent Address 3457 Discovery Request message to the "Mobile IPv6 Home-Agents" anycast 3458 address [10] for its home IP subnet prefix, using its care-of address 3459 as the Source Address of the packet. A home agent receiving such a 3460 Home Agent Address Discovery Request message that is serving this 3461 subnet (the home agent is configured with this anycast address on one 3462 of its network interfaces) SHOULD return an ICMP Home Agent Address 3463 Discovery Reply message to the mobile node (at its care-of address 3464 that was used as the Source Address of the Request message), with the 3465 Source Address of the Reply packet set to one of the global unicast 3466 addresses of the home agent. The Home Agent Addresses field in the 3467 Reply message is constructed as follows: 3469 - The Home Agent Addresses field SHOULD contain one global IP 3470 address for each home agent currently listed in this home 3471 agent's own Home Agents List (Section 4.6). However, if this 3472 home agent's own global IP address would be placed in the list 3473 (as described below) as the first entry in the list, then this 3474 home agent SHOULD NOT include its own address in the Home Agent 3475 Addresses field in the Reply message. Not placing this home 3476 agent's own IP address in the list will cause the receiving 3477 mobile node to consider this home agent as the most preferred 3478 home agent; otherwise, this home agent will be considered to be 3479 preferred in its order given by its place in the list returned. 3481 - The IP addresses in the Home Agent Addresses field SHOULD be 3482 listed in order of decreasing preference value, based either 3483 on the respective advertised preference from a Home Agent 3484 Information option or on the default preference of 0 if no 3485 preference is advertised (or on the configured home agent 3486 preference for this home agent itself). The home agent with 3487 the highest preference SHOULD be listed first in the Home Agent 3488 Addresses field, and the home agent with the lowest preference 3489 SHOULD be listed last. 3491 - Among home agents with equal preference, their IP addresses 3492 in the Home Agent Addresses field SHOULD be listed in an 3493 order randomized with respect to other home agents with equal 3494 preference, each time a Home Agent Address Discovery Reply 3495 message is returned by this home agent. 3497 - For each entry in this home agent's Home Agents List, if more 3498 than one global IP address is associated with this list entry, 3499 then one of these global IP addresses SHOULD be selected 3500 to include in the Home Agent Addresses field in the Reply 3501 message. As described in Section 4.6, one Home Agents List 3502 entry, identified by the home agent's link-local address, 3503 exists for each home agent on the link; associated with that 3504 list entry is one or more global IP addresses for this home 3505 agent, learned through Prefix Information options with the 3506 Router Address (R) bit is set, received in Router Advertisements 3507 from this link-local address. 3509 The selected global IP address for each home agent to include in 3510 forming the Home Agent Addresses field in the Reply message MUST 3511 be the global IP address of the respective home agent sharing a 3512 prefix with the Destination IP address of the Request message; 3513 if no such global IP address is known for some home agent, an 3514 entry for that home agent MUST NOT be included in the Home Agent 3515 Addresses field in the Reply message. 3517 - In order to avoid the possibility of the Reply message packet 3518 being fragmented (or rejected by an intermediate router with an 3519 ICMP Packet Too Big message [5]), if the resulting total packet 3520 size containing the complete list of home agents in the Home 3521 Agent Addresses field would exceed the minimum IPv6 MTU [6], the 3522 home agent SHOULD reduce the number of home agent IP addresses 3523 returned in the packet to the number of addresses that will fit 3524 without exceeding this limit. The home agent addresses returned 3525 in the packet SHOULD be those from the complete list with the 3526 highest preference. 3528 9.8.1. Aggregate List of Home Network Prefixes 3530 A mobile node on a remote network SHOULD autoconfigure all of the 3531 global IP addresses, which it would autoconfigure if it were attached 3532 to its home network, from network prefixes representing network 3533 addresses that are served by home agents. Site-local addresses MAY 3534 be autoconfigured if the mobile node is roaming in a network on the 3535 same site as its home addresses. Site-local addresses and addresses 3536 not served by a home agent MUST NOT be autoconfigured, since they are 3537 unusable in the remote network. 3539 To support this, the home agent monitors prefixes advertised by 3540 itself and other home agents routers on the home link, and passes 3541 this aggregated list of relevant subnet prefixes on to the mobile 3542 node in Mobile Prefix Advertisements. 3544 The home agent SHOULD construct the aggregate list of home subnet 3545 prefixes as follows: 3547 - Copy prefix information defined in the home agent's AdvPrefixList 3548 on the home subnet's interfaces to the aggregate list. Also 3549 apply any changes made to the AdvPrefixList on the home agent to 3550 the aggregate list. 3552 - Check valid prefixes received in Router Advertisements 3553 from the home network for consistency with the home agent's 3554 AdvPrefixList, as specified in section 6.2.7 of RFC 2461 3555 (Neighbor Discovery [17]). Do not update the aggregate list with 3556 any information from received prefixes that fail this check. 3558 - Check Router Advertisements which contain an `H' bit (from other 3559 home agents) for valid prefixes that are not yet in the aggregate 3560 list, and if they are usable for autoconfiguration (`A' bit set, 3561 and prefix length is valid for address autoconfiguration on the 3562 home subnet) add them and preserve the `L' flag value. Clear the 3563 `R' flag and zero the interface-id portion of the prefix field 3564 to prevent mobile nodes from treating another router's interface 3565 address as belonging to the home agent. Treat the lifetimes 3566 of these prefixes as decrementing in real time, as defined in 3567 section 6.2.7 of RFC 2461 [17]. 3569 - Do not perform consistency checks on valid prefixes received in 3570 Router Advertisements on the home network that do not exist in 3571 the home agent's AdvPrefixList. Instead, if the prefixes already 3572 exist in the aggregate list, update the prefix lifetime fields in 3573 the aggregate list according to the rules specified for hosts in 3574 section 6.3.4 of RFC 2461 (Neighbor Discovery [17]) and section 3575 5.5.3 of RFC 2462 (Stateless Address Autoconfiguration [27]). 3577 - If the L flag is set on valid prefixes received in a Router 3578 Advertisement, and that prefix already exists in the aggregate 3579 list, set the flag in the aggregate list. Ignore the flag if it 3580 is clear. 3582 - Delete prefixes from the aggregate list when their valid 3583 lifetimes expire. 3585 The home agent uses the information in the aggregate list to 3586 construct Mobile Prefix Advertisements, possibly including Binding 3587 Acknowledgement or Binding Request destination options, for delivery 3588 to a mobile node for which it is maintaining a current binding. 3589 It may be possible to construct an aggregate list by combining 3590 information contained in the home agent's AdvPrefixList and its 3591 Home Agents List used for Dynamic Home Agent Address Discovery 3592 (Section 10.8). 3594 9.8.2. Scheduling Prefix Deliveries to the Mobile Node 3596 A home agent serving a mobile node will schedule the delivery of new 3597 prefix information to that mobile node when any of the following 3598 conditions occur: 3600 MUST: 3602 - The valid or preferred lifetime or the state of the flags changes 3603 for the prefix of the mobile node's registered home address 3604 changes 3606 - The mobile node requests the information with a Router 3607 Solicitation (see section 10.16). 3609 MAY: 3611 - A new prefix is added to the aggregate list 3613 - The valid or preferred lifetime or the state of the flags changes 3614 for a prefix which is not used in any binding cache entry for 3615 this mobile node 3617 The home agent uses the following algorithm to determine when to send 3618 prefix information to the mobile node. 3620 - If the mobile node has not received the prefix information within 3621 the last HomeRtrAdvInterval seconds, then transmit the prefix 3622 information. This MAY be done according to a periodically 3623 scheduled transmission. 3625 - If a mobile node sends a solicitation, answer right away. 3627 - If a prefix in the aggregate list that matches the mobile 3628 node's home registration is added, or if its information changes 3629 in any way that does not cause the mobile node's address to 3630 go deprecated, ensure that a transmission is scheduled, and 3631 calculate RAND_ADV_DELAY in order to randomize the time at which 3632 the transmission is scheduled. 3634 - If there are any unacknowledged changes to prefix information 3635 when a Binding Update arrives for the home registration, send 3636 a Mobile Prefix Advertisement to the mobile node immediately. 3638 The Mobile Prefix Advertisement SHOULD have the Binding 3639 Acknowledgement as a Destination Option. If an advertisement 3640 was previously scheduled for the mobile node, cancel that 3641 advertisement. 3643 - If a home registration expires, cancel any scheduled 3644 advertisements to the mobile node. 3646 If the home agent already has scheduled the transmission of a Router 3647 Advertisement to the mobile node, and if the freshly calculated 3648 RAND_ADV_DELAY would cause another transmission BEFORE the Preferred 3649 Lifetime of the mobile node's home address derived from the prefix 3650 whose advertisement information has changed, then add the new 3651 information to be transmitted to the existing scheduled transmission. 3652 In this case, the home agent does not perform the following algorithm 3653 to schedule an advertisement to the mobile node. 3655 Otherwise, the home agent uses the following algorithm to compute 3656 a fresh value for RAND_ADV_DELAY, the offset from the current time 3657 for the scheduled transmission. If there is already a scheduled 3658 transmission, add the data from the existing scheduled transmission 3659 to the newly scheduled transmission, deleting the previously 3660 scheduled transmission event. 3662 If the mobile node's binding expires before the Preferred Lifetime, 3663 then return. The mobile node will get the revised information with 3664 its next Binding Acknowledgement. Otherwise, continue with the 3665 following computation. 3666 MAX_SCHEDULE_DELAY == min (MAX_PFX_ADV_DELAY, Preferred Lifetime) 3667 for the newly advertised Preferred Lifetime. 3669 Then compute RAND_ADV_DELAY == 3670 MinRtrAdvInt + rand()*(MAX_SCHEDULE_DELAY - MinRtrAdvInt) 3672 RAND_ADV_DELAY is the offset from the current time to be used 3673 to schedule the necessary advertisement to the mobile node. The 3674 computation is expected to alleviate bursts of advertisements when 3675 prefix information changes. In addition, a home agent MAY further 3676 reduce the rate of packet transmission by further delaying individual 3677 advertisements, if needed to avoid overwhelming local network 3678 resources. 3680 9.8.3. Sending Advertisements to the Mobile Node 3682 When sending a Mobile Prefix Advertisement to the mobile node, the 3683 home agent MUST construct the packet as follows: 3685 - The Source Address in the packet's IPv6 header MUST be set to 3686 the home agent's IP address to which the mobile node addressed 3687 its current home registration, or its default global home agent 3688 address if no binding exists. 3690 - If a security association exists with the mobile node's address, 3691 the packet MUST be protected by IPsec [13, 11, 12] to guard 3692 against malicious Mobile Prefix Advertisements. The IPsec 3693 protection MUST provide sender authentication, data integrity 3694 protection, and replay protection, covering the Mobile Prefix 3695 Advertisement. 3697 - The advertisement MUST include a Binding Request destination 3698 option if this is the first advertisement for a home 3699 registration, or if there was a change in prefix information 3700 since the last acknowledged advertisement was sent to the mobile 3701 node for the home registration. The Binding Request destination 3702 option MUST include a Unique Identifier Sub-Option (Section 5.5), 3703 with the unique identifier in the sub-option data set to a value 3704 different than that in any other Binding Request sent recently by 3705 this home agent. It is assumed that this requirement can be met 3706 by maintaining a simple 16-bit "wrap-around" counter to generate 3707 unique identifiers for Binding Requests that contain a Unique 3708 Identifier Sub-Option, incremented each time a Binding Request 3709 containing a Unique Identifier Sub-Option is sent. 3711 - If the advertisement was solicited, it should be destined 3712 (and authenticated, if possible) to the source address of 3713 the solicitation. If it was triggered by prefix changes or 3714 renumbering, the advertisement's destination will be the mobile 3715 node's home address in the binding which triggered the rule. 3717 - The packet MUST be sent as any other unicast IPv6 packet. If a 3718 care-of address is used, the packet will be delivered directly. 3719 If a binding exists, the home agent will send the packet with 3720 a routing header containing the care-of address, as any other 3721 packet sent to the mobile node originated by the home agent 3722 (rather than using IPv6 encapsulation, as would be used by the 3723 home agent for intercepted packets). 3725 The home agent SHOULD periodically continue to retransmit an 3726 unsolicited Advertisement to the mobile node, until it is 3727 acknowledged by the receipt from the mobile node of a Binding Update 3728 matching the Binding Request in the packet (i.e., with matching 3729 Sequence Number). The home agent MUST wait PREFIX_ADV_TIMEOUT 3730 before the first retransmission, and double the retransmission wait 3731 time for every succeeding retransmission, up until a maximum of 3732 PREFIX_ADV_RETRIES attempts. If the mobile node's bindings expire 3733 before the matching Binding Update has been received, then the home 3734 agent MUST NOT attempt any more retransmissions, even if not all 3735 PREFIX_ADV_RETRIES have been retransmitted. After another Binding 3736 Update is received from the mobile node, and if the mobile node has 3737 not returned to the home network in the meantime, the home agent 3738 SHOULD begin the process again of transmitting the unsolicited 3739 Advertisement. 3741 A Binding Update matches a Binding Request if it specifies a 3742 binding for the mobile node to which the Binding Request was sent 3743 and contains a Unique Identifier Sub-Option matching the unique 3744 identifier sent in the Unique Identifier Sub-Option in the Binding 3745 Request. In the solicited case, the mobile node will retransmit 3746 solicitations until one is received; thus, the home agent SHOULD NOT 3747 retransmit the responding advertisement. 3749 If while the home agent is still retransmitting a Mobile Prefix 3750 Advertisement to the mobile node, another condition as described 3751 above occurs on the home link causing another Prefix Advertisement to 3752 be sent to the mobile node, the home agent SHOULD combine any Prefix 3753 Information options in the unacknowledged Mobile Prefix Advertisement 3754 into the new Advertisement, discard the old Advertisement, and then 3755 begin retransmitting the new one. according to the algorithm in 3756 section 9.8.2. The home agent MUST generate a new unique identifier 3757 for use in the Unique Identifier Sub-Option in the Binding Request 3758 tunneled with the new Mobile Prefix Advertisement. 3760 9.8.4. Lifetimes for Changed Prefixes 3762 As described in Section 9.1, the lifetime returned by the home 3763 agent in a Binding Acknowledgement MUST be no greater than the 3764 remaining valid lifetime for the subnet prefix in the mobile node's 3765 home address. Furthermore, as described in Section 10.9, Binding 3766 Updates sent by the mobile node to other nodes MUST use a lifetime no 3767 greater than the remaining lifetime of its home registration of its 3768 primary care-of address. These limits on the binding lifetime serve 3769 to prohibit use of a mobile node's home address after it becomes 3770 invalid. The mobile node SHOULD further limit the lifetimes that it 3771 sends on any Binding Updates to be within the remaining preferred 3772 lifetime for the prefix in its home address. 3774 When the lifetime for a changed prefix decreases, and the change 3775 would cause cached bindings at correspondent nodes in the Binding 3776 Update List to be stored past the newly shortened lifetime, the 3777 mobile node MUST issue a Binding Update to all such correspondent 3778 nodes. 3780 10. Mobile Node Operation 3782 10.1. Sending Packets While Away from Home 3784 While a mobile node is away from home, it continues to use its home 3785 address as well as also using one or more care-of addresses. When 3786 sending a packet while away from home, a mobile node MAY choose among 3787 these in selecting the address that it will use as the source of the 3788 packet, as follows: 3790 - From the point of view of protocol layers and applications above 3791 Mobile IP (e.g., transport protocols), the mobile node will 3792 generally use its home address as the source of the packet for 3793 most packets, even while away from home, since Mobile IP is 3794 designed to make mobility transparent to such software. Doing 3795 so also makes the node's mobility---and the fact that it is 3796 currently away from home---transparent to the correspondent nodes 3797 with which it communicates. For packets sent that are part of 3798 transport-level connections established while the mobile node 3799 was at home, the mobile node MUST use its home address in this 3800 way. Likewise, for packets sent that are part of transport-level 3801 connections that the mobile node may still be using after moving 3802 to a new location, the mobile node SHOULD use its home address 3803 in this way. When sending such packets, Mobile IP will modify 3804 the packet to move the home address into a Home Address option 3805 and will set the IPv6 header's Source Address field to one of 3806 the mobile node's care-of addresses; these modifications to 3807 the packet are then reversed in the node receiving the packet, 3808 restoring the mobile node's home address to be the packet's 3809 Source Address before processing by higher protocol layers and 3810 applications. 3812 - For short-term communication, particularly for communication that 3813 may easily be retried if it fails, the mobile node MAY choose 3814 to directly use one of its care-of addresses as the source of 3815 the packet, thus not requiring the use of a Home Address option 3816 in the packet. An example of this type of communication might 3817 be DNS queries sent by the mobile node [15, 16]. Using the 3818 mobile node's care-of address as the source for such queries will 3819 generally have a lower overhead than using the mobile node's 3820 home address, since no extra options need be used in either the 3821 query or its reply, and all packets can be routed normally, 3822 directly between their source and destination without relying 3823 on Mobile IP. If the mobile node has no particular knowledge 3824 that the communication being sent fits within this general type 3825 of communication, however, the mobile node SHOULD NOT use its 3826 care-of address as the source of the packet in this way. 3828 For packets sent by a mobile node while it is at home, no special 3829 Mobile IP processing is required for sending this packet. Likewise, 3830 if the mobile node uses any address other than its home address as 3831 the source of a packet sent while away from home (from the point of 3832 view of higher protocol layers or applications, as described above), 3833 no special Mobile IP processing is required for sending that packet. 3834 In each case, the packet is simply addressed and transmitted in the 3835 same way as any normal IPv6 packet. 3837 For each other packet sent by the mobile node (i.e., packets sent 3838 while away from home, using the mobile node's home address as 3839 the source, from the point of view of higher protocol layers and 3840 applications), special Mobile IP processing of the packet is required 3841 for the insertion of the Home Address option. Specifically: 3843 - Construct the packet using the mobile node's home address as the 3844 packet's Source Address, in the same way as if the mobile node 3845 were at home. This preserves the transparency of Mobile IP to 3846 higher protocol layers (e.g., to TCP). 3848 - Insert a Home Address option into the packet, with the Home 3849 Address field copied from the original value of the Source 3850 Address field in the packet. 3852 - Change the Source Address field in the packet's IPv6 header to 3853 one of the mobile node's care-of addresses. This will typically 3854 be the mobile node's current primary care-of address, but MUST 3855 be a care-of address with a subnet prefix that is on-link on the 3856 network interface on which the mobile node will transmit the 3857 packet. 3859 By using the care-of address as the Source Address in the IPv6 3860 header, with the mobile node's home address instead in the Home 3861 Address option, the packet will be able to safely pass through any 3862 router implementing ingress filtering [7]. 3864 10.2. Interaction with Outbound IPsec Processing 3866 This section sketches the interaction between outbound Mobile IP 3867 processing and outbound IP Security (IPsec) processing for 3868 packets sent by a mobile node while away from home. Any specific 3869 implementation MAY use algorithms and data structures other than 3870 those suggested here, but its processing MUST be consistent with the 3871 effect of the operation described here and with the relevant IPsec 3872 specifications. In the steps described below, it is assumed that 3873 IPsec is being used in transport mode [13] and that the mobile node 3874 is using its home address as the source for the packet (from the 3875 point of view of higher protocol layers or applications, as described 3876 in Section 10.1): 3878 - The packet is created by higher layer protocols and applications 3879 (e.g., by TCP) as if the mobile node were at home and Mobile IP 3880 were not being used. Mobile IP is transparent to such higher 3881 layers. 3883 - As part of outbound packet processing in IP, the packet is 3884 compared against the IPsec Security Policy Database (SPD) to 3885 determine what processing is required for the packet [13]. 3887 - If IPsec processing is required, the packet is either mapped to 3888 an existing Security Association (or SA bundle), or a new SA (or 3889 SA bundle) is created for the packet, according to the procedures 3890 defined for IPsec. 3892 - Since the mobile node is away from home, the mobile node inserts 3893 a Home Address option into the packet, replacing the Source 3894 Address in the packet's IP header with a care-of address suitable 3895 for the link on which the packet is being sent, as described in 3896 Section 10.1. The Destination Options header in which the Home 3897 Address option is inserted MUST appear in the packet after the 3898 Routing Header, if present, and before the AH [11] (or ESP [12]) 3899 header, so that the Home Address option is processed by the 3900 destination node (and, possibly, intermediate routing nodes) 3901 before the AH or ESP header is processed. 3903 - If a Binding Update is being included in the packet, it is 3904 also added to a Destination Options header in the packet. The 3905 Destination Options header in which the Binding Update option is 3906 inserted MUST appear after the AH or ESP header. 3908 - Finally, once the packet is fully assembled, the necessary IPsec 3909 authentication (and encryption, if required) processing is 3910 performed on the packet, initializing the Authentication Data in 3911 the AH or ESP header. The authentication data MUST be calculated 3912 as if the following were true: 3914 * the IPv6 source address in the IPv6 header contains the 3915 mobile node's home address, 3917 * the Home Address field of the Home Address destination option 3918 (section 5.4) contains the new care-of address. 3920 This allows, but does not require, the receiver of the packet 3921 containing the Binding Update to exchange the two fields of the 3922 incoming packet, simplifying processing for all subsequent packet 3923 headers. The mechanics of implementation do not absolutely 3924 require such an exchange to occur; other implementation 3925 strategies may be more appropriate, as long as the result of the 3926 authentication calculation remain the same. 3928 In addition, when using any automated key management protocol [13] 3929 (such as IKE [8]) to create any new SA (or SA bundle) while away 3930 from home, a mobile node MUST take special care in its processing of 3931 the key management protocol. Otherwise, other nodes with which the 3932 mobile node must communicate as part of the automated key management 3933 protocol processing may be unable to correctly deliver packets to 3934 the mobile node if they and/or the mobile node's home agent do 3935 not then have a current Binding Cache entry for the mobile node. 3936 For the default case of using IKE as the automated key management 3937 protocol [8, 13], such problems can be avoided by the following 3938 requirements on the use of IKE by a mobile node while away from home: 3940 - The mobile node MUST use its care-of address as the Source 3941 Address of all packets it sends as part of the key management 3942 protocol (without use of Mobile IP for these packets, as 3943 suggested in Section 10.1). 3945 - In addition, for all security associations bound to the mobile 3946 node's home address established by way of IKE, the mobile node 3947 MUST include an ISAKMP Identification Payload [14] in the IKE 3948 exchange, giving the mobile node's home address as the initiator 3949 of the Security Association [22]. 3951 10.3. Receiving Packets While Away from Home 3953 While away from home, a mobile node will receive packets addressed to 3954 its home address, by one of three methods: 3956 - Packets sent by a correspondent node that does not have a 3957 Binding Cache entry for the mobile node, will be sent by the 3958 correspondent node in the same way as any normal IP packet. Such 3959 packets will then be intercepted by the mobile node's home agent, 3960 encapsulated using IPv6 encapsulation [4], and tunneled to the 3961 mobile node's primary care-of address. 3963 - Packets sent by a correspondent node that has a Binding Cache 3964 entry for the mobile node that contains the mobile node's current 3965 care-of address, will be sent by the correspondent node using 3966 a Routing header. The packet will be addressed to the mobile 3967 node's care-of address, with the final hop in the Routing header 3968 directing the packet to the mobile node's home address; the 3969 processing of this last hop of the Routing header is entirely 3970 internal to the mobile node, since the care-of address and home 3971 address are both addresses within the mobile node. 3973 - Packets sent by a correspondent node that has a Binding Cache 3974 entry for the mobile node that contains an out-of-date care-of 3975 address for the mobile node, will be sent by the correspondent 3976 node using a Routing header, as described above. If the mobile 3977 node sent a Binding Update to a home agent on the link on which 3978 its previous care-of address is located (Section 10.10), and 3979 if this home agent is still serving as a home agent for the 3980 mobile node's previous care-of address, then such a packet will 3981 be intercepted by this home agent, encapsulated using IPv6 3982 encapsulation [4], and tunneled to the mobile node's new care-of 3983 address (registered with this home agent). 3985 For packets received by either the first or last of these three 3986 methods, the mobile node SHOULD send a Binding Update to the original 3987 sender of the packet, as described in Section 10.9, subject to 3988 the rate limiting defined in Section 10.12. The mobile node MUST 3989 also process the received packet in the manner defined for IPv6 3990 encapsulation [4], which will result in the encapsulated (inner) 3991 packet being processed normally by upper-layer protocols within the 3992 mobile node, as if it had been addressed (only) to the mobile node's 3993 home address. 3995 For packets received by the second method above (using a Routing 3996 header), the mobile node MUST process the received packet in the 3997 manner defined for the type of IPv6 Routing header used [6], which 3998 will result in the packet being processed normally by upper-layer 3999 protocols within the mobile node, as if it had been addressed (only) 4000 to the mobile node's home address. 4002 In addition, the general procedures defined by IPv6 for Routing 4003 headers suggest that a received Routing header MAY be automatically 4004 "reversed" to construct a Routing header for use in any response 4005 packets sent by upper-layer protocols, if the received packet is 4006 authenticated [6]. If this is done for upper-layer protocol response 4007 packets sent by a mobile node while away from home, the mobile 4008 node SHOULD NOT include its own care-of address, which appears in 4009 the Routing header of the received packet, in the reversed route 4010 for the response packet. If the received Routing header contained 4011 no additional hops (other than the mobile node's home address and 4012 care-of address), then any upper-layer protocol response packet 4013 SHOULD NOT include a Routing header. 4015 10.4. Movement Detection 4017 A mobile node MAY use any combination of mechanisms available to it 4018 to detect when it has moved from one link to another. The primary 4019 movement detection mechanism for Mobile IPv6 defined here uses the 4020 facilities of IPv6 Neighbor Discovery, including Router Discovery and 4021 Neighbor Unreachability Detection, although the mobile node SHOULD 4022 supplement this mechanism with other information whenever it is 4023 available to the mobile node (e.g., from lower protocol layers). The 4024 description here is based on the conceptual model of the organization 4025 and data structures defined by Neighbor Discovery [17]. 4027 Mobile nodes SHOULD use Router Discovery to discover new routers and 4028 on-link subnet prefixes; a mobile node MAY send Router Solicitation 4029 messages, or MAY wait for unsolicited (periodic) multicast Router 4030 Advertisement messages, as specified for Router Discovery [17]. 4032 Based on received Router Advertisement messages, a mobile node (in 4033 the same way as any other node) maintains an entry in its Default 4034 Router List for each router, and an entry in its Prefix List for each 4035 subnet prefix, that it currently considers to be on-link. Each entry 4036 in these lists has an associated invalidation timer value (extracted 4037 from the Router Advertisement and Prefix Information options) used to 4038 expire the entry when it becomes invalid. 4040 While away from home, a mobile node typically selects one router 4041 from its Default Router List to use as its default router, and one 4042 subnet prefix advertised by that router from its Prefix List to use 4043 as the subnet prefix in its primary care-of address. A mobile node 4044 MAY also have associated additional care-of addresses, using other 4045 subnet prefixes from its Prefix List. The method by which a mobile 4046 node selects and forms a care-of address from the available subnet 4047 prefixes is described in Section 10.6. The mobile node registers 4048 its primary care-of address with its home agent, as described in 4049 Section 10.7. 4051 While a mobile node is away from home and using some router as its 4052 default router, it is important for the mobile node to be able to 4053 quickly detect when that router becomes unreachable, so that it 4054 can switch to a new default router and (if needed, according to 4055 prefix advertisement) to a new primary care-of address. Since some 4056 links (notably wireless) do not necessarily work equally well in 4057 both directions, it is likewise important for the mobile node to 4058 detect when it becomes unreachable for packets sent from its default 4059 router, so that the mobile node can take steps to ensure that any 4060 correspondent nodes attempting to communicate with it can still reach 4061 it through some other route. 4063 To detect when its default router becomes unreachable, a mobile 4064 node SHOULD use Neighbor Unreachability Detection. As specified in 4065 Neighbor Discovery [17], while the mobile node is actively sending 4066 packets to (or through) its default router, the mobile node can 4067 detect that the router (as its neighbor) is still reachable either 4068 through indications from upper layer protocols on the mobile node 4069 that a connection is making "forward progress" (e.g., receipt of TCP 4070 acknowledgements for new data transmitted), or through receipt of a 4071 Neighbor Advertisement message from its default router in response 4072 to an explicit Neighbor Solicitation messages to it. Note that 4073 although this mechanism detects that the mobile node's default router 4074 has become unreachable to the mobile node only while the mobile node 4075 is actively sending packets to it, this is the only time that this 4076 direction of reachability confirmation is needed. Confirmation 4077 that the mobile node is still reachable from the router is handled 4078 separately, as described below. 4080 For a mobile node to detect when it has become unreachable from its 4081 default router, the mobile node cannot efficiently rely on Neighbor 4082 Unreachability Detection alone, since the network overhead would be 4083 prohibitively high in many cases for a mobile node to continually 4084 probe its default router with Neighbor Solicitation messages even 4085 when it is not otherwise actively sending packets to it. Instead, 4086 when a mobile node receives any IPv6 packets from its current default 4087 router at all, irrespective of the source IPv6 address, it SHOULD use 4088 that as an indication that it is still reachable from the router. 4090 Since the router SHOULD be sending periodic unsolicited multicast 4091 Router Advertisement messages, the mobile node will have frequent 4092 opportunity to check if it is still reachable from its default 4093 router, even in the absence of other packets to it from the router. 4094 If Router Advertisements that the mobile node receives include 4095 an Advertisement Interval option, the mobile node MAY use its 4096 Advertisement Interval field as an indication of the frequency with 4097 which it SHOULD expect to continue to receive future Advertisements 4098 from that router. This field specifies the minimum rate (the maximum 4099 amount of time between successive Advertisements) that the mobile 4100 node SHOULD expect. If this amount of time elapses without the 4101 mobile node receiving any Advertisement from this router, the mobile 4102 node can be sure that at least one Advertisement sent by the router 4103 has been lost. It is thus possible for the mobile node to implement 4104 its own policy for determining the number of Advertisements from 4105 its current default router it is willing to tolerate losing before 4106 deciding to switch to a different router from which it may currently 4107 be correctly receiving Advertisements. 4109 On some types of network interfaces, the mobile node MAY also 4110 supplement this monitoring of Router Advertisements, by setting its 4111 network interface into "promiscuous" receive mode, so that it is able 4112 to receive all packets on the link, including those not link-level 4113 addressed to it (i.e., disabling link-level address filtering). The 4114 mobile node will then be able to detect any packets sent by the 4115 router, in order to detect reachability from the router. This use of 4116 promiscuous mode may be useful on very low bandwidth (e.g., wireless) 4117 links, but its use MUST be configurable on the mobile node since it 4118 is likely to consume additional energy resources. 4120 If the above means do not provide indication that the mobile node 4121 is still reachable from its current default router (for instance, 4122 the mobile node receives no packets from the router for a period 4123 of time), then the mobile node SHOULD attempt to actively probe 4124 the router with Neighbor Solicitation messages, even if it is not 4125 otherwise actively sending packets to the router. If it receives a 4126 solicited Neighbor Advertisement message in response from the router, 4127 then the mobile node can deduce that it is still reachable. It is 4128 expected that the mobile node will in most cases be able to determine 4129 its reachability from the router by listening for packets from the 4130 router as described above, and thus, such extra Neighbor Solicitation 4131 probes should rarely be necessary. 4133 With some types of networks, indications about link-layer mobility 4134 might be obtained from lower-layer protocol or device driver software 4135 within the mobile node. However, all link-layer mobility indications 4136 from lower layers do not necessarily indicate a movement of the 4137 mobile node to a new link, such that the mobile node would need to 4138 switch to a new default router and primary care-of address. For 4139 example, movement of a mobile node from one cell to another in many 4140 wireless LANs can be made transparent to the IP level through use of 4141 a link-layer "roaming" protocol, as long as the different wireless 4142 LAN cells all operate as part of the same IP link with the same 4143 subnet prefix. Upon lower-layer indication of link-layer mobility, 4144 the mobile node MAY send Router Solicitation messages to determine if 4145 additional on-link subnet prefixes are available on its new link. 4147 Such lower-layer information might also be useful to a mobile node in 4148 deciding to switch its primary care-of address to one of the other 4149 care-of addresses it has formed from the on-link subnet prefixes 4150 currently available through different routers from which the mobile 4151 node is reachable. For example, a mobile node MAY use signal 4152 strength or signal quality information (with suitable hysteresis) for 4153 its link with the available routers to decide when to switch to a new 4154 primary care-of address using that router rather than its current 4155 default router (and current primary care-of address). Even though 4156 the mobile node's current default router may still be reachable in 4157 terms of Neighbor Unreachability Detection, the mobile node MAY use 4158 such lower-layer information to determine that switching to a new 4159 default router would provide a better connection. 4161 10.5. Receiving Local Router Advertisement Messages 4163 Each mobile node maintains a Home Agents List recording information 4164 about all home agents from which it receives a Router Advertisement, 4165 for which the home agent lifetime indicated in that Router 4166 Advertisement has not yet expired. This list is used by the mobile 4167 node to enable it to send a Binding Update to the global unicast 4168 address of a home agent on its previous link when it moves to a new 4169 link, as described in Section 10.10. On receipt of a valid Router 4170 Advertisement, as defined in the processing algorithm specified for 4171 Neighbor Discovery [17], the mobile node performs the following 4172 steps, in addition to any steps already required of it by Neighbor 4173 Discovery. 4175 - If the Home Agent (H) bit in the Router Advertisement is not set, 4176 and the sending node currently has an entry in the node's Home 4177 Agents List, delete the corresponding entry. Subsequently, skip 4178 all of the following steps. 4180 - Otherwise, extract the Source Address from the IP header of the 4181 Router Advertisement. This is the link-local IP address on this 4182 link of the home agent sending this Advertisement [17]. 4184 - Determine from the Router Advertisement the preference for this 4185 home agent. If the Router Advertisement contains a Home Agent 4186 Information Option, then the preference is taken from the Home 4187 Agent Preference field in the option; otherwise, the default 4188 preference of 0 MUST be used. 4190 - Determine from the Router Advertisement the lifetime for 4191 this home agent. If the Router Advertisement contains a Home 4192 Agent Information Option, then the lifetime is taken from 4193 the Home Agent Lifetime field in the option; otherwise, the 4194 lifetime specified by the Router Lifetime field in the Router 4195 Advertisement SHOULD be used. 4197 - If the link-local address of the home agent sending this 4198 Advertisement is already present in this mobile node's Home 4199 Agents List and the received home agent lifetime value is zero, 4200 immediately delete this entry in the Home Agents List. 4202 - Otherwise, if the link-local address of the home agent sending 4203 this Advertisement is already present in the receiving mobile 4204 node's Home Agents List, reset its lifetime and preference to the 4205 values determined above. 4207 - If the link-local address of the home agent sending this 4208 Advertisement, as determined above, is not already present in the 4209 Home Agents List maintained by the receiving mobile node, and 4210 the lifetime for the sending home agent, as determined above, 4211 is non-zero, create a new entry in the list, and initialize its 4212 lifetime and preference to the values determined above. 4214 - If the Home Agents List entry for the link-local address of 4215 the home agent sending this Advertisement was not deleted as 4216 described above, determine any global address(es) of the home 4217 agent based on each Prefix Information option received in 4218 this Advertisement in which the Router Address (R) bit is set 4219 (Section 6.2). For each such global address determined from this 4220 Advertisement, add this global address to the list of global 4221 addresses for this home agent in this Home Agents List entry. 4223 A mobile node SHOULD maintain an entry in its Home Agents List for 4224 each such valid home agent address until that entry's lifetime 4225 expires, after which time the entry MUST be deleted. 4227 10.6. Forming New Care-of Addresses 4229 After detecting that it has moved from one link to another (i.e., its 4230 current default router has become unreachable and it has discovered 4231 a new default router), a mobile node SHOULD form a new primary 4232 care-of address using one of the on-link subnet prefixes advertised 4233 by the new router. A mobile node MAY form a new primary care-of 4234 address at any time, except that it MUST NOT do so too frequently. 4235 Specifically, a mobile node MUST NOT send a Binding Update about a 4236 new care-of address to its home agent (which is required to register 4237 the new address as its primary care-of address) more often than once 4238 per MAX_UPDATE_RATE seconds. 4240 In addition, after discovering a new on-link subnet prefix, a mobile 4241 node MAY form a new (non-primary) care-of address using that subnet 4242 prefix, even when it has not switched to a new default router. A 4243 mobile node can have only one primary care-of address at a time 4244 (which is registered with its home agent), but it MAY have an 4245 additional care-of address for any or all of the prefixes on its 4246 current link. Furthermore, since a wireless network interface may 4247 actually allow a mobile node to be reachable on more than one link at 4248 a time (i.e., within wireless transmitter range of routers on more 4249 than one separate link), a mobile node MAY have care-of addresses 4250 on more than one link at a time. The use of more than one care-of 4251 address at a time is described in Section 10.18. 4253 As described in Section 4, in order to form a new care-of address, 4254 a mobile node MAY use either stateless [27] or stateful (e.g., 4255 DHCPv6 [2]) Address Autoconfiguration. If a mobile node needs to 4256 send packets as part of the method of address autoconfiguration, 4257 it MUST use an IPv6 link-local address rather than its own IPv6 4258 home address as the Source Address in the IPv6 header of each such 4259 autoconfiguration packet. 4261 In some cases, a mobile node may already know a (constant) IPv6 4262 address that has been assigned to it for its use only while 4263 visiting a specific foreign link. For example, a mobile node may be 4264 statically configured with an IPv6 address assigned by the system 4265 administrator of some foreign link, for its use while visiting that 4266 link. If so, rather than using Address Autoconfiguration to form a 4267 new care-of address using this subnet prefix, the mobile node MAY use 4268 its own pre-assigned address as its care-of address on this link. 4270 After forming a new care-of address, a mobile node MAY perform 4271 Duplicate Address Detection [27] on that new address to confirm its 4272 uniqueness. However, doing so represents a tradeoff between safety 4273 (ensuring that the new address is not used if it is a duplicate 4274 address) and overhead (performing Duplicate Address Detection 4275 requires the sending of one or more additional packets over what 4276 may be, for example, a slow wireless link through which the mobile 4277 node is connected). Performing Duplicate Address Detection also in 4278 general would cause a delay before the mobile node could use the 4279 new care-of address, possibly causing the mobile node to be unable 4280 to continue communication with correspondent nodes for some period 4281 of time. For these reasons, a mobile node, after forming a new 4282 care-of address, MAY begin using the new care-of address without 4283 performing Duplicate Address Detection. Furthermore, the mobile node 4284 MAY continue using the address without performing Duplicate Address 4285 Detection, although it SHOULD in most cases (e.g., unless network 4286 bandwidth or battery consumption for communication is of primary 4287 concern) begin Duplicate Address Detection asynchronously when it 4288 begins use of the address, allowing the Duplicate Address Detection 4289 procedure to complete in parallel with normal communication using the 4290 address. 4292 In addition, normal processing for Duplicate Address Detection 4293 specifies that, in certain cases, the node SHOULD delay sending the 4294 initial Neighbor Solicitation message of Duplicate Address Detection 4295 by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY [17, 27]; 4296 however, in this case, the mobile node SHOULD NOT perform such a 4297 delay in its use of Duplicate Address Detection, unless the mobile 4298 node is initializing after rebooting. 4300 10.7. Sending Binding Updates to the Home Agent 4302 After deciding to change its primary care-of address as described 4303 in Sections 10.4 and 10.6, a mobile node MUST register this care-of 4304 address with its home agent in order to make this its primary care-of 4305 address. To do so, the mobile node sends a packet to its home agent 4306 containing a Binding Update option, with the packet constructed as 4307 follows: 4309 - The Home Registration (H) bit MUST be set in the Binding Update. 4311 - The Acknowledge (A) bit MUST be set in the Binding Update. 4313 - The packet MUST contain a Home Address option, giving the mobile 4314 node's home address for the binding. 4316 - The care-of address for the binding MUST be used as the Source 4317 Address in the packet's IPv6 header, unless an Alternate Care-of 4318 Address sub-option is included in the Binding Update option. 4320 - The `S' bit is set to the zero to request the mobile node's home 4321 agent to serve as a home agent for all home addresses for the 4322 mobile node based on all on-link subnet prefixes on the home 4323 link; this is the default behavior. If the mobile node desires 4324 that only a single home address should be affected by this 4325 Binding Update, the `S' bit can be set to 1. 4327 - The value specified in the Lifetime field SHOULD be less than 4328 or equal to the remaining lifetime of the home address and the 4329 care-of address specified for the binding. 4331 The Acknowledge (A) bit in the Binding Update requests the home 4332 agent to return a Binding Acknowledgement in response to this 4333 Binding Update. As described in Section 5.2, the mobile node SHOULD 4334 retransmit this Binding Update to its home agent until it receives 4335 a matching Binding Acknowledgement. Once reaching a retransmission 4336 timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart 4337 the process of delivering the Binding Update, but trying instead the 4338 next Home Agent from its Home Agent list (see section 10.8). If 4339 there is only one home agent in the Home Agent list, the mobile node 4340 instead SHOULD continue to periodically retransmit the Binding Update 4341 at this rate until acknowledged (or until it begins attempting to 4342 register a different primary care-of address). See section 10.11 for 4343 information about retransmitting Binding Updates. 4345 The Prefix Length field in the Binding Update allows the mobile node 4346 to request its home agent to serve all home addresses for the mobile 4347 node, as indicated by the interface identifier in the mobile node's 4348 home address (the remaining low-order bits after the indicated subnet 4349 prefix), together with each on-link subnet prefix on the home link. 4350 Until the lifetime of this registration expires, the home agent 4351 considers itself the home agent for each such home address of the 4352 mobile node. As the set of on-link subnet prefixes on the home link 4353 changes over time, the home agent changes the set of home addresses 4354 for this mobile node for which it is serving as the home agent. 4356 When sending a Binding Update to its home agent, the mobile node MUST 4357 also create or update the corresponding Binding Update List entry, as 4358 specified in Section 10.9. 4360 If the mobile node has additional home addresses using a different 4361 interface identifier, then the mobile node SHOULD send an additional 4362 packet containing a Binding Update to its home agent to register the 4363 care-of address for each such other home address (or set of home 4364 addresses sharing an interface identifier). These additional Binding 4365 Updates MUST each be sent as a separate packet. Each care-of address 4366 MUST be authenticated within the Binding Update as coming from the 4367 home address being associated with the care-of address, as defined in 4368 Section 4.4. 4370 While the mobile node is away from home, it relies on the home agent 4371 to participate in Duplicate Address Detection (DAD) to defend its 4372 home address against stateless autoconfiguration performed by another 4373 node. Therefore, the mobile node SHOULD set the Duplicate Address 4374 Detection (D) bit based on any requirements for DAD that would apply 4375 to the mobile node if it were at home [17, 27]. If the mobile 4376 node's recent Binding Update was accepted by the home agent, and the 4377 lifetime for that Binding Update has not yet expired, the mobile node 4378 SHOULD NOT set the `D' bit in the new Binding Update; the home agent 4379 will already be defending the home address(es) of the mobile node and 4380 does not need to perform DAD again. 4382 The home agent will only perform DAD for the mobile node's home 4383 address when the mobile node has supplied a valid binding between 4384 its home address and a care-of address. If some time elapses during 4385 which the mobile node has no binding at the home agent, it might be 4386 possible for another node to autoconfigure the mobile node's home 4387 address. Therefore, the mobile node MUST treat creation of a new 4388 binding with the home agent using an existing home address the same 4389 as creation of a new home address. In the unlikely event that the 4390 mobile node's home address is autoconfigured as the IPv6 address 4391 of another network node on the home network, the home agent will 4392 reply to the mobile node's subsequent Binding Update with a Binding 4393 Acknowledgement showing Status 138, Duplicate Address Detection 4394 failed. In this case, the mobile node MUST NOT attempt to re-use 4395 the same home address. It SHOULD continue to register care-of 4396 addresses for its other home addresses, if any. The mobile node MAY 4397 also attempt to acquire a new home address to replace the one for 4398 which Status 138 was received, for instance by using the techniques 4399 described in appendix B. 4401 10.8. Dynamic Home Agent Address Discovery 4403 Sometimes, when the mobile node needs to send a Binding Update to its 4404 home agent to register its new primary care-of address, as described 4405 in Section 10.7, the mobile node may not know the address of any 4406 router on its home link that can serve as a home agent for it. For 4407 example, some nodes on its home link may have been reconfigured while 4408 the mobile node has been away from home, such that the router that 4409 was operating as the mobile node's home agent has been replaced by a 4410 different router serving this role. 4412 In this case, the mobile node MAY attempt to discovery the address of 4413 a suitable home agent on its home link. To do so, the mobile node 4414 sends an ICMP Home Agent Address Discovery Request message to the 4415 "Mobile IPv6 Home-Agents" anycast address [10] for its home subnet 4416 prefix. This packet MUST NOT contain a Home Address option and must 4417 be sent using the mobile node's care-of address as the Source Address 4418 in the packet's IP header (the packet is sent from the care-of 4419 address, not using Mobile IP). As described in Section 9.8, the home 4420 agent on its home link that receives this Request message will return 4421 an ICMP Home Agent Address Discovery Reply message, giving this home 4422 agent's own global unicast IP address along with a list of the global 4423 unicast IP address of each other home agent operating on the home 4424 link. 4426 The mobile node, upon receiving this Home Agent Address Discovery 4427 Reply message, MAY then send its home registration Binding Update to 4428 the home agent address given as the IP Source Address of the packet 4429 carrying the Reply message or to any of the unicast IP addresses 4430 listed in the Home Agent Addresses field in the Reply. For example, 4431 if necessary, the mobile node MAY attempt its home registration 4432 with each of these home agents, in turn, by sending each a Binding 4433 Update and waiting for the matching Binding Acknowledgement, until 4434 its registration is accepted by one of these home agents. In trying 4435 each of the returned home agent addresses, the mobile node SHOULD try 4436 each in the order listed in the Home Agent Addresses field in the 4437 received Home Agent Address Discovery Reply message. If the home 4438 agent identified by the Source Address field in the IP header of the 4439 packet carrying the Home Agent Address Discovery Reply message is 4440 not listed in the Home Agent Addresses field in the Reply, it SHOULD 4441 be tried before the first address given in the list; otherwise, it 4442 SHOULD be tried in its listed order. 4444 If the mobile node has a current registration with some home agent 4445 on its home link (the Lifetime for that registration has not yet 4446 expired), then the mobile node MUST attempt any new registration 4447 first with that home agent. If that registration attempt fails 4448 (e.g., times out or is rejected), the mobile node SHOULD then 4449 reattempt this registration with another home agent on its home link. 4450 If the mobile node knows of no other suitable home agent, then it MAY 4451 attempt the dynamic home agent address discovery mechanism described 4452 above. 4454 If, after a mobile node transmits a Home Agent Address Discovery 4455 Request message to the Home Agents Anycast address, it does not 4456 receive a corresponding Home Agent Address Discovery Reply message 4457 within INITIAL_DHAAD_TIMEOUT seconds, the mobile node MAY retransmit 4458 the same Request message to the same anycast address. This 4459 retransmission MAY be repeated up to a maximum of DHAAD_RETRIES 4460 attempts. Each retransmission MUST be delayed by twice the time 4461 interval of the previous retransmission. 4463 10.9. Sending Binding Updates to Correspondent Nodes 4465 When the mobile node is assured that its home address is valid, 4466 it MAY send a Binding Update to any correspondent node at any 4467 time to allow the correspondent node to cache the mobile node's 4468 current care-of address (subject to the rate limiting defined in 4469 Section 10.12). See for example the home agent's use the 'D' bit 4470 of Binding Updates (in section 9.1) for how the mobile node can be 4471 assured that its home address is still valid. In any Binding Update 4472 sent by a mobile node, the care-of address (either the Source Address 4473 in the packet's IPv6 header or the Care-of Address in the Alternate 4474 Care-of Address Sub-Option of the Binding Update) MUST be set to one 4475 of the care-of addresses currently in use by the mobile node or to 4476 the mobile node's home address. 4478 A mobile node MAY choose to keep its location private from certain 4479 correspondent nodes, and thus need not send new Binding Updates to 4480 those correspondents. A mobile node MAY also send a Binding Update 4481 to such a correspondent node to instruct it to delete any existing 4482 binding for the mobile node from its Binding Cache, as described in 4483 Section 5.1. No other IPv6 nodes are authorized to send Binding 4484 Updates on behalf of a mobile node. 4486 If set to one of the mobile node's current care-of addresses (the 4487 care-of address given MAY differ from the mobile node's primary 4488 care-of address), the Binding Update requests the correspondent node 4489 to create or update an entry for the mobile node in the correspondent 4490 node's Binding Cache to record this care-of address for use in 4491 sending future packets to the mobile node. In this case, the value 4492 specified in the Lifetime field sent in the Binding Update SHOULD be 4493 less than or equal to the remaining lifetime of the home address and 4494 the care-of address specified for the binding. 4496 If, instead, the care-of address is set to the mobile node's home 4497 address, the Binding Update requests the correspondent node to delete 4498 any existing Binding Cache entry that it has for the mobile node. 4499 A mobile node MAY set the care-of address differently for sending 4500 Binding Updates to different correspondent nodes. 4502 When sending any Binding Update, the mobile node MUST record in its 4503 Binding Update List the following fields from the Binding Update: 4505 - The IP address of the node to which the Binding Update was sent. 4507 - The home address for which the Binding Update was sent (the value 4508 in the Home Address option in the packet carrying the Binding 4509 Update). 4511 - The initial lifetime of the binding, initialized from the 4512 Lifetime field sent in the Binding Update. 4514 - The remaining lifetime of the binding, also initialized from 4515 the Lifetime field sent in the Binding Update. This remaining 4516 lifetime value counts down and may also be reduced when the 4517 matching Binding Acknowledgement is received, based on the 4518 Lifetime value specified in that Binding Acknowledgement, as 4519 described in Section 10.13. When the remaining lifetime reaches 4520 zero, the Binding Update List entry MUST be deleted. 4522 The mobile node MUST retain in its Binding Update List information 4523 about all Binding Updates sent, for which the lifetime of the binding 4524 has not yet expired. However, when sending a Binding Update, if an 4525 entry already exists in the mobile node's Binding Update List for 4526 an earlier Binding Update sent to that same destination node, the 4527 existing Binding Update List entry is updated to reflect the new 4528 Binding Update rather than creating a new Binding Update List entry. 4530 When a mobile node sends a Binding Update to its home agent 4531 to register a new primary care-of address (as described in 4532 Section 10.7), the mobile node SHOULD also send a Binding Update 4533 to each other node for which an entry exists in the mobile node's 4534 Binding Update List, as detailed below. Thus, other relevant nodes 4535 are generally kept updated about the mobile node's binding and can 4536 send packets directly to the mobile node using the mobile node's 4537 current care-of address. 4539 The mobile node, however, need not send these Binding Updates 4540 immediately after configuring a new care-of address. For example, 4541 since the Binding Update is a destination option and can be included 4542 in any packet sent by a mobile node, the mobile node MAY delay 4543 sending a new Binding Update to any correspondent node for a 4544 short period of time, in hopes that the needed Binding Update 4545 can be included in some packet that the mobile node sends to that 4546 correspondent node for some other reason (for example, as part of 4547 some TCP connection in use). In this case, when sending a packet 4548 to some correspondent node, the mobile node SHOULD check in its 4549 Binding Update List to determine if a new Binding Update to this 4550 correspondent node is needed, and SHOULD include the new Binding 4551 Update in this packet as necessary. 4553 In addition, when a mobile node receives a packet for which the 4554 mobile node can deduce that the original sender of the packet either 4555 has no Binding Cache entry for the mobile node, or a stale entry 4556 for the mobile node in its Binding Cache, the mobile node SHOULD 4557 return a Binding Update to the sender giving its current care-of 4558 address (subject to the rate limiting defined in Section 10.12). 4559 In particular, the mobile node SHOULD return a Binding Update in 4560 response to receiving a packet that meets all of the following tests: 4562 - The packet was tunneled using IPv6 encapsulation. 4564 - The Destination Address in the tunnel (outer) IPv6 header is 4565 equal to any of the mobile node's care-of addresses. 4567 - The Destination Address in the original (inner) IPv6 header 4568 is equal to one of the mobile node's home addresses; or this 4569 Destination Address is equal to one of the mobile node's previous 4570 care-of addresses for which the mobile node has an entry in its 4571 Binding Update List, representing an unexpired Binding Update 4572 sent to a home agent on the link on which its previous care-of 4573 address is located (Section 10.10). 4575 - The Source Address in the tunnel (outer) IPv6 header differs from 4576 the Source Address in the original (inner) IPv6 header. 4578 The destination address to which the Binding Update should be sent 4579 in response to receiving a packet meeting all of the above tests is 4580 the Source Address in the original (inner) IPv6 header of the packet. 4581 The home address for which this Binding Update is sent should be the 4582 Destination Address of the original (inner) packet. 4584 Binding Updates sent to correspondent nodes are not generally 4585 required to be acknowledged. However, if the mobile node wants 4586 to be sure that its new care-of address has been entered into a 4587 correspondent node's Binding Cache, the mobile node MAY request an 4588 acknowledgement by setting the Acknowledge (A) bit in the Binding 4589 Update. In this case, however, the mobile node SHOULD NOT continue 4590 to retransmit the Binding Update once the retransmission timeout 4591 period has reached MAX_BINDACK_TIMEOUT. 4593 10.10. Establishing Forwarding from a Previous Care-of Address 4595 When a mobile node connects to a new link and forms a new care-of 4596 address, it MAY establish forwarding of packets from a previous 4597 care-of address to this new care-of address. To do so, the mobile 4598 node sends a Binding Update to any home agent on the link on which 4599 the previous care-of address is located, indicating this previous 4600 care-of address as the home address for the binding, and giving its 4601 new care-of address as the binding's care-of address. Such packet 4602 forwarding allows packets destined to the mobile node from nodes that 4603 have not yet learned the mobile node's new care-of address, to be 4604 forwarded to the mobile node rather than being lost once the mobile 4605 node is no longer reachable at this previous care-of address. 4607 In constructing this Binding Update, the mobile node utilizes the 4608 following specific steps: 4610 - The Home Address field in the Home Address option in the packet 4611 carrying the Binding Update MUST be set to the previous care-of 4612 address for which packet forwarding is being established. 4614 - The care-of address for the new binding MUST be set to the new 4615 care-of address to which packets destined to the previous care-of 4616 address are to be forwarded. Normally, this care-of address for 4617 the binding is specified by setting the Source Address of the 4618 packet carrying the Binding Update, to this address. However, 4619 the mobile node MAY instead include an Alternate Care-of Address 4620 sub-option in the Binding Update option, with its Alternate 4621 Care-of Address field set to the care-of address for the binding. 4623 - The Home Registration (H) bit MUST also be set in this Binding 4624 Update, to request this home agent to temporarily act as a home 4625 agent for this previous care-of address. 4627 This home agent will thus tunnel packets for the mobile node (packets 4628 destined to its specified previous care-of address) to its new 4629 care-of address. All of the procedures defined for home agent 4630 operation MUST be followed by this home agent for this registration. 4631 Note that this home agent does not necessarily know (and need not 4632 know) the mobile node's (permanent) home address as part of this 4633 registration. 4635 The packet carrying the Binding Update MUST be addressed to 4636 this home agent's global unicast address. Normally, this global 4637 unicast address is learned by the mobile node based on the Router 4638 Advertisements received by the mobile node (Section 6.2) while 4639 attached to the link on which this previous care-of address and this 4640 home agent are located; the mobile node obtains this home agent 4641 address from its Home Agents List (Section 4.6). Alternatively, 4642 the mobile node MAY use dynamic home agent address discovery 4643 (Section 10.8) to discover the global unicast address of a home agent 4644 on this previous link, but it SHOULD use an address from its Home 4645 Agents List if available for the prefix it used to form this previous 4646 care-of address. 4648 As with any packet containing a Binding Update (see section 5.1), the 4649 Binding Update packet to this home agent MUST meet the authentication 4650 requirements for Binding Updates, defined in Section 4.4. 4652 10.11. Retransmitting Binding Updates 4654 When the mobile node sends a Binding Update, it has to determine 4655 a value for the initial retransmission timer. If the mobile node 4656 is changing or updating an existing binding at the home agent, it 4657 should use the specified value of INITIAL_BINDACK_TIMEOUT for this 4658 initial retransmission timer. If on the other hand the mobile node 4659 does not have an existing binding at the home agent, it SHOULD use a 4660 value for the initial retransmission timer that is at least 1.5 times 4661 longer than (RetransTimer * DupAddrDetectTransmits). This value is 4662 likely to be substantially longer than the otherwise specified value 4663 of INITIAL_BINDACK_TIMEOUT that would be used by the mobile node. 4664 This longer retransmission interval will allow the the home agent 4665 to complete the DAD procedure which is mandated in this case, as 4666 detailed in section 10.7. 4668 If, after sending a Binding Update in which the care-of address has 4669 changed and the Acknowledge (A) bit is set, a mobile node fails 4670 to receive a valid, matching Binding Acknowledgement within the 4671 selected initial retransmission interval, the mobile node SHOULD 4672 retransmit the Binding Update, until a Binding Acknowledgement is 4673 received. Such a retransmitted Binding Update MUST use a Sequence 4674 Number value greater than that used for the previous transmission of 4675 this Binding Update. The retransmissions by the mobile node MUST 4676 use an exponential back-off process, in which the timeout period 4677 is doubled upon each retransmission until either the node receives 4678 a Binding Acknowledgement or the timeout period reaches the value 4679 MAX_BINDACK_TIMEOUT. 4681 10.12. Rate Limiting for Sending Binding Updates 4683 A mobile node MUST NOT send Binding Updates about the same binding to 4684 any individual node more often than once per MAX_UPDATE_RATE seconds. 4685 After sending MAX_FAST_UPDATES consecutive Binding Updates to a 4686 particular node with the same care-of address, the mobile node SHOULD 4687 reduce its rate of sending Binding Updates to that node, to the rate 4688 of SLOW_UPDATE_RATE per second. The mobile node MAY continue to send 4689 Binding Updates at this slower rate indefinitely, in hopes that the 4690 node will eventually be able to process a Binding Update and begin 4691 to route its packets directly to the mobile node at its new care-of 4692 address. 4694 10.13. Receiving Binding Acknowledgements 4696 Upon receiving a packet carrying a Binding Acknowledgement, a mobile 4697 node MUST validate the packet according to the following tests: 4699 - The packet meets the authentication requirements for Binding 4700 Acknowledgements, defined in Section 4.4. 4702 - The Option Length field in the Binding Acknowledgement option is 4703 greater than or equal to the length specified in Section 5.2. 4705 - The Sequence Number field matches the Sequence Number sent by the 4706 mobile node to this destination address in an outstanding Binding 4707 Update. 4709 Any Binding Acknowledgement not satisfying all of these tests MUST be 4710 silently ignored, although the remainder of the packet (i.e., other 4711 options, extension headers, or payload) SHOULD be processed normally 4712 according to any procedure defined for that part of the packet. 4714 When a mobile node receives a packet carrying a valid Binding 4715 Acknowledgement, the mobile node MUST examine the Status field as 4716 follows: 4718 - If the Status field indicates that the Binding Update was 4719 accepted (the Status field is less than 128), then the mobile 4720 node MUST update the corresponding entry in its Binding Update 4721 List to indicate that the Binding Update has been acknowledged; 4722 the mobile node MUST then stop retransmitting the Binding Update. 4723 In addition, if the value specified in the Lifetime field in the 4724 Binding Acknowledgement is less than the Lifetime value sent 4725 in the Binding Update being acknowledged, then the mobile node 4726 MUST subtract the difference between these two Lifetime values 4727 from the remaining lifetime for the binding as maintained in the 4728 corresponding Binding Update List entry (with a minimum value 4729 for the Binding Update List entry lifetime of 0). That is, if 4730 the Lifetime value sent in the Binding Update was L_update, the 4731 Lifetime value received in the Binding Acknowledgement was L_ack, 4732 and the current remaining lifetime of the Binding Update List 4733 entry is L_remain, then the new value for the remaining lifetime 4734 of the Binding Update List entry should be 4735 max((L_remain - (L_update - L_ack)), 0) 4737 where max(X, Y) is the maximum of X and Y. The effect of this 4738 step is to correctly manage the mobile node's view of the 4739 binding's remaining lifetime (as maintained in the corresponding 4740 Binding Update List entry) so that it correctly counts down from 4741 the Lifetime value given in the Binding Acknowledgement, but with 4742 the timer countdown beginning at the time that the Binding Update 4743 was sent. 4745 - If the Status field indicates that the Binding Update was 4746 rejected (the Status field is greater than or equal to 128), then 4747 the mobile node MUST delete the corresponding Binding Update List 4748 entry, and it MUST also stop retransmitting the Binding Update. 4749 Optionally, the mobile node MAY then take steps to correct the 4750 cause of the error and retransmit the Binding Update (with a new 4751 Sequence Number value), subject to the rate limiting restriction 4752 specified in Section 10.12. 4754 10.14. Receiving Binding Requests 4756 When a mobile node receives a packet containing a Binding Request, 4757 it SHOULD return to the sender a packet containing a Binding Update. 4758 The Lifetime field in this Binding Update SHOULD be set to a new 4759 lifetime, extending any current lifetime remaining from a previous 4760 Binding Update sent to this node (as indicated in any existing 4761 Binding Update List entry for this node), except that this lifetime 4762 MUST NOT exceed the remaining lifetime for the mobile node's primary 4763 care-of address registration at its home agent. When sending this 4764 Binding Update, the mobile node MUST update its Binding Update List 4765 in the same way as for any other Binding Update sent by the mobile 4766 node. 4768 Note, however, that the mobile node MAY choose to keep its current 4769 binding private from the sender of the Binding Request. In this 4770 case, the mobile node instead SHOULD return a Binding Update to the 4771 sender, in which the Lifetime field is set to zero and the care-of 4772 address is set to the mobile node's home address. 4774 If the Binding Request for which the Binding Update is being returned 4775 contains a Unique Identifier Sub-Option, the Binding Update MUST also 4776 include a Unique Identifier Sub-Option. The unique identifier in the 4777 Sub-Option Data field of the Unique Identifier Sub-Option MUST be 4778 copied from the unique identifier carried in the Binding Request. 4780 10.15. Receiving ICMP Error Messages 4782 The Option Type value for a Binding Update option specifies that 4783 any node receiving this option that does not recognize the Option 4784 Type SHOULD return an ICMP Parameter Problem, Code 2, message to 4785 the sender of the packet containing the Binding Update option. If 4786 a node sending a Binding Update receives such an ICMP error message 4787 in response, it SHOULD record in its Binding Update List that future 4788 Binding Updates SHOULD NOT be sent to this destination. 4790 Likewise, although ALL IPv6 nodes (whether host or router, whether 4791 mobile or stationary) MUST implement the ability to correctly process 4792 received packets containing a Home Address option, all Option Type 4793 values in IPv6 include a specification of the behavior that a node 4794 receiving a packet containing this option performs if it does not 4795 implement receipt of that type of option. For the Home Address 4796 option, the Option Type value specifies that any node receiving 4797 this option that does not recognize the Option Type SHOULD return 4798 an ICMP Parameter Problem, Code 2, message to the sender of the 4799 packet containing the Home Address option. If a mobile node receives 4800 such an ICMP error message from some node indicating that it does 4801 not recognize the mobile node's Home Address option, the mobile 4802 node SHOULD log the error and then discard the ICMP message; this 4803 error message indicates that the node to which the original packet 4804 was addressed (the node returning the ICMP error message) does not 4805 correctly implement this required part of the IPv6 protocol. 4807 10.16. Sending Mobile Prefix Solicitations 4809 When a mobile node has a home address that is about to become 4810 invalid, it sends a Mobile Prefix Solicitation to its home agent 4811 in an attempt to acquire fresh routing prefix information. The 4812 new information also enables the mobile node to participate in 4813 renumbering operations affecting the home network, as described in 4814 section 9.6. 4816 The mobile node SHOULD send a Solicitation to the home agent when 4817 its home address will become invalid within MaxRtrAdvInterval 4818 seconds, where this value is acquired in a previous Mobile Prefix 4819 Advertisement from the home agent. If no such value is known, the 4820 value MAX_PFX_ADV_DELAY seconds is used instead (see section 11). 4822 If the mobile node does not have a valid home address available for 4823 use as the IP source address, it MAY use its care-of address (but 4824 there will not be a security association between the Home Agent 4825 and the care-of address for the corresponding Advertisement to be 4826 authenticated). 4828 This solicitation follows the same retransmission rules specified for 4829 Router Solicitations [17], except that the initial retransmission 4830 interval is specified to be INITIAL_SOLICIT_TIMER (see section 11). 4832 10.17. Receiving Mobile Prefix Advertisements 4834 Section 9.6 describes the operation of a home agent to support boot 4835 time configuration and renumbering a mobile node's home subnet while 4836 the mobile node is away from home. The home agent sends Mobile 4837 Prefix Advertisement messages to the mobile node while away from 4838 home, giving "important" Prefix Information options that describe 4839 changes in the prefixes in use on the mobile node's home link. 4841 When a mobile node receives a Mobile Prefix Advertisement, it MUST 4842 validate it according to the following tests: 4844 - The Source Address of the IP packet carrying the Mobile Prefix 4845 Advertisement is the same as the home agent address to which the 4846 mobile node last sent an accepted "home registration" Binding 4847 Update to register its primary care-of address. Otherwise, if 4848 no such registrations have been made, it SHOULD be the mobile 4849 node's stored home agent address, if one exists. Otherwise, if 4850 the mobile node has not yet discovered its home agent's address, 4851 it MUST NOT accept Mobile Prefix Advertisements. 4853 - The packet MUST be protected by IPsec [13, 11, 12] to guard 4854 against malicious prefix advertisements. The IPsec protection 4855 MUST provide sender authentication, data integrity protection, 4856 and replay protection, covering the advertisement. 4858 Any received Mobile Prefix Advertisement not meeting all of these 4859 tests MUST be silently discarded. 4861 If a received Mobile Prefix Advertisement is not discarded according 4862 to the tests listed above, the mobile node MUST process the Prefix 4863 Information Options as if they arrived in a Router Advertisement 4864 on the mobile node's home link [17]. Such processing may result 4865 in the mobile node configuring a new home address, although due 4866 to separation between preferred lifetime and valid lifetime, such 4867 changes should not affect most communication by the mobile node, in 4868 the same way as for nodes that are at home. 4870 If the advertisement contains a Binding Request option, the mobile 4871 node SHOULD return a Binding Update, which will be viewed by the 4872 home agent as an acknowledgement of the corresponding Mobile Prefix 4873 Advertisement, which it can cease transmitting. 4875 In addition, if processing of this Advertisement resulted in the 4876 mobile node configuring a new home address, and if the method used 4877 for this new home address configuration would require the mobile node 4878 to perform Duplicate Address Detection [27] for the new address if 4879 the mobile node were located at home, then the mobile node MUST set 4880 the Duplicate Address Detection (D) bit in this Binding Update to 4881 its home agent, to request the home agent to perform this Duplicate 4882 Address Detection on behalf of the mobile node. 4884 10.18. Using Multiple Care-of Addresses 4886 As described in Section 10.6, a mobile node MAY use more than one 4887 care-of address at a time. Particularly in the case of many wireless 4888 networks, a mobile node effectively might be reachable through 4889 multiple links at the same time (e.g., with overlapping wireless 4890 cells), on which different on-link subnet prefixes may exist. A 4891 mobile node SHOULD select a primary care-of address from among those 4892 care-of addresses it has formed using any of these subnet prefixes, 4893 based on the movement detection mechanism in use, as described in 4894 Section 10.4. When the mobile node selects a new primary care-of 4895 address, it MUST register it with its home agent by sending it a 4896 Binding Update with the Home Registration (H) and Acknowledge (A) 4897 bits set, as described in Section 10.7. 4899 To assist with smooth handovers, a mobile node SHOULD retain 4900 its previous primary care-of address as a (non-primary) care-of 4901 address, and SHOULD still accept packets at this address, even after 4902 registering its new primary care-of address with its home agent. 4903 This is reasonable, since the mobile node could only receive packets 4904 at its previous primary care-of address if it were indeed still 4905 connected to that link. If the previous primary care-of address was 4906 allocated using stateful Address Autoconfiguration [2], the mobile 4907 node may not wish to release the address immediately upon switching 4908 to a new primary care-of address. 4910 10.19. Routing Multicast Packets 4912 A mobile node that is connected to its home link functions in the 4913 same way as any other (stationary) node. Thus, when it is at home, 4914 a mobile node functions identically to other multicast senders and 4915 receivers. This section therefore describes the behavior of a mobile 4916 node that is not on its home link. 4918 In order to receive packets sent to some multicast group, a mobile 4919 node must join that multicast group. One method by which a mobile 4920 node MAY join the group is via a (local) multicast router on the 4921 foreign link being visited. The mobile node SHOULD use one of its 4922 care-of addresses that shares a subnet prefix with the multicast 4923 router, as the source IPv6 address of its multicast group membership 4924 control messages. The mobile node MUST insert a Home Address 4925 destination option in such outgoing multicast packets, so that any 4926 multicast applications that depend on the address of the sending node 4927 will correctly use the mobile node's home address for that value. 4929 Alternatively, a mobile node MAY join multicast groups via a 4930 bi-directional tunnel to its home agent. The mobile node tunnels its 4931 multicast group membership control packets to its home agent, and the 4932 home agent forwards multicast packets down the tunnel to the mobile 4933 node. 4935 A mobile node that wishes to send packets to a multicast group 4936 also has two options: (1) send directly on the foreign link being 4937 visited; or (2) send via a tunnel to its home agent. Because 4938 multicast routing in general depends upon the Source Address used in 4939 the IPv6 header of the multicast packet, a mobile node that tunnels a 4940 multicast packet to its home agent MUST use its home address as the 4941 IPv6 Source Address of the inner multicast packet. 4943 10.20. Returning Home 4945 A mobile node detects that it has returned to its home link through 4946 the movement detection algorithm in use (Section 10.4), when the 4947 mobile node detects that its home subnet prefix is again on-link. 4948 The mobile node SHOULD then send a Binding Update to its home agent, 4949 to instruct its home agent to no longer intercept or tunnel packets 4950 for it. In this Binding Update, the mobile node MUST set the care-of 4951 address for the binding (the Source Address field in the packet's 4952 IPv6 header) to the mobile node's own home address. As with other 4953 Binding Updates sent to register with its home agent, the mobile 4954 node MUST set the Acknowledge (A) and Home Registration (H) bits, 4955 and SHOULD retransmit the Binding Update until a matching Binding 4956 Acknowledgement is received. 4958 When sending this Binding Update to its home agent, the mobile 4959 node must be careful in how it uses Neighbor Solicitation [17] (if 4960 needed) to learn the home agent's link-layer address, since the home 4961 agent will be currently configured to defend the mobile node's home 4962 address for Duplicate Address Detection. In particular, a Neighbor 4963 Solicitation from the mobile node using its home address as the 4964 Source Address would be detected by the home agent as a duplicate 4965 address. In many cases, Neighbor Solicitation by the mobile node 4966 for the home agent's address will not be necessary, since the mobile 4967 node may have already learned the home agent's link-layer address, 4968 for example from a Source Link-Layer Address option in the Router 4969 Advertisement from which it learned that its home address was on-link 4970 and that the mobile node had thus returned home. If the mobile node 4971 does Neighbor Solicitation to learn the home agent's link-layer 4972 address, in this special case of the mobile node returning home, the 4973 mobile node MUST unicast the packet, and in addition set the Source 4974 Address of this Neighbor Solicitation to the unspecified address 4975 (0:0:0:0:0:0:0:0). Since the solicitation is unicast, the home 4976 agent will be able to distinguish from a similar packet that would 4977 only be used for DAD. The home agent will send a multicast Neighbor 4978 Advertisement back to the mobile node with the Solicited flag ('S') 4979 set to zero. The mobile node SHOULD accept this advertisement, and 4980 set the state of the Neighbor Cache entry for the home agent to 4981 REACHABLE. 4983 The mobile node then sends its Binding Update using the home agent's 4984 link-layer address, instructing its home agent to no longer serve 4985 as a home agent for it. By processing this Binding Update, the 4986 home agent will cease defending the mobile node's home address for 4987 Duplicate Address Detection and will no longer respond to Neighbor 4988 Solicitations for the mobile node's home address. The mobile node 4989 is then the only node on the link receiving packets at the mobile 4990 node's home address. In addition, when returning home prior to the 4991 expiration of a current binding for its home address, and configuring 4992 its home address on its network interface on its home link, the 4993 mobile node MUST NOT perform Duplicate Address Detection on its own 4994 home address, in order to avoid confusion or conflict with its home 4995 agent's use of the same address. If the mobile node returns home 4996 after the bindings for all of its care-of addresses have expired, 4997 then it SHOULD perform DAD. 4999 After the Mobile Node sends the Binding Update, the Home Agent MUST 5000 remove the Proxy Neighbor Cache entry for the Mobile Node and MAY 5001 learn its link-layer address based on the link-layer packet or cached 5002 information, or if that is not available, it SHOULD send a Neighbor 5003 Solicitation with the target address equal to the Binding Update's 5004 source IP address. The Mobile Node MUST then reply with a unicast 5005 Neighbor Advertisement to the Home Agent with its link-layer address. 5006 While the Mobile Node is waiting for a Binding Acknowledgement, it 5007 MUST NOT respond to any Neighbor Solicitations for its Home Address 5008 other than those originating from the IP address to which it sent the 5009 Binding Update. 5011 After receiving the Binding Acknowledgement for its Binding Update 5012 to its home agent, the mobile node MUST multicast onto the home 5013 link (to the all-nodes multicast address) a Neighbor Advertisement 5014 message [17], to advertise the mobile node's own link-layer address 5015 for its own home address. The Target Address in this Neighbor 5016 Advertisement message MUST be set to the mobile node's home address, 5017 and the Advertisement MUST include a Target Link-layer Address option 5018 specifying the mobile node's link-layer address. The mobile node 5019 MUST multicast such a Neighbor Advertisement message for each of its 5020 home addresses, as defined by the current on-link prefixes, including 5021 its link-local address and site-local address. The Solicited 5022 Flag (S) in these Advertisements MUST NOT be set, since they were 5023 not solicited by any Neighbor Solicitation message. The Override 5024 Flag (O) in these Advertisements MUST be set, indicating that the 5025 Advertisements SHOULD override any existing Neighbor Cache entries at 5026 any node receiving them. 5028 Since multicasts on the local link (such as Ethernet) are typically 5029 not guaranteed to be reliable, the mobile node MAY retransmit these 5030 Neighbor Advertisement messages up to MAX_ADVERT_REXMIT times to 5031 increase their reliability. It is still possible that some nodes on 5032 the home link will not receive any of these Neighbor Advertisements, 5033 but these nodes will eventually be able to recover through use of 5034 Neighbor Unreachability Detection [17]. 5036 11. Protocol Constants 5038 HomeRtrAdvInterval 3,600 seconds 5039 DHAAD_RETRIES 3 retransmissions 5040 INITIAL_BINDACK_TIMEOUT 1 second 5041 INITIAL_DHAAD_TIMEOUT 2 seconds 5042 INITIAL_SOLICIT_TIMER 2 seconds 5043 MAX_BINDACK_TIMEOUT 256 seconds 5044 MAX_UPDATE_RATE once per second 5045 MAX_FAST_UPDATES 5 transmissions 5046 MAX_ADVERT_REXMIT 3 transmissions 5047 MAX_PFX_ADV_DELAY 1,000 seconds 5048 PREFIX_ADV_RETRIES 3 retransmissions 5049 PREFIX_ADV_TIMEOUT 5 seconds 5050 SLOW_UPDATE_RATE once per 10 second interval 5052 12. IANA Considerations 5054 This document defines four new types of IPv6 destination options, 5055 each of which must be assigned an Option Type value: 5057 - The Binding Update option, described in Section 5.1; 5059 - The Binding Acknowledgement option, described in Section 5.2; 5061 - The Binding Request option, described in Section 5.3; and 5063 - The Home Address option, described in Section 5.4. 5065 These destination options can sometimes take sub-options. The 5066 current sub-options are specified in section 5.5, "Mobile IPv6 5067 Destination Option Sub-Options", and include the following: 5069 0 Pad1 sub-option 5071 1 PadN sub-option 5073 2 Unique Identifier sub-option 5075 3 Alternate Care-of Address sub-option 5077 4 Authentication Data sub-option 5079 In addition, this document defines four ICMP message types, two used 5080 as part of the dynamic home agent address discovery mechanism and 5081 two used in lieu of router solicitations and advertisements when the 5082 mobile node is away from the home link: 5084 - The Home Agent Address Discovery Request message, described in 5085 Section 5.7; 5087 - The Home Agent Address Discovery Reply message, described in 5088 Section 5.8; 5090 - The Mobile Prefix Solicitation message, described in Section 5.9; 5091 and 5093 - The Mobile Prefix Advertisement message, described in 5094 Section 5.10. 5096 This document also defines two new Neighbor Discovery [17] options, 5097 which must be assigned Option Type values within the option numbering 5098 space for Neighbor Discovery messages: 5100 - The Advertisement Interval option, described in Section 6.3; and 5102 - The Home Agent Information option, described in Section 6.4. 5104 A new space of predefined SPIs for Binding Security Associations must 5105 be created for use with Binding Updates and Binding Acknowledgements. 5106 This space is to be managed in a way identical to that specified 5107 for IPsec [13]. Assignment of new SPIs in the reserved range 1-255 5108 requires working group approval. 5110 Finally, this document defines a new type of anycast address, which 5111 must be assigned a reserved value for use with any subnet prefix to 5112 define this anycast address on each subnet: 5114 - The "Mobile IPv6 Home-Agents" anycast address [10], used in the 5115 dynamic home agent address discovery mechanism described in 5116 Sections 9.8 and 10.8. 5118 13. Security Considerations 5120 13.1. Binding Updates, Acknowledgements, and Requests 5122 The Binding Update option described in this document will result in 5123 packets addressed to a mobile node being delivered instead to its 5124 care-of address. This ability to change the routing of these packets 5125 could be a significant vulnerability if any packet containing a 5126 Binding Update option were not authenticated. Such use of "remote 5127 redirection", for instance as performed by the Binding Update option, 5128 is widely understood to be a security problem in the current Internet 5129 if not authenticated [1]. 5131 The Binding Acknowledgement option also requires authentication, 5132 since, for example, an attacker could otherwise trick a mobile node 5133 into believing a different outcome from a registration attempt with 5134 its home agent. 5136 No authentication is required for the Binding Request option, since 5137 the use of this option does not modify or create any state in either 5138 the sender or the receiver. Issues with binding privacy stemming 5139 from use of the Binding Request option can be dealt with either 5140 through existing IPsec encryption mechanisms or through use of 5141 firewalls. 5143 After the Sequence Number space of the Binding Update and Binding 5144 Acknowledgement options has been exhausted, the mobile node and the 5145 home agent SHOULD renegotiate their security association in order to 5146 prevent any possibility of replay attacks. 5148 The only way binding cache entries can be deleted is through Binding 5149 Updates and persistent ICMP unreachables messages. The former 5150 are required to be authenticated, but not the latter. Therefore, 5151 unauthenticated ICMP messages can unfortunately be used for DoS 5152 attacks. However, the security exposure is limited, since the effect 5153 would be only to cause packets for the mobile node to be sent by way 5154 of the mobile node's home network. 5156 13.2. Security for the Home Address Option 5158 No special authentication of the Home Address option is required, 5159 except that if the IPv6 header of a packet is covered by 5160 authentication, then that authentication MUST also cover the Home 5161 Address option; this coverage is achieved automatically by the 5162 definition of the Option Type code for the Home Address option 5163 (Section 5.4), since it indicates that the option is included in the 5164 authentication computation. Thus, even when authentication is used 5165 in the IPv6 header, the security of the Source Address field in the 5166 IPv6 header is not compromised by the presence of a Home Address 5167 option. Without authentication of the packet, then any field in the 5168 IPv6 header, including the Source Address field, and any other parts 5169 of the packet, including the Home Address option, can be forged or 5170 modified in transit. In this case, the contents of the Home Address 5171 option is no more suspect than any other part of the packet. 5173 The use of the Home Address option allows packets sent by a 5174 mobile node to pass normally through routers implementing ingress 5175 filtering [7]. Since the care-of address used in the Source Address 5176 field of the packet's IPv6 header is topologically correct for the 5177 sending location of the mobile node, ingress filtering can trace the 5178 location of the mobile node in the same way as can be done with any 5179 sender when ingress filtering is in use. A node receiving a packet 5180 that includes a Home Address option MAY implement the processing of 5181 this option by physically exchanging the Home Address option field 5182 with the source IPv6 address in the IPv6 header. 5184 Notice that use of the Home Address option is conceptually similar to 5185 a particular use of encapsulation, with both the inner and outer IPv6 5186 header equal to the Correspondent Node's address. This is important 5187 for understanding the security implications surrounding the use of 5188 the Home Address option. 5190 13.3. General Mobile Computing Issues 5192 The mobile computing environment is potentially very different from 5193 the ordinary computing environment. In many cases, mobile computers 5194 will be connected to the network via wireless links. Such links 5195 are particularly vulnerable to passive eavesdropping, active replay 5196 attacks, and other active attacks. Furthermore, mobile computers 5197 are more susceptible to loss or theft than stationary computers. 5198 Any secrets such as authentication or encryption keys stored on the 5199 mobile computer are thus subject to compromise in ways generally not 5200 common in the non-mobile environment. 5202 Users who have sensitive data that they wish to keep private 5203 SHOULD protect the data using additional mechanisms (such as 5204 encryption [12]); such mechanisms are beyond the scope of this 5205 document. Users concerned about traffic analysis SHOULD consider 5206 appropriate use of link encryption. If stronger location privacy 5207 is desired, the mobile node MAY create a tunnel to its home agent. 5208 Then, packets destined for correspondent nodes will appear to emanate 5209 from the home subnet, making it more difficult to pinpoint the 5210 location of the mobile node. Such mechanisms are all beyond the 5211 scope of this document. 5213 Whether or not the mobile node is away from home is likely to 5214 influence the choice of security policy from the SPD. For instance, 5215 if a mobile node is connected to its home network and it communicates 5216 with a correspondent node on its home network, no security may be 5217 needed. If, on the other hand, the mobile node is attached to 5218 foreign network and has sent a Binding Update to its home agent, then 5219 the mobile node may need to make use of security features in order to 5220 communicate with that same correspondent node. 5222 Acknowledgements 5224 We would like to thank the members of the Mobile IP and IPng Working 5225 Groups for their comments and suggestions on this work. We would 5226 particularly like to thank (in alphabetical order) Fred Baker 5227 (Cisco), Josh Broch (Carnegie Mellon University), Robert Chalmers 5228 (University of California at Santa Barbara), Noel Chiappa (MIT), 5229 Vijay Devarapalli (Nokia Research Center), Rich Draves (Microsoft 5230 Research), Francis Dupont (ENST Bretagne), Thomas Eklund (Xelerated), 5231 Jun-Ichiro Itojun Hagino (IIJ Research Laboratory), Krishna Kumar 5232 (IBM Research), T.J. Kniveton (Nokia Research), Jiwoong Lee (KTF), 5233 Aime Lerouzic (Bull S.A.), Thomas Narten (IBM), Erik Nordmark (Sun 5234 Microsystems), Simon Nybroe (Ericsson Telebit), David Oran (Cisco), 5235 Lars Henrik Petander (HUT), Basavaraj Patil (Nokia), Ken Powell 5236 (Compaq), Phil Roberts (Motorola), Patrice Romand (Bull S.A.), 5237 Jeff Schiller (MIT) Tom Soderlund (Nokia Research), Hesham Soliman 5238 (Ericsson), Jim Solomon (RedBack Networks), Tapio Suihko (Technical 5239 Research Center of Finland), Benny Van Houdt (University of Antwerp), 5240 Jon-Olov Vatn (KTH), Alper Yegin (Sun Microsystems), and Xinhua Zhao 5241 (Stanford University) for their detailed reviews of earlier versions 5242 of this document. Their suggestions have helped to improve both the 5243 design and presentation of the protocol. 5245 We would also like to thank the participants in the Mobile IPv6 5246 testing event held at Nancy, France, September 15-17, 1999, for 5247 their valuable feedback as a result of interoperability testing 5248 of four Mobile IPv6 implementations coming from four different 5249 organizations: Bull (AIX), Ericsson Telebit (FreeBSD), NEC 5250 (FreeBSD), and INRIA (FreeBSD). Further, we would like to thank the 5251 feedback from the implementors who participated in the Mobile IPv6 5252 interoperability testing at Connectathon 2000 in San Jose, 5253 California, March 6-9, 2000. Finally, we would like to thank the 5254 participants at the ETSI interoperability testing at ETSI, in Sophia 5255 Antipolis, France, during October 2-6, 2000, including teams from 5256 Compaq, Ericsson, INRIA, Nokia, and Technical University of Helsinki. 5258 References 5260 [1] S. M. Bellovin. Security Problems in the TCP/IP Protocol Suite. 5261 ACM Computer Communications Review, 19(2), March 1989. 5263 [2] Jim Bound and Charles Perkins. Dynamic Host Configuration 5264 Protocol for IPv6 (DHCPv6), February 1999. Work in progress. 5266 [3] Scott Bradner. Key words for use in RFCs to Indicate 5267 Requirement Levels. RFC 2119, March 1997. 5269 [4] Alex Conta and Stephen Deering. Generic Packet Tunneling in 5270 IPv6 Specification. RFC 2473, December 1998. 5272 [5] Alex Conta and Stephen Deering. Internet Control Message 5273 Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) 5274 Specification. RFC 2463, December 1998. 5276 [6] Stephen E. Deering and Robert M. Hinden. Internet Protocol 5277 Version 6 (IPv6) Specification. RFC 2460, December 1998. 5279 [7] Paul Ferguson and Daniel Senie. Network Ingress Filtering: 5280 Defeating Denial of Service Attacks which employ IP Source 5281 Address Spoofing. RFC 2267, January 1998. 5283 [8] Dan Harkins and Dave Carrel. The Internet Key Exchange (IKE). 5284 RFC 2409, November 1998. 5286 [9] Robert M. Hinden and Stephen E. Deering. IP Version 6 5287 Addressing Architecture. RFC 2373, July 1998. 5289 [10] David B. Johnson and Stephen E. Deering. Reserved IPv6 Subnet 5290 Anycast Addresses. RFC 2526, March 1999. 5292 [11] Stephen Kent and Randall Atkinson. IP Authentication Header. 5293 RFC 2402, November 1998. 5295 [12] Stephen Kent and Randall Atkinson. IP Encapsulating Security 5296 Payload (ESP). RFC 2406, November 1998. 5298 [13] Stephen Kent and Randall Atkinson. Security Architecture for 5299 the Internet Protocol. RFC 2401, November 1998. 5301 [14] Douglas Maughan, Mark Schneider, Mark Schertler, and Jeff 5302 Turner. Internet Security Association and Key Management 5303 Protocol (ISAKMP). RFC 2408, November 1998. 5305 [15] P. Mockapetris. Domain Names -- Concepts and Facilities. 5306 RFC 1034, November 1987. 5308 [16] P. Mockapetris. Domain Names -- Implementation and 5309 Specification. RFC 1035, November 1987. 5311 [17] Thomas Narten, Erik Nordmark, and William Allen Simpson. 5312 Neighbor Discovery for IP Version 6 (IPv6). RFC 2461, December 5313 1998. 5315 [18] Charles Perkins. IP Encapsulation within IP. RFC 2003, October 5316 1996. 5318 [19] Charles Perkins, editor. IP Mobility Support. RFC 2002, 5319 October 1996. 5321 [20] Charles Perkins. Minimal Encapsulation within IP. RFC 2004, 5322 October 1996. 5324 [21] Charles Perkins and David B. Johnson. Route Optimization in 5325 Mobile IP, February 1999. Work in progress. 5327 [22] Derrell Piper. The Internet IP Security Domain of 5328 Interpretation for ISAKMP. RFC 2407, November 1998. 5330 [23] David C. Plummer. An Ethernet Address Resolution Protocol: 5331 Or Converting Network Protocol Addresses to 48.bit Ethernet 5332 Addresses for Transmission on Ethernet Hardware. RFC 826, 5333 November 1982. 5335 [24] J. B. Postel. User Datagram Protocol. RFC 768, August 1980. 5337 [25] J. B. Postel, editor. Transmission Control Protocol. RFC 793, 5338 September 1981. 5340 [26] Joyce K. Reynolds and Jon Postel. Assigned Numbers. RFC 1700, 5341 October 1994. See also http://www.iana.org/numbers.html. 5343 [27] Susan Thomson and Thomas Narten. IPv6 Stateless Address 5344 Autoconfiguration. RFC 2462, December 1998. 5346 A. Changes from Previous Version of the Draft 5348 This appendix briefly lists some of the major changes in this 5349 draft relative to the previous version of this same draft, 5350 draft-ietf-mobileip-ipv6-14.txt: 5352 A.1. Changes from Draft Version ...-14 5354 - Strengthened mandates for mobile nodes so that now a mobile node 5355 MUST support decapsulation and processing for routing headers 5356 (section 10.3). 5358 - Enabled ESP to be a valid way to secure reverse tunneled packets 5359 (section 9.5). 5361 - Removed mandate that mobile node select a default router, 5362 and instead described it as typical behavior (section 10.4). 5363 Also made it clear that picking a new default router does not 5364 automatically mean picking a new primary care-of address. 5366 - Added provisional ICMP numbers for the new message types, which 5367 may be reassigned by IANA, but which will be useful for testing 5368 purposes. 5370 - Removed the Mobile Router Prefix Length Sub-Option 5372 - Removed the Prefix Length field from the Binding Update, and 5373 references to error number 136. 5375 - Added the `S' bit so that the home agent can be instructed to 5376 *override* its default behavior. That is, with the `S' bit 5377 set, the home agent will not attempt to be helpful by changing 5378 multiple Binding Cache entries, for multiple routing prefixes, 5379 after receiving only one Binding Update. 5381 - Reworded the specification so that the Home Agent now has to 5382 perform Duplicate Address Detection for the mobile node's address 5383 on all the prefixes for which the router is performing home agent 5384 service. 5386 - Removed the section about Mobile Routers 5388 - Added the Authentication Data Sub-option; reorganized the section 5389 about computing authentication data. 5391 - Specified that the Home Agent lifetime is by default the same as 5392 the Router lifetime, in a Router Advertisement. 5394 - Specified that Binding Updates with zero lifetime and the 'A' bit 5395 set should cause a Binding Acknowledgement to be sent back to the 5396 Source IP address of the Binding Update. 5398 - Qualified the allowable times when a mobile node can send a 5399 Binding Update to a correspondent node 5401 - Added text allowing the correspondent node to extend an existing 5402 Routing Header by also including the care-of address as the entry 5403 of a routing header to be visited immediately before the home 5404 address. In this way, for instance, the mobile node can be an 5405 intermediate node of a path along the way to some other node. 5407 - Removed the Home Address field from the Home Agent Address 5408 Discovery Request Message. 5410 - Noted that ICMP Unreachable forms a potential mechanism by which 5411 a malicious node can cause a correspondent node to delete a valid 5412 entry from its Binding Cache. 5414 - Specified that, when a router stops offering home agent services 5415 by turning off the 'H' flag, the mobile node has to delete the 5416 corresponding entry from its Home Agent list. 5418 A.2. Changes from Previous Versions of the Draft 5420 - Clarified language about how the aggregate list of prefixes is 5421 built by the home agent, to include only prefixes with the 'H' 5422 bit set. 5424 - Specified a new error status (141) to handle cases for sequence 5425 number mismatches (e.g., when a mobile node reboots). 5427 - Moved this section to the appendix, and reorganized other 5428 appendix sections. 5430 - Reorganized some related sections to be adjacent to each other. 5432 - Changed the Prefix Length of the Binding Update to be 7-bit only, 5433 in order to reserve more flag bits for the future. 5435 - Changed the Sequence Number of the Binding Update and Binding 5436 Acknowledgement to be 8-bit only. 5438 - Inserted specification that, after returning home and sending a 5439 Neighbor Solicitation to the home agent, a mobile node should 5440 accept any Neighbor Advertisement from the home agent as an 5441 indication that the home agent is REACHABLE. 5443 - Inserted new terminology for Binding Key and Binding Security 5444 Association (BSA) in anticipation of eliminating the use of AH 5446 - Eliminated use of AH for authenticating Binding Update, and for 5447 authenticating Binding Acknowledgement 5449 - Specified that all correspondent nodes MUST implement a base 5450 protocol [?] for establishing a Binding Key. 5452 - Added the following protocol constants: 5454 INITIAL_SOLICIT_TIMER: 5456 - Created new ICMP messages for Mobile Prefix Solicitations and 5457 Advertisements (see sections 5.9 and 5.10). 5459 - Changed Network Renumbering (Section 9.6) to encompass mobile 5460 node configuration issues, remove unspecified address usage, 5461 simplify rules for prefix maintenance and sending, and use new 5462 ICMP message types noted above. 5464 - Added a paragraph to Returning Home (section 10.20) to describe 5465 how the Home Agent discovers the mobile node's link-layer address 5467 - Reworded parts of appendix B as needed. 5469 - Added the Mobile Router Prefix Length Sub-Option (section 5.5), 5470 along with text describing what a Mobile Router should do with 5471 it. 5473 B. Remote Home Address Configuration 5475 The method for initializing a mobile node's home addresses on 5476 power-up or after an extended period of being disconnected from 5477 the network is beyond the scope of this specification. Whatever 5478 procedure is used should result in the mobile node having the same 5479 stateless or stateful (e.g., DHCPv6) home address autoconfiguration 5480 information it would have if it were attached to the home network. 5481 Due to the possibility that the home network could be renumbered 5482 while the mobile node is disconnected, a robust mobile node would not 5483 rely solely on storing these addresses locally. 5485 Such a mobile node could initialize by using the following procedure: 5487 1. Generate a care-of address using stateless or stateful 5488 autoconfiguration. 5490 2. Query DNS for the home network's mobile agent anycast address. 5492 3. Send a Home Agent Address Discovery Request message to the home 5493 network. 5495 4. Receive Home Agent Address Discovery Reply message. 5497 5. Select the most preferred home agent and establish a security 5498 association between the mobile node's current care-of address and 5499 the home agent for temporary use during initialization only. 5501 6. Send a Home Prefix Solicitation message with the Request All 5502 Prefixes flag set to the home agent from the mobile node's 5503 care-of address. 5505 7. Receive a Home Prefix Advertisement message from the home agent, 5506 follow stateless address autoconfiguration rules to configure 5507 home addresses for prefixes received. 5509 8. Create a security association between the mobile node's home 5510 address and the home agent. 5512 9. Send a binding update(s) to the home agent to register the mobile 5513 node's home addresses. 5515 10. Receive binding acknowledgement(s) then begin normal 5516 communications. 5518 Chairs' Addresses 5520 The Working Group can be contacted via its current chairs: 5522 Basavaraj Patil Phil Roberts 5523 Nokia Corporation Megisto Corp. 5524 6000 Connection Drive Suite 120 5525 M/S M8-540 20251 Century Blvd 5526 Irving, TX 75039 Germantown MD 20874 5527 USA USA 5528 Phone: +1 972-894-6709 Phone: +1 847-202-9314 5529 Fax : +1 972-894-5349 Email: PRoberts@MEGISTO.com 5530 EMail: Raj.Patil@nokia.com 5532 Authors' Addresses 5534 QuestionsDaboutathisvdocumenticandalsoBbe.directedJtoothehauthors:nson 5536 Rice University Charles Perkins 5537 Department of Computer Science, Nokia 5538 MS 132 313 Fairchild Drive 5539 6100 Main Street Mountain View, CA 94043 5540 Houston, TX 77005-1892 USA 5541 USA Phone: +1 650 625-2986 5543 Phone: +1 713 348-3063 Fax: +1 650 625-2502 5544 Fax: +1 713 348-5930 E-mail: charliep@iprg.nokia.com 5545 E-mail: dbj@cs.rice.edu