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(The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- 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 (Oct 04, 2010) is 4925 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) ** Obsolete normative reference: RFC 2401 (ref. '2') (Obsoleted by RFC 4301) ** Obsolete normative reference: RFC 2402 (ref. '3') (Obsoleted by RFC 4302, RFC 4305) ** Obsolete normative reference: RFC 2406 (ref. '4') (Obsoleted by RFC 4303, RFC 4305) ** Obsolete normative reference: RFC 2460 (ref. '5') (Obsoleted by RFC 8200) ** Downref: Normative reference to an Informational RFC: RFC 3232 (ref. '10') -- Possible downref: Non-RFC (?) normative reference: ref. '11' ** Obsolete normative reference: RFC 4941 (ref. '20') (Obsoleted by RFC 8981) ** Downref: Normative reference to an Informational RFC: RFC 5014 (ref. '21') ** Obsolete normative reference: RFC 5226 (ref. '23') (Obsoleted by RFC 8126) -- Obsolete informational reference (is this intentional?): RFC 3315 (ref. '29') (Obsoleted by RFC 8415) -- Obsolete informational reference (is this intentional?): RFC 3344 (ref. '30') (Obsoleted by RFC 5944) -- Obsolete informational reference (is this intentional?): RFC 3484 (ref. '31') (Obsoleted by RFC 6724) -- Obsolete informational reference (is this intentional?): RFC 3627 (ref. '35') (Obsoleted by RFC 6547) == Outdated reference: A later version (-03) exists of draft-savola-ipv6-rh-ha-security-02 -- Obsolete informational reference (is this intentional?): RFC 4306 (ref. '41') (Obsoleted by RFC 5996) Summary: 8 errors (**), 0 flaws (~~), 4 warnings (==), 8 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IETF Mobile IP Working Group C. Perkins (Ed.) 3 Internet-Draft Tellabs Inc. 4 Obsoletes: 3775 (if approved) D. Johnson 5 Expires: April 7, 2011 Rice University 6 J. Arkko 7 Ericsson 8 Oct 04, 2010 10 Mobility Support in IPv6 11 draft-ietf-mext-rfc3775bis-07.txt 13 Abstract 15 This document specifies Mobile IPv6, a protocol which allows nodes to 16 remain reachable while moving around in the IPv6 Internet. Each 17 mobile node is always identified by its home address, regardless of 18 its current point of attachment to the Internet. While situated away 19 from its home, a mobile node is also associated with a care-of 20 address, which provides information about the mobile node's current 21 location. IPv6 packets addressed to a mobile node's home address are 22 transparently routed to its care-of address. The protocol enables 23 IPv6 nodes to cache the binding of a mobile node's home address with 24 its care-of address, and to then send any packets destined for the 25 mobile node directly to it at this care-of address. To support this 26 operation, Mobile IPv6 defines a new IPv6 protocol and a new 27 destination option. All IPv6 nodes, whether mobile or stationary, 28 can communicate with mobile nodes. This document obsoletes RFC 3775. 30 Status of this Memo 32 This Internet-Draft is submitted to IETF in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on April 7, 2011. 47 Copyright Notice 48 Copyright (c) 2010 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6 64 2. Comparison with Mobile IP for IPv4 . . . . . . . . . . . . . 8 65 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9 66 3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 9 67 3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 11 68 4. Overview of Mobile IPv6 . . . . . . . . . . . . . . . . . . . 15 69 4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 15 70 4.2. New IPv6 Protocol . . . . . . . . . . . . . . . . . . . . 17 71 4.3. New IPv6 Destination Option . . . . . . . . . . . . . . . 18 72 4.4. New IPv6 ICMP Messages . . . . . . . . . . . . . . . . . 18 73 4.5. Conceptual Data Structure Terminology . . . . . . . . . . 19 74 4.6. Unique-Local Addressability . . . . . . . . . . . . . . . 19 75 5. Overview of Mobile IPv6 Security . . . . . . . . . . . . . . 21 76 5.1. Binding Updates to Home Agents . . . . . . . . . . . . . 21 77 5.2. Binding Updates to Correspondent Nodes . . . . . . . . . 22 78 5.2.1. Node Keys . . . . . . . . . . . . . . . . . . . . . . 22 79 5.2.2. Nonces . . . . . . . . . . . . . . . . . . . . . . . 23 80 5.2.3. Cookies and Tokens . . . . . . . . . . . . . . . . . 23 81 5.2.4. Cryptographic Functions . . . . . . . . . . . . . . . 24 82 5.2.5. Return Routability Procedure . . . . . . . . . . . . 24 83 5.2.6. Authorizing Binding Management Messages . . . . . . . 29 84 5.2.7. Updating Node Keys and Nonces . . . . . . . . . . . . 31 85 5.2.8. Preventing Replay Attacks . . . . . . . . . . . . . . 32 86 5.2.9. Handling Interruptions to Return Routability . . . . 32 87 5.3. Dynamic Home Agent Address Discovery . . . . . . . . . . 33 88 5.4. Mobile Prefix Discovery . . . . . . . . . . . . . . . . . 33 89 5.5. Payload Packets . . . . . . . . . . . . . . . . . . . . . 33 90 6. New IPv6 Protocol, Message Types, and Destination Option . . 35 91 6.1. Mobility Header . . . . . . . . . . . . . . . . . . . . . 35 92 6.1.1. Format . . . . . . . . . . . . . . . . . . . . . . . 35 93 6.1.2. Binding Refresh Request Message . . . . . . . . . . . 37 94 6.1.3. Home Test Init Message . . . . . . . . . . . . . . . 38 95 6.1.4. Care-of Test Init Message . . . . . . . . . . . . . . 39 96 6.1.5. Home Test Message . . . . . . . . . . . . . . . . . . 40 97 6.1.6. Care-of Test Message . . . . . . . . . . . . . . . . 41 98 6.1.7. Binding Update Message . . . . . . . . . . . . . . . 43 99 6.1.8. Binding Acknowledgement Message . . . . . . . . . . . 45 100 6.1.9. Binding Error Message . . . . . . . . . . . . . . . . 48 101 6.2. Mobility Options . . . . . . . . . . . . . . . . . . . . 49 102 6.2.1. Format . . . . . . . . . . . . . . . . . . . . . . . 49 103 6.2.2. Pad1 . . . . . . . . . . . . . . . . . . . . . . . . 50 104 6.2.3. PadN . . . . . . . . . . . . . . . . . . . . . . . . 51 105 6.2.4. Binding Refresh Advice . . . . . . . . . . . . . . . 51 106 6.2.5. Alternate Care-of Address . . . . . . . . . . . . . . 51 107 6.2.6. Nonce Indices . . . . . . . . . . . . . . . . . . . . 52 108 6.2.7. Binding Authorization Data . . . . . . . . . . . . . 53 109 6.3. Home Address Option . . . . . . . . . . . . . . . . . . . 54 110 6.4. Type 2 Routing Header . . . . . . . . . . . . . . . . . . 56 111 6.4.1. Format . . . . . . . . . . . . . . . . . . . . . . . 56 112 6.5. ICMP Home Agent Address Discovery Request Message . . . . 58 113 6.6. ICMP Home Agent Address Discovery Reply Message . . . . . 59 114 6.7. ICMP Mobile Prefix Solicitation Message Format . . . . . 60 115 6.8. ICMP Mobile Prefix Advertisement Message Format . . . . . 61 116 7. Modifications to IPv6 Neighbor Discovery . . . . . . . . . . 65 117 7.1. Modified Router Advertisement Message Format . . . . . . 65 118 7.2. Modified Prefix Information Option Format . . . . . . . . 65 119 7.3. New Advertisement Interval Option Format . . . . . . . . 67 120 7.4. New Home Agent Information Option Format . . . . . . . . 68 121 7.5. Changes to Sending Router Advertisements . . . . . . . . 70 122 8. Requirements for Types of IPv6 Nodes . . . . . . . . . . . . 72 123 8.1. All IPv6 Nodes . . . . . . . . . . . . . . . . . . . . . 72 124 8.2. IPv6 Nodes with Support for Route Optimization . . . . . 72 125 8.3. All IPv6 Routers . . . . . . . . . . . . . . . . . . . . 74 126 8.4. IPv6 Home Agents . . . . . . . . . . . . . . . . . . . . 74 127 8.5. IPv6 Mobile Nodes . . . . . . . . . . . . . . . . . . . . 76 128 9. Correspondent Node Operation . . . . . . . . . . . . . . . . 78 129 9.1. Conceptual Data Structures . . . . . . . . . . . . . . . 78 130 9.2. Processing Mobility Headers . . . . . . . . . . . . . . . 79 131 9.3. Packet Processing . . . . . . . . . . . . . . . . . . . . 79 132 9.3.1. Receiving Packets with Home Address Option . . . . . 79 133 9.3.2. Sending Packets to a Mobile Node . . . . . . . . . . 80 134 9.3.3. Sending Binding Error Messages . . . . . . . . . . . 82 135 9.3.4. Receiving ICMP Error Messages . . . . . . . . . . . . 82 136 9.4. Return Routability Procedure . . . . . . . . . . . . . . 83 137 9.4.1. Receiving Home Test Init Messages . . . . . . . . . . 83 138 9.4.2. Receiving Care-of Test Init Messages . . . . . . . . 83 139 9.4.3. Sending Home Test Messages . . . . . . . . . . . . . 84 140 9.4.4. Sending Care-of Test Messages . . . . . . . . . . . . 84 141 9.5. Processing Bindings . . . . . . . . . . . . . . . . . . . 84 142 9.5.1. Receiving Binding Updates . . . . . . . . . . . . . . 84 143 9.5.2. Requests to Cache a Binding . . . . . . . . . . . . . 87 144 9.5.3. Requests to Delete a Binding . . . . . . . . . . . . 87 145 9.5.4. Sending Binding Acknowledgements . . . . . . . . . . 88 146 9.5.5. Sending Binding Refresh Requests . . . . . . . . . . 89 147 9.6. Cache Replacement Policy . . . . . . . . . . . . . . . . 89 148 10. Home Agent Operation . . . . . . . . . . . . . . . . . . . . 91 149 10.1. Conceptual Data Structures . . . . . . . . . . . . . . . 91 150 10.2. Processing Mobility Headers . . . . . . . . . . . . . . . 92 151 10.3. Processing Bindings . . . . . . . . . . . . . . . . . . . 92 152 10.3.1. Primary Care-of Address Registration . . . . . . . . 92 153 10.3.2. Primary Care-of Address De-Registration . . . . . . . 96 154 10.4. Packet Processing . . . . . . . . . . . . . . . . . . . . 97 155 10.4.1. Intercepting Packets for a Mobile Node . . . . . . . 97 156 10.4.2. Processing Intercepted Packets . . . . . . . . . . . 99 157 10.4.3. Multicast Membership Control . . . . . . . . . . . . 100 158 10.4.4. Stateful Address Autoconfiguration . . . . . . . . . 101 159 10.4.5. Handling Reverse Tunneled Packets . . . . . . . . . . 102 160 10.4.6. Protecting Return Routability Packets . . . . . . . . 102 161 10.5. Dynamic Home Agent Address Discovery . . . . . . . . . . 103 162 10.5.1. Receiving Router Advertisement Messages . . . . . . . 103 163 10.6. Sending Prefix Information to the Mobile Node . . . . . . 106 164 10.6.1. List of Home Network Prefixes . . . . . . . . . . . . 106 165 10.6.2. Scheduling Prefix Deliveries . . . . . . . . . . . . 106 166 10.6.3. Sending Advertisements . . . . . . . . . . . . . . . 108 167 10.6.4. Lifetimes for Changed Prefixes . . . . . . . . . . . 109 168 11. Mobile Node Operation . . . . . . . . . . . . . . . . . . . . 110 169 11.1. Conceptual Data Structures . . . . . . . . . . . . . . . 110 170 11.2. Processing Mobility Headers . . . . . . . . . . . . . . . 111 171 11.3. Packet Processing . . . . . . . . . . . . . . . . . . . . 112 172 11.3.1. Sending Packets While Away from Home . . . . . . . . 112 173 11.3.2. Interaction with Outbound IPsec Processing . . . . . 115 174 11.3.3. Receiving Packets While Away from Home . . . . . . . 117 175 11.3.4. Routing Multicast Packets . . . . . . . . . . . . . . 118 176 11.3.5. Receiving ICMP Error Messages . . . . . . . . . . . . 120 177 11.3.6. Receiving Binding Error Messages . . . . . . . . . . 120 178 11.4. Home Agent and Prefix Management . . . . . . . . . . . . 121 179 11.4.1. Dynamic Home Agent Address Discovery . . . . . . . . 121 180 11.4.2. Sending Mobile Prefix Solicitations . . . . . . . . . 122 181 11.4.3. Receiving Mobile Prefix Advertisements . . . . . . . 123 182 11.5. Movement . . . . . . . . . . . . . . . . . . . . . . . . 124 183 11.5.1. Movement Detection . . . . . . . . . . . . . . . . . 124 184 11.5.2. Home Link Detection . . . . . . . . . . . . . . . . . 127 185 11.5.3. Forming New Care-of Addresses . . . . . . . . . . . . 127 186 11.5.4. Using Multiple Care-of Addresses . . . . . . . . . . 128 187 11.5.5. Returning Home . . . . . . . . . . . . . . . . . . . 129 188 11.6. Return Routability Procedure . . . . . . . . . . . . . . 131 189 11.6.1. Sending Test Init Messages . . . . . . . . . . . . . 131 190 11.6.2. Receiving Test Messages . . . . . . . . . . . . . . . 132 191 11.6.3. Protecting Return Routability Packets . . . . . . . . 133 192 11.7. Processing Bindings . . . . . . . . . . . . . . . . . . . 133 193 11.7.1. Sending Binding Updates to the Home Agent . . . . . . 133 194 11.7.2. Correspondent Registration . . . . . . . . . . . . . 136 195 11.7.3. Receiving Binding Acknowledgements . . . . . . . . . 139 196 11.7.4. Receiving Binding Refresh Requests . . . . . . . . . 141 197 11.8. Retransmissions and Rate Limiting . . . . . . . . . . . . 142 198 12. Protocol Constants . . . . . . . . . . . . . . . . . . . . . 144 199 13. Protocol Configuration Variables . . . . . . . . . . . . . . 145 200 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 146 201 15. Security Considerations . . . . . . . . . . . . . . . . . . . 149 202 15.1. Threats . . . . . . . . . . . . . . . . . . . . . . . . . 149 203 15.2. Features . . . . . . . . . . . . . . . . . . . . . . . . 151 204 15.3. Binding Updates to Home Agent . . . . . . . . . . . . . . 153 205 15.4. Binding Updates to Correspondent Nodes . . . . . . . . . 155 206 15.4.1. Overview . . . . . . . . . . . . . . . . . . . . . . 155 207 15.4.2. Achieved Security Properties . . . . . . . . . . . . 156 208 15.4.3. Comparison to Regular IPv6 Communications . . . . . . 157 209 15.4.4. Replay Attacks . . . . . . . . . . . . . . . . . . . 159 210 15.4.5. Denial-of-Service Attacks . . . . . . . . . . . . . . 159 211 15.4.6. Key Lengths . . . . . . . . . . . . . . . . . . . . . 160 212 15.5. Dynamic Home Agent Address Discovery . . . . . . . . . . 161 213 15.6. Mobile Prefix Discovery . . . . . . . . . . . . . . . . . 162 214 15.7. Tunneling via the Home Agent . . . . . . . . . . . . . . 162 215 15.8. Home Address Option . . . . . . . . . . . . . . . . . . . 163 216 15.9. Type 2 Routing Header . . . . . . . . . . . . . . . . . . 163 217 16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 165 218 17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 166 219 18. References . . . . . . . . . . . . . . . . . . . . . . . . . 167 220 18.1. Normative References . . . . . . . . . . . . . . . . . . 167 221 18.2. Informative References . . . . . . . . . . . . . . . . . 168 222 Appendix A. Future Extensions . . . . . . . . . . . . . . . . . 171 223 A.1. Piggybacking . . . . . . . . . . . . . . . . . . . . . . 171 224 A.2. Triangular Routing . . . . . . . . . . . . . . . . . . . 171 225 A.3. New Authorization Methods . . . . . . . . . . . . . . . . 171 226 A.4. Neighbor Discovery Extensions . . . . . . . . . . . . . . 171 227 Appendix B. Changes since RFC 3775 . . . . . . . . . . . . . . . 173 228 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 175 230 1. Introduction 232 This document specifies a protocol which allows nodes to remain 233 reachable while moving around in the IPv6 Internet. Without specific 234 support for mobility in IPv6 [5], packets destined to a mobile node 235 would not be able to reach it while the mobile node is away from its 236 home link. In order to continue communication in spite of its 237 movement, a mobile node could change its IP address each time it 238 moves to a new link, but the mobile node would then not be able to 239 maintain transport and higher-layer connections when it changes 240 location. Mobility support in IPv6 is particularly important, as 241 mobile computers are likely to account for a majority or at least a 242 substantial fraction of the population of the Internet during the 243 lifetime of IPv6. 245 The protocol defined in this document, known as Mobile IPv6, allows a 246 mobile node to move from one link to another without changing the 247 mobile node's "home address". Packets may be routed to the mobile 248 node using this address regardless of the mobile node's current point 249 of attachment to the Internet. The mobile node may also continue to 250 communicate with other nodes (stationary or mobile) after moving to a 251 new link. The movement of a mobile node away from its home link is 252 thus transparent to transport and higher-layer protocols and 253 applications. 255 The Mobile IPv6 protocol is just as suitable for mobility across 256 homogeneous media as for mobility across heterogeneous media. For 257 example, Mobile IPv6 facilitates node movement from one Ethernet 258 segment to another as well as it facilitates node movement from an 259 Ethernet segment to a wireless LAN cell, with the mobile node's IP 260 address remaining unchanged in spite of such movement. 262 One can think of the Mobile IPv6 protocol as solving the network- 263 layer mobility management problem. Some mobility management 264 applications -- for example, handover among wireless transceivers, 265 each of which covers only a very small geographic area -- have been 266 solved using link-layer techniques. For example, in many current 267 wireless LAN products, link-layer mobility mechanisms allow a 268 "handover" of a mobile node from one cell to another, re-establishing 269 link-layer connectivity to the node in each new location. 271 Mobile IPv6 does not attempt to solve all general problems related to 272 the use of mobile computers or wireless networks. In particular, 273 this protocol does not attempt to solve: 275 o Handling links with unidirectional connectivity or partial 276 reachability, such as the hidden terminal problem where a host is 277 hidden from only some of the routers on the link. 279 o Access control on a link being visited by a mobile node. 281 o Local or hierarchical forms of mobility management (similar to 282 many current link-layer mobility management solutions). 284 o Assistance for adaptive applications. 286 o Mobile routers. 288 o Service Discovery. 290 o Distinguishing between packets lost due to bit errors vs. network 291 congestion. 293 This document obsoletes RFC 3775. Issues with the original document 294 have been observed during integration, testing and deployment of RFC 295 3775. A more detailed list of the changes since RFC 3775 may be 296 found in Appendix B. 298 2. Comparison with Mobile IP for IPv4 300 The design of Mobile IP support in IPv6 (Mobile IPv6) benefits both 301 from the experiences gained from the development of Mobile IP support 302 in IPv4 (Mobile IPv4) [30] [24] [25], and from the opportunities 303 provided by IPv6. Mobile IPv6 thus shares many features with Mobile 304 IPv4, but is integrated into IPv6 and offers many other improvements. 305 This section summarizes the major differences between Mobile IPv4 and 306 Mobile IPv6: 308 o There is no need to deploy special routers as "foreign agents", as 309 in Mobile IPv4. Mobile IPv6 operates in any location without any 310 special support required from the local router. 312 o Support for route optimization is a fundamental part of the 313 protocol, rather than a nonstandard set of extensions. 315 o Mobile IPv6 route optimization can operate securely even without 316 pre-arranged security associations. It is expected that route 317 optimization can be deployed on a global scale between all mobile 318 nodes and correspondent nodes. 320 o Support is also integrated into Mobile IPv6 for allowing route 321 optimization to coexist efficiently with routers that perform 322 "ingress filtering" [27]. 324 o The IPv6 Neighbor Unreachability Detection assures symmetric 325 reachability between the mobile node and its default router in the 326 current location. 328 o Most packets sent to a mobile node while away from home in Mobile 329 IPv6 are sent using an IPv6 routing header rather than IP 330 encapsulation, reducing the amount of resulting overhead compared 331 to Mobile IPv4. 333 o Mobile IPv6 is decoupled from any particular link layer, as it 334 uses IPv6 Neighbor Discovery [17] instead of ARP. This also 335 improves the robustness of the protocol. 337 o The use of IPv6 encapsulation (and the routing header) removes the 338 need in Mobile IPv6 to manage "tunnel soft state". 340 o The dynamic home agent address discovery mechanism in Mobile IPv6 341 returns a single reply to the mobile node. The directed broadcast 342 approach used in IPv4 returns separate replies from each home 343 agent. 345 3. Terminology 347 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 348 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 349 document are to be interpreted as described in RFC 2119 [1]. 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 unicast routable address 368 An identifier for a single interface such that a packet sent to it 369 from another IPv6 subnet is delivered to the interface identified 370 by that address. Accordingly, a unicast routable address must 371 either be global IPv6 address or a unique local IPv6 address. 373 host 375 Any node that is not a router. 377 link 379 A communication facility or medium over which nodes can 380 communicate at the link layer, such as an Ethernet (simple or 381 bridged). A link is the layer immediately below IP. 383 interface 385 A node's attachment to a link. 387 subnet prefix 389 A bit string that consists of some number of initial bits of an IP 390 address. 392 interface identifier 394 A number used to identify a node's interface on a link. The 395 interface identifier is the remaining low-order bits in the node's 396 IP address after the subnet prefix. 398 link-layer address 400 A link-layer identifier for an interface, such as IEEE 802 401 addresses on Ethernet links. 403 packet 405 An IP header plus payload. 407 security association 409 An IPsec security association is a cooperative relationship formed 410 by the sharing of cryptographic keying material and associated 411 context. Security associations are simplex. That is, two 412 security associations are needed to protect bidirectional traffic 413 between two nodes, one for each direction. 415 security policy database 417 A database that specifies what security services are to be offered 418 to IP packets and in what fashion. 420 destination option 422 Destination options are carried by the IPv6 Destination Options 423 extension header. Destination options include optional 424 information that need be examined only by the IPv6 node given as 425 the destination address in the IPv6 header, not by routers in 426 between. Mobile IPv6 defines one new destination option, the Home 427 Address destination option (see Section 6.3). 429 routing header 431 A routing header may be present as an IPv6 header extension, and 432 indicates that the payload has to be delivered to a destination 433 IPv6 address in some way that is different from what would be 434 carried out by standard Internet routing. In this document, use 435 of the term "routing header" typically refers to use of a type 2 436 routing header, as specified in Section 6.4. 438 "|" (concatenation) 440 Some formulas in this specification use the symbol "|" to indicate 441 bytewise concatenation, as in A | B. This concatenation requires 442 that all of the octets of the datum A appear first in the result, 443 followed by all of the octets of the datum B. 445 First (size, input) 447 Some formulas in this specification use a functional form "First 448 (size, input)" to indicate truncation of the "input" data so that 449 only the first "size" bits remain to be used. 451 3.2. Mobile IPv6 Terms 453 These terms are intended to be compatible with the definitions given 454 in RFC 3753[37]. However, if there is any conflict, the definitions 455 given here should be considered to supersede those in RFC 3753. 457 home address 459 A unicast routable address assigned to a mobile node, used as the 460 permanent address of the mobile node. This address is within the 461 mobile node's home link. Standard IP routing mechanisms will 462 deliver packets destined for a mobile node's home address to its 463 home link. Mobile nodes can have multiple home addresses, for 464 instance when there are multiple home prefixes on the home link. 466 home subnet prefix 468 The IP subnet prefix corresponding to a mobile node's home 469 address. 471 home link 473 The link on which a mobile node's home subnet prefix is defined. 475 mobile node 477 A node that can change its point of attachment from one link to 478 another, while still being reachable via its home address. 480 movement 482 A change in a mobile node's point of attachment to the Internet 483 such that it is no longer connected to the same link as it was 484 previously. If a mobile node is not currently attached to its 485 home link, the mobile node is said to be "away from home". 487 L2 handover 489 A process by which the mobile node changes from one link-layer 490 connection to another. For example, a change of wireless access 491 point is a L2 handover. 493 L3 handover 495 Subsequent to a L2 handover, a mobile node detects a change in an 496 on-link subnet prefix that would require a change in the primary 497 care-of address. For example, a change of access router 498 subsequent to a change of wireless access point typically results 499 in an L3 handover. 501 correspondent node 503 A peer node with which a mobile node is communicating. The 504 correspondent node may be either mobile or stationary. 506 foreign subnet prefix 508 Any IP subnet prefix other than the mobile node's home subnet 509 prefix. 511 foreign link 513 Any link other than the mobile node's home link. 515 care-of address 517 A unicast routable address associated with a mobile node while 518 visiting a foreign link; the subnet prefix of this IP address is a 519 foreign subnet prefix. Among the multiple care-of addresses that 520 a mobile node may have at any given time (e.g., with different 521 subnet prefixes), the one registered with the mobile node's home 522 agent for a given home address is called its "primary" care-of 523 address. 525 home agent 527 A router on a mobile node's home link with which the mobile node 528 has registered its current care-of address. While the mobile node 529 is away from home, the home agent intercepts packets on the home 530 link destined to the mobile node's home address, encapsulates 531 them, and tunnels them to the mobile node's registered care-of 532 address. 534 binding 536 The association of the home address of a mobile node with a 537 care-of address for that mobile node, along with the remaining 538 lifetime of that association. 540 registration 542 The process during which a mobile node sends a Binding Update to 543 its home agent or a correspondent node, causing a binding for the 544 mobile node to be registered. 546 mobility message 548 A message containing a Mobility Header (see Section 6.1). 550 binding authorization 552 Correspondent registration needs to be authorized to allow the 553 recipient to believe that the sender has the right to specify a 554 new binding. 556 return routability procedure 558 The return routability procedure authorizes registrations by the 559 use of a cryptographic token exchange. 561 correspondent registration 563 A return routability procedure followed by a registration, run 564 between the mobile node and a correspondent node. 566 home registration 568 A registration between the mobile node and its home agent, 569 authorized by the use of IPsec. 571 nonce 573 Nonces are random numbers used internally by the correspondent 574 node in the creation of keygen tokens related to the return 575 routability procedure. The nonces are not specific to a mobile 576 node, and are kept secret within the correspondent node. 578 nonce index 580 A nonce index is used to indicate which nonces have been used when 581 creating keygen token values, without revealing the nonces 582 themselves. 584 cookie 586 A cookie is a random number used by a mobile node to prevent 587 spoofing by a bogus correspondent node in the return routability 588 procedure. 590 care-of init cookie 592 A cookie sent to the correspondent node in the Care-of Test Init 593 message, to be returned in the Care-of Test message. 595 home init cookie 597 A cookie sent to the correspondent node in the Home Test Init 598 message, to be returned in the Home Test message. 600 keygen token 602 A keygen token is a number supplied by a correspondent node in the 603 return routability procedure to enable the mobile node to compute 604 the necessary binding management key for authorizing a Binding 605 Update. 607 care-of keygen token 609 A keygen token sent by the correspondent node in the Care-of Test 610 message. 612 home keygen token 614 A keygen token sent by the correspondent node in the Home Test 615 message. 617 binding management key (Kbm) 619 A binding management key (Kbm) is a key used for authorizing a 620 binding cache management message (e.g., Binding Update or Binding 621 Acknowledgement). Return routability provides a way to create a 622 binding management key. 624 4. Overview of Mobile IPv6 626 4.1. Basic Operation 628 A mobile node is always expected to be addressable at its home 629 address, whether it is currently attached to its home link or is away 630 from home. The "home address" is an IP address assigned to the 631 mobile node within its home subnet prefix on its home link. While a 632 mobile node is at home, packets addressed to its home address are 633 routed to the mobile node's home link, using conventional Internet 634 routing mechanisms. 636 While a mobile node is attached to some foreign link away from home, 637 it is also addressable at one or more care-of addresses. A care-of 638 address is an IP address associated with a mobile node that has the 639 subnet prefix of a particular foreign link. The mobile node can 640 acquire its care-of address through conventional IPv6 mechanisms, 641 such as stateless or stateful auto-configuration. As long as the 642 mobile node stays in this location, packets addressed to this care-of 643 address will be routed to the mobile node. The mobile node may also 644 accept packets from several care-of addresses, such as when it is 645 moving but still reachable at the previous link. 647 The association between a mobile node's home address and care-of 648 address is known as a "binding" for the mobile node. While away from 649 home, a mobile node registers its primary care-of address with a 650 router on its home link, requesting this router to function as the 651 "home agent" for the mobile node. The mobile node performs this 652 binding registration by sending a "Binding Update" message to the 653 home agent. The home agent replies to the mobile node by returning a 654 "Binding Acknowledgement" message. The operation of the mobile node 655 is specified in Section 11, and the operation of the home agent is 656 specified in Section 10. 658 Any node communicating with a mobile node is referred to in this 659 document as a "correspondent node" of the mobile node, and may itself 660 be either a stationary node or a mobile node. Mobile nodes can 661 provide information about their current location to correspondent 662 nodes. This happens through the correspondent registration. As a 663 part of this procedure, a return routability test is performed in 664 order to authorize the establishment of the binding. The operation 665 of the correspondent node is specified in Section 9. 667 There are two possible modes for communications between the mobile 668 node and a correspondent node. The first mode, bidirectional 669 tunneling, does not require Mobile IPv6 support from the 670 correspondent node and is available even if the mobile node has not 671 registered its current binding with the correspondent node. Packets 672 from the correspondent node are routed to the home agent and then 673 tunneled to the mobile node. Packets to the correspondent node are 674 tunneled from the mobile node to the home agent ("reverse tunneled") 675 and then routed normally from the home network to the correspondent 676 node. In this mode, the home agent uses proxy Neighbor Discovery to 677 intercept any IPv6 packets addressed to the mobile node's home 678 address (or home addresses) on the home link. Each intercepted 679 packet is tunneled to the mobile node's primary care-of address. 680 This tunneling is performed using IPv6 encapsulation [6]. 682 The second mode, "route optimization", requires the mobile node to 683 register its current binding at the correspondent node. Packets from 684 the correspondent node can be routed directly to the care-of address 685 of the mobile node. When sending a packet to any IPv6 destination, 686 the correspondent node checks its cached bindings for an entry for 687 the packet's destination address. If a cached binding for this 688 destination address is found, the node uses a new type of IPv6 689 routing header [5] (see Section 6.4) to route the packet to the 690 mobile node by way of the care-of address indicated in this binding. 692 Routing packets directly to the mobile node's care-of address allows 693 the shortest communications path to be used. It also eliminates 694 congestion at the mobile node's home agent and home link. In 695 addition, the impact of temporary failures of the home agent or 696 networks on the path to or from the home agent is reduced. 698 When routing packets directly to the mobile node, the correspondent 699 node sets the Destination Address in the IPv6 header to the care-of 700 address of the mobile node. A new type of IPv6 routing header (see 701 Section 6.4) is also added to the packet to carry the desired home 702 address. Similarly, the mobile node sets the Source Address in the 703 packet's IPv6 header to its current care-of addresses. The mobile 704 node adds a new IPv6 "Home Address" destination option (see 705 Section 6.3) to carry its home address. The inclusion of home 706 addresses in these packets makes the use of the care-of address 707 transparent above the network layer (e.g., at the transport layer). 709 Mobile IPv6 also provides support for multiple home agents, and a 710 limited support for the reconfiguration of the home network. In 711 these cases, the mobile node may not know the IP address of its own 712 home agent, and even the home subnet prefixes may change over time. 713 A mechanism, known as "dynamic home agent address discovery" allows a 714 mobile node to dynamically discover the IP address of a home agent on 715 its home link, even when the mobile node is away from home. Mobile 716 nodes can also learn new information about home subnet prefixes 717 through the "mobile prefix discovery" mechanism. These mechanisms 718 are described starting from Section 6.5. 720 This document is written under the assumption that the mobile node is 721 configured with the home prefix for the mobile node to be able to 722 discover a home agent and configure a home address. This might be 723 limiting in deployments where the home agent and the home address for 724 the mobile node needs to be assigned dynamically. Additional 725 mechanisms have been specified for the mobile node to dynamically 726 configure a home agent, a home address and the home prefix. These 727 mechanisms are described in "Mobile IPv6 Bootstrapping in Split 728 Scenario" [22] and "MIP6 bootstrapping for the Integrated Scenario" 729 [34]. 731 4.2. New IPv6 Protocol 733 Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header 734 (see Section 6.1). This Header is used to carry the following 735 messages: 737 Home Test Init 739 Home Test 741 Care-of Test Init 743 Care-of Test 745 These four messages are used to perform the return routability 746 procedure from the mobile node to a correspondent node. This 747 ensures authorization of subsequent Binding Updates, as described 748 in Section 5.2.5. 750 Binding Update 752 A Binding Update is used by a mobile node to notify a 753 correspondent node or the mobile node's home agent of its current 754 binding. The Binding Update sent to the mobile node's home agent 755 to register its primary care-of address is marked as a "home 756 registration". 758 Binding Acknowledgement 760 A Binding Acknowledgement is used to acknowledge receipt of a 761 Binding Update, if an acknowledgement was requested in the Binding 762 Update (e.g., the Binding Update was sent to a home agent), or an 763 error occurred. 765 Binding Refresh Request 767 A Binding Refresh Request is used by a correspondent node to 768 request that a mobile node re-establish its binding with the 769 correspondent node. This message is typically used when the 770 cached binding is in active use but the binding's lifetime is 771 close to expiration. The correspondent node may use, for 772 instance, recent traffic and open transport layer connections as 773 an indication of active use. 775 Binding Error 777 The Binding Error is used by the correspondent node to signal an 778 error related to mobility, such as an inappropriate attempt to use 779 the Home Address destination option without an existing binding. 780 The Binding Error message is also used by the Home Agent to signal 781 an error to the mobile node, if it receives an unrecognized 782 Mobility Header Message Type from the mobile node. 784 4.3. New IPv6 Destination Option 786 Mobile IPv6 defines a new IPv6 destination option, the Home Address 787 destination option. This option is described in detail in 788 Section 6.3. 790 4.4. New IPv6 ICMP Messages 792 Mobile IPv6 also introduces four new ICMP message types, two for use 793 in the dynamic home agent address discovery mechanism, and two for 794 renumbering and mobile configuration mechanisms. As described in 795 Section 10.5 and Section 11.4.1, the following two new ICMP message 796 types are used for home agent address discovery: 798 o Home Agent Address Discovery Request, described in Section 6.5. 800 o Home Agent Address Discovery Reply, described in Section 6.6. 802 The next two message types are used for network renumbering and 803 address configuration on the mobile node, as described in 804 Section 10.6: 806 o Mobile Prefix Solicitation, described in Section 6.7. 808 o Mobile Prefix Advertisement, described in Section 6.8. 810 4.5. Conceptual Data Structure Terminology 812 This document describes the Mobile IPv6 protocol in terms of the 813 following conceptual data structures: 815 Binding Cache 817 A cache of bindings for other nodes. This cache is maintained by 818 home agents and correspondent nodes. The cache contains both 819 "correspondent registration" entries (see Section 9.1) and "home 820 registration" entries (see Section 10.1). 822 Binding Update List 824 This list is maintained by each mobile node. The list has an item 825 for every binding that the mobile node has or is trying to 826 establish with a specific other node. Both correspondent and home 827 registrations are included in this list. Entries from the list 828 are deleted as the lifetime of the binding expires. See 829 Section 11.1. 831 Home Agents List 833 Home agents need to know which other home agents are on the same 834 link. This information is stored in the Home Agents List, as 835 described in more detail in Section 10.1. The list is used for 836 informing mobile nodes during dynamic home agent address 837 discovery. 839 4.6. Unique-Local Addressability 841 This specification requires that home and care-of addresses MUST be 842 unicast routable addresses. Unique-local IPv6 unicast addresses 843 (ULAs) RFC4193 [19] may be usable on networks that use such non- 844 globally routable addresses but this specification does not define 845 when such usage is safe and when it is not. Mobile nodes may not be 846 able to distinguish between their home site and the site at which 847 they are currently located. This can make it hard to prevent 848 accidental attachment to other sites, because the mobile node might 849 use the ULA at another site, which could not be used to successfully 850 send packets to the mobile node's HA. This would result in 851 unreachability between the MN and the HA, when unique-local IPv6 852 routable addresses are used as care-of addresses. Similarly, CNs 853 outside the MN's own site will not be reachable when ULAs are used as 854 home addresses. Therefore, unique-local IPv6 unicast addresses 855 SHOULD NOT be used as home or care-of addresses when other address 856 choices are available. If such addresses are used, however, 857 according to RFC4193 [19], they are treated as any global unicast 858 IPv6 address so, for the remainder of this specification, use of 859 unique-local IPv6 unicast addresses is not differentiated from other 860 globally unique IPv6 addresses. 862 5. Overview of Mobile IPv6 Security 864 This specification provides a number of security features. These 865 include the protection of Binding Updates both to home agents and 866 correspondent nodes, the protection of mobile prefix discovery, and 867 the protection of the mechanisms that Mobile IPv6 uses for 868 transporting data packets. 870 Binding Updates are protected by the use of IPsec extension headers, 871 or by the use of the Binding Authorization Data option. This option 872 employs a binding management key, Kbm, which can be established 873 through the return routability procedure. Mobile prefix discovery is 874 protected through the use of IPsec extension headers. Mechanisms 875 related to transporting payload packets - such as the Home Address 876 destination option and type 2 routing header - have been specified in 877 a manner which restricts their use in attacks. 879 5.1. Binding Updates to Home Agents 881 The mobile node and the home agent MUST use an IPsec security 882 association to protect the integrity and authenticity of the Binding 883 Updates and Acknowledgements. Both the mobile nodes and the home 884 agents MUST support and SHOULD use the Encapsulating Security Payload 885 (ESP) [4] header in transport mode and MUST use a non-NULL payload 886 authentication algorithm to provide data origin authentication, 887 connectionless integrity and optional anti-replay protection. Note 888 that Authentication Header (AH) [3] is also possible but for brevity 889 not discussed in this specification. 891 In order to protect messages exchanged between the mobile node and 892 the home agent with IPsec, appropriate security policy database 893 entries must be created. A mobile node must be prevented from using 894 its security association to send a Binding Update on behalf of 895 another mobile node using the same home agent. This MUST be achieved 896 by having the home agent check that the given home address has been 897 used with the right security association. Such a check is provided 898 in the IPsec processing, by having the security policy database 899 entries unequivocally identify a single security association for 900 protecting Binding Updates between any given home address and home 901 agent. In order to make this possible, it is necessary that the home 902 address of the mobile node is visible in the Binding Updates and 903 Acknowledgements. The home address is used in these packets as a 904 source or destination, or in the Home Address destination option or 905 the type 2 routing header. 907 As with all IPsec security associations in this specification, manual 908 configuration of security associations MUST be supported. The shared 909 secrets used MUST be random and unique for different mobile nodes, 910 and MUST be distributed off-line to the mobile nodes. Automatic key 911 management with IKEv2 [41] MAY be supported as described in [42]. 913 Section 11.3.2 discusses how IKEv2 connections to the home agent need 914 a careful treatment of the addresses used for transporting IKEv2. 915 This is necessary to ensure that a Binding Update is not needed 916 before the IKEv2 exchange which is needed for securing the Binding 917 Update. 919 More detailed descriptions and examples using IPsec to protect 920 communications between the mobile node and the home agent have been 921 published [42]. 923 5.2. Binding Updates to Correspondent Nodes 925 The protection of Binding Updates sent to correspondent nodes does 926 not require the configuration of security associations or the 927 existence of an authentication infrastructure between the mobile 928 nodes and correspondent nodes. Instead, a method called the return 929 routability procedure is used to assure that the right mobile node is 930 sending the message. This method does not protect against attackers 931 who are on the path between the home network and the correspondent 932 node. However, attackers in such a location are capable of 933 performing the same attacks even without Mobile IPv6. The main 934 advantage of the return routability procedure is that it limits the 935 potential attackers to those having an access to one specific path in 936 the Internet, and avoids forged Binding Updates from anywhere else in 937 the Internet. For a more in depth explanation of the security 938 properties of the return routability procedure, see Section 15. 939 Also, consult [40] 941 The integrity and authenticity of the Binding Update messages to 942 correspondent nodes is protected by using a keyed-hash algorithm. 943 The binding management key, Kbm, is used to key the hash algorithm 944 for this purpose. Kbm is established using data exchanged during the 945 return routability procedure. The data exchange is accomplished by 946 use of node keys, nonces, cookies, tokens, and certain cryptographic 947 functions. Section 5.2.5 outlines the basic return routability 948 procedure. Section 5.2.6 shows how the results of this procedure are 949 used to authorize a Binding Update to a correspondent node. 951 5.2.1. Node Keys 953 Each correspondent node has a secret key, Kcn, called the "node key", 954 which it uses to produce the keygen tokens sent to the mobile nodes. 955 The node key MUST be a random number, 20 octets in length. The node 956 key allows the correspondent node to verify that the keygen tokens 957 used by the mobile node in authorizing a Binding Update are indeed 958 its own. This key MUST NOT be shared with any other entity. 960 A correspondent node MAY generate a fresh node key at any time; this 961 avoids the need for secure persistent key storage. Procedures for 962 optionally updating the node key are discussed later in 963 Section 5.2.7. 965 5.2.2. Nonces 967 Each correspondent node also generates nonces at regular intervals. 968 The nonces should be generated by using a random number generator 969 that is known to have good randomness properties [14]. A 970 correspondent node may use the same Kcn and nonce with all the 971 mobiles it is in communication with. 973 Each nonce is identified by a nonce index. When a new nonce is 974 generated, it must be associated with a new nonce index; this may be 975 done, for example, by incrementing the value of the previous nonce 976 index, if the nonce index is used as an array pointer into a linear 977 array of nonces. However, there is no requirement that nonces be 978 stored that way, or that the values of subsequent nonce indices have 979 any particular relationship to each other. The index value is 980 communicated in the protocol, so that if a nonce is replaced by new 981 nonce during the run of a protocol, the correspondent node can 982 distinguish messages that should be checked against the old nonce 983 from messages that should be checked against the new nonce. Strictly 984 speaking, indices are not necessary in the authentication, but allow 985 the correspondent node to efficiently find the nonce value that it 986 used in creating a keygen token. 988 Correspondent nodes keep both the current nonce and a small set of 989 valid previous nonces whose lifetime has not yet expired. Expired 990 values MUST be discarded, and messages using stale or unknown indices 991 will be rejected. 993 The specific nonce index values cannot be used by mobile nodes to 994 determine the validity of the nonce. Expected validity times for the 995 nonces values and the procedures for updating them are discussed 996 later in Section 5.2.7. 998 A nonce is an octet string of any length. The recommended length is 999 64 bits. 1001 5.2.3. Cookies and Tokens 1003 The return routability address test procedure uses cookies and keygen 1004 tokens as opaque values within the test init and test messages, 1005 respectively. 1007 o The "home init cookie" and "care-of init cookie" are 64 bit values 1008 sent to the correspondent node from the mobile node, and later 1009 returned to the mobile node. The home init cookie is sent in the 1010 Home Test Init message, and returned in the Home Test message. 1011 The care-of init cookie is sent in the Care-of Test Init message, 1012 and returned in the Care-of Test message. 1014 o The "home keygen token" and "care-of keygen token" are 64-bit 1015 values sent by the correspondent node to the mobile node via the 1016 home agent (via the Home Test message) and the care-of address (by 1017 the Care-of Test message), respectively. 1019 The mobile node should set the home init or care-of init cookie to a 1020 newly generated random number in every Home or Care-of Test Init 1021 message it sends. The cookies are used to verify that the Home Test 1022 or Care-of Test message matches the Home Test Init or Care-of Test 1023 Init message, respectively. These cookies also serve to ensure that 1024 parties who have not seen the request cannot spoof responses. 1026 Home and care-of keygen tokens are produced by the correspondent node 1027 based on its currently active secret key (Kcn) and nonces, as well as 1028 the home or care-of address (respectively). A keygen token is valid 1029 as long as both the secret key (Kcn) and the nonce used to create it 1030 are valid. 1032 5.2.4. Cryptographic Functions 1034 By default in this specification, the function used to compute hash 1035 values is SHA1 [11]. Message Authentication Codes (MACs) are then 1036 computed using HMAC_SHA1 [26] [11]. HMAC_SHA1(K,m) denotes such a 1037 MAC computed on message m with key K. 1039 5.2.5. Return Routability Procedure 1041 The Return Routability Procedure enables the correspondent node to 1042 obtain some reasonable assurance that the mobile node is in fact 1043 addressable at its claimed care-of address as well as at its home 1044 address. Only with this assurance is the correspondent node able to 1045 accept Binding Updates from the mobile node which would then instruct 1046 the correspondent node to direct that mobile node's data traffic to 1047 its claimed care-of address. 1049 This is done by testing whether packets addressed to the two claimed 1050 addresses are routed to the mobile node. The mobile node can pass 1051 the test only if it is able to supply proof that it received certain 1052 data (the "keygen tokens") which the correspondent node sends to 1053 those addresses. These data are combined by the mobile node into a 1054 binding management key, denoted Kbm. 1056 The figure below shows the message flow for the return routability 1057 procedure. 1059 Mobile node Home agent Correspondent node 1060 | | 1061 | Home Test Init (HoTI) | | 1062 |------------------------->|------------------------->| 1063 | | | 1064 | Care-of Test Init (CoTI) | 1065 |---------------------------------------------------->| 1066 | | 1067 | | Home Test (HoT) | 1068 |<-------------------------|<-------------------------| 1069 | | | 1070 | Care-of Test (CoT) | 1071 |<----------------------------------------------------| 1072 | | 1074 The Home and Care-of Test Init messages are sent at the same time. 1075 The procedure requires very little processing at the correspondent 1076 node, and the Home and Care-of Test messages can be returned quickly, 1077 perhaps nearly simultaneously. These four messages form the return 1078 routability procedure. 1080 Home Test Init 1082 A mobile node sends a Home Test Init message to the correspondent 1083 node (via the home agent) to acquire the home keygen token. The 1084 contents of the message can be summarized as follows: 1086 * Source Address = home address 1088 * Destination Address = correspondent 1090 * Parameters: 1092 + home init cookie 1094 The Home Test Init message conveys the mobile node's home address 1095 to the correspondent node. The mobile node also sends along a 1096 home init cookie that the correspondent node must return later. 1097 The Home Test Init message is reverse tunneled through the home 1098 agent. (The headers and addresses related to reverse tunneling 1099 have been omitted from the above discussion of the message 1100 contents.) The mobile node remembers these cookie values to 1101 obtain some assurance that its protocol messages are being 1102 processed by the desired correspondent node. 1104 Care-of Test Init 1106 The mobile node sends a Care-of Test Init message to the 1107 correspondent node (directly, not via the home agent) to acquire 1108 the care-of keygen token. The contents of this message can be 1109 summarized as follows: 1111 * Source Address = care-of address 1113 * Destination Address = correspondent 1115 * Parameters: 1117 + care-of init cookie 1119 The Care-of Test Init message conveys the mobile node's care-of 1120 address to the correspondent node. The mobile node also sends 1121 along a care-of init cookie that the correspondent node must 1122 return later. The Care-of Test Init message is sent directly to 1123 the correspondent node. 1125 Home Test 1127 The Home Test message is sent in response to a Home Test Init 1128 message. It is sent via the home agent. The contents of the 1129 message are: 1131 * Source Address = correspondent 1133 * Destination Address = home address 1135 * Parameters: 1137 + home init cookie 1139 + home keygen token 1141 + home nonce index 1143 When the correspondent node receives the Home Test Init message, 1144 it generates a home keygen token as follows: 1146 home keygen token := 1147 First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0))) 1149 where | denotes concatenation. The final "0" inside the HMAC_SHA1 1150 function is a single zero octet, used to distinguish home and 1151 care-of cookies from each other. 1153 The home keygen token is formed from the first 64 bits of the MAC. 1154 The home keygen token tests that the mobile node can receive 1155 messages sent to its home address. Kcn is used in the production 1156 of home keygen token in order to allow the correspondent node to 1157 verify that it generated the home and care-of nonces, without 1158 forcing the correspondent node to remember a list of all tokens it 1159 has handed out. 1161 The Home Test message is sent to the mobile node via the home 1162 network, where it is presumed that the home agent will tunnel the 1163 message to the mobile node. This means that the mobile node needs 1164 to already have sent a Binding Update to the home agent, so that 1165 the home agent will have received and authorized the new care-of 1166 address for the mobile node before the return routability 1167 procedure. For improved security, the data passed between the 1168 home agent and the mobile node is made immune to inspection and 1169 passive attacks. Such protection is gained by encrypting the home 1170 keygen token as it is tunneled from the home agent to the mobile 1171 node as specified in Section 10.4.6. The security properties of 1172 this additional security are discussed in Section 15.4.1. 1174 The home init cookie from the mobile node is returned in the Home 1175 Test message, to ensure that the message comes from a node on the 1176 route between the home agent and the correspondent node. 1178 The home nonce index is delivered to the mobile node to later 1179 allow the correspondent node to efficiently find the nonce value 1180 that it used in creating the home keygen token. 1182 Care-of Test 1184 This message is sent in response to a Care-of Test Init message. 1185 This message is not sent via the home agent, it is sent directly 1186 to the mobile node. The contents of the message are: 1188 * Source Address = correspondent 1190 * Destination Address = care-of address 1191 * Parameters: 1193 + care-of init cookie 1195 + care-of keygen token 1197 + care-of nonce index 1199 When the correspondent node receives the Care-of Test Init 1200 message, it generates a care-of keygen token as follows: 1202 care-of keygen token := 1203 First (64, HMAC_SHA1 (Kcn, (care-of address | nonce | 1))) 1205 Here, the final "1" inside the HMAC_SHA1 function is a single 1206 octet containing the hex value 0x01, and is used to distinguish 1207 home and care-of cookies from each other. The keygen token is 1208 formed from the first 64 bits of the MAC, and sent directly to the 1209 mobile node at its care-of address. The care-of init cookie from 1210 the Care-of Test Init message is returned to ensure that the 1211 message comes from a node on the route to the correspondent node. 1213 The care-of nonce index is provided to identify the nonce used for 1214 the care-of keygen token. The home and care-of nonce indices MAY 1215 be the same, or different, in the Home and Care-of Test messages. 1217 When the mobile node has received both the Home and Care-of Test 1218 messages, the return routability procedure is complete. As a result 1219 of the procedure, the mobile node has the data it needs to send a 1220 Binding Update to the correspondent node. The mobile node hashes the 1221 tokens together to form a 20 octet binding key Kbm: 1223 Kbm = SHA1 (home keygen token | care-of keygen token) 1225 A Binding Update may also be used to delete a previously established 1226 binding (Section 6.1.7). In this case, the care-of keygen token is 1227 not used. Instead, the binding management key is generated as 1228 follows: 1230 Kbm = SHA1(home keygen token) 1232 Note that the correspondent node does not create any state specific 1233 to the mobile node, until it receives the Binding Update from that 1234 mobile node. The correspondent node does not maintain the value for 1235 the binding management key Kbm; it creates Kbm when given the nonce 1236 indices and the mobile node's addresses. 1238 5.2.6. Authorizing Binding Management Messages 1240 After the mobile node has created the binding management key (Kbm), 1241 it can supply a verifiable Binding Update to the correspondent node. 1242 This section provides an overview of this registration. The below 1243 figure shows the message flow. 1245 Mobile node Correspondent node 1246 | | 1247 | Binding Update (BU) | 1248 |---------------------------------------------->| 1249 | (MAC, seq#, nonce indices, care-of address) | 1250 | | 1251 | | 1252 | Binding Acknowledgement (BA) (if sent) | 1253 |<----------------------------------------------| 1254 | (MAC, seq#, status) | 1256 Binding Update 1258 To authorize a Binding Update, the mobile node creates a binding 1259 management key Kbm from the keygen tokens as described in the 1260 previous section. The contents of the Binding Update include the 1261 following: 1263 * Source Address = care-of address 1265 * Destination Address = correspondent 1267 * Parameters: 1269 + home address (within the Home Address destination option if 1270 different from the Source Address) 1272 + sequence number (within the Binding Update message header) 1274 + home nonce index (within the Nonce Indices option) 1276 + care-of nonce index (within the Nonce Indices option) 1278 + First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent 1279 | BU))) 1281 The Binding Update contains a Nonce Indices option, indicating to 1282 the correspondent node which home and care-of nonces to use to 1283 recompute Kbm, the binding management key. The MAC is computed as 1284 described in Section 6.2.7, using the correspondent node's address 1285 as the destination address and the Binding Update message itself 1286 ("BU" above) as the MH Data. 1288 Once the correspondent node has verified the MAC, it can create a 1289 Binding Cache entry for the mobile. 1291 Binding Acknowledgement 1293 The Binding Update is in some cases acknowledged by the 1294 correspondent node. The contents of the message are as follows: 1296 * Source Address = correspondent 1298 * Destination Address = care-of address 1300 * Parameters: 1302 + sequence number (within the Binding Update message header) 1304 + First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent 1305 | BA))) 1307 The Binding Acknowledgement contains the same sequence number as 1308 the Binding Update. The MAC is computed as described in 1309 Section 6.2.7, using the correspondent node's address as the 1310 destination address and the message itself ("BA" above) as the MH 1311 Data. 1313 Bindings established with correspondent nodes using keys created by 1314 way of the return routability procedure MUST NOT exceed 1315 MAX_RR_BINDING_LIFETIME seconds (see Section 12). 1317 The value in the Source Address field in the IPv6 header carrying the 1318 Binding Update is normally also the care-of address which is used in 1319 the binding. However, a different care-of address MAY be specified 1320 by including an Alternate Care-of Address mobility option in the 1321 Binding Update (see Section 6.2.5). When such a message is sent to 1322 the correspondent node and the return routability procedure is used 1323 as the authorization method, the Care-of Test Init and Care-of Test 1324 messages MUST have been performed for the address in the Alternate 1325 Care-of Address option (not the Source Address). The nonce indices 1326 and MAC value MUST be based on information gained in this test. 1328 Binding Updates may also be sent to delete a previously established 1329 binding. In this case, generation of the binding management key 1330 depends exclusively on the home keygen token and the care-of nonce 1331 index is ignored. 1333 5.2.7. Updating Node Keys and Nonces 1335 Correspondent nodes generate nonces at regular intervals. It is 1336 recommended to keep each nonce (identified by a nonce index) 1337 acceptable for at least MAX_TOKEN_LIFETIME seconds (see Section 12) 1338 after it has been first used in constructing a return routability 1339 message response. However, the correspondent node MUST NOT accept 1340 nonces beyond MAX_NONCE_LIFETIME seconds (see Section 12) after the 1341 first use. As the difference between these two constants is 30 1342 seconds, a convenient way to enforce the above lifetimes is to 1343 generate a new nonce every 30 seconds. The node can then continue to 1344 accept tokens that have been based on the last 8 (MAX_NONCE_LIFETIME 1345 / 30) nonces. This results in tokens being acceptable 1346 MAX_TOKEN_LIFETIME to MAX_NONCE_LIFETIME seconds after they have been 1347 sent to the mobile node, depending on whether the token was sent at 1348 the beginning or end of the first 30 second period. Note that the 1349 correspondent node may also attempt to generate new nonces on demand, 1350 or only if the old nonces have been used. This is possible, as long 1351 as the correspondent node keeps track of how long a time ago the 1352 nonces were used for the first time, and does not generate new nonces 1353 on every return routability request. 1355 Due to resource limitations, rapid deletion of bindings, or reboots 1356 the correspondent node may not in all cases recognize the nonces that 1357 the tokens were based on. If a nonce index is unrecognized, the 1358 correspondent node replies with an error code in the Binding 1359 Acknowledgement (either 136, 137, or 138 as discussed in 1360 Section 6.1.8). The mobile node can then retry the return 1361 routability procedure. 1363 An update of Kcn SHOULD be done at the same time as an update of a 1364 nonce, so that nonce indices can identify both the nonce and the key. 1365 Old Kcn values have to be therefore remembered as long as old nonce 1366 values. 1368 Given that the tokens are normally expected to be usable for 1369 MAX_TOKEN_LIFETIME seconds, the mobile node MAY use them beyond a 1370 single run of the return routability procedure until 1371 MAX_TOKEN_LIFETIME expires. After this the mobile node SHOULD NOT 1372 use the tokens. A fast moving mobile node MAY reuse a recent home 1373 keygen token from a correspondent node when moving to a new location, 1374 and just acquire a new care-of keygen token to show routability in 1375 the new location. 1377 While this does not save the number of round-trips due to the 1378 simultaneous processing of home and care-of return routability tests, 1379 there are fewer messages being exchanged, and a potentially long 1380 round-trip through the home agent is avoided. Consequently, this 1381 optimization is often useful. A mobile node that has multiple home 1382 addresses, MAY also use the same care-of keygen token for Binding 1383 Updates concerning all of these addresses. 1385 5.2.8. Preventing Replay Attacks 1387 The return routability procedure also protects the participants 1388 against replayed Binding Updates through the use of the sequence 1389 number and a MAC. Care must be taken when removing bindings at the 1390 correspondent node, however. Correspondent nodes must retain 1391 bindings and the associated sequence number information at least as 1392 long as the nonces used in the authorization of the binding are still 1393 valid. Alternatively, if memory is very constrained, the 1394 correspondent node MAY invalidate the nonces that were used for the 1395 binding being deleted (or some larger group of nonces that they 1396 belong to). This may, however, impact the ability to accept Binding 1397 Updates from mobile nodes that have recently received keygen tokens. 1398 This alternative is therefore recommended only as a last measure. 1400 5.2.9. Handling Interruptions to Return Routability 1402 In some scenarios, such as simultaneous mobility, where both 1403 correspondent host and mobile host move at the same time, or in the 1404 case where the correspondent node reboots and loses data, route 1405 optimization may not complete, or relevant data in the binding cache 1406 might be lost. 1408 o Return Routability signalling MUST be sent to the correspondent 1409 node's home address if it has one (i.e. not to the correspondent 1410 nodes care-of address if the correspondent node is also mobile). 1412 o If Return Routability signalling timed out after MAX_RO_FAILURE 1413 attempts, the mobile node MUST revert to sending packets to the 1414 correspondent node's home address through its home agent. 1416 The mobile node may run the bidirectional tunnelling in parallel with 1417 the return routability procedure until it is successful. Exponential 1418 backoff SHOULD be used for retransmission of return routability 1419 messages. 1421 The return routability procedure may be triggered by movement of the 1422 mobile node or by sustained loss of end-to-end communication with a 1423 correspondent node (e.g. based on indications from upper-layers) that 1424 has been using a route optimised connection to the mobile node. If 1425 such indications are received, the mobile node MAY revert to bi- 1426 directional tunnelling while re-starting the return routability 1427 procedure. 1429 5.3. Dynamic Home Agent Address Discovery 1431 Dynamic home agent address discovery has been designed for use in 1432 deployments where security is not needed. For this reason, no 1433 security solution is provided in this document for dynamic home agent 1434 address discovery. 1436 5.4. Mobile Prefix Discovery 1438 The mobile node and the home agent SHOULD use an IPsec security 1439 association to protect the integrity and authenticity of the Mobile 1440 Prefix Solicitations and Advertisements. Both the mobile nodes and 1441 the home agents MUST support and SHOULD use the Encapsulating 1442 Security Payload (ESP) header in transport mode with a non-NULL 1443 payload authentication algorithm to provide data origin 1444 authentication, connectionless integrity and optional anti-replay 1445 protection. 1447 5.5. Payload Packets 1449 Payload packets exchanged with mobile nodes can be protected in the 1450 usual manner, in the same way as stationary hosts can protect them. 1451 However, Mobile IPv6 introduces the Home Address destination option, 1452 a routing header, and tunneling headers in the payload packets. In 1453 the following we define the security measures taken to protect these, 1454 and to prevent their use in attacks against other parties. 1456 This specification limits the use of the Home Address destination 1457 option to the situation where the correspondent node already has a 1458 Binding Cache entry for the given home address. This avoids the use 1459 of the Home Address option in attacks described in Section 15.1. 1461 Mobile IPv6 uses a type of routing header specific to Mobile IPv6. 1462 This type provides the necessary functionality but does not open 1463 vulnerabilities discussed in Section 15.1 and RFC 5095 [43]. 1465 Tunnels between the mobile node and the home agent are protected by 1466 ensuring proper use of source addresses, and optional cryptographic 1467 protection. The mobile node verifies that the outer IP address 1468 corresponds to its home agent. The home agent verifies that the 1469 outer IP address corresponds to the current location of the mobile 1470 node (Binding Updates sent to the home agents are secure). The home 1471 agent identifies the mobile node through the source address of the 1472 inner packet. (Typically, this is the home address of the mobile 1473 node, but it can also be a link-local address, as discussed in 1474 Section 10.4.2. To recognize the latter type of addresses, the home 1475 agent requires that the Link-Local Address Compatibility (L) was set 1476 in the Binding Update.) These measures protect the tunnels against 1477 vulnerabilities discussed in Section 15.1. 1479 For traffic tunneled via the home agent, additional IPsec ESP 1480 encapsulation MAY be supported and used. If multicast group 1481 membership control protocols or stateful address autoconfiguration 1482 protocols are supported, payload data protection MUST be supported. 1484 6. New IPv6 Protocol, Message Types, and Destination Option 1486 6.1. Mobility Header 1488 The Mobility Header is an extension header used by mobile nodes, 1489 correspondent nodes, and home agents in all messaging related to the 1490 creation and management of bindings. The subsections within this 1491 section describe the message types that may be sent using the 1492 Mobility Header. 1494 Mobility Header messages MUST NOT be sent with a type 2 routing 1495 header, except as described in Section 9.5.4 for Binding 1496 Acknowledgement. Mobility Header messages also MUST NOT be used with 1497 a Home Address destination option, except as described in 1498 Section 11.7.1 and Section 11.7.2 for Binding Update. Binding Update 1499 List or Binding Cache information (when present) for the destination 1500 MUST NOT be used in sending Mobility Header messages. That is, 1501 Mobility Header messages bypass both the Binding Cache check 1502 described in Section 9.3.2 and the Binding Update List check 1503 described in Section 11.3.1 which are normally performed for all 1504 packets. This applies even to messages sent to or from a 1505 correspondent node which is itself a mobile node. 1507 6.1.1. Format 1509 The Mobility Header is identified by a Next Header value of 135 in 1510 the immediately preceding header, and has the following format: 1512 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1513 | Payload Proto | Header Len | MH Type | Reserved | 1514 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1515 | Checksum | | 1516 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1517 | | 1518 . . 1519 . Message Data . 1520 . . 1521 | | 1522 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1524 Payload Proto 1526 8-bit selector. Identifies the type of header immediately 1527 following the Mobility Header. Uses the same values as the IPv6 1528 Next Header field [5]. 1530 This field is intended to be used by a future extension (see 1531 Appendix A.1). 1533 Implementations conforming to this specification SHOULD set the 1534 payload protocol type to IPPROTO_NONE (59 decimal). 1536 Header Len 1538 8-bit unsigned integer, representing the length of the Mobility 1539 Header in units of 8 octets, excluding the first 8 octets. 1541 The length of the Mobility Header MUST be a multiple of 8 octets. 1543 MH Type 1545 8-bit selector. Identifies the particular mobility message in 1546 question. Current values are specified in Section 6.1.2 and 1547 onward. An unrecognized MH Type field causes an error indication 1548 to be sent. 1550 Reserved 1552 8-bit field reserved for future use. The value MUST be 1553 initialized to zero by the sender, and MUST be ignored by the 1554 receiver. 1556 Checksum 1558 16-bit unsigned integer. This field contains the checksum of the 1559 Mobility Header. The checksum is calculated from the octet string 1560 consisting of a "pseudo-header" followed by the entire Mobility 1561 Header starting with the Payload Proto field. The checksum is the 1562 16-bit one's complement of the one's complement sum of this 1563 string. 1565 The pseudo-header contains IPv6 header fields, as specified in 1566 Section 8.1 of RFC 2460 [5]. The Next Header value used in the 1567 pseudo-header is 135. The addresses used in the pseudo-header are 1568 the addresses that appear in the Source and Destination Address 1569 fields in the IPv6 packet carrying the Mobility Header. 1571 Note that the procedures of calculating upper layer checksums 1572 while away from home described in Section 11.3.1 apply even for 1573 the Mobility Header. If a mobility message has a Home Address 1574 destination option, then the checksum calculation uses the home 1575 address in this option as the value of the IPv6 Source Address 1576 field. The type 2 routing header is treated as explained in [5]. 1578 The Mobility Header is considered as the upper layer protocol for 1579 the purposes of calculating the pseudo-header. The Upper-Layer 1580 Packet Length field in the pseudo-header MUST be set to the total 1581 length of the Mobility Header. 1583 For computing the checksum, the checksum field is set to zero. 1585 Message Data 1587 A variable length field containing the data specific to the 1588 indicated Mobility Header type. 1590 Mobile IPv6 also defines a number of "mobility options" for use 1591 within these messages; if included, any options MUST appear after the 1592 fixed portion of the message data specified in this document. The 1593 presence of such options will be indicated by the Header Len field 1594 within the message. When the Header Len value is greater than the 1595 length required for the message specified here, the remaining octets 1596 are interpreted as mobility options. These options include padding 1597 options that can be used to ensure that other options are aligned 1598 properly, and that the total length of the message is divisible by 8. 1599 The encoding and format of defined options are described in 1600 Section 6.2. 1602 Alignment requirements for the Mobility Header are the same as for 1603 any IPv6 protocol Header. That is, they MUST be aligned on an 1604 8-octet boundary. 1606 6.1.2. Binding Refresh Request Message 1608 The Binding Refresh Request (BRR) message requests a mobile node to 1609 update its mobility binding. This message is sent by correspondent 1610 nodes according to the rules in Section 9.5.5. When a mobile node 1611 receives a packet containing a Binding Refresh Request message it 1612 processes the message according to the rules in Section 11.7.4. 1614 The Binding Refresh Request message uses the MH Type value 0. When 1615 this value is indicated in the MH Type field, the format of the 1616 Message Data field in the Mobility Header is as follows: 1618 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1619 | Reserved | 1620 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1621 | | 1622 . . 1623 . Mobility options . 1624 . . 1625 | | 1626 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1628 Reserved 1630 16-bit field reserved for future use. The value MUST be 1631 initialized to zero by the sender, and MUST be ignored by the 1632 receiver. 1634 Mobility Options 1636 Variable-length field of such length that the complete Mobility 1637 Header is an integer multiple of 8 octets long. This field 1638 contains zero or more TLV-encoded mobility options. The encoding 1639 and format of defined options are described in Section 6.2. The 1640 receiver MUST ignore and skip any options which it does not 1641 understand. 1643 There MAY be additional information, associated with this Binding 1644 Refresh Request message that need not be present in all Binding 1645 Refresh Request messages sent. Mobility options allow future 1646 extensions to the format of the Binding Refresh Request message to 1647 be defined. This specification does not define any options valid 1648 for the Binding Refresh Request message. 1650 If no actual options are present in this message, no padding is 1651 necessary and the Header Len field will be set to 0. 1653 6.1.3. Home Test Init Message 1655 A mobile node uses the Home Test Init (HoTI) message to initiate the 1656 return routability procedure and request a home keygen token from a 1657 correspondent node (see Section 11.6.1). The Home Test Init message 1658 uses the MH Type value 1. When this value is indicated in the MH 1659 Type field, the format of the Message Data field in the Mobility 1660 Header is as follows: 1662 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1663 | Reserved | 1664 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1665 | | 1666 + Home Init Cookie + 1667 | | 1668 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1669 | | 1670 . . 1671 . Mobility Options . 1672 . . 1673 | | 1674 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1676 Reserved 1678 16-bit field reserved for future use. This value MUST be 1679 initialized to zero by the sender, and MUST be ignored by the 1680 receiver. 1682 Home Init Cookie 1684 64-bit field which contains a random value, the home init cookie. 1686 Mobility Options 1688 Variable-length field of such length that the complete Mobility 1689 Header is an integer multiple of 8 octets long. This field 1690 contains zero or more TLV-encoded mobility options. The receiver 1691 MUST ignore and skip any options which it does not understand. 1692 This specification does not define any options valid for the Home 1693 Test Init message. 1695 If no actual options are present in this message, no padding is 1696 necessary and the Header Len field will be set to 1. 1698 This message is tunneled through the home agent when the mobile node 1699 is away from home. Such tunneling SHOULD employ IPsec ESP in tunnel 1700 mode between the home agent and the mobile node. This protection is 1701 indicated by the IPsec security policy database. The protection of 1702 Home Test Init messages is unrelated to the requirement to protect 1703 regular payload traffic, which MAY use such tunnels as well. 1705 6.1.4. Care-of Test Init Message 1707 A mobile node uses the Care-of Test Init (CoTI) message to initiate 1708 the return routability procedure and request a care-of keygen token 1709 from a correspondent node (see Section 11.6.1). The Care-of Test 1710 Init message uses the MH Type value 2. When this value is indicated 1711 in the MH Type field, the format of the Message Data field in the 1712 Mobility Header is as follows: 1714 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1715 | Reserved | 1716 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1717 | | 1718 + Care-of Init Cookie + 1719 | | 1720 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1721 | | 1722 . . 1723 . Mobility Options . 1724 . . 1725 | | 1726 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1728 Reserved 1730 16-bit field reserved for future use. The value MUST be 1731 initialized to zero by the sender, and MUST be ignored by the 1732 receiver. 1734 Care-of Init Cookie 1736 64-bit field which contains a random value, the care-of init 1737 cookie. 1739 Mobility Options 1741 Variable-length field of such length that the complete Mobility 1742 Header is an integer multiple of 8 octets long. This field 1743 contains zero or more TLV-encoded mobility options. The receiver 1744 MUST ignore and skip any options which it does not understand. 1745 This specification does not define any options valid for the 1746 Care-of Test Init message. 1748 If no actual options are present in this message, no padding is 1749 necessary and the Header Len field will be set to 1. 1751 6.1.5. Home Test Message 1753 The Home Test (HoT) message is a response to the Home Test Init 1754 message, and is sent from the correspondent node to the mobile node 1755 (see Section 5.2.5). The Home Test message uses the MH Type value 3. 1756 When this value is indicated in the MH Type field, the format of the 1757 Message Data field in the Mobility Header is as follows: 1759 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1760 | Home Nonce Index | 1761 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1762 | | 1763 + Home Init Cookie + 1764 | | 1765 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1766 | | 1767 + Home Keygen Token + 1768 | | 1769 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1770 | | 1771 . . 1772 . Mobility options . 1773 . . 1774 | | 1775 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1777 Home Nonce Index 1779 This field will be echoed back by the mobile node to the 1780 correspondent node in a subsequent Binding Update. 1782 Home Init Cookie 1784 64-bit field which contains the home init cookie. 1786 Home Keygen Token 1788 This field contains the 64 bit home keygen token used in the 1789 return routability procedure. 1791 Mobility Options 1793 Variable-length field of such length that the complete Mobility 1794 Header is an integer multiple of 8 octets long. This field 1795 contains zero or more TLV-encoded mobility options. The receiver 1796 MUST ignore and skip any options which it does not understand. 1797 This specification does not define any options valid for the Home 1798 Test message. 1800 If no actual options are present in this message, no padding is 1801 necessary and the Header Len field will be set to 2. 1803 6.1.6. Care-of Test Message 1805 The Care-of Test (CoT) message is a response to the Care-of Test Init 1806 message, and is sent from the correspondent node to the mobile node 1807 (see Section 11.6.2). The Care-of Test message uses the MH Type 1808 value 4. When this value is indicated in the MH Type field, the 1809 format of the Message Data field in the Mobility Header is as 1810 follows: 1812 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1813 | Care-of Nonce Index | 1814 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1815 | | 1816 + Care-of Init Cookie + 1817 | | 1818 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1819 | | 1820 + Care-of Keygen Token + 1821 | | 1822 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1823 | | 1824 . . 1825 . Mobility Options . 1826 . . 1827 | | 1828 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1830 Care-of Nonce Index 1832 This value will be echoed back by the mobile node to the 1833 correspondent node in a subsequent Binding Update. 1835 Care-of Init Cookie 1837 64-bit field which contains the care-of init cookie. 1839 Care-of Keygen Token 1841 This field contains the 64 bit care-of keygen token used in the 1842 return routability procedure. 1844 Mobility Options 1846 Variable-length field of such length that the complete Mobility 1847 Header is an integer multiple of 8 octets long. This field 1848 contains zero or more TLV-encoded mobility options. The receiver 1849 MUST ignore and skip any options which it does not understand. 1850 This specification does not define any options valid for the 1851 Care-of Test message. 1853 If no actual options are present in this message, no padding is 1854 necessary and the Header Len field will be set to 2. 1856 6.1.7. Binding Update Message 1858 The Binding Update (BU) message is used by a mobile node to notify 1859 other nodes of a new care-of address for itself. Binding Updates are 1860 sent as described in Section 11.7.1 and Section 11.7.2. 1862 The Binding Update uses the MH Type value 5. When this value is 1863 indicated in the MH Type field, the format of the Message Data field 1864 in the Mobility Header is as follows: 1866 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1867 | Sequence # | 1868 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1869 |A|H|L|K| Reserved | Lifetime | 1870 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1871 | | 1872 . . 1873 . Mobility options . 1874 . . 1875 | | 1876 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1878 Acknowledge (A) 1880 The Acknowledge (A) bit is set by the sending mobile node to 1881 request a Binding Acknowledgement (Section 6.1.8) be returned upon 1882 receipt of the Binding Update. 1884 Home Registration (H) 1886 The Home Registration (H) bit is set by the sending mobile node to 1887 request that the receiving node should act as this node's home 1888 agent. The destination of the packet carrying this message MUST 1889 be that of a router sharing the same subnet prefix as the home 1890 address of the mobile node in the binding. 1892 Link-Local Address Compatibility (L) 1894 The Link-Local Address Compatibility (L) bit is set when the home 1895 address reported by the mobile node has the same interface 1896 identifier as the mobile node's link-local address. 1898 Key Management Mobility Capability (K) 1900 If this bit is cleared, the protocol used for establishing the 1901 IPsec security associations between the mobile node and the home 1902 agent does not survive movements. It may then have to be rerun. 1903 (Note that the IPsec security associations themselves are expected 1904 to survive movements.) If manual IPsec configuration is used, the 1905 bit MUST be cleared. 1907 This bit is valid only in Binding Updates sent to the home agent, 1908 and MUST be cleared in other Binding Updates. Correspondent nodes 1909 MUST ignore this bit. 1911 Reserved 1913 These fields are unused. They MUST be initialized to zero by the 1914 sender and MUST be ignored by the receiver. 1916 Sequence # 1918 A 16-bit unsigned integer used by the receiving node to sequence 1919 Binding Updates and by the sending node to match a returned 1920 Binding Acknowledgement with this Binding Update. 1922 Lifetime 1924 16-bit unsigned integer. The number of time units remaining 1925 before the binding MUST be considered expired. A value of zero 1926 indicates that the Binding Cache entry for the mobile node MUST be 1927 deleted. One time unit is 4 seconds. 1929 Mobility Options 1931 Variable-length field of such length that the complete Mobility 1932 Header is an integer multiple of 8 octets long. This field 1933 contains zero or more TLV-encoded mobility options. The encoding 1934 and format of defined options are described in Section 6.2. The 1935 receiver MUST ignore and skip any options which it does not 1936 understand. 1938 The following options are valid in a Binding Update: 1940 * Binding Authorization Data option (this option is mandatory in 1941 Binding Updates sent to a correspondent node) 1943 * Nonce Indices option. 1945 * Alternate Care-of Address option 1947 If no options are present in this message, 4 octets of padding are 1948 necessary and the Header Len field will be set to 1. 1950 The care-of address is specified either by the Source Address field 1951 in the IPv6 header or by the Alternate Care-of Address option, if 1952 present. The care-of address MUST be a unicast routable address. 1953 IPv6 Source Address MUST be a topologically correct source address. 1954 Binding Updates for a care-of address which is not a unicast routable 1955 address MUST be silently discarded. 1957 The deletion of a binding MUST be indicated by setting the Lifetime 1958 field to 0. In deletion, the generation of the binding management 1959 key depends exclusively on the home keygen token, as explained in 1960 Section 5.2.5. 1962 Correspondent nodes SHOULD NOT delete the Binding Cache entry before 1963 the lifetime expires, if any application hosted by the correspondent 1964 node is still likely to require communication with the mobile node. 1965 A Binding Cache entry that is de-allocated prematurely might cause 1966 subsequent packets to be dropped from the mobile node, if they 1967 contain the Home Address destination option. This situation is 1968 recoverable, since a Binding Error message is sent to the mobile node 1969 (see Section 6.1.9); however, it causes unnecessary delay in the 1970 communications. 1972 6.1.8. Binding Acknowledgement Message 1974 The Binding Acknowledgement is used to acknowledge receipt of a 1975 Binding Update (Section 6.1.7). This packet is sent as described in 1976 Section 9.5.4 and Section 10.3.1. 1978 The Binding Acknowledgement has the MH Type value 6. When this value 1979 is indicated in the MH Type field, the format of the Message Data 1980 field in the Mobility Header is as follows: 1982 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1983 | Status |K| Reserved | 1984 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1985 | Sequence # | Lifetime | 1986 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1987 | | 1988 . . 1989 . Mobility options . 1990 . . 1991 | | 1992 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1994 Status 1996 8-bit unsigned integer indicating the disposition of the Binding 1997 Update. Values of the Status field less than 128 indicate that 1998 the Binding Update was accepted by the receiving node. Values 1999 greater than or equal to 128 indicate that the Binding Update was 2000 rejected by the receiving node. The following Status values are 2001 currently defined: 2003 0 Binding Update accepted 2005 1 Accepted but prefix discovery necessary 2007 128 Reason unspecified 2009 129 Administratively prohibited 2011 130 Insufficient resources 2013 131 Home registration not supported 2015 132 Not home subnet 2017 133 Not home agent for this mobile node 2019 134 Duplicate Address Detection failed 2021 135 Sequence number out of window 2023 136 Expired home nonce index 2025 137 Expired care-of nonce index 2027 138 Expired nonces 2029 139 Registration type change disallowed 2031 TBD Invalid Care-of Address 2033 Up-to-date values of the Status field are to be specified in the 2034 IANA registry of assigned numbers [10]. 2036 Key Management Mobility Capability (K) 2038 If this bit is cleared, the protocol used by the home agent for 2039 establishing the IPsec security associations between the mobile 2040 node and the home agent does not survive movements. It may then 2041 have to be rerun. (Note that the IPsec security associations 2042 themselves are expected to survive movements.) 2044 Correspondent nodes MUST set the K bit to 0. 2046 Reserved 2048 This field is unused. It MUST be initialized to zero by the 2049 sender and MUST be ignored by the receiver. 2051 Sequence # 2053 The Sequence Number in the Binding Acknowledgement is copied from 2054 the Sequence Number field in the Binding Update. It is used by 2055 the mobile node in matching this Binding Acknowledgement with an 2056 outstanding Binding Update. 2058 Lifetime 2060 The granted lifetime, in time units of 4 seconds, for which this 2061 node SHOULD retain the entry for this mobile node in its Binding 2062 Cache. 2064 The value of this field is undefined if the Status field indicates 2065 that the Binding Update was rejected. 2067 Mobility Options 2069 Variable-length field of such length that the complete Mobility 2070 Header is an integer multiple of 8 octets long. This field 2071 contains zero or more TLV-encoded mobility options. The encoding 2072 and format of defined options are described in Section 6.2. The 2073 receiver MUST ignore and skip any options which it does not 2074 understand. 2076 There MAY be additional information, associated with this Binding 2077 Acknowledgement that need not be present in all Binding 2078 Acknowledgements sent. Mobility options allow future extensions 2079 to the format of the Binding Acknowledgement to be defined. The 2080 following options are valid for the Binding Acknowledgement: 2082 * Binding Authorization Data option (this option is mandatory in 2083 Binding Acknowledgements sent by a correspondent node, except 2084 where otherwise noted in Section 9.5.4) 2086 * Binding Refresh Advice option 2088 If no options are present in this message, 4 octets of padding are 2089 necessary and the Header Len field will be set to 1. 2091 6.1.9. Binding Error Message 2093 The Binding Error (BE) message is used by the correspondent node to 2094 signal an error related to mobility, such as an inappropriate attempt 2095 to use the Home Address destination option without an existing 2096 binding; see Section 9.3.3 for details. 2098 The Binding Error message uses the MH Type value 7. When this value 2099 is indicated in the MH Type field, the format of the Message Data 2100 field in the Mobility Header is as follows: 2102 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2103 | Status | Reserved | 2104 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2105 | | 2106 + + 2107 | | 2108 + Home Address + 2109 | | 2110 + + 2111 | | 2112 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2113 . . 2114 . Mobility Options . 2115 . . 2116 | | 2117 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2119 Status 2121 8-bit unsigned integer indicating the reason for this message. 2122 The following values are currently defined: 2124 1 Unknown binding for Home Address destination option 2126 2 Unrecognized MH Type value 2128 Reserved 2130 A 8-bit field reserved for future use. The value MUST be 2131 initialized to zero by the sender, and MUST be ignored by the 2132 receiver. 2134 Home Address 2136 The home address that was contained in the Home Address 2137 destination option. The mobile node uses this information to 2138 determine which binding does not exist, in cases where the mobile 2139 node has several home addresses. 2141 Mobility Options 2143 Variable-length field of such length that the complete Mobility 2144 Header is an integer multiple of 8 octets long. This field 2145 contains zero or more TLV-encoded mobility options. The receiver 2146 MUST ignore and skip any options which it does not understand. 2148 There MAY be additional information, associated with this Binding 2149 Error message that need not be present in all Binding Error 2150 messages sent. Mobility options allow future extensions to the 2151 format of the Binding Error message to be defined. The encoding 2152 and format of defined options are described in Section 6.2. This 2153 specification does not define any options valid for the Binding 2154 Error message. 2156 If no actual options are present in this message, no padding is 2157 necessary and the Header Len field will be set to 2. 2159 6.2. Mobility Options 2161 Mobility messages can include zero or more mobility options. This 2162 allows optional fields that may not be needed in every use of a 2163 particular Mobility Header, as well as future extensions to the 2164 format of the messages. Such options are included in the Message 2165 Data field of the message itself, after the fixed portion of the 2166 message data specified in the message subsections of Section 6.1. 2168 The presence of such options will be indicated by the Header Len of 2169 the Mobility Header. If included, the Binding Authorization Data 2170 option (Section 6.2.7) MUST be the last option and MUST NOT have 2171 trailing padding. Otherwise, options can be placed in any order. 2173 6.2.1. Format 2175 Mobility options are encoded within the remaining space of the 2176 Message Data field of a mobility message, using a type-length-value 2177 (TLV) format as follows: 2179 0 1 2 3 2180 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 2181 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2182 | Option Type | Option Length | Option Data... 2183 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2185 Option Type 2187 8-bit identifier of the type of mobility option. When processing 2188 a Mobility Header containing an option for which the Option Type 2189 value is not recognized by the receiver, the receiver MUST quietly 2190 ignore and skip over the option, correctly handling any remaining 2191 options in the message. 2193 Option Length 2195 8-bit unsigned integer, representing the length in octets of the 2196 mobility option, not including the Option Type and Option Length 2197 fields. 2199 Option Data 2201 A variable length field that contains data specific to the option. 2203 The following subsections specify the Option types which are 2204 currently defined for use in the Mobility Header. 2206 Implementations MUST silently ignore any mobility options that they 2207 do not understand. 2209 Mobility options may have alignment requirements. Following the 2210 convention in IPv6, these options are aligned in a packet so that 2211 multi-octet values within the Option Data field of each option fall 2212 on natural boundaries (i.e., fields of width n octets are placed at 2213 an integer multiple of n octets from the start of the header, for n = 2214 1, 2, 4, or 8) [5]. 2216 6.2.2. Pad1 2218 The Pad1 option does not have any alignment requirements. Its format 2219 is as follows: 2221 0 2222 0 1 2 3 4 5 6 7 2223 +-+-+-+-+-+-+-+-+ 2224 | Type = 0 | 2225 +-+-+-+-+-+-+-+-+ 2227 NOTE! the format of the Pad1 option is a special case - it has 2228 neither Option Length nor Option Data fields. 2230 The Pad1 option is used to insert one octet of padding in the 2231 Mobility Options area of a Mobility Header. If more than one octet 2232 of padding is required, the PadN option, described next, should be 2233 used rather than multiple Pad1 options. 2235 6.2.3. PadN 2237 The PadN option does not have any alignment requirements. Its format 2238 is as follows: 2240 0 1 2241 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 2242 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 2243 | Type = 1 | Option Length | Option Data 2244 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 2246 The PadN option is used to insert two or more octets of padding in 2247 the Mobility Options area of a mobility message. For N octets of 2248 padding, the Option Length field contains the value N-2, and the 2249 Option Data consists of N-2 zero-valued octets. PadN Option data 2250 MUST be ignored by the receiver. 2252 6.2.4. Binding Refresh Advice 2254 The Binding Refresh Advice option has an alignment requirement of 2n. 2255 Its format is as follows: 2257 0 1 2 3 2258 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 2259 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2260 | Type = 2 | Length = 2 | 2261 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2262 | Refresh Interval | 2263 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2265 The Binding Refresh Advice option is only valid in the Binding 2266 Acknowledgement, and only on Binding Acknowledgements sent from the 2267 mobile node's home agent in reply to a home registration. The 2268 Refresh Interval is measured in units of four seconds, and indicates 2269 remaining time until the mobile node SHOULD send a new home 2270 registration to the home agent. The Refresh Interval MUST be set to 2271 indicate a smaller time interval than the Lifetime value of the 2272 Binding Acknowledgement. 2274 6.2.5. Alternate Care-of Address 2276 The Alternate Care-of Address option has an alignment requirement of 2277 8n+6. Its format is as follows: 2279 0 1 2 3 2280 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 2281 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2282 | Type = 3 | Length = 16 | 2283 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2284 | | 2285 + + 2286 | | 2287 + Alternate Care-of Address + 2288 | | 2289 + + 2290 | | 2291 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2293 Normally, a Binding Update specifies the desired care-of address in 2294 the Source Address field of the IPv6 header. However, this is not 2295 possible in some cases, such as when the mobile node wishes to 2296 indicate a care-of address which it cannot use as a topologically 2297 correct source address (Section 6.1.7 and Section 11.7.2) or when the 2298 used security mechanism does not protect the IPv6 header 2299 (Section 11.7.1). 2301 The Alternate Care-of Address option is provided for these 2302 situations. This option is valid only in Binding Update. The 2303 Alternate Care-of Address field contains an address to use as the 2304 care-of address for the binding, rather than using the Source Address 2305 of the packet as the care-of address. 2307 6.2.6. Nonce Indices 2309 The Nonce Indices option has an alignment requirement of 2n. Its 2310 format is as follows: 2312 0 1 2 3 2313 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 2314 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2315 | Type = 4 | Length = 4 | 2316 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2317 | Home Nonce Index | Care-of Nonce Index | 2318 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2320 The Nonce Indices option is valid only in the Binding Update message 2321 sent to a correspondent node, and only when present together with a 2322 Binding Authorization Data option. When the correspondent node 2323 authorizes the Binding Update, it needs to produce home and care-of 2324 keygen tokens from its stored random nonce values. 2326 The Home Nonce Index field tells the correspondent node which nonce 2327 value to use when producing the home keygen token. 2329 The Care-of Nonce Index field is ignored in requests to delete a 2330 binding. Otherwise, it tells the correspondent node which nonce 2331 value to use when producing the care-of keygen token. 2333 6.2.7. Binding Authorization Data 2335 The Binding Authorization Data option does not have alignment 2336 requirements as such. However, since this option must be the last 2337 mobility option, an implicit alignment requirement is 8n + 2. The 2338 format of this option is as follows: 2340 0 1 2 3 2341 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 2342 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2343 | Type = 5 | Option Length | 2344 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2345 | | 2346 + + 2347 | Authenticator | 2348 + + 2349 | | 2350 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2352 The Binding Authorization Data option is valid in the Binding Update 2353 and Binding Acknowledgement. 2355 The Option Length field contains the length of the authenticator in 2356 octets. 2358 The Authenticator field contains a cryptographic value which can be 2359 used to determine that the message in question comes from the right 2360 authority. Rules for calculating this value depends on the used 2361 authorization procedure. 2363 For the return routability procedure, this option can appear in the 2364 Binding Update and Binding Acknowledgements. Rules for calculating 2365 the Authenticator value are the following: 2367 Mobility Data = care-of address | correspondent | MH Data 2368 Authenticator = First (96, HMAC_SHA1 (Kbm, Mobility Data)) 2370 Where | denotes concatenation. "Care-of address" is the care-of 2371 address which will be registered for the mobile node if the Binding 2372 Update succeeds, or the home address of the mobile node if this 2373 option is used in de-registration. Note also that this address might 2374 be different from the source address of the Binding Update message, 2375 if the Alternative Care-of Address mobility option is used, or when 2376 the lifetime of the binding is set to zero. 2378 The "correspondent" is the IPv6 address of the correspondent node. 2379 Note that, if the message is sent to a destination which is itself 2380 mobile, the "correspondent" address may not be the address found in 2381 the Destination Address field of the IPv6 header; instead the home 2382 address from the type 2 Routing header should be used. 2384 "MH Data" is the content of the Mobility Header, excluding the 2385 Authenticator field itself. The Authenticator value is calculated as 2386 if the Checksum field in the Mobility Header was zero. The Checksum 2387 in the transmitted packet is still calculated in the usual manner, 2388 with the calculated Authenticator being a part of the packet 2389 protected by the Checksum. Kbm is the binding management key, which 2390 is typically created using nonces provided by the correspondent node 2391 (see Section 9.4). Note that while the contents of a potential Home 2392 Address destination option are not covered in this formula, the rules 2393 for the calculation of the Kbm do take the home address in account. 2394 This ensures that the MAC will be different for different home 2395 addresses. 2397 The first 96 bits from the MAC result are used as the Authenticator 2398 field. 2400 6.3. Home Address Option 2402 The Home Address option is carried by the Destination Option 2403 extension header (Next Header value = 60). It is used in a packet 2404 sent by a mobile node while away from home, to inform the recipient 2405 of the mobile node's home address. 2407 The Home Address option is encoded in type-length-value (TLV) format 2408 as follows: 2410 0 1 2 3 2411 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 2412 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2413 | Option Type | Option Length | 2414 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2415 | | 2416 + + 2417 | | 2418 + Home Address + 2419 | | 2420 + + 2421 | | 2422 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2424 Option Type 2426 201 = 0xC9 2428 Option Length 2430 8-bit unsigned integer. Length of the option, in octets, 2431 excluding the Option Type and Option Length fields. This field 2432 MUST be set to 16. 2434 Home Address 2436 The home address of the mobile node sending the packet. This 2437 address MUST be a unicast routable address. 2439 The alignment requirement [5] for the Home Address option is 8n+6. 2441 The three highest-order bits of the Option Type field are encoded to 2442 indicate specific processing of the option [5]; for the Home Address 2443 option, these three bits are set to 110. This indicates the 2444 following processing requirements: 2446 o Any IPv6 node that does not recognize the Option Type must discard 2447 the packet, and if the packet's Destination Address was not a 2448 multicast address, return an ICMP Parameter Problem, Code 2, 2449 message to the packet's Source Address. The Pointer field in the 2450 ICMP message SHOULD point at the Option Type field. Otherwise, 2451 for multicast addresses, the ICMP message MUST NOT be sent. 2453 o The data within the option cannot change en route to the packet's 2454 final destination. 2456 The Home Address option MUST be placed as follows: 2458 o After the routing header, if that header is present 2460 o Before the Fragment Header, if that header is present 2462 o Before the AH Header or ESP Header, if either one of those headers 2463 are present 2465 For each IPv6 packet header, the Home Address Option MUST NOT appear 2466 more than once. However, an encapsulated packet [6] MAY contain a 2467 separate Home Address option associated with each encapsulating IP 2468 header. 2470 The inclusion of a Home Address destination option in a packet 2471 affects the receiving node's processing of only this single packet. 2473 No state is created or modified in the receiving node as a result of 2474 receiving a Home Address option in a packet. In particular, the 2475 presence of a Home Address option in a received packet MUST NOT alter 2476 the contents of the receiver's Binding Cache and MUST NOT cause any 2477 changes in the routing of subsequent packets sent by this receiving 2478 node. 2480 6.4. Type 2 Routing Header 2482 Mobile IPv6 defines a new routing header variant, the type 2 routing 2483 header, to allow the packet to be routed directly from a 2484 correspondent to the mobile node's care-of address. The mobile 2485 node's care-of address is inserted into the IPv6 Destination Address 2486 field. Once the packet arrives at the care-of address, the mobile 2487 node retrieves its home address from the routing header, and this is 2488 used as the final destination address for the packet. 2490 The new routing header uses a different type than defined for 2491 "regular" IPv6 source routing, enabling firewalls to apply different 2492 rules to source routed packets than to Mobile IPv6. This routing 2493 header type (type 2) is restricted to carry only one IPv6 address. 2494 All IPv6 nodes which process this routing header MUST verify that the 2495 address contained within is the node's own home address in order to 2496 prevent packets from being forwarded outside the node. The IP 2497 address contained in the routing header, since it is the mobile 2498 node's home address, MUST be a unicast routable address. 2499 Furthermore, if the scope of the home address is smaller than the 2500 scope of the care-of address, the mobile node MUST discard the packet 2501 (see Section 4.6). 2503 6.4.1. Format 2505 The type 2 routing header has the following format: 2507 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2508 | Next Header | Hdr Ext Len=2 | Routing Type=2|Segments Left=1| 2509 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2510 | Reserved | 2511 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2512 | | 2513 + + 2514 | | 2515 + Home Address + 2516 | | 2517 + + 2518 | | 2519 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2521 Next Header 2523 8-bit selector. Identifies the type of header immediately 2524 following the routing header. Uses the same values as the IPv6 2525 Next Header field [5]. 2527 Hdr Ext Len 2529 2 (8-bit unsigned integer); length of the routing header in 2530 8-octet units, not including the first 8 octets. 2532 Routing Type 2534 2 (8-bit unsigned integer). 2536 Segments Left 2538 1 (8-bit unsigned integer). 2540 Reserved 2542 32-bit reserved field. The value MUST be initialized to zero by 2543 the sender, and MUST be ignored by the receiver. 2545 Home Address 2547 The Home Address of the destination Mobile Node. 2549 For a type 2 routing header, the Hdr Ext Len MUST be 2. The Segments 2550 Left value describes the number of route segments remaining; i.e., 2551 number of explicitly listed intermediate nodes still to be visited 2552 before reaching the final destination. Segments Left MUST be 1. The 2553 ordering rules for extension headers in an IPv6 packet are described 2554 in Section 4.1 of RFC 2460 [5]. The type 2 routing header defined 2555 for Mobile IPv6 follows the same ordering as other routing headers. 2556 If another routing header is present along with a type 2 routing 2557 header, the type 2 routing header should follow the other routing 2558 header. A packet containing such nested encapsulation should be 2559 created as if the inner (type 2) routing header was constructed first 2560 and then treated as an original packet by header construction process 2561 for the other routing header. 2563 In addition, the general procedures defined by IPv6 for routing 2564 headers suggest that a received routing header MAY be automatically 2565 "reversed" to construct a routing header for use in any response 2566 packets sent by upper-layer protocols, if the received packet is 2567 authenticated [6]. This MUST NOT be done automatically for type 2 2568 routing headers. 2570 6.5. ICMP Home Agent Address Discovery Request Message 2572 The ICMP Home Agent Address Discovery Request message is used by a 2573 mobile node to initiate the dynamic home agent address discovery 2574 mechanism, as described in Section 11.4.1. The mobile node sends the 2575 Home Agent Address Discovery Request message to the Mobile IPv6 Home- 2576 Agents anycast address [7] for its own home subnet prefix. (Note 2577 that the currently defined anycast addresses may not work with all 2578 prefix lengths other than those defined in RFC 4291 [15] [35].) 2580 0 1 2 3 2581 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 2582 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2583 | Type | Code | Checksum | 2584 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2585 | Identifier | Reserved | 2586 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2588 Type 2590 144 2592 Code 2594 0 2596 Checksum 2598 The ICMP checksum [16]. 2600 Identifier 2602 An identifier to aid in matching Home Agent Address Discovery 2603 Reply messages to this Home Agent Address Discovery Request 2604 message. 2606 Reserved 2608 This field is unused. It MUST be initialized to zero by the 2609 sender and MUST be ignored by the receiver. 2611 The Source Address of the Home Agent Address Discovery Request 2612 message packet is typically one of the mobile node's current care-of 2613 addresses. At the time of performing this dynamic home agent address 2614 discovery procedure, it is likely that the mobile node is not 2615 registered with any home agent. Therefore, neither the nature of the 2616 address nor the identity of the mobile node can be established at 2617 this time. The home agent MUST then return the Home Agent Address 2618 Discovery Reply message directly to the Source Address chosen by the 2619 mobile node. 2621 6.6. ICMP Home Agent Address Discovery Reply Message 2623 The ICMP Home Agent Address Discovery Reply message is used by a home 2624 agent to respond to a mobile node that uses the dynamic home agent 2625 address discovery mechanism, as described in Section 10.5. 2627 0 1 2 3 2628 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 2629 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2630 | Type | Code | Checksum | 2631 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2632 | Identifier | Reserved | 2633 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2634 | | 2635 + + 2636 . . 2637 . Home Agent Addresses . 2638 . . 2639 + + 2640 | | 2641 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2643 Type 2645 145 2647 Code 2649 0 2651 Checksum 2653 The ICMP checksum [16]. 2655 Identifier 2657 The identifier from the invoking Home Agent Address Discovery 2658 Request message. 2660 Reserved 2662 This field is unused. It MUST be initialized to zero by the 2663 sender and MUST be ignored by the receiver. 2665 Home Agent Addresses 2667 A list of addresses of home agents on the home link for the mobile 2668 node. The number of addresses presented in the list is indicated 2669 by the remaining length of the IPv6 packet carrying the Home Agent 2670 Address Discovery Reply message. 2672 6.7. ICMP Mobile Prefix Solicitation Message Format 2674 The ICMP Mobile Prefix Solicitation Message is sent by a mobile node 2675 to its home agent while it is away from home. The purpose of the 2676 message is to solicit a Mobile Prefix Advertisement from the home 2677 agent, which will allow the mobile node to gather prefix information 2678 about its home network. This information can be used to configure 2679 and update home address(es) according to changes in prefix 2680 information supplied by the home agent. 2682 0 1 2 3 2683 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 2684 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2685 | Type | Code | Checksum | 2686 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2687 | Identifier | Reserved | 2688 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2690 IP Fields: 2692 Source Address 2694 The mobile node's care-of address. 2696 Destination Address 2698 The address of the mobile node's home agent. This home agent must 2699 be on the link that the mobile node wishes to learn prefix 2700 information about. 2702 Hop Limit 2704 Set to an initial hop limit value, similarly to any other unicast 2705 packet sent by the mobile node. 2707 Destination Option: 2709 A Home Address destination option MUST be included. 2711 ESP header: 2713 IPsec headers MUST be supported and SHOULD be used as described in 2714 Section 5.4. 2716 ICMP Fields: 2718 Type 2720 146 2722 Code 2724 0 2726 Checksum 2728 The ICMP checksum [16]. 2730 Identifier 2732 An identifier to aid in matching a future Mobile Prefix 2733 Advertisement to this Mobile Prefix Solicitation. 2735 Reserved 2737 This field is unused. It MUST be initialized to zero by the 2738 sender and MUST be ignored by the receiver. 2740 The Mobile Prefix Solicitation messages may have options. These 2741 options MUST use the option format defined in Neighbor Discovery (RFC 2742 4861 [17]). This document does not define any option types for the 2743 Mobile Prefix Solicitation message, but future documents may define 2744 new options. Home agents MUST silently ignore any options they do 2745 not recognize and continue processing the message. 2747 6.8. ICMP Mobile Prefix Advertisement Message Format 2749 A home agent will send a Mobile Prefix Advertisement to a mobile node 2750 to distribute prefix information about the home link while the mobile 2751 node is traveling away from the home network. This will occur in 2752 response to a Mobile Prefix Solicitation with an Advertisement, or by 2753 an unsolicited Advertisement sent according to the rules in 2754 Section 10.6. 2756 0 1 2 3 2757 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 2758 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2759 | Type | Code | Checksum | 2760 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2761 | Identifier |M|O| Reserved | 2762 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2763 | Options ... 2764 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2766 IP Fields: 2768 Source Address 2770 The home agent's address as the mobile node would expect to see it 2771 (i.e., same network prefix). 2773 Destination Address 2775 If this message is a response to a Mobile Prefix Solicitation, 2776 this field contains the Source Address field from that packet. 2777 For unsolicited messages, the mobile node's care-of address SHOULD 2778 be used. Note that unsolicited messages can only be sent if the 2779 mobile node is currently registered with the home agent. 2781 Routing header: 2783 A type 2 routing header MUST be included. 2785 ESP header: 2787 IPsec headers MUST be supported and SHOULD be used as described in 2788 Section 5.4. 2790 ICMP Fields: 2792 Type 2794 147 2796 Code 2798 0 2800 Checksum 2802 The ICMP checksum [16]. 2804 Identifier 2806 An identifier to aid in matching this Mobile Prefix Advertisement 2807 to a previous Mobile Prefix Solicitation. 2809 M 2811 1-bit Managed Address Configuration flag. When set, hosts use the 2812 administered (stateful) protocol for address autoconfiguration in 2813 addition to any addresses autoconfigured using stateless address 2814 autoconfiguration. The use of this flag is described in [17] 2815 [18]. 2817 O 2819 1-bit Other Stateful Configuration flag. When set, hosts use the 2820 administered (stateful) protocol for autoconfiguration of other 2821 (non-address) information. The use of this flag is described in 2822 [17] [18]. 2824 Reserved 2826 This field is unused. It MUST be initialized to zero by the 2827 sender and MUST be ignored by the receiver. 2829 The Mobile Prefix Advertisement messages may have options. These 2830 options MUST use the option format defined in Neighbor Discovery (RFC 2831 4861 [17]). This document defines one option which may be carried in 2832 a Mobile Prefix Advertisement message, but future documents may 2833 define new options. Mobile nodes MUST silently ignore any options 2834 they do not recognize and continue processing the message. 2836 Prefix Information 2838 Each message contains one or more Prefix Information options. 2839 Each option carries the prefix(es) that the mobile node should use 2840 to configure its home address(es). Section 10.6 describes which 2841 prefixes should be advertised to the mobile node. 2843 The Prefix Information option is defined in Section 4.6.2 of 2844 Neighbor Discovery (RFC 4861 [17]), with modifications defined in 2845 Section 7.2 of this specification. The home agent MUST use this 2846 modified Prefix Information option to send home network prefixes 2847 as defined in Section 10.6.1. 2849 If the Advertisement is sent in response to a Mobile Prefix 2850 Solicitation, the home agent MUST copy the Identifier value from that 2851 message into the Identifier field of the Advertisement. 2853 The home agent MUST NOT send more than one Mobile Prefix 2854 Advertisement message per second to any mobile node. 2856 The M and O bits MUST be cleared if the Home Agent DHCPv6 support is 2857 not provided. If such support is provided then they are set in 2858 concert with the home network's administrative settings. 2860 7. Modifications to IPv6 Neighbor Discovery 2862 7.1. Modified Router Advertisement Message Format 2864 Mobile IPv6 modifies the format of the Router Advertisement message 2865 [17] by the addition of a single flag bit to indicate that the router 2866 sending the Advertisement message is serving as a home agent on this 2867 link. The format of the Router Advertisement message is as follows: 2869 0 1 2 3 2870 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 2871 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2872 | Type | Code | Checksum | 2873 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2874 | Cur Hop Limit |M|O|H| Reserved| Router Lifetime | 2875 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2876 | Reachable Time | 2877 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2878 | Retrans Timer | 2879 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2880 | Options ... 2881 +-+-+-+-+-+-+-+-+-+-+-+- 2883 This format represents the following changes over that originally 2884 specified for Neighbor Discovery [17]: 2886 Home Agent (H) 2888 The Home Agent (H) bit is set in a Router Advertisement to 2889 indicate that the router sending this Router Advertisement is also 2890 functioning as a Mobile IPv6 home agent on this link. 2892 Reserved 2894 Reduced from a 6-bit field to a 5-bit field to account for the 2895 addition of the above bit. 2897 7.2. Modified Prefix Information Option Format 2899 Mobile IPv6 requires knowledge of a router's global address in 2900 building a Home Agents List as part of the dynamic home agent address 2901 discovery mechanism. 2903 However, Neighbor Discovery [17] only advertises a router's link- 2904 local address, by requiring this address to be used as the IP Source 2905 Address of each Router Advertisement. 2907 Mobile IPv6 extends Neighbor Discovery to allow a router to advertise 2908 its global address, by the addition of a single flag bit in the 2909 format of a Prefix Information option for use in Router Advertisement 2910 messages. The format of the Prefix Information option is as follows: 2912 0 1 2 3 2913 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 2914 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2915 | Type | Length | Prefix Length |L|A|R|Reserved1| 2916 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2917 | Valid Lifetime | 2918 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2919 | Preferred Lifetime | 2920 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2921 | Reserved2 | 2922 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2923 | | 2924 + + 2925 | | 2926 + Prefix + 2927 | | 2928 + + 2929 | | 2930 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2932 This format represents the following changes over that originally 2933 specified for Neighbor Discovery [17]: 2935 Router Address (R) 2937 1-bit router address flag. When set, indicates that the Prefix 2938 field contains a complete IP address assigned to the sending 2939 router. The indicated prefix is given by the first Prefix Length 2940 bits of the Prefix field. The router IP address has the same 2941 scope and conforms to the same lifetime values as the advertised 2942 prefix. This use of the Prefix field is compatible with its use 2943 in advertising the prefix itself, since Prefix Advertisement uses 2944 only the leading bits. Interpretation of this flag bit is thus 2945 independent of the processing required for the On-Link (L) and 2946 Autonomous Address-Configuration (A) flag bits. 2948 Reserved1 2950 Reduced from a 6-bit field to a 5-bit field to account for the 2951 addition of the above bit. 2953 In a Router Advertisement, a home agent MUST, and all other routers 2954 MAY, include at least one Prefix Information option with the Router 2955 Address (R) bit set. Neighbor Discovery (RFC 4861 [17]) specifies 2956 that, when including all options in a Router Advertisement causes the 2957 size of the Advertisement to exceed the link MTU, multiple 2958 Advertisements can be sent, each containing a subset of the Neighbor 2959 Discovery options. Also, when sending unsolicited multicast Router 2960 Advertisements more frequently than the limit specified in RFC 4861, 2961 the sending router need not include all options in each of these 2962 Advertisements. However, in both of these cases the router SHOULD 2963 include at least one Prefix Information option with the Router 2964 Address (R) bit set in each such advertisement, if this bit is set in 2965 some advertisement sent by the router. 2967 In addition, the following requirement can assist mobile nodes in 2968 movement detection. Barring changes in the prefixes for the link, 2969 routers that send multiple Router Advertisements with the Router 2970 Address (R) bit set in some of the included Prefix Information 2971 options SHOULD provide at least one option and router address which 2972 stays the same in all of the Advertisements. 2974 7.3. New Advertisement Interval Option Format 2976 Mobile IPv6 defines a new Advertisement Interval option, used in 2977 Router Advertisement messages to advertise the interval at which the 2978 sending router sends unsolicited multicast Router Advertisements. 2979 The format of the Advertisement Interval option is as follows: 2981 0 1 2 3 2982 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 2983 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2984 | Type | Length | Reserved | 2985 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2986 | Advertisement Interval | 2987 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2989 Type 2991 7 2993 Length 2995 8-bit unsigned integer. The length of the option (including the 2996 type and length fields) is in units of 8 octets. The value of 2997 this field MUST be 1. 2999 Reserved 3001 This field is unused. It MUST be initialized to zero by the 3002 sender and MUST be ignored by the receiver. 3004 Advertisement Interval 3006 32-bit unsigned integer. The maximum time, in milliseconds, 3007 between successive unsolicited Router Advertisement messages sent 3008 by this router on this network interface. Using the conceptual 3009 router configuration variables defined by Neighbor Discovery [17], 3010 this field MUST be equal to the value MaxRtrAdvInterval, expressed 3011 in milliseconds. 3013 Routers MAY include this option in their Router Advertisements. A 3014 mobile node receiving a Router Advertisement containing this option 3015 SHOULD utilize the specified Advertisement Interval for that router 3016 in its movement detection algorithm, as described in Section 11.5.1. 3018 This option MUST be silently ignored for other Neighbor Discovery 3019 messages. 3021 7.4. New Home Agent Information Option Format 3023 Mobile IPv6 defines a new Home Agent Information option, used in 3024 Router Advertisements sent by a home agent to advertise information 3025 specific to this router's functionality as a home agent. The format 3026 of the Home Agent Information option is as follows: 3028 0 1 2 3 3029 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 3030 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3031 | Type | Length | Reserved | 3032 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3033 | Home Agent Preference | Home Agent Lifetime | 3034 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3036 Type 3038 8 3040 Length 3042 8-bit unsigned integer. The length of the option (including the 3043 type and length fields) in units of 8 octets. The value of this 3044 field MUST be 1. 3046 Reserved 3048 This field is unused. It MUST be initialized to zero by the 3049 sender and MUST be ignored by the receiver. 3051 Home Agent Preference 3053 16-bit unsigned integer. The preference for the home agent 3054 sending this Router Advertisement, for use in ordering the 3055 addresses returned to a mobile node in the Home Agent Addresses 3056 field of a Home Agent Address Discovery Reply message. Higher 3057 values mean more preferable. If this option is not included in a 3058 Router Advertisement in which the Home Agent (H) bit is set, the 3059 preference value for this home agent MUST be considered to be 0. 3060 Greater values indicate a more preferable home agent than lower 3061 values. 3063 The manual configuration of the Home Agent Preference value is 3064 described in Section 8.4. In addition, the sending home agent MAY 3065 dynamically set the Home Agent Preference value, for example 3066 basing it on the number of mobile nodes it is currently serving or 3067 on its remaining resources for serving additional mobile nodes; 3068 such dynamic settings are beyond the scope of this document. Any 3069 such dynamic setting of the Home Agent Preference, however, MUST 3070 set the preference appropriately, relative to the default Home 3071 Agent Preference value of 0 that may be in use by some home agents 3072 on this link (i.e., a home agent not including a Home Agent 3073 Information option in its Router Advertisements will be considered 3074 to have a Home Agent Preference value of 0). 3076 Home Agent Lifetime 3078 16-bit unsigned integer. The lifetime associated with the home 3079 agent in units of seconds. The default value is the same as the 3080 Router Lifetime, as specified in the main body of the Router 3081 Advertisement. The maximum value corresponds to 18.2 hours. A 3082 value of 0 MUST NOT be used. The Home Agent Lifetime applies only 3083 to this router's usefulness as a home agent; it does not apply to 3084 information contained in other message fields or options. 3086 Home agents MAY include this option in their Router Advertisements. 3087 This option MUST NOT be included in a Router Advertisement in which 3088 the Home Agent (H) bit (see Section 7.1) is not set. If this option 3089 is not included in a Router Advertisement in which the Home Agent (H) 3090 bit is set, the lifetime for this home agent MUST be considered to be 3091 the same as the Router Lifetime in the Router Advertisement. If 3092 multiple Advertisements are being sent instead of a single larger 3093 unsolicited multicast Advertisement, all of the multiple 3094 Advertisements with the Router Address (R) bit set MUST include this 3095 option with the same contents, otherwise this option MUST be omitted 3096 from all Advertisements. 3098 This option MUST be silently ignored for other Neighbor Discovery 3099 messages. 3101 If both the Home Agent Preference and Home Agent Lifetime are set to 3102 their default values specified above, this option SHOULD NOT be 3103 included in the Router Advertisement messages sent by this home 3104 agent. 3106 7.5. Changes to Sending Router Advertisements 3108 The Neighbor Discovery protocol specification [17] limits routers to 3109 a minimum interval of 3 seconds between sending unsolicited multicast 3110 Router Advertisement messages from any given network interface 3111 (limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that: 3113 "Routers generate Router Advertisements frequently enough that 3114 hosts will learn of their presence within a few minutes, but not 3115 frequently enough to rely on an absence of advertisements to 3116 detect router failure; a separate Neighbor Unreachability 3117 Detection algorithm provides failure detection." 3119 This limitation, however, is not suitable to providing timely 3120 movement detection for mobile nodes. Mobile nodes detect their own 3121 movement by learning the presence of new routers as the mobile node 3122 moves into wireless transmission range of them (or physically 3123 connects to a new wired network), and by learning that previous 3124 routers are no longer reachable. Mobile nodes MUST be able to 3125 quickly detect when they move to a link served by a new router, so 3126 that they can acquire a new care-of address and send Binding Updates 3127 to register this care-of address with their home agent and to notify 3128 correspondent nodes as needed. 3130 One method which can provide for faster movement detection, is to 3131 increase the rate at which unsolicited Router Advertisements are 3132 sent. Mobile IPv6 relaxes this limit such that routers MAY send 3133 unsolicited multicast Router Advertisements more frequently. This 3134 method can be applied where the router is expecting to provide 3135 service to visiting mobile nodes (e.g., wireless network interfaces), 3136 or on which it is serving as a home agent to one or more mobile nodes 3137 (who may return home and need to hear its Advertisements). 3139 Routers supporting mobility SHOULD be able to be configured with a 3140 smaller MinRtrAdvInterval value and MaxRtrAdvInterval value to allow 3141 sending of unsolicited multicast Router Advertisements more often. 3142 The minimum allowed values are: 3144 o MinRtrAdvInterval 0.03 seconds 3145 o MaxRtrAdvInterval 0.07 seconds 3147 In the case where the minimum intervals and delays are used, the mean 3148 time between unsolicited multicast router advertisements is 50ms. 3149 Use of these modified limits MUST be configurable (see also the 3150 configuration variable MinDelayBetweenRas in Section 13 which may 3151 also have to be modified accordingly). Systems where these values 3152 are available MUST NOT default to them, and SHOULD default to values 3153 specified in Neighbor Discovery (RFC 4861 [17]). Knowledge of the 3154 type of network interface and operating environment SHOULD be taken 3155 into account in configuring these limits for each network interface. 3156 This is important with some wireless links, where increasing the 3157 frequency of multicast beacons can cause considerable overhead. 3158 Routers SHOULD adhere to the intervals specified in RFC 4861 [17], if 3159 this overhead is likely to cause service degradation. 3161 Additionally, the possible low values of MaxRtrAdvInterval may cause 3162 some problems with movement detection in some mobile nodes. To 3163 ensure that this is not a problem, Routers SHOULD add 20ms to any 3164 Advertisement Intervals sent in RAs, which are below 200 ms, in order 3165 to account for scheduling granularities on both the MN and the 3166 Router. 3168 Note that multicast Router Advertisements are not always required in 3169 certain wireless networks that have limited bandwidth. Mobility 3170 detection or link changes in such networks may be done at lower 3171 layers. Router advertisements in such networks SHOULD be sent only 3172 when solicited. In such networks it SHOULD be possible to disable 3173 unsolicited multicast Router Advertisements on specific interfaces. 3174 The MinRtrAdvInterval and MaxRtrAdvInterval in such a case can be set 3175 to some high values. 3177 Home agents MUST include the Source Link-Layer Address option in all 3178 Router Advertisements they send. This simplifies the process of 3179 returning home, as discussed in Section 11.5.5. 3181 Note that according to Neighbor Discovery (RFC 4861 [17]), 3182 AdvDefaultLifetime is by default based on the value of 3183 MaxRtrAdvInterval. AdvDefaultLifetime is used in the Router Lifetime 3184 field of Router Advertisements. Given that this field is expressed 3185 in seconds, a small MaxRtrAdvInterval value can result in a zero 3186 value for this field. To prevent this, routers SHOULD keep 3187 AdvDefaultLifetime in at least one second, even if the use of 3188 MaxRtrAdvInterval would result in a smaller value. 3190 8. Requirements for Types of IPv6 Nodes 3192 Mobile IPv6 places some special requirements on the functions 3193 provided by different types of IPv6 nodes. This section summarizes 3194 those requirements, identifying the functionality each requirement is 3195 intended to support. 3197 The requirements are set for the following groups of nodes: 3199 o All IPv6 nodes. 3201 o All IPv6 nodes with support for route optimization. 3203 o All IPv6 routers. 3205 o All Mobile IPv6 home agents. 3207 o All Mobile IPv6 mobile nodes. 3209 It is outside the scope of this specification to specify which of 3210 these groups are mandatory in IPv6. We only describe what is 3211 mandatory for a node that supports, for instance, route optimization. 3212 Other specifications are expected to define the extent of IPv6. 3214 8.1. All IPv6 Nodes 3216 Any IPv6 node may at any time be a correspondent node of a mobile 3217 node, either sending a packet to a mobile node or receiving a packet 3218 from a mobile node. There are no Mobile IPv6 specific MUST 3219 requirements for such nodes, and basic IPv6 techniques are 3220 sufficient. If a mobile node attempts to set up route optimization 3221 with a node with only basic IPv6 support, an ICMP error will signal 3222 that the node does not support such optimizations (Section 11.3.5), 3223 and communications will flow through the home agent . 3225 An IPv6 node MUST NOT support the Home Address destination option, 3226 type 2 routing header, or the Mobility Header unless it fully 3227 supports the requirements listed in the next sections for either 3228 route optimization, mobile node, or home agent functionality. 3230 8.2. IPv6 Nodes with Support for Route Optimization 3232 Nodes that implement route optimization are a subset of all IPv6 3233 nodes on the Internet. The ability of a correspondent node to 3234 participate in route optimization is essential for the efficient 3235 operation of the IPv6 Internet, for the following reasons: 3237 o Avoidance of congestion in the home network, and enabling the use 3238 of lower-performance home agent equipment even for supporting 3239 thousands of mobile nodes. 3241 o Reduced network load across the entire Internet, as mobile devices 3242 begin to predominate. 3244 o Reduction of jitter and latency for the communications. 3246 o Greater likelihood of success for QoS signaling as tunneling is 3247 avoided and, again, fewer sources of congestion. 3249 o Improved robustness against network partitions, congestion, and 3250 other problems, since fewer routing path segments are traversed. 3252 These effects combine to enable much better performance and 3253 robustness for communications between mobile nodes and IPv6 3254 correspondent nodes. Route optimization introduces a small amount of 3255 additional state for the peers, some additional messaging, and up to 3256 1.5 roundtrip delays before it can be turned on. However, it is 3257 believed that the benefits far outweigh the costs in most cases. 3258 Section 11.3.1 discusses how mobile nodes may avoid route 3259 optimization for some of the remaining cases, such as very short-term 3260 communications. 3262 The following requirements apply to all correspondent nodes that 3263 support route optimization: 3265 o The node MUST be able to validate a Home Address option using an 3266 existing Binding Cache entry, as described in Section 9.3.1. 3268 o The node MUST be able to insert a type 2 routing header into 3269 packets to be sent to a mobile node, as described in 3270 Section 9.3.2. 3272 o Unless the correspondent node is also acting as a mobile node, it 3273 MUST ignore type 2 routing headers and silently discard all 3274 packets that it has received with such headers. 3276 o The node SHOULD be able to interpret ICMP messages as described in 3277 Section 9.3.4. 3279 o The node MUST be able to send Binding Error messages as described 3280 in Section 9.3.3. 3282 o The node MUST be able to process Mobility Headers as described in 3283 Section 9.2. 3285 o The node MUST be able to participate in a return routability 3286 procedure (Section 9.4). 3288 o The node MUST be able to process Binding Update messages 3289 (Section 9.5). 3291 o The node MUST be able to return a Binding Acknowledgement 3292 (Section 9.5.4). 3294 o The node MUST be able to maintain a Binding Cache of the bindings 3295 received in accepted Binding Updates, as described in Section 9.1 3296 and Section 9.6. 3298 o The node SHOULD allow route optimization to be administratively 3299 enabled or disabled. The default SHOULD be enabled. 3301 8.3. All IPv6 Routers 3303 All IPv6 routers, even those not serving as a home agent for Mobile 3304 IPv6, have an effect on how well mobile nodes can communicate: 3306 o Every IPv6 router SHOULD be able to send an Advertisement Interval 3307 option (Section 7.3) in each of its Router Advertisements [17], to 3308 aid movement detection by mobile nodes (as in Section 11.5.1). 3309 The use of this option in Router Advertisements SHOULD be 3310 configurable. 3312 o Every IPv6 router SHOULD be able to support sending unsolicited 3313 multicast Router Advertisements at the faster rate described in 3314 Section 7.5. If the router supports a faster rate, the used rate 3315 MUST be configurable. 3317 o Each router SHOULD include at least one prefix with the Router 3318 Address (R) bit set and with its full IP address in its Router 3319 Advertisements (as described in Section 7.2). 3321 o Routers supporting filtering packets with routing headers SHOULD 3322 support different rules for type 0 and type 2 routing headers (see 3323 Section 6.4) so that filtering of source routed packets (type 0) 3324 will not necessarily limit Mobile IPv6 traffic which is delivered 3325 via type 2 routing headers. 3327 8.4. IPv6 Home Agents 3329 In order for a mobile node to operate correctly while away from home, 3330 at least one IPv6 router on the mobile node's home link must function 3331 as a home agent for the mobile node. The following additional 3332 requirements apply to all IPv6 routers that serve as a home agent: 3334 o Every home agent MUST be able to maintain an entry in its Binding 3335 Cache for each mobile node for which it is serving as the home 3336 agent (Section 10.1 and Section 10.3.1). 3338 o Every home agent MUST be able to intercept packets (using proxy 3339 Neighbor Discovery [17]) addressed to a mobile node for which it 3340 is currently serving as the home agent, on that mobile node's home 3341 link, while the mobile node is away from home (Section 10.4.1). 3343 o Every home agent MUST be able to encapsulate [6] such intercepted 3344 packets in order to tunnel them to the primary care-of address for 3345 the mobile node indicated in its binding in the home agent's 3346 Binding Cache (Section 10.4.2). 3348 o Every home agent MUST support decapsulating [6] reverse tunneled 3349 packets sent to it from a mobile node's home address. Every home 3350 agent MUST also check that the source address in the tunneled 3351 packets corresponds to the currently registered location of the 3352 mobile node (Section 10.4.5). 3354 o The node MUST be able to process Mobility Headers as described in 3355 Section 10.2. 3357 o Every home agent MUST be able to return a Binding Acknowledgement 3358 in response to a Binding Update (Section 10.3.1). 3360 o Every home agent MUST maintain a separate Home Agents List for 3361 each link on which it is serving as a home agent, as described in 3362 Section 10.1 and Section 10.5.1. 3364 o Every home agent MUST be able to accept packets addressed to the 3365 Mobile IPv6 Home-Agents anycast address [7] for the subnet on 3366 which it is serving as a home agent, and MUST be able to 3367 participate in dynamic home agent address discovery 3368 (Section 10.5). 3370 o Every home agent SHOULD support a configuration mechanism to allow 3371 a system administrator to manually set the value to be sent by 3372 this home agent in the Home Agent Preference field of the Home 3373 Agent Information Option in Router Advertisements that it sends 3374 (Section 7.4). 3376 o Every home agent SHOULD support sending ICMP Mobile Prefix 3377 Advertisements (Section 6.8), and SHOULD respond to Mobile Prefix 3378 Solicitations (Section 6.7). If supported, this behavior MUST be 3379 configurable, so that home agents can be configured to avoid 3380 sending such Prefix Advertisements according to the needs of the 3381 network administration in the home domain. 3383 o Every home agent MUST support IPsec ESP for protection of packets 3384 belonging to the return routability procedure (Section 10.4.6). 3386 o Every home agent SHOULD support the multicast group membership 3387 control protocols as described in Section 10.4.3. If this support 3388 is provided, the home agent MUST be capable of using it to 3389 determine which multicast data packets to forward via the tunnel 3390 to the mobile node. 3392 o Home agents MAY support stateful address autoconfiguration for 3393 mobile nodes as described in Section 10.4.4. 3395 8.5. IPv6 Mobile Nodes 3397 Finally, the following requirements apply to all IPv6 nodes capable 3398 of functioning as mobile nodes: 3400 o The node MUST maintain a Binding Update List (Section 11.1). 3402 o The node MUST support sending packets containing a Home Address 3403 option (Section 11.3.1), and follow the required IPsec interaction 3404 (Section 11.3.2). 3406 o The node MUST be able to perform IPv6 encapsulation and 3407 decapsulation [6]. 3409 o The node MUST be able to process type 2 routing header as defined 3410 in Section 6.4 and Section 11.3.3. 3412 o The node MUST support receiving a Binding Error message 3413 (Section 11.3.6). 3415 o The node MUST support receiving ICMP errors (Section 11.3.5). 3417 o The node MUST support movement detection, care-of address 3418 formation, and returning home (Section 11.5). 3420 o The node MUST be able to process Mobility Headers as described in 3421 Section 11.2. 3423 o The node MUST support the return routability procedure 3424 (Section 11.6). 3426 o The node MUST be able to send Binding Updates, as specified in 3427 Section 11.7.1 and Section 11.7.2. 3429 o The node MUST be able to receive and process Binding 3430 Acknowledgements, as specified in Section 11.7.3. 3432 o The node MUST support receiving a Binding Refresh Request 3433 (Section 6.1.2), by responding with a Binding Update. 3435 o The node MUST support receiving Mobile Prefix Advertisements 3436 (Section 11.4.3) and reconfiguring its home address based on the 3437 prefix information contained therein. 3439 o The node SHOULD support use of the dynamic home agent address 3440 discovery mechanism, as described in Section 11.4.1. 3442 o The node MUST allow route optimization to be administratively 3443 enabled or disabled. The default SHOULD be enabled. 3445 o The node MAY support the multicast address listener part of a 3446 multicast group membership protocol as described in 3447 Section 11.3.4. If this support is provided, the mobile node MUST 3448 be able to receive tunneled multicast packets from the home agent. 3450 o The node MAY support stateful address autoconfiguration mechanisms 3451 such as DHCPv6 [29] on the interface represented by the tunnel to 3452 the home agent. 3454 9. Correspondent Node Operation 3456 9.1. Conceptual Data Structures 3458 IPv6 nodes with route optimization support maintain a Binding Cache 3459 of bindings for other nodes. A separate Binding Cache SHOULD be 3460 maintained by each IPv6 node for each of its unicast routable 3461 addresses. The Binding Cache MAY be implemented in any manner 3462 consistent with the external behavior described in this document, for 3463 example by being combined with the node's Destination Cache as 3464 maintained by Neighbor Discovery [17]. When sending a packet, the 3465 Binding Cache is searched before the Neighbor Discovery conceptual 3466 Destination Cache [17]. 3468 Each Binding Cache entry conceptually contains the following fields: 3470 o The home address of the mobile node for which this is the Binding 3471 Cache entry. This field is used as the key for searching the 3472 Binding Cache for the destination address of a packet being sent. 3474 o The care-of address for the mobile node indicated by the home 3475 address field in this Binding Cache entry. 3477 o A lifetime value, indicating the remaining lifetime for this 3478 Binding Cache entry. The lifetime value is initialized from the 3479 Lifetime field in the Binding Update that created or last modified 3480 this Binding Cache entry. A correspondent node MAY select a 3481 smaller lifetime for the Binding Cache entry, and supply that 3482 value to the mobile node in the Binding Acknowledgment message. 3484 o A flag indicating whether or not this Binding Cache entry is a 3485 home registration entry (applicable only on nodes which support 3486 home agent functionality). 3488 o The maximum value of the Sequence Number field received in 3489 previous Binding Updates for this home address. The Sequence 3490 Number field is 16 bits long. Sequence Number values MUST be 3491 compared modulo 2**16 as explained in Section 9.5.1. 3493 o Usage information for this Binding Cache entry. This is needed to 3494 implement the cache replacement policy in use in the Binding 3495 Cache. Recent use of a cache entry also serves as an indication 3496 that a Binding Refresh Request should be sent when the lifetime of 3497 this entry nears expiration. 3499 Binding Cache entries not marked as home registrations MAY be 3500 replaced at any time by any reasonable local cache replacement policy 3501 but SHOULD NOT be unnecessarily deleted. The Binding Cache for any 3502 one of a node's IPv6 addresses may contain at most one entry for each 3503 mobile node home address. The contents of a node's Binding Cache 3504 MUST NOT be changed in response to a Home Address option in a 3505 received packet. 3507 9.2. Processing Mobility Headers 3509 Mobility Header processing MUST observe the following rules: 3511 o The checksum must be verified as per Section 6.1. If invalid, the 3512 node MUST silently discard the message. 3514 o The MH Type field MUST have a known value (Section 6.1.1). 3515 Otherwise, the node MUST discard the message and issue a Binding 3516 Error message as described in Section 9.3.3, with Status field set 3517 to 2 (unrecognized MH Type value). 3519 o The Payload Proto field MUST be IPPROTO_NONE (59 decimal). 3520 Otherwise, the node MUST discard the message and SHOULD send ICMP 3521 Parameter Problem, Code 0, directly to the Source Address of the 3522 packet as specified in RFC 4443 [16]. Thus no Binding Cache 3523 information is used in sending the ICMP message. The Pointer 3524 field in the ICMP message SHOULD point at the Payload Proto field. 3526 o The Header Len field in the Mobility Header MUST NOT be less than 3527 the length specified for this particular type of message in 3528 Section 6.1. Otherwise, the node MUST discard the message and 3529 SHOULD send ICMP Parameter Problem, Code 0, directly to the Source 3530 Address of the packet as specified in RFC 4443 [16]. (The Binding 3531 Cache information is again not used.) The Pointer field in the 3532 ICMP message SHOULD point at the Header Len field. 3534 Subsequent checks depend on the particular Mobility Header. 3536 9.3. Packet Processing 3538 This section describes how the correspondent node sends packets to 3539 the mobile node, and receives packets from it. 3541 9.3.1. Receiving Packets with Home Address Option 3543 Packets containing a Home Address option MUST be dropped if the given 3544 home address is not a unicast routable address. 3546 Mobile nodes can include a Home Address destination option in a 3547 packet if they believe the correspondent node has a Binding Cache 3548 entry for the home address of a mobile node. If the Next Header 3549 value of the Destination Option is one of the following: {50 (ESP), 3550 51 (AH), 135 (Mobility Header)}, the packet SHOULD be processed 3551 normally. Otherwise, the packet MUST be dropped if there is no 3552 corresponding Binding Cache entry. A corresponding Binding Cache 3553 entry MUST have the same home address as appears in the Home Address 3554 destination option, and the currently registered care-of address MUST 3555 be equal to the source address of the packet. 3557 If the packet is dropped due to the above tests, the correspondent 3558 node MUST send the Binding Error message as described in 3559 Section 9.3.3. The Status field in this message should be set to 1 3560 (unknown binding for Home Address destination option). 3562 The correspondent node MUST process the option in a manner consistent 3563 with exchanging the Home Address field from the Home Address option 3564 into the IPv6 header and replacing the original value of the Source 3565 Address field there. After all IPv6 options have been processed, it 3566 MUST be possible for upper layers to process the packet without the 3567 knowledge that it came originally from a care-of address or that a 3568 Home Address option was used. 3570 The use of IPsec Authentication Header (AH) for the Home Address 3571 option is not required, except that if the IPv6 header of a packet is 3572 covered by AH, then the authentication MUST also cover the Home 3573 Address option; this coverage is achieved automatically by the 3574 definition of the Option Type code for the Home Address option, since 3575 it indicates that the data within the option cannot change en route 3576 to the packet's final destination, and thus the option is included in 3577 the AH computation. By requiring that any authentication of the IPv6 3578 header also cover the Home Address option, the security of the Source 3579 Address field in the IPv6 header is not compromised by the presence 3580 of a Home Address option. 3582 When attempting to verify AH authentication data in a packet that 3583 contains a Home Address option, the receiving node MUST calculate the 3584 AH authentication data as if the following were true: The Home 3585 Address option contains the care-of address, and the source IPv6 3586 address field of the IPv6 header contains the home address. This 3587 conforms with the calculation specified in Section 11.3.2. 3589 9.3.2. Sending Packets to a Mobile Node 3591 Before sending any packet, the sending node SHOULD examine its 3592 Binding Cache for an entry for the destination address to which the 3593 packet is being sent. If the sending node has a Binding Cache entry 3594 for this address, the sending node SHOULD use a type 2 routing header 3595 to route the packet to this mobile node (the destination node) by way 3596 of its care-of address. However, the sending node MUST NOT do this 3597 in the following cases: 3599 o When sending an IPv6 Neighbor Discovery [17] packet. 3601 o Where otherwise noted in Section 6.1. 3603 When calculating authentication data in a packet that contains a type 3604 2 routing header, the correspondent node MUST calculate the AH 3605 authentication data as if the following were true: The routing header 3606 contains the care-of address, the destination IPv6 address field of 3607 the IPv6 header contains the home address, and the Segments Left 3608 field is zero. The IPsec Security Policy Database lookup MUST based 3609 on the mobile node's home address. 3611 For instance, assuming there are no additional routing headers in 3612 this packet beyond those needed by Mobile IPv6, the correspondent 3613 node could set the fields in the packet's IPv6 header and routing 3614 header as follows: 3616 o The Destination Address in the packet's IPv6 header is set to the 3617 mobile node's home address (the original destination address to 3618 which the packet was being sent). 3620 o The routing header is initialized to contain a single route 3621 segment, containing the mobile node's care-of address copied from 3622 the Binding Cache entry. The Segments Left field is, however, 3623 temporarily set to zero. 3625 The IP layer will insert the routing header before performing any 3626 necessary IPsec processing. Once all IPsec processing has been 3627 performed, the node swaps the IPv6 destination field with the Home 3628 Address field in the routing header, sets the Segments Left field to 3629 one, and sends the packet. This ensures the AH calculation is done 3630 on the packet in the form it will have on the receiver after 3631 advancing the routing header. 3633 Following the definition of a type 2 routing header in Section 6.4, 3634 this packet will be routed to the mobile node's care-of address, 3635 where it will be delivered to the mobile node (the mobile node has 3636 associated the care-of address with its network interface). 3638 Note that following the above conceptual model in an implementation 3639 creates some additional requirements for path MTU discovery since the 3640 layer that determines the packet size (e.g., TCP and applications 3641 using UDP) needs to be aware of the size of the headers added by the 3642 IP layer on the sending node. 3644 If, instead, the sending node has no Binding Cache entry for the 3645 destination address to which the packet is being sent, the sending 3646 node simply sends the packet normally, with no routing header. If 3647 the destination node is not a mobile node (or is a mobile node that 3648 is currently at home), the packet will be delivered directly to this 3649 node and processed normally by it. If, however, the destination node 3650 is a mobile node that is currently away from home, the packet will be 3651 intercepted by the mobile node's home agent and tunneled to the 3652 mobile node's current primary care-of address. 3654 9.3.3. Sending Binding Error Messages 3656 Section 9.2 and Section 9.3.1 describe error conditions that lead to 3657 a need to send a Binding Error message. 3659 A Binding Error message is sent directly to the address that appeared 3660 in the IPv6 Source Address field of the offending packet. If the 3661 Source Address field does not contain a unicast address, the Binding 3662 Error message MUST NOT be sent. 3664 The Home Address field in the Binding Error message MUST be copied 3665 from the Home Address field in the Home Address destination option of 3666 the offending packet, or set to the unspecified address if no such 3667 option appeared in the packet. 3669 Note that the IPv6 Source Address and Home Address field values 3670 discussed above are the values from the wire, i.e., before any 3671 modifications possibly performed as specified in Section 9.3.1. 3673 Binding Error messages SHOULD be subject to rate limiting in the same 3674 manner as is done for ICMPv6 messages [16]. 3676 9.3.4. Receiving ICMP Error Messages 3678 When the correspondent node has a Binding Cache entry for a mobile 3679 node, all traffic destined to the mobile node goes directly to the 3680 current care-of address of the mobile node using a routing header. 3681 Any ICMP error message caused by packets on their way to the care-of 3682 address will be returned in the normal manner to the correspondent 3683 node. 3685 On the other hand, if the correspondent node has no Binding Cache 3686 entry for the mobile node, the packet will be routed through the 3687 mobile node's home link. Any ICMP error message caused by the packet 3688 on its way to the mobile node while in the tunnel, will be 3689 transmitted to the mobile node's home agent. By the definition of 3690 IPv6 encapsulation [6], the home agent MUST relay certain ICMP error 3691 messages back to the original sender of the packet, which in this 3692 case is the correspondent node. 3694 Thus, in all cases, any meaningful ICMP error messages caused by 3695 packets from a correspondent node to a mobile node will be returned 3696 to the correspondent node. If the correspondent node receives 3697 persistent ICMP Destination Unreachable messages after sending 3698 packets to a mobile node based on an entry in its Binding Cache, the 3699 correspondent node SHOULD delete this Binding Cache entry. Note that 3700 if the mobile node continues to send packets with the Home Address 3701 destination option to this correspondent node, they will be dropped 3702 due to the lack of a binding. For this reason it is important that 3703 only persistent ICMP messages lead to the deletion of the Binding 3704 Cache entry. 3706 9.4. Return Routability Procedure 3708 This subsection specifies actions taken by a correspondent node 3709 during the return routability procedure. 3711 9.4.1. Receiving Home Test Init Messages 3713 Upon receiving a Home Test Init message, the correspondent node 3714 verifies the following: 3716 o The packet MUST NOT include a Home Address destination option. 3718 Any packet carrying a Home Test Init message which fails to satisfy 3719 this test MUST be silently ignored. 3721 Otherwise, in preparation for sending the corresponding Home Test 3722 Message, the correspondent node checks that it has the necessary 3723 material to engage in a return routability procedure, as specified in 3724 Section 5.2. The correspondent node MUST have a secret Kcn and a 3725 nonce. If it does not have this material yet, it MUST produce it 3726 before continuing with the return routability procedure. 3728 Section 9.4.3 specifies further processing. 3730 9.4.2. Receiving Care-of Test Init Messages 3732 Upon receiving a Care-of Test Init message, the correspondent node 3733 verifies the following: 3735 o The packet MUST NOT include a Home Address destination option. 3737 Any packet carrying a Care-of Test Init message which fails to 3738 satisfy this test MUST be silently ignored. 3740 Otherwise, in preparation for sending the corresponding Care-of Test 3741 Message, the correspondent node checks that it has the necessary 3742 material to engage in a return routability procedure in the manner 3743 described in Section 9.4.1. 3745 Section 9.4.4 specifies further processing. 3747 9.4.3. Sending Home Test Messages 3749 The correspondent node creates a home keygen token and uses the 3750 current nonce index as the Home Nonce Index. It then creates a Home 3751 Test message (Section 6.1.5) and sends it to the mobile node at the 3752 latter's home address. 3754 9.4.4. Sending Care-of Test Messages 3756 The correspondent node creates a care-of keygen token and uses the 3757 current nonce index as the Care-of Nonce Index. It then creates a 3758 Care-of Test message (Section 6.1.6) and sends it to the mobile node 3759 at the latter's care-of address. 3761 9.5. Processing Bindings 3763 This section explains how the correspondent node processes messages 3764 related to bindings. These messages are: 3766 o Binding Update 3768 o Binding Refresh Request 3770 o Binding Acknowledgement 3772 o Binding Error 3774 9.5.1. Receiving Binding Updates 3776 Before accepting a Binding Update, the receiving node MUST validate 3777 the Binding Update according to the following tests: 3779 o The packet MUST contain a unicast routable home address, either in 3780 the Home Address option or in the Source Address, if the Home 3781 Address option is not present. 3783 o The Sequence Number field in the Binding Update is greater than 3784 the Sequence Number received in the previous valid Binding Update 3785 for this home address, if any. 3787 If the receiving node has no Binding Cache entry for the indicated 3788 home address, it MUST accept any Sequence Number value in a 3789 received Binding Update from this mobile node. 3791 This Sequence Number comparison MUST be performed modulo 2**16, 3792 i.e., the number is a free running counter represented modulo 3793 65536. A Sequence Number in a received Binding Update is 3794 considered less than or equal to the last received number if its 3795 value lies in the range of the last received number and the 3796 preceding 32768 values, inclusive. For example, if the last 3797 received sequence number was 15, then messages with sequence 3798 numbers 0 through 15, as well as 32783 through 65535, would be 3799 considered less than or equal. 3801 When the Home Registration (H) bit is not set, the following are also 3802 required: 3804 o A Nonce Indices mobility option MUST be present, and the Home and 3805 Care-of Nonce Index values in this option MUST be recent enough to 3806 be recognized by the correspondent node. (Care-of Nonce Index 3807 values are not inspected for requests to delete a binding.) 3809 o The correspondent node MUST re-generate the home keygen token and 3810 the care-of keygen token from the information contained in the 3811 packet. It then generates the binding management key Kbm and uses 3812 it to verify the authenticator field in the Binding Update as 3813 specified in Section 6.1.7. 3815 o The Binding Authorization Data mobility option MUST be present, 3816 and its contents MUST satisfy rules presented in Section 5.2.6. 3817 Note that a care-of address different from the Source Address MAY 3818 have been specified by including an Alternate Care-of Address 3819 mobility option in the Binding Update. When such a message is 3820 received and the return routability procedure is used as an 3821 authorization method, the correspondent node MUST verify the 3822 authenticator by using the address within the Alternate Care-of 3823 Address in the calculations. 3825 o The Binding Authorization Data mobility option MUST be the last 3826 option and MUST NOT have trailing padding. 3828 If the Home Registration (H) bit is set, the Nonce Indices mobility 3829 option MUST NOT be present. 3831 If the mobile node sends a sequence number which is not greater than 3832 the sequence number from the last valid Binding Update for this home 3833 address, then the receiving node MUST send back a Binding 3834 Acknowledgement with status code 135, and the last accepted sequence 3835 number in the Sequence Number field of the Binding Acknowledgement. 3837 If a binding already exists for the given home address and the home 3838 registration flag has a different value than the Home Registration 3839 (H) bit in the Binding Update, then the receiving node MUST send back 3840 a Binding Acknowledgement with status code 139 (registration type 3841 change disallowed). The home registration flag stored in the Binding 3842 Cache entry MUST NOT be changed. 3844 If the receiving node no longer recognizes the Home Nonce Index 3845 value, Care-of Nonce Index value, or both values from the Binding 3846 Update, then the receiving node MUST send back a Binding 3847 Acknowledgement with status code 136, 137, or 138, respectively. 3849 Packets carrying Binding Updates that fail to satisfy all of these 3850 tests for any reason other than insufficiency of the Sequence Number, 3851 registration type change, or expired nonce index values, MUST be 3852 silently discarded. 3854 If the Binding Update is valid according to the tests above, then the 3855 Binding Update is processed further as follows: 3857 o The Sequence Number value received from a mobile node in a Binding 3858 Update is stored by the receiving node in its Binding Cache entry 3859 for the given home address. 3861 o If the Lifetime specified in the Binding Update is not zero, then 3862 this is a request to cache a binding for the home address. If the 3863 Home Registration (H) bit is set in the Binding Update, the 3864 Binding Update is processed according to the procedure specified 3865 in Section 10.3.1; otherwise, it is processed according to the 3866 procedure specified in Section 9.5.2. 3868 o If the Lifetime specified in the Binding Update is zero, then this 3869 is a request to delete the cached binding for the home address. 3870 In this case, the Binding Update MUST include a valid home nonce 3871 index, and the care-of nonce index MUST be ignored by the 3872 correspondent node. The generation of the binding management key 3873 depends then exclusively on the home keygen token (Section 5.2.5). 3874 If the Home Registration (H) bit is set in the Binding Update, the 3875 Binding Update is processed according to the procedure specified 3876 in Section 10.3.2; otherwise, it is processed according to the 3877 procedure specified in Section 9.5.3. 3879 The specified care-of address MUST be determined as follows: 3881 o If the Alternate Care-of Address option is present, the care-of 3882 address is the address in that option. 3884 o Otherwise, the care-of address is the Source Address field in the 3885 packet's IPv6 header. 3887 The home address for the binding MUST be determined as follows: 3889 o If the Home Address destination option is present, the home 3890 address is the address in that option. 3892 o Otherwise, the home address is the Source Address field in the 3893 packet's IPv6 header. 3895 9.5.2. Requests to Cache a Binding 3897 This section describes the processing of a valid Binding Update that 3898 requests a node to cache a binding, for which the Home Registration 3899 (H) bit is not set in the Binding Update. 3901 In this case, the receiving node SHOULD create a new entry in its 3902 Binding Cache for this home address, or update its existing Binding 3903 Cache entry for this home address, if such an entry already exists. 3904 The lifetime for the Binding Cache entry is initialized from the 3905 Lifetime field specified in the Binding Update, although this 3906 lifetime MAY be reduced by the node caching the binding; the lifetime 3907 for the Binding Cache entry MUST NOT be greater than the Lifetime 3908 value specified in the Binding Update. Any Binding Cache entry MUST 3909 be deleted after the expiration of its lifetime. 3911 Note that if the mobile node did not request a Binding 3912 Acknowledgement, then it is not aware of the selected shorter 3913 lifetime. The mobile node may thus use route optimization and send 3914 packets with the Home Address destination option. As discussed in 3915 Section 9.3.1, such packets will be dropped if there is no binding. 3916 This situation is recoverable, but can cause temporary packet loss. 3918 The correspondent node MAY refuse to accept a new Binding Cache entry 3919 if it does not have sufficient resources. A new entry MAY also be 3920 refused if the correspondent node believes its resources are utilized 3921 more efficiently in some other purpose, such as serving another 3922 mobile node with higher amount of traffic. In both cases the 3923 correspondent node SHOULD return a Binding Acknowledgement with 3924 status value 130. 3926 9.5.3. Requests to Delete a Binding 3928 This section describes the processing of a valid Binding Update that 3929 requests a node to delete a binding when the Home Registration (H) 3930 bit is not set in the Binding Update. 3932 Any existing binding for the given home address MUST be deleted. A 3933 Binding Cache entry for the home address MUST NOT be created in 3934 response to receiving the Binding Update. 3936 If the Binding Cache entry was created by use of return routability 3937 nonces, the correspondent node MUST ensure that the same nonces are 3938 not used again with the particular home and care-of address. If both 3939 nonces are still valid, the correspondent node has to remember the 3940 particular combination of nonce indexes, addresses, and sequence 3941 number as illegal until at least one of the nonces has become too 3942 old. 3944 9.5.4. Sending Binding Acknowledgements 3946 A Binding Acknowledgement may be sent to indicate receipt of a 3947 Binding Update as follows: 3949 o If the Binding Update was discarded as described in Section 9.2 or 3950 Section 9.5.1, a Binding Acknowledgement MUST NOT be sent. 3951 Otherwise the treatment depends on the following rules. 3953 o If the Acknowledge (A) bit is set in the Binding Update, a Binding 3954 Acknowledgement MUST be sent. Otherwise, the treatment depends on 3955 the next rule. 3957 o If the node rejects the Binding Update due to an expired nonce 3958 index, sequence number being out of window (Section 9.5.1), or 3959 insufficiency of resources (Section 9.5.2), a Binding 3960 Acknowledgement MUST be sent. If the node accepts the Binding 3961 Update, the Binding Acknowledgement SHOULD NOT be sent. 3963 If the node accepts the Binding Update and creates or updates an 3964 entry for this binding, the Status field in the Binding 3965 Acknowledgement MUST be set to a value less than 128. Otherwise, the 3966 Status field MUST be set to a value greater than or equal to 128. 3967 Values for the Status field are described in Section 6.1.8 and in the 3968 IANA registry of assigned numbers [10]. 3970 If the Status field in the Binding Acknowledgement contains the value 3971 136 (expired home nonce index), 137 (expired care-of nonce index), or 3972 138 (expired nonces) then the message MUST NOT include the Binding 3973 Authorization Data mobility option. Otherwise, the Binding 3974 Authorization Data mobility option MUST be included, and MUST meet 3975 the specific authentication requirements for Binding Acknowledgements 3976 as defined in Section 5.2. 3978 If the Source Address field of the IPv6 header that carried the 3979 Binding Update does not contain a unicast address, the Binding 3980 Acknowledgement MUST NOT be sent and the Binding Update packet MUST 3981 be silently discarded. Otherwise, the acknowledgement MUST be sent 3982 to the Source Address. Unlike the treatment of regular packets, this 3983 addressing procedure does not use information from the Binding Cache. 3985 However, a routing header is needed in some cases. If the Source 3986 Address is the home address of the mobile node, i.e., the Binding 3987 Update did not contain a Home Address destination option, then the 3988 Binding Acknowledgement MUST be sent to that address and the routing 3989 header MUST NOT be used. Otherwise, the Binding Acknowledgement MUST 3990 be sent using a type 2 routing header which contains the mobile 3991 node's home address. 3993 9.5.5. Sending Binding Refresh Requests 3995 If a Binding Cache entry being deleted is still in active use when 3996 sending packets to a mobile node, then the next packet sent to the 3997 mobile node will be routed normally to the mobile node's home link. 3998 Communication with the mobile node continues, but the tunneling from 3999 the home network creates additional overhead and latency in 4000 delivering packets to the mobile node. 4002 If the sender knows that the Binding Cache entry is still in active 4003 use, it MAY send a Binding Refresh Request message to the mobile node 4004 in an attempt to avoid this overhead and latency due to deleting and 4005 recreating the Binding Cache entry. This message is always sent to 4006 the home address of the mobile node. 4008 The correspondent node MAY retransmit Binding Refresh Request 4009 messages as long as the rate limitation is applied. The 4010 correspondent node MUST stop retransmitting when it receives a 4011 Binding Update. 4013 9.6. Cache Replacement Policy 4015 Conceptually, a node maintains a separate timer for each entry in its 4016 Binding Cache. When creating or updating a Binding Cache entry in 4017 response to a received and accepted Binding Update, the node sets the 4018 timer for this entry to the specified Lifetime period. Any entry in 4019 a node's Binding Cache MUST be deleted after the expiration of the 4020 Lifetime specified in the Binding Update from which the entry was 4021 created or last updated. 4023 Each node's Binding Cache will, by necessity, have a finite size. A 4024 node MAY use any reasonable local policy for managing the space 4025 within its Binding Cache. 4027 A node MAY choose to drop any entry already in its Binding Cache in 4028 order to make space for a new entry. For example, a "least-recently 4029 used" (LRU) strategy for cache entry replacement among entries should 4030 work well, unless the size of the Binding Cache is substantially 4031 insufficient. When entries are deleted, the correspondent node MUST 4032 follow the rules in Section 5.2.8 in order to guard the return 4033 routability procedure against replay attacks. 4035 If the node sends a packet to a destination for which it has dropped 4036 the entry from its Binding Cache, the packet will be routed through 4037 the mobile node's home link. The mobile node can detect this and 4038 establish a new binding if necessary. 4040 However, if the mobile node believes that the binding still exists, 4041 it may use route optimization and send packets with the Home Address 4042 destination option. This can create temporary packet loss, as 4043 discussed earlier, in the context of binding lifetime reductions 4044 performed by the correspondent node (Section 9.5.2). 4046 10. Home Agent Operation 4048 10.1. Conceptual Data Structures 4050 Each home agent MUST maintain a Binding Cache and Home Agents List. 4052 The rules for maintaining a Binding Cache are the same for home 4053 agents and correspondent nodes and have already been described in 4054 Section 9.1. 4056 The Home Agents List is maintained by each home agent, recording 4057 information about each router on the same link that is acting as a 4058 home agent. This list is used by the dynamic home agent address 4059 discovery mechanism. A router is known to be acting as a home agent, 4060 if it sends a Router Advertisement in which the Home Agent (H) bit is 4061 set. When the lifetime for a list entry (defined below) expires, 4062 that entry is removed from the Home Agents List. The Home Agents 4063 List is similar to the Default Router List conceptual data structure 4064 maintained by each host for Neighbor Discovery [17]. The Home Agents 4065 List MAY be implemented in any manner consistent with the external 4066 behavior described in this document. 4068 Each home agent maintains a separate Home Agents List for each link 4069 on which it is serving as a home agent. A new entry is created or an 4070 existing entry is updated in response to receipt of a valid Router 4071 Advertisement in which the Home Agent (H) bit is set. Each Home 4072 Agents List entry conceptually contains the following fields: 4074 o The link-local IP address of a home agent on the link. This 4075 address is learned through the Source Address of the Router 4076 Advertisements [17] received from the router. 4078 o One or more global IP addresses for this home agent. Global 4079 addresses are learned through Prefix Information options with the 4080 Router Address (R) bit set and received in Router Advertisements 4081 from this link-local address. Global addresses for the router in 4082 a Home Agents List entry MUST be deleted once the prefix 4083 associated with that address is no longer valid [17]. 4085 o The remaining lifetime of this Home Agents List entry. If a Home 4086 Agent Information Option is present in a Router Advertisement 4087 received from a home agent, the lifetime of the Home Agents List 4088 entry representing that home agent is initialized from the Home 4089 Agent Lifetime field in the option (if present); otherwise, the 4090 lifetime is initialized from the Router Lifetime field in the 4091 received Router Advertisement. If Home Agents List entry lifetime 4092 reaches zero, the entry MUST be deleted from the Home Agents List. 4094 o The preference for this home agent; higher values indicate a more 4095 preferable home agent. The preference value is taken from the 4096 Home Agent Preference field in the received Router Advertisement, 4097 if the Router Advertisement contains a Home Agent Information 4098 Option and is otherwise set to the default value of 0. A home 4099 agent uses this preference in ordering the Home Agents List when 4100 it sends an ICMP Home Agent Address Discovery message. 4102 10.2. Processing Mobility Headers 4104 All IPv6 home agents MUST observe the rules described in Section 9.2 4105 when processing Mobility Headers. 4107 10.3. Processing Bindings 4109 10.3.1. Primary Care-of Address Registration 4111 When a node receives a Binding Update, it MUST validate it and 4112 determine the type of Binding Update according to the steps described 4113 in Section 9.5.1. Furthermore, it MUST authenticate the Binding 4114 Update as described in Section 5.1. An authorization step specific 4115 for the home agent is also needed to ensure that only the right node 4116 can control a particular home address. This is provided through the 4117 home address unequivocally identifying the security association that 4118 must be used. 4120 This section describes the processing of a valid and authorized 4121 Binding Update when it requests the registration of the mobile node's 4122 primary care-of address. 4124 To begin processing the Binding Update, the home agent MUST perform 4125 the following sequence of tests: 4127 o If the node implements only correspondent node functionality, or 4128 has not been configured to act as a home agent, then the node MUST 4129 reject the Binding Update. The node MUST also return a Binding 4130 Acknowledgement to the mobile node, in which the Status field is 4131 set to 131 (home registration not supported). 4133 o Else, if the home address for the binding (the Home Address field 4134 in the packet's Home Address option) is not an on-link IPv6 4135 address with respect to the home agent's current Prefix List, then 4136 the home agent MUST reject the Binding Update and SHOULD return a 4137 Binding Acknowledgement to the mobile node, in which the Status 4138 field is set to 132 (not home subnet). 4140 o Else, if the home agent chooses to reject the Binding Update for 4141 any other reason (e.g., insufficient resources to serve another 4142 mobile node as a home agent), then the home agent SHOULD return a 4143 Binding Acknowledgement to the mobile node, in which the Status 4144 field is set to an appropriate value to indicate the reason for 4145 the rejection. 4147 o A Home Address destination option MUST be present in the message. 4148 It MUST be validated as described in Section 9.3.1 with the 4149 following additional rule. The Binding Cache entry existence test 4150 MUST NOT be done for IPsec packets when the Home Address option 4151 contains an address for which the receiving node could act as a 4152 home agent. 4154 If home agent accepts the Binding Update, it MUST then create a new 4155 entry in its Binding Cache for this mobile node or update its 4156 existing Binding Cache entry, if such an entry already exists. The 4157 Home Address field as received in the Home Address option provides 4158 the home address of the mobile node. 4160 The home agent MUST mark this Binding Cache entry as a home 4161 registration to indicate that the node is serving as a home agent for 4162 this binding. Binding Cache entries marked as a home registration 4163 MUST be excluded from the normal cache replacement policy used for 4164 the Binding Cache (Section 9.6) and MUST NOT be removed from the 4165 Binding Cache until the expiration of the Lifetime period. 4167 Unless this home agent already has a binding for the given home 4168 address, the home agent MUST perform Duplicate Address Detection [18] 4169 on the mobile node's home link before returning the Binding 4170 Acknowledgement. This ensures that no other node on the home link 4171 was using the mobile node's home address when the Binding Update 4172 arrived. If this Duplicate Address Detection fails for the given 4173 home address or an associated link local address, then the home agent 4174 MUST reject the complete Binding Update and MUST return a Binding 4175 Acknowledgement to the mobile node, in which the Status field is set 4176 to 134 (Duplicate Address Detection failed). When the home agent 4177 sends a successful Binding Acknowledgement to the mobile node, the 4178 home agent assures to the mobile node that its address(es) will be 4179 kept unique by the home agent for as long as the lifetime was granted 4180 for the binding. 4182 The specific addresses, which are to be tested before accepting the 4183 Binding Update and later to be defended by performing Duplicate 4184 Address Detection, depend on the setting of the Link-Local Address 4185 Compatibility (L) bit, as follows: 4187 o L=0: Defend only the given address. Do not derive a link-local 4188 address. 4190 o L=1: Defend both the given non link-local unicast (home) address 4191 and the derived link-local. The link-local address is derived by 4192 replacing the subnet prefix in the mobile node's home address with 4193 the link-local prefix. 4195 The lifetime of the Binding Cache entry depends on a number of 4196 factors: 4198 o The lifetime for the Binding Cache entry MUST NOT be greater than 4199 the Lifetime value specified in the Binding Update. 4201 o The lifetime for the Binding Cache entry MUST NOT be greater than 4202 the remaining valid lifetime for the subnet prefix in the mobile 4203 node's home address specified with the Binding Update. The 4204 remaining valid lifetime for this prefix is determined by the home 4205 agent based on its own Prefix List entry [17]. 4207 The remaining preferred lifetime SHOULD NOT have any impact on the 4208 lifetime for the Binding Cache entry. 4210 The home agent MUST remove a binding when the valid lifetime of 4211 the prefix associated with it expires. 4213 o The home agent MAY further decrease the specified lifetime for the 4214 binding, for example based on a local policy. The resulting 4215 lifetime is stored by the home agent in the Binding Cache entry, 4216 and this Binding Cache entry MUST be deleted by the home agent 4217 after the expiration of this lifetime. 4219 Regardless of the setting of the Acknowledge (A) bit in the Binding 4220 Update, the home agent MUST return a Binding Acknowledgement to the 4221 mobile node constructed as follows: 4223 o The Status field MUST be set to a value indicating success. The 4224 value 1 (accepted but prefix discovery necessary) MUST be used if 4225 the subnet prefix of the specified home address is deprecated, or 4226 becomes deprecated during the lifetime of the binding, or becomes 4227 invalid at the end of the lifetime. The value 0 MUST be used 4228 otherwise. For the purposes of comparing the binding and prefix 4229 lifetimes, the prefix lifetimes are first converted into units of 4230 four seconds by ignoring the two least significant bits. 4232 o The Key Management Mobility Capability (K) bit is set if the 4233 following conditions are all fulfilled, and cleared otherwise: 4235 * The Key Management Mobility Capability (K) bit was set in the 4236 Binding Update. 4238 * The IPsec security associations between the mobile node and the 4239 home agent have been established dynamically. 4241 * The home agent has the capability to update its endpoint in the 4242 used key management protocol to the new care-of address every 4243 time it moves. 4245 Depending on the final value of the bit in the Binding 4246 Acknowledgement, the home agent SHOULD perform the following 4247 actions: 4249 K = 0 4251 Discard key management connections, if any, to the old care-of 4252 address. If the mobile node did not have a binding before 4253 sending this Binding Update, discard the connections to the 4254 home address. 4256 K = 1 4258 Move the peer endpoint of the key management protocol 4259 connection, if any, to the new care-of address. 4261 o The Sequence Number field MUST be copied from the Sequence Number 4262 given in the Binding Update. 4264 o The Lifetime field MUST be set to the remaining lifetime for the 4265 binding as set by the home agent in its home registration Binding 4266 Cache entry for the mobile node, as described above. 4268 o If the home agent stores the Binding Cache entry in nonvolatile 4269 storage, then the Binding Refresh Advice mobility option MUST be 4270 omitted. Otherwise, the home agent MAY include this option to 4271 suggest that the mobile node refreshes its binding before the 4272 actual lifetime of the binding ends. 4274 If the Binding Refresh Advice mobility option is present, the 4275 Refresh Interval field in the option MUST be set to a value less 4276 than the Lifetime value being returned in the Binding 4277 Acknowledgement. This indicates that the mobile node SHOULD 4278 attempt to refresh its home registration at the indicated shorter 4279 interval. The home agent MUST still retain the registration for 4280 the Lifetime period, even if the mobile node does not refresh its 4281 registration within the Refresh period. 4283 The rules for selecting the Destination IP address (and possibly 4284 routing header construction) for the Binding Acknowledgement to the 4285 mobile node are the same as in Section 9.5.4. 4287 In addition, the home agent MUST follow the procedure defined in 4288 Section 10.4.1 to intercept packets on the mobile node's home link 4289 addressed to the mobile node, while the home agent is serving as the 4290 home agent for this mobile node. The home agent MUST also be 4291 prepared to accept reverse tunneled packets from the new care-of 4292 address of the mobile node, as described in Section 10.4.5. Finally, 4293 the home agent MUST also propagate new home network prefixes, as 4294 described in Section 10.6. 4296 10.3.2. Primary Care-of Address De-Registration 4298 A binding may need to be de-registered when the mobile node returns 4299 home or when the mobile node knows that it will not have any care-of 4300 addresses in the visited network. 4302 A Binding Update is validated and authorized in the manner described 4303 in the previous section; note that when the mobile node de-registers 4304 when it is at home, it MAY choose to omit the Home Address 4305 destination option, in which case the mobile node's home address is 4306 the source IP address of the de-registration Binding Update. This 4307 section describes the processing of a valid Binding Update that 4308 requests the receiving node to no longer serve as its home agent, de- 4309 registering its primary care-of address. 4311 To begin processing the Binding Update, the home agent MUST perform 4312 the following test: 4314 o If the receiving node has no entry marked as a home registration 4315 in its Binding Cache for this mobile node, then this node MUST 4316 reject the Binding Update and SHOULD return a Binding 4317 Acknowledgement to the mobile node, in which the Status field is 4318 set to 133 (not home agent for this mobile node). 4320 If the home agent does not reject the Binding Update as described 4321 above, then the home agent MUST return a Binding Acknowledgement to 4322 the mobile node, constructed as follows: 4324 o The Status field MUST be set to a value 0, indicating success. 4326 o The Key Management Mobility Capability (K) bit is set or cleared 4327 and actions based on its value are performed as described in the 4328 previous section. The mobile node's home address is used as its 4329 new care-of address for the purposes of moving the key management 4330 connection to a new endpoint. 4332 o The Sequence Number field MUST be copied from the Sequence Number 4333 given in the Binding Update. 4335 o The Lifetime field MUST be set to zero. 4337 o The Binding Refresh Advice mobility option MUST be omitted. 4339 The rules for selecting the Destination IP address (and, if required, 4340 routing header construction) for the Binding Acknowledgement to the 4341 mobile node are the same as in the previous section. When the Status 4342 field in the Binding Acknowledgement is greater than or equal to 128 4343 and the Source Address of the Binding Update is on the home link, and 4344 the Binding Update came from a mobile node on the same link, the home 4345 agent MUST send it to the mobile node's link layer address (retrieved 4346 either from the Binding Update or through Neighbor Solicitation). 4348 When a mobile node sends a Binding Update to refresh the binding from 4349 the visited link and soon after moves to the home link and sends a 4350 de-registration Binding Update, a race condition can happen if the 4351 first Binding Update gets delayed. The delayed Binding Update can 4352 cause the home agent to create a new Binding Cache entry for a mobile 4353 node that had just attached to the home link and successfully deleted 4354 the binding. This would prevent the mobile node from using its home 4355 address from the home link. 4357 In order to prevent this, the home agent SHOULD NOT remove the 4358 Binding Cache entry immediately after receiving the deregistration 4359 Binding Update from the mobile node. It SHOULD mark the Binding 4360 Cache entry as invalid, and MUST stop intercepting packets on the 4361 mobile node's home link that are addressed to the mobile node 4362 (Section 10.4.1). The home agent should wait for 4363 MAX_DELETE_BCE_TIMEOUT (Section 12) seconds before removing the 4364 Binding Cache entry completely. In the scenario described above, if 4365 the home agent receives the delayed Binding Update that the mobile 4366 node sent from the visited link, it would reject the message since 4367 the sequence number would be less than the last received 4368 deregistration Binding Update from the home link. The home agent 4369 would then send a Binding Acknowledgment with status '135' (Sequence 4370 number out of window) to the care of address on the visited link. 4371 The mobile node can continue using the home address from the home 4372 link. 4374 10.4. Packet Processing 4376 10.4.1. Intercepting Packets for a Mobile Node 4378 While a node is serving as the home agent for a mobile node it MUST 4379 attempt to intercept packets on the mobile node's home link that are 4380 addressed to the mobile node. 4382 In order to do this, when a node begins serving as the home agent it 4383 MUST have performed Duplicate Address Detection (as specified in 4384 Section 10.3.1), and subsequently it MUST multicast onto the home 4385 link a Neighbor Advertisement message [17] on behalf of the mobile 4386 node. For the home address specified in the Binding Update, the home 4387 agent sends a Neighbor Advertisement message [17] to the all-nodes 4388 multicast address on the home link to advertise the home agent's own 4389 link-layer address for this IP address on behalf of the mobile node. 4390 If the Link-Layer Address Compatibility (L) flag has been specified 4391 in the Binding Update, the home agent MUST do the same for the link- 4392 local address of the mobile node. 4394 All fields in each Neighbor Advertisement message SHOULD be set in 4395 the same way they would be set by the mobile node if it was sending 4396 this Neighbor Advertisement [17] while at home, with the following 4397 exceptions: 4399 o The Target Address in the Neighbor Advertisement MUST be set to 4400 the specific IP address for the mobile node. 4402 o The Advertisement MUST include a Target Link-layer Address option 4403 specifying the home agent's link-layer address. 4405 o The Router (R) bit in the Advertisement MUST be set to zero. 4407 o The Solicited Flag (S) in the Advertisement MUST NOT be set, since 4408 it was not solicited by any Neighbor Solicitation. 4410 o The Override Flag (O) in the Advertisement MUST be set, indicating 4411 that the Advertisement SHOULD override any existing Neighbor Cache 4412 entry at any node receiving it. 4414 o The Source Address in the IPv6 header MUST be set to the home 4415 agent's IP address on the interface used to send the 4416 advertisement. 4418 Any node on the home link that receives one of the Neighbor 4419 Advertisement messages (described above) will update its Neighbor 4420 Cache to associate the mobile node's address with the home agent's 4421 link layer address, causing it to transmit any future packets 4422 normally destined to the mobile node to the mobile node's home agent. 4423 Since multicasting on the local link (such as Ethernet) is typically 4424 not guaranteed to be reliable, the home agent MAY retransmit this 4425 Neighbor Advertisement message up to MAX_NEIGHBOR_ADVERTISEMENT (see 4426 [17]) times to increase its reliability. It is still possible that 4427 some nodes on the home link will not receive any of the Neighbor 4428 Advertisements, but these nodes will eventually be able to detect the 4429 link-layer address change for the mobile node's address through use 4430 of Neighbor Unreachability Detection [17]. 4432 While a node is serving as a home agent for some mobile node, the 4433 home agent uses IPv6 Neighbor Discovery [17] to intercept unicast 4434 packets on the home link addressed to the mobile node. In order to 4435 intercept packets in this way, the home agent MUST act as a proxy for 4436 this mobile node and reply to any received Neighbor Solicitations for 4437 it. When a home agent receives a Neighbor Solicitation, it MUST 4438 check if the Target Address specified in the message matches the 4439 address of any mobile node for which it has a Binding Cache entry 4440 marked as a home registration. 4442 If such an entry exists in the home agent's Binding Cache, the home 4443 agent MUST reply to the Neighbor Solicitation with a Neighbor 4444 Advertisement giving the home agent's own link-layer address as the 4445 link-layer address for the specified Target Address. In addition, 4446 the Router (R) bit in the Advertisement MUST be set to zero. Acting 4447 as a proxy in this way allows other nodes on the mobile node's home 4448 link to resolve the mobile node's address and for the home agent to 4449 defend these addresses on the home link for Duplicate Address 4450 Detection [17]. 4452 10.4.2. Processing Intercepted Packets 4454 For any packet sent to a mobile node from the mobile node's home 4455 agent (in which the home agent is the original sender of the packet), 4456 the home agent is operating as a correspondent node of the mobile 4457 node for this packet and the procedures described in Section 9.3.2 4458 apply. The home agent then uses a routing header to route the packet 4459 to the mobile node by way of the primary care-of address in the home 4460 agent's Binding Cache. 4462 While the mobile node is away from home, the home agent intercepts 4463 any packets on the home link addressed to the mobile node's home 4464 address, as described in Section 10.4.1. In order to forward each 4465 intercepted packet to the mobile node, the home agent MUST tunnel the 4466 packet to the mobile node using IPv6 encapsulation [6]. When a home 4467 agent encapsulates an intercepted packet for forwarding to the mobile 4468 node, the home agent sets the Source Address in the new tunnel IP 4469 header to the home agent's own IP address and sets the Destination 4470 Address in the tunnel IP header to the mobile node's primary care-of 4471 address. When received by the mobile node, normal processing of the 4472 tunnel header [6] will result in decapsulation and processing of the 4473 original packet by the mobile node. 4475 However, packets addressed to the mobile node's link-local address 4476 MUST NOT be tunneled to the mobile node. Instead, these packets MUST 4477 be discarded and the home agent SHOULD return an ICMP Destination 4478 Unreachable, Code 3, message to the packet's Source Address (unless 4479 this Source Address is a multicast address). 4481 Interception and tunneling of the following multicast addressed 4482 packets on the home network are only done if the home agent supports 4483 multicast group membership control messages from the mobile node as 4484 described in the next section. Tunneling of multicast packets to a 4485 mobile node follows similar limitations to those defined above for 4486 unicast packets addressed to the mobile node's link-local address. 4487 Multicast packets addressed to a multicast address with link-local 4488 scope [15], to which the mobile node is subscribed, MUST NOT be 4489 tunneled to the mobile node. These packets SHOULD be silently 4490 discarded (after delivering to other local multicast recipients). 4491 Multicast packets addressed to a multicast address with a scope 4492 larger than link-local, but smaller than global (e.g., site-local and 4493 organization-local [15]), to which the mobile node is subscribed, 4494 SHOULD NOT be tunneled to the mobile node. Multicast packets 4495 addressed with a global scope, to which the mobile node has 4496 successfully subscribed, MUST be tunneled to the mobile node. 4498 Before tunneling a packet to the mobile node, the home agent MUST 4499 perform any IPsec processing as indicated by the security policy data 4500 base. 4502 10.4.3. Multicast Membership Control 4504 This section is a prerequisite for the multicast data packet 4505 forwarding, described in the previous section. If this support is 4506 not provided, multicast group membership control messages are 4507 silently ignored. 4509 In order to forward multicast data packets from the home network to 4510 all the proper mobile nodes, the home agent SHOULD be capable of 4511 receiving tunneled multicast group membership control information 4512 from the mobile node in order to determine which groups the mobile 4513 node has subscribed to. These multicast group membership messages 4514 are Listener Report messages specified in MLD [8] or in other 4515 protocols such as [38]. 4517 The messages are issued by the mobile node, but sent through the 4518 reverse tunnel to the home agent. These messages are issued whenever 4519 the mobile node decides to enable reception of packets for a 4520 multicast group or in response to an MLD Query from the home agent. 4521 The mobile node will also issue multicast group control messages to 4522 disable reception of multicast packets when it is no longer 4523 interested in receiving multicasts for a particular group. 4525 To obtain the mobile node's current multicast group membership the 4526 home agent must periodically transmit MLD Query messages through the 4527 tunnel to the mobile node. These MLD periodic transmissions will 4528 ensure the home agent has an accurate record of the groups in which 4529 the mobile node is interested despite packet losses of the mobile 4530 node's MLD group membership messages. 4532 All MLD packets are sent directly between the mobile node and the 4533 home agent. Since all of these packets are destined to a link-scope 4534 multicast address and have a hop limit of 1, there is no direct 4535 forwarding of such packets between the home network and the mobile 4536 node. The MLD packets between the mobile node and the home agent are 4537 encapsulated within the same tunnel header used for other packet 4538 flows between the mobile node and home agent. 4540 Note that at this time, even though a link-local source is used on 4541 MLD packets, no functionality depends on these addresses being 4542 unique, nor do they elicit direct responses. All MLD messages are 4543 sent to multicast destinations. To avoid ambiguity on the home 4544 agent, due to mobile nodes which may choose identical link-local 4545 source addresses for their MLD function, it is necessary for the home 4546 agent to identify which mobile node was actually the issuer of a 4547 particular MLD message. This may be accomplished by noting which 4548 tunnel such an MLD arrived by, which IPsec SA was used, or by other 4549 distinguishing means. 4551 This specification puts no requirement on how the functions in this 4552 section and the multicast forwarding in Section 10.4.2 are to be 4553 achieved. At the time of this writing it was thought that a full 4554 IPv6 multicast router function would be necessary on the home agent, 4555 but it may be possible to achieve the same effects through a "proxy 4556 MLD" application coupled with kernel multicast forwarding. This may 4557 be the subject of future specifications. 4559 10.4.4. Stateful Address Autoconfiguration 4561 This section describes how home agents support the use of stateful 4562 address autoconfiguration mechanisms such as DHCPv6 [29] from the 4563 mobile nodes. If this support is not provided, then the M and O bits 4564 must remain cleared on the Mobile Prefix Advertisement Messages. Any 4565 mobile node which sends DHCPv6 messages to the home agent without 4566 this support will not receive a response. 4568 If DHCPv6 is used, packets are sent with link-local source addresses 4569 either to a link-scope multicast address or a link-local address. 4570 Mobile nodes desiring to locate a DHCPv6 service may reverse tunnel 4571 standard DHCPv6 packets to the home agent. Since these link-scope 4572 packets cannot be forwarded onto the home network, it is necessary 4573 for the home agent to either implement a DHCPv6 relay agent or a 4574 DHCPv6 server function itself. The arriving tunnel or IPsec SA of 4575 DHCPv6 link-scope messages from the mobile node must be noted so that 4576 DHCPv6 responses may be sent back to the appropriate mobile node. 4577 DHCPv6 messages sent to the mobile node with a link-local destination 4578 must be tunneled within the same tunnel header used for other packet 4579 flows. 4581 10.4.5. Handling Reverse Tunneled Packets 4583 Unless a binding has been established between the mobile node and a 4584 correspondent node, traffic from the mobile node to the correspondent 4585 node goes through a reverse tunnel. Home agents MUST support reverse 4586 tunneling as follows: 4588 o The tunneled traffic arrives to the home agent's address using 4589 IPv6 encapsulation [6]. 4591 o Depending on the security policies used by the home agent, reverse 4592 tunneled packets MAY be discarded unless accompanied by a valid 4593 ESP header. The support for authenticated reverse tunneling 4594 allows the home agent to protect the home network and 4595 correspondent nodes from malicious nodes masquerading as a mobile 4596 node. 4598 o Otherwise, when a home agent decapsulates a tunneled packet from 4599 the mobile node, the home agent MUST verify that the Source 4600 Address in the tunnel IP header is the mobile node's primary 4601 care-of address. Otherwise, any node in the Internet could send 4602 traffic through the home agent and escape ingress filtering 4603 limitations. This simple check forces the attacker to know the 4604 current location of the real mobile node and be able to defeat 4605 ingress filtering. This check is not necessary if the reverse- 4606 tunneled packet is protected by ESP in tunnel mode. 4608 10.4.6. Protecting Return Routability Packets 4610 The return routability procedure, described in Section 5.2.5, assumes 4611 that the confidentiality of the Home Test Init and Home Test messages 4612 is protected as they are tunneled between the home agent and the 4613 mobile node. Therefore, the home agent MUST support tunnel mode 4614 IPsec ESP for the protection of packets belonging to the return 4615 routability procedure. Support for a non-null encryption transform 4616 and authentication algorithm MUST be available. It is not necessary 4617 to distinguish between different kinds of packets during the return 4618 routability procedure. 4620 Security associations are needed to provide this protection. When 4621 the care-of address for the mobile node changes as a result of an 4622 accepted Binding Update, special treatment is needed for the next 4623 packets sent using these security associations. The home agent MUST 4624 set the new care-of address as the destination address of these 4625 packets, as if the outer header destination address in the security 4626 association had changed [12]. 4628 The above protection SHOULD be used with all mobile nodes. The use 4629 is controlled by configuration of the IPsec security policy database 4630 both at the mobile node and at the home agent. 4632 As described earlier, the Binding Update and Binding Acknowledgement 4633 messages require protection between the home agent and the mobile 4634 node. The Mobility Header protocol carries both these messages as 4635 well as the return routability messages. From the point of view of 4636 the security policy database these messages are indistinguishable. 4637 When IPsec is used to protect return routability signaling or payload 4638 packets, this protection MUST only be applied to the return 4639 routability packets entering the IPv6 encapsulated tunnel interface 4640 between the mobile node and the home agent. This can be achieved, 4641 for instance, by defining the security policy database entries 4642 specifically for the tunnel interface. That is, the policy entries 4643 are not generally applied on all traffic on the physical interface(s) 4644 of the nodes, but rather only on traffic that enters the tunnel. 4645 This makes use of per-interface security policy database entries [2] 4646 specific to the tunnel interface (the node's attachment to the tunnel 4647 [5]). 4649 10.5. Dynamic Home Agent Address Discovery 4651 This section describes an optional mechanism by which a home agent 4652 can help mobile nodes to discover the addresses of other home agents 4653 on the mobile node's home network. The home agent keeps track of the 4654 other home agents on the same link and responds to queries sent by 4655 the mobile node. 4657 10.5.1. Receiving Router Advertisement Messages 4659 For each link on which a router provides service as a home agent, the 4660 router maintains a Home Agents List recording information about all 4661 other home agents on that link. This list is used in the dynamic 4662 home agent address discovery mechanism; the mobile node uses the list 4663 as described in Section 11.4.1. The information for the list is 4664 learned through receipt of the periodic unsolicited multicast Router 4665 Advertisements, in a manner similar to the Default Router List 4666 conceptual data structure maintained by each host for Neighbor 4667 Discovery [17]. In the construction of the Home Agents List, the 4668 Router Advertisements are from each (other) home agent on the link 4669 and the Home Agent (H) bit is set in them. 4671 On receipt of a valid Router Advertisement, as defined in the 4672 processing algorithm specified for Neighbor Discovery [17], the home 4673 agent performs the following steps in addition to any steps already 4674 required of it by Neighbor Discovery: 4676 o If the Home Agent (H) bit in the Router Advertisement is not set, 4677 delete the sending node's entry in the current Home Agents List 4678 (if one exists). Skip all the following steps. 4680 o Otherwise, extract the Source Address from the IP header of the 4681 Router Advertisement. This is the link-local IP address on this 4682 link of the home agent sending this Advertisement [17]. 4684 o Determine the preference for this home agent. If the Router 4685 Advertisement contains a Home Agent Information Option, then the 4686 preference is taken from the Home Agent Preference field in the 4687 option; otherwise, the default preference of 0 MUST be used. 4689 o Determine the lifetime for this home agent. If the Router 4690 Advertisement contains a Home Agent Information Option, then the 4691 lifetime is taken from the Home Agent Lifetime field in the 4692 option; otherwise, the lifetime specified by the Router Lifetime 4693 field in the Router Advertisement SHOULD be used. 4695 o If the link-local address of the home agent sending this 4696 Advertisement is already present in this home agent's Home Agents 4697 List and the received home agent lifetime value is zero, 4698 immediately delete this entry in the Home Agents List. 4700 o Otherwise, if the link-local address of the home agent sending 4701 this Advertisement is already present in the receiving home 4702 agent's Home Agents List, reset its lifetime and preference to the 4703 values determined above. 4705 o If the link-local address of the home agent sending this 4706 Advertisement is not already present in the Home Agents List 4707 maintained by the receiving home agent, and the lifetime for the 4708 sending home agent is non-zero, create a new entry in the list, 4709 and initialize its lifetime and preference to the values 4710 determined above. 4712 o If the Home Agents List entry for the link-local address of the 4713 home agent sending this Advertisement was not deleted as described 4714 above, determine any global address(es) of the home agent based on 4715 each Prefix Information option received in this Advertisement in 4716 which the Router Address (R) bit is set (Section 7.2). Add all 4717 such global addresses to the list of global addresses in this Home 4718 Agents List entry. 4720 A home agent SHOULD maintain an entry in its Home Agents List for 4721 each valid home agent address until that entry's lifetime expires, 4722 after which time the entry MUST be deleted. 4724 As described in Section 11.4.1, a mobile node attempts dynamic home 4725 agent address discovery by sending an ICMP Home Agent Address 4726 Discovery Request message to the Mobile IPv6 Home-Agents anycast 4727 address [7] for its home IP subnet prefix. A home agent receiving a 4728 Home Agent Address Discovery Request message that serves this subnet 4729 SHOULD return an ICMP Home Agent Address Discovery Reply message to 4730 the mobile node with the Source Address of the Reply packet set to 4731 one of the global unicast addresses of the home agent. The Home 4732 Agent Addresses field in the Reply message is constructed as follows: 4734 o The Home Agent Addresses field SHOULD contain all global IP 4735 addresses for each home agent currently listed in this home 4736 agent's own Home Agents List (Section 10.1). 4738 o The IP addresses in the Home Agent Addresses field SHOULD be 4739 listed in order of decreasing preference values, based either on 4740 the respective advertised preference from a Home Agent Information 4741 option or on the default preference of 0 if no preference is 4742 advertised (or on the configured home agent preference for this 4743 home agent itself). 4745 o Among home agents with equal preference, their IP addresses in the 4746 Home Agent Addresses field SHOULD be listed in an order randomized 4747 with respect to other home agents with equal preference every time 4748 a Home Agent Address Discovery Reply message is returned by this 4749 home agent. 4751 o If more than one global IP address is associated with a home 4752 agent, these addresses SHOULD be listed in a randomized order. 4754 o The home agent SHOULD reduce the number of home agent IP addresses 4755 so that the packet fits within the minimum IPv6 MTU [5]. The home 4756 agent addresses selected for inclusion in the packet SHOULD be 4757 those from the complete list with the highest preference. This 4758 limitation avoids the danger of the Reply message packet being 4759 fragmented (or rejected by an intermediate router with an ICMP 4760 Packet Too Big message [16]). 4762 10.6. Sending Prefix Information to the Mobile Node 4764 10.6.1. List of Home Network Prefixes 4766 Mobile IPv6 arranges to propagate relevant prefix information to the 4767 mobile node when it is away from home, so that it may be used in 4768 mobile node home address configuration and in network renumbering. 4769 In this mechanism, mobile nodes away from home receive Mobile Prefix 4770 Advertisement messages. These messages include Prefix Information 4771 Options for the prefixes configured on the home subnet interface(s) 4772 of the home agent. 4774 If there are multiple home agents, differences in the advertisements 4775 sent by different home agents can lead to an inability to use a 4776 particular home address when changing to another home agent. In 4777 order to ensure that the mobile nodes get the same information from 4778 different home agents, it is preferred that all of the home agents on 4779 the same link be configured in the same manner. 4781 To support this, the home agent monitors prefixes advertised by 4782 itself and other home agents on the home link. In Neighbor Discovery 4783 (RFC 4861 [17]) it is acceptable for two routers to advertise 4784 different sets of prefixes on the same link. For home agents, the 4785 differences should be detected for a given home address because the 4786 mobile node communicates only with one home agent at a time and the 4787 mobile node needs to know the full set of prefixes assigned to the 4788 home link. All other comparisons of Router Advertisements are as 4789 specified in Section 6.2.7 of RFC 4861. 4791 10.6.2. Scheduling Prefix Deliveries 4793 A home agent serving a mobile node will schedule the delivery of the 4794 new prefix information to that mobile node when any of the following 4795 conditions occur: 4797 MUST: 4799 o The state of the flags changes for the prefix of the mobile node's 4800 registered home address. 4802 o The valid or preferred lifetime is reconfigured or changes for any 4803 reason other than advancing real time. 4805 o The mobile node requests the information with a Mobile Prefix 4806 Solicitation (see Section 11.4.2). 4808 SHOULD: 4810 o A new prefix is added to the home subnet interface(s) of the home 4811 agent. 4813 MAY: 4815 o The valid or preferred lifetime or the state of the flags changes 4816 for a prefix which is not used in any Binding Cache entry for this 4817 mobile node. 4819 The home agent uses the following algorithm to determine when to send 4820 prefix information to the mobile node. 4822 o If a mobile node sends a solicitation, answer right away. 4824 o If no Mobile Prefix Advertisement has been sent to the mobile node 4825 in the last MaxMobPfxAdvInterval seconds (see Section 13), then 4826 ensure that a transmission is scheduled. The actual transmission 4827 time is randomized as described below. 4829 o If a prefix matching the mobile node's home registration is added 4830 on the home subnet interface or if its information changes in any 4831 way that does not deprecate the mobile node's address, ensure that 4832 a transmission is scheduled. The actual transmission time is 4833 randomized as described below. 4835 o If a home registration expires, cancel any scheduled 4836 advertisements to the mobile node. 4838 The list of prefixes is sent in its entirety in all cases. 4840 If the home agent has already scheduled the transmission of a Mobile 4841 Prefix Advertisement to the mobile node, then the home agent will 4842 replace the advertisement with a new one to be sent at the scheduled 4843 time. 4845 Otherwise, the home agent computes a fresh value for RAND_ADV_DELAY 4846 which offsets from the current time for the scheduled transmission. 4847 First calculate the maximum delay for the scheduled Advertisement: 4849 MaxScheduleDelay = min (MaxMobPfxAdvInterval, Preferred Lifetime), 4851 where MaxMobPfxAdvInterval is as defined in Section 12. Then compute 4852 the final delay for the advertisement: 4854 RAND_ADV_DELAY = MinMobPfxAdvInterval + 4855 (rand() % abs(MaxScheduleDelay - MinMobPfxAdvInterval)) 4857 Here rand() returns a random integer value in the range of 0 to the 4858 maximum possible integer value. This computation is expected to 4859 alleviate bursts of advertisements when prefix information changes. 4860 In addition, a home agent MAY further reduce the rate of packet 4861 transmission by further delaying individual advertisements, when 4862 necessary to avoid overwhelming local network resources. The home 4863 agent SHOULD periodically continue to retransmit an unsolicited 4864 Advertisement to the mobile node, until it is acknowledged by the 4865 receipt of a Mobile Prefix Solicitation from the mobile node. 4867 The home agent MUST wait PREFIX_ADV_TIMEOUT (see Section 12) before 4868 the first retransmission and double the retransmission wait time for 4869 every succeeding retransmission until a maximum number of 4870 PREFIX_ADV_RETRIES attempts (see Section 12) has been tried. If the 4871 mobile node's bindings expire before the matching Binding Update has 4872 been received, then the home agent MUST NOT attempt any more 4873 retransmissions, even if not all PREFIX_ADV_RETRIES have been 4874 retransmitted. In the mean time, if the mobile node sends another 4875 Binding Update without returning home, then the home agent SHOULD 4876 begin transmitting the unsolicited Advertisement again. 4878 If some condition, as described above, occurs on the home link and 4879 causes another Prefix Advertisement to be sent to the mobile node, 4880 before the mobile node acknowledges a previous transmission, the home 4881 agent SHOULD combine any Prefix Information options in the 4882 unacknowledged Mobile Prefix Advertisement into a new Advertisement. 4883 The home agent then discards the old Advertisement. 4885 10.6.3. Sending Advertisements 4887 When sending a Mobile Prefix Advertisement to the mobile node, the 4888 home agent MUST construct the packet as follows: 4890 o The Source Address in the packet's IPv6 header MUST be set to the 4891 home agent's IP address to which the mobile node addressed its 4892 current home registration or its default global home agent address 4893 if no binding exists. 4895 o If the advertisement was solicited, it MUST be destined to the 4896 source address of the solicitation. If it was triggered by prefix 4897 changes or renumbering, the advertisement's destination will be 4898 the mobile node's home address in the binding which triggered the 4899 rule. 4901 o A type 2 routing header MUST be included with the mobile node's 4902 home address. 4904 o IPsec headers MUST be supported and SHOULD be used. 4906 o The home agent MUST send the packet as it would any other unicast 4907 IPv6 packet that it originates. 4909 o Set the Managed Address Configuration (M) flag if the 4910 corresponding flag has been set in any of the Router 4911 Advertisements from which the prefix information has been learned 4912 (including the ones sent by this home agent). 4914 o Set the Other Stateful Configuration (O) flag if the corresponding 4915 flag has been set in any of the Router Advertisements from which 4916 the prefix information has been learned (including the ones sent 4917 by this home agent). 4919 10.6.4. Lifetimes for Changed Prefixes 4921 As described in Section 10.3.1, the lifetime returned by the home 4922 agent in a Binding Acknowledgement MUST NOT be greater than the 4923 remaining valid lifetime for the subnet prefix in the mobile node's 4924 home address. This limit on the binding lifetime serves to prohibit 4925 use of a mobile node's home address after it becomes invalid. 4927 11. Mobile Node Operation 4929 11.1. Conceptual Data Structures 4931 Each mobile node MUST maintain a Binding Update List. 4933 The Binding Update List records information for each Binding Update 4934 sent by this mobile node, in which the lifetime of the binding has 4935 not yet expired. The Binding Update List includes all bindings sent 4936 by the mobile node either to its home agent or correspondent nodes. 4937 It also contains Binding Updates which are waiting for the completion 4938 of the return routability procedure before they can be sent. 4939 However, for multiple Binding Updates sent to the same destination 4940 address, the Binding Update List contains only the most recent 4941 Binding Update (i.e., with the greatest Sequence Number value) sent 4942 to that destination. The Binding Update List MAY be implemented in 4943 any manner consistent with the external behavior described in this 4944 document. 4946 Each Binding Update List entry conceptually contains the following 4947 fields: 4949 o The IP address of the node to which a Binding Update was sent. 4951 o The home address for which that Binding Update was sent. 4953 o The care-of address sent in that Binding Update. This value is 4954 necessary for the mobile node to determine if it has sent a 4955 Binding Update while giving its new care-of address to this 4956 destination after changing its care-of address. 4958 o The initial value of the Lifetime field sent in that Binding 4959 Update. 4961 o The remaining lifetime of that binding. This lifetime is 4962 initialized from the Lifetime value sent in the Binding Update and 4963 is decremented until it reaches zero, at which time this entry 4964 MUST be deleted from the Binding Update List. 4966 o The maximum value of the Sequence Number field sent in previous 4967 Binding Updates to this destination. The Sequence Number field is 4968 16 bits long and all comparisons between Sequence Number values 4969 MUST be performed modulo 2**16 (see Section 9.5.1). 4971 o The time at which a Binding Update was last sent to this 4972 destination, as needed to implement the rate limiting restriction 4973 for sending Binding Updates. 4975 o The state of any retransmissions needed for this Binding Update. 4976 This state includes the time remaining until the next 4977 retransmission attempt for the Binding Update and the current 4978 state of the exponential back-off mechanism for retransmissions. 4980 o A flag specifying whether or not future Binding Updates should be 4981 sent to this destination. The mobile node sets this flag in the 4982 Binding Update List entry when it receives an ICMP Parameter 4983 Problem, Code 1, error message in response to a return routability 4984 message or Binding Update sent to that destination, as described 4985 in Section 11.3.5. 4987 The Binding Update List is used to determine whether a particular 4988 packet is sent directly to the correspondent node or tunneled via the 4989 home agent (see Section 11.3.1). 4991 The Binding Update list also conceptually contains the following data 4992 related to running the return routability procedure. This data is 4993 relevant only for Binding Updates sent to correspondent nodes. 4995 o The time at which a Home Test Init or Care-of Test Init message 4996 was last sent to this destination, as needed to implement the rate 4997 limiting restriction for the return routability procedure. 4999 o The state of any retransmissions needed for this return 5000 routability procedure. This state includes the time remaining 5001 until the next retransmission attempt and the current state of the 5002 exponential back-off mechanism for retransmissions. 5004 o Cookie values used in the Home Test Init and Care-of Test Init 5005 messages. 5007 o Home and care-of keygen tokens received from the correspondent 5008 node. 5010 o Home and care-of nonce indices received from the correspondent 5011 node. 5013 o The time at which each of the tokens and nonces were received from 5014 the correspondent node, as needed to implement reuse while moving. 5016 11.2. Processing Mobility Headers 5018 All IPv6 mobile nodes MUST observe the rules described in Section 9.2 5019 when processing Mobility Headers. 5021 11.3. Packet Processing 5023 11.3.1. Sending Packets While Away from Home 5025 While a mobile node is away from home, it continues to use its home 5026 address, as well as also using one or more care-of addresses. When 5027 sending a packet while away from home, a mobile node MAY choose among 5028 these in selecting the address that it will use as the source of the 5029 packet, as follows: 5031 o Protocols layered over IP will generally treat the mobile node's 5032 home address as its IP source address for most packets. For 5033 packets sent that are part of transport-level connections 5034 established while the mobile node was at home, the mobile node 5035 MUST use its home address. Likewise, for packets sent that are 5036 part of transport-level connections that the mobile node may still 5037 be using after moving to a new location, the mobile node SHOULD 5038 use its home address in this way. If a binding exists, the mobile 5039 node SHOULD send the packets directly to the correspondent node. 5040 Otherwise, if a binding does not exist, the mobile node MUST use 5041 reverse tunneling. 5043 o The mobile node MAY choose to directly use one of its care-of 5044 addresses as the source of the packet, not requiring the use of a 5045 Home Address option in the packet. This is particularly useful 5046 for short-term communication that may easily be retried if it 5047 fails. Using the mobile node's care-of address as the source for 5048 such queries will generally have a lower overhead than using the 5049 mobile node's home address, since no extra options need to be used 5050 in either the query or its reply. Such packets can be routed 5051 normally, directly between their source and destination without 5052 relying on Mobile IPv6. If application running on the mobile node 5053 has no particular knowledge that the communication being sent fits 5054 within this general type of communication, however, the mobile 5055 node should not use its care-of address as the source of the 5056 packet in this way. 5058 The choice of the most efficient communications method is 5059 application specific, and outside the scope of this specification. 5060 The APIs necessary for controlling the choice are also out of 5061 scope. One example of such an API is described in the IPv6 Socket 5062 API for Source Address Selection specification [21]. 5064 o While not at its home link, the mobile node MUST NOT use the Home 5065 Address destination option when communicating with link-local 5066 peers. 5068 Similarly, the mobile node MUST NOT use the Home Address 5069 destination option for IPv6 Neighbor Discovery [17] packets. 5071 Detailed operation of these cases is described later in this section 5072 and also discussed in [31]. 5074 For packets sent by a mobile node while it is at home, no special 5075 Mobile IPv6 processing is required. Likewise, if the mobile node 5076 uses any address other than one of its home addresses as the source 5077 of a packet sent while away from home, no special Mobile IPv6 5078 processing is required. In either case, the packet is simply 5079 addressed and transmitted in the same way as any normal IPv6 packet. 5081 For packets sent by the mobile node sent while away from home using 5082 the mobile node's home address as the source, special Mobile IPv6 5083 processing of the packet is required. This can be done in the 5084 following two ways: 5086 Route Optimization 5088 This manner of delivering packets does not require going through 5089 the home network, and typically will enable faster and more 5090 reliable transmission. 5092 The mobile node needs to ensure that a Binding Cache entry exists 5093 for its home address so that the correspondent node can process 5094 the packet (Section 9.3.1 specifies the rules for Home Address 5095 Destination Option Processing at a correspondent node). The 5096 mobile node SHOULD examine its Binding Update List for an entry 5097 which fulfills the following conditions: 5099 * The Source Address field of the packet being sent is equal to 5100 the home address in the entry. 5102 * The Destination Address field of the packet being sent is equal 5103 to the address of the correspondent node in the entry. 5105 * One of the current care-of addresses of the mobile node appears 5106 as the care-of address in the entry. 5108 * The entry indicates that a binding has been successfully 5109 created. 5111 * The remaining lifetime of the binding is greater than zero. 5113 When these conditions are met, the mobile node knows that the 5114 correspondent node has a suitable Binding Cache entry. 5116 A mobile node SHOULD arrange to supply the home address in a Home 5117 Address option, and MUST set the IPv6 header's Source Address 5118 field to the care-of address which the mobile node has registered 5119 to be used with this correspondent node. The correspondent node 5120 will then use the address supplied in the Home Address option to 5121 serve the function traditionally done by the Source IP address in 5122 the IPv6 header. The mobile node's home address is then supplied 5123 to higher protocol layers and applications. 5125 Specifically: 5127 * Construct the packet using the mobile node's home address as 5128 the packet's Source Address, in the same way as if the mobile 5129 node were at home. This includes the calculation of upper 5130 layer checksums using the home address as the value of the 5131 source. 5133 * Insert a Home Address option into the packet with the Home 5134 Address field copied from the original value of the Source 5135 Address field in the packet. 5137 * Change the Source Address field in the packet's IPv6 header to 5138 one of the mobile node's care-of addresses. This will 5139 typically be the mobile node's current primary care-of address, 5140 but MUST be an address assigned to the interface on the link 5141 being used. 5143 By using the care-of address as the Source Address in the IPv6 5144 header, with the mobile node's home address instead in the Home 5145 Address option, the packet will be able to safely pass through any 5146 router implementing ingress filtering [27]. 5148 Reverse Tunneling 5150 This is the mechanism which tunnels the packets via the home 5151 agent. It is not as efficient as the above mechanism, but is 5152 needed if there is no binding yet with the correspondent node. 5154 This mechanism is used for packets that have the mobile node's 5155 home address as the Source Address in the IPv6 header, or with 5156 multicast control protocol packets as described in Section 11.3.4. 5157 Specifically: 5159 * The packet is sent to the home agent using IPv6 encapsulation 5160 [6]. 5162 * The Source Address in the tunnel packet is the primary care-of 5163 address as registered with the home agent. 5165 * The Destination Address in the tunnel packet is the home 5166 agent's address. 5168 Then, the home agent will pass the encapsulated packet to the 5169 correspondent node. 5171 11.3.2. Interaction with Outbound IPsec Processing 5173 This section sketches the interaction between outbound Mobile IPv6 5174 processing and outbound IP Security (IPsec) processing for packets 5175 sent by a mobile node while away from home. Any specific 5176 implementation MAY use algorithms and data structures other than 5177 those suggested here, but its processing MUST be consistent with the 5178 effect of the operation described here and with the relevant IPsec 5179 specifications. In the steps described below, it is assumed that 5180 IPsec is being used in transport mode [2] and that the mobile node is 5181 using its home address as the source for the packet (from the point 5182 of view of higher protocol layers or applications, as described in 5183 Section 11.3.1): 5185 o The packet is created by higher layer protocols and applications 5186 (e.g., by TCP) as if the mobile node were at home and Mobile IPv6 5187 were not being used. 5189 o Determine the outgoing interface for the packet. (Note that the 5190 selection between reverse tunneling and route optimization may 5191 imply different interfaces, particularly if tunnels are considered 5192 interfaces as well.) 5194 o As part of outbound packet processing in IP, the packet is 5195 compared against the IPsec security policy database to determine 5196 what processing is required for the packet [2]. 5198 o If IPsec processing is required, the packet is either mapped to an 5199 existing Security Association (or SA bundle), or a new SA (or SA 5200 bundle) is created for the packet, according to the procedures 5201 defined for IPsec. 5203 o Since the mobile node is away from home, the mobile is either 5204 using reverse tunneling or route optimization to reach the 5205 correspondent node. 5207 If reverse tunneling is used, the packet is constructed in the 5208 normal manner and then tunneled through the home agent. 5210 If route optimization is in use, the mobile node inserts a Home 5211 Address destination option into the packet, replacing the Source 5212 Address in the packet's IP header with the care-of address used 5213 with this correspondent node, as described in Section 11.3.1. The 5214 Destination Options header in which the Home Address destination 5215 option is inserted MUST appear in the packet after the routing 5216 header, if present, and before the IPsec (AH [3] or ESP [4]) 5217 header, so that the Home Address destination option is processed 5218 by the destination node before the IPsec header is processed. 5220 Finally, once the packet is fully assembled, the necessary IPsec 5221 authentication (and encryption, if required) processing is 5222 performed on the packet, initializing the Authentication Data in 5223 the IPsec header. 5225 RFC 2402 treatment of destination options is extended as follows. 5226 The AH authentication data MUST be calculated as if the following 5227 were true: 5229 * the IPv6 source address in the IPv6 header contains the mobile 5230 node's home address, 5232 * the Home Address field of the Home Address destination option 5233 (Section 6.3) contains the new care-of address. 5235 o This allows, but does not require, the receiver of the packet 5236 containing a Home Address destination option to exchange the two 5237 fields of the incoming packet to reach the above situation, 5238 simplifying processing for all subsequent packet headers. 5239 However, such an exchange is not required, as long as the result 5240 of the authentication calculation remains the same. 5242 When an automated key management protocol is used to create new 5243 security associations for a peer, it is important to ensure that the 5244 peer can send the key management protocol packets to the mobile node. 5245 This may not be possible if the peer is the home agent of the mobile 5246 node and the purpose of the security associations would be to send a 5247 Binding Update to the home agent. Packets addressed to the home 5248 address of the mobile node cannot be used before the Binding Update 5249 has been processed. For the default case of using IKEv2 [41] as the 5250 automated key management protocol, such problems can be avoided by 5251 the following requirements when communicating with its home agent: 5253 o When the mobile node is away from home, it MUST use its care-of 5254 address as the Source Address of all packets it sends as part of 5255 the key management protocol (without use of Mobile IPv6 for these 5256 packets, as suggested in Section 11.3.1). 5258 The Key Management Mobility Capability (K) bit in Binding Updates and 5259 Acknowledgements can be used to avoid the need to rerun IKEv2 upon 5260 movements. 5262 11.3.3. Receiving Packets While Away from Home 5264 While away from home, a mobile node will receive packets addressed to 5265 its home address, by one of two methods: 5267 o Packets sent by a correspondent node, that does not have a Binding 5268 Cache entry for the mobile node, will be sent to the home address, 5269 captured by the home agent and tunneled to the mobile node. 5271 o Packets sent by a correspondent node that has a Binding Cache 5272 entry for the mobile node that contains the mobile node's current 5273 care-of address, will be sent by the correspondent node using a 5274 type 2 routing header. The packet will be addressed to the mobile 5275 node's care-of address, with the final hop in the routing header 5276 directing the packet to the mobile node's home address; the 5277 processing of this last hop of the routing header is entirely 5278 internal to the mobile node, since the care-of address and home 5279 address are both addresses within the mobile node. 5281 For packets received by the first method, the mobile node MUST check 5282 that the IPv6 source address of the tunneled packet is the IP address 5283 of its home agent. In this method, the mobile node may also send a 5284 Binding Update to the original sender of the packet as described in 5285 Section 11.7.2 and subject to the rate limiting defined in 5286 Section 11.8. The mobile node MUST also process the received packet 5287 in the manner defined for IPv6 encapsulation [6], which will result 5288 in the encapsulated (inner) packet being processed normally by upper- 5289 layer protocols within the mobile node as if it had been addressed 5290 (only) to the mobile node's home address. 5292 For packets received by the second method, the following rules will 5293 result in the packet being processed normally by upper-layer 5294 protocols within the mobile node as if it had been addressed to the 5295 mobile node's home address. 5297 A node receiving a packet addressed to itself (i.e., one of the 5298 node's addresses is in the IPv6 destination field) follows the next 5299 header chain of headers and processes them. When it encounters a 5300 type 2 routing header during this processing, it performs the 5301 following checks. If any of these checks fail, the node MUST 5302 silently discard the packet. 5304 o The length field in the routing header is exactly 2. 5306 o The segments left field in the routing header is 1 on the wire. 5307 (But implementations may process the routing header so that the 5308 value may become 0 after the routing header has been processed, 5309 but before the rest of the packet is processed.) 5311 o The Home Address field in the routing header is one of the node's 5312 home addresses, if the segments left field was 1. Thus, in 5313 particular the address field is required to be a unicast routable 5314 address. 5316 Once the above checks have been performed, the node swaps the IPv6 5317 destination field with the Home Address field in the routing header, 5318 decrements segments left by one from the value it had on the wire, 5319 and resubmits the packet to IP for processing the next header. 5320 Conceptually, this follows the same model as in RFC 2460. However, 5321 in the case of type 2 routing header this can be simplified since it 5322 is known that the packet will not be forwarded to a different node. 5324 The definition of AH requires the sender to calculate the AH 5325 integrity check value of a routing header in the same way it appears 5326 in the receiver after it has processed the header. Since IPsec 5327 headers follow the routing header, any IPsec processing will operate 5328 on the packet with the home address in the IP destination field and 5329 segments left being zero. Thus, the AH calculations at the sender 5330 and receiver will have an identical view of the packet. 5332 11.3.4. Routing Multicast Packets 5334 A mobile node that is connected to its home link functions in the 5335 same way as any other (stationary) node. Thus, when it is at home, a 5336 mobile node functions identically to other multicast senders and 5337 receivers. Therefore, this section describes the behavior of a 5338 mobile node that is not on its home link. 5340 In order to receive packets sent to some multicast group, a mobile 5341 node must join that multicast group. One method, in which a mobile 5342 node MAY join the group, is via a (local) multicast router on the 5343 foreign link being visited. In this case, the mobile node MUST use 5344 its care-of address and MUST NOT use the Home Address destination 5345 option when sending MLD packets [8]. 5347 Alternatively, a mobile node MAY join multicast groups via a bi- 5348 directional tunnel to its home agent. The mobile node tunnels its 5349 multicast group membership control packets (such as those defined in 5350 [8] or in [38]) to its home agent, and the home agent forwards 5351 multicast packets down the tunnel to the mobile node. A mobile node 5352 MUST NOT tunnel multicast group membership control packets until (1) 5353 the mobile node has a binding in place at the home agent, and (2) the 5354 latter sends at least one multicast group membership control packet 5355 via the tunnel. Once this condition is true, the mobile node SHOULD 5356 assume it does not change as long as the binding does not expire. 5358 A mobile node that wishes to send packets to a multicast group also 5359 has two options: 5361 1. Send directly on the foreign link being visited. 5363 To do this, the application uses the care-of address as a source 5364 address for multicast traffic, just as it would use a stationary 5365 address. This requires that the application either knows the 5366 care-of address, or uses an API such as the IPv6 Socket API for 5367 Source Address Selection specification [21] to request that the 5368 care-of address be used as the source address in transmitted 5369 packets. The mobile node MUST NOT use Home Address destination 5370 option in such traffic. 5372 2. Send via a tunnel to its home agent. 5374 Because multicast routing in general depends upon the Source 5375 Address used in the IPv6 header of the multicast packet, a mobile 5376 node that tunnels a multicast packet to its home agent MUST use 5377 its home address as the IPv6 Source Address of the inner 5378 multicast packet. 5380 Note that direct sending from the foreign link is only applicable 5381 while the mobile node is at that foreign link. This is because the 5382 associated multicast tree is specific to that source location and any 5383 change of location and source address will invalidate the source 5384 specific tree or branch and the application context of the other 5385 multicast group members. 5387 This specification does not provide mechanisms to enable such local 5388 multicast session to survive hand-off and to seamlessly continue from 5389 a new care-of address on each new foreign link. Any such mechanism, 5390 developed as an extension to this specification, needs to take into 5391 account the impact of fast moving mobile nodes on the Internet 5392 multicast routing protocols and their ability to maintain the 5393 integrity of source specific multicast trees and branches. 5395 While the use of bidirectional tunneling can ensure that multicast 5396 trees are independent of the mobile nodes movement, in some case such 5397 tunneling can have adverse affects. The latency of specific types of 5398 multicast applications (such as multicast based discovery protocols) 5399 will be affected when the round-trip time between the foreign subnet 5400 and the home agent is significant compared to that of the topology to 5401 be discovered. In addition, the delivery tree from the home agent in 5402 such circumstances relies on unicast encapsulation from the agent to 5403 the mobile node. Therefore, bandwidth usage is inefficient compared 5404 to the native multicast forwarding in the foreign multicast system. 5406 11.3.5. Receiving ICMP Error Messages 5408 Any node that does not recognize the Mobility header will return an 5409 ICMP Parameter Problem, Code 1, message to the sender of the packet. 5410 If the mobile node receives such an ICMP error message in response to 5411 a return routability procedure or Binding Update, it SHOULD record in 5412 its Binding Update List that future Binding Updates SHOULD NOT be 5413 sent to this destination. Such Binding Update List entries SHOULD be 5414 removed after a period of time in order to allow for retrying route 5415 optimization. 5417 New Binding Update List entries MUST NOT be created as a result of 5418 receiving ICMP error messages. 5420 Correspondent nodes that have participated in the return routability 5421 procedure MUST implement the ability to correctly process received 5422 packets containing a Home Address destination option. Therefore, 5423 correctly implemented correspondent nodes should always be able to 5424 recognize Home Address options. If a mobile node receives an ICMP 5425 Parameter Problem, Code 2, message from some node indicating that it 5426 does not support the Home Address option, the mobile node SHOULD log 5427 the error and then discard the ICMP message. 5429 11.3.6. Receiving Binding Error Messages 5431 When a mobile node receives a packet containing a Binding Error 5432 message, it should first check if the mobile node has a Binding 5433 Update List entry for the source of the Binding Error message. If 5434 the mobile node does not have such an entry, it MUST ignore the 5435 message. This is necessary to prevent a waste of resources on, e.g., 5436 return routability procedure due to spoofed Binding Error messages. 5438 Otherwise, if the message Status field was 1 (unknown binding for 5439 Home Address destination option), the mobile node should perform one 5440 of the following three actions: 5442 o If the Binding Error Message was sent by the Home Agent, the 5443 Mobile Node SHOULD send a Binding Update to the Home Agent 5444 according to Section 11.7.1. 5446 o If the mobile node has recent upper layer progress information, 5447 which indicates that communications with the correspondent node 5448 are progressing, it MAY ignore the message. This can be done in 5449 order to limit the damage that spoofed Binding Error messages can 5450 cause to ongoing communications. 5452 o If the mobile node has no upper layer progress information, it 5453 MUST remove the entry and route further communications through the 5454 home agent. It MAY also optionally start a return routability 5455 procedure (see Section 5.2). 5457 If the message Status field was 2 (unrecognized MH Type value), the 5458 mobile node should perform one of the following two actions: 5460 o If the mobile node is not expecting an acknowledgement or response 5461 from the correspondent node, the mobile node SHOULD ignore this 5462 message. 5464 o Otherwise, the mobile node SHOULD cease the use of any extensions 5465 to this specification. If no extensions had been used, the mobile 5466 node should cease the attempt to use route optimization. 5468 11.4. Home Agent and Prefix Management 5470 11.4.1. Dynamic Home Agent Address Discovery 5472 Sometimes when the mobile node needs to send a Binding Update to its 5473 home agent to register its new primary care-of address, as described 5474 in Section 11.7.1, the mobile node may not know the address of any 5475 router on its home link that can serve as a home agent for it. For 5476 example, some nodes on its home link may have been reconfigured while 5477 the mobile node has been away from home, such that the router that 5478 was operating as the mobile node's home agent has been replaced by a 5479 different router serving this role. 5481 In this case, the mobile node MAY attempt to discover the address of 5482 a suitable home agent on its home link. To do so, the mobile node 5483 sends an ICMP Home Agent Address Discovery Request message to the 5484 Mobile IPv6 Home-Agents anycast address [7] for its home subnet 5485 prefix. As described in Section 10.5, the home agent on its home 5486 link that receives this Request message will return an ICMP Home 5487 Agent Address Discovery Reply message. This message gives the 5488 addresses for the home agents operating on the home link. 5490 The mobile node, upon receiving this Home Agent Address Discovery 5491 Reply message, MAY then send its home registration Binding Update to 5492 any of the unicast IP addresses listed in the Home Agent Addresses 5493 field in the Reply. For example, the mobile node MAY attempt its 5494 home registration to each of these addresses, in turn, until its 5495 registration is accepted. The mobile node sends a Binding Update to 5496 an address and waits for the matching Binding Acknowledgement, moving 5497 on to the next address if there is no response. The mobile node 5498 MUST, however, wait at least InitialBindackTimeoutFirstReg seconds 5499 (see Section 13) before sending a Binding Update to the next home 5500 agent. In trying each of the returned home agent addresses, the 5501 mobile node SHOULD try each of them in the order they appear in the 5502 Home Agent Addresses field in the received Home Agent Address 5503 Discovery Reply message. In order to do this, the mobile node SHOULD 5504 store the list of home agents for later use in case the home agent 5505 currently managing the mobile node's care-of address forwarding 5506 should become unavailable. The list MAY be stored, along with any 5507 available lifetime information for the home agent addresses, in 5508 nonvolatile memory to survive reboots by the mobile node. 5510 If the mobile node has a current registration with some home agent 5511 (the Lifetime for that registration has not yet expired), then the 5512 mobile node MUST attempt any new registration first with that home 5513 agent. If that registration attempt fails (e.g., timed out or 5514 rejected), the mobile node SHOULD then reattempt this registration 5515 with another home agent. If the mobile node knows of no other 5516 suitable home agent, then it MAY attempt the dynamic home agent 5517 address discovery mechanism described above. 5519 If, after a mobile node transmits a Home Agent Address Discovery 5520 Request message to the Home Agents Anycast address, it does not 5521 receive a corresponding Home Agent Address Discovery Reply message 5522 within INITIAL_DHAAD_TIMEOUT (see Section 12) seconds, the mobile 5523 node MAY retransmit the same Request message to the same anycast 5524 address. This retransmission MAY be repeated up to a maximum of 5525 DHAAD_RETRIES (see Section 12) attempts. Each retransmission MUST be 5526 delayed by twice the time interval of the previous retransmission. 5528 11.4.2. Sending Mobile Prefix Solicitations 5530 When a mobile node has a home address that is about to become 5531 invalid, it SHOULD send a Mobile Prefix Solicitation to its home 5532 agent in an attempt to acquire fresh routing prefix information. The 5533 new information also enables the mobile node to participate in 5534 renumbering operations affecting the home network, as described in 5535 Section 10.6. 5537 The mobile node MUST use the Home Address destination option to carry 5538 its home address. The mobile node MUST support and SHOULD use IPsec 5539 to protect the solicitation. The mobile node MUST set the Identifier 5540 field in the ICMP header to a random value. 5542 As described in Section 11.7.2, Binding Updates sent by the mobile 5543 node to other nodes MUST use a lifetime no greater than the remaining 5544 lifetime of its home registration of its primary care-of address. 5545 The mobile node SHOULD further limit the lifetimes that it sends on 5546 any Binding Updates to be within the remaining valid lifetime (see 5547 Section 10.6.2) for the prefix in its home address. 5549 When the lifetime for a changed prefix decreases, and the change 5550 would cause cached bindings at correspondent nodes in the Binding 5551 Update List to be stored past the newly shortened lifetime, the 5552 mobile node MUST issue a Binding Update to all such correspondent 5553 nodes. 5555 These limits on the binding lifetime serve to prohibit use of a 5556 mobile node's home address after it becomes invalid. 5558 11.4.3. Receiving Mobile Prefix Advertisements 5560 Section 10.6 describes the operation of a home agent to support boot 5561 time configuration and renumbering a mobile node's home subnet while 5562 the mobile node is away from home. The home agent sends Mobile 5563 Prefix Advertisements to the mobile node while away from home, giving 5564 "important" Prefix Information options that describe changes in the 5565 prefixes in use on the mobile node's home link. 5567 The Mobile Prefix Solicitation is similar to the Router Solicitation 5568 used in Neighbor Discovery [17], except it is routed from the mobile 5569 node on the visited network to the home agent on the home network by 5570 usual unicast routing rules. 5572 When a mobile node receives a Mobile Prefix Advertisement, it MUST 5573 validate it according to the following test: 5575 o The Source Address of the IP packet carrying the Mobile Prefix 5576 Advertisement is the same as the home agent address to which the 5577 mobile node last sent an accepted home registration Binding Update 5578 to register its primary care-of address. Otherwise, if no such 5579 registrations have been made, it SHOULD be the mobile node's 5580 stored home agent address, if one exists. Otherwise, if the 5581 mobile node has not yet discovered its home agent's address, it 5582 MUST NOT accept Mobile Prefix Advertisements. 5584 o The packet MUST have a type 2 routing header and SHOULD be 5585 protected by an IPsec header as described in Section 5.4 and 5586 Section 6.8. 5588 o If the ICMP Identifier value matches the ICMP Identifier value of 5589 the most recently sent Mobile Prefix Solicitation and no other 5590 advertisement has yet been received for this value, then the 5591 advertisement is considered to be solicited and will be processed 5592 further. 5594 Otherwise, the advertisement is unsolicited, and MUST be 5595 discarded. In this case the mobile node SHOULD send a Mobile 5596 Prefix Solicitation. 5598 Any received Mobile Prefix Advertisement not meeting these tests MUST 5599 be silently discarded. 5601 For an accepted Mobile Prefix Advertisement, the mobile node MUST 5602 process Managed Address Configuration (M), Other Stateful 5603 Configuration (O), and the Prefix Information Options as if they 5604 arrived in a Router Advertisement [17] on the mobile node's home 5605 link. (This specification does not, however, describe how to acquire 5606 home addresses through stateful protocols.) Such processing may 5607 result in the mobile node configuring a new home address, although 5608 due to separation between preferred lifetime and valid lifetime, such 5609 changes should not affect most communications by the mobile node, in 5610 the same way as for nodes that are at home. 5612 This specification assumes that any security associations and 5613 security policy entries that may be needed for new prefixes have been 5614 pre-configured in the mobile node. Note that while dynamic key 5615 management avoids the need to configure new security associations, it 5616 is still necessary to add policy entries to protect the 5617 communications involving the home address(es). Mechanisms for 5618 setting up these entries are outside the scope of this specification. 5620 11.5. Movement 5622 11.5.1. Movement Detection 5624 The primary goal of movement detection is to detect L3 handovers. 5625 This section does not attempt to specify a fast movement detection 5626 algorithm which will function optimally for all types of 5627 applications, link-layers and deployment scenarios; instead, it 5628 describes a generic method that uses the facilities of IPv6 Neighbor 5629 Discovery, including Router Discovery and Neighbor Unreachability 5630 Detection. At the time of this writing, this method is considered 5631 well enough understood to recommend for standardization, however it 5632 is expected that future versions of this specification or other 5633 specifications may contain updated versions of the movement detection 5634 algorithm that have better performance. 5636 Generic movement detection uses Neighbor Unreachability Detection to 5637 detect when the default router is no longer bi-directionally 5638 reachable, in which case the mobile node must discover a new default 5639 router (usually on a new link). However, this detection only occurs 5640 when the mobile node has packets to send, and in the absence of 5641 frequent Router Advertisements or indications from the link-layer, 5642 the mobile node might become unaware of an L3 handover that occurred. 5643 Therefore, the mobile node should supplement this method with other 5644 information whenever it is available to the mobile node (e.g., from 5645 lower protocol layers). 5647 When the mobile node detects an L3 handover, it performs Duplicate 5648 Address Detection [18] on its link-local address, selects a new 5649 default router as a consequence of Router Discovery, and then 5650 performs Prefix Discovery with that new router to form new care-of 5651 address(es) as described in Section 11.5.3. It then registers its 5652 new primary care-of address with its home agent as described in 5653 Section 11.7.1. After updating its home registration, the mobile 5654 node then updates associated mobility bindings in correspondent nodes 5655 that it is performing route optimization with as specified in 5656 Section 11.7.2. 5658 Due to the temporary packet flow disruption and signaling overhead 5659 involved in updating mobility bindings, the mobile node should avoid 5660 performing an L3 handover until it is strictly necessary. 5661 Specifically, when the mobile node receives a Router Advertisement 5662 from a new router that contains a different set of on-link prefixes, 5663 if the mobile node detects that the currently selected default router 5664 on the old link is still bi-directionally reachable, it should 5665 generally continue to use the old router on the old link rather than 5666 switch away from it to use a new default router. 5668 Mobile nodes can use the information in received Router 5669 Advertisements to detect L3 handovers. In doing so the mobile node 5670 needs to consider the following issues: 5672 o There might be multiple routers on the same link, thus hearing a 5673 new router does not necessarily constitute an L3 handover. 5675 o When there are multiple routers on the same link they might 5676 advertise different prefixes. Thus even hearing a new router with 5677 a new prefix might not be a reliable indication of an L3 handover. 5679 o The link-local addresses of routers are not globally unique, hence 5680 after completing an L3 handover the mobile node might continue to 5681 receive Router Advertisements with the same link-local source 5682 address. This might be common if routers use the same link-local 5683 address on multiple interfaces. This issue can be avoided when 5684 routers use the Router Address (R) bit, since that provides a 5685 global address of the router. 5687 In addition, the mobile node should consider the following events as 5688 indications that an L3 handover may have occurred. Upon receiving 5689 such indications, the mobile node needs to perform Router Discovery 5690 to discover routers and prefixes on the new link, as described in 5691 Section 6.3.7 of Neighbor Discovery (RFC 4861 [17]). 5693 o If Router Advertisements that the mobile node receives include an 5694 Advertisement Interval option, the mobile node may use its 5695 Advertisement Interval field as an indication of the frequency 5696 with which it should expect to continue to receive future 5697 Advertisements from that router. This field specifies the minimum 5698 rate (the maximum amount of time between successive 5699 Advertisements) that the mobile node should expect. If this 5700 amount of time elapses without the mobile node receiving any 5701 Advertisement from this router, the mobile node can be sure that 5702 at least one Advertisement sent by the router has been lost. The 5703 mobile node can then implement its own policy to determine how 5704 many lost Advertisements from its current default router 5705 constitute an L3 handover indication. 5707 o Neighbor Unreachability Detection determines that the default 5708 router is no longer reachable. 5710 o With some types of networks, notification that a L2 handover has 5711 occurred might be obtained from lower layer protocols or device 5712 driver software within the mobile node. While further details 5713 around handling L2 indications as movement hints is an item for 5714 further study, at the time of writing this specification the 5715 following is considered reasonable: 5717 A L2 handover indication may or may not imply L2 movement and L2 5718 movement may or may not imply L3 movement; the correlations might 5719 be a function of the type of L2 but might also be a function of 5720 actual deployment of the wireless topology. 5722 Unless it is well-known that a L2 handover indication is likely to 5723 imply L3 movement, instead of immediately multicasting a router 5724 solicitation it may be better to attempt to verify whether the 5725 default router is still bi-directionally reachable. This can be 5726 accomplished by sending a unicast Neighbor Solicitation and 5727 waiting for a Neighbor Advertisement with the solicited flag set. 5728 Note that this is similar to Neighbor Unreachability detection but 5729 it does not have the same state machine, such as the STALE state. 5731 If the default router does not respond to the Neighbor 5732 Solicitation it makes sense to proceed to multicasting a Router 5733 Solicitation. 5735 11.5.2. Home Link Detection 5737 When an MN detects that it has arrived on a new link using the 5738 movement detection algorithm in use (Section 11.5.1,) or on 5739 bootstrapping, it performs the following steps to determine if it is 5740 on the home link. 5742 o The MN performs the procedure described in Section 11.5.3 and 5743 configures an address. It also keeps track of all the on-link 5744 prefix(es) received in the RA along with their prefix lengths. 5746 o If the home prefix has not been statically configured the MN uses 5747 some form of bootstrapping procedure (e.g. RFC5026 [22]) to 5748 determine the home prefix. 5750 o Given the availability of the home prefix, the MN checks whether 5751 or not the home prefix matches one of the prefixes received in the 5752 RA. If it does, the MN concludes that it is connected to the home 5753 link. 5755 11.5.3. Forming New Care-of Addresses 5757 After detecting that it has moved a mobile node SHOULD generate a new 5758 primary care-of address using normal IPv6 mechanisms. This SHOULD 5759 also be done when the current primary care-of address becomes 5760 deprecated. A mobile node MAY form a new primary care-of address at 5761 any time, but a mobile node MUST NOT send a Binding Update about a 5762 new care-of address to its home agent more than MAX_UPDATE_RATE times 5763 within a second. 5765 In addition, a mobile node MAY form new non-primary care-of addresses 5766 even when it has not switched to a new default router. A mobile node 5767 can have only one primary care-of address at a time (which is 5768 registered with its home agent), but it MAY have an additional 5769 care-of address for any or all of the prefixes on its current link. 5770 Furthermore, since a wireless network interface may actually allow a 5771 mobile node to be reachable on more than one link at a time (i.e., 5772 within wireless transmitter range of routers on more than one 5773 separate link), a mobile node MAY have care-of addresses on more than 5774 one link at a time. The use of more than one care-of address at a 5775 time is described in Section 11.5.4. 5777 As described in Section 4, in order to form a new care-of address, a 5778 mobile node MAY use either stateless [18] or stateful (e.g., DHCPv6 5779 [29]) Address Autoconfiguration. If a mobile node needs to use a 5780 source address (other than the unspecified address) in packets sent 5781 as a part of address autoconfiguration, it MUST use an IPv6 link- 5782 local address rather than its own IPv6 home address. 5784 RFC 4862 [18] specifies that in normal processing for Duplicate 5785 Address Detection, the node SHOULD delay sending the initial Neighbor 5786 Solicitation message by a random delay between 0 and 5787 MAX_RTR_SOLICITATION_DELAY. Since delaying DAD can result in 5788 significant delays in configuring a new care-of address when the 5789 Mobile Node moves to a new link, the Mobile Node preferably SHOULD 5790 NOT delay DAD when configuring a new care-of address. The Mobile 5791 Node SHOULD delay according to the mechanisms specified in RFC 4862 5792 unless the implementation has a behavior that desynchronizes the 5793 steps that happen before the DAD in the case that multiple nodes 5794 experience handover at the same time. Such desynchronizing behaviors 5795 might be due to random delays in the L2 protocols or device drivers, 5796 or due to the movement detection mechanism that is used. 5798 11.5.4. Using Multiple Care-of Addresses 5800 As described in Section 11.5.3, a mobile node MAY use more than one 5801 care-of address at a time. Particularly in the case of many wireless 5802 networks, a mobile node effectively might be reachable through 5803 multiple links at the same time (e.g., with overlapping wireless 5804 cells), on which different on-link subnet prefixes may exist. The 5805 mobile node MUST ensure that its primary care-of address always has a 5806 prefix that is advertised by its current default router. After 5807 selecting a new primary care-of address, the mobile node MUST send a 5808 Binding Update containing that care-of address to its home agent. 5809 The Binding Update MUST have the Home Registration (H) and 5810 Acknowledge (A) bits set its home agent, as described on 5811 Section 11.7.1. 5813 To assist with smooth handovers, a mobile node SHOULD retain its 5814 previous primary care-of address as a (non-primary) care-of address, 5815 and SHOULD still accept packets at this address, even after 5816 registering its new primary care-of address with its home agent. 5817 This is reasonable, since the mobile node could only receive packets 5818 at its previous primary care-of address if it were indeed still 5819 connected to that link. If the previous primary care-of address was 5820 allocated using stateful Address Autoconfiguration [29], the mobile 5821 node may not wish to release the address immediately upon switching 5822 to a new primary care-of address. 5824 Whenever a mobile node determines that it is no longer reachable 5825 through a given link, it SHOULD invalidate all care-of addresses 5826 associated with address prefixes that it discovered from routers on 5827 the unreachable link which are not in the current set of address 5828 prefixes advertised by the (possibly new) current default router. 5830 11.5.5. Returning Home 5832 A mobile node detects that it has returned to its home link through 5833 the movement detection algorithm in use (Section 11.5.2), when the 5834 mobile node detects that its home subnet prefix is again on-link. To 5835 be able to send and receive packets using its home address from the 5836 home link, the mobile node MUST send a Binding Update to its home 5837 agent to instruct its home agent to no longer intercept or tunnel 5838 packets for it. Until the mobile node sends such a de-registration 5839 Binding Update, it MUST NOT attempt to send and receive packets using 5840 its home address from the home link. The home agent will continue to 5841 intercept all packets sent to the mobile's home address and tunnel 5842 them to the previously registered care-of address. 5844 In this home registration, the mobile node MUST set the Acknowledge 5845 (A) and Home Registration (H) bits, set the Lifetime field to zero, 5846 and set the care-of address for the binding to the mobile node's own 5847 home address. The mobile node MUST use its home address as the 5848 source address in the Binding Update. 5850 When sending this Binding Update to its home agent, the mobile node 5851 must be careful in how it uses Neighbor Solicitation [17] (if needed) 5852 to learn the home agent's link-layer address, since the home agent 5853 will be currently configured to intercept packets to the mobile 5854 node's home address using Proxy Neighbor Discovery (Proxy ND). In 5855 particular, the mobile node is unable to use its home address as the 5856 Source Address in the Neighbor Solicitation until the home agent 5857 stops defending the home address. 5859 Neighbor Solicitation by the mobile node for the home agent's address 5860 will normally not be necessary, since the mobile node has already 5861 learned the home agent's link-layer address from a Source Link-Layer 5862 Address option in a Router Advertisement. However, if there are 5863 multiple home agents it may still be necessary to send a 5864 solicitation. In this special case of the mobile node returning 5865 home, the mobile node MUST multicast the packet, and in addition set 5866 the Source Address of this Neighbor Solicitation to the unspecified 5867 address (0:0:0:0:0:0:0:0). The target of the Neighbor Solicitation 5868 MUST be set to the mobile node's home address. The destination IP 5869 address MUST be set to the Solicited-Node multicast address [15]. 5870 The home agent will send a multicast Neighbor Advertisement back to 5871 the mobile node with the Solicited flag (S) set to zero. In any 5872 case, the mobile node SHOULD record the information from the Source 5873 Link-Layer Address option or from the advertisement, and set the 5874 state of the Neighbor Cache entry for the home agent to REACHABLE. 5876 The mobile node then sends its Binding Update to the home agent's 5877 link-layer address, instructing its home agent to no longer serve as 5878 a home agent for it. By processing this Binding Update, the home 5879 agent will cease defending the mobile node's home address for 5880 Duplicate Address Detection and will no longer respond to Neighbor 5881 Solicitations for the mobile node's home address. The mobile node is 5882 then the only node on the link receiving packets at the mobile node's 5883 home address. In addition, when returning home prior to the 5884 expiration of a current binding for its home address, and configuring 5885 its home address on its network interface on its home link, the 5886 mobile node MUST NOT perform Duplicate Address Detection on its own 5887 home address, in order to avoid confusion or conflict with its home 5888 agent's use of the same address. This rule also applies to the 5889 derived link-local address of the mobile node, if the Link Local 5890 Address Compatibility (L) bit was set when the binding was created. 5891 If the mobile node returns home after the bindings for all of its 5892 care-of addresses have expired, then it SHOULD perform DAD. 5894 After the Mobile Node sends the Binding Update, it MUST be prepared 5895 to reply to Neighbor Solicitations for its home address. Such 5896 replies MUST be sent using a unicast Neighbor Advertisement to the 5897 sender's link-layer address. It is necessary to reply, since sending 5898 the Binding Acknowledgement from the home agent may require 5899 performing Neighbor Discovery, and the mobile node may not be able to 5900 distinguish Neighbor Solicitations coming from the home agent from 5901 other Neighbor Solicitations. Note that a race condition exists 5902 where both the mobile node and the home agent respond to the same 5903 solicitations sent by other nodes; this will be only temporary, 5904 however, until the Binding Update is accepted. 5906 After receiving the Binding Acknowledgement for its Binding Update to 5907 its home agent, the mobile node MUST multicast onto the home link (to 5908 the all-nodes multicast address) a Neighbor Advertisement [17], to 5909 advertise the mobile node's own link-layer address for its own home 5910 address. The Target Address in this Neighbor Advertisement MUST be 5911 set to the mobile node's home address, and the Advertisement MUST 5912 include a Target Link-layer Address option specifying the mobile 5913 node's link-layer address. The mobile node MUST multicast such a 5914 Neighbor Advertisement for each of its home addresses, as defined by 5915 the current on-link prefixes, including its link-local address. The 5916 Solicited Flag (S) in these Advertisements MUST NOT be set, since 5917 they were not solicited by any Neighbor Solicitation. The Override 5918 Flag (O) in these Advertisements MUST be set, indicating that the 5919 Advertisements SHOULD override any existing Neighbor Cache entries at 5920 any node receiving them. 5922 Since multicasting on the local link (such as Ethernet) is typically 5923 not guaranteed to be reliable, the mobile node MAY retransmit these 5924 Neighbor Advertisements [17] up to MAX_NEIGHBOR_ADVERTISEMENT times 5925 to increase their reliability. It is still possible that some nodes 5926 on the home link will not receive any of these Neighbor 5927 Advertisements, but these nodes will eventually be able to recover 5928 through use of Neighbor Unreachability Detection [17]. 5930 Note that the tunnel via the home agent typically stops operating at 5931 the same time that the home registration is deleted. 5933 11.6. Return Routability Procedure 5935 This section defines the rules that the mobile node must follow when 5936 performing the return routability procedure. Section 11.7.2 5937 describes the rules when the return routability procedure needs to be 5938 initiated. 5940 11.6.1. Sending Test Init Messages 5942 A mobile node that initiates a return routability procedure MUST send 5943 (in parallel) a Home Test Init message and a Care-of Test Init 5944 messages. However, if the mobile node has recently received (see 5945 Section 5.2.7) one or both home or care-of keygen tokens, and 5946 associated nonce indices for the desired addresses, it MAY reuse 5947 them. Therefore, the return routability procedure may in some cases 5948 be completed with only one message pair. It may even be completed 5949 without any messages at all, if the mobile node has a recent home 5950 keygen token and has previously visited the same care-of address so 5951 that it also has a recent care-of keygen token. If the mobile node 5952 intends to send a Binding Update with the Lifetime set to zero and 5953 the care-of address equal to its home address - such as when 5954 returning home - sending a Home Test Init message is sufficient. In 5955 this case, generation of the binding management key depends 5956 exclusively on the home keygen token (Section 5.2.5). 5958 A Home Test Init message MUST be created as described in 5959 Section 6.1.3. 5961 A Care-of Test Init message MUST be created as described in 5962 Section 6.1.4. When sending a Home Test Init or Care-of Test Init 5963 message the mobile node MUST record in its Binding Update List the 5964 following fields from the messages: 5966 o The IP address of the node to which the message was sent. 5968 o The home address of the mobile node. This value will appear in 5969 the Source Address field of the Home Test Init message. When 5970 sending the Care-of Test Init message, this address does not 5971 appear in the message, but represents the home address for which 5972 the binding is desired. 5974 o The time at which each of these messages was sent. 5976 o The cookies used in the messages. 5978 Note that a single Care-of Test Init message may be sufficient even 5979 when there are multiple home addresses. In this case the mobile node 5980 MAY record the same information in multiple Binding Update List 5981 entries. 5983 11.6.2. Receiving Test Messages 5985 Upon receiving a packet carrying a Home Test message, a mobile node 5986 MUST validate the packet according to the following tests: 5988 o The Source Address of the packet belongs to a correspondent node 5989 for which the mobile node has a Binding Update List entry with a 5990 state indicating that return routability procedure is in progress. 5991 Note that there may be multiple such entries. 5993 o The Binding Update List indicates that no home keygen token has 5994 been received yet. 5996 o The Destination Address of the packet has the home address of the 5997 mobile node, and the packet has been received in a tunnel from the 5998 home agent. 6000 o The Home Init Cookie field in the message matches the value stored 6001 in the Binding Update List. 6003 Any Home Test message not satisfying all of these tests MUST be 6004 silently ignored. Otherwise, the mobile node MUST record the Home 6005 Nonce Index and home keygen token in the Binding Update List. If the 6006 Binding Update List entry does not have a care-of keygen token, the 6007 mobile node SHOULD continue waiting for the Care-of Test message. 6009 Upon receiving a packet carrying a Care-of Test message, a mobile 6010 node MUST validate the packet according to the following tests: 6012 o The Source Address of the packet belongs to a correspondent node 6013 for which the mobile node has a Binding Update List entry with a 6014 state indicating that return routability procedure is in progress. 6015 Note that there may be multiple such entries. 6017 o The Binding Update List indicates that no care-of keygen token has 6018 been received yet. 6020 o The Destination Address of the packet is the current care-of 6021 address of the mobile node. 6023 o The Care-of Init Cookie field in the message matches the value 6024 stored in the Binding Update List. 6026 Any Care-of Test message not satisfying all of these tests MUST be 6027 silently ignored. Otherwise, the mobile node MUST record the Care-of 6028 Nonce Index and care-of keygen token in the Binding Update List. If 6029 the Binding Update List entry does not have a home keygen token, the 6030 mobile node SHOULD continue waiting for the Home Test message. 6032 If after receiving either the Home Test or the Care-of Test message 6033 and performing the above actions, the Binding Update List entry has 6034 both the home and the care-of keygen tokens, the return routability 6035 procedure is complete. The mobile node SHOULD then proceed with 6036 sending a Binding Update as described in Section 11.7.2. 6038 Correspondent nodes from the time before this specification was 6039 published may not support the Mobility Header protocol. These nodes 6040 will respond to Home Test Init and Care-of Test Init messages with an 6041 ICMP Parameter Problem code 1. The mobile node SHOULD take such 6042 messages as an indication that the correspondent node cannot provide 6043 route optimization, and revert back to the use of bidirectional 6044 tunneling. 6046 11.6.3. Protecting Return Routability Packets 6048 The mobile node MUST support the protection of Home Test and Home 6049 Test Init messages as described in Section 10.4.6. 6051 When IPsec is used to protect return routability signaling or payload 6052 packets, the mobile node MUST set the source address it uses for the 6053 outgoing tunnel packets to the current primary care-of address. The 6054 mobile node starts to use a new primary care-of address immediately 6055 after sending a Binding Update to the home agent to register this new 6056 address. 6058 11.7. Processing Bindings 6060 11.7.1. Sending Binding Updates to the Home Agent 6062 In order to change its primary care-of address as described in 6063 Section 11.5.1 and Section 11.5.3, a mobile node MUST register this 6064 care-of address with its home agent in order to make this its primary 6065 care-of address. 6067 Also, if the mobile node wants the services of the home agent beyond 6068 the current registration period, the mobile node should send a new 6069 Binding Update to it well before the expiration of this period, even 6070 if it is not changing its primary care-of address. However, if the 6071 home agent returned a Binding Acknowledgement for the current 6072 registration with Status field set to 1 (accepted but prefix 6073 discovery necessary), the mobile node should not try to register 6074 again before it has learned the validity of its home prefixes through 6075 mobile prefix discovery. This is typically necessary every time this 6076 Status value is received, because information learned earlier may 6077 have changed. 6079 To register a care-of address or to extend the lifetime of an 6080 existing registration, the mobile node sends a packet to its home 6081 agent containing a Binding Update, with the packet constructed as 6082 follows: 6084 o The Home Registration (H) bit MUST be set in the Binding Update. 6086 o The Acknowledge (A) bit MUST be set in the Binding Update. 6088 o The packet MUST contain a Home Address destination option, giving 6089 the mobile node's home address for the binding. 6091 o The care-of address for the binding MUST be used as the Source 6092 Address in the packet's IPv6 header, unless an Alternate Care-of 6093 Address mobility option is included in the Binding Update. This 6094 option MUST be included in all home registrations, as the ESP 6095 protocol will not be able to protect care-of addresses in the IPv6 6096 header. (Mobile IPv6 implementations that know they are using 6097 IPsec AH to protect a particular message might avoid this option. 6098 For brevity the usage of AH is not discussed in this document.) 6100 o If the mobile node's link-local address has the same interface 6101 identifier as the home address for which it is supplying a new 6102 care-of address, then the mobile node SHOULD set the Link-Local 6103 Address Compatibility (L) bit. 6105 o If the home address was generated using RFC 4941 [20], then the 6106 link local address is unlikely to have a compatible interface 6107 identifier. In this case, the mobile node MUST clear the Link- 6108 Local Address Compatibility (L) bit. 6110 o If the IPsec security associations between the mobile node and the 6111 home agent have been established dynamically, and the mobile node 6112 has the capability to update its endpoint in the used key 6113 management protocol to the new care-of address every time it 6114 moves, the mobile node SHOULD set the Key Management Mobility 6115 Capability (K) bit in the Binding Update. Otherwise, the mobile 6116 node MUST clear the bit. 6118 o The value specified in the Lifetime field MUST be non-zero and 6119 SHOULD be less than or equal to the remaining valid lifetime of 6120 the home address and the care-of address specified for the 6121 binding. 6123 Mobile nodes that use dynamic home agent address discovery should 6124 be careful with long lifetimes. If the mobile node loses the 6125 knowledge of its binding with a specific home agent, registering a 6126 new binding with another home agent may be impossible as the 6127 previous home agent is still defending the existing binding. 6128 Therefore, to ensure that mobile nodes using home agent address 6129 discovery do not lose information about their binding, they SHOULD 6130 de-register before losing this information, or use small 6131 lifetimes. 6133 The Acknowledge (A) bit in the Binding Update requests the home agent 6134 to return a Binding Acknowledgement in response to this Binding 6135 Update. As described in Section 6.1.8, the mobile node SHOULD 6136 retransmit this Binding Update to its home agent until it receives a 6137 matching Binding Acknowledgement. Once reaching a retransmission 6138 timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart 6139 the process of delivering the Binding Update, but trying instead the 6140 next home agent returned during dynamic home agent address discovery 6141 (see Section 11.4.1). If there was only one home agent, the mobile 6142 node instead SHOULD continue to periodically retransmit the Binding 6143 Update at this rate until acknowledged (or until it begins attempting 6144 to register a different primary care-of address). See Section 11.8 6145 for information about retransmitting Binding Updates. 6147 With the Binding Update, the mobile node requests the home agent to 6148 serve as the home agent for the given home address. Until the 6149 lifetime of this registration expires, the home agent considers 6150 itself the home agent for this home address. 6152 Each Binding Update MUST be authenticated as coming from the right 6153 mobile node, as defined in Section 5.1. The mobile node MUST use its 6154 home address - either in the Home Address destination option or in 6155 the Source Address field of the IPv6 header - in Binding Updates sent 6156 to the home agent. This is necessary in order to allow the IPsec 6157 policies to be matched with the correct home address. 6159 When sending a Binding Update to its home agent, the mobile node MUST 6160 also create or update the corresponding Binding Update List entry, as 6161 specified in Section 11.7.2. 6163 The last Sequence Number value sent to the home agent in a Binding 6164 Update is stored by the mobile node. If the sending mobile node has 6165 no knowledge of the correct Sequence Number value, it may start at 6166 any value. If the home agent rejects the value, it sends back a 6167 Binding Acknowledgement with a status code 135, and the last accepted 6168 sequence number in the Sequence Number field of the Binding 6169 Acknowledgement. The mobile node MUST store this information and use 6170 the next Sequence Number value for the next Binding Update it sends. 6172 If the mobile node has additional home addresses, then the mobile 6173 node SHOULD send an additional packet containing a Binding Update to 6174 its home agent to register the care-of address for each such other 6175 home address. 6177 The home agent will only perform DAD for the mobile node's home 6178 address when the mobile node has supplied a valid binding between its 6179 home address and a care-of address. If some time elapses during 6180 which the mobile node has no binding at the home agent, it might be 6181 possible for another node to autoconfigure the mobile node's home 6182 address. Therefore, the mobile node MUST treat the creation of a new 6183 binding with the home agent using an existing home address, the same 6184 as creation of a new home address. In the unlikely event that the 6185 mobile node's home address is autoconfigured as the IPv6 address of 6186 another network node on the home network, the home agent will reply 6187 to the mobile node's subsequent Binding Update with a Binding 6188 Acknowledgement containing a Status of 134 (Duplicate Address 6189 Detection failed). In this case, the mobile node MUST NOT attempt to 6190 re-use the same home address. It SHOULD continue to register the 6191 care-of addresses for its other home addresses, if any. Mechanisms 6192 outlined in "Mobile IPv6 Bootstrapping in Split Scenario" [22] allow 6193 mobile nodes to acquire new home addresses to replace the one for 6194 which Status 134 was received. 6196 11.7.2. Correspondent Registration 6198 When the mobile node is assured that its home address is valid, it 6199 can initiate a correspondent registration with the purpose of 6200 allowing the correspondent node to cache the mobile node's current 6201 care-of address. This procedure consists of the return routability 6202 procedure followed by a registration. 6204 This section defines when the correspondent registration is to be 6205 initiated and the rules to follow while it is being performed. 6207 After the mobile node has sent a Binding Update to its home agent, 6208 registering a new primary care-of address (as described in 6209 Section 11.7.1), the mobile node SHOULD initiate a correspondent 6210 registration for each node that already appears in the mobile node's 6211 Binding Update List. The initiated procedures can be used to either 6212 update or delete binding information in the correspondent node. 6214 For nodes that do not appear in the mobile node's Binding Update 6215 List, the mobile node MAY initiate a correspondent registration at 6216 any time after sending the Binding Update to its home agent. 6217 Considerations regarding when (and if) to initiate the procedure 6218 depend on the specific movement and traffic patterns of the mobile 6219 node and are outside the scope of this document. 6221 In addition, the mobile node MAY initiate the correspondent 6222 registration in response to receiving a packet that meets all of the 6223 following tests: 6225 o The packet was tunneled using IPv6 encapsulation. 6227 o The Destination Address in the tunnel (outer) IPv6 header is equal 6228 to any of the mobile node's care-of addresses. 6230 o The Destination Address in the original (inner) IPv6 header is 6231 equal to one of the mobile node's home addresses. 6233 o The Source Address in the tunnel (outer) IPv6 header differs from 6234 the Source Address in the original (inner) IPv6 header. 6236 o The packet does not contain a Home Test, Home Test Init, Care-of 6237 Test, or Care-of Test Init message. 6239 If a mobile node has multiple home addresses, it becomes important to 6240 select the right home address to use in the correspondent 6241 registration. The used home address MUST be the Destination Address 6242 of the original (inner) packet. 6244 The peer address used in the procedure MUST be determined as follows: 6246 o If a Home Address destination option is present in the original 6247 (inner) packet, the address from this option is used. 6249 o Otherwise, the Source Address in the original (inner) IPv6 header 6250 of the packet is used. 6252 Note that the validity of the original packet is checked before 6253 attempting to initiate a correspondent registration. For instance, 6254 if a Home Address destination option appeared in the original packet, 6255 then rules in Section 9.3.1 are followed. 6257 A mobile node MAY also choose to keep its topological location 6258 private from certain correspondent nodes, and thus need not initiate 6259 the correspondent registration. 6261 Upon successfully completing the return routability procedure, and 6262 after receiving a successful Binding Acknowledgement from the Home 6263 Agent, a Binding Update MAY be sent to the correspondent node. 6265 In any Binding Update sent by a mobile node, the care-of address 6266 (either the Source Address in the packet's IPv6 header or the Care-of 6267 Address in the Alternate Care-of Address mobility option of the 6268 Binding Update) MUST be set to one of the care-of addresses currently 6269 in use by the mobile node or to the mobile node's home address. A 6270 mobile node MAY set the care-of address differently for sending 6271 Binding Updates to different correspondent nodes. 6273 A mobile node MAY also send a Binding Update to such a correspondent 6274 node, instructing it to delete any existing binding for the mobile 6275 node from its Binding Cache, as described in Section 6.1.7. Even in 6276 this case a successful completion of the return routability procedure 6277 is required first. 6279 If the care-of address is not set to the mobile node's home address, 6280 the Binding Update requests that the correspondent node create or 6281 update an entry for the mobile node in the correspondent node's 6282 Binding Cache. This is done in order to record a care-of address for 6283 use in sending future packets to the mobile node. In this case, the 6284 value specified in the Lifetime field sent in the Binding Update 6285 SHOULD be less than or equal to the remaining lifetime of the home 6286 registration and the care-of address specified for the binding. The 6287 care-of address given in the Binding Update MAY differ from the 6288 mobile node's primary care-of address. 6290 If the Binding Update is sent to the correspondent node, requesting 6291 the deletion of any existing Binding Cache entry it has for the 6292 mobile node, the care-of address is set to the mobile node's home 6293 address and the Lifetime field set to zero. In this case, generation 6294 of the binding management key depends exclusively on the home keygen 6295 token (Section 5.2.5). The care-of nonce index SHOULD be set to zero 6296 in this case. In keeping with the Binding Update creation rules 6297 below, the care-of address MUST be set to the home address if the 6298 mobile node is at home, or to the current care-of address if it is 6299 away from home. 6301 If the mobile node wants to ensure that its new care-of address has 6302 been entered into a correspondent node's Binding Cache, the mobile 6303 node needs to request an acknowledgement by setting the Acknowledge 6304 (A) bit in the Binding Update. 6306 A Binding Update is created as follows: 6308 o The current care-of address of the mobile node MUST be sent either 6309 in the Source Address of the IPv6 header, or in the Alternate 6310 Care-of Address mobility option. 6312 o The Destination Address of the IPv6 header MUST contain the 6313 address of the correspondent node. 6315 o The Mobility Header is constructed according to rules in 6316 Section 6.1.7 and Section 5.2.6, including the Binding 6317 Authorization Data (calculated as defined in Section 6.2.7) and 6318 possibly the Nonce Indices mobility options. 6320 o The home address of the mobile node MUST be added to the packet in 6321 a Home Address destination option, unless the Source Address is 6322 the home address. 6324 Each Binding Update MUST have a Sequence Number greater than the 6325 Sequence Number value sent in the previous Binding Update to the same 6326 destination address (if any). The sequence numbers are compared 6327 modulo 2**16, as described in Section 9.5.1. There is no 6328 requirement, however, that the Sequence Number value strictly 6329 increase by 1 with each new Binding Update sent or received, as long 6330 as the value stays within the window. The last Sequence Number value 6331 sent to a destination in a Binding Update is stored by the mobile 6332 node in its Binding Update List entry for that destination. If the 6333 sending mobile node has no Binding Update List entry, the Sequence 6334 Number SHOULD start at a random value. The mobile node MUST NOT use 6335 the same Sequence Number in two different Binding Updates to the same 6336 correspondent node, even if the Binding Updates provide different 6337 care-of addresses. 6339 The mobile node is responsible for the completion of the 6340 correspondent registration, as well as any retransmissions that may 6341 be needed (subject to the rate limitation defined in Section 11.8). 6343 11.7.3. Receiving Binding Acknowledgements 6345 Upon receiving a packet carrying a Binding Acknowledgement, a mobile 6346 node MUST validate the packet according to the following tests: 6348 o The packet meets the authentication requirements for Binding 6349 Acknowledgements defined in Section 6.1.8 and Section 5. That is, 6350 if the Binding Update was sent to the home agent, the underlying 6351 IPsec protection is used. If the Binding Update was sent to the 6352 correspondent node, the Binding Authorization Data mobility option 6353 MUST be present and have a valid value. 6355 o The Binding Authorization Data mobility option, if present, MUST 6356 be the last option and MUST NOT have trailing padding. 6358 o The Sequence Number field matches the Sequence Number sent by the 6359 mobile node to this destination address in an outstanding Binding 6360 Update, and the Status field is not 135. 6362 Any Binding Acknowledgement not satisfying all of these tests MUST be 6363 silently ignored. 6365 When a mobile node receives a packet carrying a valid Binding 6366 Acknowledgement, the mobile node MUST examine the Status field as 6367 follows: 6369 o If the Status field indicates that the Binding Update was accepted 6370 (the Status field is less than 128), then the mobile node MUST 6371 update the corresponding entry in its Binding Update List to 6372 indicate that the Binding Update has been acknowledged; the mobile 6373 node MUST then stop retransmitting the Binding Update. In 6374 addition, if the value specified in the Lifetime field in the 6375 Binding Acknowledgement is less than the Lifetime value sent in 6376 the Binding Update being acknowledged, the mobile node MUST 6377 subtract the difference between these two Lifetime values from the 6378 remaining lifetime for the binding as maintained in the 6379 corresponding Binding Update List entry (with a minimum value for 6380 the Binding Update List entry lifetime of 0). That is, if the 6381 Lifetime value sent in the Binding Update was L_update, the 6382 Lifetime value received in the Binding Acknowledgement was L_ack, 6383 and the current remaining lifetime of the Binding Update List 6384 entry is L_remain, then the new value for the remaining lifetime 6385 of the Binding Update List entry should be 6387 max((L_remain - (L_update - L_ack)), 0) 6389 where max(X, Y) is the maximum of X and Y. The effect of this step 6390 is to correctly manage the mobile node's view of the binding's 6391 remaining lifetime (as maintained in the corresponding Binding 6392 Update List entry) so that it correctly counts down from the 6393 Lifetime value given in the Binding Acknowledgement, but with the 6394 timer countdown beginning at the time that the Binding Update was 6395 sent. 6397 Mobile nodes SHOULD send a new Binding Update well before the 6398 expiration of this period in order to extend the lifetime. This 6399 helps to avoid disruptions in communications which might otherwise 6400 be caused by network delays or clock drift. 6402 o If the Binding Acknowledgement correctly passes authentication and 6403 the Status field value is 135 (Sequence Number out of window), 6404 then the mobile node MUST update its binding sequence number 6405 appropriately to match the sequence number given in the Binding 6406 Acknowledgement. Otherwise, if the Status field value is 135 but 6407 the Binding Acknowledgement does not pass authentication, the 6408 message MUST be silently ignored. 6410 o If the Status field value is 1 (accepted but prefix discovery 6411 necessary), the mobile node SHOULD send a Mobile Prefix 6412 Solicitation message to update its information about the available 6413 prefixes. 6415 o If the Status field indicates that the Binding Update was rejected 6416 (the Status field is greater than or equal to 128), then the 6417 mobile node can take steps to correct the cause of the error and 6418 retransmit the Binding Update (with a new Sequence Number value), 6419 subject to the rate limiting restriction specified in 6420 Section 11.8. If this is not done or it fails, then the mobile 6421 node SHOULD record in its Binding Update List that future Binding 6422 Updates SHOULD NOT be sent to this destination. 6424 The treatment of a Binding Refresh Advice mobility option within the 6425 Binding Acknowledgement depends on where the acknowledgement came 6426 from. This option MUST be ignored if the acknowledgement came from a 6427 correspondent node. If it came from the home agent, the mobile node 6428 uses the Refresh Interval field in the option as a suggestion that it 6429 SHOULD attempt to refresh its home registration at the indicated 6430 shorter interval. 6432 If the acknowledgement came from the home agent, the mobile node 6433 examines the value of the Key Management Mobility Capability (K) bit. 6434 If this bit is not set, the mobile node SHOULD discard key management 6435 protocol connections, if any, to the home agent. The mobile node MAY 6436 also initiate a new key management connection. 6438 If this bit is set, the mobile node SHOULD move its own endpoint in 6439 the key management protocol connections to the home agent, if any. 6440 The mobile node's new endpoint should be the new care-of address. 6442 11.7.4. Receiving Binding Refresh Requests 6444 When a mobile node receives a packet containing a Binding Refresh 6445 Request message, if the mobile node has a Binding Update List entry 6446 for the source of the Binding Refresh Request, and the mobile node 6447 wants to retain its Binding Cache entry at the correspondent node, 6448 then the mobile node should start a return routability procedure. If 6449 the mobile node wants to have its Binding Cache entry removed, it can 6450 either ignore the Binding Refresh Request and wait for the binding to 6451 time out, or at any time, it can delete its binding from a 6452 correspondent node with an explicit Binding Update with a zero 6453 lifetime and the care-of address set to the home address. If the 6454 mobile node does not know if it needs the Binding Cache entry, it can 6455 make the decision in an implementation dependent manner, such as 6456 based on available resources. 6458 Note that the mobile node should be careful to not respond to Binding 6459 Refresh Requests for addresses not in the Binding Update List to 6460 avoid being subjected to a denial of service attack. 6462 If the return routability procedure completes successfully, a Binding 6463 Update message SHOULD be sent, as described in Section 11.7.2. The 6464 Lifetime field in this Binding Update SHOULD be set to a new 6465 lifetime, extending any current lifetime remaining from a previous 6466 Binding Update sent to this node (as indicated in any existing 6467 Binding Update List entry for this node), and the lifetime SHOULD 6468 again be less than or equal to the remaining lifetime of the home 6469 registration and the care-of address specified for the binding. When 6470 sending this Binding Update, the mobile node MUST update its Binding 6471 Update List in the same way as for any other Binding Update sent by 6472 the mobile node. 6474 11.8. Retransmissions and Rate Limiting 6476 The mobile node is responsible for retransmissions and rate limiting 6477 in the return routability procedure, registrations, and in solicited 6478 prefix discovery. 6480 When the mobile node sends a Mobile Prefix Solicitation, Home Test 6481 Init, Care-of Test Init or Binding Update for which it expects a 6482 response, the mobile node has to determine a value for the initial 6483 retransmission timer: 6485 o If the mobile node is sending a Mobile Prefix Solicitation, it 6486 SHOULD use an initial retransmission interval of 6487 INITIAL_SOLICIT_TIMER (see Section 12). 6489 o If the mobile node is sending a Binding Update and does not have 6490 an existing binding at the home agent, it SHOULD use 6491 InitialBindackTimeoutFirstReg (see Section 13) as a value for the 6492 initial retransmission timer. This long retransmission interval 6493 will allow the home agent to complete the Duplicate Address 6494 Detection procedure mandated in this case, as detailed in 6495 Section 11.7.1. 6497 o Otherwise, the mobile node should use the specified value of 6498 INITIAL_BINDACK_TIMEOUT for the initial retransmission timer. 6500 If the mobile node fails to receive a valid matching response within 6501 the selected initial retransmission interval, the mobile node SHOULD 6502 retransmit the message until a response is received. 6504 The retransmissions by the mobile node MUST use an exponential back- 6505 off process in which the timeout period is doubled upon each 6506 retransmission, until either the node receives a response or the 6507 timeout period reaches the value MAX_BINDACK_TIMEOUT. The mobile 6508 node MAY continue to send these messages at this slower rate 6509 indefinitely. 6511 The mobile node SHOULD start a separate back-off process for 6512 different message types, different home addresses and different 6513 care-of addresses. However, in addition an overall rate limitation 6514 applies for messages sent to a particular correspondent node. This 6515 ensures that the correspondent node has a sufficient amount of time 6516 to respond when bindings for multiple home addresses are registered, 6517 for instance. The mobile node MUST NOT send Mobility Header messages 6518 of a particular type to a particular correspondent node more than 6519 MAX_UPDATE_RATE times within a second. 6521 Retransmitted Binding Updates MUST use a Sequence Number value 6522 greater than that used for the previous transmission of this Binding 6523 Update. Retransmitted Home Test Init and Care-of Test Init messages 6524 MUST use new cookie values. 6526 12. Protocol Constants 6528 DHAAD_RETRIES 4 retransmissions 6529 INITIAL_BINDACK_TIMEOUT 1 second 6530 INITIAL_DHAAD_TIMEOUT 3 seconds 6531 INITIAL_SOLICIT_TIMER 3 seconds 6532 MAX_BINDACK_TIMEOUT 32 seconds 6533 MAX_DELETE_BCE_TIMEOUT 10 seconds 6534 MAX_NONCE_LIFETIME 240 seconds 6535 MAX_TOKEN_LIFETIME 210 seconds 6536 MAX_RO_FAILURE 3 retries 6537 MAX_RR_BINDING_LIFETIME 420 seconds 6538 MAX_UPDATE_RATE 3 times 6539 PREFIX_ADV_RETRIES 3 retransmissions 6540 PREFIX_ADV_TIMEOUT 3 seconds 6542 13. Protocol Configuration Variables 6544 MaxMobPfxAdvInterval Default: 86,400 seconds 6545 MinDelayBetweenRAs Default: 3 seconds, 6546 Min: 0.03 seconds 6547 MinMobPfxAdvInterval Default: 600 seconds 6548 InitialBindackTimeoutFirstReg Default: 1.5 seconds 6550 Home agents MUST allow the first three variables to be configured by 6551 system management, and mobile nodes MUST allow the last variable to 6552 be configured by system management. 6554 The default value for InitialBindackTimeoutFirstReg has been 6555 calculated as 1.5 times the default value of RetransTimer, as 6556 specified in Neighbor Discovery (RFC 4861 [17]) times the default 6557 value of DupAddrDetectTransmits, as specified in Stateless Address 6558 Autoconfiguration (RFC 4862 [18]) 6560 The value MinDelayBetweenRAs overrides the value of the protocol 6561 constant MIN_DELAY_BETWEEN_RAS, as specified in Neighbor Discovery 6562 (RFC 4861 [17]). This variable SHOULD be set to MinRtrAdvInterval, 6563 if MinRtrAdvInterval is less than 3 seconds. 6565 14. IANA Considerations 6567 This document defines a new IPv6 protocol, the Mobility Header, 6568 described in Section 6.1. This protocol has been assigned protocol 6569 number 135. 6571 This document also creates a new name space "Mobility Header Type", 6572 for the MH Type field in the Mobility Header. The current message 6573 types are described starting from Section 6.1.2, and are the 6574 following: 6576 0 Binding Refresh Request 6578 1 Home Test Init 6580 2 Care-of Test Init 6582 3 Home Test 6584 4 Care-of Test 6586 5 Binding Update 6588 6 Binding Acknowledgement 6590 7 Binding Error 6592 Future values of the MH Type can be allocated using Standards Action 6593 or IESG Approval [23]. 6595 Furthermore, each mobility message may contain mobility options as 6596 described in Section 6.2. This document defines a new name space 6597 "Mobility Option" to identify these options. The current mobility 6598 options are defined starting from Section 6.2.2 and are the 6599 following: 6601 0 Pad1 6603 1 PadN 6605 2 Binding Refresh Advice 6607 3 Alternate Care-of Address 6609 4 Nonce Indices 6610 5 Authorization Data 6612 Future values of the Option Type can be allocated using Standards 6613 Action or IESG Approval [23]. 6615 Finally, this document creates a third new name space "Status Code" 6616 for the Status field in the Binding Acknowledgement message. The 6617 current values are listed in Section 6.1.8 and are the following: 6619 0 Binding Update accepted 6621 1 Accepted but prefix discovery necessary 6623 128 Reason unspecified 6625 129 Administratively prohibited 6627 130 Insufficient resources 6629 131 Home registration not supported 6631 132 Not home subnet 6633 133 Not home agent for this mobile node 6635 134 Duplicate Address Detection failed 6637 135 Sequence number out of window 6639 136 Expired home nonce index 6641 137 Expired care-of nonce index 6643 138 Expired nonces 6645 139 Registration type change disallowed 6647 TBD Invalid Care-of Address 6649 Future values of the Status field can be allocated using Standards 6650 Action or IESG Approval [23]. 6652 All fields labeled "Reserved" are only to be assigned through 6653 Standards Action or IESG Approval. 6655 This document also defines a new IPv6 destination option, the Home 6656 Address option, described in Section 6.3. This option has been 6657 assigned the Option Type value 0xC9. 6659 This document also defines a new IPv6 type 2 routing header, 6660 described in Section 6.4. The value 2 has been allocated by IANA. 6662 In addition, this document defines four ICMP message types, two used 6663 as part of the dynamic home agent address discovery mechanism, and 6664 two used in lieu of Router Solicitations and Advertisements when the 6665 mobile node is away from the home link. These messages have been 6666 assigned ICMPv6 type numbers from the informational message range: 6668 o The Home Agent Address Discovery Request message, described in 6669 Section 6.5; 6671 o The Home Agent Address Discovery Reply message, described in 6672 Section 6.6; 6674 o The Mobile Prefix Solicitation, described in Section 6.7; and 6676 o The Mobile Prefix Advertisement, described in Section 6.8. 6678 This document also defines two new Neighbor Discovery [17] options, 6679 which have been assigned Option Type values within the option 6680 numbering space for Neighbor Discovery messages: 6682 o The Advertisement Interval option, described in Section 7.3; and 6684 o The Home Agent Information option, described in Section 7.4. 6686 15. Security Considerations 6688 15.1. Threats 6690 Any mobility solution must protect itself against misuses of the 6691 mobility features and mechanisms. In Mobile IPv6, most of the 6692 potential threats are concerned with false Bindings, usually 6693 resulting in Denial-of-Service attacks. Some of the threats also 6694 pose potential for Man-in-the-Middle, Hijacking, Confidentiality, and 6695 Impersonation attacks. The main threats this protocol protects 6696 against are the following: 6698 o Threats involving Binding Updates sent to home agents and 6699 correspondent nodes. For instance, an attacker might claim that a 6700 certain mobile node is currently at a different location than it 6701 really is. If a home agent accepts such spoofed information sent 6702 to it, the mobile node might not get traffic destined to it. 6703 Similarly, a malicious (mobile) node might use the home address of 6704 a victim node in a forged Binding Update sent to a correspondent 6705 node. 6707 These pose threats against confidentiality, integrity, and 6708 availability. That is, an attacker might learn the contents of 6709 packets destined to another node by redirecting the traffic to 6710 itself. Furthermore, an attacker might use the redirected packets 6711 in an attempt to set itself as a Man-in-the-Middle between a 6712 mobile and a correspondent node. This would allow the attacker to 6713 impersonate the mobile node, leading to integrity and availability 6714 problems. 6716 A malicious (mobile) node might also send Binding Updates in which 6717 the care-of address is set to the address of a victim node. If 6718 such Binding Updates were accepted, the malicious node could lure 6719 the correspondent node into sending potentially large amounts of 6720 data to the victim; the correspondent node's replies to messages 6721 sent by the malicious mobile node will be sent to the victim host 6722 or network. This could be used to cause a Distributed Denial-of- 6723 Service attack. For example, the correspondent node might be a 6724 site that will send a high-bandwidth stream of video to anyone who 6725 asks for it. Note that the use of flow-control protocols such as 6726 TCP does not necessarily defend against this type of attack, 6727 because the attacker can fake the acknowledgements. Even keeping 6728 TCP initial sequence numbers secret does not help, because the 6729 attacker can receive the first few segments (including the ISN) at 6730 its own address, and only then redirect the stream to the victim's 6731 address. These types of attacks may also be directed to networks 6732 instead of nodes. Further variations of this threat are described 6733 elsewhere [28] [33]. 6735 An attacker might also attempt to disrupt a mobile node's 6736 communications by replaying a Binding Update that the node had 6737 sent earlier. If the old Binding Update was accepted, packets 6738 destined for the mobile node would be sent to its old location as 6739 opposed to its current location. 6741 A malicious mobile node associated to multiple home agents could 6742 create a routing loop amongst them. This can be achieved when a 6743 mobile node binds one home address located on a first home agent 6744 to another home address on a second home agent. This type of 6745 binding will force the home agents to route the same packet among 6746 each other without knowledge that a routing loop has been created. 6747 Such looping problem is limited to cases where a mobile node has 6748 multiple home agents and is permitted to be associated with the 6749 multiple home agents. For the single home agent case, a policy at 6750 the home agent would prevent the binding of one home address to 6751 another home address hosted by the same home agent. 6753 The potential problems caused by such routing loops in this 6754 scenario can be substantially reduced by use of the Tunnel-Limit 6755 Option specified in RFC 2473 [6]. 6757 In conclusion, there are Denial-of-Service, Man-in-the-Middle, 6758 Confidentiality, and Impersonation threats against the parties 6759 involved in sending legitimate Binding Updates, the threat of 6760 routing loops when there are multiple home agents, and Denial-of- 6761 Service threats against any other party. 6763 o Threats associated with payload packets: Payload packets exchanged 6764 with mobile nodes are exposed to similar threats as that of 6765 regular IPv6 traffic. However, Mobile IPv6 introduces the Home 6766 Address destination option, a new routing header type (type 2), 6767 and uses tunneling headers in the payload packets. The protocol 6768 must protect against potential new threats involving the use of 6769 these mechanisms. 6771 Third parties become exposed to a reflection threat via the Home 6772 Address destination option, unless appropriate security 6773 precautions are followed. The Home Address destination option 6774 could be used to direct response traffic toward a node whose IP 6775 address appears in the option. In this case, ingress filtering 6776 would not catch the forged "return address" [36] [40]. 6778 A similar threat exists with the tunnels between the mobile node 6779 and the home agent. An attacker might forge tunnel packets 6780 between the mobile node and the home agent, making it appear that 6781 the traffic is coming from the mobile node when it is not. Note 6782 that an attacker who is able to forge tunnel packets would 6783 typically also be able to forge packets that appear to come 6784 directly from the mobile node. This is not a new threat as such. 6785 However, it may make it easier for attackers to escape detection 6786 by avoiding ingress filtering and packet tracing mechanisms. 6787 Furthermore, spoofed tunnel packets might be used to gain access 6788 to the home network. 6790 Finally, a routing header could also be used in reflection 6791 attacks, and in attacks designed to bypass firewalls. The 6792 generality of the regular routing header would allow circumvention 6793 of IP-address based rules in firewalls. It would also allow 6794 reflection of traffic to other nodes. These threats exist with 6795 routing headers in general, even if the usage that Mobile IPv6 6796 requires is safe. 6798 o Threats associated with dynamic home agent and mobile prefix 6799 discovery. 6801 o Threats against the Mobile IPv6 security mechanisms themselves: An 6802 attacker might, for instance, lure the participants into executing 6803 expensive cryptographic operations or allocating memory for the 6804 purpose of keeping state. The victim node would have no resources 6805 left to handle other tasks. 6807 As a fundamental service in an IPv6 stack, Mobile IPv6 is expected to 6808 be deployed in most nodes of the IPv6 Internet. The above threats 6809 should therefore be considered as being applicable to the whole 6810 Internet. 6812 It should also be noted that some additional threats result from 6813 movements as such, even without the involvement of mobility 6814 protocols. Mobile nodes must be capable to defend themselves in the 6815 networks that they visit, as typical perimeter defenses applied in 6816 the home network no longer protect them. 6818 15.2. Features 6820 This specification provides a series of features designed to mitigate 6821 the risk introduced by the threats listed above. The main security 6822 features are the following: 6824 o Reverse Tunneling as a mandatory feature. 6826 o Protection of Binding Updates sent to home agents. 6828 o Protection of Binding Updates sent to correspondent nodes. 6830 o Protection against reflection attacks that use the Home Address 6831 destination option. 6833 o Protection of tunnels between the mobile node and the home agent. 6835 o Closing routing header vulnerabilities. 6837 o Mitigating Denial-of-Service threats to the Mobile IPv6 security 6838 mechanisms themselves. 6840 The support for encrypted reverse tunneling (see Section 11.3.1) 6841 allows mobile nodes to defeat certain kinds of traffic analysis. 6843 Protecting those Binding Updates that are sent to home agents and 6844 those that are sent to arbitrary correspondent nodes requires very 6845 different security solutions due to the different situations. Mobile 6846 nodes and home agents are naturally expected to be subject to the 6847 network administration of the home domain. 6849 Thus, they can and are supposed to have a security association that 6850 can be used to reliably authenticate the exchanged messages. See 6851 Section 5.1 for the description of the protocol mechanisms, and 6852 Section 15.3 below for a discussion of the resulting level of 6853 security. 6855 It is expected that Mobile IPv6 route optimization will be used on a 6856 global basis between nodes belonging to different administrative 6857 domains. It would be a very demanding task to build an 6858 authentication infrastructure on this scale. Furthermore, a 6859 traditional authentication infrastructure cannot be easily used to 6860 authenticate IP addresses because IP addresses can change often. It 6861 is not sufficient to just authenticate the mobile nodes; 6862 Authorization to claim the right to use an address is needed as well. 6863 Thus, an "infrastructureless" approach is necessary. The chosen 6864 infrastructureless method is described in Section 5.2, and 6865 Section 15.4 discusses the resulting security level and the design 6866 rationale of this approach. 6868 Specific rules guide the use of the Home Address destination option, 6869 the routing header, and the tunneling headers in the payload packets. 6870 These rules are necessary to remove the vulnerabilities associated 6871 with their unrestricted use. The effect of the rules is discussed in 6872 Section 15.7, Section 15.8, and Section 15.9. 6874 Denial-of-Service threats against Mobile IPv6 security mechanisms 6875 themselves concern mainly the Binding Update procedures with 6876 correspondent nodes. The protocol has been designed to limit the 6877 effects of such attacks, as will be described in Section 15.4.5. 6879 15.3. Binding Updates to Home Agent 6881 Signaling between the mobile node and the home agent requires message 6882 integrity. This is necessary to assure the home agent that a Binding 6883 Update is from a legitimate mobile node. In addition, correct 6884 ordering and anti-replay protection are optionally needed. 6886 IPsec ESP protects the integrity of the Binding Updates and Binding 6887 Acknowledgements by securing mobility messages between the mobile 6888 node and the home agent. 6890 IPsec can provide anti-replay protection only if dynamic keying is 6891 used (which may not always be the case). IPsec does not guarantee 6892 correct ordering of packets, only that they have not been replayed. 6893 Because of this, sequence numbers within the Mobile IPv6 messages are 6894 used to ensure correct ordering (see Section 5.1). However, if the 6895 16 bit Mobile IPv6 sequence number space is cycled through, or the 6896 home agent reboots and loses its state regarding the sequence 6897 numbers, replay and reordering attacks become possible. The use of 6898 dynamic keying, IPsec anti-replay protection, and the Mobile IPv6 6899 sequence numbers can together prevent such attacks. It is also 6900 recommended that use of non-volatile storage be considered for home 6901 agents, to avoid losing their state. 6903 A sliding window scheme is used for the sequence numbers. The 6904 protection against replays and reordering attacks without a key 6905 management mechanism works when the attacker remembers up to a 6906 maximum of 2**15 Binding Updates. 6908 The above mechanisms do not show that the care-of address given in 6909 the Binding Update is correct. This opens the possibility for 6910 Denial-of-Service attacks against third parties. However, since the 6911 mobile node and home agent have a security association, the home 6912 agent can always identify an ill-behaving mobile node. This allows 6913 the home agent operator to discontinue the mobile node's service, and 6914 possibly take further actions based on the business relationship with 6915 the mobile node's owner. 6917 Note that the use of a single pair of manually keyed security 6918 associations conflicts with the generation of a new home address [20] 6919 for the mobile node, or with the adoption of a new home subnet 6920 prefix. This is because IPsec security associations are bound to the 6921 used addresses. While certificate-based automatic keying alleviates 6922 this problem to an extent, it is still necessary to ensure that a 6923 given mobile node cannot send Binding Updates for the address of 6924 another mobile node. In general, this leads to the inclusion of home 6925 addresses in certificates in the Subject AltName field. This again 6926 limits the introduction of new addresses without either manual or 6927 automatic procedures to establish new certificates. Therefore, this 6928 specification restricts the generation of new home addresses (for any 6929 reason) to those situations where a security association or 6930 certificate for the new address already exists. 6932 Support for IKEv2 has been specified as optional. The following 6933 should be observed about the use of manual keying: 6935 o As discussed above, with manually keyed IPsec, only a limited form 6936 of protection exists against replay and reordering attacks. A 6937 vulnerability exists if either the sequence number space is cycled 6938 through, or if the home agent reboots and forgets its sequence 6939 numbers (and uses volatile memory to store the sequence numbers). 6941 Assuming the mobile node moves continuously every 10 minutes, it 6942 takes roughly 455 days before the sequence number space has been 6943 cycled through. Typical movement patterns rarely reach this high 6944 frequency today. 6946 o A mobile node and its home agent belong to the same domain. If 6947 this were not the case, manual keying would not be possible [39], 6948 but in Mobile IPv6 only these two parties need to know the 6949 manually configured keys. Similarly, we note that Mobile IPv6 6950 employs standard block ciphers in IPsec, and is not vulnerable to 6951 problems associated with stream ciphers and manual keying. 6953 o It is expected that the owner of the mobile node and the 6954 administrator of the home agent agree on the used keys and other 6955 parameters with some off-line mechanism. 6957 The use of IKEv2 with Mobile IPv6 is documented in more detail in 6958 [42]. The following should be observed regarding the use of IKEv2: 6960 o It is necessary to prevent a mobile node from claiming another 6961 mobile node's home address. The home agent must verify that the 6962 mobile node trying to negotiate the SA for a particular home 6963 address is authorized for that home address. This implies that 6964 even with the use of IKEv2, a policy entry needs to be configured 6965 for each home address served by the home agent. 6967 It may be possible to include home addresses in the Subject 6968 AltName field of certificate to avoid this. However, 6969 implementations are not guaranteed to support the use of a 6970 particular IP address (care-of address) while another address 6971 (home address) appears in the certificate. In any case, even this 6972 approach would require user-specific tasks in the certificate 6973 authority. 6975 o Due to the problems outlined in Section 11.3.2, the IKEv2 SA 6976 between the mobile node and its home agent is established using 6977 the mobile node's current care-of address. This implies that when 6978 the mobile node moves to a new location, it may have to re- 6979 establish an IKEv2 Security Association. A Key Management 6980 Mobility Capability (K) flag is provided for implementations that 6981 can update the IKEv2 endpoints without re-establishing an IKEv2 6982 Security Association, but the support for this behavior is 6983 optional. 6985 o Nevertheless, even if per-mobile node configuration is required 6986 with IKEv2, an important benefit of IKEv2 is that it automates the 6987 negotiation of cryptographic parameters, including the SPIs, 6988 cryptographic algorithms, and so on. Thus, less configuration 6989 information is needed. 6991 o The frequency of movements in some link layers or deployment 6992 scenarios may be high enough to make replay and reordering attacks 6993 possible, if only manual keying is used. IKEv2 SHOULD be used in 6994 such cases. Potentially vulnerable scenarios involve continuous 6995 movement through small cells, or uncontrolled alternation between 6996 available network attachment points. 6998 o Similarly, in some deployment scenarios the number of mobile nodes 6999 may be very large. In these cases, it can be necessary to use 7000 automatic mechanisms to reduce the management effort in the 7001 administration of cryptographic parameters, even if some per- 7002 mobile node configuration is always needed. IKEv2 SHOULD also be 7003 used in such cases. 7005 15.4. Binding Updates to Correspondent Nodes 7007 The motivation for designing the return routability procedure was to 7008 have sufficient support for Mobile IPv6, without creating significant 7009 new security problems. The goal for this procedure was not to 7010 protect against attacks that were already possible before the 7011 introduction of Mobile IPv6. 7013 The next sections will describe the security properties of the used 7014 method, both from the point of view of possible on-path attackers who 7015 can see those cryptographic values that have been sent in the clear 7016 (Section 15.4.2 and Section 15.4.3) and from the point of view of 7017 other attackers (Section 15.4.6). 7019 15.4.1. Overview 7021 The chosen infrastructureless method verifies that the mobile node is 7022 "live" (that is, it responds to probes) at its home and care-of 7023 addresses. Section 5.2 describes the return routability procedure in 7024 detail. The procedure uses the following principles: 7026 o A message exchange verifies that the mobile node is reachable at 7027 its addresses, i.e., is at least able to transmit and receive 7028 traffic at both the home and care-of addresses. 7030 o The eventual Binding Update is cryptographically bound to the 7031 tokens supplied in the exchanged messages. 7033 o Symmetric exchanges are employed to avoid the use of this protocol 7034 in reflection attacks. In a symmetric exchange, the responses are 7035 always sent to the same address the request was sent from. 7037 o The correspondent node operates in a stateless manner until it 7038 receives a fully authorized Binding Update. 7040 o Some additional protection is provided by encrypting the tunnels 7041 between the mobile node and home agent with IPsec ESP. As the 7042 tunnel also transports the nonce exchanges, the ability of 7043 attackers to see these nonces is limited. For instance, this 7044 prevents attacks from being launched from the mobile node's 7045 current foreign link, even when no link-layer confidentiality is 7046 available. 7048 The resulting level of security is in theory the same even without 7049 this additional protection: the return routability tokens are 7050 still exposed only to one path within the whole Internet. 7051 However, the mobile nodes are often found on an insecure link, 7052 such as a public access Wireless LAN. Thus, in many cases, this 7053 addition makes a practical difference. 7055 For further information about the design rationale of the return 7056 routability procedure, see [28] [33] [32] [40]. The mechanisms used 7057 have been adopted from these documents. 7059 15.4.2. Achieved Security Properties 7061 The return routability procedure protects Binding Updates against all 7062 attackers who are unable to monitor the path between the home agent 7063 and the correspondent node. The procedure does not defend against 7064 attackers who can monitor this path. Note that such attackers are in 7065 any case able to mount an active attack against the mobile node when 7066 it is at its home location. The possibility of such attacks is not 7067 an impediment to the deployment of Mobile IPv6 because these attacks 7068 are possible regardless of whether or not Mobile IPv6 is in use. 7070 This procedure also protects against Denial-of-Service attacks in 7071 which the attacker pretends to be mobile, but uses the victim's 7072 address as the care-of address. This would cause the correspondent 7073 node to send the victim some unexpected traffic. This procedure 7074 defends against these attacks by requiring at least the passive 7075 presence of the attacker at the care-of address or on the path from 7076 the correspondent to the care-of address. Normally, this will be the 7077 mobile node. 7079 15.4.3. Comparison to Regular IPv6 Communications 7081 This section discusses the protection offered by the return 7082 routability method by comparing it to the security of regular IPv6 7083 communications. We will divide vulnerabilities into three classes: 7084 (1) those related to attackers on the local network of the mobile 7085 node, home agent, or the correspondent node, (2) those related to 7086 attackers on the path between the home network and the correspondent 7087 node, and (3) off-path attackers, i.e., the rest of the Internet. 7089 We will now discuss the vulnerabilities of regular IPv6 7090 communications. The on-link vulnerabilities of IPv6 communications 7091 include Denial-of-Service, Masquerading, Man-in-the-Middle, 7092 Eavesdropping, and other attacks. These attacks can be launched 7093 through spoofing Router Discovery, Neighbor Discovery and other IPv6 7094 mechanisms. Some of these attacks can be prevented with the use of 7095 cryptographic protection in the packets. 7097 A similar situation exists with on-path attackers. That is, without 7098 cryptographic protection, the traffic is completely vulnerable. 7100 Assuming that attackers have not penetrated the security of the 7101 Internet routing protocols, attacks are much harder to launch from 7102 off-path locations. Attacks that can be launched from these 7103 locations are mainly Denial-of-Service attacks, such as flooding 7104 and/or reflection attacks. It is not possible for an off-path 7105 attacker to become a Man-in-the-Middle. 7107 Next, we will consider the vulnerabilities that exist when IPv6 is 7108 used together with Mobile IPv6 and the return routability procedure. 7109 On the local link, the vulnerabilities are the same as those in IPv6, 7110 but Masquerade and Man-in-the-Middle attacks can now also be launched 7111 against future communications, and not just against current 7112 communications. If a Binding Update was sent while the attacker was 7113 present on the link, its effects remain for the lifetime of the 7114 binding. This happens even if the attacker moves away from the link. 7115 In contrast, an attacker who uses only plain IPv6 generally has to 7116 stay on the link in order to continue the attack. Note that in order 7117 to launch these new attacks, the IP address of the victim must be 7118 known. This makes this attack feasible, mainly in the context of 7119 well-known interface IDs, such as those already appearing in the 7120 traffic on the link or registered in the DNS. 7122 On-path attackers can exploit similar vulnerabilities as in regular 7123 IPv6. There are some minor differences, however. Masquerade, Man- 7124 in-the-Middle, and Denial-of-Service attacks can be launched with 7125 just the interception of a few packets, whereas in regular IPv6 it is 7126 necessary to intercept every packet. The effect of the attacks is 7127 the same regardless of the method, however. In any case, the most 7128 difficult task an attacker faces in these attacks is getting on the 7129 right path. 7131 The vulnerabilities for off-path attackers are the same as in regular 7132 IPv6. Those nodes that are not on the path between the home agent 7133 and the correspondent node will not be able to receive the home 7134 address probe messages. 7136 In conclusion, we can state the following main results from this 7137 comparison: 7139 o Return routability prevents any off-path attacks beyond those that 7140 are already possible in regular IPv6. This is the most important 7141 result, preventing attackers on the Internet from exploiting any 7142 vulnerabilities. 7144 o Vulnerabilities to attackers on the home agent link, the 7145 correspondent node link, and the path between them are roughly the 7146 same as in regular IPv6. 7148 o However, one difference is that in basic IPv6 an on-path attacker 7149 must be constantly present on the link or the path, whereas with 7150 Mobile IPv6, an attacker can leave a binding behind after moving 7151 away. 7153 For this reason, this specification limits the creation of 7154 bindings to at most MAX_TOKEN_LIFETIME seconds after the last 7155 routability check has been performed, and limits the duration of a 7156 binding to at most MAX_RR_BINDING_LIFETIME seconds. With these 7157 limitations, attackers cannot take any practical advantages of 7158 this vulnerability. 7160 o There are some other minor differences, such as an effect to the 7161 Denial-of-Service vulnerabilities. These can be considered to be 7162 insignificant. 7164 o The path between the home agent and a correspondent node is 7165 typically easiest to attack on the links at either end, in 7166 particular if these links are publicly accessible wireless LANs. 7168 Attacks against the routers or switches on the path are typically 7169 harder to accomplish. The security on layer 2 of the links plays 7170 then a major role in the resulting overall network security. 7171 Similarly, security of IPv6 Neighbor and Router Discovery on these 7172 links has a large impact. If these were secured using some new 7173 technology in the future, this could change the situation 7174 regarding the easiest point of attack. 7176 For a more in-depth discussion of these issues, see [40]. 7178 15.4.4. Replay Attacks 7180 The return routability procedure also protects the participants 7181 against replayed Binding Updates. The attacker is unable replay the 7182 same message due to the sequence number which is a part of the 7183 Binding Update. It is also unable to modify the Binding Update since 7184 the MAC verification would fail after such a modification. 7186 Care must be taken when removing bindings at the correspondent node, 7187 however. If a binding is removed while the nonce used in its 7188 creation is still valid, an attacker could replay the old Binding 7189 Update. Rules outlined in Section 5.2.8 ensure that this cannot 7190 happen. 7192 15.4.5. Denial-of-Service Attacks 7194 The return routability procedure has protection against resource 7195 exhaustion Denial-of-Service attacks. The correspondent nodes do not 7196 retain any state about individual mobile nodes until an authentic 7197 Binding Update arrives. This is achieved through the construct of 7198 keygen tokens from the nonces and node keys that are not specific to 7199 individual mobile nodes. The keygen tokens can be reconstructed by 7200 the correspondent node, based on the home and care-of address 7201 information that arrives with the Binding Update. This means that 7202 the correspondent nodes are safe against memory exhaustion attacks 7203 except where on-path attackers are concerned. Due to the use of 7204 symmetric cryptography, the correspondent nodes are relatively safe 7205 against CPU resource exhaustion attacks as well. 7207 Nevertheless, as [28] describes, there are situations in which it is 7208 impossible for the mobile and correspondent nodes to determine if 7209 they actually need a binding or whether they just have been fooled 7210 into believing so by an attacker. Therefore, it is necessary to 7211 consider situations where such attacks are being made. 7213 Even if route optimization is a very important optimization, it is 7214 still only an optimization. A mobile node can communicate with a 7215 correspondent node even if the correspondent refuses to accept any 7216 Binding Updates. However, performance will suffer because packets 7217 from the correspondent node to the mobile node will be routed via the 7218 mobile's home agent rather than a more direct route. A correspondent 7219 node can protect itself against some of these resource exhaustion 7220 attacks as follows. If the correspondent node is flooded with a 7221 large number of Binding Updates that fail the cryptographic integrity 7222 checks, it can stop processing Binding Updates. If a correspondent 7223 node finds that it is spending more resources on checking bogus 7224 Binding Updates than it is likely to save by accepting genuine 7225 Binding Updates, then it may silently discard some or all Binding 7226 Updates without performing any cryptographic operations. 7228 Layers above IP can usually provide additional information to help 7229 determine whether there is a need to establish a binding with a 7230 specific peer. For example, TCP knows if the node has a queue of 7231 data that it is trying to send to a peer. An implementation of this 7232 specification is not required to make use of information from higher 7233 protocol layers, but some implementations are likely to be able to 7234 manage resources more effectively by making use of such information. 7236 We also require that all implementations be capable of 7237 administratively disabling route optimization. 7239 15.4.6. Key Lengths 7241 Attackers can try to break the return routability procedure in many 7242 ways. Section 15.4.2 discusses the situation where the attacker can 7243 see the cryptographic values sent in the clear, and Section 15.4.3 7244 discusses the impact this has on IPv6 communications. This section 7245 discusses whether attackers can guess the correct values without 7246 seeing them. 7248 While the return routability procedure is in progress, 64 bit cookies 7249 are used to protect spoofed responses. This is believed to be 7250 sufficient, given that to blindly spoof a response a very large 7251 number of messages would have to be sent before success would be 7252 probable. 7254 The tokens used in the return routability procedure provide together 7255 128 bits of information. This information is used internally as 7256 input to a hash function to produce a 160 bit quantity suitable for 7257 producing the keyed hash in the Binding Update using the HMAC_SHA1 7258 algorithm. The final keyed hash length is 96 bits. The limiting 7259 factors in this case are the input token lengths and the final keyed 7260 hash length. The internal hash function application does not reduce 7261 the entropy. 7263 The 96 bit final keyed hash is of typical size and is believed to be 7264 secure. The 128 bit input from the tokens is broken in two pieces, 7265 the home keygen token and the care-of keygen token. An attacker can 7266 try to guess the correct cookie value, but again this would require a 7267 large number of messages (an the average 2**63 messages for one or 7268 2**127 for two). Furthermore, given that the cookies are valid only 7269 for a short period of time, the attack has to keep a high constant 7270 message rate to achieve a lasting effect. This does not appear 7271 practical. 7273 When the mobile node is returning home, it is allowed to use just the 7274 home keygen token of 64 bits. This is less than 128 bits, but 7275 attacking it blindly would still require a large number of messages 7276 to be sent. If the attacker is on the path and capable of seeing the 7277 Binding Update, it could conceivably break the keyed hash with brute 7278 force. However, in this case the attacker has to be on the path, 7279 which appears to offer easier ways for denial-of-service than 7280 preventing route optimization. 7282 15.5. Dynamic Home Agent Address Discovery 7284 The dynamic home agent address discovery function could be used to 7285 learn the addresses of home agents in the home network. 7287 The ability to learn addresses of nodes may be useful to attackers 7288 because brute-force scanning of the address space is not practical 7289 with IPv6. Thus, they could benefit from any means which make 7290 mapping the networks easier. For example, if a security threat 7291 targeted at routers or even home agents is discovered, having a 7292 simple ICMP mechanism to easily find out possible targets may prove 7293 to be an additional (though minor) security risk. 7295 This document does not define any authentication mechanism for 7296 dynamic home agent address discovery messages. Therefore the home 7297 agent cannot verify the home address of the mobile node that 7298 requested the list of home agents. 7300 Apart from discovering the address(es) of home agents, attackers will 7301 not be able to learn much from this information, and mobile nodes 7302 cannot be tricked into using wrong home agents, as all other 7303 communication with the home agents is secure. 7305 In cases where additional security is needed, one may consider 7306 instead the use of MIPv6 bootstrapping [22], (based on DNS SRV 7307 Resource Records [9]) in conjunction with security mechanisms 7308 suggested in these specifications. In that solution, security is 7309 provided by the DNSSEC [13] framework. The needed pre-configured 7310 data on the mobile node for this mechanism is the domain name of the 7311 mobile service provider, which is marginally better than the home 7312 subnet prefix. For the security, a trust anchor which dominates the 7313 domain is needed. 7315 15.6. Mobile Prefix Discovery 7317 The mobile prefix discovery function may leak interesting information 7318 about network topology and prefix lifetimes to eavesdroppers; for 7319 this reason, requests for this information have to be authenticated. 7320 Responses and unsolicited prefix information needs to be 7321 authenticated to prevent the mobile nodes from being tricked into 7322 believing false information about the prefixes and possibly 7323 preventing communications with the existing addresses. Optionally, 7324 encryption may be applied to prevent leakage of the prefix 7325 information. 7327 15.7. Tunneling via the Home Agent 7329 Tunnels between the mobile node and the home agent can be protected 7330 by ensuring proper use of source addresses, and optional 7331 cryptographic protection. These procedures are discussed in 7332 Section 5.5. 7334 Binding Updates to the home agents are secure. When receiving 7335 tunneled traffic, the home agent verifies that the outer IP address 7336 corresponds to the current location of the mobile node. This acts as 7337 a weak form of protection against spoofing packets that appear to 7338 come from the mobile node. This is particularly useful, if no end- 7339 to-end security is being applied between the mobile and correspondent 7340 nodes. The outer IP address check prevents attacks where the 7341 attacker is controlled by ingress filtering. It also prevents 7342 attacks when the attacker does not know the current care-of address 7343 of the mobile node. Attackers who know the care-of address and are 7344 not controlled by ingress filtering could still send traffic through 7345 the home agent. This includes attackers on the same local link as 7346 the mobile node is currently on. But such attackers could send 7347 packets that appear to come from the mobile node without attacking 7348 the tunnel; the attacker could simply send packets with the source 7349 address set to the mobile node's home address. However, this attack 7350 does not work if the final destination of the packet is in the home 7351 network, and some form of perimeter defense is being applied for 7352 packets sent to those destinations. In such cases it is recommended 7353 that either end-to-end security or additional tunnel protection be 7354 applied, as is usual in remote access situations. 7356 Home agents and mobile nodes may use IPsec ESP to protect payload 7357 packets tunneled between themselves. This is useful for protecting 7358 communications against attackers on the path of the tunnel. 7360 When a Unique-Local Address (ULA) RFC4193 [19] is used as a home 7361 address, reverse tunneling can be used to send local traffic from 7362 another location. Administrators should be aware of this when 7363 allowing such home addresses. In particular, the outer IP address 7364 check described above is not sufficient against all attackers. The 7365 use of encrypted tunnels is particularly useful for these kinds of 7366 home addresses. 7368 15.8. Home Address Option 7370 When the mobile node sends packets directly to the correspondent 7371 node, the Source Address field of the packet's IPv6 header is the 7372 care-of address. Therefore, ingress filtering [27] works in the 7373 usual manner even for mobile nodes, as the Source Address is 7374 topologically correct. The Home Address option is used to inform the 7375 correspondent node of the mobile node's home address. 7377 However, the care-of address in the Source Address field does not 7378 survive in replies sent by the correspondent node unless it has a 7379 binding for this mobile node. Also, not all attacker tracing 7380 mechanisms work when packets are being reflected through 7381 correspondent nodes using the Home Address option. For these 7382 reasons, this specification restricts the use of the Home Address 7383 option. It may only be used when a binding has already been 7384 established with the participation of the node at the home address, 7385 as described in Section 5.5 and Section 6.3. This prevents 7386 reflection attacks through the use of the Home Address option. It 7387 also ensures that the correspondent nodes reply to the same address 7388 that the mobile node sends traffic from. 7390 No special authentication of the Home Address option is required 7391 beyond the above, but note that if the IPv6 header of a packet is 7392 covered by IPsec Authentication Header, then that authentication 7393 covers the Home Address option as well. Thus, even when 7394 authentication is used in the IPv6 header, the security of the Source 7395 Address field in the IPv6 header is not compromised by the presence 7396 of a Home Address option. Without authentication of the packet, any 7397 field in the IPv6 header, including the Source Address field or any 7398 other part of the packet and the Home Address option can be forged or 7399 modified in transit. In this case, the contents of the Home Address 7400 option is no more suspect than any other part of the packet. 7402 15.9. Type 2 Routing Header 7404 The definition of the type 2 routing header is described in 7405 Section 6.4. This definition and the associated processing rules 7406 have been chosen so that the header cannot be used for what is 7407 traditionally viewed as source routing. In particular, the Home 7408 Address in the routing header will always have to be assigned to the 7409 home address of the receiving node; otherwise the packet will be 7410 dropped. 7412 Generally, source routing has a number of security concerns. These 7413 include the automatic reversal of unauthenticated source routes 7414 (which is an issue for IPv4, but not for IPv6). Another concern is 7415 the ability to use source routing to "jump" between nodes inside, as 7416 well as outside a firewall. These security concerns are not issues 7417 in Mobile IPv6, due to the rules mentioned above. 7419 In essence the semantics of the type 2 routing header is the same as 7420 a special form of IP-in-IP tunneling where the inner and outer source 7421 addresses are the same. 7423 This implies that a device which implements the filtering of packets 7424 should be able to distinguish between a type 2 routing header and 7425 other routing headers, as required in Section 8.3. This is necessary 7426 in order to allow Mobile IPv6 traffic while still having the option 7427 of filtering out other uses of routing headers. 7429 16. Contributors 7431 Work done by Tuomas Aura, Mike Roe, Greg O'Shea, Pekka Nikander, Erik 7432 Nordmark, and Michael Thomas shaped the return routability protocols 7433 described in [33]. 7435 Significant contributions were made by members of the Mobile IPv6 7436 Security Design Team, including (in alphabetical order) Gabriel 7437 Montenegro, Erik Nordmark and Pekka Nikander. 7439 17. Acknowledgements 7441 We would like to thank the members of the Mobile IP, Mobility 7442 Extensions for IPv6, and IPng Working Groups for their comments and 7443 suggestions on this work. We would particularly like to thank (in 7444 alphabetical order) Fred Baker, Josh Broch, Samita Chakrabarti, 7445 Robert Chalmers, Noel Chiappa, Jean-Michel Combes, Greg Daley, Vijay 7446 Devarapalli, Rich Draves, Francis Dupont, Ashutosh Dutta, Arnaud 7447 Ebalard, Wesley Eddy, Thomas Eklund, Jun-Ichiro Itojun Hagino, Brian 7448 Haley, Marc Hasson, John Ioannidis, James Kempf, Rajeev Koodli, 7449 Suresh Krishnan, Krishna Kumar, T.J. Kniveton, Joe Lau, Aime Le 7450 Rouzic, Julien Laganier, Jiwoong Lee, Benjamin Lim, Vesa-Matti 7451 Mantyla, Kevin Miles, Glenn Morrow, Ahmad Muhanna, Thomas Narten, 7452 Karen Nielsen, Simon Nybroe, David Oran, Mohan Parthasarathy, 7453 Basavaraj Patil, Brett Pentland, Lars Henrik Petander, Alexandru 7454 Petrescu, Mattias Petterson, Ken Powell, Ed Remmell, Phil Roberts, 7455 Patrice Romand, Luis A. Sanchez, Pekka Savola, Jeff Schiller, Arvind 7456 Sevalkar, Keiichi Shima, Tom Soderlund, Hesham Soliman, Jim Solomon, 7457 Tapio Suihko, Dave Thaler, Pascal Thubert, Benny Van Houdt, Jon-Olov 7458 Vatn, Ryuji Wakikawa, Kilian Weniger, Carl E. Williams, Vladislav 7459 Yasevich, Alper Yegin, and Xinhua Zhao, for their detailed reviews of 7460 earlier versions of this document. Their suggestions have helped to 7461 improve both the design and presentation of the protocol. 7463 We would also like to thank the participants of the Mobile IPv6 7464 testing event (1999), implementors who participated in Mobile IPv6 7465 interoperability testing at Connectathons (2000, 2001, 2002, and 7466 2003), and the participants at the ETSI interoperability testing 7467 (2000, 2002). Finally, we would like to thank the TAHI project who 7468 has provided test suites for Mobile IPv6. 7470 18. References 7472 18.1. Normative References 7474 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 7475 Levels", BCP 14, RFC 2119, March 1997. 7477 [2] Kent, S. and R. Atkinson, "Security Architecture for the 7478 Internet Protocol", RFC 2401, November 1998. 7480 [3] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, 7481 November 1998. 7483 [4] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload 7484 (ESP)", RFC 2406, November 1998. 7486 [5] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) 7487 Specification", RFC 2460, December 1998. 7489 [6] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 7490 Specification", RFC 2473, December 1998. 7492 [7] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast 7493 Addresses", RFC 2526, March 1999. 7495 [8] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener 7496 Discovery (MLD) for IPv6", RFC 2710, October 1999. 7498 [9] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 7499 specifying the location of services (DNS SRV)", RFC 2782, 7500 February 2000. 7502 [10] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On- 7503 line Database", RFC 3232, January 2002. 7505 [11] National Institute of Standards and Technology, "Secure Hash 7506 Standard", FIPS PUB 180-1, April 1995, 7507 . 7509 [12] Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to 7510 Protect Mobile IPv6 Signaling Between Mobile Nodes and Home 7511 Agents", RFC 3776, June 2004. 7513 [13] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, 7514 "DNS Security Introduction and Requirements", RFC 4033, 7515 March 2005. 7517 [14] Eastlake, D., Schiller, J., and S. Crocker, "Randomness 7518 Requirements for Security", BCP 106, RFC 4086, June 2005. 7520 [15] Hinden, R. and S. Deering, "IP Version 6 Addressing 7521 Architecture", RFC 4291, February 2006. 7523 [16] Conta, A., Deering, S., and M. Gupta, "Internet Control Message 7524 Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) 7525 Specification", RFC 4443, March 2006. 7527 [17] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 7528 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 7529 September 2007. 7531 [18] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address 7532 Autoconfiguration", RFC 4862, September 2007. 7534 [19] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast 7535 Addresses", RFC 4193, October 2005. 7537 [20] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions 7538 for Stateless Address Autoconfiguration in IPv6", RFC 4941, 7539 September 2007. 7541 [21] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6 Socket 7542 API for Source Address Selection", RFC 5014, September 2007. 7544 [22] Giaretta, G., Kempf, J., and V. Devarapalli, "Mobile IPv6 7545 Bootstrapping in Split Scenario", RFC 5026, October 2007. 7547 [23] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA 7548 Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 7550 18.2. Informative References 7552 [24] Perkins, C., "IP Encapsulation within IP", RFC 2003, 7553 October 1996. 7555 [25] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, 7556 October 1996. 7558 [26] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing 7559 for Message Authentication", RFC 2104, February 1997. 7561 [27] Ferguson, P. and D. Senie, "Network Ingress Filtering: 7562 Defeating Denial of Service Attacks which employ IP Source 7563 Address Spoofing", BCP 38, RFC 2827, May 2000. 7565 [28] Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses", 7566 draft-aura-mipv6-bu-attacks-01 (work in progress), March 2002. 7568 [29] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. 7569 Carney, "Dynamic Host Configuration Protocol for IPv6 7570 (DHCPv6)", RFC 3315, July 2003. 7572 [30] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, 7573 August 2002. 7575 [31] Draves, R., "Default Address Selection for Internet Protocol 7576 version 6 (IPv6)", RFC 3484, February 2003. 7578 [32] Nordmark, E., "Securing MIPv6 BUs using return routability 7579 (BU3WAY)", draft-nordmark-mobileip-bu3way-00 (work in 7580 progress), November 2001. 7582 [33] Roe, M., "Authentication of Mobile IPv6 Binding Updates and 7583 Acknowledgments", draft-roe-mobileip-updateauth-02 (work in 7584 progress), March 2002. 7586 [34] Chowdhury, K. and A. Yegin, "MIP6-bootstrapping for the 7587 Integrated Scenario", 7588 draft-ietf-mip6-bootstrapping-integrated-dhc-06 (work in 7589 progress), April 2008. 7591 [35] Savola, P., "Use of /127 Prefix Length Between Routers 7592 Considered Harmful", RFC 3627, September 2003. 7594 [36] Savola, P., "Security of IPv6 Routing Header and Home Address 7595 Options", draft-savola-ipv6-rh-ha-security-02 (work in 7596 progress), March 2002. 7598 [37] Manner, J. and M. Kojo, "Mobility Related Terminology", 7599 RFC 3753, June 2004. 7601 [38] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 7602 (MLDv2) for IPv6", RFC 3810, June 2004. 7604 [39] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key 7605 Management", BCP 107, RFC 4107, June 2005. 7607 [40] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E. 7608 Nordmark, "Mobile IP Version 6 Route Optimization Security 7609 Design Background", RFC 4225, December 2005. 7611 [41] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", 7612 RFC 4306, December 2005. 7614 [42] Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with 7615 IKEv2 and the Revised IPsec Architecture", RFC 4877, 7616 April 2007. 7618 [43] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation of 7619 Type 0 Routing Headers in IPv6", RFC 5095, December 2007. 7621 Appendix A. Future Extensions 7623 A.1. Piggybacking 7625 This document does not specify how to piggyback payload packets on 7626 the binding related messages. However, it is envisioned that this 7627 can be specified in a separate document when issues such as the 7628 interaction between piggybacking and IPsec are fully resolved (see 7629 also Appendix A.3). The return routability messages can indicate 7630 support for piggybacking with a new mobility option. 7632 A.2. Triangular Routing 7634 Due to the concerns about opening reflection attacks with the Home 7635 Address destination option, this specification requires that this 7636 option be verified against the Binding Cache, i.e., there must be a 7637 Binding Cache entry for the Home Address and Care-of Address. 7639 Future extensions may be specified that allow the use of unverified 7640 Home Address destination options in ways that do not introduce 7641 security issues. 7643 A.3. New Authorization Methods 7645 While the return routability procedure provides a good level of 7646 security, there exist methods that have even higher levels of 7647 security. Secondly, as discussed in Section 15.4, future 7648 enhancements of IPv6 security may cause a need to also improve the 7649 security of the return routability procedure. Using IPsec as the 7650 sole method for authorizing Binding Updates to correspondent nodes is 7651 also possible. The protection of the Mobility Header for this 7652 purpose is easy, though one must ensure that the IPsec SA was created 7653 with appropriate authorization to use the home address referenced in 7654 the Binding Update. For instance, a certificate used by IKEv2 to 7655 create the security association might contain the home address. A 7656 future specification may specify how this is done. 7658 A.4. Neighbor Discovery Extensions 7660 Future specifications may improve the efficiency of Neighbor 7661 Discovery tasks, which could be helpful for fast movements. One 7662 factor is currently being looked at: the delays caused by the 7663 Duplicate Address Detection mechanism. Currently, Duplicate Address 7664 Detection needs to be performed for every new care-of address as the 7665 mobile node moves, and for the mobile node's link-local address on 7666 every new link. In particular, the need and the trade-offs of re- 7667 performing Duplicate Address Detection for the link-local address 7668 every time the mobile node moves on to new links will need to be 7669 examined. Improvements in this area are, however, generally 7670 applicable and progress independently from the Mobile IPv6 7671 specification. 7673 Future functional improvements may also be relevant for Mobile IPv6 7674 and other applications. For instance, mechanisms that would allow 7675 recovery from a Duplicate Address Detection collision would be useful 7676 for link-local, care-of, and home addresses. 7678 Appendix B. Changes since RFC 3775 7680 The following issues were identified during the evolution of the 7681 current document. Discussion about the issues can be found on the 7682 [mext] working group page 7683 http://trac.tools.ietf.org/wg/mext/trac/report/6 7685 Issue #1 Last Accepted SQN [Ahmad Muhanna] 7687 Solution: specify that the mobile node update its binding sequence 7688 number to match the sequence number given in the Binding 7689 Acknowledgement (if the Binding Acknowledgement correctly passes 7690 authentication and the status is 135 (Sequence Number out of 7691 window). See Section 11.7.3. 7693 Issue #4 Remove references to site-local addresses [George 7694 Tsirtsis]. 7696 Fixed. 7698 Issue #5 Wrong protocol number (2 instead of 135) used in discussion 7699 about checksum pseudo-header. 7701 Fixed. See Section 6.1.1. 7703 Issue #8 Application using the care-of address [Julien Laganier] 7705 Cite IPv6 Socket API for Source Address Selection specification 7706 [21]. See Section 11.3.4. 7708 Issue #10 The usage of "HA lifetime" [Ryuji Wakikawa] 7710 The mobile node SHOULD store the list of home agents for later use 7711 in case the home agent currently managing the mobile node's 7712 care-of address forwarding should become unavailable. See 7713 Section 11.4.1. 7715 Issue #11 De-registration when returning home [Vijay Devarapalli] 7717 To be able to send and receive packets using its home address from 7718 the home link, the mobile node MUST send a Binding Update to its 7719 home agent to instruct its home agent to no longer intercept or 7720 tunnel packets for it. Until the mobile node sends such a de- 7721 registration Binding Update, it MUST NOT attempt to send and 7722 receive packets using its home address from the home link. See 7723 Section 11.5.5. 7725 Issue #12 BErr sent by HA too, not only by CN [Alexandru Petrescu] 7727 Fixed. See Section 4.2. 7729 Issue #13 Home Link Detection [Suresh Krishnan] 7731 Proposal: add Section 11.5.2 for Home Link Detection, drawing on 7732 Internet Draft draft-krishnan-mext-hld. 7734 Issue #14 References to Bootstrapping [Vijay Devarapalli] 7736 Cite "Mobile IPv6 Bootstrapping in Split Scenario" [22] and "MIP6 7737 bootstrapping for the Integrated Scenario" [34]. See Section 4.1. 7739 Issue #17 Multi-homed mobile node can cause routing loop between 7740 home agents [Benjamin Lim] 7742 Added security advisory in Section 15.1, to highlight risk of 7743 routing loop among HAs (e.g., in 3GPP): 7745 A malicious mobile node associated to multiple home agents could 7746 create a routing loop amongst them. This would happen when a 7747 mobile node binds one home address located on a first home agent 7748 to another home address on a second home agent. 7750 Issue #18 Subject: Issues regarding Home Address Option and ICMP / 7751 Binding Errors [Fabian Mauchle] 7753 Proposal: Use the value in the Next Header field {50 (ESP), 51 7754 (AH), 135 (Mobility Header)} to determine, if a Binding Cache 7755 entry is required. See Section 9.3.1. 7757 Proposal: If the Binding Error Message was sent by the Home Agent, 7758 the Mobile Node SHOULD send a Binding Update to the Home Agent 7759 according to Section 11.7.1. See Section 11.3.6. 7761 Issue #19 BU de-registration race condition [Kilian Weniger] 7763 Problem arises if de-registration arrives at Home Agent before an 7764 immediately preceding Binding Update. 7766 Solution: Home Agent defers BCE removal after sending the Binding 7767 Acknowledgement. See Section 10.3.2. 7769 Issue #6 Minor editorial corrections and updates 7771 Authors' Addresses 7773 Charles E. Perkins 7774 Tellabs Inc. 7775 3590 N. 1st Street, Suite 300 7776 San Jose CA 95134 7777 USA 7779 Email: charliep@computer.org 7781 David B. Johnson 7782 Rice University 7783 Dept. of Computer Science, MS 132 7784 6100 Main Street 7785 Houston TX 77005-1892 7786 USA 7788 Email: dbj@cs.rice.edu 7790 Jari Arkko 7791 Ericsson 7792 Jorvas 02420 7793 Finland 7795 Email: jari.arkko@ericsson.com