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Miscellaneous warnings: ---------------------------------------------------------------------------- == The document seems to lack the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. (The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: Packets sent by a mobile node while away from home generally include a Home Address option. When any node receives a packet containing a Home Address option, it MUST process the option in a manner consistent with copying the Home Address field from the Home Address option into the IPv6 header, replacing the original value of the Source Address field there. However, any actual modifications to the Source Address field in the packet's IPv6 header MUST not be performed until after all processing of other options contained in this same Destination Options extension header is completed. -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (22 October 1999) is 8953 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. '1' -- Possible downref: Non-RFC (?) normative reference: ref. '2' ** Obsolete normative reference: RFC 2460 (ref. '5') (Obsoleted by RFC 8200) ** Obsolete normative reference: RFC 2267 (ref. '6') (Obsoleted by RFC 2827) ** Obsolete normative reference: RFC 2373 (ref. '7') (Obsoleted by RFC 3513) ** Obsolete normative reference: RFC 2402 (ref. '9') (Obsoleted by RFC 4302, RFC 4305) ** Obsolete normative reference: RFC 2406 (ref. '10') (Obsoleted by RFC 4303, RFC 4305) ** Obsolete normative reference: RFC 2401 (ref. '11') (Obsoleted by RFC 4301) ** Obsolete normative reference: RFC 2461 (ref. '14') (Obsoleted by RFC 4861) ** Obsolete normative reference: RFC 2002 (ref. '16') (Obsoleted by RFC 3220) -- Possible downref: Non-RFC (?) normative reference: ref. '18' ** Obsolete normative reference: RFC 793 (ref. '21') (Obsoleted by RFC 9293) ** Obsolete normative reference: RFC 1700 (ref. '22') (Obsoleted by RFC 3232) ** Obsolete normative reference: RFC 2462 (ref. '23') (Obsoleted by RFC 4862) Summary: 14 errors (**), 0 flaws (~~), 5 warnings (==), 5 comments (--). 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 Carnegie Mellon University 3 Charles Perkins 4 Nokia 5 22 October 1999 7 Mobility Support in IPv6 9 11 Status of This Memo 13 This document is an Internet-Draft and is in full conformance with 14 all provisions of Section 10 of RFC 2026. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note 18 that other groups may also distribute working documents as 19 Internet-Drafts. 21 Internet-Drafts are draft documents, valid for a maximum of six 22 months, and may be updated, replaced, or obsoleted by other documents 23 at any time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 Abstract 34 This document specifies the operation of mobile computers using IPv6. 35 Each mobile node is always identified by its home address, regardless 36 of its current point of attachment to the Internet. While situated 37 away from its home, a mobile node is also associated with a care-of 38 address, which provides information about the mobile node's current 39 location. IPv6 packets addressed to a mobile node's home address are 40 transparently routed to its care-of address. The protocol enables 41 IPv6 nodes to cache the binding of a mobile node's home address with 42 its care-of address, and to then send any packets destined for the 43 mobile node directly to it at this care-of address. To support this 44 operation, Mobile IPv6 defines four new IPv6 destination options, 45 including one that MUST be supported in packets received by any node, 46 whether mobile or stationary. 48 Contents 50 Status of This Memo i 52 Abstract i 54 1. Introduction 1 56 2. Comparison with Mobile IP for IPv4 3 58 3. Terminology 6 59 3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 6 60 3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 7 61 3.3. Specification Language . . . . . . . . . . . . . . . . . 8 63 4. Overview of Mobile IPv6 9 64 4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 9 65 4.2. New IPv6 Destination Options . . . . . . . . . . . . . . 11 66 4.3. Conceptual Data Structures . . . . . . . . . . . . . . . 13 67 4.4. Binding Management . . . . . . . . . . . . . . . . . . . 17 69 5. New IPv6 Destination Options 19 70 5.1. Binding Update Option Format . . . . . . . . . . . . . . 19 71 5.2. Binding Acknowledgement Option Format . . . . . . . . . . 23 72 5.3. Binding Request Option Format . . . . . . . . . . . . . . 27 73 5.4. Home Address Option Format . . . . . . . . . . . . . . . 29 74 5.5. Mobile IPv6 Destination Option Sub-Options . . . . . . . 31 76 6. Modifications to IPv6 Neighbor Discovery 34 77 6.1. Modified Router Advertisement Message Format . . . . . . 34 78 6.2. Modified Prefix Information Option Format . . . . . . . . 36 79 6.3. New Advertisement Interval Option Format . . . . . . . . 38 80 6.4. New Home Agent Information Option Format . . . . . . . . 39 81 6.5. Changes to Sending Router Advertisements . . . . . . . . 41 82 6.6. Changes to Sending Router Solicitations . . . . . . . . . 42 84 7. Requirements for IPv6 Nodes 44 85 7.1. Requirements for All IPv6 Hosts and Routers . . . . . . . 44 86 7.2. Requirements for All IPv6 Routers . . . . . . . . . . . . 44 87 7.3. Requirements for IPv6 Home Agents . . . . . . . . . . . . 44 88 7.4. Requirements for IPv6 Mobile Nodes . . . . . . . . . . . 45 90 8. Correspondent Node Operation 47 91 8.1. Receiving Packets from a Mobile Node . . . . . . . . . . 47 92 8.2. Receiving Binding Updates . . . . . . . . . . . . . . . . 47 93 8.3. Requests to Cache a Binding . . . . . . . . . . . . . . . 48 94 8.4. Requests to Delete a Binding . . . . . . . . . . . . . . 49 95 8.5. Sending Binding Acknowledgements . . . . . . . . . . . . 49 96 8.6. Sending Binding Requests . . . . . . . . . . . . . . . . 50 97 8.7. Cache Replacement Policy . . . . . . . . . . . . . . . . 50 98 8.8. Receiving ICMP Error Messages . . . . . . . . . . . . . . 51 99 8.9. Sending Packets to a Mobile Node . . . . . . . . . . . . 52 101 9. Home Agent Operation 54 102 9.1. Receiving Router Advertisement Messages . . . . . . . . . 54 103 9.2. Dynamic Home Agent Address Discovery . . . . . . . . . . 55 104 9.3. Primary Care-of Address Registration . . . . . . . . . . 57 105 9.4. Primary Care-of Address De-registration . . . . . . . . . 59 106 9.5. Intercepting Packets for a Mobile Node . . . . . . . . . 60 107 9.6. Tunneling Intercepted Packets to a Mobile Node . . . . . 62 108 9.7. Renumbering the Home Subnet . . . . . . . . . . . . . . . 63 110 10. Mobile Node Operation 67 111 10.1. Sending Packets While Away from Home . . . . . . . . . . 67 112 10.2. Interaction with Outbound IPsec Processing . . . . . . . 68 113 10.3. Receiving Packets While Away from Home . . . . . . . . . 69 114 10.4. Movement Detection . . . . . . . . . . . . . . . . . . . 71 115 10.5. Forming New Care-of Addresses . . . . . . . . . . . . . . 74 116 10.6. Sending Binding Updates to the Home Agent . . . . . . . . 75 117 10.7. Dynamic Home Agent Address Discovery . . . . . . . . . . 76 118 10.8. Sending Binding Updates to Correspondent Nodes . . . . . 76 119 10.9. Establishing Forwarding from a Previous Care-of Address . 79 120 10.10. Retransmitting Binding Updates . . . . . . . . . . . . . 80 121 10.11. Rate Limiting for Sending Binding Updates . . . . . . . . 80 122 10.12. Receiving Binding Acknowledgements . . . . . . . . . . . 81 123 10.13. Receiving Binding Requests . . . . . . . . . . . . . . . 82 124 10.14. Receiving ICMP Error Messages . . . . . . . . . . . . . . 82 125 10.15. Receiving Tunneled Router Advertisements . . . . . . . . 83 126 10.16. Using Multiple Care-of Addresses . . . . . . . . . . . . 84 127 10.17. Routing Multicast Packets . . . . . . . . . . . . . . . . 84 128 10.18. Returning Home . . . . . . . . . . . . . . . . . . . . . 85 130 11. Constants 86 132 12. IANA Considerations 87 134 13. Security Considerations 88 135 13.1. Binding Updates, Acknowledgements, and Requests . . . . . 88 136 13.2. Home Address Option . . . . . . . . . . . . . . . . . . . 88 137 13.3. General Mobile Computing Issues . . . . . . . . . . . . . 89 139 Changes from Previous Version of the Draft 91 141 Acknowledgements 93 143 References 94 145 Chair's Address 96 147 Authors' Addresses 97 148 1. Introduction 150 This document specifies the operation of mobile computers using 151 Internet Protocol Version 6 (IPv6) [5]. Without specific support 152 for mobility in IPv6, packets destined to a mobile node (host or 153 router) would not be able to reach it while the mobile node is away 154 from its home link (the link on which its home IPv6 subnet prefix is 155 in use), since routing is based on the subnet prefix in a packet's 156 destination IP address. In order to continue communication in spite 157 of its movement, a mobile node could change its IP address each time 158 it moves to a new link, but the mobile node would then not be able 159 to maintain transport and higher-layer connections when it changes 160 location. Mobility support in IPv6 is particularly important, as 161 mobile computers are likely to account for a majority or at least a 162 substantial fraction of the population of the Internet during the 163 lifetime of IPv6. 165 The protocol operation defined here, known as Mobile IPv6, allows a 166 mobile node to move from one link to another without changing the 167 mobile node's IP address. A mobile node is always addressable by 168 its "home address", an IP address assigned to the mobile node within 169 its home subnet prefix on its home link. Packets may be routed to 170 the mobile node using this address regardless of the mobile node's 171 current point of attachment to the Internet, and the mobile node may 172 continue to communicate with other nodes (stationary or mobile) after 173 moving to a new link. The movement of a mobile node away from its 174 home link is thus transparent to transport and higher-layer protocols 175 and applications. 177 The Mobile IPv6 protocol is just as suitable for mobility across 178 homogeneous media as for mobility across heterogeneous media. For 179 example, Mobile IPv6 facilitates node movement from one Ethernet 180 segment to another as well as it facilitates node movement from an 181 Ethernet segment to a wireless LAN cell, with the mobile node's IP 182 address remaining unchanged in spite of such movement. 184 One can think of the Mobile IPv6 protocol as solving the "macro" 185 mobility management problem. More "micro" mobility management 186 applications -- for example, handoff among wireless transceivers, 187 each of which covers only a very small geographic area -- are 188 possibly more suited to other solutions. For example, in many 189 current wireless LAN products, link-layer mobility mechanisms allow a 190 "handoff" of a mobile node from one cell to another, reestablishing 191 link-layer connectivity to the node in each new location. As long 192 as such handoff occurs only within cells of the mobile node's home 193 link, such link-layer mobility mechanisms are likely to offer faster 194 convergence and lower overhead than Mobile IPv6. Extensions to the 195 Mobile IPv6 protocol are also possible to support a more local, 196 hierarchical form of mobility management, but such extensions are 197 beyond the scope of this document. 199 The protocol specified in this document solves the problem of 200 transparently routing packets to and from mobile nodes while away 201 from home. However, it does not attempt to solve all general 202 problems related to the use of mobile computers or wireless networks. 203 In particular, this protocol does not attempt to solve: 205 - Handling links with partial reachability, such as typical 206 wireless networks. Some aspects of this problem are addressed 207 by the movement detection procedure described in Section 10.4, 208 but no attempt has been made to fully solve this problem in its 209 general form. Most aspects of this problem can be solved by the 210 workaround of restricting such networks to only one router per 211 link, although there are still possible hidden terminal problems 212 when two nodes on the same link (on opposite sides of the router) 213 attempt to communicate directly. 215 - Access control on a link being visited by a mobile node. This 216 is a general problem any time an untrusted node is allowed 217 to connect to any link layer. It is independent whether the 218 connecting node uses Mobile IP, DHCP [2], or just "borrows" an IP 219 address on the link. 221 2. Comparison with Mobile IP for IPv4 223 The design of Mobile IP support in IPv6 (Mobile IPv6) represents a 224 natural combination of the experiences gained from the development 225 of Mobile IP support in IPv4 (Mobile IPv4) [16, 15, 17], together 226 with the opportunities provided by the design and deployment of a new 227 version of IP itself (IPv6) and the new protocol features offered 228 by IPv6. Mobile IPv6 thus shares many features with Mobile IPv4, 229 but the protocol is now fully integrated into IP and provides many 230 improvements over Mobile IPv4. This section summarizes the major 231 differences between Mobile IPv4 and Mobile IPv6: 233 - Support for what is known in Mobile IPv4 as "Route 234 Optimization" [18] is now built in as a fundamental part 235 of the protocol, rather than being added on as a optional 236 set of extensions that may not be supported by all nodes 237 as in Mobile IPv4. This integration of Route Optimization 238 functionality allows direct routing from any correspondent node 239 to any mobile node, without needing to pass through the mobile 240 node's home network and be forwarded by its home agent, and thus 241 eliminates the problem of "triangle routing" present in the base 242 Mobile IPv4 protocol [16]. This integration also allows the 243 Mobile IPv4 "registration" functionality and the Mobile IPv4 244 Route Optimization functionality to be performed by a single 245 protocol rather than two separate (and different) protocols. 247 - Support is also integrated into Mobile IPv6 -- and into IPv6 248 itself -- for allowing mobile nodes and Mobile IP to coexist 249 efficiently with routers that perform "ingress filtering" [6]. A 250 mobile node now uses its care-of address as the Source Address in 251 the IP header of packets it sends, allowing the packets to pass 252 normally through ingress filtering routers. The home address 253 of the mobile node is carried in the packet in a Home Address 254 destination option, allowing the use of the care-of address in 255 the packet to be transparent above the IP layer. The ability 256 to correctly process a Home Address option in a received packet 257 is required in all IPv6 nodes, whether mobile nor stationary, 258 whether host or router. 260 - The use of the care-of address as the Source Address in each 261 packet's IP header also simplifies routing of multicast packets 262 sent by a mobile node. With Mobile IPv4, the mobile node 263 had to tunnel multicast packets to its home agent in order to 264 transparently use its home address as the source of the multicast 265 packets. With Mobile IPv6, the use of the Home Address option 266 allows the home address to be used but still be compatible with 267 multicast routing that is based in part on the packet's Source 268 Address. 270 - There is no longer any need to deploy special routers as 271 "foreign agents" as are used in Mobile IPv4. In Mobile IPv6, 272 mobile nodes make use of the enhanced features of IPv6, such 273 as Neighbor Discovery [14] and Address Autoconfiguration [23], 274 to operate in any location away from home without any special 275 support required from its local router. 277 - Unlike Mobile IPv4, Mobile IPv6 utilizes IP Security 278 (IPsec) [9, 10, 11] for all security requirements (sender 279 authentication, data integrity protection, and replay protection) 280 for Binding Updates (which serve the role of both registration 281 and Route Optimization in Mobile IPv4). Mobile IPv4 relies 282 on its own security mechanisms for these functions, based on 283 statically configured "mobility security associations". 285 - The movement detection mechanism in Mobile IPv6 provides 286 bidirectional confirmation of a mobile node's ability to 287 communicate with its default router in its current location 288 (packets that the router sends are reaching the mobile node, and 289 packets that the mobile node sends are reaching the router). 290 This confirmation provides a detection of the "black hole" 291 situation that may exist in some wireless environments where the 292 link to the router does not work equally well in both directions, 293 such as when the mobile node has moved out of good wireless 294 transmission range from the router. The mobile node may then 295 attempt to find a new router and begin using a new care-of 296 address if its link to its current router is not working well. 297 In contrast, in Mobile IPv4, only the forward direction (packets 298 from the router are reaching the mobile node) is confirmed, 299 allowing the black hole condition to persist. 301 - Most packets sent to a mobile node while away from home in 302 Mobile IPv6 are tunneled using an IPv6 Routing header rather than 303 IP encapsulation, whereas Mobile IPv4 must use encapsulation 304 for all packets. The use of a Routing header requires less 305 additional header bytes to be added to the packet, reducing the 306 overhead of Mobile IP packet delivery. To avoid modifying the 307 packet in flight, however, packets intercepted and tunneled 308 by a mobile node's home agent in Mobile IPv6 must still use 309 encapsulation for tunneling. 311 - While a mobile node is away from home, its home agent intercepts 312 any packets for the mobile node that arrive at the home network, 313 using IPv6 Neighbor Discovery [14] rather than ARP [19] as is 314 used in Mobile IPv4. The use of Neighbor Discovery improves 315 the robustness of the protocol (e.g., due to the Neighbor 316 Advertisement "override" bit) and simplifies implementation 317 of Mobile IP due to the ability to not be concerned with any 318 particular link layer as is required in ARP. 320 - The use of IPv6 encapsulation (and the Routing header) removes 321 the need in Mobile IPv6 to manage "tunnel soft state", which was 322 required in Mobile IPv4 due to limitations in ICMP for IPv4. Due 323 to the definition of ICMP for IPv6, the use of tunnel soft state 324 is no longer required in IPv6 for correctly relaying ICMP error 325 messages from within the tunnel back to the original sender of 326 the packet. 328 - The dynamic home agent address discovery mechanism in Mobile IPv6 329 uses IPv6 anycast [8] and returns a single reply to the mobile 330 node, rather than the corresponding Mobile IPv4 mechanism that 331 used IPv4 directed broadcast and returned a separate reply from 332 each home agent on the mobile node's home link. The Mobile IPv6 333 mechanism is more efficient and more reliable, since only 334 one packet need be sent back to the mobile node and since the 335 mobile node is less likely to lose one of the replies because no 336 "implosion" of replies is required by the protocol. 338 - Mobile IPv6 defines an Advertisement Interval option on 339 Router Advertisements (equivalent to Agent Advertisements in 340 Mobile IPv4), allowing a mobile node to decide for itself how 341 many Router Advertisements (Agent Advertisements) it is willing 342 to miss before declaring its current router unreachable. 344 - The use of IPv6 destination options allows all Mobile IPv6 345 control traffic to be piggybacked on any existing IPv6 packets, 346 whereas in Mobile IPv4 and its Route Optimization extensions, 347 separate UDP packets were required for each control message. 349 3. Terminology 351 3.1. General Terms 353 IP 355 Internet Protocol Version 6 (IPv6). 357 node 359 A device that implements IP. 361 router 363 A node that forwards IP packets not explicitly addressed to 364 itself. 366 host 368 Any node that is not a router. 370 link 372 A communication facility or medium over which nodes can 373 communicate at the link layer, such as an Ethernet (simple or 374 bridged). A link is the layer immediately below IP. 376 interface 378 A node's attachment to a link. 380 subnet prefix 382 A bit string that consists of some number of initial bits of an 383 IP address. 385 interface identifier 387 A number used to identify a node's interface on a link. The 388 interface identifier is the remaining low-order bits in the 389 node's IP address after the subnet prefix. 391 link-layer address 393 A link-layer identifier for an interface, such as IEEE 802 394 addresses on Ethernet links. 396 packet 398 An IP header plus payload. 400 3.2. Mobile IPv6 Terms 402 home address 404 An IP address assigned to a mobile node within its home link. 406 home subnet prefix 408 The IP subnet prefix corresponding to a mobile node's home 409 address. 411 home link 413 The link on which a mobile node's home subnet prefix is 414 defined. Standard IP routing mechanisms will deliver packets 415 destined for a mobile node's home address to its home link. 417 mobile node 419 A node that can change its point of attachment from one link to 420 another, while still being reachable via its home address. 422 movement 424 A change in a mobile node's point of attachment to the Internet 425 such that it is no longer connected to the same link as it was 426 previously. If a mobile node is not currently attached to its 427 home link, the mobile node is said to be "away from home". 429 correspondent node 431 A peer node with which a mobile node is communicating. The 432 correspondent node may be either mobile or stationary. 434 foreign subnet prefix 436 Any IP subnet prefix other than the mobile node's home subnet 437 prefix. 439 foreign link 441 Any link other than the mobile node's home link. 443 home agent 445 A router on a mobile node's home link with which the mobile 446 node has registered its current care-of address. While the 447 mobile node is away from home, the home agent intercepts 448 packets on the home link destined to the mobile node's home 449 address, encapsulates them, and tunnels them to the mobile 450 node's registered care-of address. 452 care-of address 454 An IP address associated with a mobile node while visiting a 455 foreign link; the subnet prefix of this IP address is a foreign 456 subnet prefix. Among the multiple care-of addresses that a 457 mobile node may have at a time (e.g., with different subnet 458 prefixes), the one registered with the mobile node's home agent 459 is called its "primary" care-of address. 461 binding 463 The association of the home address of a mobile node with a 464 care-of address for that mobile node, along with the remaining 465 lifetime of that association. 467 3.3. Specification Language 469 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 470 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 471 document are to be interpreted as described in RFC 2119 [3]. 473 4. Overview of Mobile IPv6 475 4.1. Basic Operation 477 A mobile node is always addressable by its home address, whether it 478 is currently attached to its home link or is away from home. While 479 a mobile node is at home, packets addressed to its home address are 480 routed to it using conventional Internet routing mechanisms in the 481 same way as if the node were never mobile. Since the subnet prefix 482 of a mobile node's home address is the subnet prefix (or one of the 483 subnet prefixes) on the mobile node's home link (it is the mobile 484 node's home subnet prefix), packets addressed to it will be routed to 485 its home link. 487 While a mobile node is attached to some foreign link away from home, 488 it is also addressable by one or more care-of addresses, in addition 489 to its home address. A care-of address is an IP address associated 490 with a mobile node while visiting a particular foreign link. The 491 subnet prefix of a mobile node's care-of address is the subnet prefix 492 (or one of the subnet prefixes) on the foreign link being visited by 493 the mobile node; if the mobile node is connected to this foreign link 494 while using that care-of address, packets addressed to this care-of 495 address will be routed to the mobile node in its location away from 496 home. 498 The association between a mobile node's home address and care-of 499 address is known as a "binding" for the mobile node. A mobile node 500 typically acquires its care-of address through stateless [23] or 501 stateful (e.g., DHCPv6 [2]) Address Autoconfiguration, according 502 to the methods of IPv6 Neighbor Discovery [14]. Other methods 503 of acquiring a care-of address are also possible, such as static 504 pre-assignment by the owner or manager of a particular foreign link, 505 but details of such other methods are beyond the scope of this 506 document. 508 While away from home, a mobile node registers one of its care-of 509 addresses with a router on its home link, requesting this router 510 to function as the "home agent" for the mobile node. This binding 511 registration is done by the mobile node sending to the home agent 512 a packet containing a "Binding Update" destination option; the 513 home agent then replies to the mobile node by returning a packet 514 containing a "Binding Acknowledgement" destination option. The 515 care-of address in this binding registered with its home agent is 516 known as the mobile node's "primary care-of address". The mobile 517 node's home agent thereafter uses proxy Neighbor Discovery to 518 intercept any IPv6 packets addressed to the mobile node's home 519 address (or home addresses) on the home link, and tunnels each 520 intercepted packet to the mobile node's primary care-of address. 521 To tunnel each intercepted packet, the home agent encapsulates the 522 packet using IPv6 encapsulation [4], with the outer IPv6 header 523 addressed to the mobile node's primary care-of address. 525 Section 10.16 discusses the reasons why it may be desirable for 526 a mobile node to use more than one care-of address at the same 527 time. However, a mobile node's primary care-of address is distinct 528 among these in that the home agent maintains only a single care-of 529 address registered for each mobile node, and always tunnels a mobile 530 node's packets intercepted from its home link to this mobile node's 531 registered primary care-of address. The home agent thus need not 532 implement any policy to determine which of possibly many care-of 533 addresses to which to tunnel each intercepted packet, leaving the 534 mobile node entirely in control of this policy by which of its 535 care-of addresses it registers with its home agent. 537 It is possible that while a mobile node is away from home, some nodes 538 on its home link may be reconfigured, such that the router that was 539 operating as the mobile node's home agent is replaced by a different 540 router serving this role. In this case, the mobile node may not 541 know the IP address of its own home agent. Mobile IPv6 provides a 542 mechanism, known as "dynamic home agent address discovery", that 543 allows a mobile node to dynamically discover the IP address of a home 544 agent on its home link with which it may register its care-of address 545 while away from home. The mobile node sends a Binding Update to the 546 "Mobile IPv6 Home-Agents" anycast address for its own home subnet 547 prefix [8] and thus reaches one of the (possibly many) routers on 548 its home link currently operating as a home agent. This home agent 549 rejects the mobile node's Binding Update, but returns in the Binding 550 Acknowledgement in response a list of all home agents on the home 551 link. This list of home agents is maintained by each home agent on 552 the home link through use of the Home Agent (H) bit in each home 553 agent's periodic unsolicited multicast Router Advertisements. 555 The Binding Update and Binding Acknowledgement destination options, 556 together with a "Binding Request" destination option, are also used 557 to allow IPv6 nodes communicating with a mobile node, to dynamically 558 learn and cache the mobile node's binding. When sending a packet 559 to any IPv6 destination, a node checks its cached bindings for an 560 entry for the packet's destination address. If a cached binding for 561 this destination address is found, the node uses an IPv6 Routing 562 header [5] (instead of IPv6 encapsulation) to route the packet to 563 the mobile node by way of the care-of address indicated in this 564 binding. If, instead, the sending node has no cached binding for 565 this destination address, the node sends the packet normally (with 566 no Routing header), and the packet is subsequently intercepted and 567 tunneled by the mobile node's home agent as described above. Any 568 node communicating with a mobile node is referred to in this document 569 as a "correspondent node" of the mobile node, and may itself be 570 either a stationary node or a mobile node. 572 Since a Binding Update, Binding Acknowledgement, and Binding Request 573 are each represented in a packet as an IPv6 destination option [5], 574 they may be included in any IPv6 packet. Any of these options can be 575 sent in either of two ways: 577 - A Binding Update, Binding Acknowledgement, or Binding Request can 578 be included within any IPv6 packet carrying any payload such as 579 TCP [21] or UDP [20]. 581 - A Binding Update, Binding Acknowledgement, or Binding Request can 582 be sent as a separate IPv6 packet containing no payload. In this 583 case, the Next Header field in the last extension header in the 584 packet is set to the value 59, to indicate "No Next Header" [5]. 586 Mobile IPv6 also defines one additional IPv6 destination option. 587 When a mobile node sends a packet while away from home, it will 588 generally set the Source Address in the packet's IPv6 header to one 589 of its current care-of addresses, and will also include a "Home 590 Address" destination option in the packet, giving the mobile node's 591 home address. Many routers implement security policies such as 592 "ingress filtering" [6] that do not allow forwarding of packets that 593 appear to have a Source Address that is not topologically correct. 594 By using the care-of address as the IPv6 header Source Address, 595 the packet will be able to pass normally through such routers, 596 yet ingress filtering rules will still be able to locate the true 597 topological source of the packet in the same way as packets from 598 non-mobile nodes. By also including the Home Address option in each 599 packet, the sending mobile node can communicate its home address to 600 the correspondent node receiving this packet, allowing the use of 601 the care-of address to be transparent above the Mobile IPv6 support 602 level (e.g., at the transport layer). The inclusion of a Home 603 Address option in a packet affects only the correspondent node's 604 receipt of this single packet; no state is created or modified in the 605 correspondent node as a result of receiving a Home Address option in 606 a packet. 608 4.2. New IPv6 Destination Options 610 As discussed in general in Section 4.1, the following four new IPv6 611 destination options are defined for Mobile IPv6: 613 Binding Update 615 A Binding Update option is used by a mobile node to notify 616 a correspondent node or the mobile node's home agent of 617 its current binding. The Binding Update sent to the mobile 618 node's home agent to register its primary care-of address is 619 marked as a "home registration". Any packet that includes a 620 Binding Update option MUST also include either an AH [9] or 621 ESP [10] header providing sender authentication, data integrity 622 protection, and replay protection. The Binding Update option 623 is described in detail in Section 5.1. 625 Binding Acknowledgement 627 A Binding Acknowledgement option is used to acknowledge receipt 628 of a Binding Update, if an acknowledgement was requested 629 in the Binding Update. Any packet that includes a Binding 630 Acknowledgement option MUST also include either an AH [9] or 631 ESP [10] header providing sender authentication, data integrity 632 protection, and replay protection. The Binding Acknowledgement 633 option is described in detail in Section 5.2. 635 Binding Request 637 A Binding Request option is used to request a mobile node to 638 send to the requesting node a Binding Update containing the 639 mobile node's current binding. This option is typically used 640 by a correspondent node to refresh a cached binding for a 641 mobile node, when the cached binding is in active use but the 642 binding's lifetime is close to expiration. No authentication 643 is required for the Binding Request option. The Binding 644 Request option is described in detail in Section 5.3. 646 Home Address 648 A Home Address option is used in a packet sent by a mobile 649 node to inform the recipient of that packet of the mobile 650 node's home address. For packets sent by a mobile node while 651 away from home, the mobile node generally uses one of its 652 care-of addresses as the Source Address in the packet's IPv6 653 header. By including a Home Address option in the packet, the 654 correspondent node receiving the packet is able to substitute 655 the mobile node's home address for this care-of address when 656 processing the packet, thus making the use of the care-of 657 address transparent to the correspondent node. If the IP 658 header of a packet carrying a Home Address option is covered 659 by authentication, then the Home Address option MUST also be 660 covered by this authentication, but no other authentication is 661 required for the Home Address option. The Home Address option 662 is described in detail in Section 5.4. 664 Sub-options within the format of these options MAY be included after 665 the fixed portion of the option data specified in this document. The 666 presence of such sub-options will be indicated by the Option Length 667 field within the option. When the Option Length is greater than the 668 length required for the option specified here, the remaining octets 669 are interpreted as sub-options. The encoding and format of defined 670 sub-options are described in Section 5.5. 672 4.3. Conceptual Data Structures 674 This document describes the Mobile IPv6 protocol in terms of the 675 following three conceptual data structures: 677 Binding Cache 679 A cache, maintained by each IPv6 node, of bindings for other 680 nodes. The Binding Cache MAY be implemented in any manner 681 consistent with the external behavior described in this 682 document, for example by being combined with the node's 683 Destination Cache as maintained by Neighbor Discovery [14]. 684 When sending a packet, the Binding Cache is searched before the 685 Neighbor Discovery conceptual Destination Cache [14] (i.e., any 686 Binding Cache entry for this destination SHOULD take precedence 687 over any Destination Cache entry for the same destination). 688 Each Binding Cache entry conceptually contains the following 689 fields: 691 - The home address of the mobile node for which this is the 692 Binding Cache entry. This field is used as the key for 693 searching the Binding Cache for the destination address of 694 a packet being sent. If the destination address of the 695 packet matches the home address in the Binding Cache entry, 696 this entry SHOULD be used in routing that packet. 698 - The care-of address for the mobile node indicated by 699 the home address field in this Binding Cache entry. If 700 the destination address of a packet being routed by a 701 node matches the home address in this entry, the packet 702 SHOULD be routed to this care-of address, as described in 703 Section 8.9, for packets originated by this node, or in 704 Section 9.6, if this node is the mobile node's home agent 705 and the packet was intercepted by it on the home link. 707 - A lifetime value, indicating the remaining lifetime 708 for this Binding Cache entry. The lifetime value is 709 initialized from the Lifetime field in the Binding Update 710 that created or last modified this Binding Cache entry. 711 Once the lifetime on this entry expires, the entry MUST be 712 deleted from the Binding Cache. 714 - A flag indicating whether or not this Binding Cache entry 715 is a "home registration" entry. 717 - A flag indicating whether or not this Binding Cache entry 718 represents a mobile node that should be advertised as a 719 router in proxy Neighbor Advertisements sent by this node 720 on its behalf. This flag is only valid if the Binding 721 Cache entry indicates that this is a "home registration" 722 entry. 724 - The value of the Prefix Length field received in the 725 Binding Update that created or last modified this Binding 726 Cache entry. This field is only valid if the "home 727 registration" flag is set on this Binding Cache entry. 729 - The maximum value of the Sequence Number field received 730 in previous Binding Updates for this mobile node home 731 address. The Sequence Number field is 16 bits long, and 732 all comparisons between Sequence Number values MUST be 733 performed modulo 2**16. 735 - Recent usage information for this Binding Cache entry, as 736 needed to implement the cache replacement policy in use in 737 the Binding Cache and to assist in determining whether a 738 Binding Request should be sent when the lifetime on this 739 entry nears expiration. 741 - The time at which a Binding Request was last sent for this 742 entry, as needed to implement the rate limiting restriction 743 for sending Binding Requests. 745 An entry in a node's Binding Cache for which the node is 746 serving as a home agent is marked as a "home registration" 747 entry and SHOULD NOT be deleted by the home agent until the 748 expiration of its binding lifetime. Other Binding Cache 749 entries MAY be replaced at any time by any reasonable local 750 cache replacement policy but SHOULD NOT be unnecessarily 751 deleted. Any node's Binding Cache may contain at most one 752 entry for each mobile node home address. The contents of a 753 node's Binding Cache MUST NOT be changed in response to a Home 754 Address option in a received packet. 756 Binding Update List 758 A list, maintained by each mobile node, recording information 759 for each Binding Update sent by this mobile node, for which 760 the Lifetime sent in that Binding Update has not yet expired. 761 The Binding Update List includes all bindings sent by the 762 mobile node: those to correspondent nodes, to the mobile 763 node's home agent, and to a previous default router of the 764 mobile node. The Binding Update List MAY be implemented in any 765 manner consistent with the external behavior described in this 766 document. Each Binding Update List entry conceptually contains 767 the following fields: 769 - The IP address of the node to which a Binding Update was 770 sent. This node might still have a Binding Cache entry 771 created or updated from this Binding Update, if the Binding 772 Update was successfully received by that node (e.g., not 773 lost by the network) and if that node has not deleted the 774 entry before its expiration (e.g., to reclaim space in its 775 Binding Cache for other entries). 777 - The home address for which that Binding Update was sent. 778 This will be one of the mobile node's home addresses for 779 most Binding Updates (Sections 10.6 and 10.8), but will 780 be the mobile node's previous care-of address for Binding 781 Updates sent to the mobile node's previous default router 782 (Section 10.9). 784 - The care-of address sent in that Binding Update. This 785 value is necessary for the mobile node to determine if it 786 has sent a Binding Update giving its new care-of address to 787 this destination after changing its care-of address. 789 - The remaining lifetime of that binding. This lifetime is 790 initialized from the Lifetime value sent in the Binding 791 Update and is decremented until it reaches zero, at which 792 time this entry MUST be deleted from the Binding Update 793 List. 795 - The maximum value of the Sequence Number field sent in 796 previous Binding Updates to this destination. The Sequence 797 Number field is 16 bits long, and all comparisons between 798 Sequence Number values MUST be performed modulo 2**16. 800 - The time at which a Binding Update was last sent to this 801 destination, as needed to implement the rate limiting 802 restriction for sending Binding Updates. 804 - The state of any retransmissions needed for this Binding 805 Update, if the Acknowledge (A) bit was set in this Binding 806 Update. This state includes the time remaining until the 807 next retransmission attempt for the Binding Update, and the 808 current state of the exponential back-off mechanism for 809 retransmissions. 811 - A flag that, when set, indicates that future Binding 812 Updates should not be sent to this destination. The 813 mobile node sets this flag in the Binding Update List 814 entry when it receives an ICMP Parameter Problem, Code 2, 815 error message in response to a Binding Update sent to that 816 destination, as described in Section 10.14. 818 Home Agents List 820 A list, maintained by each home agent, recording information 821 about each other home agent on a link on which this node 822 is serving as a home agent; each home agent maintains a 823 separate Home Agents List for each such link on which it is 824 serving. This list is used in the dynamic home agent address 825 discovery mechanism. The information for the list is learned 826 through receipt of the periodic unsolicited multicast Router 827 Advertisements from each other home agent on the link, in which 828 the Home Agent (H) bit is set, in a manner similar to the 829 Default Router List conceptual data structure maintained by 830 each host for Neighbor Discovery [14]. The Home Agents List 831 MAY be implemented in any manner consistent with the external 832 behavior described in this document. Each Home Agents List 833 entry conceptually contains the following fields: 835 - The link-local IP address of another router on the home 836 link that this node currently believes is operating as 837 a home agent for this link. A new entry is created or 838 an existing entry is updated in the Home Agents List in 839 response to receipt of a valid Router Advertisement in 840 which the Home Agent (H) bit is set. The link-local 841 address of the home agent is learned through the Source 842 Address of the Router Advertisements received from it [14]. 844 - One or more global IP addresses for this home agent, 845 learned through Prefix Information options with the 846 Router Address (R) bit is set, received in Router 847 Advertisements from this link-local address. Global 848 addresses for the router in a Home Agents List entry MUST 849 be deleted once the prefix associated with that address is 850 no longer valid [14]. 852 - The remaining lifetime of this Home Agents List entry. If 853 a Home Agent Information Option is present in a Router 854 Advertisement received from a home agent, the lifetime of 855 the Home Agents List entry representing this home agent 856 is initialized from the Home Agent Lifetime field in the 857 option; otherwise, the lifetime is initialized from the 858 Router Lifetime field in the received Router Advertisement. 859 The Home Agents List entry lifetime is decremented until it 860 reaches zero, at which time this entry MUST be deleted from 861 the Home Agents List. 863 - The preference for this home agent, for use in ordering the 864 Home Agents List returned in a Binding Acknowledgement; 865 higher values indicate a more preferable home agent. The 866 preference value is taken from the Home Agent Preference 867 field (a signed, twos-complement integer) in the received 868 Router Advertisement, if the Router Advertisement contains 869 a Home Agent Information Option, and is otherwise set to 870 the default value of 0. 872 4.4. Binding Management 874 When a mobile node configures a new care-of address and decides to 875 use this new address as its primary care-of address, the mobile 876 node registers this new binding with its home agent by sending 877 the home agent a Binding Update. The mobile node indicates 878 that an acknowledgement is needed for this Binding Update and 879 continues to periodically retransmit it until acknowledged. The 880 home agent acknowledges the Binding Update by returning a Binding 881 Acknowledgement to the mobile node. 883 When a mobile node receives a packet tunneled to it from its 884 home agent, the mobile node assumes that the original sending 885 correspondent node has no Binding Cache entry for the mobile node, 886 since the correspondent node would otherwise have sent the packet 887 directly to the mobile node using a Routing header. The mobile node 888 thus returns a Binding Update to the correspondent node, allowing 889 it to cache the mobile node's binding for routing future packets to 890 it. Although the mobile node may request an acknowledgement for 891 this Binding Update, it need not, since subsequent packets from the 892 correspondent node will continue to be intercepted and tunneled by 893 the mobile node's home agent, effectively causing any needed Binding 894 Update retransmission. 896 A correspondent node with a Binding Cache entry for a mobile node 897 may refresh this binding, for example if the binding's lifetime 898 is near expiration, by sending a Binding Request to the mobile 899 node. Normally, a correspondent node will only refresh a Binding 900 Cache entry in this way if it is actively communicating with the 901 mobile node and has indications, such as an open TCP connection to 902 the mobile node, that it will continue this communication in the 903 future. When a mobile node receives a Binding Request, it replies by 904 returning a Binding Update to the node sending the Binding Request. 906 A mobile node may use more than one care-of address at the same 907 time, although only one care-of address may be registered for it at 908 its home agent as its primary care-of address. The mobile node's 909 home agent will tunnel all intercepted packets for the mobile node 910 to its (single) registered primary care-of address, but the mobile 911 node will accept packets that it receives at any of its current 912 care-of addresses. Use of more than one care-of address by a mobile 913 node may be useful, for example, to improve smooth handoff when the 914 mobile node moves from one wireless link to another. If each of 915 these wireless links is connected to the Internet through a separate 916 base station, such that the wireless transmission range from the 917 two base stations overlap, the mobile node may be able to remain 918 connected to both links while in the area of overlap. In this case, 919 the mobile node could acquire a new care-of address on the new link 920 before moving out of transmission range and disconnecting from the 921 old link. The mobile node may thus still accept packets at its 922 old care-of address while it works to update its home agent and 923 correspondent nodes, notifying them of its new care-of address on the 924 new link. 926 Since correspondent nodes cache bindings, it is expected that 927 correspondent nodes usually will route packets directly to the mobile 928 node's care-of address, so that the home agent is rarely involved 929 with packet transmission to the mobile node. This is essential for 930 scalability and reliability, and for minimizing overall network load. 931 By caching the care-of address of a mobile node, optimal routing of 932 packets can be achieved from the correspondent node to the mobile 933 node. Routing packets directly to the mobile node's care-of address 934 also eliminates congestion at the mobile node's home agent and home 935 link. In addition, the impact of any possible failure of the home 936 agent, the home link, or intervening networks leading to or from the 937 home link is reduced, since these nodes and links are not involved in 938 the delivery of most packets to the mobile node. 940 5. New IPv6 Destination Options 942 5.1. Binding Update Option Format 944 The Binding Update destination option is used by a mobile node 945 to notify other nodes of a new care-of address for itself. As a 946 destination option, it MAY be included in any existing packet being 947 sent to this same destination or MAY be sent in a packet by itself; 948 a packet containing a Binding Update is sent in the same way as any 949 packet sent by a mobile node (Section 10.1). 951 The Binding Update option is encoded in type-length-value (TLV) 952 format as follows: 954 0 1 2 3 955 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 1 956 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 957 | Option Type | Option Length | 958 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 959 |A|H|R| Reserved| Prefix Length | Sequence Number | 960 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 961 | Lifetime | 962 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 963 | Sub-Options... 964 +-+-+-+-+-+-+-+-+-+-+-+- 966 Option Type 968 198 = 0xC6 970 Option Length 972 8-bit unsigned integer. Length of the option, in octets, 973 excluding the Option Type and Option Length fields. This field 974 MUST be set to 8 plus the total length of all sub-options 975 present, including their Sub-Option Type and Sub-Option Len 976 fields. 978 Acknowledge (A) 980 The Acknowledge (A) bit is set by the sending mobile node to 981 request a Binding Acknowledgement (Section 5.2) be returned 982 upon receipt of the Binding Update. 984 Home Registration (H) 986 The Home Registration (H) bit is set by the sending mobile node 987 to request the receiving node to act as this node's home agent. 988 The destination of the packet carrying this option MUST be that 989 of a router sharing the same subnet prefix as the home address 990 of the mobile node in the binding (given by the Home Address 991 field in the Home Address option in the packet). 993 Router (R) 995 The Router (R) bit, when set, indicates that the sending 996 mobile node is a router. This bit is only valid when the 997 Home Registration (H) bit is also set, and MUST NOT be set 998 otherwise. This bit is saved in the home agent's "home 999 registration" Binding Cache entry for the mobile node, and 1000 is copied into the corresponding bit in all proxy Neighbor 1001 Advertisement messages sent on behalf of this mobile node by 1002 the home agent using this Binding Cache entry. 1004 Reserved 1006 Reserved. Sent as 0; ignored on reception. 1008 Prefix Length 1010 The Prefix Length field is valid only for a "home registration" 1011 Binding Update. This field MUST be zero if the Home 1012 Registration (H) bit is not set in the Binding Update. The 1013 Prefix Length field is set by the sending mobile node to the 1014 (nonzero) length of its subnet prefix in its home address 1015 (given in the Home Address option in the packet) to request 1016 its home agent to use the interface identifier in the mobile 1017 node's home address (the remaining low-order bits after the 1018 indicated subnet prefix) to form all other home addresses for 1019 the mobile node on the home link. The home agent becomes the 1020 home agent not only for the individual home address given in 1021 this binding, but also for all other home addresses for this 1022 mobile node formed from this interface identifier. That is, 1023 for each on-link prefix on the home link, the home agent uses 1024 the interface identifier to form other valid addresses for the 1025 mobile node on the home link, and acts as a home agent also 1026 for those addresses. In addition, the home agent forms the 1027 link-local address and site-local address corresponding to 1028 this interface identifier, and defends each for purposes of 1029 Duplicate Address Detection. Details of this operation are 1030 described in Section 9.3. 1032 Sequence Number 1034 Used by the receiving node to sequence Binding Updates and by 1035 the sending node to match a returned Binding Acknowledgement 1036 with this Binding Update. Each Binding Update sent by a mobile 1037 node MUST use a Sequence Number greater than the Sequence 1038 Number value sent in the previous Binding Update (if any) to 1039 the same destination address (modulo 2**16). There is no 1040 requirement, however, that the Sequence Number value strictly 1041 increase by 1 with each new Binding Update sent or received. 1043 Lifetime 1045 32-bit unsigned integer. The number of seconds remaining 1046 before the binding must be considered expired. A value of all 1047 one bits (0xffffffff) indicates infinity. A value of zero 1048 indicates that the Binding Cache entry for the mobile node 1049 should be deleted. 1051 Sub-Options 1053 Additional information, associated with this Binding Update 1054 option, that need not be present in all Binding Updates sent. 1055 This use of sub-options also allows for future extensions to 1056 the format of the Binding Update option to be defined. The 1057 encoding and format of defined sub-options are described in 1058 Section 5.5. The following sub-options are valid in a Binding 1059 Update option: 1061 - Unique Identifier Sub-Option 1063 - Alternate Care-of Address Sub-Option 1065 The alignment requirement [5] for the Binding Update option is 4n+2. 1067 Any packet that includes a Binding Update option MUST also include 1068 a Home Address option. The home address of the mobile node in the 1069 binding given in the Binding Update option is indicated by the Home 1070 Address field in the Home Address option in the packet. 1072 The care-of address for the binding given in the Binding Update 1073 option is normally specified by the Source Address field in the IPv6 1074 header of the packet carrying the Binding Update option. However, a 1075 care-of address different from the Source Address MAY be specified 1076 by including an Alternate Care-of Address sub-option in the Binding 1077 Update option. 1079 Any packet that includes a Binding Update option MUST also include 1080 either an AH [9] or ESP [10] header providing sender authentication, 1081 data integrity protection, and replay protection. 1083 If the care-of address for the binding (specified either in an 1084 Alternate Care-of Address sub-option in the Binding Update option, if 1085 present, or in the Source Address field in the packet's IPv6 header) 1086 is equal to the home address of the mobile node, the Binding Update 1087 option indicates that any existing binding for the mobile node MUST 1088 be deleted. Likewise, if the Lifetime field in the Binding Update 1089 option is equal to 0, the Binding Update option indicates that any 1090 existing binding for the mobile node MUST be deleted. In each of 1091 these cases, a Binding Cache entry for the mobile node MUST NOT be 1092 created in response to receiving the Binding Update. 1094 The last Sequence Number value sent to a destination in a Binding 1095 Update is stored by the mobile node in its Binding Update List entry 1096 for that destination; the last Sequence Number value received from 1097 a mobile node in a Binding Update is stored by a correspondent node 1098 in its Binding Cache entry for that mobile node. Thus, the mobile 1099 node's and the correspondent node's knowledge of the last sequence 1100 number expire at the same time. If the sending mobile node has no 1101 Binding Update List entry, the Sequence Number may start at any 1102 value; if the receiving correspondent node has no Binding Cache entry 1103 for the sending mobile node, it MUST accept any Sequence Number value 1104 in a received Binding Update from this mobile node. 1106 The three highest-order bits of the Option Type are encoded to 1107 indicate specific processing of the option [5]. For the Binding 1108 Update option, these three bits are set to 110, indicating that any 1109 IPv6 node processing this option that does not recognize the Option 1110 Type must discard the packet and, only if the packet's Destination 1111 Address was not a multicast address, return an ICMP Parameter 1112 Problem, Code 2, message to the packet's Source Address; and that the 1113 data within the option cannot change en-route to the packet's final 1114 destination. 1116 5.2. Binding Acknowledgement Option Format 1118 The Binding Acknowledgement destination option is used to acknowledge 1119 receipt of a Binding Update option (Section 5.1). When a node 1120 receives a packet containing a Binding Update option, with this 1121 node being the destination of the packet (only the destination node 1122 processes the option since it is a destination option), this node 1123 MUST return a Binding Acknowledgement to the source of the packet, 1124 if the Acknowledge (A) bit is set in the Binding Update. As a 1125 destination option, this node MAY include the Binding Acknowledgement 1126 in any existing packet being sent to the mobile node or MAY send it 1127 in a packet by itself. A packet containing a Binding Acknowledgement 1128 is sent in the same way as any packet to a mobile node, using a 1129 Routing header to route the packet to the mobile node by way of the 1130 care-of address in the binding (Section 8.9). 1132 The Binding Acknowledgement option is encoded in type-length-value 1133 (TLV) format as follows: 1135 0 1 2 3 1136 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 1137 +-+-+-+-+-+-+-+-+ 1138 | Option Type | 1139 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1140 | Option Length | Status | Sequence Number | 1141 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1142 | Lifetime | 1143 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1144 | Refresh | 1145 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1146 | Sub-Options... 1147 +-+-+-+-+-+-+-+-+-+-+-+- 1149 Option Type 1151 7 1153 Option Length 1155 8-bit unsigned integer. Length of the option, in octets, 1156 excluding the Option Type and Option Length fields. This field 1157 MUST be set to 11 plus the total length of all sub-options 1158 present, including their Sub-Option Type and Sub-Option Len 1159 fields. 1161 Status 1163 8-bit unsigned integer indicating the disposition of the 1164 Binding Update. Values of the Status field less than 128 1165 indicate that the Binding Update was accepted by the receiving 1166 node. The following such Status values are currently defined: 1168 0 Binding Update accepted 1170 Values of the Status field greater than or equal to 128 1171 indicate that the Binding Update was rejected by the receiving 1172 node. The following such Status values are currently defined: 1174 128 Reason unspecified 1175 130 Administratively prohibited 1176 131 Insufficient resources 1177 132 Home registration not supported 1178 133 Not home subnet 1179 135 Dynamic home agent address discovery response 1180 136 Incorrect interface identifier length 1181 137 Not home agent for this mobile node 1183 Up-to-date values of the Status field are to be specified in 1184 the most recent "Assigned Numbers" [22]. 1186 Sequence Number 1188 The Sequence Number in the Binding Acknowledgement is copied 1189 from the Sequence Number field in the Binding Update being 1190 acknowledged, for use by the mobile node in matching this 1191 Acknowledgement with an outstanding Binding Update. 1193 Lifetime 1195 The granted lifetime, in seconds, for which this node will 1196 attempt to retain the entry for this mobile node in its Binding 1197 Cache. If the node sending the Binding Acknowledgement is 1198 serving as the mobile node's home agent, the Lifetime period 1199 also indicates the period for which this node will continue 1200 this service; if the mobile node requires home agent service 1201 from this node beyond this period, the mobile node MUST send a 1202 new Binding Update to it before the expiration of this period 1203 (even if it is not changing its primary care-of address), in 1204 order to extend the lifetime. The value of this field is 1205 undefined if the Status field indicates that the Binding Update 1206 was rejected. 1208 Refresh 1210 The recommended interval, in seconds, at which the mobile 1211 node SHOULD send a new Binding Update to this node in order 1212 to "refresh" the mobile node's binding in this node's Binding 1213 Cache. This refreshing of the binding is useful in case the 1214 node fails and loses its cache state. The Refresh period is 1215 determined by the node sending the Binding Acknowledgement 1216 (the node caching the binding). If this node is serving as 1217 the mobile node's home agent, the Refresh value may be set, 1218 for example, based on whether the node stores its Binding 1219 Cache in volatile storage or in nonvolatile storage. If the 1220 node sending the Binding Acknowledgement is not serving as the 1221 mobile node's home agent, the Refresh period SHOULD be set 1222 equal to the Lifetime period in the Binding Acknowledgement; 1223 even if this node loses this cache entry due to a failure of 1224 the node, packets from it can still reach the mobile node 1225 through the mobile node's home agent, causing a new Binding 1226 Update to this node to allow it to recreate this cache entry. 1227 The value of this field is undefined if the Status field 1228 indicates that the Binding Update was rejected. 1230 Sub-Options 1232 Additional information, associated with this Binding 1233 Acknowledgement option, that need not be present in all Binding 1234 Acknowledgements sent. This use of sub-options also allows for 1235 future extensions to the format of the Binding Acknowledgement 1236 option to be defined. The encoding and format of defined 1237 sub-options are described in Section 5.5. The following 1238 sub-options are valid in a Binding Acknowledgement option: 1240 - Home Agents List Sub-Option 1242 The alignment requirement [5] for the Binding Acknowledgement option 1243 is 4n+3. 1245 Any packet that includes a Binding Acknowledgement option MUST 1246 also include either an AH [9] or ESP [10] header providing sender 1247 authentication, data integrity protection, and replay protection. 1249 If the node returning the Binding Acknowledgement accepted the 1250 Binding Update for which the Acknowledgement is being returned (the 1251 value of the Status field in the Acknowledgement is less than 128), 1252 this node will have an entry for the mobile node in its Binding Cache 1253 and MUST use this entry (which includes the care-of address received 1254 in the Binding Update) in sending the packet containing the Binding 1255 Acknowledgement to the mobile node. The details of sending this 1256 packet to the mobile node are the same as for sending any packet to a 1257 mobile node using a binding, and are described in Section 8.9. The 1258 packet is sent using a Routing header, routing the packet to the 1259 mobile node by way of its care-of address recorded in the Binding 1260 Cache entry. 1262 If the node returning the Binding Acknowledgement instead 1263 rejected the Binding Update (the value of the Status field in the 1264 Acknowledgement is greater than or equal to 128), this node MUST 1265 similarly use a Routing header in sending the packet containing the 1266 Binding Acknowledgement, as described in Section 8.9, but MUST NOT 1267 use its Binding Cache in forming the IP header or Routing header 1268 in this packet. Rather, the care-of address used by this node in 1269 sending the packet containing the Binding Acknowledgement MUST be 1270 copied from the care-of address received in the rejected Binding 1271 Update; this node MUST NOT modify its Binding Cache in response 1272 to receiving this rejected Binding Update and MUST ignore its 1273 Binding Cache in sending the packet in which it returns this Binding 1274 Acknowledgement. The packet is sent using a Routing header, routing 1275 the packet to the home address of the rejected Binding Update by 1276 way of the care-of address indicated in the packet containing the 1277 Binding Update. When sending a Binding Acknowledgement to reject a 1278 Binding Update, the Binding Acknowledgement MUST be sent in an IPv6 1279 packet containing no payload (with the Next Header field in the last 1280 extension header in the packet set to indicate "No Next Header" [5]). 1282 The three highest-order bits of the Option Type are encoded to 1283 indicate specific processing of the option [5]. For the Binding 1284 Acknowledgement option, these three bits are set to 000, indicating 1285 that any IPv6 node processing this option that does not recognize the 1286 Option Type must skip over this option and continue processing the 1287 header, and that the data within the option cannot change en-route to 1288 the packet's final destination. 1290 5.3. Binding Request Option Format 1292 The Binding Request destination option is used to request a mobile 1293 node's binding from the mobile node. As a destination option, it 1294 MAY be included in any existing packet being sent to the mobile 1295 node or MAY be sent in a packet by itself; a packet containing a 1296 Binding Request option is sent in the same way as any packet to a 1297 mobile node (Section 8.9). When a mobile node receives a packet 1298 containing a Binding Request option, it SHOULD return a Binding 1299 Update (Section 5.1) to the source of the Binding Request. 1301 The Binding Request option is encoded in type-length-value (TLV) 1302 format as follows: 1304 0 1 1305 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1306 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 1307 | Option Type | Option Length | Sub-Options... 1308 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 1310 Option Type 1312 8 1314 Option Length 1316 8-bit unsigned integer. Length of the option, in octets, 1317 excluding the Option Type and Option Length fields. This field 1318 MUST be set to 0 plus the total length of all sub-options 1319 present, including their Sub-Option Type and Sub-Option Len 1320 fields. 1322 Sub-Options 1324 Additional information, associated with this Binding Request 1325 option, that need not be present in all Binding Requests sent. 1326 This use of sub-options also allows for future extensions to 1327 the format of the Binding Request option to be defined. The 1328 encoding and format of defined sub-options are described in 1329 Section 5.5. The following sub-options are valid in a Binding 1330 Request option: 1332 - Unique Identifier Sub-Option 1334 There is no requirement for alignment [5] of the Binding Request 1335 option. 1337 The three highest-order bits of the Option Type are encoded to 1338 indicate specific processing of the option [5]. For the Binding 1339 Request option, these three bits are set to 000, indicating that any 1340 IPv6 node processing this option that does not recognize the Option 1341 Type must skip over this option and continue processing the header, 1342 and that the data within the option cannot change en-route to the 1343 packet's final destination. 1345 5.4. Home Address Option Format 1347 The Home Address destination option is used in a packet sent by a 1348 mobile node while away from home, to inform the recipient of that 1349 packet of the mobile node's home address. For packets sent by a 1350 mobile node while away from home, the mobile node generally uses 1351 one of its care-of addresses as the Source Address in the packet's 1352 IPv6 header. By including a Home Address option in the packet, the 1353 correspondent node receiving the packet is able to substitute the 1354 mobile node's home address for this care-of address when processing 1355 the packet, thus making the use of the care-of address transparent to 1356 the correspondent node. 1358 The Home Address option is encoded in type-length-value (TLV) format 1359 as follows: 1361 0 1 2 3 1362 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 1363 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1364 | Option Type | Option Length | 1365 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1366 | | 1367 + + 1368 | | 1369 + Home Address + 1370 | | 1371 + + 1372 | | 1373 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1374 | Sub-Options... 1375 +-+-+-+-+-+-+-+-+-+-+-+- 1377 Option Type 1379 201 = 0xC9 1381 Option Length 1383 8-bit unsigned integer. Length of the option, in octets, 1384 excluding the Option Type and Option Length fields. This field 1385 MUST be set to 16 plus the total length of all sub-options 1386 present, including their Sub-Option Type and Sub-Option Len 1387 fields. 1389 Home Address 1391 The home address of the mobile node sending the packet. 1393 Sub-Options 1395 Additional information, associated with this Home Address 1396 option, that need not be present in all Home Address options 1397 sent. This use of sub-options also allows for future 1398 extensions to the format of the Home Address option to be 1399 defined. The encoding and format of defined sub-options are 1400 described in Section 5.5. Currently, no valid sub-options are 1401 defined for use in a Home Address option. 1403 The alignment requirement [5] for the Home Address option is 8n+6. 1405 The inclusion of a Home Address option in a packet affects the 1406 receiving node's processing of only this single packet; no state is 1407 created or modified in the receiving node as a result of receiving a 1408 Home Address option in a packet. In particular, the presence of a 1409 Home Address option in a received packet MUST NOT alter the contents 1410 of the receiver's Binding Cache and MUST NOT cause any changes in the 1411 routing of subsequent packets sent by this receiving node. 1413 No authentication of the Home Address option is required, except that 1414 if the IPv6 header of a packet is covered by authentication, then 1415 that authentication MUST also cover the Home Address option; this 1416 coverage is achieved automatically by the definition of the Option 1417 Type code for the Home Address option, since it indicates that the 1418 data within the option cannot change en-route to the packet's final 1419 destination, and thus the option is included in the authentication 1420 computation. If the packet carries no IP authentication, then the 1421 contents of the Home Address option, as well as the Source Address 1422 field or any other field in the IPv6 header, may have been forged or 1423 altered during transit. 1425 Upon receipt of a packet containing a Home Address option, the 1426 receiving node replaces the Source Address in the IPv6 header with 1427 the Home Address in the Home Address option. By requiring that any 1428 authentication of the IPv6 header also cover the Home Address option, 1429 the security of the Source Address field in the IPv6 header is not 1430 compromised by the presence of a Home Address option. Security 1431 issues related to the Home Address option are discussed further in 1432 Section 13. 1434 The three highest-order bits of the Option Type are encoded to 1435 indicate specific processing of the option [5]. For the Home Address 1436 option, these three bits are set to 110, indicating that any IPv6 1437 node processing this option that does not recognize the Option Type 1438 must discard the packet and, only if the packet's Destination Address 1439 was not a multicast address, return an ICMP Parameter Problem, 1440 Code 2, message to the packet's Source Address; and that the data 1441 within the option cannot change en-route to the packet's final 1442 destination. 1444 5.5. Mobile IPv6 Destination Option Sub-Options 1446 In order to allow optional fields that may not be needed in most uses 1447 of any given Mobile IPv6 destination option, and to allow future 1448 extensions to the format of these destination options to be defined, 1449 any of the Mobile IPv6 destination options defined in this document 1450 MAY include one or more sub-options. 1452 Such sub-options are included in the data portion of the destination 1453 option itself, after the fixed portion of the option data specified 1454 for that particular destination option (Sections 5.1 through 5.4). 1455 The presence of such sub-options will be indicated by the Option 1456 Length field. When the Option Length is greater than the standard 1457 length defined for that destination option, the remaining octets are 1458 interpreted as sub-options. 1460 These sub-options are encoded within the remaining space of the 1461 option data for that option, using a type-length-value (TLV) format 1462 as follows: 1464 0 1 2 3 1465 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 1466 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1467 |Sub-Option Type| Sub-Option Len| Sub-Option Data... 1468 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1470 Sub-Option Type 1472 8-bit identifier of the type of sub-option. In processing a 1473 Mobile IPv6 destination option containing a sub-option for 1474 which the Sub-Option Type value is not recognized by the 1475 receiver, the receiver SHOULD quietly ignore and skip over the 1476 sub-option, correctly handling any remaining sub-options in the 1477 option. 1479 Sub-Option Length 1481 8-bit unsigned integer. Length of the Sub-Option Data field 1482 of this sub-option, in octets. The Sub-Option Len does not 1483 include the length of the Sub-Option Type and Sub-Option Len 1484 fields. 1486 Sub-Option Data 1488 Variable-length field. Sub-Option-Type-specific data. 1490 As with IPv6 options appearing in a Hop-by-Hop Options header 1491 or Destination Options header [5], individual sub-options within 1492 a Mobile IPv6 destination option may have specific alignment 1493 requirements, to ensure that multi-octet values within Sub-Option 1494 Data fields fall on natural boundaries. The alignment requirement 1495 of a sub-option is specified using the same notation as is used to 1496 specify alignment requirements for IPv6 options [5]. 1498 Each section above defining the Mobile IPv6 destination options 1499 specifies which of the defined sub-options is valid for that 1500 destination option. In addition, there are two padding sub-options, 1501 Pad1 and PadN (defined below), which are used when necessary to align 1502 subsequent sub-options. The Pad1 and PadN sub-options are valid for 1503 all Mobile IPv6 destination options. Unlike the padding options 1504 used in Hop-by-Hop Options header or Destination Options header [5], 1505 there is no requirement for padding the total size of any Mobile IPv6 1506 destination option to a multiple of 8 octets in length, and the 1507 Pad1 and PadN sub-options SHOULD NOT be used for this purpose. All 1508 Mobile IPv6 sub-options defined in this document MUST be recognized 1509 by all Mobile IPv6 implementations. 1511 Currently, the following sub-option types are defined for use in 1512 Mobile IPv6 destination options: 1514 Pad1 Sub-Option (alignment requirement: none) 1516 0 1517 0 1 2 3 4 5 6 7 1518 +-+-+-+-+-+-+-+-+ 1519 | 0 | 1520 +-+-+-+-+-+-+-+-+ 1522 NOTE! the format of the Pad1 sub-option is a special 1523 case -- it does not have Sub-Option Len and Sub-Option Data 1524 fields. 1526 The Pad1 sub-option is used to insert one octet of padding 1527 into the Sub-Options area of a Mobile IPv6 option. If more 1528 than one octet of padding is required, the PadN sub-option, 1529 described next, should be used, rather than multiple Pad1 1530 sub-options. 1532 PadN Sub-Option (alignment requirement: none) 1534 0 1 1535 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1536 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 1537 | 1 | Sub-Option Len| Sub-Option Data 1538 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 1540 The PadN sub-option is used to insert two or more octets of 1541 padding into the Sub-Options area of a Mobile IPv6 option. 1542 For N octets of padding, the Sub-Option Len field contains 1543 the value N-2, and the Sub-Option Data consists of N-2 1544 zero-valued octets. 1546 Unique Identifier Sub-Option (alignment requirement: 2n) 1548 0 1 2 3 1549 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 1550 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1551 | 2 | 2 | Unique Identifier | 1552 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1554 The Unique Identifier sub-option is valid only in Binding 1555 Request and Binding Update destination options. The Unique 1556 Identifier field contains a 16-bit value that serves to 1557 uniquely identify a Binding Request among those sent by this 1558 Source Address, and to allow the Binding Update to identify 1559 the specific Binding Request to which it responds. This 1560 matching of Binding Updates to Binding Requests is required 1561 in the procedure for renumbering the home subnet while a 1562 mobile node is away from home (Section 9.7). 1564 Home Agents List Sub-Option (alignment requirement: 8n+6) 1566 0 1 2 3 1567 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 1568 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1569 | 3 | 16 * N | 1570 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1571 | | 1572 + + 1573 . Home Agent Addresses . 1574 . (N = number of addresses present) . 1575 . . 1576 + + 1577 | | 1578 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1580 The Home Agents List sub-option is valid only in a Binding 1581 Acknowledgement destination option. The Home Agents 1582 Addresses field contains a list of N addresses of home 1583 agents on the home link for the mobile node to which this 1584 Binding Acknowledgement is sent. This sub-option MUST NOT 1585 be present unless the Binding Acknowledgement is sent in 1586 response to a "Mobile IPv6 Home-Agents" anycast Binding 1587 Update sent by a mobile node attempting dynamic home agent 1588 address discovery [8]. In this case, the Status field in 1589 the Binding Acknowledgement MUST be set to 135 (dynamic 1590 home agent address discovery response). The specific 1591 construction of the Home Agent Addresses field for this 1592 sub-option is defined in Section 9.2. 1594 Alternate Care-of Address Sub-Option (alignment requirement: 8n+6) 1596 0 1 2 3 1597 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 1598 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1599 | 4 | 16 | 1600 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1601 | | 1602 | Alternate Care-of Addresses | 1603 | | 1604 | | 1605 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1607 The Alternate Care-of Address sub-option is valid only in 1608 Binding Update destination options. The Alternate Care-of 1609 Address field contains an address to use as the care-of 1610 address for the binding, rather than using the Source 1611 Address of the packet as the care-of address. 1613 6. Modifications to IPv6 Neighbor Discovery 1615 6.1. Modified Router Advertisement Message Format 1617 Mobile IPv6 modifies the format of the Router Advertisement 1618 message [14] by the addition of a single flag bit for use in the 1619 dynamic home agent address discovery mechanism (Sections 9.2 1620 and 10.7). The format of the Router Advertisement message is 1621 as follows: 1623 0 1 2 3 1624 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 1625 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1626 | Type | Code | Checksum | 1627 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1628 | Cur Hop Limit |M|O|H| Reserved| Router Lifetime | 1629 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1630 | Reachable Time | 1631 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1632 | Retrans Timer | 1633 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1634 | Options ... 1635 +-+-+-+-+-+-+-+-+-+-+-+- 1637 This format represents the following changes over that originally 1638 specified for Neighbor Discovery [14]: 1640 Home Agent (H) 1642 The Home Agent (H) bit is set in a Router Advertisement to 1643 indicate that the router sending this Router Advertisement is 1644 also functioning as a Mobile IP home agent. 1646 Reserved 1648 Reduced from a 6-bit field to a 5-bit field to account for the 1649 addition of the Home Agent (H) bit. 1651 6.2. Modified Prefix Information Option Format 1653 Mobile IPv6 requires knowledge of a router's global address for two 1654 reasons: 1656 - To allow a home agent (a router) to learn the address of all 1657 other home agents on the link for which it is providing home 1658 agent service, for use in building its Home Agents List as 1659 part of the dynamic home agent address discovery mechanism 1660 (Sections 9.2 and 10.7). 1662 - To allow a mobile node to send a Binding Update to its previous 1663 default router, after moving to a new subnet and acquiring a new 1664 care-of address (Section 10.9). 1666 However, Neighbor Discovery [14] only advertises a router's 1667 link-local address, by requiring this address to be used as the IP 1668 Source Address of each Router Advertisement. 1670 Mobile IPv6 extends Neighbor Discovery to allow a router to easily 1671 and efficiently advertise its global address, by the addition of a 1672 single flag bit in the format of a Prefix Information option for 1673 use in Router Advertisement messages. The format of the Prefix 1674 Information option is as follows: 1676 0 1 2 3 1677 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 1678 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1679 | Type | Length | Prefix Length |L|A|R|Reserved1| 1680 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1681 | Valid Lifetime | 1682 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1683 | Preferred Lifetime | 1684 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1685 | Reserved2 | 1686 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1687 | | 1688 + + 1689 | | 1690 + Prefix + 1691 | | 1692 + + 1693 | | 1694 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1696 This format represents the following changes over that originally 1697 specified for Neighbor Discovery [14]: 1699 Router Address (R) 1701 1-bit router address flag. When set, indicates that the 1702 Prefix field, in addition to advertising the indicated prefix, 1703 contains a complete IP address assigned to the sending router. 1704 This router IP address has the same scope and conforms to the 1705 same lifetime values as the advertised prefix. This use of 1706 the Prefix field is compatible with its use in advertising 1707 the prefix itself, since prefix advertisement uses only the 1708 leading number Prefix bits specified by the Prefix Length 1709 field. Interpretation of this flag bit is thus independent 1710 of the processing required for the On-Link (L) and Autonomous 1711 Address-Configuration (A) flag bits. 1713 Reserved1 1715 Reduced from a 6-bit field to a 5-bit field to account for the 1716 addition of the Router Address (R) bit. 1718 In a solicited Router Advertisement, a router MUST include at least 1719 one Prefix Information option with the Router Address (R) bit set. 1720 Neighbor Discovery specifies that, if including all options in a 1721 Router Advertisement causes the size of the Advertisement to exceed 1722 the link MTU, multiple Advertisements can be sent, each containing 1723 a subset of the options [14]. In this case, at least one of these 1724 multiple Advertisements being sent instead of a single larger 1725 solicited Advertisement, MUST include a Prefix Information option 1726 with the Router Address (R) bit set. 1728 All routers SHOULD include at least one Prefix Information option 1729 with the Router Address (R) bit set, in each unsolicited multicast 1730 Router Advertisement that they send. If multiple Advertisements 1731 are being sent instead of a single larger unsolicited multicast 1732 Advertisement, at least one of these multiple Advertisements SHOULD 1733 include a Prefix Information option with the Router Address (R) bit 1734 set. 1736 6.3. New Advertisement Interval Option Format 1738 Mobile IPv6 defines a new Advertisement Interval option, used in 1739 Router Advertisement messages to advertise the interval at which the 1740 sending router sends unsolicited multicast Router Advertisements. 1741 The format of the Advertisement Interval option is as follows: 1743 0 1 2 3 1744 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 1745 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1746 | Type | Length | Reserved | 1747 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1748 | Advertisement Interval | 1749 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1751 Type 1753 7 1755 Length 1757 8-bit unsigned integer. The length of the option (including 1758 the type and length fields) in units of 8 octets. The value of 1759 this field MUST be 1. 1761 Reserved 1763 This field is unused. It MUST be initialized to zero by the 1764 sender and MUST be ignored by the receiver. 1766 Advertisement Interval 1768 32-bit unsigned integer. The maximum time, in milliseconds, 1769 between successive unsolicited router Router Advertisement 1770 messages sent by this router on this network interface. Using 1771 the conceptual router configuration variables defined by 1772 Neighbor Discovery [14], this field MUST be equal to the value 1773 MaxRtrAdvInterval, expressed in milliseconds. 1775 Routers MAY include this option in their Router Advertisements. A 1776 mobile node receiving a Router Advertisement containing this option 1777 SHOULD utilize the specified Advertisement Interval for that router 1778 in its movement detection algorithm, as described in Section 10.4. 1780 This option MUST be silently ignored for other Neighbor Discovery 1781 messages. 1783 6.4. New Home Agent Information Option Format 1785 Mobile IPv6 defines a new Home Agent Information option, used in 1786 Router Advertisement messages sent by a home agent to advertise 1787 information specific to this router's functionality as a home agent. 1788 The format of the Home Agent Information option is as follows: 1790 0 1 2 3 1791 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 1792 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1793 | Type | Length | Reserved | 1794 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1795 | Home Agent Preference | Home Agent Lifetime | 1796 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1798 Type 1800 8 1802 Length 1804 8-bit unsigned integer. The length of the option (including 1805 the type and length fields) in units of 8 octets. The value of 1806 this field MUST be 1. 1808 Reserved 1810 This field is unused. It MUST be initialized to zero by the 1811 sender and MUST be ignored by the receiver. 1813 Home Agent Preference 1815 16-bit signed, twos-complement integer. The preference for 1816 the home agent sending this Router Advertisement, for use 1817 in ordering the addresses contained in the Home Agents List 1818 Sub-Option returned in a Binding Acknowledgement; higher values 1819 mean more preferable. If this option is not included in a 1820 Router Advertisement in which the Home Agent (H) bit is set, 1821 the preference value for this home agent SHOULD be considered 1822 to be 0. Values greater than 0 indicate a home agent more 1823 preferable than this default value, and values less than 0 1824 indicate a less preferable home agent. In addition to the 1825 manual configuration of the Home Agent Preference value as 1826 described in Section 7.3, the Home Agent Preference sent by 1827 a home agent could, for example, be set dynamically by the 1828 sending home agent based on the number of mobile nodes it is 1829 currently serving or on its remaining resources for serving 1830 additional mobile nodes, but such dynamic settings are beyond 1831 the scope of this document. 1833 Home Agent Lifetime 1835 16-bit unsigned integer. The lifetime associated with the home 1836 agent in units of seconds. The maximum value corresponds to 1837 18.2 hours. A value of 0 MUST NOT be used. The Home Agent 1838 Lifetime applies only to this router's usefulness as a home 1839 agent; it does not apply to information contained in other 1840 message fields or options. If this option is not included in 1841 a Router Advertisement in which the Home Agent (H) bit is set, 1842 the lifetime for this home agent SHOULD be considered to be the 1843 same as the Router Lifetime specified in the main body of the 1844 Router Advertisement message. 1846 Home agents MAY include this option in their Router Advertisements. 1847 This option MUST NOT be included in a Router Advertisement in which 1848 the Home Agent (H) bit (Section 6.1) is not set. 1850 This option MUST be silently ignored for other Neighbor Discovery 1851 messages. 1853 If both the Home Agent Preference and Home Agent Lifetime are set 1854 to their default values specified above, this option SHOULD NOT be 1855 included in the Router Advertisement messages sent by this home 1856 agent. 1858 6.5. Changes to Sending Router Advertisements 1860 The Neighbor Discovery protocol specification [14] limits routers to 1861 a minimum interval of 3 seconds between sending unsolicited multicast 1862 Router Advertisement messages from any given network interface 1863 (limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that: 1865 "Routers generate Router Advertisements frequently enough 1866 that hosts will learn of their presence within a few 1867 minutes, but not frequently enough to rely on an absence 1868 of advertisements to detect router failure; a separate 1869 Neighbor Unreachability Detection algorithm provides failure 1870 detection." 1872 This limitation, however, is not suitable to providing timely 1873 movement detection for mobile nodes. Mobile nodes detect their 1874 own movement by learning the presence of new routers as the mobile 1875 node moves into wireless transmission range of them (or physically 1876 connects to a new wired network), and by learning that previous 1877 routers are no longer reachable. Mobile nodes MUST be able to 1878 quickly detect when they move to a link served by a new router, so 1879 that they can acquire a new care-of address and send Binding Updates 1880 to register this care-of address with their home agent and to notify 1881 correspondent nodes as needed. 1883 Thus, to provide good support for mobile nodes, Mobile IPv6 relaxes 1884 this limit such that routers MAY send unsolicited multicast Router 1885 Advertisements more frequently. In particular, on network interfaces 1886 where the router is expecting to provide service to visiting mobile 1887 nodes (e.g., wireless network interfaces), or on which it is serving 1888 as a home agent to one or more mobile nodes (who may return home and 1889 need to hear its Advertisements), the home agent SHOULD be configured 1890 with a smaller MinRtrAdvInterval value and MaxRtrAdvInterval value, 1891 to allow sending of unsolicited multicast Router Advertisements more 1892 often. Recommended values for these limits are: 1894 - MinRtrAdvInterval 0.5 seconds 1896 - MaxRtrAdvInterval 1.5 seconds 1898 Use of these modified limits MUST be configurable, and specific 1899 knowledge of the type of network interface in use SHOULD be taken 1900 into account in configuring these limits for each network interface. 1902 When sending unsolicited multicast Router Advertisements more 1903 frequently than the standard limit on unsolicited multicast 1904 Advertisement frequency, the sending router need not include all 1905 options in each of these Advertisements, but it SHOULD include at 1906 least one Prefix Information option with the Router Address (R) bit 1907 set (Section 6.2) in each. 1909 6.6. Changes to Sending Router Solicitations 1911 In addition to the limit on routers sending unsolicited multicast 1912 Router Advertisement messages (Section 6.5), Neighbor Discovery 1913 defines limits on nodes sending Router Solicitation messages, such 1914 that a node SHOULD send no more than 3 Router Solicitations, and that 1915 these 3 transmissions SHOULD be spaced at least 4 seconds apart. 1916 However, these limits prevent a mobile node from finding a new 1917 default router (and thus a new care-of address) quickly as it moves 1918 about. 1920 Mobile IPv6 relaxes this limit such that, while a mobile node is away 1921 from home, it MAY send Router Solicitations more frequently. The 1922 following limits for sending Router Solicitations are recommended for 1923 mobile nodes while away from home: 1925 - A mobile node that is not configured with any current care-of 1926 address (e.g., the mobile node has moved since its previous 1927 care-of address was configured), MAY send more than the defined 1928 Neighbor Discovery limit of MAX_RTR_SOLICITATIONS Router 1929 Solicitations. 1931 - The rate at which a mobile node sends Router Solicitations MUST 1932 be limited, although a mobile node MAY send Router Solicitations 1933 more frequently than the defined Neighbor Discovery limit of 1934 RTR_SOLICITATION_INTERVAL seconds. The minimum interval MUST 1935 be configurable, and specific knowledge of the type of network 1936 interface in use SHOULD be taken into account in configuring this 1937 limit for each network interface. A recommended minimum interval 1938 is 1 second. 1940 - After sending at most MAX_RTR_SOLICITATIONS Router Solicitations, 1941 a mobile node MUST reduce the rate at which it sends subsequent 1942 Router Solicitations. Subsequent Router Solicitations SHOULD 1943 be sent using a binary exponential backoff mechanism, doubling 1944 the interval between consecutive Router Solicitations, up to a 1945 maximum interval. The maximum interval MUST be configurable and 1946 SHOULD be chosen appropriately based on the characteristics of 1947 the type of network interface in use. 1949 - While still searching for a new default router and care-of 1950 address, a mobile node MUST NOT increase the rate at which it 1951 sends Router Solicitations unless it has received a positive 1952 indication (such as from lower network layers) that it has moved 1953 to a new link. After successfully acquiring a new care-of 1954 address, the mobile node SHOULD also increase the rate at which 1955 it will send Router Solicitations when it next begins searching 1956 for a new default router and care-of address. 1958 - A mobile node that is currently configured with a care-of 1959 address SHOULD NOT send Router Solicitations, until its movement 1960 detection algorithm (Section 10.4) determines that it has moved 1961 and that its current care-of address might no longer be valid. 1963 7. Requirements for IPv6 Nodes 1965 Mobile IPv6 places some special requirements on the functions 1966 provided by different types of IPv6 nodes. This section summarizes 1967 those requirements, identifying the functionality each requirement 1968 is intended to support. Further details on this functionality is 1969 provided in the following sections. 1971 7.1. Requirements for All IPv6 Hosts and Routers 1973 Since any IPv6 node may at any time be a correspondent node of a 1974 mobile node, either sending a packet to a mobile node or receiving a 1975 packet from a mobile node, the following requirements apply to ALL 1976 IPv6 nodes (whether host or router, whether mobile or stationary): 1978 - Every IPv6 node MUST be able to process a Home Address option 1979 received in any IPv6 packet. 1981 - Every IPv6 node SHOULD be able to process a Binding Update option 1982 received in a packet, and to return a Binding Acknowledgement 1983 option if the Acknowledge (A) bit is set in the received Binding 1984 Update. 1986 - Every IPv6 node SHOULD be able to maintain a Binding Cache of the 1987 bindings received in accepted Binding Updates. 1989 7.2. Requirements for All IPv6 Routers 1991 The following requirements apply to all IPv6 routers, even those not 1992 serving as a home agent for Mobile IPv6: 1994 - Every IPv6 router SHOULD be able to send an Advertisement 1995 Interval option in its Router Advertisements, to aid movement 1996 detection by mobile nodes. The use of this option in Router 1997 Advertisements MUST be configurable. 1999 - Every IPv6 router SHOULD be able to support sending unsolicited 2000 multicast Router Advertisements at the faster rate described in 2001 Section 6.5. The use of this faster rate MUST be configurable. 2003 7.3. Requirements for IPv6 Home Agents 2005 In order for a mobile node to operate correctly while away from home, 2006 at least one IPv6 router on the mobile node's home link must function 2007 as a home agent for the mobile node. The following additional 2008 requirements apply to all IPv6 routers capable of serving as a home 2009 agent: 2011 - Every home agent MUST be able to maintain an entry in its Binding 2012 Cache for each mobile node for which it is serving as the home 2013 agent. Each such Binding Cache entry records the mobile node's 2014 binding with its primary care-of address and is marked as a "home 2015 registration". 2017 - Every home agent MUST be able to intercept packets (using proxy 2018 Neighbor Discovery) addressed to a mobile node for which it is 2019 currently serving as the home agent, on that mobile node's home 2020 link, while the mobile node is away from home. 2022 - Every home agent MUST be able to encapsulate such intercepted 2023 packets in order to tunnel them to the primary care-of address 2024 for the mobile node indicated in its binding in the home agent's 2025 Binding Cache. 2027 - Every home agent MUST be able to return a Binding Acknowledgement 2028 option in response to a Binding Update option received with the 2029 Acknowledge (A) bit set. 2031 - Every home agent MUST be able to accept packets addressed 2032 to the "Mobile IPv6 Home-Agents" anycast address for the 2033 subnet on which it is serving as a home agent [8], and MUST be 2034 able to participate in dynamic home agent address discovery 2035 (Section 9.2). 2037 - Every home agent SHOULD support a configuration mechanism to 2038 allow a system administrator to manually set the value to be sent 2039 by this home agent in the Home Agent Preference field of the Home 2040 Agent Information Option in Router Advertisements that it sends. 2042 7.4. Requirements for IPv6 Mobile Nodes 2044 Finally, the following requirements apply to all IPv6 nodes capable 2045 of functioning as mobile nodes: 2047 - Every IPv6 mobile node MUST be able to perform IPv6 2048 decapsulation [4]. 2050 - Every IPv6 mobile node MUST support sending Binding Update 2051 options, as specified in Sections 10.6, 10.8, and 10.9; and MUST 2052 be able to receive and process Binding Acknowledgement options, 2053 as specified in Section 10.12. 2055 - Every IPv6 mobile node MUST support use of the dynamic home agent 2056 address discovery mechanism, as described in Section 10.7. 2058 - Every IPv6 mobile node MUST maintain a Binding Update List in 2059 which it records the IP address of each other node to which it 2060 has sent a Binding Update, for which the Lifetime sent in that 2061 binding has not yet expired. 2063 - Every IPv6 mobile node MUST support receiving a Binding Request 2064 option, by responding with a Binding Update option. 2066 - Every IPv6 mobile node MUST support sending packets containing a 2067 Home Address option; this option MUST be included in all packets 2068 sent while away from home, if the packet would otherwise have 2069 been sent with the mobile node's home address as the IP Source 2070 Address. 2072 8. Correspondent Node Operation 2074 A correspondent node is any node communicating with a mobile node. 2075 The correspondent node, itself, may be stationary or mobile, and may 2076 possibly also be functioning as a home agent for Mobile IPv6. The 2077 procedures in this section thus apply to all IPv6 nodes. 2079 8.1. Receiving Packets from a Mobile Node 2081 Packets sent by a mobile node while away from home generally include 2082 a Home Address option. When any node receives a packet containing 2083 a Home Address option, it MUST process the option in a manner 2084 consistent with copying the Home Address field from the Home Address 2085 option into the IPv6 header, replacing the original value of the 2086 Source Address field there. However, any actual modifications to 2087 the Source Address field in the packet's IPv6 header MUST not be 2088 performed until after all processing of other options contained in 2089 this same Destination Options extension header is completed. 2091 Further processing of such a packet after option processing (e.g., 2092 at the transport layer) thus need not know that the original Source 2093 Address was a care-of address, or that the Home Address option was 2094 used in the packet. Since the sending mobile node uses its home 2095 address at the transport layer when sending such a packet, the use of 2096 the care-of address and Home Address option is transparent to both 2097 the mobile node and the correspondent node above the level of the 2098 Home Address option generation and processing. 2100 8.2. Receiving Binding Updates 2102 Upon receiving a Binding Update option in some packet, the receiving 2103 node MUST validate the Binding Update according to the following 2104 tests: 2106 - The packet contains a valid AH [9] or ESP [10] header that 2107 provides sender authentication, integrity protection, and replay 2108 protection. 2110 - The packet MUST contain a valid Home Address option. The home 2111 address for the binding is specified by the Home Address field of 2112 the Home Address option. 2114 - The Option Length field in the Binding Update option is greater 2115 than or equal to the length specified in Section 5.1. 2117 - The Sequence Number field in the Binding Update option is greater 2118 than the Sequence Number received in the previous Binding Update 2119 for this home address, if any. The Sequence Number comparison is 2120 performed modulo 2**16. 2122 Any Binding Update not satisfying all of these tests MUST be 2123 silently ignored, and the packet carrying the Binding Update MUST be 2124 discarded. 2126 If the Binding Update is valid according to the tests above, then the 2127 Binding Update is processed further as follows: 2129 - If the Destination Address in the packet's IPv6 header is the 2130 "Mobile IPv6 Home-Agents" anycast address [8] for an on-link 2131 prefix and this address is assigned to one of this node's network 2132 interfaces, then the mobile node sending this Binding Update is 2133 attempting dynamic home agent address discovery. Processing for 2134 this type of received Binding Update is described in Section 9.2. 2135 (If the Destination Address is not assigned to one of this node's 2136 network interfaces, then the packet would have been forwarded as 2137 a normal packet and the Binding Update, as a destination option, 2138 would not be processed in any way by this node.) 2140 - If the Lifetime specified in the Binding Update is nonzero and 2141 the specified Care-of Address is not equal to the home address 2142 for the binding (as given in the Home Address option in the 2143 packet), then this is a request to cache a binding for the 2144 mobile node. If the Home Registration (H) bit is set in the 2145 Binding Update, the Binding Update is processed according to the 2146 procedure specified in Section 9.3; otherwise, it is processed 2147 according to the procedure specified in Section 8.3. 2149 - If the Lifetime specified in the Binding Update is zero or the 2150 specified Care-of Address matches the home address for the 2151 binding, then this is a request to delete the mobile node's 2152 cached binding. If the Home Registration (H) bit is set in the 2153 Binding Update, the Binding Update is processed according to the 2154 procedure specified in Section 9.4; otherwise, it is processed 2155 according to the procedure specified in Section 8.4. 2157 8.3. Requests to Cache a Binding 2159 When a node receives a Binding Update, it MUST validate it and 2160 determine the type of Binding Update according to the steps described 2161 in Section 8.2. This section describes the processing of a valid 2162 Binding Update that requests a node to cache a mobile node's binding, 2163 for which the Home Registration (H) bit is not set in the Binding 2164 Update. 2166 In this case, the receiving node SHOULD create a new entry in its 2167 Binding Cache for this mobile node (or update its existing Binding 2168 Cache entry for this mobile node, if such an entry already exists). 2169 The home address of the mobile node is taken from the Home Address 2170 field in the packet's Home Address option. The new Binding Cache 2171 entry records the association between this home address and the 2172 care-of address for the binding, as specified in either the Care-of 2173 Address field of the Binding Update or in the Source Address field 2174 in the packet's IPv6 header. Any Binding Cache entry created or 2175 updated in response to processing this Binding Update MUST be deleted 2176 after the expiration of the Lifetime period specified in the Binding 2177 Update. 2179 8.4. Requests to Delete a Binding 2181 When a node receives a Binding Update, it MUST validate it and 2182 determine the type of Binding Update according to the steps described 2183 in Section 8.2. This section describes the processing of a valid 2184 Binding Update that requests a node to delete a mobile node's binding 2185 from its Binding Cache, for which the Home Registration (H) bit is 2186 not set in the Binding Update. 2188 In this case, the receiving node MUST delete any existing entry in 2189 its Binding Cache for this mobile node. The home address of the 2190 mobile node is taken from the Home Address field in the packet's Home 2191 Address option. 2193 8.5. Sending Binding Acknowledgements 2195 When any node receives a packet containing a Binding Update option 2196 in which the Acknowledge (A) bit is set, it SHOULD return a Binding 2197 Acknowledgement option acknowledging receipt of the Binding Update. 2198 If the node accepts the Binding Update and creates or updates an 2199 entry in its Binding Cache for this binding, the Status field in 2200 the Binding Acknowledgement MUST be set to a value less than 128; 2201 if the node rejects the Binding Update and does not create or 2202 update an entry for this binding, the Status field in the Binding 2203 Acknowledgement MUST be set to a value greater than or equal to 128. 2204 Specific values for the Status field are described in Section 5.2 and 2205 in the most recent "Assigned Numbers" [22]. 2207 As described in Section 5.2, the packet in which the Binding 2208 Acknowledgement is returned MUST include either an AH [9] or ESP [10] 2209 header providing sender authentication, data integrity protection, 2210 and replay protection; and the packet MUST be sent using a Routing 2211 header in the same way as any other packet sent to a mobile node 2212 using a care-of address (even if the binding was rejected), as 2213 described in Section 8.9. The packet is routed first to the care-of 2214 address contained in the Binding Update being acknowledged, and 2215 then to the mobile node's home address. This use of the Routing 2216 header ensures that the Binding Acknowledgement will be routed to the 2217 current location of the node sending the Binding Update, whether the 2218 Binding Update was accepted or rejected. 2220 8.6. Sending Binding Requests 2222 Entries in a node's Binding Cache MUST be deleted when their lifetime 2223 expires. If such an entry is still in active use in sending packets 2224 to a mobile node, the next packet sent to the mobile node will be 2225 routed normally to the mobile node's home link, where it will be 2226 intercepted and tunneled to the mobile node. The mobile node will 2227 then return a Binding Update to the sender, allowing it to create 2228 a new Binding Cache entry for sending future packets to the mobile 2229 node. Communication with the mobile node continues uninterrupted, 2230 but the forwarding of this packet through the mobile node's home 2231 agent creates additional overhead and latency in delivering packets 2232 to the mobile node. 2234 If the sender knows that the Binding Cache entry is still in active 2235 use, it MAY send a Binding Request option to the mobile node in 2236 an attempt to avoid this overhead and latency due to deleting and 2237 recreating the Binding Cache entry. Since a Binding Request is a 2238 destination option, it may, for example, be included in any packet 2239 already being sent to the mobile node, such as a packet that is part 2240 of ongoing TCP communication with the mobile node. When the mobile 2241 node receives a packet from some sender containing a Binding Request 2242 option, it returns a Binding Update option to that sender, giving its 2243 current binding and a new lifetime. 2245 8.7. Cache Replacement Policy 2247 Any entry in a node's Binding Cache MUST be deleted after the 2248 expiration of the Lifetime specified in the Binding Update from 2249 which the entry was created or last updated. Conceptually, a node 2250 maintains a separate timer for each entry in its Binding Cache. When 2251 creating or updating a Binding Cache entry in response to a received 2252 and accepted Binding Update, the node sets the timer for this entry 2253 to the specified Lifetime period. When a Binding Cache entry's timer 2254 expires, the node deletes the entry. 2256 Each node's Binding Cache will, by necessity, have a finite size. 2257 A node MAY use any reasonable local policy for managing the space 2258 within its Binding Cache, except that any entry marked as a "home 2259 registration" (Section 9.3) MUST NOT be deleted from the cache until 2260 the expiration of its lifetime period. When attempting to add a 2261 new "home registration" entry in response to a Binding Update with 2262 the Home Registration (H) bit set, if insufficient space exists (or 2263 can be reclaimed) in the node's Binding Cache, the node MUST reject 2264 the Binding Update and SHOULD return a Binding Acknowledgement to 2265 the sending mobile node, in which the Status field is set to 131 2266 (insufficient resources). When otherwise attempting to add a new 2267 entry to its Binding Cache, a node MAY, if needed, choose to drop any 2268 entry already in its Binding Cache, other than a "home registration" 2269 entry, in order to make space for the new entry. For example, a 2270 "least-recently used" (LRU) strategy for cache entry replacement 2271 among entries not marked as a "home registration" is likely to work 2272 well. 2274 Any binding dropped from a node's Binding Cache due to lack of cache 2275 space will be rediscovered and a new cache entry created, if the 2276 binding is still in active use by the node for sending packets. If 2277 the node sends a packet to a destination for which it has dropped the 2278 entry from its Binding Cache, the packet will be routed normally, 2279 leading to the mobile node's home link. There, the packet will be 2280 intercepted by the mobile node's home agent and tunneled to the 2281 mobile node's current primary care-of address. As when a Binding 2282 Cache entry is initially created, this indirect routing to the mobile 2283 node through its home agent will result in the mobile node sending 2284 a Binding Update to this sending node when it receives the tunneled 2285 packet, allowing it to add an entry again for this destination mobile 2286 node to its Binding Cache. 2288 8.8. Receiving ICMP Error Messages 2290 When a correspondent node sends a packet to a mobile node, if the 2291 correspondent node has a Binding Cache entry for the destination 2292 address of the packet, then the correspondent node uses a Routing 2293 header to deliver the packet to the mobile node through the care-of 2294 address in the binding recorded in the Binding Cache entry. Any ICMP 2295 error message caused by the packet on its way to the mobile node will 2296 be returned normally to the correspondent node. 2298 On the other hand, if the correspondent node has no Binding Cache 2299 entry for the mobile node, the packet will be routed to the mobile 2300 node's home link. There, it will be intercepted by the mobile node's 2301 home agent, encapsulated, and tunneled to the mobile node's primary 2302 care-of address. Any ICMP error message caused by the packet on 2303 its way to the mobile node while in the tunnel, will be returned to 2304 the mobile node's home agent (the source of the tunnel). By the 2305 definition of IPv6 encapsulation [4], this encapsulating node MUST 2306 relay certain ICMP error messages back to the original sender of the 2307 packet, which in this case is the correspondent node. 2309 Likewise, if a packet for a mobile node arrives at the mobile node's 2310 previous default router (e.g., the mobile node moved after the packet 2311 was sent), the router will encapsulate and tunnel the packet to the 2312 mobile node's new care-of address (if it has a Binding Cache entry 2313 for the mobile node). As above, any ICMP error message caused by the 2314 packet while in this tunnel will be returned to the previous default 2315 router (the source of the tunnel), which MUST relay certain ICMP 2316 error messages back to the correspondent node [4]. 2318 Thus, in all cases, any meaningful ICMP error messages caused 2319 by packets from a correspondent node to a mobile node will be 2320 returned to the correspondent node. If the correspondent node 2321 receives persistent ICMP Destination Unreachable messages after 2322 sending packets to a mobile node based on an entry in its Binding 2323 Cache, the correspondent node SHOULD delete this Binding Cache 2324 entry. If the correspondent node subsequently transmits another 2325 packet to the mobile node, the packet will be routed to the mobile 2326 node's home link, intercepted by the mobile node's home agent, and 2327 tunneled to the mobile node's primary care-of address using IPv6 2328 encapsulation. The mobile node will then return a Binding Update to 2329 the correspondent node, allowing it to recreate a (correct) Binding 2330 Cache entry for the mobile node. 2332 8.9. Sending Packets to a Mobile Node 2334 Before sending any packet, the sending node SHOULD examine its 2335 Binding Cache for an entry for the destination address to which the 2336 packet is being sent. If the sending node has a Binding Cache entry 2337 for this address, the sending node SHOULD use a Routing header to 2338 route the packet to this mobile node (the destination node) by way 2339 of the care-of address in the binding recorded in that Binding Cache 2340 entry. For example, assuming use of a Type 0 Routing header [5], if 2341 no other use of a Routing header is involved in the routing of this 2342 packet, the mobile node sets the fields in the packet's IPv6 header 2343 and Routing header as follows: 2345 - The Destination Address in the packet's IPv6 header is set to 2346 the mobile node's care-of address copied from the Binding Cache 2347 entry. 2349 - The Routing header is initialized to contain a single route 2350 segment, with an Address of the mobile node's home address (the 2351 original destination address to which the packet was being sent). 2353 Following the definition of a Type 0 Routing header [5], this packet 2354 will be routed to the mobile node's care-of address, where it will 2355 be delivered to the mobile node (the mobile node has associated the 2356 care-of address with its network interface). Normal processing of 2357 the Routing header by the mobile node will then proceed as follows: 2359 - The mobile node swaps the Destination Address in the packet's 2360 IPv6 header and the Address specified in the Routing header. 2362 This results in the packet's IP Destination Address being set to 2363 the mobile node's home address. 2365 - The mobile node then resubmits the packet to its IPv6 module for 2366 further processing, "looping back" the packet inside the mobile 2367 node. Since the mobile node recognizes its own home address as 2368 one of its current IP addresses, the packet is processed further 2369 within the mobile node, in the same way then as if the mobile 2370 node was at home. 2372 If, instead, the sending node has no Binding Cache entry for the 2373 destination address to which the packet is being sent, the sending 2374 node simply sends the packet normally, with no Routing header. If 2375 the destination node is not a mobile node (or is a mobile node that 2376 is currently at home), the packet will be delivered directly to this 2377 node and processed normally by it. If, however, the destination node 2378 is a mobile node that is currently away from home, the packet will 2379 be intercepted by the mobile node's home agent and tunneled (using 2380 IPv6 encapsulation [4]) to the mobile node's current primary care-of 2381 address, as described in Section 9.6. The mobile node will then send 2382 a Binding Update to the sending node, as described in Section 10.8, 2383 allowing the sending node to create a Binding Cache entry for its use 2384 in sending subsequent packets to this mobile node. 2386 9. Home Agent Operation 2388 9.1. Receiving Router Advertisement Messages 2390 For each link on which a router provides service as a home agent, the 2391 router maintains a Home Agents List recording information about all 2392 other home agents on that link. This list is used in the dynamic 2393 home agent address discovery mechanism, described in Section 9.2. 2394 The information for the list is learned through receipt of the 2395 periodic unsolicited multicast Router Advertisements from each other 2396 home agent on the link, in which the Home Agent (H) bit is set, in a 2397 manner similar to the Default Router List conceptual data structure 2398 maintained by each host for Neighbor Discovery [14]. 2400 On receipt of a valid Router Advertisement, as defined in the 2401 processing algorithm specified for Neighbor Discovery [14], the home 2402 agent performs the following steps, in addition to any steps already 2403 required of it by Neighbor Discovery: 2405 - If the Home Agent (H) bit in the Router Advertisement is not set, 2406 skip all of the following steps. There are no special processing 2407 steps required by Mobile IP for this Router Advertisement, since 2408 the Advertisement was not sent by a home agent. 2410 - Otherwise, extract the Source Address from the IP header of the 2411 Router Advertisement. This is the link-local IP address on this 2412 link of the home agent sending this Advertisement [14]. 2414 - Determine from the Router Advertisement the preference for this 2415 home agent. If the Router Advertisement contains a Home Agent 2416 Information Option, then the preference is taken from the Home 2417 Agent Preference field in the option; otherwise, the default 2418 preference of 0 SHOULD be used. 2420 - Determine from the Router Advertisement the lifetime for 2421 this home agent. If the Router Advertisement contains a Home 2422 Agent Information Option, then the lifetime is taken from 2423 the Home Agent Lifetime field in the option; otherwise, the 2424 lifetime specified by the Router Lifetime field in the Router 2425 Advertisement SHOULD be used. 2427 - If the link-local address of the home agent sending this 2428 Advertisement is already present in this home agent's Home 2429 Agents List and the received home agent lifetime value is zero, 2430 immediately delete this entry in the Home Agents List. 2432 - Otherwise, if the link-local address of the home agent sending 2433 this Advertisement is already present in the receiving home 2434 agent's Home Agents List, reset its lifetime and preference to 2435 the values determined above. 2437 - If the link-local address of the home agent sending this 2438 Advertisement, as determined above, is not already present in 2439 the Home Agents List maintained by the receiving home agent, and 2440 the lifetime for the sending home agent, as determined above, 2441 is non-zero, create a new entry in the list, and initialize its 2442 lifetime and preference to the values determined above. 2444 - If the Home Agents List entry for the link-local address of 2445 the home agent sending this Advertisement was not deleted as 2446 described above, determine any global address(es) of the home 2447 agent based on each Prefix Information option received in 2448 this Advertisement in which the Router Address (R) bit is set 2449 (Section 6.2). For each such global address determined from this 2450 Advertisement, add this global address to the list of global 2451 addresses for this home agent in this Home Agents List entry. 2453 A home agent SHOULD maintain an entry in its Home Agents List for 2454 each such valid home agent address until that entry's lifetime 2455 expires, after which time the entry MUST be deleted. 2457 9.2. Dynamic Home Agent Address Discovery 2459 When a node receives a Binding Update, it MUST validate it and 2460 determine the type of Binding Update according to the steps described 2461 in Section 8.2. This section describes the processing of a valid 2462 Binding Update that indicates that the mobile node sending it is 2463 attempting dynamic home agent address discovery. 2465 As described in Section 10.7, a mobile node attempts dynamic home 2466 agent address discovery by sending its "home registration" Binding 2467 Update to the "Mobile IPv6 Home-Agents" anycast address [8] for its 2468 home IP subnet prefix (the packet MUST also include a Home Address 2469 option). A home agent receiving such a Binding Update that is 2470 serving this subnet (the home agent is configured with this anycast 2471 address on one of its network interfaces) MUST reject the Binding 2472 Update and SHOULD return a Binding Acknowledgement indicating this 2473 rejection, with the Source Address of the packet carrying the Binding 2474 Acknowledgement set to one of the global unicast addresses of the 2475 home agent. The Status field in the Binding Acknowledgement MUST be 2476 set to 135 (dynamic home agent address discovery response). 2478 In this Binding Acknowledgement rejecting the dynamic home agent 2479 address discovery Binding Update, this home agent SHOULD include a 2480 Home Agents List Sub-Option as follows: 2482 - The Home Agents List Sub-Option in this Binding Acknowledgement 2483 SHOULD contain one global IP address for each home agent 2484 currently listed in this home agent's own Home Agents List 2485 (Section 4.3). However, if this home agent's own global IP 2486 address would be placed in the list (as described below) as the 2487 first entry in the list, then this home agent SHOULD NOT include 2488 its own address in the list in the sub-option in the Binding 2489 Acknowledgement. Not placing this home agent's own IP address in 2490 the list will cause the receiving mobile node to consider this 2491 home agent as the most preferred home agent; otherwise, this home 2492 agent will be considered to be preferred in its order given by 2493 its place in the list returned. 2495 - The IP addresses in the Home Agents List should be placed in 2496 the Home Agents List Sub-Option in the Binding Acknowledgement 2497 in order of decreasing preference value, based either on the 2498 respective advertised preference from a Home Agent Information 2499 option or on the default preference of 0 if no preference is 2500 advertised (or on the configured home agent preference for this 2501 home agent itself). The home agent with the highest preference 2502 SHOULD be listed first, and the home agent with the lowest 2503 preference SHOULD be listed last. 2505 - Among home agents with equal preference, their IP addresses in 2506 the Home Agents List SHOULD be listed in an order randomized with 2507 respect to other home agents with equal preference, each time a 2508 Binding Acknowledgement with a Home Agents List Sub-Option is 2509 returned by this home agent. 2511 - For each entry in this home agent's Home Agents List, if more 2512 than one global IP address is associated with this list entry, 2513 then one of these global IP addresses SHOULD be selected to 2514 include in the Home Agents List Sub-Option to be returned in the 2515 Binding Acknowledgement. As described in Section 4.3, one Home 2516 Agents List entry, identified by the home agent's link-local 2517 address, exists for each home agent on the link; associated with 2518 that list entry is one or more global IP addresses for this 2519 home agent, learned through Prefix Information options with the 2520 Router Address (R) bit is set, received in Router Advertisements 2521 from this link-local address. The selected global IP address 2522 for each home agent to include in forming the Home Agents List 2523 Sub-Option to be returned in the Binding Acknowledgement MUST 2524 be the global IP address of the respective home agent sharing a 2525 prefix with the mobile node's home address for which the Binding 2526 Acknowledgement is being returned; if no such global IP address 2527 is known for some home agent, an entry for that home agent MUST 2528 NOT be included in the Home Agents List Sub-Option returned in 2529 the Binding Acknowledgement. 2531 - In order to avoid the possibility of the packet carrying the 2532 Binding Acknowledgement being fragmented, if the resulting 2533 total packet size containing the complete Home Agents List 2534 Sub-Option would exceed the minimum IPv6 MTU [5], the home agent 2535 SHOULD reduce the number of home agent IP addresses returned 2536 in the packet to the number of addresses that will fit without 2537 exceeding this limit. The home agent addresses returned in the 2538 packet SHOULD be those from the complete list with the highest 2539 preference. 2541 The mobile node, upon receiving this Binding Acknowledgement, MAY 2542 then resend its Binding Update to the home agent address given as the 2543 IP Source Address of the packet carrying the Binding Acknowledgement 2544 or to any of the unicast IP addresses listed in the Home Agents List 2545 Sub-Option in the Acknowledgement. For example, the mobile node may 2546 re-attempt its home registration with each of these home agents in 2547 turn, by sending each a Binding Update and waiting for the matching 2548 Binding Acknowledgement, until its registration is accepted by one 2549 of these home agents. In trying each of the returned home agent 2550 addresses, the mobile node SHOULD try each in the order listed in the 2551 Home Agents List Sub-Option in the Binding Acknowledgement. If the 2552 home agent identified by the Source Address field in the IP header 2553 of the packet carrying the Binding Acknowledgement is not listed in 2554 the Home Agents List Sub-Option, it SHOULD be tried before the first 2555 address given in the list; otherwise, it SHOULD be tried in the in 2556 its listed order. 2558 9.3. Primary Care-of Address Registration 2560 When a node receives a Binding Update, it MUST validate it and 2561 determine the type of Binding Update according to the steps described 2562 in Section 8.2. This section describes the processing of a valid 2563 Binding Update that requests the receiving node to serve as its home 2564 agent, registering its primary care-of address. 2566 To begin processing the Binding Update, the home agent MUST perform 2567 the following sequence of tests: 2569 - If the node is not a router that implements home agent 2570 functionality, then the node MUST reject the Binding Update and 2571 SHOULD return a Binding Acknowledgement to the mobile node, in 2572 which the Status field is set to 132 (home registration not 2573 supported). 2575 - Else, if the home address for the binding (the Home Address field 2576 in the packet's Home Address option) is not an on-link IPv6 2577 address with respect to the home agent's current Prefix List, 2578 then the home agent MUST reject the Binding Update and SHOULD 2579 return a Binding Acknowledgement to the mobile node, in which the 2580 Status field is set to 133 (not home subnet). 2582 - Else, if the Prefix Length field is nonzero in the Binding Update 2583 and this length differs from the length of the home agent's own 2584 knowledge of the corresponding subnet prefix on the home link, 2585 then the home agent MUST reject the Binding Update and SHOULD 2586 return a Binding Acknowledgement to the mobile node, in which the 2587 Status field is set to 136 (incorrect subnet prefix length). 2589 - Else, if the home agent chooses to reject the Binding Update for 2590 any other reason (e.g., insufficient resources to serve another 2591 mobile node as a home agent), then the home agent SHOULD return a 2592 Binding Acknowledgement to the mobile node, in which the Status 2593 field is set to an appropriate value to indicate the reason for 2594 the rejection. 2596 If the home agent does not reject the Binding Update as described 2597 above, then it becomes the home agent for the mobile node. The new 2598 home agent (the receiving node) MUST then create a new entry in its 2599 Binding Cache for this mobile node (or update its existing Binding 2600 Cache entry for this mobile node, if such an entry already exists) 2601 The home address of the mobile node is taken from the Home Address 2602 field in the packet's Home Address option. The care-of address for 2603 this Binding Cache entry is taken from the Alternate Care-of Address 2604 sub-option in the Binding Update option, if present, or from the 2605 Source Address field in the packet's IPv6 header, otherwise. 2607 The home agent MUST mark this Binding Cache entry as a "home 2608 registration" to indicate that the node is serving as a home 2609 agent for this binding. Binding Cache entries marked as a "home 2610 registration" MUST be excluded from the normal cache replacement 2611 policy used for the Binding Cache (Section 8.7) and MUST NOT be 2612 removed from the Binding Cache until the expiration of the Lifetime 2613 period. 2615 In addition, the home agent MUST copy the Router (R) bit from the 2616 Binding Update into the corresponding bit in this Binding Cache entry 2617 for this mobile node. 2619 The lifetime for the Binding Cache entry MUST NOT be greater than 2620 the remaining valid lifetime for the subnet prefix in the mobile 2621 node's home address specified with the Binding Update. The remaining 2622 valid lifetime for this prefix is determined by the home agent based 2623 on its own Prefix List entry for this prefix [14]. Furthermore, 2624 if the Prefix Length field in the Binding Update is nonzero, then 2625 the lifetime for the Binding Cache entry MUST NOT be greater than 2626 the minimum remaining valid lifetime for all subnet prefixes on 2627 the mobile node's home link. If the value of the Lifetime field 2628 specified by the mobile node in its Binding Update is greater than 2629 this prefix lifetime, the home agent MUST decrease the binding 2630 lifetime to less than or equal to the prefix valid lifetime. The 2631 home agent MAY further decrease the specified lifetime for the 2632 binding, for example based on a local policy implemented by the home 2633 agent. The resulting lifetime is stored by the home agent in the 2634 Binding Cache entry, and this Binding Cache entry MUST be deleted by 2635 the home agent after the expiration of this lifetime. 2637 The Prefix Length in the Binding Update MUST also be saved in the 2638 Binding Cache entry. 2640 If the Acknowledge (A) bit is set in the Binding Update (it SHOULD 2641 be), then the home agent MUST return a Binding Acknowledgement to the 2642 mobile node, constructed as follows: 2644 - The Status field MUST be set to a value indicating success (the 2645 value MUST be less than 128). The only currently defined success 2646 Status value is 0, indicating simply that the Binding Update was 2647 accepted. 2649 - The Sequence Number field MUST be copied from the Sequence Number 2650 given in the Binding Update. 2652 - The Lifetime field MUST be set to the remaining lifetime for 2653 the binding as set by the home agent in its "home registration" 2654 Binding Cache entry for the mobile node. As described above, 2655 this lifetime MUST NOT be greater than the remaining valid 2656 lifetime for the subnet prefix in the mobile node's home address. 2658 - The Refresh field MUST be set to a value less than or equal to 2659 the Lifetime value being returned in the Binding Update. If the 2660 home agent stores the Binding Cache entry in nonvolatile storage 2661 (that survives the crash or other failure of the home agent), 2662 then the Refresh field SHOULD be set to the same value as the 2663 Lifetime field; otherwise, the home agent MAY set the Refresh 2664 field to a value less than the Lifetime field, to indicate that 2665 the mobile node SHOULD attempt to refresh its home registration 2666 at the indicated shorter interval (although the home agent will 2667 still retain the registration for the Lifetime period, even if 2668 the mobile node does not refresh its registration within the 2669 Refresh period). 2671 In addition, the home agent MUST follow the procedure defined in 2672 Section 9.5 to intercept packets on the mobile node's home link 2673 addressed to the mobile node, while the home agent is serving as the 2674 home agent for this mobile node. 2676 9.4. Primary Care-of Address De-registration 2678 When a node receives a Binding Update, it MUST validate it and 2679 determine the type of Binding Update according to the steps described 2680 in Section 8.2. This section describes the processing of a valid 2681 Binding Update that requests the receiving node to no longer serve as 2682 its home agent, de-registering its primary care-of address. 2684 To begin processing the Binding Update, the home agent MUST perform 2685 the following test: 2687 - If the receiving node has no entry in its Binding Cache for this 2688 mobile node that is marked as a "home registration", then this 2689 node MUST reject the Binding Update and SHOULD return a Binding 2690 Acknowledgement to the mobile node, in which the Status field is 2691 set to 137 (not home agent for this mobile node). 2693 If the home agent does not reject the Binding Update as described 2694 above, then it MUST delete any existing entry in its Binding Cache 2695 for this mobile node. 2697 If the Acknowledge (A) bit is set in the Binding Update (it SHOULD 2698 be), then the home agent MUST return a Binding Acknowledgement to the 2699 mobile node, constructed as follows: 2701 - The Status field MUST be set to a value indicating success (the 2702 value MUST be less than 128). The only currently defined success 2703 Status value is 0, indicating simply that the Binding Update was 2704 accepted. 2706 - The Sequence Number field MUST be copied from the Sequence Number 2707 given in the Binding Update. 2709 - The Lifetime field MUST be set to zero. 2711 - The Refresh field MUST be set to zero. 2713 In addition, the home agent MUST stop intercepting packets on the 2714 mobile node's home link addressed to the mobile node (Section 9.5). 2716 9.5. Intercepting Packets for a Mobile Node 2718 While a node is serving as the home agent for mobile node (while the 2719 node has an entry in its Binding Cache for this mobile node that is 2720 marked as a "home registration"), this node MUST attempt to intercept 2721 packets on the mobile node's home link addressed to the mobile node, 2722 and MUST tunnel each intercepted packet to the mobile node using 2723 using IPv6 encapsulation [4]. 2725 In order to intercept such packets on the home link, when a node 2726 becomes the home agent for some mobile node (it did not already 2727 have a Binding Cache entry for this mobile node marked as a "home 2728 registration"), then the home agent MUST multicast onto the home link 2729 a "gratuitous" Neighbor Advertisement message [14] on behalf of the 2730 mobile node. Specifically, the home agent performs the following 2731 steps: 2733 - The home agent examines the value of the Prefix Length field 2734 in the new "home registration" Binding Cache entry. If this 2735 value is zero, the following step is carried out only for the 2736 individual home address specified for this binding. If, instead, 2737 this field is nonzero, then the following step is carried out 2738 for each address for the mobile node formed from the interface 2739 identifier in the mobile node's home address in this binding 2740 (the remaining low-order bits in the address after the indicated 2741 subnet prefix), together with each one of the subnet prefixes 2742 currently considered by the home agent to be on-link (including 2743 both the link-local and site-local prefix). 2745 - For each specific IP address for the mobile node determined 2746 in the first step above, the home agent multicasts onto the 2747 home link (to the all-nodes multicast address) a Neighbor 2748 Advertisement message [14] on behalf of the mobile node, to 2749 advertise the home agent's own link-layer address for this IP 2750 address. The Target Address in the Neighbor Advertisement 2751 message MUST be set to this IP address for the mobile node, and 2752 the Advertisement MUST include a Target Link-layer Address option 2753 specifying the home agent's link-layer address. In addition, 2754 the Router (R) bit in the Advertisement MUST be copied from the 2755 corresponding bit in the home agent's Binding Cache entry for 2756 the mobile node. The Solicited Flag (S) in the Advertisement 2757 MUST NOT be set, since it was not solicited by any Neighbor 2758 Solicitation message. The Override Flag (O) in the Advertisement 2759 MUST be set, indicating that the Advertisement SHOULD override 2760 any existing Neighbor Cache entry at any node receiving it. 2762 Any node on the home link receiving one of the Neighbor Advertisement 2763 messages described above will thus update its Neighbor Cache to 2764 associate the mobile node's address with the home agent's link 2765 layer address, causing it to transmit any future packets for the 2766 mobile node normally destined to this address instead to the mobile 2767 node's home agent. Since multicasts on the local link (such as 2768 Ethernet) are typically not guaranteed to be reliable, the home 2769 agent MAY retransmit this Neighbor Advertisement message up to 2770 MAX_ADVERT_REXMIT times to increase its reliability. It is still 2771 possible that some nodes on the home link will not receive any of 2772 these Neighbor Advertisements, but these nodes will eventually be 2773 able to detect the link-layer address change for the mobile node's 2774 home address, through use of Neighbor Unreachability Detection [14]. 2776 While a node is serving as a home agent for some mobile node (it 2777 still has a "home registration" entry for this mobile node in its 2778 Binding Cache), the home agent uses IPv6 Neighbor Discovery [14] 2779 to intercept unicast packets on the home link addressed the mobile 2780 node's home address. In order to intercept packets in this way, 2781 the home agent MUST act as a proxy for this mobile node to reply to 2782 any received Neighbor Solicitation messages for it. When a home 2783 agent receives a Neighbor Solicitation message, it MUST check if the 2784 Target Address specified in the message matches the home address 2785 of any mobile node for which it has a Binding Cache entry marked 2786 as a "home registration". This check MUST include all possible 2787 home addresses for the mobile node, based on the subnet prefixes 2788 currently considered to be on-link by the home agent (including the 2789 corresponding link-local address and site-local address), if the 2790 Prefix Length in the Binding Cache entry for this mobile node (from 2791 the Binding Update that created this Cache entry) is nonzero. 2793 If such an entry exists in the home agent's Binding Cache, the home 2794 agent MUST reply to the Neighbor Solicitation message with a Neighbor 2795 Advertisement message, giving the home agent's own link-layer address 2796 as the link-layer address for the specified Target Address. In 2797 addition, the Router (R) bit in the Advertisement MUST be copied from 2798 the corresponding bit in the home agent's Binding Cache entry for the 2799 mobile node. Acting as a proxy in this way allows other nodes on 2800 the mobile node's home link to resolve the mobile node's IPv6 home 2801 address, and allows the home agent to to defend these addresses on 2802 the home link for Duplicate Address Detection [14]. 2804 9.6. Tunneling Intercepted Packets to a Mobile Node 2806 For any packet sent to a mobile node from the mobile node's home 2807 agent (for which the home agent is the original sender of the 2808 packet), the home agent is operating as a correspondent node of 2809 the mobile node for this packet and the procedures described in 2810 Section 8.9 apply. The home agent (as a correspondent node) uses a 2811 Routing header to route the packet to the mobile node by way of the 2812 care-of address in the home agent's Binding Cache (the mobile node's 2813 primary care-of address, in this case). 2815 While the mobile node is away from home and this node is acting 2816 as the mobile node's home agent, the home agent intercepts any 2817 packets on the home link addressed to the mobile node's home address 2818 (including addresses formed from other on-link prefixes, if the 2819 Prefix Length field was nonzero in the Binding Update), as described 2820 in Section 9.5. The home agent cannot use a Routing header to 2821 forward these intercepted packets to the mobile node, since it cannot 2822 modify the packet in flight without invalidating any existing IPv6 2823 AH [9] or ESP [10] header present in the packet. 2825 For forwarding each intercepted packet to the mobile node, the 2826 home agent MUST tunnel the packet to the mobile node using IPv6 2827 encapsulation [4]; the tunnel entry point node is the home agent, 2828 and the tunnel exit point node is the primary care-of address as 2829 registered with the home agent (which is an address of the mobile 2830 node itself). When a home agent encapsulates an intercepted packet 2831 for forwarding to the mobile node, the home agent sets the Source 2832 Address in the prepended tunnel IP header to the home agent's own IP 2833 address, and sets the Destination Address in the tunnel IP header 2834 to the mobile node's primary care-of address. When received by the 2835 mobile node (using its primary care-of address), normal processing of 2836 the tunnel header [4] will result in decapsulation and processing of 2837 the original packet by the mobile node. 2839 However, packets addressed to the mobile node's link-local address 2840 MUST NOT be tunneled to the mobile node. Instead, such a packet MUST 2841 be discarded, and the home agent SHOULD return an ICMP Destination 2842 Unreachable, Code 3, message to the packet's Source Address (unless 2843 this Source Address is a multicast address). Packets addressed to 2844 the mobile node's site-local address SHOULD be tunneled to the mobile 2845 node by default, but this behavior MUST be configurable to disable 2846 it; currently, the exact definition and semantics of a "site" and a 2847 site-local address are undefined in IPv6, and this default behavior 2848 might change at some point in the future. 2850 Tunneling of multicast packets to a mobile node follows similar 2851 limitations to those defined above for unicast packets addressed to 2852 the mobile node's link-local and site-local addresses. Multicast 2853 packets addressed to a multicast address with link-local scope [7], 2854 to which the mobile node is subscribed, MUST NOT be tunneled 2855 to the mobile node; such packets SHOULD be silently discarded 2856 (after delivering to other local multicast recipients). Multicast 2857 packets addressed to a multicast address with scope larger 2858 than link-local but smaller than global (e.g., site-local and 2859 organization-local) [7], to which the mobile node is subscribed, 2860 SHOULD be tunneled to the mobile node by default, but this behavior 2861 MUST be configurable to disable it; this default behavior might 2862 change at some point in the future as the definition of these scopes 2863 become better defined in IPv6. 2865 9.7. Renumbering the Home Subnet 2867 IPv6 provides mechanisms through Neighbor Discovery [14] and Address 2868 Autoconfiguration [23] to aid in renumbering a subnet, such as when a 2869 site switches to a new network service provider. In renumbering, new 2870 prefixes and addresses can be introduced for the subnet and old ones 2871 can be deprecated and removed. These mechanisms are defined to work 2872 while all nodes using the old prefixes are at home, connected to the 2873 link using these prefixes. Mobile IPv6 extends these mechanisms for 2874 the case in which one or more mobile nodes using the old prefixes are 2875 away from home while the renumbering takes place. 2877 The IPv6 renumbering mechanisms are based on nodes on the link 2878 receiving Prefix Information options in Router Advertisement messages 2879 giving the valid lifetime and preferred lifetime for different 2880 prefixes on the link [14]. Mobile IPv6 arranges to tunnel certain 2881 Router Advertisements giving "important" Prefix Information options 2882 to mobile nodes while away from home. To avoid the need to tunnel 2883 all Router Advertisements from the home link to a mobile node away 2884 from home, those Router Advertisements that are tunneled to the 2885 mobile node are retransmitted until acknowledged. To avoid possible 2886 security attacks from forged Router Advertisements tunneled to 2887 the mobile node, all such tunneled Router Advertisements must be 2888 authenticated to the mobile node by its home agent using AH [9] or 2889 ESP [10]. 2891 Specifically, a home agent serving some mobile node SHOULD construct 2892 and tunnel to the mobile node a new Router Advertisement when any of 2893 the following conditions occur: 2895 - The preferred or valid lifetime for an existing prefix on the 2896 home link is reduced. 2898 - A new prefix is introduced on the home link. 2900 - The state of the home agent's AdvManagedFlag flag [14] changes 2901 from FALSE to TRUE or from TRUE to FALSE. 2903 The home agent determines these conditions based on its own 2904 configuration as a router and based on the Router Advertisements 2905 that it receives on the home link. The home agent constructs a new 2906 Router Advertisement message containing no options other than the 2907 Prefix Information options describing the prefixes for which one of 2908 the conditions above has occurred since the last Router Advertisement 2909 tunneled to and acknowledged by the mobile node. When multiple 2910 conditions occur at or near the same time, the home agent SHOULD 2911 attempt to combine them into a single Router Advertisement message to 2912 the mobile node. 2914 In tunneling each such Router Advertisement to the mobile node, the 2915 home agent MUST construct the packet as follows: 2917 - The Source Address in the packet's IPv6 header MUST be set to the 2918 home agent's IP address to which the mobile node addressed its 2919 current home registration. 2921 - The packet MUST include either an AH [9] or ESP [10] header 2922 providing sender authentication, data integrity protection, and 2923 replay protection. 2925 - The packet MUST include a Binding Request destination option. 2927 - The Binding Request destination option MUST include a Unique 2928 Identifier Sub-Option (Section 5.5), with the unique identifier 2929 in the sub-option data set to a value different than that in 2930 any other Binding Request sent recently by this node. The word 2931 "recently" here means within the maximum likely lifetime of a 2932 packet, including transit time from source to destination and 2933 time spent awaiting reassembly with other fragments of the same 2934 packet, if fragmented. However, it is not required that a source 2935 node know the maximum packet lifetime. Rather, it is assumed 2936 that the requirement can be met by maintaining a simple 16-bit 2937 "wrap-around" counter to generate unique identifiers for Binding 2938 Requests that contain a Unique Identifier Sub-Option, incremented 2939 each time a Binding Request containing a Unique Identifier 2940 Sub-Option is sent. 2942 - The packet MUST be tunneled to the mobile node's primary care-of 2943 address using a Routing header, in the same way as any packet 2944 sent to the mobile node originated by the home agent (rather than 2945 using IPv6 encapsulation, as would be used by the home agent for 2946 intercepted packets). 2948 The home agent SHOULD periodically continue to retransmit this 2949 tunneled packet to the mobile node, until it is acknowledged by 2950 the receipt from the mobile node of a Binding Update matching 2951 the Binding Request in the packet (i.e., with matching Sequence 2952 Number). A Binding Update matches a Binding Request if it specifies 2953 a binding for the mobile node to which the Binding Request was sent 2954 and contains a Unique Identifier Sub-Option matching the unique 2955 identifier sent in the Unique Identifier Sub-Option in the Binding 2956 Request. 2958 If while the home agent is still retransmitting a Router 2959 Advertisement to the mobile node, another condition as described 2960 above occurs on the home link causing another Router Advertisement 2961 to be tunneled to the mobile node, the home agent SHOULD combine any 2962 Prefix Information options in the unacknowledged Router Advertisement 2963 into the new Router Advertisement and then begin retransmitting the 2964 new Router Advertisement rather than the old one. When tunneling 2965 a new Router Advertisement, even if it contains Prefix Information 2966 options sent previously in an unacknowledged tunneled Router 2967 Advertisement, the home agent MUST generate a new unique identifer 2968 for use in the Unique Identifier Sub-Option in the Binding Request 2969 tunneled with the new Router Advertisement. 2971 In addition, as described in Section 9.3, the lifetime returned by a 2972 mobile node's home agent in its Binding Acknowledgement in response 2973 to registration of a new primary care-of address by the mobile node 2974 MUST be no greater than the remaining valid lifetime for the subnet 2975 prefix in the mobile node's home address. Furthermore, as described 2976 in Section 10.8, Binding Updates sent by the mobile node to other 2977 nodes MUST use a lifetime no greater than the remaining lifetime of 2978 its home registration of its primary care-of address. These limits 2979 on a binding lifetimes ensure that no node uses a mobile node's home 2980 address beyond the time that it becomes invalid. The mobile node 2981 SHOULD further limit the lifetimes that it sends on any Binding 2982 Updates to be within the remaining preferred lifetime for the prefix 2983 in its home address. 2985 10. Mobile Node Operation 2987 10.1. Sending Packets While Away from Home 2989 While a mobile node is away from home, it continues to use its home 2990 address as well as also using one or more care-of addresses. When 2991 sending a packet while away from home, a mobile node MAY choose among 2992 these in selecting the address that it will use as the source of the 2993 packet, as follows: 2995 - From the point of view of protocol layers and applications 2996 above Mobile IP (e.g., transport protocols), the mobile node 2997 will generally use its home address as the source of the packet 2998 for most packets, even while away from home, since Mobile IP 2999 is designed to make mobility transparent to such software. 3000 Doing so also makes the node's mobility and the fact that it is 3001 currently away from home transparent to the correspondent nodes 3002 with which it communicates. For packets sent that are part of 3003 transport-level connections established while the mobile node 3004 was at home, the mobile node MUST use its home address in this 3005 way. Likewise, for packets sent that are part of transport-level 3006 connections that the mobile node may still be using after moving 3007 to a new location, the mobile node SHOULD use its home address 3008 in this way. When sending such packets, Mobile IP will modify 3009 the packet to move the home address into a Home Address option 3010 and will set the IPv6 header's Source Address field to one of 3011 the mobile node's care-of addresses; these modifications to 3012 the packet are then reversed in the node receiving the packet, 3013 restoring the mobile node's home address to be the packet's 3014 Source Address before processing by higher protocol layers and 3015 applications. 3017 - For short-term communication, particularly for communication that 3018 may easily be retried if it fails, the mobile node MAY choose 3019 to directly use one of its care-of addresses as the source of 3020 the packet, thus not requiring the use of a Home Address option 3021 in the packet. An example of this type of communication might 3022 be DNS queries sent by the mobile node [12, 13]. Using the 3023 mobile node's care-of address as the source for such queries will 3024 generally have a lower overhead than using the mobile node's 3025 home address, since no extra options need be used in either the 3026 query or its reply, and all packets can be routed normally, 3027 directly between their source and destination without relying 3028 on Mobile IP. If the mobile node has no particular knowledge 3029 that the communication being sent fits within this general type 3030 of communication, however, the mobile node SHOULD NOT use its 3031 care-of address as the source of the packet in this way. 3033 For packets sent by a mobile node while it is at home, no special 3034 Mobile IP processing is required for sending this packet. Likewise, 3035 if the mobile node uses any address other than its home address as 3036 the source of a packet sent while away from home (from the point of 3037 view of higher protocol layers or applications, as described above), 3038 no special Mobile IP processing is required for sending that packet. 3039 In each case, the packet is simply addressed and transmitted in the 3040 same way as any normal IPv6 packet. 3042 For each other packet sent by the mobile node (i.e., packets sent 3043 while away from home, using the mobile node's home address as 3044 the source, from the point of view of higher protocol layers and 3045 applications), special Mobile IP processing of the packet is required 3046 for the insertion of the Home Address option. Specifically: 3048 - Since Mobile IP is transparent to higher protocol layers (e.g., 3049 to TCP), the packet is initially constructed using the mobile 3050 node's home address as the packet's Source Address, in the same 3051 way as if the mobile node were at home. 3053 - Insert a Home Address option into the packet, with the Home 3054 Address field copied from the original value of the Source 3055 Address field in the packet. 3057 - Change the Source Address field in the packet's IPv6 header to 3058 one of the mobile node's care-of addresses. This will typically 3059 be the mobile node's current primary care-of address, but MUST 3060 be a care-of address with a subnet prefix that is on-link on the 3061 network interface on which the mobile node will transmit the 3062 packet. 3064 By using the care-of address as the Source Address in the IPv6 3065 header, with the mobile node's home address instead in the Home 3066 Address option, the packet will be able to safely pass through any 3067 router implementing ingress filtering [6]. 3069 10.2. Interaction with Outbound IPsec Processing 3071 As a guidance to implementors, this section sketches the interaction 3072 between outbound Mobile IP processing and outbound IP Security 3073 (IPsec) processing for packets sent by a mobile node while away 3074 from home. Any specific implementation MAY use algorithms and data 3075 structures other than those suggested here, but its processing 3076 MUST be consistent with these steps and with the relevant IPsec 3077 specifications. In the steps described below, it is assumed that 3078 IPsec is being used in transport mode [11] and that the mobile node 3079 is using its home address as the source for the packet (from the 3080 point of view of higher protocol layers or applications, as described 3081 in Section 10.1): 3083 - The packet is created by higher layer protocols and applications 3084 (e.g., by TCP) as if the mobile node were at home and Mobile IP 3085 were not being used. Mobile IP is transparent to such higher 3086 layers. 3088 - As part of outbound packet processing in IP, the packet is 3089 compared against the IPsec Security Policy Database (SPD) to 3090 determine what processing is required for the packet [11]. 3092 - As a special case for Mobile IP, if a Binding Update or 3093 Binding Acknowledgement is being included in the packet, IPsec 3094 authentication and replay protection using AH [9] or ESP [10] 3095 MUST be applied to the packet. If the SPD check above has 3096 already indicated that authentication and replay protection 3097 are required, this processing is sufficient for the Mobile IP 3098 requirement that all packets containing Binding Updates or 3099 Binding Acknowledgements be authenticated and covered by replay 3100 protection. Otherwise, an implementation can force the required 3101 IPsec processing on this individual packet by, for example, 3102 creating a temporary SPD entry for handling of this packet. 3104 - If IPsec processing is required, the packet is either mapped to 3105 an existing Security Association (or SA bundle), or a new SA (or 3106 SA bundle) is created for the packet, according to the procedures 3107 defined for IPsec. 3109 - Since the mobile node is away from home, the mobile node inserts 3110 a Home Address option into the packet, replacing the Source 3111 Address in the packet's IP header with a care-of address suitable 3112 for the link on which the packet is being sent, as described in 3113 Section 10.1. The Home Address option MUST be inserted in a 3114 Destination Options header that appears in the packet after the 3115 AH or ESP header. 3117 - If a Binding Update is being included in the packet, it is also 3118 added to the Destination Options header in the packet. 3120 - Finally, once the packet is fully assembled, the necessary IPsec 3121 authentication (and encryption, if required) processing is 3122 performed on the packet, initializing the Authentication Data in 3123 the AH or ESP header. 3125 10.3. Receiving Packets While Away from Home 3127 While away from home, a mobile node will receive packets addressed to 3128 its home address, by one of three methods: 3130 - Packets sent by a correspondent node that does not have a 3131 Binding Cache entry for the mobile node, will be sent by the 3132 correspondent node in the same way as any normal IP packet. Such 3133 packets will then be intercepted by the mobile node's home agent, 3134 encapsulated using IPv6 encapsulation [4], and tunneled to the 3135 mobile node's primary care-of address. 3137 - Packets sent by a correspondent node that has a Binding Cache 3138 entry for the mobile node that contains the mobile node's current 3139 care-of address, will be sent by the correspondent node using 3140 a Routing header. The packet will be addressed to the mobile 3141 node's care-of address, with the final hop in the Routing header 3142 directing the packet to the mobile node's home address; the 3143 processing of this last hop of the Routing header is entirely 3144 internal to the mobile node, since the care-of address and home 3145 address are both addresses within the mobile node. 3147 - Packets sent by a correspondent node that has a Binding Cache 3148 entry for the mobile node that contains an out-of-date care-of 3149 address for the mobile node, will be sent by the correspondent 3150 node using a Routing header, as described above. If the mobile 3151 node sent a Binding Update to its previous default router when 3152 moving from this care-of address to another, and if the Binding 3153 Cache entry that was created from this Binding Update is still 3154 present in this router's Binding Cache, then such a packet 3155 will be intercepted by this router, encapsulated using IPv6 3156 encapsulation [4], and tunneled to the mobile node's primary 3157 care-of address (registered with this router, acting as a home 3158 agent for this out-of-date care-of address). 3160 For packets received by either the first or last of these three 3161 methods, the mobile node SHOULD send a Binding Update to the original 3162 sender of the packet, as described in Section 10.8, subject to the 3163 rate limiting defined in Section 10.11. The mobile node SHOULD 3164 also process the received packet in the manner defined for IPv6 3165 encapsulation [4], which will result in the encapsulated (inner) 3166 packet being processed normally by upper-layer protocols within the 3167 mobile node, as if it had been addressed (only) to the mobile node's 3168 home address. 3170 For packets received by the second method above (using a Routing 3171 header), the mobile node SHOULD process the received packet in the 3172 manner defined for the type of IPv6 Routing header used [5], which 3173 will result in the packet being processed normally by upper-layer 3174 protocols within the mobile node, as if it had been addressed (only) 3175 to the mobile node's home address. 3177 In addition, the general procedures defined by IPv6 for Routing 3178 headers suggest that a received Routing header MAY be automatically 3179 "reversed" to construct a Routing header for use in any response 3180 packets sent by upper-layer protocols, if the received packet is 3181 authenticated [5]. If this is done for upper-layer protocol response 3182 packets sent by a mobile node while away from home, the mobile 3183 node SHOULD NOT include its own care-of address, which appears in 3184 the Routing header of the received packet, in the reversed route 3185 for the response packet. If the received Routing header contained 3186 no additional hops (other than the mobile node's home address and 3187 care-of address), then any upper-layer protocol response packet 3188 SHOULD NOT include a Routing header. 3190 10.4. Movement Detection 3192 A mobile node MAY use any combination of mechanisms available to it 3193 to detect when it has moved from one link to another. The primary 3194 movement detection mechanism for Mobile IPv6 defined here uses the 3195 facilities of IPv6 Neighbor Discovery, including Router Discovery and 3196 Neighbor Unreachability Detection. The description here is based on 3197 the conceptual model of the organization and data structures defined 3198 by Neighbor Discovery [14]. 3200 Mobile nodes SHOULD use Router Discovery to discover new routers and 3201 on-link subnet prefixes; a mobile node MAY send Router Solicitation 3202 messages, or MAY wait for unsolicited (periodic) multicast Router 3203 Advertisement messages, as specified for Router Discovery [14]. 3204 Based on received Router Advertisement messages, a mobile node (in 3205 the same way as any other node) maintains an entry in its Default 3206 Router List for each router, and an entry in its Prefix List for each 3207 subnet prefix, that it currently considers to be on-link. Each entry 3208 in these lists has an associated invalidation timer value (extracted 3209 from the Router Advertisement) used to expire the entry when it 3210 becomes invalid. 3212 While away from home, a mobile node SHOULD select one router from 3213 its Default Router List to use as its default router, and one subnet 3214 prefix advertised by that router from its Prefix List to use as 3215 the subnet prefix in its primary care-of address. A mobile node 3216 MAY also have associated additional care-of addresses, using other 3217 subnet prefixes from its Prefix List. The method by which a mobile 3218 node selects and forms a care-of address from the available subnet 3219 prefixes is described in Section 10.5. The mobile node registers 3220 its primary care-of address with its home agent, as described in 3221 Section 10.6. 3223 While a mobile node is away from home and using some router as its 3224 default router, it is important for the mobile node to be able to 3225 quickly detect when that router becomes unreachable, so that it can 3226 switch to a new default router and to a new primary care-of address. 3227 Since some links (notably wireless) do not necessarily work equally 3228 well in both directions, it is likewise important for the mobile 3229 node to detect when it becomes unreachable to packets sent from its 3230 default router, so that the mobile node can take steps to ensure that 3231 any correspondent nodes attempting to communicate with it can still 3232 reach it through some other route. 3234 To detect when its default router becomes unreachable, a mobile 3235 node SHOULD use Neighbor Unreachability Detection. As specified in 3236 Neighbor Discovery [14], while the mobile node is actively sending 3237 packets to (or through) its default router, the mobile node can 3238 detect that the router (as its neighbor) is still reachable either 3239 through indications from upper layer protocols on the mobile node 3240 that a connection is making "forward progress" (e.g., receipt of TCP 3241 acknowledgements for new data transmitted), or through receipt of a 3242 Neighbor Advertisement message from its default router in response 3243 to an explicit Neighbor Solicitation messages to it. Note that 3244 although this mechanism detects that the mobile node's default router 3245 has become unreachable to the mobile node only while the mobile node 3246 is actively sending packets to it, this is the only time that this 3247 direction of reachability confirmation is needed. Confirmation 3248 that the mobile node is still reachable from the router is handled 3249 separately, as described below. 3251 For a mobile node to detect when it has become unreachable from its 3252 default router, the mobile node cannot efficiently rely on Neighbor 3253 Unreachability Detection alone, since the network overhead would be 3254 prohibitively high in many cases for a mobile node to continually 3255 probe its default router with Neighbor Solicitation messages even 3256 when it is not otherwise actively sending packets to it. Instead, 3257 a mobile node SHOULD consider receipt of any IPv6 packets from its 3258 current default router as an indication that it is still reachable 3259 from the router. Both packets from the router's IP address and 3260 (IPv6) packets from its link-layer address (e.g., those forwarded but 3261 not originated by the router) SHOULD be considered. 3263 Since the router SHOULD be sending periodic unsolicited multicast 3264 Router Advertisement messages, the mobile node will have frequent 3265 opportunity to check if it is still reachable from its default 3266 router, even in the absence of other packets to it from the router. 3267 If Router Advertisements that the mobile node receives include 3268 an Advertisement Interval option, the mobile node MAY use its 3269 Advertisement Interval field as an indication of the frequency with 3270 which it should expect to continue to receive future Advertisements 3271 from that router. This field specifies the minimum rate (the maximum 3272 amount of time between successive Advertisements) that the mobile 3273 node should expect. If this amount of time elapses without the 3274 mobile node receiving any Advertisement from this router, the mobile 3275 node can be sure that at least one Advertisement sent by the router 3276 has been lost. It is thus possible for the mobile node to implement 3277 its own policy for determining the number of Advertisements from 3278 its current default router it is willing to tolerate losing before 3279 deciding to switch to a different router from which it may currently 3280 be correctly receiving Advertisements. 3282 On some types of network interfaces, the mobile node MAY also 3283 supplement this monitoring of Router Advertisements, by setting its 3284 network interface into "promiscuous" receive mode, so that it is able 3285 to receive all packets on the link, including those not link-level 3286 addressed to it (i.e., disabling link-level address filtering). The 3287 mobile node will then be able to detect any packets sent by the 3288 router, in order to to detect reachability from the router. This 3289 use of promiscuous mode may be useful on very low bandwidth (e.g., 3290 wireless) links, but its use MUST be configurable on the mobile node. 3292 If the above means do not provide indication that the mobile node is 3293 still reachable from its current default router (i.e., the mobile 3294 node receives no packets from the router for a period of time), then 3295 the mobile node SHOULD attempt to actively probe the router with 3296 Neighbor Solicitation messages, even if it is not otherwise actively 3297 sending packets to the router. If it receives a solicited Neighbor 3298 Advertisement message in response from the router, then the mobile 3299 node can deduce that it is still reachable. It is expected that the 3300 mobile node will in most cases be able to determine its reachability 3301 from the router by listening for packets from the router as described 3302 above, and thus, such extra Neighbor Solicitation probes should 3303 rarely be necessary. 3305 With some types of networks, it is possible that additional 3306 indications about link-layer mobility can be obtained from 3307 lower-layer protocol or device driver software within the mobile 3308 node. However, a mobile node MUST NOT assume that all link-layer 3309 mobility indications from lower layers indicate a movement of the 3310 mobile node to a new link, such that the mobile node would need to 3311 switch to a new default router and primary care-of address. For 3312 example, movement of a mobile node from one cell to another in many 3313 wireless LANs can be made transparent to the IP level through use of 3314 a link-layer "roaming" protocol, as long as the different wireless 3315 LAN cells all operate as part of the same IP link with the same 3316 subnet prefix. Upon lower-layer indication of link-layer mobility, 3317 the mobile node MAY send Router Solicitation messages to determine if 3318 new routers (and new on-link subnet prefixes) are present on its new 3319 link. 3321 Such lower-layer information might also be useful to a mobile node in 3322 deciding to switch its primary care-of address to one of the other 3323 care-of addresses it has formed from the on-link subnet prefixes 3324 currently available through different routers from which the mobile 3325 node is reachable. For example, a mobile node MAY use signal 3326 strength or signal quality information (with suitable hysteresis) for 3327 its link with the available routers to decide when to switch to a new 3328 primary care-of address using that router rather than its current 3329 default router (and current primary care-of address). Even though 3330 the mobile node's current default router may still be reachable in 3331 terms of Neighbor Unreachability Detection, the mobile node MAY use 3332 such lower-layer information to determine that switching to a new 3333 default router would provide a better connection. 3335 10.5. Forming New Care-of Addresses 3337 After detecting that it has moved from one link to another (i.e., its 3338 current default router has become unreachable and it has discovered 3339 a new default router), a mobile node SHOULD form a new primary 3340 care-of address using one of the on-link subnet prefixes advertised 3341 by the new router. A mobile node MAY form a new primary care-of 3342 address at any time, except that it MUST NOT do so too frequently. 3343 Specifically, a mobile node MUST NOT send a Binding Update about a 3344 new care-of address to its home agent (which is required to register 3345 the new address as its primary care-of address) more often than once 3346 per MAX_UPDATE_RATE seconds. 3348 In addition, after discovering a new on-link subnet prefix, a mobile 3349 node MAY form a new (non-primary) care-of address using that subnet 3350 prefix, even when it has not switched to a new default router. A 3351 mobile node can have only one primary care-of address at a time 3352 (which is registered with its home agent), but it MAY have an 3353 additional care-of address for any or all of the prefixes on its 3354 current link. Furthermore, since a wireless network interface may 3355 actually allow a mobile node to be reachable on more than one link at 3356 a time (i.e., within wireless transmitter range of routers on more 3357 than one separate link), a mobile node MAY have care-of addresses 3358 on more than one link at a time. The use of more than one care-of 3359 address at a time is described in Section 10.16. 3361 As described in Section 4, in order to form a new care-of address, 3362 a mobile node MAY use either stateless [23] or stateful (e.g., 3363 DHCPv6 [2]) Address Autoconfiguration. If a mobile node needs to 3364 send packets as part of the method of address autoconfiguration, 3365 it MUST use an IPv6 link-local address rather than its own IPv6 3366 home address as the Source Address in the IPv6 header of each such 3367 autoconfiguration packet. 3369 In some cases, a mobile node may already know a (constant) IPv6 3370 address that has been assigned to it for its use only while 3371 visiting a specific foreign link. For example, a mobile node may be 3372 statically configured with an IPv6 address assigned by the system 3373 administrator of some foreign link, for its use while visiting that 3374 link. If so, rather than using Address Autoconfiguration to form a 3375 new care-of address using this subnet prefix, the mobile node MAY use 3376 its own pre-assigned address as its care-of address on this link. 3378 10.6. Sending Binding Updates to the Home Agent 3380 After deciding to change its primary care-of address as described 3381 in Sections 10.4 and 10.5, a mobile node MUST register this care-of 3382 address with its home agent in order to make this its primary care-of 3383 address. To do so, the mobile node sends a packet to its home agent 3384 containing a Binding Update option, with the packet constructed as 3385 follows: 3387 - The Home Registration (H) bit MUST be set in the Binding Update. 3389 - The Acknowledge (A) bit MUST be set in the Binding Update. 3391 - The packet MUST contain a Home Address option, giving the mobile 3392 node's home address for the binding. 3394 - The care-of address for the binding MUST be used as the Source 3395 Address in the packet's IPv6 header, unless an Alternate Care-of 3396 Address sub-option is included in the Binding Update option. 3398 - The Prefix Length field SHOULD be set to the length of the mobile 3399 node's subnet prefix in its home address, to request the mobile 3400 node's home agent to serve as a home agent for all home addresses 3401 for the mobile node based on all on-link subnet prefixes on the 3402 home link. Otherwise, this field MUST be set to zero. 3404 The Acknowledge (A) bit in the Binding Update requests the home 3405 agent to return a Binding Acknowledgement in response to this 3406 Binding Update. As described in Section 5.2, the mobile node SHOULD 3407 retransmit this Binding Update to its home agent until it receives 3408 a matching Binding Acknowledgement. Once reaching a retransmission 3409 timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD 3410 continue to periodically retransmit the Binding Update at this rate 3411 until acknowledged (or until it begins attempting to register a 3412 different primary care-of address). 3414 The Prefix Length field in the Binding Update allows the mobile node 3415 to request its home agent to serve all home addresses for the mobile 3416 node, as indicated by the interface identifier in the mobile node's 3417 home address (the remaining low-order bits after the indicated subnet 3418 prefix), together with each on-link subnet prefix on the home link. 3419 Until the lifetime of this registration expires, the home agent 3420 considers itself the home agent for each such home address of the 3421 mobile node. As the set of on-link subnet prefixes on the home link 3422 changes over time, the home agent changes the set of home addresses 3423 for this mobile node for which it is serving as the home agent. 3425 If the mobile node has additional home addresses using a different 3426 interface identifier, then the mobile node SHOULD send an additional 3427 packet containing a Binding Update to its home agent to register 3428 the care-of address for each such other home address (or set of 3429 home addresses sharing an interface identifier). These additional 3430 Binding Updates MUST each be sent as a separate packet, since each 3431 MUST contain an AH [9] or ESP [10] header to authenticate the Binding 3432 Update as coming from the home address being bound. 3434 10.7. Dynamic Home Agent Address Discovery 3436 It is possible that when the mobile node needs to send a Binding 3437 Update to its home agent to register its new primary care-of address, 3438 as described in Section 10.6, the mobile node may not know the 3439 address of any router on its home link that can serve as a home agent 3440 for it. For example, some nodes on its home link may have been 3441 reconfigured while the mobile node has been away from home, such that 3442 the router that was operating as the mobile node's home agent has 3443 been replaced by a different router serving this role. 3445 In this case, the mobile node SHOULD use the dynamic home agent 3446 address discovery procedure to find the address of a suitable home 3447 agent on its home link. To do so, the mobile node sends the packet, 3448 as described above, with the Destination Address in the packet's 3449 IPv6 header set to the "Mobile IPv6 Home-Agents" anycast address [8] 3450 for its home subnet prefix. As described in Section 9.2, the 3451 home agent on its home link that receives this Binding Update will 3452 reject the Update, returning to the mobile node the home agent's own 3453 global unicast IP address along with a list of the global unicast IP 3454 addresses of each other home agent operating on the home link. The 3455 mobile node SHOULD then retransmit its Binding Update to one of these 3456 homes agent using the provided global unicast address; the mobile 3457 node MAY re-attempt this home registration with each of these home 3458 agents in turn, by sending each a Binding Update and waiting for the 3459 matching Binding Acknowledgement, until its registration is accepted 3460 by one of these home agents. 3462 If the mobile node has a current registration with some home agent 3463 on its home link (the Lifetime for that registration has not yet 3464 expired), then the mobile node MUST attempt any new registration 3465 first with that home agent. If that registration attempt fails 3466 (e.g., times out or is rejected), the mobile node SHOULD then 3467 reattempt this registration with another home agent on its home link. 3468 If the mobile node knows of no other suitable home agent, then it MAY 3469 attempt the dynamic home agent address discovery procedure described 3470 above. 3472 10.8. Sending Binding Updates to Correspondent Nodes 3474 A mobile node MAY send a Binding Update to any correspondent node at 3475 any time to allow the correspondent node to cache the mobile node's 3476 current care-of address (subject to the rate limiting defined in 3477 Section 10.11). In any Binding Update sent by a mobile node, the 3478 care-of address (either the Source Address in the packet's IPv6 3479 header or the Care-of Address field in the Binding Update) MUST be 3480 set to one of the care-of addresses currently in use by the mobile 3481 node or to the mobile node's home address. 3483 If set to one of the mobile node's current care-of addresses (the 3484 care-of address given MAY differ from the mobile node's primary 3485 care-of address), the Binding Update requests the correspondent node 3486 to create or update an entry for the mobile node in the correspondent 3487 node's Binding Cache to record this care-of address for use in 3488 sending future packets to the mobile node. In this case, the 3489 Lifetime value sent in the Binding Update MUST be no greater than 3490 the remaining lifetime of the mobile node's home registration of its 3491 primary care-of address at its home agent. 3493 If, instead, the care-of address is set to the mobile node's home 3494 address, the Binding Update requests the correspondent node to delete 3495 any existing Binding Cache entry that it has for the mobile node. 3496 A mobile node MAY set the care-of address differently for sending 3497 Binding Updates to different correspondent nodes. 3499 When sending any Binding Update, the mobile node MUST record in its 3500 Binding Update List the following fields from the Binding Update: 3502 - The IP address of the node to which the Binding Update was sent. 3504 - The home address for which the Binding Update was sent (the value 3505 in the Home Address option in the packet carrying the Binding 3506 Update). 3508 - The remaining lifetime of the binding, initialized from the 3509 Lifetime field sent in the Binding Update. 3511 The mobile node MUST retain in its Binding Update List information 3512 about all Binding Updates sent, for which the lifetime of the binding 3513 has not yet expired. However, when sending a Binding Update, if an 3514 entry already exists in the mobile node's Binding Update List for 3515 an earlier Binding Update sent to that same destination node, the 3516 existing Binding Update List entry is updated to reflect the new 3517 Binding Update rather than creating a new Binding Update List entry. 3519 In general, when a mobile node sends a Binding Update to its home 3520 agent to register a new primary care-of address (as described in 3521 Section 10.6), the mobile node will also send a Binding Update to 3522 each other node for which an entry exists in the mobile node's 3523 Binding Update List. Thus, other relevant nodes are generally kept 3524 updated about the mobile node's binding and can send packets directly 3525 to the mobile node using the mobile node's current care-of address. 3527 The mobile node, however, need not send these Binding Updates 3528 immediately after configuring a new care-of address. For example, 3529 since the Binding Update is a destination option and can be included 3530 in any packet sent by a mobile node, the mobile node MAY delay 3531 sending a new Binding Update to any correspondent node for a 3532 short period of time, in hopes that the needed Binding Update 3533 can be included in some packet that the mobile node sends to that 3534 correspondent node for some other reason (for example, as part of 3535 some TCP connection in use). In this case, when sending a packet 3536 to some correspondent node, the mobile node SHOULD check in its 3537 Binding Update List to determine if a new Binding Update to this 3538 correspondent node is needed, and SHOULD include the new Binding 3539 Update in this packet as necessary. 3541 In addition, when a mobile node receives a packet for which the 3542 mobile node can deduce that the original sender of the packet has 3543 no Binding Cache entry for the mobile node, or for which the mobile 3544 node can deduce that the original sender of the packet has an 3545 out-of-date care-of address for the mobile node in its Binding Cache, 3546 the mobile node SHOULD return a Binding Update to the sender giving 3547 its current care-of address (subject to the rate limiting defined 3548 in Section 10.11). In particular, the mobile node SHOULD return a 3549 Binding Update in response to receiving a packet that meets all of 3550 the following tests: 3552 - The packet was tunneled using IPv6 encapsulation. 3554 - The Destination Address in the tunnel (outer) IPv6 header is 3555 equal to any of the mobile node's care-of addresses. 3557 - The Destination Address in the original (inner) IPv6 header 3558 is equal to one of the mobile node's home addresses; or this 3559 Destination Address is equal to one of the mobile node's previous 3560 care-of addresses, if the mobile node has an entry in its Binding 3561 Update List representing an unexpired Binding Update sent to 3562 a previous default router for this previous care-of address 3563 (Section 10.9). 3565 - The Source Address in the tunnel (outer) IPv6 header differs from 3566 the Source Address in the original (inner) IPv6 header. 3568 The destination address to which the Binding Update should be sent 3569 in response to receiving a packet meeting all of the above tests is 3570 the Source Address in the original (inner) IPv6 header of the packet. 3571 The home address for which this Binding Update is sent should be the 3572 Destination Address of the original (inner) packet. 3574 Binding Updates sent to correspondent nodes are not generally 3575 required to be acknowledged. However, if the mobile node wants 3576 to be sure that its new care-of address has been entered into a 3577 correspondent node's Binding Cache, the mobile node MAY request an 3578 acknowledgement by setting the Acknowledge (A) bit in the Binding 3579 Update. In this case, however, the mobile node SHOULD NOT continue 3580 to retransmit the Binding Update once the retransmission timeout 3581 period has reached MAX_BINDACK_TIMEOUT. 3583 A mobile node MAY choose to keep its location private from certain 3584 correspondent nodes, and thus need not send new Binding Updates to 3585 those correspondents. A mobile node MAY also send a Binding Update 3586 to such a correspondent node to instruct it to delete any existing 3587 binding for the mobile node from its Binding Cache, as described in 3588 Section 5.1. No other IPv6 nodes are authorized to send Binding 3589 Updates on behalf of a mobile node. 3591 10.9. Establishing Forwarding from a Previous Care-of Address 3593 When a mobile node connects to a new link and forms a new care-of 3594 address, it MAY establish forwarding of packets from a previous 3595 care-of address to this new care-of address. To do so, the mobile 3596 node sends a Binding Update to any home agent on the link on which 3597 the previous care-of address is located, indicating this previous 3598 care-of address as the home address for the binding, and giving its 3599 new care-of address as the binding's care-of address. Such packet 3600 forwarding allows packets destined to the mobile node from nodes that 3601 have not yet learned the mobile node's new care-of address, to be 3602 forwarded to the mobile node rather than being lost once the mobile 3603 node is no longer reachable at this previous care-of address. 3605 In constructing this Binding Update, the mobile node utilizes the 3606 following specific steps: 3608 - The Home Address field in the Home Address option in the packet 3609 carrying the Binding Update MUST be set to the previous care-of 3610 address for which packet forwarding is being established. 3612 - The care-of address for the new binding MUST be set to the new 3613 care-of address to which packets destined to the previous care-of 3614 address are to be forwarded. Normally, this care-of address for 3615 the binding is specified by setting the Source Address of the 3616 packet carrying the Binding Update, to this address. However, 3617 the mobile node MAY instead include an Alternate Care-of Address 3618 sub-option in the Binding Update option, with its Alternate 3619 Care-of Address field set to the care-of address for the binding. 3621 - The Home Registration (H) bit MUST also be set in this Binding 3622 Update, to request this home agent to temporarily act as a home 3623 agent for this previous care-of address. 3625 This home agent will thus tunnel packets for the mobile node (packets 3626 destined to its specified previous care-of address) to its new 3627 care-of address. All of the procedures defined for home agent 3628 operation MUST be followed by this home agent for this registration. 3629 Note that this home agent does not necessarily know (and need not 3630 know) the mobile node's (permanent) home address as part of this 3631 registration. 3633 The packet carrying the Binding Update MUST be addressed to 3634 this home agent's global unicast address. Normally, this global 3635 unicast address is learned by the mobile node based on the Router 3636 Advertisements received by the mobile node (Section 6.2) while 3637 attached to the link on which this previous care-of address and this 3638 home agent are located; the mobile node obtains this home agent 3639 address from its Home Agents List (Section 4.3) before clearing the 3640 list upon connecting to the new link. Alternatively, the mobile 3641 node MAY use dynamic home agent address discovery (Section 10.7) to 3642 discover the global unicast address of a home agent on this previous 3643 link, but it SHOULD use the address from its Home Agents List if 3644 available. 3646 10.10. Retransmitting Binding Updates 3648 If, after sending a Binding Update in which the Acknowledge (A) bit 3649 is set, a mobile node fails to receive a Binding Acknowledgement 3650 within INITIAL_BINDACK_TIMEOUT seconds, the mobile node SHOULD 3651 retransmit the Binding Update until a Binding Acknowledgement 3652 is received. Such a retransmitted Binding Update MUST use the 3653 same Sequence Number value as the original transmission. The 3654 retransmissions by the mobile node MUST use an exponential 3655 back-off process, in which the timeout period is doubled 3656 upon each retransmission until either the node receives a 3657 Binding Acknowledgement or the timeout period reaches the value 3658 MAX_BINDACK_TIMEOUT. 3660 10.11. Rate Limiting for Sending Binding Updates 3662 A mobile node MUST NOT send Binding Updates about the same binding 3663 to any node more often than once per MAX_UPDATE_RATE seconds. After 3664 sending MAX_FAST_UPDATES consecutive Binding Updates to a particular 3665 node with the same care-of address, the mobile node SHOULD reduce 3666 its rate of sending Binding Updates to that node, to the rate of 3667 SLOW_UPDATE_RATE per second. The mobile node MAY continue to send 3668 Binding Updates at this slower rate indefinitely, in hopes that the 3669 node will eventually be able to process a Binding Update and begin 3670 to route its packets directly to the mobile node at its new care-of 3671 address. 3673 10.12. Receiving Binding Acknowledgements 3675 Upon receiving a packet carrying a Binding Acknowledgement, a mobile 3676 node MUST validate the packet according to the following tests: 3678 - The packet contains a valid AH [9] or ESP [10] header providing 3679 sender authentication, data integrity protection, and replay 3680 protection. 3682 - The Option Length field in the option is greater than or equal to 3683 11 octets. 3685 - The Sequence Number field matches the Sequence Number sent by the 3686 mobile node to this destination address in an outstanding Binding 3687 Update. 3689 Any Binding Acknowledgement not satisfying all of these tests MUST be 3690 silently ignored, although the remainder of the packet (i.e., other 3691 options, extension headers, or payload) SHOULD be processed normally 3692 according to any procedure defined for that part of the packet. 3694 When a mobile node receives a packet carrying a valid Binding 3695 Acknowledgement, the mobile node MUST examine the Status field as 3696 follows: 3698 - If the Status field indicates that the Binding Update was 3699 accepted (the Status field is less than 128), then the mobile 3700 node MUST update the corresponding entry in its Binding Update 3701 List to indicate that the Binding Update has been acknowledged. 3702 The mobile node MUST then stop retransmitting the Binding Update. 3704 - If the Status field indicates that the Binding Update was 3705 rejected (the Status field is greater than or equal to 128), then 3706 the mobile node MUST delete the corresponding Binding Update List 3707 entry (and MUST also stop retransmitting the Binding Update). 3708 Optionally, the mobile node MAY then take steps to correct the 3709 cause of the error and retransmit the Binding Update (with a new 3710 Sequence Number value), subject to the rate limiting restriction 3711 specified in Section 10.11. In particular, if the Status field 3712 is equal to 135 (dynamic home agent address discovery response), 3713 then the mobile node MAY reattempt its home registration with 3714 the home agent address given in the Source Address field of the 3715 packet carrying the Binding Acknowledgement or with any of the 3716 home agent IP addresses listed in the Home Agents List Sub-Option 3717 in the Binding Acknowledgement. If any of these addresses is not 3718 a global unicast address or does not have a subnet prefix equal 3719 to the mobile node's own subnet prefix, then that particular 3720 address MUST be ignored and the mobile node MUST NOT reattempt 3721 its home registration with that home agent. 3723 10.13. Receiving Binding Requests 3725 When a mobile node receives a packet containing a Binding Request, 3726 it SHOULD return to the sender a packet containing a Binding Update. 3727 The Lifetime field in this Binding Update SHOULD be set to a new 3728 lifetime, extending any current lifetime remaining from a previous 3729 Binding Update sent to this node (as indicated in any existing 3730 Binding Update List entry for this node), except that this lifetime 3731 MUST NOT exceed the remaining lifetime for the mobile node's primary 3732 care-of address registration at its home agent. When sending this 3733 Binding Update, the mobile node MUST update its Binding Update List 3734 in the same way as for any other Binding Update sent by the mobile 3735 node. 3737 Note, however, that the mobile node MAY choose to keep its current 3738 binding private from the sender of the Binding Request. In this 3739 case, the mobile node instead SHOULD returns a Binding Update to the 3740 sender, in which the Lifetime field is set to zero and the care-of 3741 address is set to the mobile node's home address. 3743 If the Binding Request for which the Binding Update is being returned 3744 contains a Unique Identifer Sub-Option, the Binding Update MUST also 3745 include a Unique Identifier Sub-Option. The unique identifier in the 3746 SUb-Option Data field of the Unique Identifier Sub-Option MUST be 3747 copied from the unique identifier carried in the Binding Request. 3749 10.14. Receiving ICMP Error Messages 3751 The Option Type value for a Binding Update option specifies that 3752 any node receiving this option that does not recognize the Option 3753 Type SHOULD return an ICMP Parameter Problem, Code 2, message to 3754 the sender of the packet containing the Binding Update option. If 3755 a node sending a Binding Update receives such an ICMP error message 3756 in response, it should record in its Binding Update List that future 3757 Binding Updates should not be sent to this destination. 3759 Likewise, although ALL IPv6 nodes (whether host or router, whether 3760 mobile or stationary) MUST implement the ability to correctly process 3761 received packets containing a Home Address option, all Option Type 3762 values in IPv6 include a specification of the behavior that a node 3763 receiving a packet containing this option performs if it does not 3764 implement receipt of that type of option. For the Home Address 3765 option, the Option Type value specifies that any node receiving 3766 this option that does not recognize the Option Type SHOULD return 3767 an ICMP Parameter Problem, Code 2, message to the sender of the 3768 packet containing the Home Address option. If a mobile node receives 3769 such an ICMP error message from some node indicating that it does 3770 not recognize the mobile node's Home Address option, the mobile 3771 node SHOULD log the error and then discard the ICMP message; this 3772 error message indicates that the node to which the original packet 3773 was addressed (the node returning the ICMP error message) does not 3774 correctly implement this required part of the IPv6 protocol. 3776 10.15. Receiving Tunneled Router Advertisements 3778 Section 9.7 describes the operation of a home agent to support 3779 renumbering a mobile node's home subnet while the mobile node is 3780 away from home. The home agent tunnels certain Router Advertisement 3781 messages to the mobile node while away from home, giving "important" 3782 Prefix Information options that describe changes in the prefixes in 3783 use on the mobile node's home link. 3785 When a mobile node receives a tunneled Router Advertisement, it MUST 3786 validate it according to the following tests: 3788 - The Source Address of the IP packet carrying the Router 3789 Advertisement is the same as the home agent address to which the 3790 mobile node last sent an accepted "home registration" Binding 3791 Update to register its primary care-of address. 3793 - The packet contains either an AH [9] or ESP [10] header providing 3794 sender authentication, data integrity protection, and replay 3795 protection. 3797 - The packet contains a Binding Request destination option. 3799 - The Binding Request option contains a Unique Identifier 3800 Sub-Option. 3802 Any received tunneled Router Advertisement not meeting all of these 3803 tests MUST be silently discarded. 3805 If a received tunneled Router Advertisement is not discarded 3806 according to the tests listed above, the mobile node MUST process the 3807 Router Advertisement as if it were connected to its home link [14]. 3808 Such processing MAY result in the mobile node configuring a new home 3809 address, although due to separation between preferred lifetime and 3810 valid lifetime, such changes should not affect most communication by 3811 the mobile node, in the same way as for nodes that are at home. 3813 In addition, in processing the packet containing this Router 3814 Advertisement, the mobile node SHOULD return to the home agent a 3815 Binding Update in response to the Binding Request carried in the 3816 packet. The correct formation of this Binding Update by the mobile 3817 node and processing of it by the home agent will be viewed by the 3818 home agent as an acknowledgement of this Router Advertisement, 3819 confirming to it that this Router Advertisement was received by the 3820 mobile node. 3822 10.16. Using Multiple Care-of Addresses 3824 As described in Section 10.5, a mobile node MAY use more than one 3825 care-of address at a time. Particularly in the case of many wireless 3826 networks, a mobile node effectively might be reachable through 3827 multiple links at the same time (e.g., with overlapping wireless 3828 cells), on which different on-link subnet prefixes may exist. A 3829 mobile node SHOULD select a primary care-of address from among those 3830 care-of addresses it has formed using any of these subnet prefixes, 3831 based on the movement detection mechanism in use, as described in 3832 Section 10.4. When the mobile node selects a new primary care-of 3833 address, it MUST register it with its home agent by sending it a 3834 Binding Update with the Home Registration (H) and Acknowledge (A) 3835 bits set, as described in Section 10.6. 3837 To assist with smooth handoffs, a mobile node SHOULD retain 3838 its previous primary care-of address as a (non-primary) care-of 3839 address, and SHOULD still accept packets at this address, even after 3840 registering its new primary care-of address with its home agent. 3841 This is reasonable, since the mobile node could only receive packets 3842 at its previous primary care-of address if it were indeed still 3843 connected to that link. If the previous primary care-of address was 3844 allocated using stateful Address Autoconfiguration [2], the mobile 3845 node may not wish to release the address immediately upon switching 3846 to a new primary care-of address. 3848 10.17. Routing Multicast Packets 3850 A mobile node that is connected to its home link functions in the 3851 same way as any other (stationary) node. Thus, when it is at home, 3852 a mobile node functions identically to other multicast senders and 3853 receivers. This section therefore describes the behavior of a mobile 3854 node that is not on its home link. 3856 In order to receive packets sent to some multicast group, a mobile 3857 node must join that multicast group. One method by which a mobile 3858 node MAY join the group is via a (local) multicast router on the 3859 foreign link being visited. The mobile node SHOULD use its care-of 3860 address sharing a subnet prefix with the multicast router, as 3861 the source IPv6 address of its multicast group membership control 3862 messages. 3864 Alternatively, a mobile node MAY join multicast groups via a 3865 bi-directional tunnel to its home agent. The mobile node tunnels its 3866 multicast group membership control packets to its home agent, and the 3867 home agent forwards multicast packets down the tunnel to the mobile 3868 node. 3870 A mobile node that wishes to send packets to a multicast group 3871 also has two options: (1) send directly on the foreign link being 3872 visited; or (2) send via a tunnel to its home agent. Because 3873 multicast routing in general depends upon the Source Address used in 3874 the IPv6 header of the multicast packet, a mobile node that tunnels a 3875 multicast packet to its home agent MUST use its home address as the 3876 IPv6 Source Address of the inner multicast packet. 3878 10.18. Returning Home 3880 A mobile node detects that it has returned to its home link through 3881 the movement detection algorithm in use (Section 10.4), when the 3882 mobile node detects that its home subnet prefix is again on-link. 3883 The mobile node SHOULD then send a Binding Update to its home agent, 3884 to instruct its home agent to no longer intercept or tunnel packets 3885 for it. In this Binding Update, the mobile node MUST set the care-of 3886 address for the binding (the Source Address field in the packet's 3887 IPv6 header) to the mobile node's own home address. As with other 3888 Binding Updates sent to register with its home agent, the mobile 3889 node MUST set the Acknowledge (A) and Home Registration (H) bits, 3890 and SHOULD retransmit the Binding Update until a matching Binding 3891 Acknowledgement is received. 3893 In addition, the mobile node MUST multicast onto the home link 3894 (to the all-nodes multicast address) a Neighbor Advertisement 3895 message [14], to advertise the mobile node's own link-layer address 3896 for its own home address. The Target Address in this Neighbor 3897 Advertisement message MUST be set to the mobile node's home address, 3898 and the Advertisement MUST include a Target Link-layer Address option 3899 specifying the mobile node's link-layer address. The mobile node 3900 MUST multicast such a Neighbor Advertisement message for each of its 3901 home addresses, as defined by the current on-link prefixes, including 3902 its link-local address and site-local address. The Solicited 3903 Flag (S) in these Advertisements MUST NOT be set, since they were 3904 not solicited by any Neighbor Solicitation message. The Override 3905 Flag (O) in these Advertisements MUST be set, indicating that the 3906 Advertisements SHOULD override any existing Neighbor Cache entries at 3907 any node receiving them. 3909 Since multicasts on the local link (such as Ethernet) are typically 3910 not guaranteed to be reliable, the mobile node MAY retransmit these 3911 Neighbor Advertisement messages up to MAX_ADVERT_REXMIT times to 3912 increase their reliability. It is still possible that some nodes on 3913 the home link will not receive any of these Neighbor Advertisements, 3914 but these nodes will eventually be able to recover through use of 3915 Neighbor Unreachability Detection [14]. 3917 11. Constants 3919 INITIAL_BINDACK_TIMEOUT 1 second 3921 MAX_BINDACK_TIMEOUT 256 seconds 3923 MAX_UPDATE_RATE once per second 3925 SLOW_UPDATE_RATE once per 10 seconds 3927 MAX_FAST_UPDATES 5 transmissions 3929 MAX_ADVERT_REXMIT 3 transmissions 3931 12. IANA Considerations 3933 This document defines four new types of IPv6 destination options, 3934 each of which must be assigned an Option Type value: 3936 - The Binding Update option, described in Section 5.1 3938 - The Binding Acknowledgement option, described in Section 5.2 3940 - The Binding Request option, described in Section 5.3 3942 - The Home Address option, described in Section 5.4 3944 In addition, this document defines two new Neighbor Discovery [14] 3945 options, which must be assigned Option Type values within the option 3946 numbering space for Neighbor Discovery messages: 3948 - The Advertisement Interval option, described in Section 6.3. 3950 - The Home Agent Information option, described in Section 6.4. 3952 Finally, this document defines a new type of anycast address, which 3953 must be assigned a reserved value for use with any subnet prefix to 3954 define this anycast address on each subnet: 3956 - The "Mobile IPv6 Home-Agents" anycast address [8], used in the 3957 dynamic home agent address discovery procedure described in 3958 Sections 9.2 and 10.7. 3960 13. Security Considerations 3962 13.1. Binding Updates, Acknowledgements, and Requests 3964 The Binding Update option described in this document will result 3965 in packets addressed to a mobile node being delivered instead to 3966 its care-of address. This ability to change the routing of these 3967 packets could be a significant vulnerability if any packet containing 3968 a Binding Update option was not authenticated. Such use of "remote 3969 redirection", for instance as performed by the Binding Update option, 3970 is widely understood to be a security problem in the current Internet 3971 if not authenticated [1]. 3973 The Binding Acknowledgement option also requires authentication, 3974 since, for example, an attacker could otherwise trick a mobile node 3975 into believing a different outcome from a registration attempt with 3976 its home agent. 3978 No authentication is required for the Binding Request option, since 3979 the use of this option does not modify or create any state in either 3980 the sender or the receiver. The Binding Request option does open 3981 some issues with binding privacy, but those issues can be dealt with 3982 either through existing IPsec encryption mechanisms or through use of 3983 firewalls. 3985 The existing IPsec replay protection mechanisms allow a "replay 3986 protection window" to support receiving packets out of order. 3987 Although appropriate for many forms of communication, Binding Updates 3988 MUST be applied only in the order sent. The Binding Update option 3989 thus includes a Sequence Number field to provide this necessary 3990 sequencing. The use of this Sequence Number together with IPsec 3991 replay protection is similar in many ways, for example, to the the 3992 sequence number in TCP. IPsec provides strong replay protection but 3993 no ordering, and the sequence number provides ordering but need not 3994 worry about replay protection such as through the sequence number 3995 wrapping around. 3997 13.2. Home Address Option 3999 No special authentication of the Home Address option is required, 4000 except that if the IPv6 header of a packet is covered by 4001 authentication, then that authentication MUST also cover the Home 4002 Address option; this coverage is achieved automatically by the 4003 definition of the Option Type code for the Home Address option 4004 (Section 5.4), since it indicates that the option is included in the 4005 authentication computation. Thus, even when authentication is used 4006 in the IPv6 header, the security of the Source Address field in the 4007 IPv6 header is not compromised by the presence of a Home Address 4008 option. Without authentication of the packet, then any field in the 4009 IPv6 header, including the Source Address field, and any other parts 4010 of the packet, including the Home Address option, can be forged or 4011 modified in transit. In this case, the contents of the Home Address 4012 option is no more suspect than any other part of the packet. 4014 The use of the Home Address option allows packets sent by a 4015 mobile node to pass normally through routers implementing ingress 4016 filtering [6]. Since the care-of address used in the Source Address 4017 field of the packet's IPv6 header is topologically correct for the 4018 sending location of the mobile node, ingress filtering can trace the 4019 location of the mobile node in the same way as can be done with any 4020 sender when ingress filtering is in use. 4022 However, if a node receiving a packet that includes a Home Address 4023 option implements the processing of this option by physically 4024 copying the Home Address field from the option into the IPv6 header, 4025 replacing the Source Address field there, then the ability to 4026 trace the true location of the sender is removed once this step 4027 in the processing is performed. This diminishing of the power of 4028 ingress filtering only occurs once the packet has been received at 4029 its ultimate destination, and does not affect the capability of 4030 ingress filtering while the packet is in transit. Furthermore, this 4031 diminishing can be entirely eliminated by appropriate implementation 4032 techniques in the receiving node. For example, the original contents 4033 of the Source Address field (the sending care-of address) could be 4034 saved elsewhere in memory with the packet, until all processing of 4035 the packet is completed. 4037 13.3. General Mobile Computing Issues 4039 The mobile computing environment is potentially very different from 4040 the ordinary computing environment. In many cases, mobile computers 4041 will be connected to the network via wireless links. Such links 4042 are particularly vulnerable to passive eavesdropping, active replay 4043 attacks, and other active attacks. Furthermore, mobile computers 4044 are more susceptible to loss or theft than stationary computers. 4045 Any secrets such as authentication or encryption keys stored on the 4046 mobile computer are thus subject to compromise in ways generally not 4047 common in the non-mobile environment. 4049 Users who have sensitive data that they do not wish others to have 4050 access to should use additional mechanisms (such as encryption) to 4051 provide privacy protection, but such mechanisms are beyond the scope 4052 of this document. Users concerned about traffic analysis should 4053 consider appropriate use of link encryption. If stronger location 4054 privacy is desired, the mobile node can create a tunnel to its home 4055 agent. Then, packets destined for correspondent nodes will appear 4056 to emanate from the home subnet, and it may be more difficult to 4057 pinpoint the location of the mobile node. Such mechanisms are all 4058 beyond the scope of this document. 4060 Changes from Previous Version of the Draft 4062 This appendix briefly lists some of the major changes in this 4063 draft relative to the previous version of this same draft, 4064 draft-ietf-mobileip-ipv6-08.txt: 4066 - Added a new section (Section 10.2), giving guidance to 4067 implementors on the interaction between outbound Mobile IP 4068 processing and outbound IPsec processing for packets sent by a 4069 mobile node while away from home. 4071 - Changed the optional Care-of Address field in the Binding Update 4072 option format (Section 5.1) to instead be a sub-option. This 4073 better matches the use of other sub-options to encode optional 4074 information in an option, and also makes the specification of the 4075 alignment requirement for the Binding Update option easier. 4077 - Also removed the Care-of Address Present (C) bit in the Binding 4078 Update option format, as it is no longer needed with the use of a 4079 sub-option to specify the alternate care-of address. NOTE: This 4080 change also resulted in moving the Router (R) bit over by one bit 4081 position in the Binding Update option format. 4083 - Added explicit specification of the alignment requirements [5] 4084 for each of the Mobile IPv6 destination options and sub-options. 4086 - Introduced Pad1 and PadN sub-options (Section 5.5) to allow 4087 the sub-options in Mobile IPv6 options to be aligned properly. 4088 To match the option numbering used for the Pad1 and PadN 4089 options defined for use in Hop-by-Hop Options and Destination 4090 Options extension headers [5], these two sub-options have been 4091 numbered 0 and 1, respectively. NOTE: This change also resulted 4092 in changing the sub-option numbers of the other Mobile IPv6 4093 sub-options. 4095 - Added an explicit specification of the units in which 4096 the Lifetime and Refresh fields are expressed in the 4097 Binding Acknowledgement option (Section 5.2). As with the 4098 corresponding Lifetime field in the Binding Update option and the 4099 Valid Lifetime and Preferred Lifetime in the Prefix Information 4100 option used by Neighbor Discovery [14], these fields in the 4101 Binding Acknowledgement option are expressed in seconds. 4103 - Rewrote Section 10.9 to indicate that the Binding Update is sent 4104 to any home agent on the link on which the specified previous 4105 care-of address is located, rather than (necessarily) to the 4106 router that it used as its default router on that link. Also, 4107 the mobile node can establish packet forwarding from any previous 4108 care-of address, not just from its previous primary care-of 4109 address. 4111 - Removed the Status value of 129 (Poorly formed Binding Update) 4112 for the Binding Acknowledgement option, since this value is not 4113 used. Instead, poorly formed Binding Updates have already been 4114 defined (Section 8.2) to be silently discarded. 4116 - Removed the Status value of 134 (Sequence Number field value too 4117 small) for the Binding Acknowledgement option, since this value 4118 is not used. Instead, received Binding Updates in which the 4119 Sequence Number field is not greater than the Sequence Number 4120 received in the previous Binding Update for this home address, 4121 if any, have already been defined (Section 8.2) to be silently 4122 discarded. 4124 - Corrected a few more minor typographical errors in places. 4126 Acknowledgements 4128 We would like to thank the members of the Mobile IP and IPng 4129 Working Groups for their comments and suggestions on this work. 4130 We would particularly like to thank (in alphabetical order) 4131 Josh Broch (Carnegie Mellon University), Rich Draves (Microsoft 4132 Research), Jun-Ichiro Hagino (IIJ Research Laboratory), Thomas Narten 4133 (IBM), Erik Nordmark (Sun Microsystems), Simon Nybroe (Telebit 4134 Communications), Patrice Romand (Bull S.A.), Tom Soderlund (Nokia 4135 Research), and Jim Solomon (RedBack Networks) for their detailed 4136 reviews of earlier versions of this draft. Their suggestions have 4137 helped to improve both the design and presentation of the protocol. 4139 References 4141 [1] S. M. Bellovin. Security problems in the TCP/IP protocol suite. 4142 ACM Computer Communications Review, 19(2), March 1989. 4144 [2] Jim Bound and Charles Perkins. Dynamic Host Configuration 4145 Protocol for IPv6 (DHCPv6), February 1999. Work in progress. 4147 [3] Scott Bradner. Key words for use in RFCs to indicate 4148 requirement levels. RFC 2119, March 1997. 4150 [4] Alex Conta and Stephen Deering. Generic packet tunneling in 4151 IPv6 specification. RFC 2473, December 1998. 4153 [5] Stephen E. Deering and Robert M. Hinden. Internet Protocol 4154 version 6 (IPv6) specification. RFC 2460, December 1998. 4156 [6] Paul Ferguson and Daniel Senie. Network ingress filtering: 4157 Defeating denial of service attacks which employ IP source 4158 address spoofing. RFC 2267, January 1998. 4160 [7] Robert M. Hinden and Stephen E. Deering. IP Version 6 4161 addressing architecture. RFC 2373, July 1998. 4163 [8] David B. Johnson and Stephen E. Deering. Reserved IPv6 subnet 4164 anycast addresses. RFC 2526, March 1999. 4166 [9] Stephen Kent and Randall Atkinson. IP Authentication header. 4167 RFC 2402, November 1998. 4169 [10] Stephen Kent and Randall Atkinson. IP Encapsulating Security 4170 Payload (ESP). RFC 2406, November 1998. 4172 [11] Stephen Kent and Randall Atkinson. Security architecture for 4173 the Internet Protocol. RFC 2401, November 1998. 4175 [12] P. Mockapetris. Domain Names -- concepts and facilities. 4176 RFC 1034, November 1987. 4178 [13] P. Mockapetris. Domain Names -- implementation and 4179 specification. RFC 1035, November 1987. 4181 [14] Thomas Narten, Erik Nordmark, and William Allen Simpson. 4182 Neighbor Discovery for IP version 6 (IPv6). RFC 2461, December 4183 1998. 4185 [15] Charles Perkins. IP encapsulation within IP. RFC 2003, October 4186 1996. 4188 [16] Charles Perkins, editor. IP mobility support. RFC 2002, 4189 October 1996. 4191 [17] Charles Perkins. Minimal encapsulation within IP. RFC 2004, 4192 October 1996. 4194 [18] Charles Perkins and David B. Johnson. Route optimization in 4195 Mobile IP, February 1999. Work in progress. 4197 [19] David C. Plummer. An Ethernet address resolution protocol: 4198 Or converting network protocol addresses to 48.bit Ethernet 4199 addresses for transmission on Ethernet hardware. RFC 826, 4200 November 1982. 4202 [20] J. B. Postel. User Datagram Protocol. RFC 768, August 1980. 4204 [21] J. B. Postel, editor. Transmission Control Protocol. RFC 793, 4205 September 1981. 4207 [22] Joyce K. Reynolds and Jon Postel. Assigned numbers. RFC 1700, 4208 October 1994. See also http://www.iana.org/numbers.html. 4210 [23] Susan Thomson and Thomas Narten. IPv6 stateless address 4211 autoconfiguration. RFC 2462, December 1998. 4213 Chair's Address 4215 The Working Group can be contacted via its current chairs: 4217 Phil Roberts 4218 Motorola 4219 1501 West Shure Drive 4220 Arlington Heights, IL 60004 4222 Phone: +1 847 632-3148 4223 E-mail: qa3445@email.mot.com 4225 Basavaraj Patil 4226 Nortel Networks, Inc. 4227 2201 Lakeside Blvd. 4228 Richardson, TX 75082-4399 4229 USA 4231 Phone: +1 972 684-1489 4232 Fax: +1 972 685-3207 4233 E-mail: bpatil@nortelnetworks.com 4235 Authors' Addresses 4237 Questions about this document can also be directed to the authors: 4239 David B. Johnson 4240 Carnegie Mellon University 4241 Computer Science Department 4242 5000 Forbes Avenue 4243 Pittsburgh, PA 15213-3891 4244 USA 4246 Phone: +1 412 268-7399 4247 Fax: +1 412 268-5576 4248 E-mail: dbj@cs.cmu.edu 4250 Charles Perkins 4251 Nokia 4252 313 Fairchild Drive 4253 Mountain View, CA 94043 4254 USA 4256 Phone: +1 650 625-2986 4257 Fax: +1 650 691-2170 4258 E-mail: charliep@iprg.nokia.com