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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 (October 1, 2008) is 5686 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 2407 (ref. '5') (Obsoleted by RFC 4306) ** Obsolete normative reference: RFC 2408 (ref. '6') (Obsoleted by RFC 4306) ** Obsolete normative reference: RFC 2409 (ref. '7') (Obsoleted by RFC 4306) ** Obsolete normative reference: RFC 2460 (ref. '8') (Obsoleted by RFC 8200) ** Downref: Normative reference to an Informational RFC: RFC 3232 (ref. '13') -- Possible downref: Non-RFC (?) normative reference: ref. '14' ** Obsolete normative reference: RFC 4941 (ref. '23') (Obsoleted by RFC 8981) ** Downref: Normative reference to an Informational RFC: RFC 5014 (ref. '24') ** Obsolete normative reference: RFC 5226 (ref. '26') (Obsoleted by RFC 8126) -- Obsolete informational reference (is this intentional?): RFC 3315 (ref. '32') (Obsoleted by RFC 8415) -- Obsolete informational reference (is this intentional?): RFC 3344 (ref. '33') (Obsoleted by RFC 5944) -- Obsolete informational reference (is this intentional?): RFC 3484 (ref. '34') (Obsoleted by RFC 6724) -- Obsolete informational reference (is this intentional?): RFC 3627 (ref. '38') (Obsoleted by RFC 6547) == Outdated reference: A later version (-03) exists of draft-savola-ipv6-rh-ha-security-02 -- Obsolete informational reference (is this intentional?): RFC 4306 (ref. '43') (Obsoleted by RFC 5996) Summary: 12 errors (**), 0 flaws (~~), 4 warnings (==), 14 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IETF Mobile IP Working Group D. Johnson 3 Internet-Draft Rice University 4 Obsoletes: 3775 (if approved) C. Perkins (Ed.) 5 Expires: April 4, 2009 WiChorus Inc. 6 J. Arkko 7 Ericsson 8 October 1, 2008 10 Mobility Support in IPv6 11 draft-ietf-mext-rfc3775bis-02.txt 13 Status of this Memo 15 By submitting this Internet-Draft, each author represents that any 16 applicable patent or other IPR claims of which he or she is aware 17 have been or will be disclosed, and any of which he or she becomes 18 aware will be disclosed, in accordance with Section 6 of BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt. 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 This Internet-Draft will expire on April 4, 2009. 38 Abstract 40 This document specifies a protocol which allows nodes to remain 41 reachable while moving around in the IPv6 Internet. Each mobile node 42 is always identified by its home address, regardless of its current 43 point of attachment to the Internet. While situated away from its 44 home, a mobile node is also associated with a care-of address, which 45 provides information about the mobile node's current location. IPv6 46 packets addressed to a mobile node's home address are transparently 47 routed to its care-of address. The protocol enables IPv6 nodes to 48 cache the binding of a mobile node's home address with its care-of 49 address, and to then send any packets destined for the mobile node 50 directly to it at this care-of address. To support this operation, 51 Mobile IPv6 defines a new IPv6 protocol and a new destination option. 52 All IPv6 nodes, whether mobile or stationary, can communicate with 53 mobile nodes. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7 58 2. Comparison with Mobile IP for IPv4 . . . . . . . . . . . . . 9 59 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 10 60 3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 10 61 3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 12 62 4. Overview of Mobile IPv6 . . . . . . . . . . . . . . . . . . . 16 63 4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 16 64 4.2. New IPv6 Protocol . . . . . . . . . . . . . . . . . . . . 18 65 4.3. New IPv6 Destination Option . . . . . . . . . . . . . . . 19 66 4.4. New IPv6 ICMP Messages . . . . . . . . . . . . . . . . . 19 67 4.5. Conceptual Data Structure Terminology . . . . . . . . . . 19 68 4.6. Unique-Local Addressability . . . . . . . . . . . . . . . 20 69 5. Overview of Mobile IPv6 Security . . . . . . . . . . . . . . 21 70 5.1. Binding Updates to Home Agents . . . . . . . . . . . . . 21 71 5.2. Binding Updates to Correspondent Nodes . . . . . . . . . 22 72 5.2.1. Node Keys . . . . . . . . . . . . . . . . . . . . . . 23 73 5.2.2. Nonces . . . . . . . . . . . . . . . . . . . . . . . 23 74 5.2.3. Cookies and Tokens . . . . . . . . . . . . . . . . . 24 75 5.2.4. Cryptographic Functions . . . . . . . . . . . . . . . 24 76 5.2.5. Return Routability Procedure . . . . . . . . . . . . 25 77 5.2.6. Authorizing Binding Management Messages . . . . . . . 29 78 5.2.7. Updating Node Keys and Nonces . . . . . . . . . . . . 31 79 5.2.8. Preventing Replay Attacks . . . . . . . . . . . . . . 32 80 5.3. Dynamic Home Agent Address Discovery . . . . . . . . . . 32 81 5.4. Mobile Prefix Discovery . . . . . . . . . . . . . . . . . 32 82 5.5. Payload Packets . . . . . . . . . . . . . . . . . . . . . 33 83 6. New IPv6 Protocol, Message Types, and Destination Option . . 34 84 6.1. Mobility Header . . . . . . . . . . . . . . . . . . . . . 34 85 6.1.1. Format . . . . . . . . . . . . . . . . . . . . . . . 34 86 6.1.2. Binding Refresh Request Message . . . . . . . . . . . 36 87 6.1.3. Home Test Init Message . . . . . . . . . . . . . . . 37 88 6.1.4. Care-of Test Init Message . . . . . . . . . . . . . . 38 89 6.1.5. Home Test Message . . . . . . . . . . . . . . . . . . 39 90 6.1.6. Care-of Test Message . . . . . . . . . . . . . . . . 40 91 6.1.7. Binding Update Message . . . . . . . . . . . . . . . 42 92 6.1.8. Binding Acknowledgement Message . . . . . . . . . . . 44 93 6.1.9. Binding Error Message . . . . . . . . . . . . . . . . 47 94 6.2. Mobility Options . . . . . . . . . . . . . . . . . . . . 48 95 6.2.1. Format . . . . . . . . . . . . . . . . . . . . . . . 49 96 6.2.2. Pad1 . . . . . . . . . . . . . . . . . . . . . . . . 49 97 6.2.3. PadN . . . . . . . . . . . . . . . . . . . . . . . . 50 98 6.2.4. Binding Refresh Advice . . . . . . . . . . . . . . . 50 99 6.2.5. Alternate Care-of Address . . . . . . . . . . . . . . 51 100 6.2.6. Nonce Indices . . . . . . . . . . . . . . . . . . . . 51 101 6.2.7. Binding Authorization Data . . . . . . . . . . . . . 52 102 6.3. Home Address Option . . . . . . . . . . . . . . . . . . . 53 103 6.4. Type 2 Routing Header . . . . . . . . . . . . . . . . . . 55 104 6.4.1. Format . . . . . . . . . . . . . . . . . . . . . . . 56 105 6.5. ICMP Home Agent Address Discovery Request Message . . . . 57 106 6.6. ICMP Home Agent Address Discovery Reply Message . . . . . 58 107 6.7. ICMP Mobile Prefix Solicitation Message Format . . . . . 59 108 6.8. ICMP Mobile Prefix Advertisement Message Format . . . . . 61 109 7. Modifications to IPv6 Neighbor Discovery . . . . . . . . . . 64 110 7.1. Modified Router Advertisement Message Format . . . . . . 64 111 7.2. Modified Prefix Information Option Format . . . . . . . . 64 112 7.3. New Advertisement Interval Option Format . . . . . . . . 66 113 7.4. New Home Agent Information Option Format . . . . . . . . 67 114 7.5. Changes to Sending Router Advertisements . . . . . . . . 69 115 8. Requirements for Types of IPv6 Nodes . . . . . . . . . . . . 71 116 8.1. All IPv6 Nodes . . . . . . . . . . . . . . . . . . . . . 71 117 8.2. IPv6 Nodes with Support for Route Optimization . . . . . 71 118 8.3. All IPv6 Routers . . . . . . . . . . . . . . . . . . . . 73 119 8.4. IPv6 Home Agents . . . . . . . . . . . . . . . . . . . . 73 120 8.5. IPv6 Mobile Nodes . . . . . . . . . . . . . . . . . . . . 75 121 9. Correspondent Node Operation . . . . . . . . . . . . . . . . 77 122 9.1. Conceptual Data Structures . . . . . . . . . . . . . . . 77 123 9.2. Processing Mobility Headers . . . . . . . . . . . . . . . 78 124 9.3. Packet Processing . . . . . . . . . . . . . . . . . . . . 78 125 9.3.1. Receiving Packets with Home Address Option . . . . . 78 126 9.3.2. Sending Packets to a Mobile Node . . . . . . . . . . 79 127 9.3.3. Sending Binding Error Messages . . . . . . . . . . . 81 128 9.3.4. Receiving ICMP Error Messages . . . . . . . . . . . . 81 129 9.4. Return Routability Procedure . . . . . . . . . . . . . . 82 130 9.4.1. Receiving Home Test Init Messages . . . . . . . . . . 82 131 9.4.2. Receiving Care-of Test Init Messages . . . . . . . . 82 132 9.4.3. Sending Home Test Messages . . . . . . . . . . . . . 83 133 9.4.4. Sending Care-of Test Messages . . . . . . . . . . . . 83 134 9.5. Processing Bindings . . . . . . . . . . . . . . . . . . . 83 135 9.5.1. Receiving Binding Updates . . . . . . . . . . . . . . 83 136 9.5.2. Requests to Cache a Binding . . . . . . . . . . . . . 86 137 9.5.3. Requests to Delete a Binding . . . . . . . . . . . . 86 138 9.5.4. Sending Binding Acknowledgements . . . . . . . . . . 87 139 9.5.5. Sending Binding Refresh Requests . . . . . . . . . . 88 140 9.6. Cache Replacement Policy . . . . . . . . . . . . . . . . 88 141 10. Home Agent Operation . . . . . . . . . . . . . . . . . . . . 90 142 10.1. Conceptual Data Structures . . . . . . . . . . . . . . . 90 143 10.2. Processing Mobility Headers . . . . . . . . . . . . . . . 91 144 10.3. Processing Bindings . . . . . . . . . . . . . . . . . . . 91 145 10.3.1. Primary Care-of Address Registration . . . . . . . . 91 146 10.3.2. Primary Care-of Address De-Registration . . . . . . . 95 147 10.4. Packet Processing . . . . . . . . . . . . . . . . . . . . 96 148 10.4.1. Intercepting Packets for a Mobile Node . . . . . . . 96 149 10.4.2. Processing Intercepted Packets . . . . . . . . . . . 98 150 10.4.3. Multicast Membership Control . . . . . . . . . . . . 99 151 10.4.4. Stateful Address Autoconfiguration . . . . . . . . . 100 152 10.4.5. Handling Reverse Tunneled Packets . . . . . . . . . . 101 153 10.4.6. Protecting Return Routability Packets . . . . . . . . 101 154 10.5. Dynamic Home Agent Address Discovery . . . . . . . . . . 102 155 10.5.1. Receiving Router Advertisement Messages . . . . . . . 102 156 10.6. Sending Prefix Information to the Mobile Node . . . . . . 104 157 10.6.1. List of Home Network Prefixes . . . . . . . . . . . . 104 158 10.6.2. Scheduling Prefix Deliveries . . . . . . . . . . . . 105 159 10.6.3. Sending Advertisements . . . . . . . . . . . . . . . 107 160 10.6.4. Lifetimes for Changed Prefixes . . . . . . . . . . . 108 161 11. Mobile Node Operation . . . . . . . . . . . . . . . . . . . . 109 162 11.1. Conceptual Data Structures . . . . . . . . . . . . . . . 109 163 11.2. Processing Mobility Headers . . . . . . . . . . . . . . . 110 164 11.3. Packet Processing . . . . . . . . . . . . . . . . . . . . 111 165 11.3.1. Sending Packets While Away from Home . . . . . . . . 111 166 11.3.2. Interaction with Outbound IPsec Processing . . . . . 114 167 11.3.3. Receiving Packets While Away from Home . . . . . . . 116 168 11.3.4. Routing Multicast Packets . . . . . . . . . . . . . . 117 169 11.3.5. Receiving ICMP Error Messages . . . . . . . . . . . . 119 170 11.3.6. Receiving Binding Error Messages . . . . . . . . . . 119 171 11.4. Home Agent and Prefix Management . . . . . . . . . . . . 120 172 11.4.1. Dynamic Home Agent Address Discovery . . . . . . . . 120 173 11.4.2. Sending Mobile Prefix Solicitations . . . . . . . . . 121 174 11.4.3. Receiving Mobile Prefix Advertisements . . . . . . . 122 175 11.5. Movement . . . . . . . . . . . . . . . . . . . . . . . . 123 176 11.5.1. Movement Detection . . . . . . . . . . . . . . . . . 123 177 11.5.2. Home Link Detection . . . . . . . . . . . . . . . . . 125 178 11.5.3. Forming New Care-of Addresses . . . . . . . . . . . . 126 179 11.5.4. Using Multiple Care-of Addresses . . . . . . . . . . 127 180 11.5.5. Returning Home . . . . . . . . . . . . . . . . . . . 127 181 11.6. Return Routability Procedure . . . . . . . . . . . . . . 129 182 11.6.1. Sending Test Init Messages . . . . . . . . . . . . . 129 183 11.6.2. Receiving Test Messages . . . . . . . . . . . . . . . 130 184 11.6.3. Protecting Return Routability Packets . . . . . . . . 132 185 11.7. Processing Bindings . . . . . . . . . . . . . . . . . . . 132 186 11.7.1. Sending Binding Updates to the Home Agent . . . . . . 132 187 11.7.2. Correspondent Registration . . . . . . . . . . . . . 135 188 11.7.3. Receiving Binding Acknowledgements . . . . . . . . . 138 189 11.7.4. Receiving Binding Refresh Requests . . . . . . . . . 140 190 11.8. Retransmissions and Rate Limiting . . . . . . . . . . . . 141 191 12. Protocol Constants . . . . . . . . . . . . . . . . . . . . . 143 192 13. Protocol Configuration Variables . . . . . . . . . . . . . . 144 193 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 145 194 15. Security Considerations . . . . . . . . . . . . . . . . . . . 148 195 15.1. Threats . . . . . . . . . . . . . . . . . . . . . . . . . 148 196 15.2. Features . . . . . . . . . . . . . . . . . . . . . . . . 150 197 15.3. Binding Updates to Home Agent . . . . . . . . . . . . . . 151 198 15.4. Binding Updates to Correspondent Nodes . . . . . . . . . 154 199 15.4.1. Overview . . . . . . . . . . . . . . . . . . . . . . 154 200 15.4.2. Achieved Security Properties . . . . . . . . . . . . 155 201 15.4.3. Comparison to Regular IPv6 Communications . . . . . . 156 202 15.4.4. Replay Attacks . . . . . . . . . . . . . . . . . . . 158 203 15.4.5. Denial-of-Service Attacks . . . . . . . . . . . . . . 158 204 15.4.6. Key Lengths . . . . . . . . . . . . . . . . . . . . . 159 205 15.5. Dynamic Home Agent Address Discovery . . . . . . . . . . 160 206 15.6. Mobile Prefix Discovery . . . . . . . . . . . . . . . . . 161 207 15.7. Tunneling via the Home Agent . . . . . . . . . . . . . . 161 208 15.8. Home Address Option . . . . . . . . . . . . . . . . . . . 162 209 15.9. Type 2 Routing Header . . . . . . . . . . . . . . . . . . 162 210 16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 164 211 17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 165 212 18. References . . . . . . . . . . . . . . . . . . . . . . . . . 166 213 18.1. Normative References . . . . . . . . . . . . . . . . . . 166 214 18.2. Informative References . . . . . . . . . . . . . . . . . 167 215 Appendix A. Future Extensions . . . . . . . . . . . . . . . . . 170 216 A.1. Piggybacking . . . . . . . . . . . . . . . . . . . . . . 170 217 A.2. Triangular Routing . . . . . . . . . . . . . . . . . . . 170 218 A.3. New Authorization Methods . . . . . . . . . . . . . . . . 170 219 A.4. Dynamically Generated Home Addresses . . . . . . . . . . 170 220 A.5. Remote Home Address Configuration . . . . . . . . . . . . 170 221 A.6. Neighbor Discovery Extensions . . . . . . . . . . . . . . 171 222 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 173 223 Intellectual Property and Copyright Statements . . . . . . . . . 174 225 1. Introduction 227 This document specifies a protocol which allows nodes to remain 228 reachable while moving around in the IPv6 Internet. Without specific 229 support for mobility in IPv6 [8], packets destined to a mobile node 230 would not be able to reach it while the mobile node is away from its 231 home link. In order to continue communication in spite of its 232 movement, a mobile node could change its IP address each time it 233 moves to a new link, but the mobile node would then not be able to 234 maintain transport and higher-layer connections when it changes 235 location. Mobility support in IPv6 is particularly important, as 236 mobile computers are likely to account for a majority or at least a 237 substantial fraction of the population of the Internet during the 238 lifetime of IPv6. 240 The protocol defined in this document, known as Mobile IPv6, allows a 241 mobile node to move from one link to another without changing the 242 mobile node's "home address". Packets may be routed to the mobile 243 node using this address regardless of the mobile node's current point 244 of attachment to the Internet. The mobile node may also continue to 245 communicate with other nodes (stationary or mobile) after moving to a 246 new link. The movement of a mobile node away from its home link is 247 thus transparent to transport and higher-layer protocols and 248 applications. 250 The Mobile IPv6 protocol is just as suitable for mobility across 251 homogeneous media as for mobility across heterogeneous media. For 252 example, Mobile IPv6 facilitates node movement from one Ethernet 253 segment to another as well as it facilitates node movement from an 254 Ethernet segment to a wireless LAN cell, with the mobile node's IP 255 address remaining unchanged in spite of such movement. 257 One can think of the Mobile IPv6 protocol as solving the network- 258 layer mobility management problem. Some mobility management 259 applications -- for example, handover among wireless transceivers, 260 each of which covers only a very small geographic area -- have been 261 solved using link-layer techniques. For example, in many current 262 wireless LAN products, link-layer mobility mechanisms allow a 263 "handover" of a mobile node from one cell to another, re-establishing 264 link-layer connectivity to the node in each new location. 266 Mobile IPv6 does not attempt to solve all general problems related to 267 the use of mobile computers or wireless networks. In particular, 268 this protocol does not attempt to solve: 270 o Handling links with unidirectional connectivity or partial 271 reachability, such as the hidden terminal problem where a host is 272 hidden from only some of the routers on the link. 274 o Access control on a link being visited by a mobile node. 276 o Local or hierarchical forms of mobility management (similar to 277 many current link-layer mobility management solutions). 279 o Assistance for adaptive applications. 281 o Mobile routers. 283 o Service Discovery. 285 o Distinguishing between packets lost due to bit errors vs. network 286 congestion. 288 2. Comparison with Mobile IP for IPv4 290 The design of Mobile IP support in IPv6 (Mobile IPv6) benefits both 291 from the experiences gained from the development of Mobile IP support 292 in IPv4 (Mobile IPv4) [33] [27] [28], and from the opportunities 293 provided by IPv6. Mobile IPv6 thus shares many features with Mobile 294 IPv4, but is integrated into IPv6 and offers many other improvements. 295 This section summarizes the major differences between Mobile IPv4 and 296 Mobile IPv6: 298 o There is no need to deploy special routers as "foreign agents", as 299 in Mobile IPv4. Mobile IPv6 operates in any location without any 300 special support required from the local router. 302 o Support for route optimization is a fundamental part of the 303 protocol, rather than a nonstandard set of extensions. 305 o Mobile IPv6 route optimization can operate securely even without 306 pre-arranged security associations. It is expected that route 307 optimization can be deployed on a global scale between all mobile 308 nodes and correspondent nodes. 310 o Support is also integrated into Mobile IPv6 for allowing route 311 optimization to coexist efficiently with routers that perform 312 "ingress filtering" [30]. 314 o The IPv6 Neighbor Unreachability Detection assures symmetric 315 reachability between the mobile node and its default router in the 316 current location. 318 o Most packets sent to a mobile node while away from home in Mobile 319 IPv6 are sent using an IPv6 routing header rather than IP 320 encapsulation, reducing the amount of resulting overhead compared 321 to Mobile IPv4. 323 o Mobile IPv6 is decoupled from any particular link layer, as it 324 uses IPv6 Neighbor Discovery [20] instead of ARP. This also 325 improves the robustness of the protocol. 327 o The use of IPv6 encapsulation (and the routing header) removes the 328 need in Mobile IPv6 to manage "tunnel soft state". 330 o The dynamic home agent address discovery mechanism in Mobile IPv6 331 returns a single reply to the mobile node. The directed broadcast 332 approach used in IPv4 returns separate replies from each home 333 agent. 335 3. Terminology 337 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 338 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 339 document are to be interpreted as described in RFC 2119 [1]. 341 3.1. General Terms 343 IP 345 Internet Protocol Version 6 (IPv6). 347 node 349 A device that implements IP. 351 router 353 A node that forwards IP packets not explicitly addressed to 354 itself. 356 unicast routable address 358 An identifier for a single interface such that a packet sent to it 359 from another IPv6 subnet is delivered to the interface identified 360 by that address. Accordingly, a unicast routable address must 361 either be global IPv6 address or a unique local IPv6 address. 363 host 365 Any node that is not a router. 367 link 369 A communication facility or medium over which nodes can 370 communicate at the link layer, such as an Ethernet (simple or 371 bridged). A link is the layer immediately below IP. 373 interface 375 A node's attachment to a link. 377 subnet prefix 379 A bit string that consists of some number of initial bits of an IP 380 address. 382 interface identifier 384 A number used to identify a node's interface on a link. The 385 interface identifier is the remaining low-order bits in the node's 386 IP address after the subnet prefix. 388 link-layer address 390 A link-layer identifier for an interface, such as IEEE 802 391 addresses on Ethernet links. 393 packet 395 An IP header plus payload. 397 security association 399 An IPsec security association is a cooperative relationship formed 400 by the sharing of cryptographic keying material and associated 401 context. Security associations are simplex. That is, two 402 security associations are needed to protect bidirectional traffic 403 between two nodes, one for each direction. 405 security policy database 407 A database that specifies what security services are to be offered 408 to IP packets and in what fashion. 410 destination option 412 Destination options are carried by the IPv6 Destination Options 413 extension header. Destination options include optional 414 information that need be examined only by the IPv6 node given as 415 the destination address in the IPv6 header, not by routers in 416 between. Mobile IPv6 defines one new destination option, the Home 417 Address destination option (see Section 6.3). 419 routing header 421 A routing header may be present as an IPv6 header extension, and 422 indicates that the payload has to be delivered to a destination 423 IPv6 address in some way that is different from what would be 424 carried out by standard Internet routing. In this document, use 425 of the term "routing header" typically refers to use of a type 2 426 routing header, as specified in Section 6.4. 428 "|" (concatenation) 430 Some formulas in this specification use the symbol "|" to indicate 431 bytewise concatenation, as in A | B. This concatenation requires 432 that all of the octets of the datum A appear first in the result, 433 followed by all of the octets of the datum B. 435 First (size, input) 437 Some formulas in this specification use a functional form "First 438 (size, input)" to indicate truncation of the "input" data so that 439 only the first "size" bits remain to be used. 441 3.2. Mobile IPv6 Terms 443 home address 445 A unicast routable address assigned to a mobile node, used as the 446 permanent address of the mobile node. This address is within the 447 mobile node's home link. Standard IP routing mechanisms will 448 deliver packets destined for a mobile node's home address to its 449 home link. Mobile nodes can have multiple home addresses, for 450 instance when there are multiple home prefixes on the home link. 452 home subnet prefix 454 The IP subnet prefix corresponding to a mobile node's home 455 address. 457 home link 459 The link on which a mobile node's home subnet prefix is defined. 461 mobile node 463 A node that can change its point of attachment from one link to 464 another, while still being reachable via its home address. 466 movement 468 A change in a mobile node's point of attachment to the Internet 469 such that it is no longer connected to the same link as it was 470 previously. If a mobile node is not currently attached to its 471 home link, the mobile node is said to be "away from home". 473 L2 handover 475 A process by which the mobile node changes from one link-layer 476 connection to another. For example, a change of wireless access 477 point is an L2 handover. 479 L3 handover 481 Subsequent to an L2 handover, a mobile node detects a change in an 482 on-link subnet prefix that would require a change in the primary 483 care-of address. For example, a change of access router 484 subsequent to a change of wireless access point typically results 485 in an L3 handover. 487 correspondent node 489 A peer node with which a mobile node is communicating. The 490 correspondent node may be either mobile or stationary. 492 foreign subnet prefix 494 Any IP subnet prefix other than the mobile node's home subnet 495 prefix. 497 foreign link 499 Any link other than the mobile node's home link. 501 care-of address 503 A unicast routable address associated with a mobile node while 504 visiting a foreign link; the subnet prefix of this IP address is a 505 foreign subnet prefix. Among the multiple care-of addresses that 506 a mobile node may have at any given time (e.g., with different 507 subnet prefixes), the one registered with the mobile node's home 508 agent for a given home address is called its "primary" care-of 509 address. 511 home agent 513 A router on a mobile node's home link with which the mobile node 514 has registered its current care-of address. While the mobile node 515 is away from home, the home agent intercepts packets on the home 516 link destined to the mobile node's home address, encapsulates 517 them, and tunnels them to the mobile node's registered care-of 518 address. 520 binding 522 The association of the home address of a mobile node with a 523 care-of address for that mobile node, along with the remaining 524 lifetime of that association. 526 registration 528 The process during which a mobile node sends a Binding Update to 529 its home agent or a correspondent node, causing a binding for the 530 mobile node to be registered. 532 mobility message 534 A message containing a Mobility Header (see Section 6.1). 536 binding authorization 538 Correspondent registration needs to be authorized to allow the 539 recipient to believe that the sender has the right to specify a 540 new binding. 542 return routability procedure 544 The return routability procedure authorizes registrations by the 545 use of a cryptographic token exchange. 547 correspondent registration 549 A return routability procedure followed by a registration, run 550 between the mobile node and a correspondent node. 552 home registration 554 A registration between the mobile node and its home agent, 555 authorized by the use of IPsec. 557 nonce 559 Nonces are random numbers used internally by the correspondent 560 node in the creation of keygen tokens related to the return 561 routability procedure. The nonces are not specific to a mobile 562 node, and are kept secret within the correspondent node. 564 nonce index 566 A nonce index is used to indicate which nonces have been used when 567 creating keygen token values, without revealing the nonces 568 themselves. 570 cookie 572 A cookie is a random number used by a mobile node to prevent 573 spoofing by a bogus correspondent node in the return routability 574 procedure. 576 care-of init cookie 578 A cookie sent to the correspondent node in the Care-of Test Init 579 message, to be returned in the Care-of Test message. 581 home init cookie 583 A cookie sent to the correspondent node in the Home Test Init 584 message, to be returned in the Home Test message. 586 keygen token 588 A keygen token is a number supplied by a correspondent node in the 589 return routability procedure to enable the mobile node to compute 590 the necessary binding management key for authorizing a Binding 591 Update. 593 care-of keygen token 595 A keygen token sent by the correspondent node in the Care-of Test 596 message. 598 home keygen token 600 A keygen token sent by the correspondent node in the Home Test 601 message. 603 binding management key (Kbm) 605 A binding management key (Kbm) is a key used for authorizing a 606 binding cache management message (e.g., Binding Update or Binding 607 Acknowledgement). Return routability provides a way to create a 608 binding management key. 610 4. Overview of Mobile IPv6 612 4.1. Basic Operation 614 A mobile node is always expected to be addressable at its home 615 address, whether it is currently attached to its home link or is away 616 from home. The "home address" is an IP address assigned to the 617 mobile node within its home subnet prefix on its home link. While a 618 mobile node is at home, packets addressed to its home address are 619 routed to the mobile node's home link, using conventional Internet 620 routing mechanisms. 622 While a mobile node is attached to some foreign link away from home, 623 it is also addressable at one or more care-of addresses. A care-of 624 address is an IP address associated with a mobile node that has the 625 subnet prefix of a particular foreign link. The mobile node can 626 acquire its care-of address through conventional IPv6 mechanisms, 627 such as stateless or stateful auto-configuration. As long as the 628 mobile node stays in this location, packets addressed to this care-of 629 address will be routed to the mobile node. The mobile node may also 630 accept packets from several care-of addresses, such as when it is 631 moving but still reachable at the previous link. 633 The association between a mobile node's home address and care-of 634 address is known as a "binding" for the mobile node. While away from 635 home, a mobile node registers its primary care-of address with a 636 router on its home link, requesting this router to function as the 637 "home agent" for the mobile node. The mobile node performs this 638 binding registration by sending a "Binding Update" message to the 639 home agent. The home agent replies to the mobile node by returning a 640 "Binding Acknowledgement" message. The operation of the mobile node 641 is specified in Section 11, and the operation of the home agent is 642 specified in Section 10. 644 Any node communicating with a mobile node is referred to in this 645 document as a "correspondent node" of the mobile node, and may itself 646 be either a stationary node or a mobile node. Mobile nodes can 647 provide information about their current location to correspondent 648 nodes. This happens through the correspondent registration. As a 649 part of this procedure, a return routability test is performed in 650 order to authorize the establishment of the binding. The operation 651 of the correspondent node is specified in Section 9. 653 There are two possible modes for communications between the mobile 654 node and a correspondent node. The first mode, bidirectional 655 tunneling, does not require Mobile IPv6 support from the 656 correspondent node and is available even if the mobile node has not 657 registered its current binding with the correspondent node. Packets 658 from the correspondent node are routed to the home agent and then 659 tunneled to the mobile node. Packets to the correspondent node are 660 tunneled from the mobile node to the home agent ("reverse tunneled") 661 and then routed normally from the home network to the correspondent 662 node. In this mode, the home agent uses proxy Neighbor Discovery to 663 intercept any IPv6 packets addressed to the mobile node's home 664 address (or home addresses) on the home link. Each intercepted 665 packet is tunneled to the mobile node's primary care-of address. 666 This tunneling is performed using IPv6 encapsulation [9]. 668 The second mode, "route optimization", requires the mobile node to 669 register its current binding at the correspondent node. Packets from 670 the correspondent node can be routed directly to the care-of address 671 of the mobile node. When sending a packet to any IPv6 destination, 672 the correspondent node checks its cached bindings for an entry for 673 the packet's destination address. If a cached binding for this 674 destination address is found, the node uses a new type of IPv6 675 routing header [8] (see Section 6.4) to route the packet to the 676 mobile node by way of the care-of address indicated in this binding. 678 Routing packets directly to the mobile node's care-of address allows 679 the shortest communications path to be used. It also eliminates 680 congestion at the mobile node's home agent and home link. In 681 addition, the impact of any possible failure of the home agent or 682 networks on the path to or from it is reduced. 684 When routing packets directly to the mobile node, the correspondent 685 node sets the Destination Address in the IPv6 header to the care-of 686 address of the mobile node. A new type of IPv6 routing header (see 687 Section 6.4) is also added to the packet to carry the desired home 688 address. Similarly, the mobile node sets the Source Address in the 689 packet's IPv6 header to its current care-of addresses. The mobile 690 node adds a new IPv6 "Home Address" destination option (see 691 Section 6.3) to carry its home address. The inclusion of home 692 addresses in these packets makes the use of the care-of address 693 transparent above the network layer (e.g., at the transport layer). 695 Mobile IPv6 also provides support for multiple home agents, and a 696 limited support for the reconfiguration of the home network. In 697 these cases, the mobile node may not know the IP address of its own 698 home agent, and even the home subnet prefixes may change over time. 699 A mechanism, known as "dynamic home agent address discovery" allows a 700 mobile node to dynamically discover the IP address of a home agent on 701 its home link, even when the mobile node is away from home. Mobile 702 nodes can also learn new information about home subnet prefixes 703 through the "mobile prefix discovery" mechanism. These mechanisms 704 are described starting from Section 6.5. 706 This document assumes that the mobile node is configured with the 707 home prefix for the mobile node to be able to discover a home agent 708 and configure a home address. This might be limiting in deployments 709 where the home agent and the home address for the mobile node needs 710 to be assigned dynamically. Additional mechanisms have been 711 specified for the mobile node to dynamically configure a home agent, 712 a home address and the home prefix. These mechanisms are described 713 in "Mobile IPv6 Bootstrapping in Split Scenario" [25] and "MIP6 714 bootstrapping for the Integrated Scenario" [37]. 716 4.2. New IPv6 Protocol 718 Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header 719 (see Section 6.1). This Header is used to carry the following 720 messages: 722 Home Test Init 724 Home Test 726 Care-of Test Init 728 Care-of Test 730 These four messages are used to perform the return routability 731 procedure from the mobile node to a correspondent node. This 732 ensures authorization of subsequent Binding Updates, as described 733 in Section 5.2.5. 735 Binding Update 737 A Binding Update is used by a mobile node to notify a 738 correspondent node or the mobile node's home agent of its current 739 binding. The Binding Update sent to the mobile node's home agent 740 to register its primary care-of address is marked as a "home 741 registration". 743 Binding Acknowledgement 745 A Binding Acknowledgement is used to acknowledge receipt of a 746 Binding Update, if an acknowledgement was requested in the Binding 747 Update, the binding update was sent to a home agent, or an error 748 occurred. 750 Binding Refresh Request 752 A Binding Refresh Request is used by a correspondent node to 753 request a mobile node to re-establish its binding with the 754 correspondent node. This message is typically used when the 755 cached binding is in active use but the binding's lifetime is 756 close to expiration. The correspondent node may use, for 757 instance, recent traffic and open transport layer connections as 758 an indication of active use. 760 Binding Error 762 The Binding Error is used by the correspondent node or the home 763 agent to signal an error related to mobility, such as an 764 inappropriate attempt to use the Home Address destination option 765 without an existing binding, or when an unrecognized Mobility 766 Header is received. 768 4.3. New IPv6 Destination Option 770 Mobile IPv6 defines a new IPv6 destination option, the Home Address 771 destination option. This option is described in detail in 772 Section 6.3. 774 4.4. New IPv6 ICMP Messages 776 Mobile IPv6 also introduces four new ICMP message types, two for use 777 in the dynamic home agent address discovery mechanism, and two for 778 renumbering and mobile configuration mechanisms. As described in 779 Section 10.5 and Section 11.4.1, the following two new ICMP message 780 types are used for home agent address discovery: 782 o Home Agent Address Discovery Request, described in Section 6.5. 784 o Home Agent Address Discovery Reply, described in Section 6.6. 786 The next two message types are used for network renumbering and 787 address configuration on the mobile node, as described in 788 Section 10.6: 790 o Mobile Prefix Solicitation, described in Section 6.7. 792 o Mobile Prefix Advertisement, described in Section 6.8. 794 4.5. Conceptual Data Structure Terminology 796 This document describes the Mobile IPv6 protocol in terms of the 797 following conceptual data structures: 799 Binding Cache 801 A cache of bindings for other nodes. This cache is maintained by 802 home agents and correspondent nodes. The cache contains both 803 "correspondent registration" entries (see Section 9.1) and "home 804 registration" entries (see Section 10.1). 806 Binding Update List 808 This list is maintained by each mobile node. The list has an item 809 for every binding that the mobile node has or is trying to 810 establish with a specific other node. Both correspondent and home 811 registrations are included in this list. Entries from the list 812 are deleted as the lifetime of the binding expires. See 813 Section 11.1. 815 Home Agents List 817 Home agents need to know which other home agents are on the same 818 link. This information is stored in the Home Agents List, as 819 described in more detail in Section 10.1. The list is used for 820 informing mobile nodes during dynamic home agent address 821 discovery. 823 4.6. Unique-Local Addressability 825 This specification requires that home and care-of addresses MUST be 826 unicast routable addresses. Unique-local IPv6 unicast addresses 827 RFC4193 [22] may be usable on networks that use such non-globaly 828 routable addresses but this specification does not define when such 829 usage is safe and when it is not. Mobile nodes may not be aware of 830 which site they are currently making it hard to prevent accidental 831 attachment to other sites, resulting in possible unrechability 832 between the MN and the HA, when unique-local IPv6 routable addresses 833 are used as care-of addresses. Also, CNs outside the MN's own site 834 are not going to be reachable when unique-local IPv6 routable 835 addresses are used as home addresses. Therefore, unique-local IPv6 836 unicast addresses SHOULD NOT be used as home or care-of addresses. 837 If such addresses are used, however, according to RFC4193 [22], they 838 are treated as any global unicast IPv6 address so, for the remainder 839 of this specification, use of unique-local IPv6 unicast addresses is 840 not differentiated from other globally unique IPv6 addresses. 842 5. Overview of Mobile IPv6 Security 844 This specification provides a number of security features. These 845 include the protection of Binding Updates both to home agents and 846 correspondent nodes, the protection of mobile prefix discovery, and 847 the protection of the mechanisms that Mobile IPv6 uses for 848 transporting data packets. 850 Binding Updates are protected by the use of IPsec extension headers, 851 or by the use of the Binding Authorization Data option. This option 852 employs a binding management key, Kbm, which can be established 853 through the return routability procedure. Mobile prefix discovery is 854 protected through the use of IPsec extension headers. Mechanisms 855 related to transporting payload packets - such as the Home Address 856 destination option and type 2 routing header - have been specified in 857 a manner which restricts their use in attacks. 859 5.1. Binding Updates to Home Agents 861 The mobile node and the home agent MUST use an IPsec security 862 association to protect the integrity and authenticity of the Binding 863 Updates and Acknowledgements. Both the mobile nodes and the home 864 agents MUST support and SHOULD use the Encapsulating Security Payload 865 (ESP) [4] header in transport mode and MUST use a non-NULL payload 866 authentication algorithm to provide data origin authentication, 867 connectionless integrity and optional anti-replay protection. Note 868 that Authentication Header (AH) [3] is also possible but for brevity 869 not discussed in this specification. 871 In order to protect messages exchanged between the mobile node and 872 the home agent with IPsec, appropriate security policy database 873 entries must be created. A mobile node must be prevented from using 874 its security association to send a Binding Update on behalf of 875 another mobile node using the same home agent. This MUST be achieved 876 by having the home agent check that the given home address has been 877 used with the right security association. Such a check is provided 878 in the IPsec processing, by having the security policy database 879 entries unequivocally identify a single security association for 880 protecting Binding Updates between any given home address and home 881 agent. In order to make this possible, it is necessary that the home 882 address of the mobile node is visible in the Binding Updates and 883 Acknowledgements. The home address is used in these packets as a 884 source or destination, or in the Home Address Destination option or 885 the type 2 routing header. 887 As with all IPsec security associations in this specification, manual 888 configuration of security associations MUST be supported. The used 889 shared secrets MUST be random and unique for different mobile nodes, 890 and MUST be distributed off-line to the mobile nodes. 892 Automatic key management with IKE [7] MAY be supported. When IKE is 893 used, either the security policy database entries or the Mobile IPv6 894 processing MUST unequivocally identify the IKE phase 1 credentials 895 which can be used to authorize the creation of security associations 896 for protecting Binding Updates for a particular home address. How 897 these mappings are maintained is outside the scope of this 898 specification, but they may be maintained, for instance, as a locally 899 administered table in the home agent. If the phase 1 identity is a 900 Fully Qualified Domain Name (FQDN), secure forms of DNS may also be 901 used. 903 Section 11.3.2 discusses how IKE connections to the home agent need a 904 careful treatment of the addresses used for transporting IKE. This 905 is necessary to ensure that a Binding Update is not needed before the 906 IKE exchange which is needed for securing the Binding Update. 908 When IKE version 1 is used with preshared secret authentication 909 between the mobile node and the home agent, aggressive mode MUST be 910 used. 912 The ID_IPV6_ADDR Identity Payload MUST NOT be used in IKEv1 phase 1. 914 Reference [15] contains a more detailed description and examples on 915 using IPsec to protect the communications between the mobile node and 916 the home agent. 918 5.2. Binding Updates to Correspondent Nodes 920 The protection of Binding Updates sent to correspondent nodes does 921 not require the configuration of security associations or the 922 existence of an authentication infrastructure between the mobile 923 nodes and correspondent nodes. Instead, a method called the return 924 routability procedure is used to assure that the right mobile node is 925 sending the message. This method does not protect against attackers 926 who are on the path between the home network and the correspondent 927 node. However, attackers in such a location are capable of 928 performing the same attacks even without Mobile IPv6. The main 929 advantage of the return routability procedure is that it limits the 930 potential attackers to those having an access to one specific path in 931 the Internet, and avoids forged Binding Updates from anywhere else in 932 the Internet. For a more in depth explanation of the security 933 properties of the return routability procedure, see Section 15. 935 The integrity and authenticity of the Binding Updates messages to 936 correspondent nodes is protected by using a keyed-hash algorithm. 937 The binding management key, Kbm, is used to key the hash algorithm 938 for this purpose. Kbm is established using data exchanged during the 939 return routability procedure. The data exchange is accomplished by 940 use of node keys, nonces, cookies, tokens, and certain cryptographic 941 functions. Section 5.2.5 outlines the basic return routability 942 procedure. Section 5.2.6 shows how the results of this procedure are 943 used to authorize a Binding Update to a correspondent node. 945 5.2.1. Node Keys 947 Each correspondent node has a secret key, Kcn, called the "node key", 948 which it uses to produce the keygen tokens sent to the mobile nodes. 949 The node key MUST be a random number, 20 octets in length. The node 950 key allows the correspondent node to verify that the keygen tokens 951 used by the mobile node in authorizing a Binding Update are indeed 952 its own. This key MUST NOT be shared with any other entity. 954 A correspondent node MAY generate a fresh node key at any time; this 955 avoids the need for secure persistent key storage. Procedures for 956 optionally updating the node key are discussed later in 957 Section 5.2.7. 959 5.2.2. Nonces 961 Each correspondent node also generates nonces at regular intervals. 962 The nonces should be generated by using a random number generator 963 that is known to have good randomness properties [17]. A 964 correspondent node may use the same Kcn and nonce with all the 965 mobiles it is in communication with. 967 Each nonce is identified by a nonce index. When a new nonce is 968 generated, it must be associated with a new nonce index; this may be 969 done, for example, by incrementing the value of the previous nonce 970 index, if the nonce index is used as an array pointer into a linear 971 array of nonces. However, there is no requirement that nonces be 972 stored that way, or that the values of subsequent nonce indices have 973 any particular relationship to each other. The index value is 974 communicated in the protocol, so that if a nonce is replaced by new 975 nonce during the run of a protocol, the correspondent node can 976 distinguish messages that should be checked against the old nonce 977 from messages that should be checked against the new nonce. Strictly 978 speaking, indices are not necessary in the authentication, but allow 979 the correspondent node to efficiently find the nonce value that it 980 used in creating a keygen token. 982 Correspondent nodes keep both the current nonce and a small set of 983 valid previous nonces whose lifetime has not yet expired. Expired 984 values MUST be discarded, and messages using stale or unknown indices 985 will be rejected. 987 The specific nonce index values cannot be used by mobile nodes to 988 determine the validity of the nonce. Expected validity times for the 989 nonces values and the procedures for updating them are discussed 990 later in Section 5.2.7. 992 A nonce is an octet string of any length. The recommended length is 993 64 bits. 995 5.2.3. Cookies and Tokens 997 The return routability address test procedure uses cookies and keygen 998 tokens as opaque values within the test init and test messages, 999 respectively. 1001 o The "home init cookie" and "care-of init cookie" are 64 bit values 1002 sent to the correspondent node from the mobile node, and later 1003 returned to the mobile node. The home init cookie is sent in the 1004 Home Test Init message, and returned in the Home Test message. 1005 The care-of init cookie is sent in the Care-of Test Init message, 1006 and returned in the Care-of Test message. 1008 o The "home keygen token" and "care-of keygen token" are 64-bit 1009 values sent by the correspondent node to the mobile node via the 1010 home agent (via the Home Test message) and the care-of address (by 1011 the Care-of Test message), respectively. 1013 The mobile node should set the home init or care-of init cookie to a 1014 newly generated random number in every Home or Care-of Test Init 1015 message it sends. The cookies are used to verify that the Home Test 1016 or Care-of Test message matches the Home Test Init or Care-of Test 1017 Init message, respectively. These cookies also serve to ensure that 1018 parties who have not seen the request cannot spoof responses. 1020 Home and care-of keygen tokens are produced by the correspondent node 1021 based on its currently active secret key (Kcn) and nonces, as well as 1022 the home or care-of address (respectively). A keygen token is valid 1023 as long as both the secret key (Kcn) and the nonce used to create it 1024 are valid. 1026 5.2.4. Cryptographic Functions 1028 In this specification, the function used to compute hash values is 1029 SHA1 [14]. Message Authentication Codes (MACs) are computed using 1030 HMAC_SHA1 [29] [14]. HMAC_SHA1(K,m) denotes such a MAC computed on 1031 message m with key K. 1033 5.2.5. Return Routability Procedure 1035 The Return Routability Procedure enables the correspondent node to 1036 obtain some reasonable assurance that the mobile node is in fact 1037 addressable at its claimed care-of address as well as at its home 1038 address. Only with this assurance is the correspondent node able to 1039 accept Binding Updates from the mobile node which would then instruct 1040 the correspondent node to direct that mobile node's data traffic to 1041 its claimed care-of address. 1043 This is done by testing whether packets addressed to the two claimed 1044 addresses are routed to the mobile node. The mobile node can pass 1045 the test only if it is able to supply proof that it received certain 1046 data (the "keygen tokens") which the correspondent node sends to 1047 those addresses. These data are combined by the mobile node into a 1048 binding management key, denoted Kbm. 1050 The figure below shows the message flow for the return routability 1051 procedure. 1053 Mobile node Home agent Correspondent node 1054 | | 1055 | Home Test Init (HoTI) | | 1056 |------------------------->|------------------------->| 1057 | | | 1058 | Care-of Test Init (CoTI) | 1059 |---------------------------------------------------->| 1060 | | 1061 | | Home Test (HoT) | 1062 |<-------------------------|<-------------------------| 1063 | | | 1064 | Care-of Test (CoT) | 1065 |<----------------------------------------------------| 1066 | | 1068 The Home and Care-of Test Init messages are sent at the same time. 1069 The procedure requires very little processing at the correspondent 1070 node, and the Home and Care-of Test messages can be returned quickly, 1071 perhaps nearly simultaneously. These four messages form the return 1072 routability procedure. 1074 Home Test Init 1076 A mobile node sends a Home Test Init message to the correspondent 1077 node (via the home agent) to acquire the home keygen token. The 1078 contents of the message can be summarized as follows: 1080 * Source Address = home address 1082 * Destination Address = correspondent 1084 * Parameters: 1086 + home init cookie 1088 The Home Test Init message conveys the mobile node's home address 1089 to the correspondent node. The mobile node also sends along a 1090 home init cookie that the correspondent node must return later. 1091 The Home Test Init message is reverse tunneled through the home 1092 agent. (The headers and addresses related to reverse tunneling 1093 have been omitted from the above discussion of the message 1094 contents.) The mobile node remembers these cookie values to 1095 obtain some assurance that its protocol messages are being 1096 processed by the desired correspondent node. 1098 Care-of Test Init 1100 The mobile node sends a Care-of Test Init message to the 1101 correspondent node (directly, not via the home agent) to acquire 1102 the care-of keygen token. The contents of this message can be 1103 summarized as follows: 1105 * Source Address = care-of address 1107 * Destination Address = correspondent 1109 * Parameters: 1111 + care-of init cookie 1113 The Care-of Test Init message conveys the mobile node's care-of 1114 address to the correspondent node. The mobile node also sends 1115 along a care-of init cookie that the correspondent node must 1116 return later. The Care-of Test Init message is sent directly to 1117 the correspondent node. 1119 Home Test 1121 The Home Test message is sent in response to a Home Test Init 1122 message. It is sent via the home agent. The contents of the 1123 message are: 1125 * Source Address = correspondent 1127 * Destination Address = home address 1129 * Parameters: 1131 + home init cookie 1133 + home keygen token 1135 + home nonce index 1137 When the correspondent node receives the Home Test Init message, 1138 it generates a home keygen token as follows: 1140 home keygen token := 1141 First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0))) 1143 where | denotes concatenation. The final "0" inside the HMAC_SHA1 1144 function is a single zero octet, used to distinguish home and 1145 care-of cookies from each other. 1147 The home keygen token is formed from the first 64 bits of the MAC. 1148 The home keygen token tests that the mobile node can receive were 1149 messages sent to its home address. Kcn is used in the production 1150 of home keygen token in order to allow the correspondent node to 1151 verify that it generated the home and care-of nonces, without 1152 forcing the correspondent node to remember a list of all tokens it 1153 has handed out. 1155 The Home Test message is sent to the mobile node via the home 1156 network, where it is presumed that the home agent will tunnel the 1157 message to the mobile node. This means that the mobile node needs 1158 to already have sent a Binding Update to the home agent, so that 1159 the home agent will have received and authorized the new care-of 1160 address for the mobile node before the return routability 1161 procedure. For improved security, the data passed between the 1162 home agent and the mobile node is made immune to inspection and 1163 passive attacks. Such protection is gained by encrypting the home 1164 keygen token as it is tunneled from the home agent to the mobile 1165 node as specified in Section 10.4.6. The security properties of 1166 this additional security are discussed in Section 15.4.1. 1168 The home init cookie from the mobile node is returned in the Home 1169 Test message, to ensure that the message comes from a node on the 1170 route between the home agent and the correspondent node. 1172 The home nonce index is delivered to the mobile node to later 1173 allow the correspondent node to efficiently find the nonce value 1174 that it used in creating the home keygen token. 1176 Care-of Test 1178 This message is sent in response to a Care-of Test Init message. 1179 This message is not sent via the home agent, it is sent directly 1180 to the mobile node. The contents of the message are: 1182 * Source Address = correspondent 1184 * Destination Address = care-of address 1186 * Parameters: 1188 + care-of init cookie 1190 + care-of keygen token 1192 + care-of nonce index 1194 When the correspondent node receives the Care-of Test Init 1195 message, it generates a care-of keygen token as follows: 1197 care-of keygen token := 1198 First (64, HMAC_SHA1 (Kcn, (care-of address | nonce | 1))) 1200 Here, the final "1" inside the HMAC_SHA1 function is a single 1201 octet containing the hex value 0x01, and is used to distinguish 1202 home and care-of cookies from each other. The keygen token is 1203 formed from the first 64 bits of the MAC, and sent directly to the 1204 mobile node at its care-of address. The care-of init cookie from 1205 the Care-of Test Init message is returned to ensure that the 1206 message comes from a node on the route to the correspondent node. 1208 The care-of nonce index is provided to identify the nonce used for 1209 the care-of keygen token. The home and care-of nonce indices MAY 1210 be the same, or different, in the Home and Care-of Test messages. 1212 When the mobile node has received both the Home and Care-of Test 1213 messages, the return routability procedure is complete. As a result 1214 of the procedure, the mobile node has the data it needs to send a 1215 Binding Update to the correspondent node. The mobile node hashes the 1216 tokens together to form a 20 octet binding key Kbm: 1218 Kbm = SHA1 (home keygen token | care-of keygen token) 1220 A Binding Update may also be used to delete a previously established 1221 binding (Section 6.1.7). In this case, the care-of keygen token is 1222 not used. Instead, the binding management key is generated as 1223 follows: 1225 Kbm = SHA1(home keygen token) 1227 Note that the correspondent node does not create any state specific 1228 to the mobile node, until it receives the Binding Update from that 1229 mobile node. The correspondent node does not maintain the value for 1230 the binding management key Kbm; it creates Kbm when given the nonce 1231 indices and the mobile node's addresses. 1233 5.2.6. Authorizing Binding Management Messages 1235 After the mobile node has created the binding management key (Kbm), 1236 it can supply a verifiable Binding Update to the correspondent node. 1237 This section provides an overview of this registration. The below 1238 figure shows the message flow. 1240 Mobile node Correspondent node 1241 | | 1242 | Binding Update (BU) | 1243 |---------------------------------------------->| 1244 | (MAC, seq#, nonce indices, care-of address) | 1245 | | 1246 | | 1247 | Binding Acknowledgement (BA) (if sent) | 1248 |<----------------------------------------------| 1249 | (MAC, seq#, status) | 1251 Binding Update 1253 To authorize a Binding Update, the mobile node creates a binding 1254 management key Kbm from the keygen tokens as described in the 1255 previous section. The contents of the Binding Update include the 1256 following: 1258 * Source Address = care-of address 1260 * Destination Address = correspondent 1262 * Parameters: 1264 + home address (within the Home Address destination option if 1265 different from the Source Address) 1267 + sequence number (within the Binding Update message header) 1269 + home nonce index (within the Nonce Indices option) 1271 + care-of nonce index (within the Nonce Indices option) 1273 + First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent 1274 | BU))) 1276 The Binding Update contains a Nonce Indices option, indicating to 1277 the correspondent node which home and care-of nonces to use to 1278 recompute Kbm, the binding management key. The MAC is computed as 1279 described in Section 6.2.7, using the correspondent node's address 1280 as the destination address and the Binding Update message itself 1281 ("BU" above) as the MH Data. 1283 Once the correspondent node has verified the MAC, it can create a 1284 Binding Cache entry for the mobile. 1286 Binding Acknowledgement 1288 The Binding Update is in some cases acknowledged by the 1289 correspondent node. The contents of the message are as follows: 1291 * Source Address = correspondent 1293 * Destination Address = care-of address 1295 * Parameters: 1297 + sequence number (within the Binding Update message header) 1299 + First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent 1300 | BA))) 1302 The Binding Acknowledgement contains the same sequence number as 1303 the Binding Update. The MAC is computed as described in 1304 Section 6.2.7, using the correspondent node's address as the 1305 destination address and the message itself ("BA" above) as the MH 1306 Data. 1308 Bindings established with correspondent nodes using keys created by 1309 way of the return routability procedure MUST NOT exceed 1310 MAX_RR_BINDING_LIFETIME seconds (see Section 12). 1312 The value in the Source Address field in the IPv6 header carrying the 1313 Binding Update is normally also the care-of address which is used in 1314 the binding. However, a different care-of address MAY be specified 1315 by including an Alternate Care-of Address mobility option in the 1316 Binding Update (see Section 6.2.5). When such a message is sent to 1317 the correspondent node and the return routability procedure is used 1318 as the authorization method, the Care-of Test Init and Care-of Test 1319 messages MUST have been performed for the address in the Alternate 1320 Care-of Address option (not the Source Address). The nonce indices 1321 and MAC value MUST be based on information gained in this test. 1323 Binding Updates may also be sent to delete a previously established 1324 binding. In this case, generation of the binding management key 1325 depends exclusively on the home keygen token and the care-of nonce 1326 index is ignored. 1328 5.2.7. Updating Node Keys and Nonces 1330 Correspondent nodes generate nonces at regular intervals. It is 1331 recommended to keep each nonce (identified by a nonce index) 1332 acceptable for at least MAX_TOKEN_LIFETIME seconds (see Section 12) 1333 after it has been first used in constructing a return routability 1334 message response. However, the correspondent node MUST NOT accept 1335 nonces beyond MAX_NONCE_LIFETIME seconds (see Section 12) after the 1336 first use. As the difference between these two constants is 30 1337 seconds, a convenient way to enforce the above lifetimes is to 1338 generate a new nonce every 30 seconds. The node can then continue to 1339 accept tokens that have been based on the last 8 (MAX_NONCE_LIFETIME 1340 / 30) nonces. This results in tokens being acceptable 1341 MAX_TOKEN_LIFETIME to MAX_NONCE_LIFETIME seconds after they have been 1342 sent to the mobile node, depending on whether the token was sent at 1343 the beginning or end of the first 30 second period. Note that the 1344 correspondent node may also attempt to generate new nonces on demand, 1345 or only if the old nonces have been used. This is possible, as long 1346 as the correspondent node keeps track of how long a time ago the 1347 nonces were used for the first time, and does not generate new nonces 1348 on every return routability request. 1350 Due to resource limitations, rapid deletion of bindings, or reboots 1351 the correspondent node may not in all cases recognize the nonces that 1352 the tokens were based on. If a nonce index is unrecognized, the 1353 correspondent node replies with an error code in the Binding 1354 Acknowledgement (either 136, 137, or 138 as discussed in 1355 Section 6.1.8). The mobile node can then retry the return 1356 routability procedure. 1358 An update of Kcn SHOULD be done at the same time as an update of a 1359 nonce, so that nonce indices can identify both the nonce and the key. 1360 Old Kcn values have to be therefore remembered as long as old nonce 1361 values. 1363 Given that the tokens are normally expected to be usable for 1364 MAX_TOKEN_LIFETIME seconds, the mobile node MAY use them beyond a 1365 single run of the return routability procedure until 1366 MAX_TOKEN_LIFETIME expires. After this the mobile node SHOULD NOT 1367 use the tokens. A fast moving mobile node MAY reuse a recent home 1368 keygen token from a correspondent node when moving to a new location, 1369 and just acquire a new care-of keygen token to show routability in 1370 the new location. 1372 While this does not save the number of round-trips due to the 1373 simultaneous processing of home and care-of return routability tests, 1374 there are fewer messages being exchanged, and a potentially long 1375 round-trip through the home agent is avoided. Consequently, this 1376 optimization is often useful. A mobile node that has multiple home 1377 addresses, MAY also use the same care-of keygen token for Binding 1378 Updates concerning all of these addresses. 1380 5.2.8. Preventing Replay Attacks 1382 The return routability procedure also protects the participants 1383 against replayed Binding Updates through the use of the sequence 1384 number and a MAC. Care must be taken when removing bindings at the 1385 correspondent node, however. Correspondent nodes must retain 1386 bindings and the associated sequence number information at least as 1387 long as the nonces used in the authorization of the binding are still 1388 valid. Alternatively, if memory is very constrained, the 1389 correspondent node MAY invalidate the nonces that were used for the 1390 binding being deleted (or some larger group of nonces that they 1391 belong to). This may, however, impact the ability to accept Binding 1392 Updates from mobile nodes that have recently received keygen tokens. 1393 This alternative is therefore recommended only as a last measure. 1395 5.3. Dynamic Home Agent Address Discovery 1397 No security is required for dynamic home agent address discovery. 1399 5.4. Mobile Prefix Discovery 1401 The mobile node and the home agent SHOULD use an IPsec security 1402 association to protect the integrity and authenticity of the Mobile 1403 Prefix Solicitations and Advertisements. Both the mobile nodes and 1404 the home agents MUST support and SHOULD use the Encapsulating 1405 Security Payload (ESP) header in transport mode with a non-NULL 1406 payload authentication algorithm to provide data origin 1407 authentication, connectionless integrity and optional anti-replay 1408 protection. 1410 5.5. Payload Packets 1412 Payload packets exchanged with mobile nodes can be protected in the 1413 usual manner, in the same way as stationary hosts can protect them. 1414 However, Mobile IPv6 introduces the Home Address destination option, 1415 a routing header, and tunneling headers in the payload packets. In 1416 the following we define the security measures taken to protect these, 1417 and to prevent their use in attacks against other parties. 1419 This specification limits the use of the Home Address destination 1420 option to the situation where the correspondent node already has a 1421 Binding Cache entry for the given home address. This avoids the use 1422 of the Home Address option in attacks described in Section 15.1. 1424 Mobile IPv6 uses a Mobile IPv6 specific type of a routing header. 1425 This type provides the necessary functionality but does not open 1426 vulnerabilities discussed in Section 15.1. 1428 Tunnels between the mobile node and the home agent are protected by 1429 ensuring proper use of source addresses, and optional cryptographic 1430 protection. The mobile node verifies that the outer IP address 1431 corresponds to its home agent. The home agent verifies that the 1432 outer IP address corresponds to the current location of the mobile 1433 node (Binding Updates sent to the home agents are secure). The home 1434 agent identifies the mobile node through the source address of the 1435 inner packet. (Typically, this is the home address of the mobile 1436 node, but it can also be a link-local address, as discussed in 1437 Section 10.4.2. To recognize the latter type of addresses, the home 1438 agent requires that the Link-Local Address Compatibility (L) was set 1439 in the Binding Update.) These measures protect the tunnels against 1440 vulnerabilities discussed in Section 15.1. 1442 For traffic tunneled via the home agent, additional IPsec ESP 1443 encapsulation MAY be supported and used. If multicast group 1444 membership control protocols or stateful address autoconfiguration 1445 protocols are supported, payload data protection MUST be supported. 1447 6. New IPv6 Protocol, Message Types, and Destination Option 1449 6.1. Mobility Header 1451 The Mobility Header is an extension header used by mobile nodes, 1452 correspondent nodes, and home agents in all messaging related to the 1453 creation and management of bindings. The subsections within this 1454 section describe the message types that may be sent using the 1455 Mobility Header. 1457 Mobility Header messages MUST NOT be sent with a type 2 routing 1458 header, except as described in Section 9.5.4 for Binding 1459 Acknowledgement. Mobility Header messages also MUST NOT be used with 1460 a Home Address destination option, except as described in 1461 Section 11.7.1 and Section 11.7.2 for Binding Update. Binding Update 1462 List or Binding Cache information (when present) for the destination 1463 MUST NOT be used in sending Mobility Header messages. That is, 1464 Mobility Header messages bypass both the Binding Cache check 1465 described in Section 9.3.2 and the Binding Update List check 1466 described in Section 11.3.1 which are normally performed for all 1467 packets. This applies even to messages sent to or from a 1468 correspondent node which is itself a mobile node. 1470 6.1.1. Format 1472 The Mobility Header is identified by a Next Header value of 135 in 1473 the immediately preceding header, and has the following format: 1475 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1476 | Payload Proto | Header Len | MH Type | Reserved | 1477 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1478 | Checksum | | 1479 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1480 | | 1481 . . 1482 . Message Data . 1483 . . 1484 | | 1485 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1487 Payload Proto 1489 8-bit selector. Identifies the type of header immediately 1490 following the Mobility Header. Uses the same values as the IPv6 1491 Next Header field [8]. 1493 This field is intended to be used by a future extension (see 1494 Appendix A.1). 1496 Implementations conforming to this specification SHOULD set the 1497 payload protocol type to IPPROTO_NONE (59 decimal). 1499 Header Len 1501 8-bit unsigned integer, representing the length of the Mobility 1502 Header in units of 8 octets, excluding the first 8 octets. 1504 The length of the Mobility Header MUST be a multiple of 8 octets. 1506 MH Type 1508 8-bit selector. Identifies the particular mobility message in 1509 question. Current values are specified in Section 6.1.2 and 1510 onward. An unrecognized MH Type field causes an error indication 1511 to be sent. 1513 Reserved 1515 8-bit field reserved for future use. The value MUST be 1516 initialized to zero by the sender, and MUST be ignored by the 1517 receiver. 1519 Checksum 1521 16-bit unsigned integer. This field contains the checksum of the 1522 Mobility Header. The checksum is calculated from the octet string 1523 consisting of a "pseudo-header" followed by the entire Mobility 1524 Header starting with the Payload Proto field. The checksum is the 1525 16-bit one's complement of the one's complement sum of this 1526 string. 1528 The pseudo-header contains IPv6 header fields, as specified in 1529 Section 8.1 of RFC 2460 [8]. The Next Header value used in the 1530 pseudo-header is 135. The addresses used in the pseudo-header are 1531 the addresses that appear in the Source and Destination Address 1532 fields in the IPv6 packet carrying the Mobility Header. 1534 Note that the procedures of calculating upper layer checksums 1535 while away from home described in Section 11.3.1 apply even for 1536 the Mobility Header. If a mobility message has a Home Address 1537 destination option, then the checksum calculation uses the home 1538 address in this option as the value of the IPv6 Source Address 1539 field. The type 2 routing header is treated as explained in [8]. 1541 The Mobility Header is considered as the upper layer protocol for 1542 the purposes of calculating the pseudo-header. The Upper-Layer 1543 Packet Length field in the pseudo-header MUST be set to the total 1544 length of the Mobility Header. 1546 For computing the checksum, the checksum field is set to zero. 1548 Message Data 1550 A variable length field containing the data specific to the 1551 indicated Mobility Header type. 1553 Mobile IPv6 also defines a number of "mobility options" for use 1554 within these messages; if included, any options MUST appear after the 1555 fixed portion of the message data specified in this document. The 1556 presence of such options will be indicated by the Header Len field 1557 within the message. When the Header Len value is greater than the 1558 length required for the message specified here, the remaining octets 1559 are interpreted as mobility options. These options include padding 1560 options that can be used to ensure that other options are aligned 1561 properly, and that the total length of the message is divisible by 8. 1562 The encoding and format of defined options are described in 1563 Section 6.2. 1565 Alignment requirements for the Mobility Header are the same as for 1566 any IPv6 protocol Header. That is, they MUST be aligned on an 1567 8-octet boundary. 1569 6.1.2. Binding Refresh Request Message 1571 The Binding Refresh Request (BRR) message requests a mobile node to 1572 update its mobility binding. This message is sent by correspondent 1573 nodes according to the rules in Section 9.5.5. When a mobile node 1574 receives a packet containing a Binding Refresh Request message it 1575 processes the message according to the rules in Section 11.7.4. 1577 The Binding Refresh Request message uses the MH Type value 0. When 1578 this value is indicated in the MH Type field, the format of the 1579 Message Data field in the Mobility Header is as follows: 1581 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1582 | Reserved | 1583 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1584 | | 1585 . . 1586 . Mobility options . 1587 . . 1588 | | 1589 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1591 Reserved 1593 16-bit field reserved for future use. The value MUST be 1594 initialized to zero by the sender, and MUST be ignored by the 1595 receiver. 1597 Mobility Options 1599 Variable-length field of such length that the complete Mobility 1600 Header is an integer multiple of 8 octets long. This field 1601 contains zero or more TLV-encoded mobility options. The encoding 1602 and format of defined options are described in Section 6.2. The 1603 receiver MUST ignore and skip any options which it does not 1604 understand. 1606 There MAY be additional information, associated with this Binding 1607 Refresh Request message that need not be present in all Binding 1608 Refresh Request messages sent. Mobility options allow future 1609 extensions to the format of the Binding Refresh Request message to 1610 be defined. This specification does not define any options valid 1611 for the Binding Refresh Request message. 1613 If no actual options are present in this message, no padding is 1614 necessary and the Header Len field will be set to 0. 1616 6.1.3. Home Test Init Message 1618 A mobile node uses the Home Test Init (HoTI) message to initiate the 1619 return routability procedure and request a home keygen token from a 1620 correspondent node (see Section 11.6.1). The Home Test Init message 1621 uses the MH Type value 1. When this value is indicated in the MH 1622 Type field, the format of the Message Data field in the Mobility 1623 Header is as follows: 1625 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1626 | Reserved | 1627 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1628 | | 1629 + Home Init Cookie + 1630 | | 1631 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1632 | | 1633 . . 1634 . Mobility Options . 1635 . . 1636 | | 1637 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1639 Reserved 1641 16-bit field reserved for future use. This value MUST be 1642 initialized to zero by the sender, and MUST be ignored by the 1643 receiver. 1645 Home Init Cookie 1647 64-bit field which contains a random value, the home init cookie. 1649 Mobility Options 1651 Variable-length field of such length that the complete Mobility 1652 Header is an integer multiple of 8 octets long. This field 1653 contains zero or more TLV-encoded mobility options. The receiver 1654 MUST ignore and skip any options which it does not understand. 1655 This specification does not define any options valid for the Home 1656 Test Init message. 1658 If no actual options are present in this message, no padding is 1659 necessary and the Header Len field will be set to 1. 1661 This message is tunneled through the home agent when the mobile node 1662 is away from home. Such tunneling SHOULD employ IPsec ESP in tunnel 1663 mode between the home agent and the mobile node. This protection is 1664 indicated by the IPsec security policy database. The protection of 1665 Home Test Init messages is unrelated to the requirement to protect 1666 regular payload traffic, which MAY use such tunnels as well. 1668 6.1.4. Care-of Test Init Message 1670 A mobile node uses the Care-of Test Init (CoTI) message to initiate 1671 the return routability procedure and request a care-of keygen token 1672 from a correspondent node (see Section 11.6.1). The Care-of Test 1673 Init message uses the MH Type value 2. When this value is indicated 1674 in the MH Type field, the format of the Message Data field in the 1675 Mobility Header is as follows: 1677 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1678 | Reserved | 1679 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1680 | | 1681 + Care-of Init Cookie + 1682 | | 1683 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1684 | | 1685 . . 1686 . Mobility Options . 1687 . . 1688 | | 1689 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1691 Reserved 1693 16-bit field reserved for future use. The value MUST be 1694 initialized to zero by the sender, and MUST be ignored by the 1695 receiver. 1697 Care-of Init Cookie 1699 64-bit field which contains a random value, the care-of init 1700 cookie. 1702 Mobility Options 1704 Variable-length field of such length that the complete Mobility 1705 Header is an integer multiple of 8 octets long. This field 1706 contains zero or more TLV-encoded mobility options. The receiver 1707 MUST ignore and skip any options which it does not understand. 1708 This specification does not define any options valid for the 1709 Care-of Test Init message. 1711 If no actual options are present in this message, no padding is 1712 necessary and the Header Len field will be set to 1. 1714 6.1.5. Home Test Message 1716 The Home Test (HoT) message is a response to the Home Test Init 1717 message, and is sent from the correspondent node to the mobile node 1718 (see Section 5.2.5). The Home Test message uses the MH Type value 3. 1719 When this value is indicated in the MH Type field, the format of the 1720 Message Data field in the Mobility Header is as follows: 1722 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1723 | Home Nonce Index | 1724 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1725 | | 1726 + Home Init Cookie + 1727 | | 1728 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1729 | | 1730 + Home Keygen Token + 1731 | | 1732 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1733 | | 1734 . . 1735 . Mobility options . 1736 . . 1737 | | 1738 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1740 Home Nonce Index 1742 This field will be echoed back by the mobile node to the 1743 correspondent node in a subsequent Binding Update. 1745 Home Init Cookie 1747 64-bit field which contains the home init cookie. 1749 Home Keygen Token 1751 This field contains the 64 bit home keygen token used in the 1752 return routability procedure. 1754 Mobility Options 1756 Variable-length field of such length that the complete Mobility 1757 Header is an integer multiple of 8 octets long. This field 1758 contains zero or more TLV-encoded mobility options. The receiver 1759 MUST ignore and skip any options which it does not understand. 1760 This specification does not define any options valid for the Home 1761 Test message. 1763 If no actual options are present in this message, no padding is 1764 necessary and the Header Len field will be set to 2. 1766 6.1.6. Care-of Test Message 1768 The Care-of Test (CoT) message is a response to the Care-of Test Init 1769 message, and is sent from the correspondent node to the mobile node 1770 (see Section 11.6.2). The Care-of Test message uses the MH Type 1771 value 4. When this value is indicated in the MH Type field, the 1772 format of the Message Data field in the Mobility Header is as 1773 follows: 1775 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1776 | Care-of Nonce Index | 1777 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1778 | | 1779 + Care-of Init Cookie + 1780 | | 1781 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1782 | | 1783 + Care-of Keygen Token + 1784 | | 1785 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1786 | | 1787 . . 1788 . Mobility Options . 1789 . . 1790 | | 1791 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1793 Care-of Nonce Index 1795 This value will be echoed back by the mobile node to the 1796 correspondent node in a subsequent Binding Update. 1798 Care-of Init Cookie 1800 64-bit field which contains the care-of init cookie. 1802 Care-of Keygen Token 1804 This field contains the 64 bit care-of keygen token used in the 1805 return routability procedure. 1807 Mobility Options 1809 Variable-length field of such length that the complete Mobility 1810 Header is an integer multiple of 8 octets long. This field 1811 contains zero or more TLV-encoded mobility options. The receiver 1812 MUST ignore and skip any options which it does not understand. 1813 This specification does not define any options valid for the 1814 Care-of Test message. 1816 If no actual options are present in this message, no padding is 1817 necessary and the Header Len field will be set to 2. 1819 6.1.7. Binding Update Message 1821 The Binding Update (BU) message is used by a mobile node to notify 1822 other nodes of a new care-of address for itself. Binding Updates are 1823 sent as described in Section 11.7.1 and Section 11.7.2. 1825 The Binding Update uses the MH Type value 5. When this value is 1826 indicated in the MH Type field, the format of the Message Data field 1827 in the Mobility Header is as follows: 1829 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1830 | Sequence # | 1831 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1832 |A|H|L|K| Reserved | Lifetime | 1833 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1834 | | 1835 . . 1836 . Mobility options . 1837 . . 1838 | | 1839 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1841 Acknowledge (A) 1843 The Acknowledge (A) bit is set by the sending mobile node to 1844 request a Binding Acknowledgement (Section 6.1.8) be returned upon 1845 receipt of the Binding Update. 1847 Home Registration (H) 1849 The Home Registration (H) bit is set by the sending mobile node to 1850 request that the receiving node should act as this node's home 1851 agent. The destination of the packet carrying this message MUST 1852 be that of a router sharing the same subnet prefix as the home 1853 address of the mobile node in the binding. 1855 Link-Local Address Compatibility (L) 1857 The Link-Local Address Compatibility (L) bit is set when the home 1858 address reported by the mobile node has the same interface 1859 identifier as the mobile node's link-local address. 1861 Key Management Mobility Capability (K) 1863 If this bit is cleared, the protocol used for establishing the 1864 IPsec security associations between the mobile node and the home 1865 agent does not survive movements. It may then have to be rerun. 1866 (Note that the IPsec security associations themselves are expected 1867 to survive movements.) If manual IPsec configuration is used, the 1868 bit MUST be cleared. 1870 This bit is valid only in Binding Updates sent to the home agent, 1871 and MUST be cleared in other Binding Updates. Correspondent nodes 1872 MUST ignore this bit. 1874 Reserved 1876 These fields are unused. They MUST be initialized to zero by the 1877 sender and MUST be ignored by the receiver. 1879 Sequence # 1881 A 16-bit unsigned integer used by the receiving node to sequence 1882 Binding Updates and by the sending node to match a returned 1883 Binding Acknowledgement with this Binding Update. 1885 Lifetime 1887 16-bit unsigned integer. The number of time units remaining 1888 before the binding MUST be considered expired. A value of zero 1889 indicates that the Binding Cache entry for the mobile node MUST be 1890 deleted. (In this case the specified care-of address MUST also be 1891 set equal to the home address.) One time unit is 4 seconds. 1893 Mobility Options 1895 Variable-length field of such length that the complete Mobility 1896 Header is an integer multiple of 8 octets long. This field 1897 contains zero or more TLV-encoded mobility options. The encoding 1898 and format of defined options are described in Section 6.2. The 1899 receiver MUST ignore and skip any options which it does not 1900 understand. 1902 The following options are valid in a Binding Update: 1904 * Binding Authorization Data option (this option is mandatory in 1905 Binding Updates sent to a correspondent node) 1907 * Nonce Indices option. 1909 * Alternate Care-of Address option 1911 If no options are present in this message, 4 octets of padding are 1912 necessary and the Header Len field will be set to 1. 1914 The care-of address is specified either by the Source Address field 1915 in the IPv6 header or by the Alternate Care-of Address option, if 1916 present. The care-of address MUST be a unicast routable address. 1917 IPv6 Source Address MUST be a topologically correct source address. 1918 Binding Updates for a care-of address which is not a unicast routable 1919 address MUST be silently discarded. Similarly, the Binding Update 1920 MUST be silently discarded if the care-of address appears as a home 1921 address in an existing Binding Cache entry, with its current location 1922 creating a circular reference back to the home address specified in 1923 the Binding Update (possibly through additional entries). 1925 The deletion of a binding can be indicated by setting the Lifetime 1926 field to 0 and by setting the care-of address equal to the home 1927 address. In deletion, the generation of the binding management key 1928 depends exclusively on the home keygen token, as explained in 1929 Section 5.2.5. (Note that while the senders are required to set both 1930 the Lifetime field to 0 and the care-of address equal to the home 1931 address, Section 9.5.1 rules for receivers are more liberal, and 1932 interpret either condition as a deletion.) 1934 Correspondent nodes SHOULD NOT delete the Binding Cache entry before 1935 the lifetime expires, if any application hosted by the correspondent 1936 node is still likely to require communication with the mobile node. 1937 A Binding Cache entry that is de-allocated prematurely might cause 1938 subsequent packets to be dropped from the mobile node, if they 1939 contain the Home Address destination option. This situation is 1940 recoverable, since a Binding Error message is sent to the mobile node 1941 (see Section 6.1.9); however, it causes unnecessary delay in the 1942 communications. 1944 6.1.8. Binding Acknowledgement Message 1946 The Binding Acknowledgement is used to acknowledge receipt of a 1947 Binding Update (Section 6.1.7). This packet is sent as described in 1948 Section 9.5.4 and Section 10.3.1. 1950 The Binding Acknowledgement has the MH Type value 6. When this value 1951 is indicated in the MH Type field, the format of the Message Data 1952 field in the Mobility Header is as follows: 1954 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1955 | Status |K| Reserved | 1956 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1957 | Sequence # | Lifetime | 1958 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1959 | | 1960 . . 1961 . Mobility options . 1962 . . 1963 | | 1964 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1966 Key Management Mobility Capability (K) 1968 If this bit is cleared, the protocol used by the home agent for 1969 establishing the IPsec security associations between the mobile 1970 node and the home agent does not survive movements. It may then 1971 have to be rerun. (Note that the IPsec security associations 1972 themselves are expected to survive movements.) 1974 Correspondent nodes MUST set the K bit to 0. 1976 Reserved 1978 These fields are unused. They MUST be initialized to zero by the 1979 sender and MUST be ignored by the receiver. 1981 Status 1983 8-bit unsigned integer indicating the disposition of the Binding 1984 Update. Values of the Status field less than 128 indicate that 1985 the Binding Update was accepted by the receiving node. Values 1986 greater than or equal to 128 indicate that the Binding Update was 1987 rejected by the receiving node. The following Status values are 1988 currently defined: 1990 0 Binding Update accepted 1992 1 Accepted but prefix discovery necessary 1994 128 Reason unspecified 1996 129 Administratively prohibited 1998 130 Insufficient resources 1999 131 Home registration not supported 2001 132 Not home subnet 2003 133 Not home agent for this mobile node 2005 134 Duplicate Address Detection failed 2007 135 Sequence number out of window 2009 136 Expired home nonce index 2011 137 Expired care-of nonce index 2013 138 Expired nonces 2015 139 Registration type change disallowed 2017 Up-to-date values of the Status field are to be specified in the 2018 IANA registry of assigned numbers [13]. 2020 Sequence # 2022 The Sequence Number in the Binding Acknowledgement is copied from 2023 the Sequence Number field in the Binding Update. It is used by 2024 the mobile node in matching this Binding Acknowledgement with an 2025 outstanding Binding Update. 2027 Lifetime 2029 The granted lifetime, in time units of 4 seconds, for which this 2030 node SHOULD retain the entry for this mobile node in its Binding 2031 Cache. 2033 The value of this field is undefined if the Status field indicates 2034 that the Binding Update was rejected. 2036 Mobility Options 2038 Variable-length field of such length that the complete Mobility 2039 Header is an integer multiple of 8 octets long. This field 2040 contains zero or more TLV-encoded mobility options. The encoding 2041 and format of defined options are described in Section 6.2. The 2042 receiver MUST ignore and skip any options which it does not 2043 understand. 2045 There MAY be additional information, associated with this Binding 2046 Acknowledgement that need not be present in all Binding 2047 Acknowledgements sent. Mobility options allow future extensions 2048 to the format of the Binding Acknowledgement to be defined. The 2049 following options are valid for the Binding Acknowledgement: 2051 * Binding Authorization Data option (this option is mandatory in 2052 Binding Acknowledgements sent by a correspondent node, except 2053 where otherwise noted in Section 9.5.4) 2055 * Binding Refresh Advice option 2057 If no options are present in this message, 4 octets of padding are 2058 necessary and the Header Len field will be set to 1. 2060 6.1.9. Binding Error Message 2062 The Binding Error (BE) message is used by the correspondent node to 2063 signal an error related to mobility, such as an inappropriate attempt 2064 to use the Home Address destination option without an existing 2065 binding; see Section 9.3.3 for details. 2067 The Binding Error message uses the MH Type value 7. When this value 2068 is indicated in the MH Type field, the format of the Message Data 2069 field in the Mobility Header is as follows: 2071 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2072 | Status | Reserved | 2073 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2074 | | 2075 + + 2076 | | 2077 + Home Address + 2078 | | 2079 + + 2080 | | 2081 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2082 . . 2083 . Mobility Options . 2084 . . 2085 | | 2086 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2088 Status 2090 8-bit unsigned integer indicating the reason for this message. 2091 The following values are currently defined: 2093 1 Unknown binding for Home Address destination option 2095 2 Unrecognized MH Type value 2097 Reserved 2099 A 8-bit field reserved for future use. The value MUST be 2100 initialized to zero by the sender, and MUST be ignored by the 2101 receiver. 2103 Home Address 2105 The home address that was contained in the Home Address 2106 destination option. The mobile node uses this information to 2107 determine which binding does not exist, in cases where the mobile 2108 node has several home addresses. 2110 Mobility Options 2112 Variable-length field of such length that the complete Mobility 2113 Header is an integer multiple of 8 octets long. This field 2114 contains zero or more TLV-encoded mobility options. The receiver 2115 MUST ignore and skip any options which it does not understand. 2117 There MAY be additional information, associated with this Binding 2118 Error message that need not be present in all Binding Error 2119 messages sent. Mobility options allow future extensions to the 2120 format of the format of the Binding Error message to be defined. 2121 The encoding and format of defined options are described in 2122 Section 6.2. This specification does not define any options valid 2123 for the Binding Error message. 2125 If no actual options are present in this message, no padding is 2126 necessary and the Header Len field will be set to 2. 2128 6.2. Mobility Options 2130 Mobility messages can include zero or more mobility options. This 2131 allows optional fields that may not be needed in every use of a 2132 particular Mobility Header, as well as future extensions to the 2133 format of the messages. Such options are included in the Message 2134 Data field of the message itself, after the fixed portion of the 2135 message data specified in the message subsections of Section 6.1. 2137 The presence of such options will be indicated by the Header Len of 2138 the Mobility Header. If included, the Binding Authorization Data 2139 option (Section 6.2.7) MUST be the last option and MUST NOT have 2140 trailing padding. Otherwise, options can be placed in any order. 2142 6.2.1. Format 2144 Mobility options are encoded within the remaining space of the 2145 Message Data field of a mobility message, using a type-length-value 2146 (TLV) format as follows: 2148 0 1 2 3 2149 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 2150 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2151 | Option Type | Option Length | Option Data... 2152 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2154 Option Type 2156 8-bit identifier of the type of mobility option. When processing 2157 a Mobility Header containing an option for which the Option Type 2158 value is not recognized by the receiver, the receiver MUST quietly 2159 ignore and skip over the option, correctly handling any remaining 2160 options in the message. 2162 Option Length 2164 8-bit unsigned integer, representing the length in octets of the 2165 mobility option, not including the Option Type and Option Length 2166 fields. 2168 Option Data 2170 A variable length field that contains data specific to the option. 2172 The following subsections specify the Option types which are 2173 currently defined for use in the Mobility Header. 2175 Implementations MUST silently ignore any mobility options that they 2176 do not understand. 2178 Mobility options may have alignment requirements. Following the 2179 convention in IPv6, these options are aligned in a packet so that 2180 multi-octet values within the Option Data field of each option fall 2181 on natural boundaries (i.e., fields of width n octets are placed at 2182 an integer multiple of n octets from the start of the header, for n = 2183 1, 2, 4, or 8) [8]. 2185 6.2.2. Pad1 2187 The Pad1 option does not have any alignment requirements. Its format 2188 is as follows: 2190 0 2191 0 1 2 3 4 5 6 7 2192 +-+-+-+-+-+-+-+-+ 2193 | Type = 0 | 2194 +-+-+-+-+-+-+-+-+ 2196 NOTE! the format of the Pad1 option is a special case - it has 2197 neither Option Length nor Option Data fields. 2199 The Pad1 option is used to insert one octet of padding in the 2200 Mobility Options area of a Mobility Header. If more than one octet 2201 of padding is required, the PadN option, described next, should be 2202 used rather than multiple Pad1 options. 2204 6.2.3. PadN 2206 The PadN option does not have any alignment requirements. Its format 2207 is as follows: 2209 0 1 2210 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 2211 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 2212 | Type = 1 | Option Length | Option Data 2213 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - 2215 The PadN option is used to insert two or more octets of padding in 2216 the Mobility Options area of a mobility message. For N octets of 2217 padding, the Option Length field contains the value N-2, and the 2218 Option Data consists of N-2 zero-valued octets. PadN Option data 2219 MUST be ignored by the receiver. 2221 6.2.4. Binding Refresh Advice 2223 The Binding Refresh Advice option has an alignment requirement of 2n. 2224 Its format is as follows: 2226 0 1 2 3 2227 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 2228 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2229 | Type = 2 | Length = 2 | 2230 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2231 | Refresh Interval | 2232 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2234 The Binding Refresh Advice option is only valid in the Binding 2235 Acknowledgement, and only on Binding Acknowledgements sent from the 2236 mobile node's home agent in reply to a home registration. The 2237 Refresh Interval is measured in units of four seconds, and indicates 2238 remaining time until the mobile node SHOULD send a new home 2239 registration to the home agent. The Refresh Interval MUST be set to 2240 indicate a smaller time interval than the Lifetime value of the 2241 Binding Acknowledgement. 2243 6.2.5. Alternate Care-of Address 2245 The Alternate Care-of Address option has an alignment requirement of 2246 8n+6. Its format is as follows: 2248 0 1 2 3 2249 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 2250 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2251 | Type = 3 | Length = 16 | 2252 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2253 | | 2254 + + 2255 | | 2256 + Alternate Care-of Address + 2257 | | 2258 + + 2259 | | 2260 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2262 Normally, a Binding Update specifies the desired care-of address in 2263 the Source Address field of the IPv6 header. However, this is not 2264 possible in some cases, such as when the mobile node wishes to 2265 indicate a care-of address which it cannot use as a topologically 2266 correct source address (Section 6.1.7 and Section 11.7.2) or when the 2267 used security mechanism does not protect the IPv6 header 2268 (Section 11.7.1). 2270 The Alternate Care-of Address option is provided for these 2271 situations. This option is valid only in Binding Update. The 2272 Alternate Care-of Address field contains an address to use as the 2273 care-of address for the binding, rather than using the Source Address 2274 of the packet as the care-of address. 2276 6.2.6. Nonce Indices 2278 The Nonce Indices option has an alignment requirement of 2n. Its 2279 format is as follows: 2281 0 1 2 3 2282 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 2283 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2284 | Type = 4 | Length = 4 | 2285 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2286 | Home Nonce Index | Care-of Nonce Index | 2287 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2289 The Nonce Indices option is valid only in the Binding Update message 2290 sent to a correspondent node, and only when present together with a 2291 Binding Authorization Data option. When the correspondent node 2292 authorizes the Binding Update, it needs to produce home and care-of 2293 keygen tokens from its stored random nonce values. 2295 The Home Nonce Index field tells the correspondent node which nonce 2296 value to use when producing the home keygen token. 2298 The Care-of Nonce Index field is ignored in requests to delete a 2299 binding. Otherwise, it tells the correspondent node which nonce 2300 value to use when producing the care-of keygen token. 2302 6.2.7. Binding Authorization Data 2304 The Binding Authorization Data option does not have alignment 2305 requirements as such. However, since this option must be the last 2306 mobility option, an implicit alignment requirement is 8n + 2. The 2307 format of this option is as follows: 2309 0 1 2 3 2310 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 2311 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2312 | Type = 5 | Option Length | 2313 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2314 | | 2315 + + 2316 | Authenticator | 2317 + + 2318 | | 2319 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2321 The Binding Authorization Data option is valid in the Binding Update 2322 and Binding Acknowledgement. 2324 The Option Length field contains the length of the authenticator in 2325 octets. 2327 The Authenticator field contains a cryptographic value which can be 2328 used to determine that the message in question comes from the right 2329 authority. Rules for calculating this value depends on the used 2330 authorization procedure. 2332 For the return routability procedure, this option can appear in the 2333 Binding Update and Binding Acknowledgements. Rules for calculating 2334 the Authenticator value are the following: 2336 Mobility Data = care-of address | correspondent | MH Data 2337 Authenticator = First (96, HMAC_SHA1 (Kbm, Mobility Data)) 2339 Where | denotes concatenation. "Care-of address" is the care-of 2340 address which will be registered for the mobile node if the Binding 2341 Update succeeds, or the home address of the mobile node if this 2342 option is used in de-registration. Note also that this address might 2343 be different from the source address of the Binding Update message, 2344 if the Alternative Care-of Address mobility option is used, or when 2345 the lifetime of the binding is set to zero. 2347 The "correspondent" is the IPv6 address of the correspondent node. 2348 Note that, if the message is sent to a destination which is itself 2349 mobile, the "correspondent" address may not be the address found in 2350 the Destination Address field of the IPv6 header; instead the home 2351 address from the type 2 Routing header should be used. 2353 "MH Data" is the content of the Mobility Header, excluding the 2354 Authenticator field itself. The Authenticator value is calculated as 2355 if the Checksum field in the Mobility Header was zero. The Checksum 2356 in the transmitted packet is still calculated in the usual manner, 2357 with the calculated Authenticator being a part of the packet 2358 protected by the Checksum. Kbm is the binding management key, which 2359 is typically created using nonces provided by the correspondent node 2360 (see Section 9.4). Note that while the contents of a potential Home 2361 Address destination option are not covered in this formula, the rules 2362 for the calculation of the Kbm do take the home address in account. 2363 This ensures that the MAC will be different for different home 2364 addresses. 2366 The first 96 bits from the MAC result are used as the Authenticator 2367 field. 2369 6.3. Home Address Option 2371 The Home Address option is carried by the Destination Option 2372 extension header (Next Header value = 60). It is used in a packet 2373 sent by a mobile node while away from home, to inform the recipient 2374 of the mobile node's home address. 2376 The Home Address option is encoded in type-length-value (TLV) format 2377 as follows: 2379 0 1 2 3 2380 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 2381 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2382 | Option Type | Option Length | 2383 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2384 | | 2385 + + 2386 | | 2387 + Home Address + 2388 | | 2389 + + 2390 | | 2391 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2393 Option Type 2395 201 = 0xC9 2397 Option Length 2399 8-bit unsigned integer. Length of the option, in octets, 2400 excluding the Option Type and Option Length fields. This field 2401 MUST be set to 16. 2403 Home Address 2405 The home address of the mobile node sending the packet. This 2406 address MUST be a unicast routable address. 2408 The alignment requirement [8] for the Home Address option is 8n+6. 2410 The three highest-order bits of the Option Type field are encoded to 2411 indicate specific processing of the option [8]; for the Home Address 2412 option, these three bits are set to 110. This indicates the 2413 following processing requirements: 2415 o Any IPv6 node that does not recognize the Option Type must discard 2416 the packet, and if the packet's Destination Address was not a 2417 multicast address, return an ICMP Parameter Problem, Code 2, 2418 message to the packet's Source Address. The Pointer field in the 2419 ICMP message SHOULD point at the Option Type field. Otherwise, 2420 for multicast addresses, the ICMP message MUST NOT be sent. 2422 o The data within the option cannot change en route to the packet's 2423 final destination. 2425 The Home Address option MUST be placed as follows: 2427 o After the routing header, if that header is present 2429 o Before the Fragment Header, if that header is present 2431 o Before the AH Header or ESP Header, if either one of those headers 2432 are present 2434 For each IPv6 packet header, the Home Address Option MUST NOT appear 2435 more than once. However, an encapsulated packet [9] MAY contain a 2436 separate Home Address option associated with each encapsulating IP 2437 header. 2439 The inclusion of a Home Address destination option in a packet 2440 affects the receiving node's processing of only this single packet. 2441 No state is created or modified in the receiving node as a result of 2442 receiving a Home Address option in a packet. In particular, the 2443 presence of a Home Address option in a received packet MUST NOT alter 2444 the contents of the receiver's Binding Cache and MUST NOT cause any 2445 changes in the routing of subsequent packets sent by this receiving 2446 node. 2448 6.4. Type 2 Routing Header 2450 Mobile IPv6 defines a new routing header variant, the type 2 routing 2451 header, to allow the packet to be routed directly from a 2452 correspondent to the mobile node's care-of address. The mobile 2453 node's care-of address is inserted into the IPv6 Destination Address 2454 field. Once the packet arrives at the care-of address, the mobile 2455 node retrieves its home address from the routing header, and this is 2456 used as the final destination address for the packet. 2458 The new routing header uses a different type than defined for 2459 "regular" IPv6 source routing, enabling firewalls to apply different 2460 rules to source routed packets than to Mobile IPv6. This routing 2461 header type (type 2) is restricted to carry only one IPv6 address. 2462 All IPv6 nodes which process this routing header MUST verify that the 2463 address contained within is the node's own home address in order to 2464 prevent packets from being forwarded outside the node. The IP 2465 address contained in the routing header, since it is the mobile 2466 node's home address, MUST be a unicast routable address. 2467 Furthermore, if the scope of the home address is smaller than the 2468 scope of the care-of address, the mobile node MUST discard the packet 2469 (see Section 4.6). 2471 6.4.1. Format 2473 The type 2 routing header has the following format: 2475 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2476 | Next Header | Hdr Ext Len=2 | Routing Type=2|Segments Left=1| 2477 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2478 | Reserved | 2479 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2480 | | 2481 + + 2482 | | 2483 + Home Address + 2484 | | 2485 + + 2486 | | 2487 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2489 Next Header 2491 8-bit selector. Identifies the type of header immediately 2492 following the routing header. Uses the same values as the IPv6 2493 Next Header field [8]. 2495 Hdr Ext Len 2497 2 (8-bit unsigned integer); length of the routing header in 2498 8-octet units, not including the first 8 octets. 2500 Routing Type 2502 2 (8-bit unsigned integer). 2504 Segments Left 2506 1 (8-bit unsigned integer). 2508 Reserved 2510 32-bit reserved field. The value MUST be initialized to zero by 2511 the sender, and MUST be ignored by the receiver. 2513 Home Address 2515 The Home Address of the destination Mobile Node. 2517 For a type 2 routing header, the Hdr Ext Len MUST be 2. The Segments 2518 Left value describes the number of route segments remaining; i.e., 2519 number of explicitly listed intermediate nodes still to be visited 2520 before reaching the final destination. Segments Left MUST be 1. The 2521 ordering rules for extension headers in an IPv6 packet are described 2522 in Section 4.1 of RFC 2460 [8]. The type 2 routing header defined 2523 for Mobile IPv6 follows the same ordering as other routing headers. 2524 If both a type 0 and a type 2 routing header are present, the type 2 2525 routing header should follow the other routing header. A packet 2526 containing such nested encapsulation should be created as if the 2527 inner (type 2) routing header was constructed first and then treated 2528 as an original packet by the outer (type 0) routing header 2529 construction process. 2531 In addition, the general procedures defined by IPv6 for routing 2532 headers suggest that a received routing header MAY be automatically 2533 "reversed" to construct a routing header for use in any response 2534 packets sent by upper-layer protocols, if the received packet is 2535 authenticated [6]. This MUST NOT be done automatically for type 2 2536 routing headers. 2538 6.5. ICMP Home Agent Address Discovery Request Message 2540 The ICMP Home Agent Address Discovery Request message is used by a 2541 mobile node to initiate the dynamic home agent address discovery 2542 mechanism, as described in Section 11.4.1. The mobile node sends the 2543 Home Agent Address Discovery Request message to the Mobile IPv6 Home- 2544 Agents anycast address [10] for its own home subnet prefix. (Note 2545 that the currently defined anycast addresses may not work with all 2546 prefix lengths other than those defined in RFC 4291 [18] [38].) 2548 0 1 2 3 2549 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 2550 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2551 | Type | Code | Checksum | 2552 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2553 | Identifier | Reserved | 2554 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2556 Type 2558 144 2560 Code 2562 0 2564 Checksum 2566 The ICMP checksum [19]. 2568 Identifier 2570 An identifier to aid in matching Home Agent Address Discovery 2571 Reply messages to this Home Agent Address Discovery Request 2572 message. 2574 Reserved 2576 This field is unused. It MUST be initialized to zero by the 2577 sender and MUST be ignored by the receiver. 2579 The Source Address of the Home Agent Address Discovery Request 2580 message packet is typically one of the mobile node's current care-of 2581 addresses. At the time of performing this dynamic home agent address 2582 discovery procedure, it is likely that the mobile node is not 2583 registered with any home agent. Therefore, neither the nature of the 2584 address nor the identity of the mobile node can be established at 2585 this time. The home agent MUST then return the Home Agent Address 2586 Discovery Reply message directly to the Source Address chosen by the 2587 mobile node. 2589 6.6. ICMP Home Agent Address Discovery Reply Message 2591 The ICMP Home Agent Address Discovery Reply message is used by a home 2592 agent to respond to a mobile node that uses the dynamic home agent 2593 address discovery mechanism, as described in Section 10.5. 2595 0 1 2 3 2596 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 2597 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2598 | Type | Code | Checksum | 2599 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2600 | Identifier | Reserved | 2601 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2602 | | 2603 + + 2604 . . 2605 . Home Agent Addresses . 2606 . . 2607 + + 2608 | | 2609 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2611 Type 2613 145 2615 Code 2617 0 2619 Checksum 2621 The ICMP checksum [19]. 2623 Identifier 2625 The identifier from the invoking Home Agent Address Discovery 2626 Request message. 2628 Reserved 2630 This field is unused. It MUST be initialized to zero by the 2631 sender and MUST be ignored by the receiver. 2633 Home Agent Addresses 2635 A list of addresses of home agents on the home link for the mobile 2636 node. The number of addresses presented in the list is indicated 2637 by the remaining length of the IPv6 packet carrying the Home Agent 2638 Address Discovery Reply message. 2640 6.7. ICMP Mobile Prefix Solicitation Message Format 2642 The ICMP Mobile Prefix Solicitation Message is sent by a mobile node 2643 to its home agent while it is away from home. The purpose of the 2644 message is to solicit a Mobile Prefix Advertisement from the home 2645 agent, which will allow the mobile node to gather prefix information 2646 about its home network. This information can be used to configure 2647 and update home address(es) according to changes in prefix 2648 information supplied by the home agent. 2650 0 1 2 3 2651 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 2652 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2653 | Type | Code | Checksum | 2654 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2655 | Identifier | Reserved | 2656 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2658 IP Fields: 2660 Source Address 2662 The mobile node's care-of address. 2664 Destination Address 2666 The address of the mobile node's home agent. This home agent must 2667 be on the link that the mobile node wishes to learn prefix 2668 information about. 2670 Hop Limit 2672 Set to an initial hop limit value, similarly to any other unicast 2673 packet sent by the mobile node. 2675 Destination Option: 2677 A Home Address destination option MUST be included. 2679 ESP header: 2681 IPsec headers MUST be supported and SHOULD be used as described in 2682 Section 5.4. 2684 ICMP Fields: 2686 Type 2688 146 2690 Code 2692 0 2694 Checksum 2696 The ICMP checksum [19]. 2698 Identifier 2700 An identifier to aid in matching a future Mobile Prefix 2701 Advertisement to this Mobile Prefix Solicitation. 2703 Reserved 2705 This field is unused. It MUST be initialized to zero by the 2706 sender and MUST be ignored by the receiver. 2708 The Mobile Prefix Solicitation messages may have options. These 2709 options MUST use the option format defined in RFC 4861 [20]. This 2710 document does not define any option types for the Mobile Prefix 2711 Solicitation message, but future documents may define new options. 2712 Home agents MUST silently ignore any options they do not recognize 2713 and continue processing the message. 2715 6.8. ICMP Mobile Prefix Advertisement Message Format 2717 A home agent will send a Mobile Prefix Advertisement to a mobile node 2718 to distribute prefix information about the home link while the mobile 2719 node is traveling away from the home network. This will occur in 2720 response to a Mobile Prefix Solicitation with an Advertisement, or by 2721 an unsolicited Advertisement sent according to the rules in 2722 Section 10.6. 2724 0 1 2 3 2725 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 2726 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2727 | Type | Code | Checksum | 2728 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2729 | Identifier |M|O| Reserved | 2730 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2731 | Options ... 2732 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2734 IP Fields: 2736 Source Address 2738 The home agent's address as the mobile node would expect to see it 2739 (i.e., same network prefix). 2741 Destination Address 2743 If this message is a response to a Mobile Prefix Solicitation, 2744 this field contains the Source Address field from that packet. 2745 For unsolicited messages, the mobile node's care-of address SHOULD 2746 be used. Note that unsolicited messages can only be sent if the 2747 mobile node is currently registered with the home agent. 2749 Routing header: 2751 A type 2 routing header MUST be included. 2753 ESP header: 2755 IPsec headers MUST be supported and SHOULD be used as described in 2756 Section 5.4. 2758 ICMP Fields: 2760 Type 2762 147 2764 Code 2766 0 2768 Checksum 2770 The ICMP checksum [19]. 2772 Identifier 2774 An identifier to aid in matching this Mobile Prefix Advertisement 2775 to a previous Mobile Prefix Solicitation. 2777 M 2779 1-bit Managed Address Configuration flag. When set, hosts use the 2780 administered (stateful) protocol for address autoconfiguration in 2781 addition to any addresses autoconfigured using stateless address 2782 autoconfiguration. The use of this flag is described in [20] 2783 [21]. 2785 O 2787 1-bit Other Stateful Configuration flag. When set, hosts use the 2788 administered (stateful) protocol for autoconfiguration of other 2789 (non-address) information. The use of this flag is described in 2790 [20] [21]. 2792 Reserved 2794 This field is unused. It MUST be initialized to zero by the 2795 sender and MUST be ignored by the receiver. 2797 The Mobile Prefix Advertisement messages may have options. These 2798 options MUST use the option format defined in RFC 4861 [20]. This 2799 document defines one option which may be carried in a Mobile Prefix 2800 Advertisement message, but future documents may define new options. 2801 Mobile nodes MUST silently ignore any options they do not recognize 2802 and continue processing the message. 2804 Prefix Information 2806 Each message contains one or more Prefix Information options. 2807 Each option carries the prefix(es) that the mobile node should use 2808 to configure its home address(es). Section 10.6 describes which 2809 prefixes should be advertised to the mobile node. 2811 The Prefix Information option is defined in Section 4.6.2 of RFC 2812 4861 [20], with modifications defined in Section 7.2 of this 2813 specification. The home agent MUST use this modified Prefix 2814 Information option to send home network prefixes as defined in 2815 Section 10.6.1. 2817 If the Advertisement is sent in response to a Mobile Prefix 2818 Solicitation, the home agent MUST copy the Identifier value from that 2819 message into the Identifier field of the Advertisement. 2821 The home agent MUST NOT send more than one Mobile Prefix 2822 Advertisement message per second to any mobile node. 2824 The M and O bits MUST be cleared if the Home Agent DHCPv6 support is 2825 not provided. If such support is provided then they are set in 2826 concert with the home network's administrative settings. 2828 7. Modifications to IPv6 Neighbor Discovery 2830 7.1. Modified Router Advertisement Message Format 2832 Mobile IPv6 modifies the format of the Router Advertisement message 2833 [20] by the addition of a single flag bit to indicate that the router 2834 sending the Advertisement message is serving as a home agent on this 2835 link. The format of the Router Advertisement message is as follows: 2837 0 1 2 3 2838 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 2839 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2840 | Type | Code | Checksum | 2841 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2842 | Cur Hop Limit |M|O|H| Reserved| Router Lifetime | 2843 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2844 | Reachable Time | 2845 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2846 | Retrans Timer | 2847 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2848 | Options ... 2849 +-+-+-+-+-+-+-+-+-+-+-+- 2851 This format represents the following changes over that originally 2852 specified for Neighbor Discovery [20]: 2854 Home Agent (H) 2856 The Home Agent (H) bit is set in a Router Advertisement to 2857 indicate that the router sending this Router Advertisement is also 2858 functioning as a Mobile IPv6 home agent on this link. 2860 Reserved 2862 Reduced from a 6-bit field to a 5-bit field to account for the 2863 addition of the above bit. 2865 7.2. Modified Prefix Information Option Format 2867 Mobile IPv6 requires knowledge of a router's global address in 2868 building a Home Agents List as part of the dynamic home agent address 2869 discovery mechanism. 2871 However, Neighbor Discovery [20] only advertises a router's link- 2872 local address, by requiring this address to be used as the IP Source 2873 Address of each Router Advertisement. 2875 Mobile IPv6 extends Neighbor Discovery to allow a router to advertise 2876 its global address, by the addition of a single flag bit in the 2877 format of a Prefix Information option for use in Router Advertisement 2878 messages. The format of the Prefix Information option is as follows: 2880 0 1 2 3 2881 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 2882 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2883 | Type | Length | Prefix Length |L|A|R|Reserved1| 2884 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2885 | Valid Lifetime | 2886 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2887 | Preferred Lifetime | 2888 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2889 | Reserved2 | 2890 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2891 | | 2892 + + 2893 | | 2894 + Prefix + 2895 | | 2896 + + 2897 | | 2898 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2900 This format represents the following changes over that originally 2901 specified for Neighbor Discovery [20]: 2903 Router Address (R) 2905 1-bit router address flag. When set, indicates that the Prefix 2906 field contains a complete IP address assigned to the sending 2907 router. The indicated prefix is the first Prefix Length bits of 2908 the Prefix field. The router IP address has the same scope and 2909 conforms to the same lifetime values as the advertised prefix. 2910 This use of the Prefix field is compatible with its use in 2911 advertising the prefix itself, since Prefix Advertisement uses 2912 only the leading bits. Interpretation of this flag bit is thus 2913 independent of the processing required for the On-Link (L) and 2914 Autonomous Address-Configuration (A) flag bits. 2916 Reserved1 2918 Reduced from a 6-bit field to a 5-bit field to account for the 2919 addition of the above bit. 2921 In a Router Advertisement, a home agent MUST, and all other routers 2922 MAY, include at least one Prefix Information option with the Router 2923 Address (R) bit set. Neighbor Discovery specifies that, if including 2924 all options in a Router Advertisement causes the size of the 2925 Advertisement to exceed the link MTU, multiple Advertisements can be 2926 sent, each containing a subset of the options [20]. Also, when 2927 sending unsolicited multicast Router Advertisements more frequently 2928 than the limit specified in RFC 4861 [20], the sending router need 2929 not include all options in each of these Advertisements. However, in 2930 both of these cases the router SHOULD include at least one Prefix 2931 Information option with the Router Address (R) bit set in each such 2932 advertisement, if this bit is set in some advertisement sent by the 2933 router. 2935 In addition, the following requirement can assist mobile nodes in 2936 movement detection. Barring changes in the prefixes for the link, 2937 routers that send multiple Router Advertisements with the Router 2938 Address (R) bit set in some of the included Prefix Information 2939 options SHOULD provide at least one option and router address which 2940 stays the same in all of the Advertisements. 2942 7.3. New Advertisement Interval Option Format 2944 Mobile IPv6 defines a new Advertisement Interval option, used in 2945 Router Advertisement messages to advertise the interval at which the 2946 sending router sends unsolicited multicast Router Advertisements. 2947 The format of the Advertisement Interval option is as follows: 2949 0 1 2 3 2950 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 2951 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2952 | Type | Length | Reserved | 2953 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2954 | Advertisement Interval | 2955 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2957 Type 2959 7 2961 Length 2963 8-bit unsigned integer. The length of the option (including the 2964 type and length fields) is in units of 8 octets. The value of 2965 this field MUST be 1. 2967 Reserved 2969 This field is unused. It MUST be initialized to zero by the 2970 sender and MUST be ignored by the receiver. 2972 Advertisement Interval 2974 32-bit unsigned integer. The maximum time, in milliseconds, 2975 between successive unsolicited Router Advertisement messages sent 2976 by this router on this network interface. Using the conceptual 2977 router configuration variables defined by Neighbor Discovery [20], 2978 this field MUST be equal to the value MaxRtrAdvInterval, expressed 2979 in milliseconds. 2981 Routers MAY include this option in their Router Advertisements. A 2982 mobile node receiving a Router Advertisement containing this option 2983 SHOULD utilize the specified Advertisement Interval for that router 2984 in its movement detection algorithm, as described in Section 11.5.1. 2986 This option MUST be silently ignored for other Neighbor Discovery 2987 messages. 2989 7.4. New Home Agent Information Option Format 2991 Mobile IPv6 defines a new Home Agent Information option, used in 2992 Router Advertisements sent by a home agent to advertise information 2993 specific to this router's functionality as a home agent. The format 2994 of the Home Agent Information option is as follows: 2996 0 1 2 3 2997 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 2998 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2999 | Type | Length | Reserved | 3000 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3001 | Home Agent Preference | Home Agent Lifetime | 3002 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3004 Type 3006 8 3008 Length 3010 8-bit unsigned integer. The length of the option (including the 3011 type and length fields) in units of 8 octets. The value of this 3012 field MUST be 1. 3014 Reserved 3016 This field is unused. It MUST be initialized to zero by the 3017 sender and MUST be ignored by the receiver. 3019 Home Agent Preference 3021 16-bit unsigned integer. The preference for the home agent 3022 sending this Router Advertisement, for use in ordering the 3023 addresses returned to a mobile node in the Home Agent Addresses 3024 field of a Home Agent Address Discovery Reply message. Higher 3025 values mean more preferable. If this option is not included in a 3026 Router Advertisement in which the Home Agent (H) bit is set, the 3027 preference value for this home agent MUST be considered to be 0. 3028 Greater values indicate a more preferable home agent than lower 3029 values. 3031 The manual configuration of the Home Agent Preference value is 3032 described in Section 8.4. In addition, the sending home agent MAY 3033 dynamically set the Home Agent Preference value, for example 3034 basing it on the number of mobile nodes it is currently serving or 3035 on its remaining resources for serving additional mobile nodes; 3036 such dynamic settings are beyond the scope of this document. Any 3037 such dynamic setting of the Home Agent Preference, however, MUST 3038 set the preference appropriately, relative to the default Home 3039 Agent Preference value of 0 that may be in use by some home agents 3040 on this link (i.e., a home agent not including a Home Agent 3041 Information option in its Router Advertisements will be considered 3042 to have a Home Agent Preference value of 0). 3044 Home Agent Lifetime 3046 16-bit unsigned integer. The lifetime associated with the home 3047 agent in units of seconds. The default value is the same as the 3048 Router Lifetime, as specified in the main body of the Router 3049 Advertisement. The maximum value corresponds to 18.2 hours. A 3050 value of 0 MUST NOT be used. The Home Agent Lifetime applies only 3051 to this router's usefulness as a home agent; it does not apply to 3052 information contained in other message fields or options. 3054 Home agents MAY include this option in their Router Advertisements. 3055 This option MUST NOT be included in a Router Advertisement in which 3056 the Home Agent (H) bit (see Section 7.1) is not set. If this option 3057 is not included in a Router Advertisement in which the Home Agent (H) 3058 bit is set, the lifetime for this home agent MUST be considered to be 3059 the same as the Router Lifetime in the Router Advertisement. If 3060 multiple Advertisements are being sent instead of a single larger 3061 unsolicited multicast Advertisement, all of the multiple 3062 Advertisements with the Router Address (R) bit set MUST include this 3063 option with the same contents, otherwise this option MUST be omitted 3064 from all Advertisements. 3066 This option MUST be silently ignored for other Neighbor Discovery 3067 messages. 3069 If both the Home Agent Preference and Home Agent Lifetime are set to 3070 their default values specified above, this option SHOULD NOT be 3071 included in the Router Advertisement messages sent by this home 3072 agent. 3074 7.5. Changes to Sending Router Advertisements 3076 The Neighbor Discovery protocol specification [20] limits routers to 3077 a minimum interval of 3 seconds between sending unsolicited multicast 3078 Router Advertisement messages from any given network interface 3079 (limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that: 3081 "Routers generate Router Advertisements frequently enough that 3082 hosts will learn of their presence within a few minutes, but not 3083 frequently enough to rely on an absence of advertisements to 3084 detect router failure; a separate Neighbor Unreachability 3085 Detection algorithm provides failure detection." 3087 This limitation, however, is not suitable to providing timely 3088 movement detection for mobile nodes. Mobile nodes detect their own 3089 movement by learning the presence of new routers as the mobile node 3090 moves into wireless transmission range of them (or physically 3091 connects to a new wired network), and by learning that previous 3092 routers are no longer reachable. Mobile nodes MUST be able to 3093 quickly detect when they move to a link served by a new router, so 3094 that they can acquire a new care-of address and send Binding Updates 3095 to register this care-of address with their home agent and to notify 3096 correspondent nodes as needed. 3098 One method which can provide for faster movement detection, is to 3099 increase the rate at which unsolicited Router Advertisements are 3100 sent. Mobile IPv6 relaxes this limit such that routers MAY send 3101 unsolicited multicast Router Advertisements more frequently. This 3102 method can be applied where the router is expecting to provide 3103 service to visiting mobile nodes (e.g., wireless network interfaces), 3104 or on which it is serving as a home agent to one or more mobile nodes 3105 (who may return home and need to hear its Advertisements). 3107 Routers supporting mobility SHOULD be able to be configured with a 3108 smaller MinRtrAdvInterval value and MaxRtrAdvInterval value to allow 3109 sending of unsolicited multicast Router Advertisements more often. 3110 The minimum allowed values are: 3112 o MinRtrAdvInterval 0.03 seconds 3113 o MaxRtrAdvInterval 0.07 seconds 3115 In the case where the minimum intervals and delays are used, the mean 3116 time between unsolicited multicast router advertisements is 50ms. 3117 Use of these modified limits MUST be configurable (see also the 3118 configuration variable MinDelayBetweenRas in Section 13 which may 3119 also have to be modified accordingly). Systems where these values 3120 are available MUST NOT default to them, and SHOULD default to values 3121 specified in RFC 4861. Knowledge of the type of network interface 3122 and operating environment SHOULD be taken into account in configuring 3123 these limits for each network interface. This is important with some 3124 wireless links, where increasing the frequency of multicast beacons 3125 can cause considerable overhead. Routers SHOULD adhere to the 3126 intervals specified in RFC 4861 [20], if this overhead is likely to 3127 cause service degradation. 3129 Additionally, the possible low values of MaxRtrAdvInterval may cause 3130 some problems with movement detection in some mobile nodes. To 3131 ensure that this is not a problem, Routers SHOULD add 20ms to any 3132 Advertisement Intervals sent in RAs, which are below 200 ms, in order 3133 to account for scheduling granularities on both the MN and the 3134 Router. 3136 Note that multicast Router Advertisements are not always required in 3137 certain wireless networks that have limited bandwidth. Mobility 3138 detection or link changes in such networks may be done at lower 3139 layers. Router advertisements in such networks SHOULD be sent only 3140 when solicited. In such networks it SHOULD be possible to disable 3141 unsolicited multicast Router Advertisements on specific interfaces. 3142 The MinRtrAdvInterval and MaxRtrAdvInterval in such a case can be set 3143 to some high values. 3145 Home agents MUST include the Source Link-Layer Address option in all 3146 Router Advertisements they send. This simplifies the process of 3147 returning home, as discussed in Section 11.5.5. 3149 Note that according to RFC 4861 [20], AdvDefaultLifetime is by 3150 default based on the value of MaxRtrAdvInterval. AdvDefaultLifetime 3151 is used in the Router Lifetime field of Router Advertisements. Given 3152 that this field is expressed in seconds, a small MaxRtrAdvInterval 3153 value can result in a zero value for this field. To prevent this, 3154 routers SHOULD keep AdvDefaultLifetime in at least one second, even 3155 if the use of MaxRtrAdvInterval would result in a smaller value. 3157 8. Requirements for Types of IPv6 Nodes 3159 Mobile IPv6 places some special requirements on the functions 3160 provided by different types of IPv6 nodes. This section summarizes 3161 those requirements, identifying the functionality each requirement is 3162 intended to support. 3164 The requirements are set for the following groups of nodes: 3166 o All IPv6 nodes. 3168 o All IPv6 nodes with support for route optimization. 3170 o All IPv6 routers. 3172 o All Mobile IPv6 home agents. 3174 o All Mobile IPv6 mobile nodes. 3176 It is outside the scope of this specification to specify which of 3177 these groups are mandatory in IPv6. We only describe what is 3178 mandatory for a node that supports, for instance, route optimization. 3179 Other specifications are expected to define the extent of IPv6. 3181 8.1. All IPv6 Nodes 3183 Any IPv6 node may at any time be a correspondent node of a mobile 3184 node, either sending a packet to a mobile node or receiving a packet 3185 from a mobile node. There are no Mobile IPv6 specific MUST 3186 requirements for such nodes, and basic IPv6 techniques are 3187 sufficient. If a mobile node attempts to set up route optimization 3188 with a node with only basic IPv6 support, an ICMP error will signal 3189 that the node does not support such optimizations (Section 11.3.5), 3190 and communications will flow through the home agent . 3192 An IPv6 node MUST NOT support the Home Address destination option, 3193 type 2 routing header, or the Mobility Header unless it fully 3194 supports the requirements listed in the next sections for either 3195 route optimization, mobile node, or home agent functionality. 3197 8.2. IPv6 Nodes with Support for Route Optimization 3199 Nodes that implement route optimization are a subset of all IPv6 3200 nodes on the Internet. The ability of a correspondent node to 3201 participate in route optimization is essential for the efficient 3202 operation of the IPv6 Internet, for the following reasons: 3204 o Avoidance of congestion in the home network, and enabling the use 3205 of lower-performance home agent equipment even for supporting 3206 thousands of mobile nodes. 3208 o Reduced network load across the entire Internet, as mobile devices 3209 begin to predominate. 3211 o Reduction of jitter and latency for the communications. 3213 o Greater likelihood of success for QoS signaling as tunneling is 3214 avoided and, again, fewer sources of congestion. 3216 o Improved robustness against network partitions, congestion, and 3217 other problems, since fewer routing path segments are traversed. 3219 These effects combine to enable much better performance and 3220 robustness for communications between mobile nodes and IPv6 3221 correspondent nodes. Route optimization introduces a small amount of 3222 additional state for the peers, some additional messaging, and up to 3223 1.5 roundtrip delays before it can be turned on. However, it is 3224 believed that the benefits far outweigh the costs in most cases. 3225 Section 11.3.1 discusses how mobile nodes may avoid route 3226 optimization for some of the remaining cases, such as very short-term 3227 communications. 3229 The following requirements apply to all correspondent nodes that 3230 support route optimization: 3232 o The node MUST be able to validate a Home Address option using an 3233 existing Binding Cache entry, as described in Section 9.3.1. 3235 o The node MUST be able to insert a type 2 routing header into 3236 packets to be sent to a mobile node, as described in 3237 Section 9.3.2. 3239 o Unless the correspondent node is also acting as a mobile node, it 3240 MUST ignore type 2 routing headers and silently discard all 3241 packets that it has received with such headers. 3243 o The node SHOULD be able to interpret ICMP messages as described in 3244 Section 9.3.4. 3246 o The node MUST be able to send Binding Error messages as described 3247 in Section 9.3.3. 3249 o The node MUST be able to process Mobility Headers as described in 3250 Section 9.2. 3252 o The node MUST be able to participate in a return routability 3253 procedure (Section 9.4). 3255 o The node MUST be able to process Binding Update messages 3256 (Section 9.5). 3258 o The node MUST be able to return a Binding Acknowledgement 3259 (Section 9.5.4). 3261 o The node MUST be able to maintain a Binding Cache of the bindings 3262 received in accepted Binding Updates, as described in Section 9.1 3263 and Section 9.6. 3265 o The node SHOULD allow route optimization to be administratively 3266 enabled or disabled. The default SHOULD be enabled. 3268 8.3. All IPv6 Routers 3270 All IPv6 routers, even those not serving as a home agent for Mobile 3271 IPv6, have an effect on how well mobile nodes can communicate: 3273 o Every IPv6 router SHOULD be able to send an Advertisement Interval 3274 option (Section 7.3) in each of its Router Advertisements [20], to 3275 aid movement detection by mobile nodes (as in Section 11.5.1). 3276 The use of this option in Router Advertisements SHOULD be 3277 configurable. 3279 o Every IPv6 router SHOULD be able to support sending unsolicited 3280 multicast Router Advertisements at the faster rate described in 3281 Section 7.5. If the router supports a faster rate, the used rate 3282 MUST be configurable. 3284 o Each router SHOULD include at least one prefix with the Router 3285 Address (R) bit set and with its full IP address in its Router 3286 Advertisements (as described in Section 7.2). 3288 o Routers supporting filtering packets with routing headers SHOULD 3289 support different rules for type 0 and type 2 routing headers (see 3290 Section 6.4) so that filtering of source routed packets (type 0) 3291 will not necessarily limit Mobile IPv6 traffic which is delivered 3292 via type 2 routing headers. 3294 8.4. IPv6 Home Agents 3296 In order for a mobile node to operate correctly while away from home, 3297 at least one IPv6 router on the mobile node's home link must function 3298 as a home agent for the mobile node. The following additional 3299 requirements apply to all IPv6 routers that serve as a home agent: 3301 o Every home agent MUST be able to maintain an entry in its Binding 3302 Cache for each mobile node for which it is serving as the home 3303 agent (Section 10.1 and Section 10.3.1). 3305 o Every home agent MUST be able to intercept packets (using proxy 3306 Neighbor Discovery [20]) addressed to a mobile node for which it 3307 is currently serving as the home agent, on that mobile node's home 3308 link, while the mobile node is away from home (Section 10.4.1). 3310 o Every home agent MUST be able to encapsulate [9] such intercepted 3311 packets in order to tunnel them to the primary care-of address for 3312 the mobile node indicated in its binding in the home agent's 3313 Binding Cache (Section 10.4.2). 3315 o Every home agent MUST support decapsulating [9] reverse tunneled 3316 packets sent to it from a mobile node's home address. Every home 3317 agent MUST also check that the source address in the tunneled 3318 packets corresponds to the currently registered location of the 3319 mobile node (Section 10.4.5). 3321 o The node MUST be able to process Mobility Headers as described in 3322 Section 10.2. 3324 o Every home agent MUST be able to return a Binding Acknowledgement 3325 in response to a Binding Update (Section 10.3.1). 3327 o Every home agent MUST maintain a separate Home Agents List for 3328 each link on which it is serving as a home agent, as described in 3329 Section 10.1 and Section 10.5.1. 3331 o Every home agent MUST be able to accept packets addressed to the 3332 Mobile IPv6 Home-Agents anycast address [10] for the subnet on 3333 which it is serving as a home agent, and MUST be able to 3334 participate in dynamic home agent address discovery 3335 (Section 10.5). 3337 o Every home agent SHOULD support a configuration mechanism to allow 3338 a system administrator to manually set the value to be sent by 3339 this home agent in the Home Agent Preference field of the Home 3340 Agent Information Option in Router Advertisements that it sends 3341 (Section 7.4). 3343 o Every home agent SHOULD support sending ICMP Mobile Prefix 3344 Advertisements (Section 6.8), and SHOULD respond to Mobile Prefix 3345 Solicitations (Section 6.7). If supported, this behavior MUST be 3346 configurable, so that home agents can be configured to avoid 3347 sending such Prefix Advertisements according to the needs of the 3348 network administration in the home domain. 3350 o Every home agent MUST support IPsec ESP for protection of packets 3351 belonging to the return routability procedure (Section 10.4.6). 3353 o Every home agent SHOULD support the multicast group membership 3354 control protocols as described in Section 10.4.3. If this support 3355 is provided, the home agent MUST be capable of using it to 3356 determine which multicast data packets to forward via the tunnel 3357 to the mobile node. 3359 o Home agents MAY support stateful address autoconfiguration for 3360 mobile nodes as described in Section 10.4.4. 3362 8.5. IPv6 Mobile Nodes 3364 Finally, the following requirements apply to all IPv6 nodes capable 3365 of functioning as mobile nodes: 3367 o The node MUST maintain a Binding Update List (Section 11.1). 3369 o The node MUST support sending packets containing a Home Address 3370 option (Section 11.3.1), and follow the required IPsec interaction 3371 (Section 11.3.2). 3373 o The node MUST be able to perform IPv6 encapsulation and 3374 decapsulation [9]. 3376 o The node MUST be able to process type 2 routing header as defined 3377 in Section 6.4 and Section 11.3.3. 3379 o The node MUST support receiving a Binding Error message 3380 (Section 11.3.6). 3382 o The node MUST support receiving ICMP errors (Section 11.3.5). 3384 o The node MUST support movement detection, care-of address 3385 formation, and returning home (Section 11.5). 3387 o The node MUST be able to process Mobility Headers as described in 3388 Section 11.2. 3390 o The node MUST support the return routability procedure 3391 (Section 11.6). 3393 o The node MUST be able to send Binding Updates, as specified in 3394 Section 11.7.1 and Section 11.7.2. 3396 o The node MUST be able to receive and process Binding 3397 Acknowledgements, as specified in Section 11.7.3. 3399 o The node MUST support receiving a Binding Refresh Request 3400 (Section 6.1.2), by responding with a Binding Update. 3402 o The node MUST support receiving Mobile Prefix Advertisements 3403 (Section 11.4.3) and reconfiguring its home address based on the 3404 prefix information contained therein. 3406 o The node SHOULD support use of the dynamic home agent address 3407 discovery mechanism, as described in Section 11.4.1. 3409 o The node MUST allow route optimization to be administratively 3410 enabled or disabled. The default SHOULD be enabled. 3412 o The node MAY support the multicast address listener part of a 3413 multicast group membership protocol as described in 3414 Section 11.3.4. If this support is provided, the mobile node MUST 3415 be able to receive tunneled multicast packets from the home agent. 3417 o The node MAY support stateful address autoconfiguration mechanisms 3418 such as DHCPv6 [32] on the interface represented by the tunnel to 3419 the home agent. 3421 9. Correspondent Node Operation 3423 9.1. Conceptual Data Structures 3425 IPv6 nodes with route optimization support maintain a Binding Cache 3426 of bindings for other nodes. A separate Binding Cache SHOULD be 3427 maintained by each IPv6 node for each of its unicast routable 3428 addresses. The Binding Cache MAY be implemented in any manner 3429 consistent with the external behavior described in this document, for 3430 example by being combined with the node's Destination Cache as 3431 maintained by Neighbor Discovery [20]. When sending a packet, the 3432 Binding Cache is searched before the Neighbor Discovery conceptual 3433 Destination Cache [20]. 3435 Each Binding Cache entry conceptually contains the following fields: 3437 o The home address of the mobile node for which this is the Binding 3438 Cache entry. This field is used as the key for searching the 3439 Binding Cache for the destination address of a packet being sent. 3441 o The care-of address for the mobile node indicated by the home 3442 address field in this Binding Cache entry. 3444 o A lifetime value, indicating the remaining lifetime for this 3445 Binding Cache entry. The lifetime value is initialized from the 3446 Lifetime field in the Binding Update that created or last modified 3447 this Binding Cache entry. 3449 o A flag indicating whether or not this Binding Cache entry is a 3450 home registration entry (applicable only on nodes which support 3451 home agent functionality). 3453 o The maximum value of the Sequence Number field received in 3454 previous Binding Updates for this home address. The Sequence 3455 Number field is 16 bits long. Sequence Number values MUST be 3456 compared modulo 2**16 as explained in Section 9.5.1. 3458 o Usage information for this Binding Cache entry. This is needed to 3459 implement the cache replacement policy in use in the Binding 3460 Cache. Recent use of a cache entry also serves as an indication 3461 that a Binding Refresh Request should be sent when the lifetime of 3462 this entry nears expiration. 3464 Binding Cache entries not marked as home registrations MAY be 3465 replaced at any time by any reasonable local cache replacement policy 3466 but SHOULD NOT be unnecessarily deleted. The Binding Cache for any 3467 one of a node's IPv6 addresses may contain at most one entry for each 3468 mobile node home address. The contents of a node's Binding Cache 3469 MUST NOT be changed in response to a Home Address option in a 3470 received packet. 3472 9.2. Processing Mobility Headers 3474 Mobility Header processing MUST observe the following rules: 3476 o The checksum must be verified as per Section 6.1. Otherwise, the 3477 node MUST silently discard the message. 3479 o The MH Type field MUST have a known value (Section 6.1.1). 3480 Otherwise, the node MUST discard the message and issue a Binding 3481 Error message as described in Section 9.3.3, with Status field set 3482 to 2 (unrecognized MH Type value). 3484 o The Payload Proto field MUST be IPPROTO_NONE (59 decimal). 3485 Otherwise, the node MUST discard the message and SHOULD send ICMP 3486 Parameter Problem, Code 0, directly to the Source Address of the 3487 packet as specified in RFC 4443 [19]. Thus no Binding Cache 3488 information is used in sending the ICMP message. The Pointer 3489 field in the ICMP message SHOULD point at the Payload Proto field. 3491 o The Header Len field in the Mobility Header MUST NOT be less than 3492 the length specified for this particular type of message in 3493 Section 6.1. Otherwise, the node MUST discard the message and 3494 SHOULD send ICMP Parameter Problem, Code 0, directly to the Source 3495 Address of the packet as specified in RFC 4443 [19]. (The Binding 3496 Cache information is again not used.) The Pointer field in the 3497 ICMP message SHOULD point at the Header Len field. 3499 Subsequent checks depend on the particular Mobility Header. 3501 9.3. Packet Processing 3503 This section describes how the correspondent node sends packets to 3504 the mobile node, and receives packets from it. 3506 9.3.1. Receiving Packets with Home Address Option 3508 Packets containing a Home Address option MUST be dropped if the given 3509 home address is not a unicast routable address. 3511 Mobile nodes can include a Home Address destination option in a 3512 packet if they believe the correspondent node has a Binding Cache 3513 entry for the home address of a mobile node. Packets containing a 3514 Home Address option MUST be dropped if there is no corresponding 3515 Binding Cache entry. A corresponding Binding Cache entry MUST have 3516 the same home address as appears in the Home Address destination 3517 option, and the currently registered care-of address MUST be equal to 3518 the source address of the packet. These tests MUST NOT be done for 3519 packets that contain a Home Address option and a Binding Update. 3521 If the packet is dropped due the above tests, the correspondent node 3522 MUST send the Binding Error message as described in Section 9.3.3. 3523 The Status field in this message should be set to 1 (unknown binding 3524 for Home Address destination option). 3526 The correspondent node MUST process the option in a manner consistent 3527 with exchanging the Home Address field from the Home Address option 3528 into the IPv6 header and replacing the original value of the Source 3529 Address field there. After all IPv6 options have been processed, it 3530 MUST be possible for upper layers to process the packet without the 3531 knowledge that it came originally from a care-of address or that a 3532 Home Address option was used. 3534 The use of IPsec Authentication Header (AH) for the Home Address 3535 option is not required, except that if the IPv6 header of a packet is 3536 covered by AH, then the authentication MUST also cover the Home 3537 Address option; this coverage is achieved automatically by the 3538 definition of the Option Type code for the Home Address option, since 3539 it indicates that the data within the option cannot change en route 3540 to the packet's final destination, and thus the option is included in 3541 the AH computation. By requiring that any authentication of the IPv6 3542 header also cover the Home Address option, the security of the Source 3543 Address field in the IPv6 header is not compromised by the presence 3544 of a Home Address option. 3546 When attempting to verify AH authentication data in a packet that 3547 contains a Home Address option, the receiving node MUST calculate the 3548 AH authentication data as if the following were true: The Home 3549 Address option contains the care-of address, and the source IPv6 3550 address field of the IPv6 header contains the home address. This 3551 conforms with the calculation specified in Section 11.3.2. 3553 9.3.2. Sending Packets to a Mobile Node 3555 Before sending any packet, the sending node SHOULD examine its 3556 Binding Cache for an entry for the destination address to which the 3557 packet is being sent. If the sending node has a Binding Cache entry 3558 for this address, the sending node SHOULD use a type 2 routing header 3559 to route the packet to this mobile node (the destination node) by way 3560 of its care-of address. However, the sending node MUST NOT do this 3561 in the following cases: 3563 o When sending an IPv6 Neighbor Discovery [20] packet. 3565 o Where otherwise noted in Section 6.1. 3567 When calculating authentication data in a packet that contains a type 3568 2 routing header, the correspondent node MUST calculate the AH 3569 authentication data as if the following were true: The routing header 3570 contains the care-of address, the destination IPv6 address field of 3571 the IPv6 header contains the home address, and the Segments Left 3572 field is zero. The IPsec Security Policy Database lookup MUST based 3573 on the mobile node's home address. 3575 For instance, assuming there are no additional routing headers in 3576 this packet beyond those needed by Mobile IPv6, the correspondent 3577 node could set the fields in the packet's IPv6 header and routing 3578 header as follows: 3580 o The Destination Address in the packet's IPv6 header is set to the 3581 mobile node's home address (the original destination address to 3582 which the packet was being sent). 3584 o The routing header is initialized to contain a single route 3585 segment, containing the mobile node's care-of address copied from 3586 the Binding Cache entry. The Segments Left field is, however, 3587 temporarily set to zero. 3589 The IP layer will insert the routing header before performing any 3590 necessary IPsec processing. Once all IPsec processing has been 3591 performed, the node swaps the IPv6 destination field with the Home 3592 Address field in the routing header, sets the Segments Left field to 3593 one, and sends the packet. This ensures the AH calculation is done 3594 on the packet in the form it will have on the receiver after 3595 advancing the routing header. 3597 Following the definition of a type 2 routing header in Section 6.4, 3598 this packet will be routed to the mobile node's care-of address, 3599 where it will be delivered to the mobile node (the mobile node has 3600 associated the care-of address with its network interface). 3602 Note that following the above conceptual model in an implementation 3603 creates some additional requirements for path MTU discovery since the 3604 layer that decides the packet size (e.g., TCP and applications using 3605 UDP) needs to be aware of the size of the headers added by the IP 3606 layer on the sending node. 3608 If, instead, the sending node has no Binding Cache entry for the 3609 destination address to which the packet is being sent, the sending 3610 node simply sends the packet normally, with no routing header. If 3611 the destination node is not a mobile node (or is a mobile node that 3612 is currently at home), the packet will be delivered directly to this 3613 node and processed normally by it. If, however, the destination node 3614 is a mobile node that is currently away from home, the packet will be 3615 intercepted by the mobile node's home agent and tunneled to the 3616 mobile node's current primary care-of address. 3618 9.3.3. Sending Binding Error Messages 3620 Section 9.2 and Section 9.3.1 describe error conditions that lead to 3621 a need to send a Binding Error message. 3623 A Binding Error message is sent directly to the address that appeared 3624 in the IPv6 Source Address field of the offending packet. If the 3625 Source Address field does not contain a unicast address, the Binding 3626 Error message MUST NOT be sent. 3628 The Home Address field in the Binding Error message MUST be copied 3629 from the Home Address field in the Home Address destination option of 3630 the offending packet, or set to the unspecified address if no such 3631 option appeared in the packet. 3633 Note that the IPv6 Source Address and Home Address field values 3634 discussed above are the values from the wire, i.e., before any 3635 modifications possibly performed as specified in Section 9.3.1. 3637 Binding Error messages SHOULD be subject to rate limiting in the same 3638 manner as is done for ICMPv6 messages [19]. 3640 9.3.4. Receiving ICMP Error Messages 3642 When the correspondent node has a Binding Cache entry for a mobile 3643 node, all traffic destined to the mobile node goes directly to the 3644 current care-of address of the mobile node using a routing header. 3645 Any ICMP error message caused by packets on their way to the care-of 3646 address will be returned in the normal manner to the correspondent 3647 node. 3649 On the other hand, if the correspondent node has no Binding Cache 3650 entry for the mobile node, the packet will be routed through the 3651 mobile node's home link. Any ICMP error message caused by the packet 3652 on its way to the mobile node while in the tunnel, will be 3653 transmitted to the mobile node's home agent. By the definition of 3654 IPv6 encapsulation [9], the home agent MUST relay certain ICMP error 3655 messages back to the original sender of the packet, which in this 3656 case is the correspondent node. 3658 Thus, in all cases, any meaningful ICMP error messages caused by 3659 packets from a correspondent node to a mobile node will be returned 3660 to the correspondent node. If the correspondent node receives 3661 persistent ICMP Destination Unreachable messages after sending 3662 packets to a mobile node based on an entry in its Binding Cache, the 3663 correspondent node SHOULD delete this Binding Cache entry. Note that 3664 if the mobile node continues to send packets with the Home Address 3665 destination option to this correspondent node, they will be dropped 3666 due to the lack of a binding. For this reason it is important that 3667 only persistent ICMP messages lead to the deletion of the Binding 3668 Cache entry. 3670 9.4. Return Routability Procedure 3672 This subsection specifies actions taken by a correspondent node 3673 during the return routability procedure. 3675 9.4.1. Receiving Home Test Init Messages 3677 Upon receiving a Home Test Init message, the correspondent node 3678 verifies the following: 3680 o The packet MUST NOT include a Home Address destination option. 3682 Any packet carrying a Home Test Init message which fails to satisfy 3683 all of these tests MUST be silently ignored. 3685 Otherwise, in preparation for sending the corresponding Home Test 3686 Message, the correspondent node checks that it has the necessary 3687 material to engage in a return routability procedure, as specified in 3688 Section 5.2. The correspondent node MUST have a secret Kcn and a 3689 nonce. If it does not have this material yet, it MUST produce it 3690 before continuing with the return routability procedure. 3692 Section 9.4.3 specifies further processing. 3694 9.4.2. Receiving Care-of Test Init Messages 3696 Upon receiving a Care-of Test Init message, the correspondent node 3697 verifies the following: 3699 o The packet MUST NOT include a Home Address destination option. 3701 Any packet carrying a Care-of Test Init message which fails to 3702 satisfy all of these tests MUST be silently ignored. 3704 Otherwise, in preparation for sending the corresponding Care-of Test 3705 Message, the correspondent node checks that it has the necessary 3706 material to engage in a return routability procedure in the manner 3707 described in Section 9.4.1. 3709 Section 9.4.4 specifies further processing. 3711 9.4.3. Sending Home Test Messages 3713 The correspondent node creates a home keygen token and uses the 3714 current nonce index as the Home Nonce Index. It then creates a Home 3715 Test message (Section 6.1.5) and sends it to the mobile node at the 3716 latter's home address. 3718 9.4.4. Sending Care-of Test Messages 3720 The correspondent node creates a care-of keygen token and uses the 3721 current nonce index as the Care-of Nonce Index. It then creates a 3722 Care-of Test message (Section 6.1.6) and sends it to the mobile node 3723 at the latter's care-of address. 3725 9.5. Processing Bindings 3727 This section explains how the correspondent node processes messages 3728 related to bindings. These messages are: 3730 o Binding Update 3732 o Binding Refresh Request 3734 o Binding Acknowledgement 3736 o Binding Error 3738 9.5.1. Receiving Binding Updates 3740 Before accepting a Binding Update, the receiving node MUST validate 3741 the Binding Update according to the following tests: 3743 o The packet MUST contain a unicast routable home address, either in 3744 the Home Address option or in the Source Address, if the Home 3745 Address option is not present. 3747 o The Sequence Number field in the Binding Update is greater than 3748 the Sequence Number received in the previous valid Binding Update 3749 for this home address, if any. 3751 If the receiving node has no Binding Cache entry for the indicated 3752 home address, it MUST accept any Sequence Number value in a 3753 received Binding Update from this mobile node. 3755 This Sequence Number comparison MUST be performed modulo 2**16, 3756 i.e., the number is a free running counter represented modulo 3757 65536. A Sequence Number in a received Binding Update is 3758 considered less than or equal to the last received number if its 3759 value lies in the range of the last received number and the 3760 preceding 32768 values, inclusive. For example, if the last 3761 received sequence number was 15, then messages with sequence 3762 numbers 0 through 15, as well as 32783 through 65535, would be 3763 considered less than or equal. 3765 When the Home Registration (H) bit is not set, the following are also 3766 required: 3768 o A Nonce Indices mobility option MUST be present, and the Home and 3769 Care-of Nonce Index values in this option MUST be recent enough to 3770 be recognized by the correspondent node. (Care-of Nonce Index 3771 values are not inspected for requests to delete a binding.) 3773 o The correspondent node MUST re-generate the home keygen token and 3774 the care-of keygen token from the information contained in the 3775 packet. It then generates the binding management key Kbm and uses 3776 it to verify the authenticator field in the Binding Update as 3777 specified in Section 6.1.7. 3779 o The Binding Authorization Data mobility option MUST be present, 3780 and its contents MUST satisfy rules presented in Section 5.2.6. 3781 Note that a care-of address different from the Source Address MAY 3782 have been specified by including an Alternate Care-of Address 3783 mobility option in the Binding Update. When such a message is 3784 received and the return routability procedure is used as an 3785 authorization method, the correspondent node MUST verify the 3786 authenticator by using the address within the Alternate Care-of 3787 Address in the calculations. 3789 o The Binding Authorization Data mobility option MUST be the last 3790 option and MUST NOT have trailing padding. 3792 If the Home Registration (H) bit is set, the Nonce Indices mobility 3793 option MUST NOT be present. 3795 If the mobile node sends a sequence number which is not greater than 3796 the sequence number from the last valid Binding Update for this home 3797 address, then the receiving node MUST send back a Binding 3798 Acknowledgement with status code 135, and the last accepted sequence 3799 number in the Sequence Number field of the Binding Acknowledgement. 3801 If a binding already exists for the given home address and the home 3802 registration flag has a different value than the Home Registration 3803 (H) bit in the Binding Update, then the receiving node MUST send back 3804 a Binding Acknowledgement with status code 139 (registration type 3805 change disallowed). The home registration flag stored in the Binding 3806 Cache entry MUST NOT be changed. 3808 If the receiving node no longer recognizes the Home Nonce Index 3809 value, Care-of Nonce Index value, or both values from the Binding 3810 Update, then the receiving node MUST send back a Binding 3811 Acknowledgement with status code 136, 137, or 138, respectively. 3813 Packets carrying Binding Updates that fail to satisfy all of these 3814 tests for any reason other than insufficiency of the Sequence Number, 3815 registration type change, or expired nonce index values, MUST be 3816 silently discarded. 3818 If the Binding Update is valid according to the tests above, then the 3819 Binding Update is processed further as follows: 3821 o The Sequence Number value received from a mobile node in a Binding 3822 Update is stored by the receiving node in its Binding Cache entry 3823 for the given home address. 3825 o If the Lifetime specified in the Binding Update is nonzero and the 3826 specified care-of address is not equal to the home address for the 3827 binding, then this is a request to cache a binding for the home 3828 address. If the Home Registration (H) bit is set in the Binding 3829 Update, the Binding Update is processed according to the procedure 3830 specified in Section 10.3.1; otherwise, it is processed according 3831 to the procedure specified in Section 9.5.2. 3833 o If the Lifetime specified in the Binding Update is zero or the 3834 specified care-of address matches the home address for the 3835 binding, then this is a request to delete the cached binding for 3836 the home address. In this case, the Binding Update MUST include a 3837 valid home nonce index, and the care-of nonce index MUST be 3838 ignored by the correspondent node. The generation of the binding 3839 management key depends then exclusively on the home keygen token 3840 (Section 5.2.5). If the Home Registration (H) bit is set in the 3841 Binding Update, the Binding Update is processed according to the 3842 procedure specified in Section 10.3.2; otherwise, it is processed 3843 according to the procedure specified in Section 9.5.3. 3845 The specified care-of address MUST be determined as follows: 3847 o If the Alternate Care-of Address option is present, the care-of 3848 address is the address in that option. 3850 o Otherwise, the care-of address is the Source Address field in the 3851 packet's IPv6 header. 3853 The home address for the binding MUST be determined as follows: 3855 o If the Home Address destination option is present, the home 3856 address is the address in that option. 3858 o Otherwise, the home address is the Source Address field in the 3859 packet's IPv6 header. 3861 9.5.2. Requests to Cache a Binding 3863 This section describes the processing of a valid Binding Update that 3864 requests a node to cache a binding, for which the Home Registration 3865 (H) bit is not set in the Binding Update. 3867 In this case, the receiving node SHOULD create a new entry in its 3868 Binding Cache for this home address, or update its existing Binding 3869 Cache entry for this home address, if such an entry already exists. 3870 The lifetime for the Binding Cache entry is initialized from the 3871 Lifetime field specified in the Binding Update, although this 3872 lifetime MAY be reduced by the node caching the binding; the lifetime 3873 for the Binding Cache entry MUST NOT be greater than the Lifetime 3874 value specified in the Binding Update. Any Binding Cache entry MUST 3875 be deleted after the expiration of its lifetime. 3877 Note that if the mobile node did not request a Binding 3878 Acknowledgement, then it is not aware of the selected shorter 3879 lifetime. The mobile node may thus use route optimization and send 3880 packets with the Home Address destination option. As discussed in 3881 Section 9.3.1, such packets will be dropped if there is no binding. 3882 This situation is recoverable, but can cause temporary packet loss. 3884 The correspondent node MAY refuse to accept a new Binding Cache entry 3885 if it does not have sufficient resources. A new entry MAY also be 3886 refused if the correspondent node believes its resources are utilized 3887 more efficiently in some other purpose, such as serving another 3888 mobile node with higher amount of traffic. In both cases the 3889 correspondent node SHOULD return a Binding Acknowledgement with 3890 status value 130. 3892 9.5.3. Requests to Delete a Binding 3894 This section describes the processing of a valid Binding Update that 3895 requests a node to delete a binding when the Home Registration (H) 3896 bit is not set in the Binding Update. 3898 Any existing binding for the given home address MUST be deleted. A 3899 Binding Cache entry for the home address MUST NOT be created in 3900 response to receiving the Binding Update. 3902 If the Binding Cache entry was created by use of return routability 3903 nonces, the correspondent node MUST ensure that the same nonces are 3904 not used again with the particular home and care-of address. If both 3905 nonces are still valid, the correspondent node has to remember the 3906 particular combination of nonce indexes, addresses, and sequence 3907 number as illegal until at least one of the nonces has become too 3908 old. 3910 9.5.4. Sending Binding Acknowledgements 3912 A Binding Acknowledgement may be sent to indicate receipt of a 3913 Binding Update as follows: 3915 o If the Binding Update was discarded as described in Section 9.2 or 3916 Section 9.5.1, a Binding Acknowledgement MUST NOT be sent. 3917 Otherwise the treatment depends on the following rules. 3919 o If the Acknowledge (A) bit set is set in the Binding Update, a 3920 Binding Acknowledgement MUST be sent. Otherwise, the treatment 3921 depends on the below rule. 3923 o If the node rejects the Binding Update due to an expired nonce 3924 index, sequence number being out of window (Section 9.5.1), or 3925 insufficiency of resources (Section 9.5.2), a Binding 3926 Acknowledgement MUST be sent. If the node accepts the Binding 3927 Update, the Binding Acknowledgement SHOULD NOT be sent. 3929 If the node accepts the Binding Update and creates or updates an 3930 entry for this binding, the Status field in the Binding 3931 Acknowledgement MUST be set to a value less than 128. Otherwise, the 3932 Status field MUST be set to a value greater than or equal to 128. 3933 Values for the Status field are described in Section 6.1.8 and in the 3934 IANA registry of assigned numbers [13]. 3936 If the Status field in the Binding Acknowledgement contains the value 3937 136 (expired home nonce index), 137 (expired care-of nonce index), or 3938 138 (expired nonces) then the message MUST NOT include the Binding 3939 Authorization Data mobility option. Otherwise, the Binding 3940 Authorization Data mobility option MUST be included, and MUST meet 3941 the specific authentication requirements for Binding Acknowledgements 3942 as defined in Section 5.2. 3944 If the Source Address field of the IPv6 header that carried the 3945 Binding Update does not contain a unicast address, the Binding 3946 Acknowledgement MUST NOT be sent and the Binding Update packet MUST 3947 be silently discarded. Otherwise, the acknowledgement MUST be sent 3948 to the Source Address. Unlike the treatment of regular packets, this 3949 addressing procedure does not use information from the Binding Cache. 3951 However, a routing header is needed in some cases. If the Source 3952 Address is the home address of the mobile node, i.e., the Binding 3953 Update did not contain a Home Address destination option, then the 3954 Binding Acknowledgement MUST be sent to that address and the routing 3955 header MUST NOT be used. Otherwise, the Binding Acknowledgement MUST 3956 be sent using a type 2 routing header which contains the mobile 3957 node's home address. 3959 9.5.5. Sending Binding Refresh Requests 3961 If a Binding Cache entry being deleted is still in active use when 3962 sending packets to a mobile node, then the next packet sent to the 3963 mobile node will be routed normally to the mobile node's home link. 3964 Communication with the mobile node continues, but the tunneling from 3965 the home network creates additional overhead and latency in 3966 delivering packets to the mobile node. 3968 If the sender knows that the Binding Cache entry is still in active 3969 use, it MAY send a Binding Refresh Request message to the mobile node 3970 in an attempt to avoid this overhead and latency due to deleting and 3971 recreating the Binding Cache entry. This message is always sent to 3972 the home address of the mobile node. 3974 The correspondent node MAY retransmit Binding Refresh Request 3975 messages as long as the rate limitation is applied. The 3976 correspondent node MUST stop retransmitting when it receives a 3977 Binding Update. 3979 9.6. Cache Replacement Policy 3981 Conceptually, a node maintains a separate timer for each entry in its 3982 Binding Cache. When creating or updating a Binding Cache entry in 3983 response to a received and accepted Binding Update, the node sets the 3984 timer for this entry to the specified Lifetime period. Any entry in 3985 a node's Binding Cache MUST be deleted after the expiration of the 3986 Lifetime specified in the Binding Update from which the entry was 3987 created or last updated. 3989 Each node's Binding Cache will, by necessity, have a finite size. A 3990 node MAY use any reasonable local policy for managing the space 3991 within its Binding Cache. 3993 A node MAY choose to drop any entry already in its Binding Cache in 3994 order to make space for a new entry. For example, a "least-recently 3995 used" (LRU) strategy for cache entry replacement among entries should 3996 work well, unless the size of the Binding Cache is substantially 3997 insufficient. When entries are deleted, the correspondent node MUST 3998 follow the rules in Section 5.2.8 in order to guard the return 3999 routability procedure against replay attacks. 4001 If the node sends a packet to a destination for which it has dropped 4002 the entry from its Binding Cache, the packet will be routed through 4003 the mobile node's home link. The mobile node can detect this and 4004 establish a new binding if necessary. 4006 However, if the mobile node believes that the binding still exists, 4007 it may use route optimization and send packets with the Home Address 4008 destination option. This can create temporary packet loss, as 4009 discussed earlier, in the context of binding lifetime reductions 4010 performed by the correspondent node (Section 9.5.2). 4012 10. Home Agent Operation 4014 10.1. Conceptual Data Structures 4016 Each home agent MUST maintain a Binding Cache and Home Agents List. 4018 The rules for maintaining a Binding Cache are the same for home 4019 agents and correspondent nodes and have already been described in 4020 Section 9.1. 4022 The Home Agents List is maintained by each home agent, recording 4023 information about each router on the same link that is acting as a 4024 home agent. This list is used by the dynamic home agent address 4025 discovery mechanism. A router is known to be acting as a home agent, 4026 if it sends a Router Advertisement in which the Home Agent (H) bit is 4027 set. When the lifetime for a list entry (defined below) expires, 4028 that entry is removed from the Home Agents List. The Home Agents 4029 List is similar to the Default Router List conceptual data structure 4030 maintained by each host for Neighbor Discovery [20]. The Home Agents 4031 List MAY be implemented in any manner consistent with the external 4032 behavior described in this document. 4034 Each home agent maintains a separate Home Agents List for each link 4035 on which it is serving as a home agent. A new entry is created or an 4036 existing entry is updated in response to receipt of a valid Router 4037 Advertisement in which the Home Agent (H) bit is set. Each Home 4038 Agents List entry conceptually contains the following fields: 4040 o The link-local IP address of a home agent on the link. This 4041 address is learned through the Source Address of the Router 4042 Advertisements [20] received from the router. 4044 o One or more global IP addresses for this home agent. Global 4045 addresses are learned through Prefix Information options with the 4046 Router Address (R) bit set and received in Router Advertisements 4047 from this link-local address. Global addresses for the router in 4048 a Home Agents List entry MUST be deleted once the prefix 4049 associated with that address is no longer valid [20]. 4051 o The remaining lifetime of this Home Agents List entry. If a Home 4052 Agent Information Option is present in a Router Advertisement 4053 received from a home agent, the lifetime of the Home Agents List 4054 entry representing that home agent is initialized from the Home 4055 Agent Lifetime field in the option (if present); otherwise, the 4056 lifetime is initialized from the Router Lifetime field in the 4057 received Router Advertisement. If Home Agents List entry lifetime 4058 reaches zero, the entry MUST be deleted from the Home Agents List. 4060 o The preference for this home agent; higher values indicate a more 4061 preferable home agent. The preference value is taken from the 4062 Home Agent Preference field in the received Router Advertisement, 4063 if the Router Advertisement contains a Home Agent Information 4064 Option and is otherwise set to the default value of 0. A home 4065 agent uses this preference in ordering the Home Agents List when 4066 it sends an ICMP Home Agent Address Discovery message. 4068 10.2. Processing Mobility Headers 4070 All IPv6 home agents MUST observe the rules described in Section 9.2 4071 when processing Mobility Headers. 4073 10.3. Processing Bindings 4075 10.3.1. Primary Care-of Address Registration 4077 When a node receives a Binding Update, it MUST validate it and 4078 determine the type of Binding Update according to the steps described 4079 in Section 9.5.1. Furthermore, it MUST authenticate the Binding 4080 Update as described in Section 5.1. An authorization step specific 4081 for the home agent is also needed to ensure that only the right node 4082 can control a particular home address. This is provided through the 4083 home address unequivocally identifying the security association that 4084 must be used. 4086 This section describes the processing of a valid and authorized 4087 Binding Update when it requests the registration of the mobile node's 4088 primary care-of address. 4090 To begin processing the Binding Update, the home agent MUST perform 4091 the following sequence of tests: 4093 o If the node implements only correspondent node functionality, or 4094 has not been configured to act as a home agent, then the node MUST 4095 reject the Binding Update. The node MUST also return a Binding 4096 Acknowledgement to the mobile node, in which the Status field is 4097 set to 131 (home registration not supported). 4099 o Else, if the home address for the binding (the Home Address field 4100 in the packet's Home Address option) is not an on-link IPv6 4101 address with respect to the home agent's current Prefix List, then 4102 the home agent MUST reject the Binding Update and SHOULD return a 4103 Binding Acknowledgement to the mobile node, in which the Status 4104 field is set to 132 (not home subnet). 4106 o Else, if the home agent chooses to reject the Binding Update for 4107 any other reason (e.g., insufficient resources to serve another 4108 mobile node as a home agent), then the home agent SHOULD return a 4109 Binding Acknowledgement to the mobile node, in which the Status 4110 field is set to an appropriate value to indicate the reason for 4111 the rejection. 4113 o A Home Address destination option MUST be present in the message. 4114 It MUST be validated as described in Section 9.3.1 with the 4115 following additional rule. The Binding Cache entry existence test 4116 MUST NOT be done for IPsec packets when the Home Address option 4117 contains an address for which the receiving node could act as a 4118 home agent. 4120 If home agent accepts the Binding Update, it MUST then create a new 4121 entry in its Binding Cache for this mobile node or update its 4122 existing Binding Cache entry, if such an entry already exists. The 4123 Home Address field as received in the Home Address option provides 4124 the home address of the mobile node. 4126 The home agent MUST mark this Binding Cache entry as a home 4127 registration to indicate that the node is serving as a home agent for 4128 this binding. Binding Cache entries marked as a home registration 4129 MUST be excluded from the normal cache replacement policy used for 4130 the Binding Cache (Section 9.6) and MUST NOT be removed from the 4131 Binding Cache until the expiration of the Lifetime period. 4133 Unless this home agent already has a binding for the given home 4134 address, the home agent MUST perform Duplicate Address Detection [21] 4135 on the mobile node's home link before returning the Binding 4136 Acknowledgement. This ensures that no other node on the home link 4137 was using the mobile node's home address when the Binding Update 4138 arrived. If this Duplicate Address Detection fails for the given 4139 home address or an associated link local address, then the home agent 4140 MUST reject the complete Binding Update and MUST return a Binding 4141 Acknowledgement to the mobile node, in which the Status field is set 4142 to 134 (Duplicate Address Detection failed). When the home agent 4143 sends a successful Binding Acknowledgement to the mobile node, the 4144 home agent assures to the mobile node that its address(es) will be 4145 kept unique by the home agent for as long as the lifetime was granted 4146 for the binding. 4148 The specific addresses, which are to be tested before accepting the 4149 Binding Update and later to be defended by performing Duplicate 4150 Address Detection, depend on the setting of the Link-Local Address 4151 Compatibility (L) bit, as follows: 4153 o L=0: Defend only the given address. Do not derive a link-local 4154 address. 4156 o L=1: Defend both the given non link-local unicast (home) address 4157 and the derived link-local. The link-local address is derived by 4158 replacing the subnet prefix in the mobile node's home address with 4159 the link-local prefix. 4161 The lifetime of the Binding Cache entry depends on a number of 4162 factors: 4164 o The lifetime for the Binding Cache entry MUST NOT be greater than 4165 the Lifetime value specified in the Binding Update. 4167 o The lifetime for the Binding Cache entry MUST NOT be greater than 4168 the remaining valid lifetime for the subnet prefix in the mobile 4169 node's home address specified with the Binding Update. The 4170 remaining valid lifetime for this prefix is determined by the home 4171 agent based on its own Prefix List entry [20]. 4173 The remaining preferred lifetime SHOULD NOT have any impact on the 4174 lifetime for the binding cache entry. 4176 The home agent MUST remove a binding when the valid lifetime of 4177 the prefix associated with it expires. 4179 o The home agent MAY further decrease the specified lifetime for the 4180 binding, for example based on a local policy. The resulting 4181 lifetime is stored by the home agent in the Binding Cache entry, 4182 and this Binding Cache entry MUST be deleted by the home agent 4183 after the expiration of this lifetime. 4185 Regardless of the setting of the Acknowledge (A) bit in the Binding 4186 Update, the home agent MUST return a Binding Acknowledgement to the 4187 mobile node constructed as follows: 4189 o The Status field MUST be set to a value indicating success. The 4190 value 1 (accepted but prefix discovery necessary) MUST be used if 4191 the subnet prefix of the specified home address is deprecated, or 4192 becomes deprecated during the lifetime of the binding, or becomes 4193 invalid at the end of the lifetime. The value 0 MUST be used 4194 otherwise. For the purposes of comparing the binding and prefix 4195 lifetimes, the prefix lifetimes are first converted into units of 4196 four seconds by ignoring the two least significant bits. 4198 o The Key Management Mobility Capability (K) bit is set if the 4199 following conditions are all fulfilled, and cleared otherwise: 4201 * The Key Management Mobility Capability (K) bit was set in the 4202 Binding Update. 4204 * The IPsec security associations between the mobile node and the 4205 home agent have been established dynamically. 4207 * The home agent has the capability to update its endpoint in the 4208 used key management protocol to the new care-of address every 4209 time it moves. 4211 Depending on the final value of the bit in the Binding 4212 Acknowledgement, the home agent SHOULD perform the following 4213 actions: 4215 K = 0 4217 Discard key management connections, if any, to the old care-of 4218 address. If the mobile node did not have a binding before 4219 sending this Binding Update, discard the connections to the 4220 home address. 4222 K = 1 4224 Move the peer endpoint of the key management protocol 4225 connection, if any, to the new care-of address. For an IKE 4226 phase 1 connection, this means that any IKE packets sent to the 4227 peer are sent to this address, and packets from this address 4228 with the original ISAKMP cookies are accepted. 4230 Note that RFC 2408 [6] Section 2.5.3 gives specific rules that 4231 ISAKMP cookies must satisfy: they must depend on specific 4232 parties and can only be generated by the entity itself. Then 4233 it recommends a particular way to do this, namely a hash of IP 4234 addresses. With the K bit set to 1, the recommended 4235 implementation technique does not work directly. To satisfy 4236 the two rules, the specific parties must be treated as the 4237 original IP addresses, not the ones in use at the specific 4238 moment. 4240 o The Sequence Number field MUST be copied from the Sequence Number 4241 given in the Binding Update. 4243 o The Lifetime field MUST be set to the remaining lifetime for the 4244 binding as set by the home agent in its home registration Binding 4245 Cache entry for the mobile node, as described above. 4247 o If the home agent stores the Binding Cache entry in nonvolatile 4248 storage, then the Binding Refresh Advice mobility option MUST be 4249 omitted. Otherwise, the home agent MAY include this option to 4250 suggest that the mobile node refreshes its binding before the 4251 actual lifetime of the binding ends. 4253 If the Binding Refresh Advice mobility option is present, the 4254 Refresh Interval field in the option MUST be set to a value less 4255 than the Lifetime value being returned in the Binding 4256 Acknowledgement. This indicates that the mobile node SHOULD 4257 attempt to refresh its home registration at the indicated shorter 4258 interval. The home agent MUST still retain the registration for 4259 the Lifetime period, even if the mobile node does not refresh its 4260 registration within the Refresh period. 4262 The rules for selecting the Destination IP address (and possibly 4263 routing header construction) for the Binding Acknowledgement to the 4264 mobile node are the same as in Section 9.5.4. 4266 In addition, the home agent MUST follow the procedure defined in 4267 Section 10.4.1 to intercept packets on the mobile node's home link 4268 addressed to the mobile node, while the home agent is serving as the 4269 home agent for this mobile node. The home agent MUST also be 4270 prepared to accept reverse tunneled packets from the new care-of 4271 address of the mobile node, as described in Section 10.4.5. Finally, 4272 the home agent MUST also propagate new home network prefixes, as 4273 described in Section 10.6. 4275 10.3.2. Primary Care-of Address De-Registration 4277 A binding may need to be de-registered when the mobile node returns 4278 home or when the mobile node knows that it will not have any care-of 4279 addresses in the visited network. 4281 A Binding Update is validated and authorized in the manner described 4282 in the previous section; note that when the mobile node de-registers 4283 when it is at home, it may not include the Home Address destination 4284 option, in which case the mobile node's home address is the source IP 4285 address of the de-registration Binding Update. This section 4286 describes the processing of a valid Binding Update that requests the 4287 receiving node to no longer serve as its home agent, de-registering 4288 its primary care-of address. 4290 To begin processing the Binding Update, the home agent MUST perform 4291 the following test: 4293 o If the receiving node has no entry marked as a home registration 4294 in its Binding Cache for this mobile node, then this node MUST 4295 reject the Binding Update and SHOULD return a Binding 4296 Acknowledgement to the mobile node, in which the Status field is 4297 set to 133 (not home agent for this mobile node). 4299 If the home agent does not reject the Binding Update as described 4300 above, then it MUST delete any existing entry in its Binding Cache 4301 for this mobile node. Then, the home agent MUST return a Binding 4302 Acknowledgement to the mobile node, constructed as follows: 4304 o The Status field MUST be set to a value 0, indicating success. 4306 o The Key Management Mobility Capability (K) bit is set or cleared 4307 and actions based on its value are performed as described in the 4308 previous section. The mobile node's home address is used as its 4309 new care-of address for the purposes of moving the key management 4310 connection to a new endpoint. 4312 o The Sequence Number field MUST be copied from the Sequence Number 4313 given in the Binding Update. 4315 o The Lifetime field MUST be set to zero. 4317 o The Binding Refresh Advice mobility option MUST be omitted. 4319 In addition, the home agent MUST stop intercepting packets on the 4320 mobile node's home link that are addressed to the mobile node 4321 (Section 10.4.1). 4323 The rules for selecting the Destination IP address (and, if required, 4324 routing header construction) for the Binding Acknowledgement to the 4325 mobile node are the same as in the previous section. When the Status 4326 field in the Binding Acknowledgement is greater than or equal to 128 4327 and the Source Address of the Binding Update is on the home link, the 4328 home agent MUST send it to the mobile node's link layer address 4329 (retrieved either from the Binding Update or through Neighbor 4330 Solicitation). 4332 10.4. Packet Processing 4334 10.4.1. Intercepting Packets for a Mobile Node 4336 While a node is serving as the home agent for mobile node it MUST 4337 attempt to intercept packets on the mobile node's home link that are 4338 addressed to the mobile node. 4340 In order to do this, when a node begins serving as the home agent it 4341 MUST multicast onto the home link a Neighbor Advertisement message 4342 [20] on behalf of the mobile node. For the home address specified in 4343 the Binding Update, the home agent sends a Neighbor Advertisement 4344 message [20] to the all-nodes multicast address on the home link to 4345 advertise the home agent's own link-layer address for this IP address 4346 on behalf of the mobile node. If the Link-Layer Address 4347 Compatibility (L) flag has been specified in the Binding Update, the 4348 home agent MUST do the same for the link-local address of the mobile 4349 node. 4351 All fields in each Neighbor Advertisement message SHOULD be set in 4352 the same way they would be set by the mobile node if it was sending 4353 this Neighbor Advertisement [20] while at home, with the following 4354 exceptions: 4356 o The Target Address in the Neighbor Advertisement MUST be set to 4357 the specific IP address for the mobile node. 4359 o The Advertisement MUST include a Target Link-layer Address option 4360 specifying the home agent's link-layer address. 4362 o The Router (R) bit in the Advertisement MUST be set to zero. 4364 o The Solicited Flag (S) in the Advertisement MUST NOT be set, since 4365 it was not solicited by any Neighbor Solicitation. 4367 o The Override Flag (O) in the Advertisement MUST be set, indicating 4368 that the Advertisement SHOULD override any existing Neighbor Cache 4369 entry at any node receiving it. 4371 o The Source Address in the IPv6 header MUST be set to the home 4372 agent's IP address on the interface used to send the 4373 advertisement. 4375 Any node on the home link that receives one of the Neighbor 4376 Advertisement messages (described above) will update its Neighbor 4377 Cache to associate the mobile node's address with the home agent's 4378 link layer address, causing it to transmit any future packets 4379 normally destined to the mobile node to the mobile node's home agent. 4380 Since multicasting on the local link (such as Ethernet) is typically 4381 not guaranteed to be reliable, the home agent MAY retransmit this 4382 Neighbor Advertisement message up to MAX_NEIGHBOR_ADVERTISEMENT (see 4383 [20]) times to increase its reliability. It is still possible that 4384 some nodes on the home link will not receive any of the Neighbor 4385 Advertisements, but these nodes will eventually be able to detect the 4386 link-layer address change for the mobile node's address through use 4387 of Neighbor Unreachability Detection [20]. 4389 While a node is serving as a home agent for some mobile node, the 4390 home agent uses IPv6 Neighbor Discovery [20] to intercept unicast 4391 packets on the home link addressed to the mobile node. In order to 4392 intercept packets in this way, the home agent MUST act as a proxy for 4393 this mobile node and reply to any received Neighbor Solicitations for 4394 it. When a home agent receives a Neighbor Solicitation, it MUST 4395 check if the Target Address specified in the message matches the 4396 address of any mobile node for which it has a Binding Cache entry 4397 marked as a home registration. 4399 If such an entry exists in the home agent's Binding Cache, the home 4400 agent MUST reply to the Neighbor Solicitation with a Neighbor 4401 Advertisement giving the home agent's own link-layer address as the 4402 link-layer address for the specified Target Address. In addition, 4403 the Router (R) bit in the Advertisement MUST be set to zero. Acting 4404 as a proxy in this way allows other nodes on the mobile node's home 4405 link to resolve the mobile node's address and for the home agent to 4406 defend these addresses on the home link for Duplicate Address 4407 Detection [20]. 4409 10.4.2. Processing Intercepted Packets 4411 For any packet sent to a mobile node from the mobile node's home 4412 agent (in which the home agent is the original sender of the packet), 4413 the home agent is operating as a correspondent node of the mobile 4414 node for this packet and the procedures described in Section 9.3.2 4415 apply. The home agent then uses a routing header to route the packet 4416 to the mobile node by way of the primary care-of address in the home 4417 agent's Binding Cache. 4419 While the mobile node is away from home, the home agent intercepts 4420 any packets on the home link addressed to the mobile node's home 4421 address, as described in Section 10.4.1. In order to forward each 4422 intercepted packet to the mobile node, the home agent MUST tunnel the 4423 packet to the mobile node using IPv6 encapsulation [9]. When a home 4424 agent encapsulates an intercepted packet for forwarding to the mobile 4425 node, the home agent sets the Source Address in the new tunnel IP 4426 header to the home agent's own IP address and sets the Destination 4427 Address in the tunnel IP header to the mobile node's primary care-of 4428 address. When received by the mobile node, normal processing of the 4429 tunnel header [9] will result in decapsulation and processing of the 4430 original packet by the mobile node. 4432 However, packets addressed to the mobile node's link-local address 4433 MUST NOT be tunneled to the mobile node. Instead, these packets MUST 4434 be discarded and the home agent SHOULD return an ICMP Destination 4435 Unreachable, Code 3, message to the packet's Source Address (unless 4436 this Source Address is a multicast address). 4438 Interception and tunneling of the following multicast addressed 4439 packets on the home network are only done if the home agent supports 4440 multicast group membership control messages from the mobile node as 4441 described in the next section. Tunneling of multicast packets to a 4442 mobile node follows similar limitations to those defined above for 4443 unicast packets addressed to the mobile node's link-local address. 4444 Multicast packets addressed to a multicast address with link-local 4445 scope [18], to which the mobile node is subscribed, MUST NOT be 4446 tunneled to the mobile node. These packets SHOULD be silently 4447 discarded (after delivering to other local multicast recipients). 4448 Multicast packets addressed to a multicast address with a scope 4449 larger than link-local, but smaller than global (e.g., site-local and 4450 organization-local [18]), to which the mobile node is subscribed, 4451 SHOULD NOT be tunneled to the mobile node. Multicast packets 4452 addressed with a global scope, to which the mobile node has 4453 successfully subscribed, MUST be tunneled to the mobile node. 4455 Before tunneling a packet to the mobile node, the home agent MUST 4456 perform any IPsec processing as indicated by the security policy data 4457 base. 4459 10.4.3. Multicast Membership Control 4461 This section is a prerequisite for the multicast data packet 4462 forwarding, described in the previous section. If this support is 4463 not provided, multicast group membership control messages are 4464 silently ignored. 4466 In order to forward multicast data packets from the home network to 4467 all the proper mobile nodes, the home agent SHOULD be capable of 4468 receiving tunneled multicast group membership control information 4469 from the mobile node in order to determine which groups the mobile 4470 node has subscribed to. These multicast group membership messages 4471 are Listener Report messages specified in MLD [11] or in other 4472 protocols such as [40]. 4474 The messages are issued by the mobile node, but sent through the 4475 reverse tunnel to the home agent. These messages are issued whenever 4476 the mobile node decides to enable reception of packets for a 4477 multicast group or in response to an MLD Query from the home agent. 4478 The mobile node will also issue multicast group control messages to 4479 disable reception of multicast packets when it is no longer 4480 interested in receiving multicasts for a particular group. 4482 To obtain the mobile node's current multicast group membership the 4483 home agent must periodically transmit MLD Query messages through the 4484 tunnel to the mobile node. These MLD periodic transmissions will 4485 ensure the home agent has an accurate record of the groups in which 4486 the mobile node is interested despite packet losses of the mobile 4487 node's MLD group membership messages. 4489 All MLD packets are sent directly between the mobile node and the 4490 home agent. Since all of these packets are destined to a link-scope 4491 multicast address and have a hop limit of 1, there is no direct 4492 forwarding of such packets between the home network and the mobile 4493 node. The MLD packets between the mobile node and the home agent are 4494 encapsulated within the same tunnel header used for other packet 4495 flows between the mobile node and home agent. 4497 Note that at this time, even though a link-local source is used on 4498 MLD packets, no functionality depends on these addresses being 4499 unique, nor do they elicit direct responses. All MLD messages are 4500 sent to multicast destinations. To avoid ambiguity on the home 4501 agent, due to mobile nodes which may choose identical link-local 4502 source addresses for their MLD function, it is necessary for the home 4503 agent to identify which mobile node was actually the issuer of a 4504 particular MLD message. This may be accomplished by noting which 4505 tunnel such an MLD arrived by, which IPsec SA was used, or by other 4506 distinguishing means. 4508 This specification puts no requirement on how the functions in this 4509 section and the multicast forwarding in Section 10.4.2 are to be 4510 achieved. At the time of this writing it was thought that a full 4511 IPv6 multicast router function would be necessary on the home agent, 4512 but it may be possible to achieve the same effects through a "proxy 4513 MLD" application coupled with kernel multicast forwarding. This may 4514 be the subject of future specifications. 4516 10.4.4. Stateful Address Autoconfiguration 4518 This section describes how home agents support the use of stateful 4519 address autoconfiguration mechanisms such as DHCPv6 [32] from the 4520 mobile nodes. If this support is not provided, then the M and O bits 4521 must remain cleared on the Mobile Prefix Advertisement Messages. Any 4522 mobile node which sends DHCPv6 messages to the home agent without 4523 this support will not receive a response. 4525 If DHCPv6 is used, packets are sent with link-local source addresses 4526 either to a link-scope multicast address or a link-local address. 4527 Mobile nodes desiring to locate a DHCPv6 service may reverse tunnel 4528 standard DHCPv6 packets to the home agent. Since these link-scope 4529 packets cannot be forwarded onto the home network, it is necessary 4530 for the home agent to either implement a DHCPv6 relay agent or a 4531 DHCPv6 server function itself. The arriving tunnel or IPsec SA of 4532 DHCPv6 link-scope messages from the mobile node must be noted so that 4533 DHCPv6 responses may be sent back to the appropriate mobile node. 4534 DHCPv6 messages sent to the mobile node with a link-local destination 4535 must be tunneled within the same tunnel header used for other packet 4536 flows. 4538 10.4.5. Handling Reverse Tunneled Packets 4540 Unless a binding has been established between the mobile node and a 4541 correspondent node, traffic from the mobile node to the correspondent 4542 node goes through a reverse tunnel. Home agents MUST support reverse 4543 tunneling as follows: 4545 o The tunneled traffic arrives to the home agent's address using 4546 IPv6 encapsulation [9]. 4548 o Depending on the security policies used by the home agent, reverse 4549 tunneled packets MAY be discarded unless accompanied by a valid 4550 ESP header. The support for authenticated reverse tunneling 4551 allows the home agent to protect the home network and 4552 correspondent nodes from malicious nodes masquerading as a mobile 4553 node. 4555 o Otherwise, when a home agent decapsulates a tunneled packet from 4556 the mobile node, the home agent MUST verify that the Source 4557 Address in the tunnel IP header is the mobile node's primary 4558 care-of address. Otherwise, any node in the Internet could send 4559 traffic through the home agent and escape ingress filtering 4560 limitations. This simple check forces the attacker to know the 4561 current location of the real mobile node and be able to defeat 4562 ingress filtering. This check is not necessary if the reverse- 4563 tunneled packet is protected by ESP in tunnel mode. 4565 10.4.6. Protecting Return Routability Packets 4567 The return routability procedure, described in Section 5.2.5, assumes 4568 that the confidentiality of the Home Test Init and Home Test messages 4569 is protected as they are tunneled between the home agent and the 4570 mobile node. Therefore, the home agent MUST support tunnel mode 4571 IPsec ESP for the protection of packets belonging to the return 4572 routability procedure. Support for a non-null encryption transform 4573 and authentication algorithm MUST be available. It is not necessary 4574 to distinguish between different kinds of packets during the return 4575 routability procedure. 4577 Security associations are needed to provide this protection. When 4578 the care-of address for the mobile node changes as a result of an 4579 accepted Binding Update, special treatment is needed for the next 4580 packets sent using these security associations. The home agent MUST 4581 set the new care-of address as the destination address of these 4582 packets, as if the outer header destination address in the security 4583 association had changed [15]. 4585 The above protection SHOULD be used with all mobile nodes. The use 4586 is controlled by configuration of the IPsec security policy database 4587 both at the mobile node and at the home agent. 4589 As described earlier, the Binding Update and Binding Acknowledgement 4590 messages require protection between the home agent and the mobile 4591 node. The Mobility Header protocol carries both these messages as 4592 well as the return routability messages. From the point of view of 4593 the security policy database these messages are indistinguishable. 4594 When IPsec is used to protect return routability signaling or payload 4595 packets, this protection MUST only be applied to the return 4596 routability packets entering the IPv6 encapsulated tunnel interface 4597 between the mobile node and the home agent. This can be achieved, 4598 for instance, by defining the security policy database entries 4599 specifically for the tunnel interface. That is, the policy entries 4600 are not generally applied on all traffic on the physical interface(s) 4601 of the nodes, but rather only on traffic that enters the tunnel. 4602 This makes use of per-interface security policy database entries [2] 4603 specific to the tunnel interface (the node's attachment to the tunnel 4604 [8]). 4606 10.5. Dynamic Home Agent Address Discovery 4608 This section describes how a home agent can help mobile nodes to 4609 discover the addresses of the home agents. The home agent keeps 4610 track of the other home agents on the same link and responds to 4611 queries sent by the mobile node. 4613 10.5.1. Receiving Router Advertisement Messages 4615 For each link on which a router provides service as a home agent, the 4616 router maintains a Home Agents List recording information about all 4617 other home agents on that link. This list is used in the dynamic 4618 home agent address discovery mechanism, described in Section 10.5. 4619 The information for the list is learned through receipt of the 4620 periodic unsolicited multicast Router Advertisements, in a manner 4621 similar to the Default Router List conceptual data structure 4622 maintained by each host for Neighbor Discovery [20]. In the 4623 construction of the Home Agents List, the Router Advertisements are 4624 from each (other) home agent on the link and the Home Agent (H) bit 4625 is set in them. 4627 On receipt of a valid Router Advertisement, as defined in the 4628 processing algorithm specified for Neighbor Discovery [20], the home 4629 agent performs the following steps in addition to any steps already 4630 required of it by Neighbor Discovery: 4632 o If the Home Agent (H) bit in the Router Advertisement is not set, 4633 delete the sending node's entry in the current Home Agents List 4634 (if one exists). Skip all the following steps. 4636 o Otherwise, extract the Source Address from the IP header of the 4637 Router Advertisement. This is the link-local IP address on this 4638 link of the home agent sending this Advertisement [20]. 4640 o Determine the preference for this home agent. If the Router 4641 Advertisement contains a Home Agent Information Option, then the 4642 preference is taken from the Home Agent Preference field in the 4643 option; otherwise, the default preference of 0 MUST be used. 4645 o Determine the lifetime for this home agent. If the Router 4646 Advertisement contains a Home Agent Information Option, then the 4647 lifetime is taken from the Home Agent Lifetime field in the 4648 option; otherwise, the lifetime specified by the Router Lifetime 4649 field in the Router Advertisement SHOULD be used. 4651 o If the link-local address of the home agent sending this 4652 Advertisement is already present in this home agent's Home Agents 4653 List and the received home agent lifetime value is zero, 4654 immediately delete this entry in the Home Agents List. 4656 o Otherwise, if the link-local address of the home agent sending 4657 this Advertisement is already present in the receiving home 4658 agent's Home Agents List, reset its lifetime and preference to the 4659 values determined above. 4661 o If the link-local address of the home agent sending this 4662 Advertisement is not already present in the Home Agents List 4663 maintained by the receiving home agent, and the lifetime for the 4664 sending home agent is non-zero, create a new entry in the list, 4665 and initialize its lifetime and preference to the values 4666 determined above. 4668 o If the Home Agents List entry for the link-local address of the 4669 home agent sending this Advertisement was not deleted as described 4670 above, determine any global address(es) of the home agent based on 4671 each Prefix Information option received in this Advertisement in 4672 which the Router Address (R) bit is set (Section 7.2). Add all 4673 such global addresses to the list of global addresses in this Home 4674 Agents List entry. 4676 A home agent SHOULD maintain an entry in its Home Agents List for 4677 each valid home agent address until that entry's lifetime expires, 4678 after which time the entry MUST be deleted. 4680 As described in Section 11.4.1, a mobile node attempts dynamic home 4681 agent address discovery by sending an ICMP Home Agent Address 4682 Discovery Request message to the Mobile IPv6 Home-Agents anycast 4683 address [10] for its home IP subnet prefix. A home agent receiving a 4684 Home Agent Address Discovery Request message that serves this subnet 4685 SHOULD return an ICMP Home Agent Address Discovery Reply message to 4686 the mobile node with the Source Address of the Reply packet set to 4687 one of the global unicast addresses of the home agent. The Home 4688 Agent Addresses field in the Reply message is constructed as follows: 4690 o The Home Agent Addresses field SHOULD contain all global IP 4691 addresses for each home agent currently listed in this home 4692 agent's own Home Agents List (Section 10.1). 4694 o The IP addresses in the Home Agent Addresses field SHOULD be 4695 listed in order of decreasing preference values, based either on 4696 the respective advertised preference from a Home Agent Information 4697 option or on the default preference of 0 if no preference is 4698 advertised (or on the configured home agent preference for this 4699 home agent itself). 4701 o Among home agents with equal preference, their IP addresses in the 4702 Home Agent Addresses field SHOULD be listed in an order randomized 4703 with respect to other home agents with equal preference every time 4704 a Home Agent Address Discovery Reply message is returned by this 4705 home agent. 4707 o If more than one global IP address is associated with a home 4708 agent, these addresses SHOULD be listed in a randomized order. 4710 o The home agent SHOULD reduce the number of home agent IP addresses 4711 so that the packet fits within the minimum IPv6 MTU [8]. The home 4712 agent addresses selected for inclusion in the packet SHOULD be 4713 those from the complete list with the highest preference. This 4714 limitation avoids the danger of the Reply message packet being 4715 fragmented (or rejected by an intermediate router with an ICMP 4716 Packet Too Big message [19]). 4718 10.6. Sending Prefix Information to the Mobile Node 4720 10.6.1. List of Home Network Prefixes 4722 Mobile IPv6 arranges to propagate relevant prefix information to the 4723 mobile node when it is away from home, so that it may be used in 4724 mobile node home address configuration and in network renumbering. 4725 In this mechanism, mobile nodes away from home receive Mobile Prefix 4726 Advertisements messages. These messages include Prefix Information 4727 Options for the prefixes configured on the home subnet interface(s) 4728 of the home agent. 4730 If there are multiple home agents, differences in the advertisements 4731 sent by different home agents can lead to an inability to use a 4732 particular home address when changing to another home agent. In 4733 order to ensure that the mobile nodes get the same information from 4734 different home agents, it is preferred that all of the home agents on 4735 the same link be configured in the same manner. 4737 To support this, the home agent monitors prefixes advertised by 4738 itself and other home agents on the home link. In RFC 4861 [20] it 4739 is acceptable for two routers to advertise different sets of prefixes 4740 on the same link. For home agents, the differences should be 4741 detected for a given home address because the mobile node 4742 communicates only with one home agent at a time and the mobile node 4743 needs to know the full set of prefixes assigned to the home link. 4744 All other comparisons of Router Advertisements are as specified in 4745 Section 6.2.7 of RFC 4861. 4747 10.6.2. Scheduling Prefix Deliveries 4749 A home agent serving a mobile node will schedule the delivery of the 4750 new prefix information to that mobile node when any of the following 4751 conditions occur: 4753 MUST: 4755 o The state of the flags changes for the prefix of the mobile node's 4756 registered home address. 4758 o The valid or preferred lifetime is reconfigured or changes for any 4759 reason other than advancing real time. 4761 o The mobile node requests the information with a Mobile Prefix 4762 Solicitation (see Section 11.4.2). 4764 SHOULD: 4766 o A new prefix is added to the home subnet interface(s) of the home 4767 agent. 4769 MAY: 4771 o The valid or preferred lifetime or the state of the flags changes 4772 for a prefix which is not used in any Binding Cache entry for this 4773 mobile node. 4775 The home agent uses the following algorithm to determine when to send 4776 prefix information to the mobile node. 4778 o If a mobile node sends a solicitation, answer right away. 4780 o If no Mobile Prefix Advertisement has been sent to the mobile node 4781 in the last MaxMobPfxAdvInterval seconds (see Section 13), then 4782 ensure that a transmission is scheduled. The actual transmission 4783 time is randomized as described below. 4785 o If a prefix matching the mobile node's home registration is added 4786 on the home subnet interface or if its information changes in any 4787 way that does not deprecate the mobile node's address, ensure that 4788 a transmission is scheduled. The actual transmission time is 4789 randomized as described below. 4791 o If a home registration expires, cancel any scheduled 4792 advertisements to the mobile node. 4794 The list of prefixes is sent in its entirety in all cases. 4796 If the home agent has already scheduled the transmission of a Mobile 4797 Prefix Advertisement to the mobile node, then the home agent will 4798 replace the advertisement with a new one to be sent at the scheduled 4799 time. 4801 Otherwise, the home agent computes a fresh value for RAND_ADV_DELAY 4802 which offsets from the current time for the scheduled transmission. 4803 First calculate the maximum delay for the scheduled Advertisement: 4805 MaxScheduleDelay = min (MaxMobPfxAdvInterval, Preferred Lifetime), 4807 where MaxMobPfxAdvInterval is as defined in Section 12. Then compute 4808 the final delay for the advertisement: 4810 RAND_ADV_DELAY = MinMobPfxAdvInterval + 4811 (rand() % abs(MaxScheduleDelay - MinMobPfxAdvInterval)) 4813 Here rand() returns a random integer value in the range of 0 to the 4814 maximum possible integer value. This computation is expected to 4815 alleviate bursts of advertisements when prefix information changes. 4816 In addition, a home agent MAY further reduce the rate of packet 4817 transmission by further delaying individual advertisements, when 4818 necessary to avoid overwhelming local network resources. The home 4819 agent SHOULD periodically continue to retransmit an unsolicited 4820 Advertisement to the mobile node, until it is acknowledged by the 4821 receipt of a Mobile Prefix Solicitation from the mobile node. 4823 The home agent MUST wait PREFIX_ADV_TIMEOUT (see Section 12) before 4824 the first retransmission and double the retransmission wait time for 4825 every succeeding retransmission until a maximum number of 4826 PREFIX_ADV_RETRIES attempts (see Section 12) has been tried. If the 4827 mobile node's bindings expire before the matching Binding Update has 4828 been received, then the home agent MUST NOT attempt any more 4829 retransmissions, even if not all PREFIX_ADV_RETRIES have been 4830 retransmitted. In the mean time, if the mobile node sends another 4831 Binding Update without returning home, then the home agent SHOULD 4832 begin transmitting the unsolicited Advertisement again. 4834 If some condition, as described above, occurs on the home link and 4835 causes another Prefix Advertisement to be sent to the mobile node, 4836 before the mobile node acknowledges a previous transmission, the home 4837 agent SHOULD combine any Prefix Information options in the 4838 unacknowledged Mobile Prefix Advertisement into a new Advertisement. 4839 The home agent then discards the old Advertisement. 4841 10.6.3. Sending Advertisements 4843 When sending a Mobile Prefix Advertisement to the mobile node, the 4844 home agent MUST construct the packet as follows: 4846 o The Source Address in the packet's IPv6 header MUST be set to the 4847 home agent's IP address to which the mobile node addressed its 4848 current home registration or its default global home agent address 4849 if no binding exists. 4851 o If the advertisement was solicited, it MUST be destined to the 4852 source address of the solicitation. If it was triggered by prefix 4853 changes or renumbering, the advertisement's destination will be 4854 the mobile node's home address in the binding which triggered the 4855 rule. 4857 o A type 2 routing header MUST be included with the mobile node's 4858 home address. 4860 o IPsec headers MUST be supported and SHOULD be used. 4862 o The home agent MUST send the packet as it would any other unicast 4863 IPv6 packet that it originates. 4865 o Set the Managed Address Configuration (M) flag if the 4866 corresponding flag has been set in any of the Router 4867 Advertisements from which the prefix information has been learned 4868 (including the ones sent by this home agent). 4870 o Set the Other Stateful Configuration (O) flag if the corresponding 4871 flag has been set in any of the Router Advertisements from which 4872 the prefix information has been learned (including the ones sent 4873 by this home agent). 4875 10.6.4. Lifetimes for Changed Prefixes 4877 As described in Section 10.3.1, the lifetime returned by the home 4878 agent in a Binding Acknowledgement MUST NOT be greater than the 4879 remaining valid lifetime for the subnet prefix in the mobile node's 4880 home address. This limit on the binding lifetime serves to prohibit 4881 use of a mobile node's home address after it becomes invalid. 4883 11. Mobile Node Operation 4885 11.1. Conceptual Data Structures 4887 Each mobile node MUST maintain a Binding Update List. 4889 The Binding Update List records information for each Binding Update 4890 sent by this mobile node, in which the lifetime of the binding has 4891 not yet expired. The Binding Update List includes all bindings sent 4892 by the mobile node either to its home agent or correspondent nodes. 4893 It also contains Binding Updates which are waiting for the completion 4894 of the return routability procedure before they can be sent. 4895 However, for multiple Binding Updates sent to the same destination 4896 address, the Binding Update List contains only the most recent 4897 Binding Update (i.e., with the greatest Sequence Number value) sent 4898 to that destination. The Binding Update List MAY be implemented in 4899 any manner consistent with the external behavior described in this 4900 document. 4902 Each Binding Update List entry conceptually contains the following 4903 fields: 4905 o The IP address of the node to which a Binding Update was sent. 4907 o The home address for which that Binding Update was sent. 4909 o The care-of address sent in that Binding Update. This value is 4910 necessary for the mobile node to determine if it has sent a 4911 Binding Update while giving its new care-of address to this 4912 destination after changing its care-of address. 4914 o The initial value of the Lifetime field sent in that Binding 4915 Update. 4917 o The remaining lifetime of that binding. This lifetime is 4918 initialized from the Lifetime value sent in the Binding Update and 4919 is decremented until it reaches zero, at which time this entry 4920 MUST be deleted from the Binding Update List. 4922 o The maximum value of the Sequence Number field sent in previous 4923 Binding Updates to this destination. The Sequence Number field is 4924 16 bits long and all comparisons between Sequence Number values 4925 MUST be performed modulo 2**16 (see Section 9.5.1). 4927 o The time at which a Binding Update was last sent to this 4928 destination, as needed to implement the rate limiting restriction 4929 for sending Binding Updates. 4931 o The state of any retransmissions needed for this Binding Update. 4932 This state includes the time remaining until the next 4933 retransmission attempt for the Binding Update and the current 4934 state of the exponential back-off mechanism for retransmissions. 4936 o A flag specifying whether or not future Binding Updates should be 4937 sent to this destination. The mobile node sets this flag in the 4938 Binding Update List entry when it receives an ICMP Parameter 4939 Problem, Code 1, error message in response to a return routability 4940 message or Binding Update sent to that destination, as described 4941 in Section 11.3.5. 4943 The Binding Update List is used to determine whether a particular 4944 packet is sent directly to the correspondent node or tunneled via the 4945 home agent (see Section 11.3.1). 4947 The Binding Update list also conceptually contains the following data 4948 related to running the return routability procedure. This data is 4949 relevant only for Binding Updates sent to correspondent nodes. 4951 o The time at which a Home Test Init or Care-of Test Init message 4952 was last sent to this destination, as needed to implement the rate 4953 limiting restriction for the return routability procedure. 4955 o The state of any retransmissions needed for this return 4956 routability procedure. This state includes the time remaining 4957 until the next retransmission attempt and the current state of the 4958 exponential back-off mechanism for retransmissions. 4960 o Cookie values used in the Home Test Init and Care-of Test Init 4961 messages. 4963 o Home and care-of keygen tokens received from the correspondent 4964 node. 4966 o Home and care-of nonce indices received from the correspondent 4967 node. 4969 o The time at which each of the tokens and nonces were received from 4970 the correspondent node, as needed to implement reuse while moving. 4972 11.2. Processing Mobility Headers 4974 All IPv6 mobile nodes MUST observe the rules described in Section 9.2 4975 when processing Mobility Headers. 4977 11.3. Packet Processing 4979 11.3.1. Sending Packets While Away from Home 4981 While a mobile node is away from home, it continues to use its home 4982 address, as well as also using one or more care-of addresses. When 4983 sending a packet while away from home, a mobile node MAY choose among 4984 these in selecting the address that it will use as the source of the 4985 packet, as follows: 4987 o Protocols layered over IP will generally treat the mobile node's 4988 home address as its IP address for most packets. For packets sent 4989 that are part of transport-level connections established while the 4990 mobile node was at home, the mobile node MUST use its home 4991 address. Likewise, for packets sent that are part of transport- 4992 level connections that the mobile node may still be using after 4993 moving to a new location, the mobile node SHOULD use its home 4994 address in this way. If a binding exists, the mobile node SHOULD 4995 send the packets directly to the correspondent node. Otherwise, 4996 if a binding does not exist, the mobile node MUST use reverse 4997 tunneling. 4999 o The mobile node MAY choose to directly use one of its care-of 5000 addresses as the source of the packet, not requiring the use of a 5001 Home Address option in the packet. This is particularly useful 5002 for short-term communication that may easily be retried if it 5003 fails. Using the mobile node's care-of address as the source for 5004 such queries will generally have a lower overhead than using the 5005 mobile node's home address, since no extra options need be used in 5006 either the query or its reply. Such packets can be routed 5007 normally, directly between their source and destination without 5008 relying on Mobile IPv6. If application running on the mobile node 5009 has no particular knowledge that the communication being sent fits 5010 within this general type of communication, however, the mobile 5011 node should not use its care-of address as the source of the 5012 packet in this way. 5014 The choice of the most efficient communications method is 5015 application specific, and outside the scope of this specification. 5016 The APIs necessary for controlling the choice are also out of 5017 scope. One example of such an API is described in the IPv6 Socket 5018 API for Source Address Selection specification [24]. 5020 o While not at its home link, the mobile node MUST NOT use the Home 5021 Address destination option when communicating with link-local 5022 peers. 5024 Similarly, the mobile node MUST NOT use the Home Address 5025 destination option for IPv6 Neighbor Discovery [20] packets. 5027 Detailed operation of these cases is described later in this section 5028 and also discussed in [34]. 5030 For packets sent by a mobile node while it is at home, no special 5031 Mobile IPv6 processing is required. Likewise, if the mobile node 5032 uses any address other than one of its home addresses as the source 5033 of a packet sent while away from home, no special Mobile IPv6 5034 processing is required. In either case, the packet is simply 5035 addressed and transmitted in the same way as any normal IPv6 packet. 5037 For packets sent by the mobile node sent while away from home using 5038 the mobile node's home address as the source, special Mobile IPv6 5039 processing of the packet is required. This can be done in the 5040 following two ways: 5042 Route Optimization 5044 This manner of delivering packets does not require going through 5045 the home network, and typically will enable faster and more 5046 reliable transmission. 5048 The mobile node needs to ensure that a Binding Cache entry exists 5049 for its home address so that the correspondent node can process 5050 the packet (Section 9.3.1 specifies the rules for Home Address 5051 Destination Option Processing at a correspondent node). The 5052 mobile node SHOULD examine its Binding Update List for an entry 5053 which fulfills the following conditions: 5055 * The Source Address field of the packet being sent is equal to 5056 the home address in the entry. 5058 * The Destination Address field of the packet being sent is equal 5059 to the address of the correspondent node in the entry. 5061 * One of the current care-of addresses of the mobile node appears 5062 as the care-of address in the entry. 5064 * The entry indicates that a binding has been successfully 5065 created. 5067 * The remaining lifetime of the binding is greater than zero. 5069 When these conditions are met, the mobile node knows that the 5070 correspondent node has a suitable Binding Cache entry. 5072 A mobile node SHOULD arrange to supply the home address in a Home 5073 Address option, and MUST set the IPv6 header's Source Address 5074 field to the care-of address which the mobile node has registered 5075 to be used with this correspondent node. The correspondent node 5076 will then use the address supplied in the Home Address option to 5077 serve the function traditionally done by the Source IP address in 5078 the IPv6 header. The mobile node's home address is then supplied 5079 to higher protocol layers and applications. 5081 Specifically: 5083 * Construct the packet using the mobile node's home address as 5084 the packet's Source Address, in the same way as if the mobile 5085 node were at home. This includes the calculation of upper 5086 layer checksums using the home address as the value of the 5087 source. 5089 * Insert a Home Address option into the packet with the Home 5090 Address field copied from the original value of the Source 5091 Address field in the packet. 5093 * Change the Source Address field in the packet's IPv6 header to 5094 one of the mobile node's care-of addresses. This will 5095 typically be the mobile node's current primary care-of address, 5096 but MUST be an address assigned to the interface on the link 5097 being used. 5099 By using the care-of address as the Source Address in the IPv6 5100 header, with the mobile node's home address instead in the Home 5101 Address option, the packet will be able to safely pass through any 5102 router implementing ingress filtering [30]. 5104 Reverse Tunneling 5106 This is the mechanism which tunnels the packets via the home 5107 agent. It is not as efficient as the above mechanism, but is 5108 needed if there is no binding yet with the correspondent node. 5110 This mechanism is used for packets that have the mobile node's 5111 home address as the Source Address in the IPv6 header, or with 5112 multicast control protocol packets as described in Section 11.3.4. 5113 Specifically: 5115 * The packet is sent to the home agent using IPv6 encapsulation 5116 [9]. 5118 * The Source Address in the tunnel packet is the primary care-of 5119 address as registered with the home agent. 5121 * The Destination Address in the tunnel packet is the home 5122 agent's address. 5124 Then, the home agent will pass the encapsulated packet to the 5125 correspondent node. 5127 11.3.2. Interaction with Outbound IPsec Processing 5129 This section sketches the interaction between outbound Mobile IPv6 5130 processing and outbound IP Security (IPsec) processing for packets 5131 sent by a mobile node while away from home. Any specific 5132 implementation MAY use algorithms and data structures other than 5133 those suggested here, but its processing MUST be consistent with the 5134 effect of the operation described here and with the relevant IPsec 5135 specifications. In the steps described below, it is assumed that 5136 IPsec is being used in transport mode [2] and that the mobile node is 5137 using its home address as the source for the packet (from the point 5138 of view of higher protocol layers or applications, as described in 5139 Section 11.3.1): 5141 o The packet is created by higher layer protocols and applications 5142 (e.g., by TCP) as if the mobile node were at home and Mobile IPv6 5143 were not being used. 5145 o Determine the outgoing interface for the packet. (Note that the 5146 selection between reverse tunneling and route optimization may 5147 imply different interfaces, particularly if tunnels are considered 5148 interfaces as well.) 5150 o As part of outbound packet processing in IP, the packet is 5151 compared against the IPsec security policy database to determine 5152 what processing is required for the packet [2]. 5154 o If IPsec processing is required, the packet is either mapped to an 5155 existing Security Association (or SA bundle), or a new SA (or SA 5156 bundle) is created for the packet, according to the procedures 5157 defined for IPsec. 5159 o Since the mobile node is away from home, the mobile is either 5160 using reverse tunneling or route optimization to reach the 5161 correspondent node. 5163 If reverse tunneling is used, the packet is constructed in the 5164 normal manner and then tunneled through the home agent. 5166 If route optimization is in use, the mobile node inserts a Home 5167 Address destination option into the packet, replacing the Source 5168 Address in the packet's IP header with the care-of address used 5169 with this correspondent node, as described in Section 11.3.1. The 5170 Destination Options header in which the Home Address destination 5171 option is inserted MUST appear in the packet after the routing 5172 header, if present, and before the IPsec (AH [3] or ESP [4]) 5173 header, so that the Home Address destination option is processed 5174 by the destination node before the IPsec header is processed. 5176 Finally, once the packet is fully assembled, the necessary IPsec 5177 authentication (and encryption, if required) processing is 5178 performed on the packet, initializing the Authentication Data in 5179 the IPsec header. 5181 RFC 2402 treatment of destination options is extended as follows. 5182 The AH authentication data MUST be calculated as if the following 5183 were true: 5185 * the IPv6 source address in the IPv6 header contains the mobile 5186 node's home address, 5188 * the Home Address field of the Home Address destination option 5189 (Section 6.3) contains the new care-of address. 5191 o This allows, but does not require, the receiver of the packet 5192 containing a Home Address destination option to exchange the two 5193 fields of the incoming packet to reach the above situation, 5194 simplifying processing for all subsequent packet headers. 5195 However, such an exchange is not required, as long as the result 5196 of the authentication calculation remains the same. 5198 When an automated key management protocol is used to create new 5199 security associations for a peer, it is important to ensure that the 5200 peer can send the key management protocol packets to the mobile node. 5201 This may not be possible if the peer is the home agent of the mobile 5202 node and the purpose of the security associations would be to send a 5203 Binding Update to the home agent. Packets addressed to the home 5204 address of the mobile node cannot be used before the Binding Update 5205 has been processed. For the default case of using IKE [7] as the 5206 automated key management protocol, such problems can be avoided by 5207 the following requirements when communicating with its home agent: 5209 o When the mobile node is away from home, it MUST use its care-of 5210 address as the Source Address of all packets it sends as part of 5211 the key management protocol (without use of Mobile IPv6 for these 5212 packets, as suggested in Section 11.3.1). 5214 o In addition, for all security associations bound to the mobile 5215 node's home address established by IKE, the mobile node MUST 5216 include an ISAKMP Identification Payload [6] in the IKE phase 2 5217 exchange, giving the mobile node's home address as the initiator 5218 of the Security Association [5]. 5220 The Key Management Mobility Capability (K) bit in Binding Updates and 5221 Acknowledgements can be used to avoid the need to rerun IKE upon 5222 movements. 5224 11.3.3. Receiving Packets While Away from Home 5226 While away from home, a mobile node will receive packets addressed to 5227 its home address, by one of two methods: 5229 o Packets sent by a correspondent node, that does not have a Binding 5230 Cache entry for the mobile node, will be sent to the home address, 5231 captured by the home agent and tunneled to the mobile node. 5233 o Packets sent by a correspondent node that has a Binding Cache 5234 entry for the mobile node that contains the mobile node's current 5235 care-of address, will be sent by the correspondent node using a 5236 type 2 routing header. The packet will be addressed to the mobile 5237 node's care-of address, with the final hop in the routing header 5238 directing the packet to the mobile node's home address; the 5239 processing of this last hop of the routing header is entirely 5240 internal to the mobile node, since the care-of address and home 5241 address are both addresses within the mobile node. 5243 For packets received by the first method, the mobile node MUST check 5244 that the IPv6 source address of the tunneled packet is the IP address 5245 of its home agent. In this method, the mobile node may also send a 5246 Binding Update to the original sender of the packet as described in 5247 Section 11.7.2 and subject to the rate limiting defined in 5248 Section 11.8. The mobile node MUST also process the received packet 5249 in the manner defined for IPv6 encapsulation [9], which will result 5250 in the encapsulated (inner) packet being processed normally by upper- 5251 layer protocols within the mobile node as if it had been addressed 5252 (only) to the mobile node's home address. 5254 For packets received by the second method, the following rules will 5255 result in the packet being processed normally by upper-layer 5256 protocols within the mobile node as if it had been addressed to the 5257 mobile node's home address. 5259 A node receiving a packet addressed to itself (i.e., one of the 5260 node's addresses is in the IPv6 destination field) follows the next 5261 header chain of headers and processes them. When it encounters a 5262 type 2 routing header during this processing, it performs the 5263 following checks. If any of these checks fail, the node MUST 5264 silently discard the packet. 5266 o The length field in the routing header is exactly 2. 5268 o The segments left field in the routing header is 1 on the wire. 5269 (But implementations may process the routing header so that the 5270 value may become 0 after the routing header has been processed, 5271 but before the rest of the packet is processed.) 5273 o The Home Address field in the routing header is one of the node's 5274 home addresses, if the segments left field was 1. Thus, in 5275 particular the address field is required to be a unicast routable 5276 address. 5278 Once the above checks have been performed, the node swaps the IPv6 5279 destination field with the Home Address field in the routing header, 5280 decrements segments left by one from the value it had on the wire, 5281 and resubmits the packet to IP for processing the next header. 5282 Conceptually, this follows the same model as in RFC 2460. However, 5283 in the case of type 2 routing header this can be simplified since it 5284 is known that the packet will not be forwarded to a different node. 5286 The definition of AH requires the sender to calculate the AH 5287 integrity check value of a routing header in the same way it appears 5288 in the receiver after it has processed the header. Since IPsec 5289 headers follow the routing header, any IPsec processing will operate 5290 on the packet with the home address in the IP destination field and 5291 segments left being zero. Thus, the AH calculations at the sender 5292 and receiver will have an identical view of the packet. 5294 11.3.4. Routing Multicast Packets 5296 A mobile node that is connected to its home link functions in the 5297 same way as any other (stationary) node. Thus, when it is at home, a 5298 mobile node functions identically to other multicast senders and 5299 receivers. Therefore, this section describes the behavior of a 5300 mobile node that is not on its home link. 5302 In order to receive packets sent to some multicast group, a mobile 5303 node must join that multicast group. One method, in which a mobile 5304 node MAY join the group, is via a (local) multicast router on the 5305 foreign link being visited. In this case, the mobile node MUST use 5306 its care-of address and MUST NOT use the Home Address destination 5307 option when sending MLD packets [11]. 5309 Alternatively, a mobile node MAY join multicast groups via a bi- 5310 directional tunnel to its home agent. The mobile node tunnels its 5311 multicast group membership control packets (such as those defined in 5312 [11] or in [40]) to its home agent, and the home agent forwards 5313 multicast packets down the tunnel to the mobile node. A mobile node 5314 MUST NOT tunnel multicast group membership control packets until (1) 5315 the mobile node has a binding in place at the home agent, and (2) the 5316 latter sends at least one multicast group membership control packet 5317 via the tunnel. Once this condition is true, the mobile node SHOULD 5318 assume it does not change as long as the binding does not expire. 5320 A mobile node that wishes to send packets to a multicast group also 5321 has two options: 5323 1. Send directly on the foreign link being visited. 5325 The application uses the care-of address as a source address for 5326 multicast traffic, just like it would use a stationary address. 5327 This requires that the application either knows the care-of 5328 address, or uses an API such as the IPv6 Socket API for Source 5329 Address Selection specification [24]. to request the stack to use 5330 the care-of address as a source address in sent packets. The 5331 mobile node MUST NOT use Home Address destination option in such 5332 traffic. 5334 2. Send via a tunnel to its home agent. 5336 Because multicast routing in general depends upon the Source 5337 Address used in the IPv6 header of the multicast packet, a mobile 5338 node that tunnels a multicast packet to its home agent MUST use 5339 its home address as the IPv6 Source Address of the inner 5340 multicast packet. 5342 Note that direct sending from the foreign link is only applicable 5343 while the mobile node is at that foreign link. This is because the 5344 associated multicast tree is specific to that source location and any 5345 change of location and source address will invalidate the source 5346 specific tree or branch and the application context of the other 5347 multicast group members. 5349 This specification does not provide mechanisms to enable such local 5350 multicast session to survive hand-off and to seamlessly continue from 5351 a new care-of address on each new foreign link. Any such mechanism, 5352 developed as an extension to this specification, needs to take into 5353 account the impact of fast moving mobile nodes on the Internet 5354 multicast routing protocols and their ability to maintain the 5355 integrity of source specific multicast trees and branches. 5357 While the use of bidirectional tunneling can ensure that multicast 5358 trees are independent of the mobile nodes movement, in some case such 5359 tunneling can have adverse affects. The latency of specific types of 5360 multicast applications (such as multicast based discovery protocols) 5361 will be affected when the round-trip time between the foreign subnet 5362 and the home agent is significant compared to that of the topology to 5363 be discovered. In addition, the delivery tree from the home agent in 5364 such circumstances relies on unicast encapsulation from the agent to 5365 the mobile node. Therefore, bandwidth usage is inefficient compared 5366 to the native multicast forwarding in the foreign multicast system. 5368 11.3.5. Receiving ICMP Error Messages 5370 Any node that does not recognize the Mobility header will return an 5371 ICMP Parameter Problem, Code 1, message to the sender of the packet. 5372 If the mobile node receives such an ICMP error message in response to 5373 a return routability procedure or Binding Update, it SHOULD record in 5374 its Binding Update List that future Binding Updates SHOULD NOT be 5375 sent to this destination. Such Binding Update List entries SHOULD be 5376 removed after a period of time in order to allow for retrying route 5377 optimization. 5379 New Binding Update List entries MUST NOT be created as a result of 5380 receiving ICMP error messages. 5382 Correspondent nodes that have participated in the return routability 5383 procedure MUST implement the ability to correctly process received 5384 packets containing a Home Address destination option. Therefore, 5385 correctly implemented correspondent nodes should always be able to 5386 recognize Home Address options. If a mobile node receives an ICMP 5387 Parameter Problem, Code 2, message from some node indicating that it 5388 does not support the Home Address option, the mobile node SHOULD log 5389 the error and then discard the ICMP message. 5391 11.3.6. Receiving Binding Error Messages 5393 When a mobile node receives a packet containing a Binding Error 5394 message, it should first check if the mobile node has a Binding 5395 Update List entry for the source of the Binding Error message. If 5396 the mobile node does not have such an entry, it MUST ignore the 5397 message. This is necessary to prevent a waste of resources on, e.g., 5398 return routability procedure due to spoofed Binding Error messages. 5400 Otherwise, if the message Status field was 1 (unknown binding for 5401 Home Address destination option), the mobile node should perform one 5402 of the following two actions: 5404 o If the mobile node has recent upper layer progress information, 5405 which indicates that communications with the correspondent node 5406 are progressing, it MAY ignore the message. This can be done in 5407 order to limit the damage that spoofed Binding Error messages can 5408 cause to ongoing communications. 5410 o If the mobile node has no upper layer progress information, it 5411 MUST remove the entry and route further communications through the 5412 home agent. It MAY also optionally start a return routability 5413 procedure (see Section 5.2). 5415 If the message Status field was 2 (unrecognized MH Type value), the 5416 mobile node should perform one of the following two actions: 5418 o If the mobile node is not expecting an acknowledgement or response 5419 from the correspondent node, the mobile node SHOULD ignore this 5420 message. 5422 o Otherwise, the mobile node SHOULD cease the use of any extensions 5423 to this specification. If no extensions had been used, the mobile 5424 node should cease the attempt to use route optimization. 5426 11.4. Home Agent and Prefix Management 5428 11.4.1. Dynamic Home Agent Address Discovery 5430 Sometimes when the mobile node needs to send a Binding Update to its 5431 home agent to register its new primary care-of address, as described 5432 in Section 11.7.1, the mobile node may not know the address of any 5433 router on its home link that can serve as a home agent for it. For 5434 example, some nodes on its home link may have been reconfigured while 5435 the mobile node has been away from home, such that the router that 5436 was operating as the mobile node's home agent has been replaced by a 5437 different router serving this role. 5439 In this case, the mobile node MAY attempt to discover the address of 5440 a suitable home agent on its home link. To do so, the mobile node 5441 sends an ICMP Home Agent Address Discovery Request message to the 5442 Mobile IPv6 Home-Agents anycast address [10] for its home subnet 5443 prefix. As described in Section 10.5, the home agent on its home 5444 link that receives this Request message will return an ICMP Home 5445 Agent Address Discovery Reply message. This message gives the 5446 addresses for the home agents operating on the home link. 5448 The mobile node, upon receiving this Home Agent Address Discovery 5449 Reply message, MAY then send its home registration Binding Update to 5450 any of the unicast IP addresses listed in the Home Agent Addresses 5451 field in the Reply. For example, the mobile node MAY attempt its 5452 home registration to each of these addresses, in turn, until its 5453 registration is accepted. The mobile node sends a Binding Update to 5454 an address and waits for the matching Binding Acknowledgement, moving 5455 on to the next address if there is no response. The mobile node 5456 MUST, however, wait at least InitialBindackTimeoutFirstReg seconds 5457 (see Section 13) before sending a Binding Update to the next home 5458 agent. In trying each of the returned home agent addresses, the 5459 mobile node SHOULD try each of them in the order they appear in the 5460 Home Agent Addresses field in the received Home Agent Address 5461 Discovery Reply message. 5463 If the mobile node has a current registration with some home agent 5464 (the Lifetime for that registration has not yet expired), then the 5465 mobile node MUST attempt any new registration first with that home 5466 agent. If that registration attempt fails (e.g., timed out or 5467 rejected), the mobile node SHOULD then reattempt this registration 5468 with another home agent. If the mobile node knows of no other 5469 suitable home agent, then it MAY attempt the dynamic home agent 5470 address discovery mechanism described above. 5472 If, after a mobile node transmits a Home Agent Address Discovery 5473 Request message to the Home Agents Anycast address, it does not 5474 receive a corresponding Home Agent Address Discovery Reply message 5475 within INITIAL_DHAAD_TIMEOUT (see Section 12) seconds, the mobile 5476 node MAY retransmit the same Request message to the same anycast 5477 address. This retransmission MAY be repeated up to a maximum of 5478 DHAAD_RETRIES (see Section 12) attempts. Each retransmission MUST be 5479 delayed by twice the time interval of the previous retransmission. 5481 11.4.2. Sending Mobile Prefix Solicitations 5483 When a mobile node has a home address that is about to become 5484 invalid, it SHOULD send a Mobile Prefix Solicitation to its home 5485 agent in an attempt to acquire fresh routing prefix information. The 5486 new information also enables the mobile node to participate in 5487 renumbering operations affecting the home network, as described in 5488 Section 10.6. 5490 The mobile node MUST use the Home Address destination option to carry 5491 its home address. The mobile node MUST support and SHOULD use IPsec 5492 to protect the solicitation. The mobile node MUST set the Identifier 5493 field in the ICMP header to a random value. 5495 As described in Section 11.7.2, Binding Updates sent by the mobile 5496 node to other nodes MUST use a lifetime no greater than the remaining 5497 lifetime of its home registration of its primary care-of address. 5498 The mobile node SHOULD further limit the lifetimes that it sends on 5499 any Binding Updates to be within the remaining valid lifetime (see 5500 Section 10.6.2) for the prefix in its home address. 5502 When the lifetime for a changed prefix decreases, and the change 5503 would cause cached bindings at correspondent nodes in the Binding 5504 Update List to be stored past the newly shortened lifetime, the 5505 mobile node MUST issue a Binding Update to all such correspondent 5506 nodes. 5508 These limits on the binding lifetime serve to prohibit use of a 5509 mobile node's home address after it becomes invalid. 5511 11.4.3. Receiving Mobile Prefix Advertisements 5513 Section 10.6 describes the operation of a home agent to support boot 5514 time configuration and renumbering a mobile node's home subnet while 5515 the mobile node is away from home. The home agent sends Mobile 5516 Prefix Advertisements to the mobile node while away from home, giving 5517 "important" Prefix Information options that describe changes in the 5518 prefixes in use on the mobile node's home link. 5520 The Mobile Prefix Solicitation is similar to the Router Solicitation 5521 used in Neighbor Discovery [20], except it is routed from the mobile 5522 node on the visited network to the home agent on the home network by 5523 usual unicast routing rules. 5525 When a mobile node receives a Mobile Prefix Advertisement, it MUST 5526 validate it according to the following test: 5528 o The Source Address of the IP packet carrying the Mobile Prefix 5529 Advertisement is the same as the home agent address to which the 5530 mobile node last sent an accepted home registration Binding Update 5531 to register its primary care-of address. Otherwise, if no such 5532 registrations have been made, it SHOULD be the mobile node's 5533 stored home agent address, if one exists. Otherwise, if the 5534 mobile node has not yet discovered its home agent's address, it 5535 MUST NOT accept Mobile Prefix Advertisements. 5537 o The packet MUST have a type 2 routing header and SHOULD be 5538 protected by an IPsec header as described in Section 5.4 and 5539 Section 6.8. 5541 o If the ICMP Identifier value matches the ICMP Identifier value of 5542 the most recently sent Mobile Prefix Solicitation and no other 5543 advertisement has yet been received for this value, then the 5544 advertisement is considered to be solicited and will be processed 5545 further. 5547 Otherwise, the advertisement is unsolicited, and MUST be 5548 discarded. In this case the mobile node SHOULD send a Mobile 5549 Prefix Solicitation. 5551 Any received Mobile Prefix Advertisement not meeting these tests MUST 5552 be silently discarded. 5554 For an accepted Mobile Prefix Advertisement, the mobile node MUST 5555 process Managed Address Configuration (M), Other Stateful 5556 Configuration (O), and the Prefix Information Options as if they 5557 arrived in a Router Advertisement [20] on the mobile node's home 5558 link. (This specification does not, however, describe how to acquire 5559 home addresses through stateful protocols.) Such processing may 5560 result in the mobile node configuring a new home address, although 5561 due to separation between preferred lifetime and valid lifetime, such 5562 changes should not affect most communications by the mobile node, in 5563 the same way as for nodes that are at home. 5565 This specification assumes that any security associations and 5566 security policy entries that may be needed for new prefixes have been 5567 pre-configured in the mobile node. Note that while dynamic key 5568 management avoids the need to create new security associations, it is 5569 still necessary to add policy entries to protect the communications 5570 involving the home address(es). Mechanisms for automatic set-up of 5571 these entries are outside the scope of this specification. 5573 11.5. Movement 5575 11.5.1. Movement Detection 5577 The primary goal of movement detection is to detect L3 handovers. 5578 This section does not attempt to specify a fast movement detection 5579 algorithm which will function optimally for all types of 5580 applications, link-layers and deployment scenarios; instead, it 5581 describes a generic method that uses the facilities of IPv6 Neighbor 5582 Discovery, including Router Discovery and Neighbor Unreachability 5583 Detection. At the time of this writing, this method is considered 5584 well enough understood to recommend for standardization, however it 5585 is expected that future versions of this specification or other 5586 specifications may contain updated versions of the movement detection 5587 algorithm that have better performance. 5589 Generic movement detection uses Neighbor Unreachability Detection to 5590 detect when the default router is no longer bi-directionally 5591 reachable, in which case the mobile node must discover a new default 5592 router (usually on a new link). However, this detection only occurs 5593 when the mobile node has packets to send, and in the absence of 5594 frequent Router Advertisements or indications from the link-layer, 5595 the mobile node might become unaware of an L3 handover that occurred. 5596 Therefore, the mobile node should supplement this method with other 5597 information whenever it is available to the mobile node (e.g., from 5598 lower protocol layers). 5600 When the mobile node detects an L3 handover, it performs Duplicate 5601 Address Detection [21] on its link-local address, selects a new 5602 default router as a consequence of Router Discovery, and then 5603 performs Prefix Discovery with that new router to form new care-of 5604 address(es) as described in Section 11.5.3. It then registers its 5605 new primary care-of address with its home agent as described in 5606 Section 11.7.1. After updating its home registration, the mobile 5607 node then updates associated mobility bindings in correspondent nodes 5608 that it is performing route optimization with as specified in 5609 Section 11.7.2. 5611 Due to the temporary packet flow disruption and signaling overhead 5612 involved in updating mobility bindings, the mobile node should avoid 5613 performing an L3 handover until it is strictly necessary. 5614 Specifically, when the mobile node receives a Router Advertisement 5615 from a new router that contains a different set of on-link prefixes, 5616 if the mobile node detects that the currently selected default router 5617 on the old link is still bi-directionally reachable, it should 5618 generally continue to use the old router on the old link rather than 5619 switch away from it to use a new default router. 5621 Mobile nodes can use the information in received Router 5622 Advertisements to detect L3 handovers. In doing so the mobile node 5623 needs to consider the following issues: 5625 o There might be multiple routers on the same link, thus hearing a 5626 new router does not necessarily constitute an L3 handover. 5628 o When there are multiple routers on the same link they might 5629 advertise different prefixes. Thus even hearing a new router with 5630 a new prefix might not be a reliable indication of an L3 handover. 5632 o The link-local addresses of routers are not globally unique, hence 5633 after completing an L3 handover the mobile node might continue to 5634 receive Router Advertisements with the same link-local source 5635 address. This might be common if routers use the same link-local 5636 address on multiple interfaces. This issue can be avoided when 5637 routers use the Router Address (R) bit, since that provides a 5638 global address of the router. 5640 In addition, the mobile node should consider the following events as 5641 indications that an L3 handover may have occurred. Upon receiving 5642 such indications, the mobile node needs to perform Router Discovery 5643 to discover routers and prefixes on the new link, as described in 5644 Section 6.3.7 of RFC 4861 [20]. 5646 o If Router Advertisements that the mobile node receives include an 5647 Advertisement Interval option, the mobile node may use its 5648 Advertisement Interval field as an indication of the frequency 5649 with which it should expect to continue to receive future 5650 Advertisements from that router. This field specifies the minimum 5651 rate (the maximum amount of time between successive 5652 Advertisements) that the mobile node should expect. If this 5653 amount of time elapses without the mobile node receiving any 5654 Advertisement from this router, the mobile node can be sure that 5655 at least one Advertisement sent by the router has been lost. The 5656 mobile node can then implement its own policy to determine how 5657 many lost Advertisements from its current default router 5658 constitute an L3 handover indication. 5660 o Neighbor Unreachability Detection determines that the default 5661 router is no longer reachable. 5663 o With some types of networks, notification that an L2 handover has 5664 occurred might be obtained from lower layer protocols or device 5665 driver software within the mobile node. While further details 5666 around handling L2 indications as movement hints is an item for 5667 further study, at the time of writing this specification the 5668 following is considered reasonable: 5670 An L2 handover indication may or may not imply L2 movement and L2 5671 movement may or may not imply L3 movement; the correlations might 5672 be a function of the type of L2 but might also be a function of 5673 actual deployment of the wireless topology. 5675 Unless it is well-known that an L2 handover indication is likely 5676 to imply L3 movement, instead of immediately multicasting a router 5677 solicitation it may be better to attempt to verify whether the 5678 default router is still bi-directionally reachable. This can be 5679 accomplished by sending a unicast Neighbor Solicitation and 5680 waiting for a Neighbor Advertisement with the solicited flag set. 5681 Note that this is similar to Neighbor Unreachability detection but 5682 it does not have the same state machine, such as the STALE state. 5684 If the default router does not respond to the Neighbor 5685 Solicitation it makes sense to proceed to multicasting a Router 5686 Solicitation. 5688 11.5.2. Home Link Detection 5690 When an MN detects that it has arrived on a new link using the 5691 movement detection algorithm in use (Section Section 11.5.1,) it 5692 performs the following steps to determine if it is on the home link. 5694 o The MN performs the procedure described in Section 11.5.2x and 5695 configures an address. It also keeps track of all the on-link 5696 prefix(es) received in the RA along with their prefix lengths. 5698 o If the home prefix has not been statically configured the MN uses 5699 some form of bootstrapping procedure (e.g. RFC5026 [25]) to 5700 determine the home prefix. 5702 o Given the availability of the home prefix, the MN checks whether 5703 or not the home prefix matches one of the prefixes received in the 5704 RA. If it does, the MN concludes that it has returned home. 5706 11.5.3. Forming New Care-of Addresses 5708 After detecting that it has moved a mobile node SHOULD generate a new 5709 primary care-of address using normal IPv6 mechanisms. This SHOULD 5710 also be done when the current primary care-of address becomes 5711 deprecated. A mobile node MAY form a new primary care-of address at 5712 any time, but a mobile node MUST NOT send a Binding Update about a 5713 new care-of address to its home agent more than MAX_UPDATE_RATE times 5714 within a second. 5716 In addition, a mobile node MAY form new non-primary care-of addresses 5717 even when it has not switched to a new default router. A mobile node 5718 can have only one primary care-of address at a time (which is 5719 registered with its home agent), but it MAY have an additional 5720 care-of address for any or all of the prefixes on its current link. 5721 Furthermore, since a wireless network interface may actually allow a 5722 mobile node to be reachable on more than one link at a time (i.e., 5723 within wireless transmitter range of routers on more than one 5724 separate link), a mobile node MAY have care-of addresses on more than 5725 one link at a time. The use of more than one care-of address at a 5726 time is described in Section 11.5.4. 5728 As described in Section 4, in order to form a new care-of address, a 5729 mobile node MAY use either stateless [21] or stateful (e.g., DHCPv6 5730 [32]) Address Autoconfiguration. If a mobile node needs to use a 5731 source address (other than the unspecified address) in packets sent 5732 as a part of address autoconfiguration, it MUST use an IPv6 link- 5733 local address rather than its own IPv6 home address. 5735 RFC 4862 [21] specifies that in normal processing for Duplicate 5736 Address Detection, the node SHOULD delay sending the initial Neighbor 5737 Solicitation message by a random delay between 0 and 5738 MAX_RTR_SOLICITATION_DELAY. Since delaying DAD can result in 5739 significant delays in configuring a new care-of address when the 5740 Mobile Node moves to a new link, the Mobile Node preferably SHOULD 5741 NOT delay DAD when configuring a new care-of address. The Mobile 5742 Node SHOULD delay according to the mechanisms specified in RFC 4862 5743 unless the implementation has a behavior that desynchronizes the 5744 steps that happen before the DAD in the case that multiple nodes 5745 experience handover at the same time. Such desynchronizing behaviors 5746 might be due to random delays in the L2 protocols or device drivers, 5747 or due to the movement detection mechanism that is used. 5749 11.5.4. Using Multiple Care-of Addresses 5751 As described in Section 11.5.3, a mobile node MAY use more than one 5752 care-of address at a time. Particularly in the case of many wireless 5753 networks, a mobile node effectively might be reachable through 5754 multiple links at the same time (e.g., with overlapping wireless 5755 cells), on which different on-link subnet prefixes may exist. The 5756 mobile node MUST ensure that its primary care-of address always has a 5757 prefix that is advertised by its current default router. After 5758 selecting a new primary care-of address, the mobile node MUST send a 5759 Binding Update containing that care-of address to its home agent. 5760 The Binding Update MUST have the Home Registration (H) and 5761 Acknowledge (A) bits set its home agent, as described on 5762 Section 11.7.1. 5764 To assist with smooth handovers, a mobile node SHOULD retain its 5765 previous primary care-of address as a (non-primary) care-of address, 5766 and SHOULD still accept packets at this address, even after 5767 registering its new primary care-of address with its home agent. 5768 This is reasonable, since the mobile node could only receive packets 5769 at its previous primary care-of address if it were indeed still 5770 connected to that link. If the previous primary care-of address was 5771 allocated using stateful Address Autoconfiguration [32], the mobile 5772 node may not wish to release the address immediately upon switching 5773 to a new primary care-of address. 5775 Whenever a mobile node determines that it is no longer reachable 5776 through a given link, it SHOULD invalidate all care-of addresses 5777 associated with address prefixes that it discovered from routers on 5778 the unreachable link which are not in the current set of address 5779 prefixes advertised by the (possibly new) current default router. 5781 11.5.5. Returning Home 5783 A mobile node detects that it has returned to its home link through 5784 the movement detection algorithm in use (Section 11.5.2). The mobile 5785 node SHOULD then send a Binding Update to its home agent, to instruct 5786 its home agent to no longer intercept or tunnel packets for it. In 5787 this home registration, the mobile node MUST set the Acknowledge (A) 5788 and Home Registration (H) bits, set the Lifetime field to zero, and 5789 set the care-of address for the binding to the mobile node's own home 5790 address. The mobile node MUST use its home address as the source 5791 address in the Binding Update. 5793 When sending this Binding Update to its home agent, the mobile node 5794 must be careful in how it uses Neighbor Solicitation [20] (if needed) 5795 to learn the home agent's link-layer address, since the home agent 5796 will be currently configured to intercept packets to the mobile 5797 node's home address using Duplicate Address Detection (DAD). In 5798 particular, the mobile node is unable to use its home address as the 5799 Source Address in the Neighbor Solicitation until the home agent 5800 stops defending the home address. 5802 Neighbor Solicitation by the mobile node for the home agent's address 5803 will normally not be necessary, since the mobile node has already 5804 learned the home agent's link-layer address from a Source Link-Layer 5805 Address option in a Router Advertisement. However, if there are 5806 multiple home agents it may still be necessary to send a 5807 solicitation. In this special case of the mobile node returning 5808 home, the mobile node MUST multicast the packet, and in addition set 5809 the Source Address of this Neighbor Solicitation to the unspecified 5810 address (0:0:0:0:0:0:0:0). The target of the Neighbor Solicitation 5811 MUST be set to the mobile node's home address. The destination IP 5812 address MUST be set to the Solicited-Node multicast address [18]. 5813 The home agent will send a multicast Neighbor Advertisement back to 5814 the mobile node with the Solicited flag (S) set to zero. In any 5815 case, the mobile node SHOULD record the information from the Source 5816 Link-Layer Address option or from the advertisement, and set the 5817 state of the Neighbor Cache entry for the home agent to REACHABLE. 5819 The mobile node then sends its Binding Update to the home agent's 5820 link-layer address, instructing its home agent to no longer serve as 5821 a home agent for it. By processing this Binding Update, the home 5822 agent will cease defending the mobile node's home address for 5823 Duplicate Address Detection and will no longer respond to Neighbor 5824 Solicitations for the mobile node's home address. The mobile node is 5825 then the only node on the link receiving packets at the mobile node's 5826 home address. In addition, when returning home prior to the 5827 expiration of a current binding for its home address, and configuring 5828 its home address on its network interface on its home link, the 5829 mobile node MUST NOT perform Duplicate Address Detection on its own 5830 home address, in order to avoid confusion or conflict with its home 5831 agent's use of the same address. This rule also applies to the 5832 derived link-local address of the mobile node, if the Link Local 5833 Address Compatibility (L) bit was set when the binding was created. 5834 If the mobile node returns home after the bindings for all of its 5835 care-of addresses have expired, then it SHOULD perform DAD. 5837 After the Mobile Node sends the Binding Update, it MUST be prepared 5838 to reply to Neighbor Solicitations for its home address. Such 5839 replies MUST be sent using a unicast Neighbor Advertisement to the 5840 sender's link-layer address. It is necessary to reply, since sending 5841 the Binding Acknowledgement from the home agent may require 5842 performing Neighbor Discovery, and the mobile node may not be able to 5843 distinguish Neighbor Solicitations coming from the home agent from 5844 other Neighbor Solicitations. Note that a race condition exists 5845 where both the mobile node and the home agent respond to the same 5846 solicitations sent by other nodes; this will be only temporary, 5847 however, until the Binding Update is accepted. 5849 After receiving the Binding Acknowledgement for its Binding Update to 5850 its home agent, the mobile node MUST multicast onto the home link (to 5851 the all-nodes multicast address) a Neighbor Advertisement [20], to 5852 advertise the mobile node's own link-layer address for its own home 5853 address. The Target Address in this Neighbor Advertisement MUST be 5854 set to the mobile node's home address, and the Advertisement MUST 5855 include a Target Link-layer Address option specifying the mobile 5856 node's link-layer address. The mobile node MUST multicast such a 5857 Neighbor Advertisement for each of its home addresses, as defined by 5858 the current on-link prefixes, including its link-local address. The 5859 Solicited Flag (S) in these Advertisements MUST NOT be set, since 5860 they were not solicited by any Neighbor Solicitation. The Override 5861 Flag (O) in these Advertisements MUST be set, indicating that the 5862 Advertisements SHOULD override any existing Neighbor Cache entries at 5863 any node receiving them. 5865 Since multicasting on the local link (such as Ethernet) is typically 5866 not guaranteed to be reliable, the mobile node MAY retransmit these 5867 Neighbor Advertisements [20] up to MAX_NEIGHBOR_ADVERTISEMENT times 5868 to increase their reliability. It is still possible that some nodes 5869 on the home link will not receive any of these Neighbor 5870 Advertisements, but these nodes will eventually be able to recover 5871 through use of Neighbor Unreachability Detection [20]. 5873 Note that the tunnel via the home agent typically stops operating at 5874 the same time that the home registration is deleted. 5876 11.6. Return Routability Procedure 5878 This section defines the rules that the mobile node must follow when 5879 performing the return routability procedure. Section 11.7.2 5880 describes the rules when the return routability procedure needs to be 5881 initiated. 5883 11.6.1. Sending Test Init Messages 5885 A mobile node that initiates a return routability procedure MUST send 5886 (in parallel) a Home Test Init message and a Care-of Test Init 5887 messages. However, if the mobile node has recently received (see 5888 Section 5.2.7) one or both home or care-of keygen tokens, and 5889 associated nonce indices for the desired addresses, it MAY reuse 5890 them. Therefore, the return routability procedure may in some cases 5891 be completed with only one message pair. It may even be completed 5892 without any messages at all, if the mobile node has a recent home 5893 keygen token and has previously visited the same care-of address so 5894 that it also has a recent care-of keygen token. If the mobile node 5895 intends to send a Binding Update with the Lifetime set to zero and 5896 the care-of address equal to its home address - such as when 5897 returning home - sending a Home Test Init message is sufficient. In 5898 this case, generation of the binding management key depends 5899 exclusively on the home keygen token (Section 5.2.5). 5901 A Home Test Init message MUST be created as described in 5902 Section 6.1.3. 5904 A Care-of Test Init message MUST be created as described in 5905 Section 6.1.4. When sending a Home Test Init or Care-of Test Init 5906 message the mobile node MUST record in its Binding Update List the 5907 following fields from the messages: 5909 o The IP address of the node to which the message was sent. 5911 o The home address of the mobile node. This value will appear in 5912 the Source Address field of the Home Test Init message. When 5913 sending the Care-of Test Init message, this address does not 5914 appear in the message, but represents the home address for which 5915 the binding is desired. 5917 o The time at which each of these messages was sent. 5919 o The cookies used in the messages. 5921 Note that a single Care-of Test Init message may be sufficient even 5922 when there are multiple home addresses. In this case the mobile node 5923 MAY record the same information in multiple Binding Update List 5924 entries. 5926 11.6.2. Receiving Test Messages 5928 Upon receiving a packet carrying a Home Test message, a mobile node 5929 MUST validate the packet according to the following tests: 5931 o The Source Address of the packet belongs to a correspondent node 5932 for which the mobile node has a Binding Update List entry with a 5933 state indicating that return routability procedure is in progress. 5934 Note that there may be multiple such entries. 5936 o The Binding Update List indicates that no home keygen token has 5937 been received yet. 5939 o The Destination Address of the packet has the home address of the 5940 mobile node, and the packet has been received in a tunnel from the 5941 home agent. 5943 o The Home Init Cookie field in the message matches the value stored 5944 in the Binding Update List. 5946 Any Home Test message not satisfying all of these tests MUST be 5947 silently ignored. Otherwise, the mobile node MUST record the Home 5948 Nonce Index and home keygen token in the Binding Update List. If the 5949 Binding Update List entry does not have a care-of keygen token, the 5950 mobile node SHOULD continue waiting for the Care-of Test message. 5952 Upon receiving a packet carrying a Care-of Test message, a mobile 5953 node MUST validate the packet according to the following tests: 5955 o The Source Address of the packet belongs to a correspondent node 5956 for which the mobile node has a Binding Update List entry with a 5957 state indicating that return routability procedure is in progress. 5958 Note that there may be multiple such entries. 5960 o The Binding Update List indicates that no care-of keygen token has 5961 been received yet. 5963 o The Destination Address of the packet is the current care-of 5964 address of the mobile node. 5966 o The Care-of Init Cookie field in the message matches the value 5967 stored in the Binding Update List. 5969 Any Care-of Test message not satisfying all of these tests MUST be 5970 silently ignored. Otherwise, the mobile node MUST record the Care-of 5971 Nonce Index and care-of keygen token in the Binding Update List. If 5972 the Binding Update List entry does not have a home keygen token, the 5973 mobile node SHOULD continue waiting for the Home Test message. 5975 If after receiving either the Home Test or the Care-of Test message 5976 and performing the above actions, the Binding Update List entry has 5977 both the home and the care-of keygen tokens, the return routability 5978 procedure is complete. The mobile node SHOULD then proceed with 5979 sending a Binding Update as described in Section 11.7.2. 5981 Correspondent nodes from the time before this specification was 5982 published may not support the Mobility Header protocol. These nodes 5983 will respond to Home Test Init and Care-of Test Init messages with an 5984 ICMP Parameter Problem code 1. The mobile node SHOULD take such 5985 messages as an indication that the correspondent node cannot provide 5986 route optimization, and revert back to the use of bidirectional 5987 tunneling. 5989 11.6.3. Protecting Return Routability Packets 5991 The mobile node MUST support the protection of Home Test and Home 5992 Test Init messages as described in Section 10.4.6. 5994 When IPsec is used to protect return routability signaling or payload 5995 packets, the mobile node MUST set the source address it uses for the 5996 outgoing tunnel packets to the current primary care-of address. The 5997 mobile node starts to use a new primary care-of address immediately 5998 after sending a Binding Update to the home agent to register this new 5999 address. 6001 11.7. Processing Bindings 6003 11.7.1. Sending Binding Updates to the Home Agent 6005 After deciding to change its primary care-of address as described in 6006 Section 11.5.1 and Section 11.5.3, a mobile node MUST register this 6007 care-of address with its home agent in order to make this its primary 6008 care-of address. 6010 Also, if the mobile node wants the services of the home agent beyond 6011 the current registration period, the mobile node should send a new 6012 Binding Update to it well before the expiration of this period, even 6013 if it is not changing its primary care-of address. However, if the 6014 home agent returned a Binding Acknowledgement for the current 6015 registration with Status field set to 1 (accepted but prefix 6016 discovery necessary), the mobile node should not try to register 6017 again before it has learned the validity of its home prefixes through 6018 mobile prefix discovery. This is typically necessary every time this 6019 Status value is received, because information learned earlier may 6020 have changed. 6022 To register a care-of address or to extend the lifetime of an 6023 existing registration, the mobile node sends a packet to its home 6024 agent containing a Binding Update, with the packet constructed as 6025 follows: 6027 o The Home Registration (H) bit MUST be set in the Binding Update. 6029 o The Acknowledge (A) bit MUST be set in the Binding Update. 6031 o The packet MUST contain a Home Address destination option, giving 6032 the mobile node's home address for the binding. 6034 o The care-of address for the binding MUST be used as the Source 6035 Address in the packet's IPv6 header, unless an Alternate Care-of 6036 Address mobility option is included in the Binding Update. This 6037 option MUST be included in all home registrations, as the ESP 6038 protocol will not be able to protect care-of addresses in the IPv6 6039 header. (Mobile IPv6 implementations that know they are using 6040 IPsec AH to protect a particular message might avoid this option. 6041 For brevity the usage of AH is not discussed in this document.) 6043 o If the mobile node's link-local address has the same interface 6044 identifier as the home address for which it is supplying a new 6045 care-of address, then the mobile node SHOULD set the Link-Local 6046 Address Compatibility (L) bit. 6048 o If the home address was generated using RFC 4941 [23], then the 6049 link local address is unlikely to have a compatible interface 6050 identifier. In this case, the mobile node MUST clear the Link- 6051 Local Address Compatibility (L) bit. 6053 o If the IPsec security associations between the mobile node and the 6054 home agent have been established dynamically, and the mobile node 6055 has the capability to update its endpoint in the used key 6056 management protocol to the new care-of address every time it 6057 moves, the mobile node SHOULD set the Key Management Mobility 6058 Capability (K) bit in the Binding Update. Otherwise, the mobile 6059 node MUST clear the bit. 6061 o The value specified in the Lifetime field MUST be non-zero and 6062 SHOULD be less than or equal to the remaining valid lifetime of 6063 the home address and the care-of address specified for the 6064 binding. 6066 Mobile nodes that use dynamic home agent address discovery should 6067 be careful with long lifetimes. If the mobile node loses the 6068 knowledge of its binding with a specific home agent, registering a 6069 new binding with another home agent may be impossible as the 6070 previous home agent is still defending the existing binding. 6071 Therefore, to ensure that mobile nodes using home agent address 6072 discovery do not lose information about their binding, they SHOULD 6073 de-register before losing this information, or use small 6074 lifetimes. 6076 The Acknowledge (A) bit in the Binding Update requests the home agent 6077 to return a Binding Acknowledgement in response to this Binding 6078 Update. As described in Section 6.1.8, the mobile node SHOULD 6079 retransmit this Binding Update to its home agent until it receives a 6080 matching Binding Acknowledgement. Once reaching a retransmission 6081 timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart 6082 the process of delivering the Binding Update, but trying instead the 6083 next home agent returned during dynamic home agent address discovery 6084 (see Section 11.4.1). If there was only one home agent, the mobile 6085 node instead SHOULD continue to periodically retransmit the Binding 6086 Update at this rate until acknowledged (or until it begins attempting 6087 to register a different primary care-of address). See Section 11.8 6088 for information about retransmitting Binding Updates. 6090 With the Binding Update, the mobile node requests the home agent to 6091 serve as the home agent for the given home address. Until the 6092 lifetime of this registration expires, the home agent considers 6093 itself the home agent for this home address. 6095 Each Binding Update MUST be authenticated as coming from the right 6096 mobile node, as defined in Section 5.1. The mobile node MUST use its 6097 home address - either in the Home Address destination option or in 6098 the Source Address field of the IPv6 header - in Binding Updates sent 6099 to the home agent. This is necessary in order to allow the IPsec 6100 policies to be matched with the correct home address. 6102 When sending a Binding Update to its home agent, the mobile node MUST 6103 also create or update the corresponding Binding Update List entry, as 6104 specified in Section 11.7.2. 6106 The last Sequence Number value sent to the home agent in a Binding 6107 Update is stored by the mobile node. If the sending mobile node has 6108 no knowledge of the correct Sequence Number value, it may start at 6109 any value. If the home agent rejects the value, it sends back a 6110 Binding Acknowledgement with a status code 135, and the last accepted 6111 sequence number in the Sequence Number field of the Binding 6112 Acknowledgement. The mobile node MUST store this information and use 6113 the next Sequence Number value for the next Binding Update it sends. 6115 If the mobile node has additional home addresses, then the mobile 6116 node SHOULD send an additional packet containing a Binding Update to 6117 its home agent to register the care-of address for each such other 6118 home address. 6120 The home agent will only perform DAD for the mobile node's home 6121 address when the mobile node has supplied a valid binding between its 6122 home address and a care-of address. If some time elapses during 6123 which the mobile node has no binding at the home agent, it might be 6124 possible for another node to autoconfigure the mobile node's home 6125 address. Therefore, the mobile node MUST treat the creation of a new 6126 binding with the home agent using an existing home address, the same 6127 as creation of a new home address. In the unlikely event that the 6128 mobile node's home address is autoconfigured as the IPv6 address of 6129 another network node on the home network, the home agent will reply 6130 to the mobile node's subsequent Binding Update with a Binding 6131 Acknowledgement containing a Status of 134 (Duplicate Address 6132 Detection failed). In this case, the mobile node MUST NOT attempt to 6133 re-use the same home address. It SHOULD continue to register the 6134 care-of addresses for its other home addresses, if any. (Mechanisms 6135 outlined in Appendix A.5 may in the future allow mobile nodes to 6136 acquire new home addresses to replace the one for which Status 134 6137 was received.) 6139 11.7.2. Correspondent Registration 6141 When the mobile node is assured that its home address is valid, it 6142 can initiate a correspondent registration with the purpose of 6143 allowing the correspondent node to cache the mobile node's current 6144 care-of address. This procedure consists of the return routability 6145 procedure followed by a registration. 6147 This section defines when the correspondent registration is to be 6148 initiated and the rules to follow while it is being performed. 6150 After the mobile node has sent a Binding Update to its home agent, 6151 registering a new primary care-of address (as described in 6152 Section 11.7.1), the mobile node SHOULD initiate a correspondent 6153 registration for each node that already appears in the mobile node's 6154 Binding Update List. The initiated procedures can be used to either 6155 update or delete binding information in the correspondent node. 6157 For nodes that do not appear in the mobile node's Binding Update 6158 List, the mobile node MAY initiate a correspondent registration at 6159 any time after sending the Binding Update to its home agent. 6160 Considerations regarding when (and if) to initiate the procedure 6161 depend on the specific movement and traffic patterns of the mobile 6162 node and are outside the scope of this document. 6164 In addition, the mobile node MAY initiate the correspondent 6165 registration in response to receiving a packet that meets all of the 6166 following tests: 6168 o The packet was tunneled using IPv6 encapsulation. 6170 o The Destination Address in the tunnel (outer) IPv6 header is equal 6171 to any of the mobile node's care-of addresses. 6173 o The Destination Address in the original (inner) IPv6 header is 6174 equal to one of the mobile node's home addresses. 6176 o The Source Address in the tunnel (outer) IPv6 header differs from 6177 the Source Address in the original (inner) IPv6 header. 6179 o The packet does not contain a Home Test, Home Test Init, Care-of 6180 Test, or Care-of Test Init message. 6182 If a mobile node has multiple home addresses, it becomes important to 6183 select the right home address to use in the correspondent 6184 registration. The used home address MUST be the Destination Address 6185 of the original (inner) packet. 6187 The peer address used in the procedure MUST be determined as follows: 6189 o If a Home Address destination option is present in the original 6190 (inner) packet, the address from this option is used. 6192 o Otherwise, the Source Address in the original (inner) IPv6 header 6193 of the packet is used. 6195 Note that the validity of the original packet is checked before 6196 attempting to initiate a correspondent registration. For instance, 6197 if a Home Address destination option appeared in the original packet, 6198 then rules in Section 9.3.1 are followed. 6200 A mobile node MAY also choose to keep its topological location 6201 private from certain correspondent nodes, and thus need not initiate 6202 the correspondent registration. 6204 Upon successfully completing the return routability procedure, and 6205 after receiving a successful Binding Acknowledgement from the Home 6206 Agent, a Binding Update MAY be sent to the correspondent node. 6208 In any Binding Update sent by a mobile node, the care-of address 6209 (either the Source Address in the packet's IPv6 header or the Care-of 6210 Address in the Alternate Care-of Address mobility option of the 6211 Binding Update) MUST be set to one of the care-of addresses currently 6212 in use by the mobile node or to the mobile node's home address. A 6213 mobile node MAY set the care-of address differently for sending 6214 Binding Updates to different correspondent nodes. 6216 A mobile node MAY also send a Binding Update to such a correspondent 6217 node, instructing it to delete any existing binding for the mobile 6218 node from its Binding Cache, as described in Section 6.1.7. Even in 6219 this case a successful completion of the return routability procedure 6220 is required first. 6222 If the care-of address is not set to the mobile node's home address, 6223 the Binding Update requests that the correspondent node create or 6224 update an entry for the mobile node in the correspondent node's 6225 Binding Cache. This is done in order to record a care-of address for 6226 use in sending future packets to the mobile node. In this case, the 6227 value specified in the Lifetime field sent in the Binding Update 6228 SHOULD be less than or equal to the remaining lifetime of the home 6229 registration and the care-of address specified for the binding. The 6230 care-of address given in the Binding Update MAY differ from the 6231 mobile node's primary care-of address. 6233 If the Binding Update is sent to the correspondent node, requesting 6234 the deletion of any existing Binding Cache entry it has for the 6235 mobile node, the care-of address is set to the mobile node's home 6236 address and the Lifetime field set to zero. In this case, generation 6237 of the binding management key depends exclusively on the home keygen 6238 token (Section 5.2.5). The care-of nonce index SHOULD be set to zero 6239 in this case. In keeping with the Binding Update creation rules 6240 below, the care-of address MUST be set to the home address if the 6241 mobile node is at home, or to the current care-of address if it is 6242 away from home. 6244 If the mobile node wants to ensure that its new care-of address has 6245 been entered into a correspondent node's Binding Cache, the mobile 6246 node needs to request an acknowledgement by setting the Acknowledge 6247 (A) bit in the Binding Update. 6249 A Binding Update is created as follows: 6251 o The current care-of address of the mobile node MUST be sent either 6252 in the Source Address of the IPv6 header, or in the Alternate 6253 Care-of Address mobility option. 6255 o The Destination Address of the IPv6 header MUST contain the 6256 address of the correspondent node. 6258 o The Mobility Header is constructed according to rules in 6259 Section 6.1.7 and Section 5.2.6, including the Binding 6260 Authorization Data (calculated as defined in Section 6.2.7) and 6261 possibly the Nonce Indices mobility options. 6263 o The home address of the mobile node MUST be added to the packet in 6264 a Home Address destination option, unless the Source Address is 6265 the home address. 6267 Each Binding Update MUST have a Sequence Number greater than the 6268 Sequence Number value sent in the previous Binding Update to the same 6269 destination address (if any). The sequence numbers are compared 6270 modulo 2**16, as described in Section 9.5.1. There is no 6271 requirement, however, that the Sequence Number value strictly 6272 increase by 1 with each new Binding Update sent or received, as long 6273 as the value stays within the window. The last Sequence Number value 6274 sent to a destination in a Binding Update is stored by the mobile 6275 node in its Binding Update List entry for that destination. If the 6276 sending mobile node has no Binding Update List entry, the Sequence 6277 Number SHOULD start at a random value. The mobile node MUST NOT use 6278 the same Sequence Number in two different Binding Updates to the same 6279 correspondent node, even if the Binding Updates provide different 6280 care-of addresses. 6282 The mobile node is responsible for the completion of the 6283 correspondent registration, as well as any retransmissions that may 6284 be needed (subject to the rate limitation defined in Section 11.8). 6286 11.7.3. Receiving Binding Acknowledgements 6288 Upon receiving a packet carrying a Binding Acknowledgement, a mobile 6289 node MUST validate the packet according to the following tests: 6291 o The packet meets the authentication requirements for Binding 6292 Acknowledgements defined in Section 6.1.8 and Section 5. That is, 6293 if the Binding Update was sent to the home agent, underlying IPsec 6294 protection is used. If the Binding Update was sent to the 6295 correspondent node, the Binding Authorization Data mobility option 6296 MUST be present and have a valid value. 6298 o The Binding Authorization Data mobility option, if present, MUST 6299 be the last option and MUST NOT have trailing padding. 6301 o The Sequence Number field matches the Sequence Number sent by the 6302 mobile node to this destination address in an outstanding Binding 6303 Update, and the Status field is not 135. 6305 Any Binding Acknowledgement not satisfying all of these tests MUST be 6306 silently ignored. 6308 When a mobile node receives a packet carrying a valid Binding 6309 Acknowledgement, the mobile node MUST examine the Status field as 6310 follows: 6312 o If the Status field indicates that the Binding Update was accepted 6313 (the Status field is less than 128), then the mobile node MUST 6314 update the corresponding entry in its Binding Update List to 6315 indicate that the Binding Update has been acknowledged; the mobile 6316 node MUST then stop retransmitting the Binding Update. In 6317 addition, if the value specified in the Lifetime field in the 6318 Binding Acknowledgement is less than the Lifetime value sent in 6319 the Binding Update being acknowledged, the mobile node MUST 6320 subtract the difference between these two Lifetime values from the 6321 remaining lifetime for the binding as maintained in the 6322 corresponding Binding Update List entry (with a minimum value for 6323 the Binding Update List entry lifetime of 0). That is, if the 6324 Lifetime value sent in the Binding Update was L_update, the 6325 Lifetime value received in the Binding Acknowledgement was L_ack, 6326 and the current remaining lifetime of the Binding Update List 6327 entry is L_remain, then the new value for the remaining lifetime 6328 of the Binding Update List entry should be 6330 max((L_remain - (L_update - L_ack)), 0) 6332 where max(X, Y) is the maximum of X and Y. The effect of this step 6333 is to correctly manage the mobile node's view of the binding's 6334 remaining lifetime (as maintained in the corresponding Binding 6335 Update List entry) so that it correctly counts down from the 6336 Lifetime value given in the Binding Acknowledgement, but with the 6337 timer countdown beginning at the time that the Binding Update was 6338 sent. 6340 Mobile nodes SHOULD send a new Binding Update well before the 6341 expiration of this period in order to extend the lifetime. This 6342 helps to avoid disruptions in communications which might otherwise 6343 be caused by network delays or clock drift. 6345 o If the Binding Acknowledgement correctly passes authentication and 6346 the Status field value is 135 (Sequence Number out of window), 6347 then the mobile node MUST update its binding sequence number 6348 appropriately to match the sequence number given in the Binding 6349 Acknowledgement. Otherwise, if the Status field value is 135 but 6350 the Binding Acknowledgement does not pass authentication, the 6351 message MUST be silently ignored. 6353 o If the Status field value is 1 (accepted but prefix discovery 6354 necessary), the mobile node SHOULD send a Mobile Prefix 6355 Solicitation message to update its information about the available 6356 prefixes. 6358 o If the Status field indicates that the Binding Update was rejected 6359 (the Status field is greater than or equal to 128), then the 6360 mobile node can take steps to correct the cause of the error and 6361 retransmit the Binding Update (with a new Sequence Number value), 6362 subject to the rate limiting restriction specified in 6363 Section 11.8. If this is not done or it fails, then the mobile 6364 node SHOULD record in its Binding Update List that future Binding 6365 Updates SHOULD NOT be sent to this destination. 6367 The treatment of a Binding Refresh Advice mobility option within the 6368 Binding Acknowledgement depends on where the acknowledgement came 6369 from. This option MUST be ignored if the acknowledgement came from a 6370 correspondent node. If it came from the home agent, the mobile node 6371 uses the Refresh Interval field in the option as a suggestion that it 6372 SHOULD attempt to refresh its home registration at the indicated 6373 shorter interval. 6375 If the acknowledgement came from the home agent, the mobile node 6376 examines the value of the Key Management Mobility Capability (K) bit. 6377 If this bit is not set, the mobile node SHOULD discard key management 6378 protocol connections, if any, to the home agent. The mobile node MAY 6379 also initiate a new key management connection. 6381 If this bit is set, the mobile node SHOULD move its own endpoint in 6382 the key management protocol connections to the home agent, if any. 6383 The mobile node's new endpoint should be the new care-of address. 6384 For an IKE phase 1 connection, this means that packets sent to this 6385 address with the original ISAKMP cookies are accepted. 6387 11.7.4. Receiving Binding Refresh Requests 6389 When a mobile node receives a packet containing a Binding Refresh 6390 Request message, the mobile node has a Binding Update List entry for 6391 the source of the Binding Refresh Request, and the mobile node wants 6392 to retain its binding cache entry at the correspondent node, then the 6393 mobile node should start a return routability procedure. If the 6394 mobile node wants to have its binding cache entry removed, it can 6395 either ignore the Binding Refresh Request and wait for the binding to 6396 time out, or at any time, it can delete its binding from a 6397 correspondent node with an explicit binding update with a zero 6398 lifetime and the care-of address set to the home address. If the 6399 mobile node does not know if it needs the binding cache entry, it can 6400 make the decision in an implementation dependent manner, such as 6401 based on available resources. 6403 Note that the mobile node should be careful to not respond to Binding 6404 Refresh Requests for addresses not in the Binding Update List to 6405 avoid being subjected to a denial of service attack. 6407 If the return routability procedure completes successfully, a Binding 6408 Update message SHOULD be sent, as described in Section 11.7.2. The 6409 Lifetime field in this Binding Update SHOULD be set to a new 6410 lifetime, extending any current lifetime remaining from a previous 6411 Binding Update sent to this node (as indicated in any existing 6412 Binding Update List entry for this node), and the lifetime SHOULD 6413 again be less than or equal to the remaining lifetime of the home 6414 registration and the care-of address specified for the binding. When 6415 sending this Binding Update, the mobile node MUST update its Binding 6416 Update List in the same way as for any other Binding Update sent by 6417 the mobile node. 6419 11.8. Retransmissions and Rate Limiting 6421 The mobile node is responsible for retransmissions and rate limiting 6422 in the return routability procedure, registrations, and in solicited 6423 prefix discovery. 6425 When the mobile node sends a Mobile Prefix Solicitation, Home Test 6426 Init, Care-of Test Init or Binding Update for which it expects a 6427 response, the mobile node has to determine a value for the initial 6428 retransmission timer: 6430 o If the mobile node is sending a Mobile Prefix Solicitation, it 6431 SHOULD use an initial retransmission interval of 6432 INITIAL_SOLICIT_TIMER (see Section 12). 6434 o If the mobile node is sending a Binding Update and does not have 6435 an existing binding at the home agent, it SHOULD use 6436 InitialBindackTimeoutFirstReg (see Section 13) as a value for the 6437 initial retransmission timer. This long retransmission interval 6438 will allow the home agent to complete the Duplicate Address 6439 Detection procedure mandated in this case, as detailed in 6440 Section 11.7.1. 6442 o Otherwise, the mobile node should use the specified value of 6443 INITIAL_BINDACK_TIMEOUT for the initial retransmission timer. 6445 If the mobile node fails to receive a valid matching response within 6446 the selected initial retransmission interval, the mobile node SHOULD 6447 retransmit the message until a response is received. 6449 The retransmissions by the mobile node MUST use an exponential back- 6450 off process in which the timeout period is doubled upon each 6451 retransmission, until either the node receives a response or the 6452 timeout period reaches the value MAX_BINDACK_TIMEOUT. The mobile 6453 node MAY continue to send these messages at this slower rate 6454 indefinitely. 6456 The mobile node SHOULD start a separate back-off process for 6457 different message types, different home addresses and different 6458 care-of addresses. However, in addition an overall rate limitation 6459 applies for messages sent to a particular correspondent node. This 6460 ensures that the correspondent node has a sufficient amount of time 6461 to respond when bindings for multiple home addresses are registered, 6462 for instance. The mobile node MUST NOT send Mobility Header messages 6463 of a particular type to a particular correspondent node more than 6464 MAX_UPDATE_RATE times within a second. 6466 Retransmitted Binding Updates MUST use a Sequence Number value 6467 greater than that used for the previous transmission of this Binding 6468 Update. Retransmitted Home Test Init and Care-of Test Init messages 6469 MUST use new cookie values. 6471 12. Protocol Constants 6473 DHAAD_RETRIES 4 retransmissions 6474 INITIAL_BINDACK_TIMEOUT 1 second 6475 INITIAL_DHAAD_TIMEOUT 3 seconds 6476 INITIAL_SOLICIT_TIMER 3 seconds 6477 MAX_BINDACK_TIMEOUT 32 seconds 6478 MAX_NONCE_LIFETIME 240 seconds 6479 MAX_TOKEN_LIFETIME 210 seconds 6480 MAX_RR_BINDING_LIFETIME 420 seconds 6481 MAX_UPDATE_RATE 3 times 6482 PREFIX_ADV_RETRIES 3 retransmissions 6483 PREFIX_ADV_TIMEOUT 3 seconds 6485 13. Protocol Configuration Variables 6487 MaxMobPfxAdvInterval Default: 86,400 seconds 6488 MinDelayBetweenRAs Default: 3 seconds, 6489 Min: 0.03 seconds 6490 MinMobPfxAdvInterval Default: 600 seconds 6491 InitialBindackTimeoutFirstReg Default: 1.5 seconds 6493 Home agents MUST allow the first three variables to be configured by 6494 system management, and mobile nodes MUST allow the last variable to 6495 be configured by system management. 6497 The default value for InitialBindackTimeoutFirstReg has been 6498 calculated as 1.5 times the default value of RetransTimer [20] times 6499 the default value of DupAddrDetectTransmits [21]. 6501 The value MinDelayBetweenRAs overrides the value of the protocol 6502 constant MIN_DELAY_BETWEEN_RAS, as specified in RFC 4861 [20]. This 6503 variable SHOULD be set to MinRtrAdvInterval, if MinRtrAdvInterval is 6504 less than 3 seconds. 6506 14. IANA Considerations 6508 This document defines a new IPv6 protocol, the Mobility Header, 6509 described in Section 6.1. This protocol has been assigned protocol 6510 number 135. 6512 This document also creates a new name space "Mobility Header Type", 6513 for the MH Type field in the Mobility Header. The current message 6514 types are described starting from Section 6.1.2, and are the 6515 following: 6517 0 Binding Refresh Request 6519 1 Home Test Init 6521 2 Care-of Test Init 6523 3 Home Test 6525 4 Care-of Test 6527 5 Binding Update 6529 6 Binding Acknowledgement 6531 7 Binding Error 6533 Future values of the MH Type can be allocated using Standards Action 6534 or IESG Approval [26]. 6536 Furthermore, each mobility message may contain mobility options as 6537 described in Section 6.2. This document defines a new name space 6538 "Mobility Option" to identify these options. The current mobility 6539 options are defined starting from Section 6.2.2 and are the 6540 following: 6542 0 Pad1 6544 1 PadN 6546 2 Binding Refresh Advice 6548 3 Alternate Care-of Address 6550 4 Nonce Indices 6551 5 Authorization Data 6553 Future values of the Option Type can be allocated using Standards 6554 Action or IESG Approval [26]. 6556 Finally, this document creates a third new name space "Status Code" 6557 for the Status field in the Binding Acknowledgement message. The 6558 current values are described in Section 6.1.8, and are the following: 6560 0 Binding Update accepted 6562 1 Accepted but prefix discovery necessary 6564 128 Reason unspecified 6566 129 Administratively prohibited 6568 130 Insufficient resources 6570 131 Home registration not supported 6572 132 Not home subnet 6574 133 Not home agent for this mobile node 6576 134 Duplicate Address Detection failed 6578 135 Sequence number out of window 6580 136 Expired home nonce index 6582 137 Expired care-of nonce index 6584 138 Expired nonces 6586 139 Registration type change disallowed 6588 Future values of the Status field can be allocated using Standards 6589 Action or IESG Approval [26]. 6591 All fields labeled "Reserved" are only to be assigned through 6592 Standards Action or IESG Approval. 6594 This document also defines a new IPv6 destination option, the Home 6595 Address option, described in Section 6.3. This option has been 6596 assigned the Option Type value 0xC9. 6598 This document also defines a new IPv6 type 2 routing header, 6599 described in Section 6.4. The value 2 has been allocated by IANA. 6601 In addition, this document defines four ICMP message types, two used 6602 as part of the dynamic home agent address discovery mechanism, and 6603 two used in lieu of Router Solicitations and Advertisements when the 6604 mobile node is away from the home link. These messages have been 6605 assigned ICMPv6 type numbers from the informational message range: 6607 o The Home Agent Address Discovery Request message, described in 6608 Section 6.5; 6610 o The Home Agent Address Discovery Reply message, described in 6611 Section 6.6; 6613 o The Mobile Prefix Solicitation, described in Section 6.7; and 6615 o The Mobile Prefix Advertisement, described in Section 6.8. 6617 This document also defines two new Neighbor Discovery [20] options, 6618 which have been assigned Option Type values within the option 6619 numbering space for Neighbor Discovery messages: 6621 o The Advertisement Interval option, described in Section 7.3; and 6623 o The Home Agent Information option, described in Section 7.4. 6625 15. Security Considerations 6627 15.1. Threats 6629 Any mobility solution must protect itself against misuses of the 6630 mobility features and mechanisms. In Mobile IPv6, most of the 6631 potential threats are concerned with false Bindings, usually 6632 resulting in Denial-of-Service attacks. Some of the threats also 6633 pose potential for Man-in-the-Middle, Hijacking, Confidentiality, and 6634 Impersonation attacks. The main threats this protocol protects 6635 against are the following: 6637 o Threats involving Binding Updates sent to home agents and 6638 correspondent nodes. For instance, an attacker might claim that a 6639 certain mobile node is currently at a different location than it 6640 really is. If a home agent accepts such spoofed information sent 6641 to it, the mobile node might not get traffic destined to it. 6642 Similarly, a malicious (mobile) node might use the home address of 6643 a victim node in a forged Binding Update sent to a correspondent 6644 node. 6646 These pose threats against confidentiality, integrity, and 6647 availability. That is, an attacker might learn the contents of 6648 packets destined to another node by redirecting the traffic to 6649 itself. Furthermore, an attacker might use the redirected packets 6650 in an attempt to set itself as a Man-in-the-Middle between a 6651 mobile and a correspondent node. This would allow the attacker to 6652 impersonate the mobile node, leading to integrity and availability 6653 problems. 6655 A malicious (mobile) node might also send Binding Updates in which 6656 the care-of address is set to the address of a victim node. If 6657 such Binding Updates were accepted, the malicious node could lure 6658 the correspondent node into sending potentially large amounts of 6659 data to the victim; the correspondent node's replies to messages 6660 sent by the malicious mobile node will be sent to the victim host 6661 or network. This could be used to cause a Distributed Denial-of- 6662 Service attack. For example, the correspondent node might be a 6663 site that will send a high-bandwidth stream of video to anyone who 6664 asks for it. Note that the use of flow-control protocols such as 6665 TCP does not necessarily defend against this type of attack, 6666 because the attacker can fake the acknowledgements. Even keeping 6667 TCP initial sequence numbers secret does not help, because the 6668 attacker can receive the first few segments (including the ISN) at 6669 its own address, and only then redirect the stream to the victim's 6670 address. These types of attacks may also be directed to networks 6671 instead of nodes. Further variations of this threat are described 6672 elsewhere [31] [36]. 6674 An attacker might also attempt to disrupt a mobile node's 6675 communications by replaying a Binding Update that the node had 6676 sent earlier. If the old Binding Update was accepted, packets 6677 destined for the mobile node would be sent to its old location as 6678 opposed to its current location. 6680 In conclusion, there are Denial-of-Service, Man-in-the-Middle, 6681 Confidentiality, and Impersonation threats against the parties 6682 involved in sending legitimate Binding Updates, and Denial-of- 6683 Service threats against any other party. 6685 o Threats associated with payload packets: Payload packets exchanged 6686 with mobile nodes are exposed to similar threats as that of 6687 regular IPv6 traffic. However, Mobile IPv6 introduces the Home 6688 Address destination option, a new routing header type (type 2), 6689 and uses tunneling headers in the payload packets. The protocol 6690 must protect against potential new threats involving the use of 6691 these mechanisms. 6693 Third parties become exposed to a reflection threat via the Home 6694 Address destination option, unless appropriate security 6695 precautions are followed. The Home Address destination option 6696 could be used to direct response traffic toward a node whose IP 6697 address appears in the option. In this case, ingress filtering 6698 would not catch the forged "return address" [39] [42]. 6700 A similar threat exists with the tunnels between the mobile node 6701 and the home agent. An attacker might forge tunnel packets 6702 between the mobile node and the home agent, making it appear that 6703 the traffic is coming from the mobile node when it is not. Note 6704 that an attacker who is able to forge tunnel packets would 6705 typically also be able to forge packets that appear to come 6706 directly from the mobile node. This is not a new threat as such. 6707 However, it may make it easier for attackers to escape detection 6708 by avoiding ingress filtering and packet tracing mechanisms. 6709 Furthermore, spoofed tunnel packets might be used to gain access 6710 to the home network. 6712 Finally, a routing header could also be used in reflection 6713 attacks, and in attacks designed to bypass firewalls. The 6714 generality of the regular routing header would allow circumvention 6715 of IP-address based rules in firewalls. It would also allow 6716 reflection of traffic to other nodes. These threats exist with 6717 routing headers in general, even if the usage that Mobile IPv6 6718 requires is safe. 6720 o Threats associated with dynamic home agent and mobile prefix 6721 discovery. 6723 o Threats against the Mobile IPv6 security mechanisms themselves: An 6724 attacker might, for instance, lure the participants into executing 6725 expensive cryptographic operations or allocating memory for the 6726 purpose of keeping state. The victim node would have no resources 6727 left to handle other tasks. 6729 As a fundamental service in an IPv6 stack, Mobile IPv6 is expected to 6730 be deployed in most nodes of the IPv6 Internet. The above threats 6731 should therefore be considered as being applicable to the whole 6732 Internet. 6734 It should also be noted that some additional threats result from 6735 movements as such, even without the involvement of mobility 6736 protocols. Mobile nodes must be capable to defend themselves in the 6737 networks that they visit, as typical perimeter defenses applied in 6738 the home network no longer protect them. 6740 15.2. Features 6742 This specification provides a series of features designed to mitigate 6743 the risk introduced by the threats listed above. The main security 6744 features are the following: 6746 o Reverse Tunneling as a mandatory feature. 6748 o Protection of Binding Updates sent to home agents. 6750 o Protection of Binding Updates sent to correspondent nodes. 6752 o Protection against reflection attacks that use the Home Address 6753 destination option. 6755 o Protection of tunnels between the mobile node and the home agent. 6757 o Closing routing header vulnerabilities. 6759 o Mitigating Denial-of-Service threats to the Mobile IPv6 security 6760 mechanisms themselves. 6762 The support for encrypted reverse tunneling (see Section 11.3.1) 6763 allows mobile nodes to defeat certain kinds of traffic analysis. 6765 Protecting those Binding Updates that are sent to home agents and 6766 those that are sent to arbitrary correspondent nodes requires very 6767 different security solutions due to the different situations. Mobile 6768 nodes and home agents are naturally expected to be subject to the 6769 network administration of the home domain. 6771 Thus, they can and are supposed to have a security association that 6772 can be used to reliably authenticate the exchanged messages. See 6773 Section 5.1 for the description of the protocol mechanisms, and 6774 Section 15.3 below for a discussion of the resulting level of 6775 security. 6777 It is expected that Mobile IPv6 route optimization will be used on a 6778 global basis between nodes belonging to different administrative 6779 domains. It would be a very demanding task to build an 6780 authentication infrastructure on this scale. Furthermore, a 6781 traditional authentication infrastructure cannot be easily used to 6782 authenticate IP addresses because IP addresses can change often. It 6783 is not sufficient to just authenticate the mobile nodes; 6784 Authorization to claim the right to use an address is needed as well. 6785 Thus, an "infrastructureless" approach is necessary. The chosen 6786 infrastructureless method is described in Section 5.2, and 6787 Section 15.4 discusses the resulting security level and the design 6788 rationale of this approach. 6790 Specific rules guide the use of the Home Address destination option, 6791 the routing header, and the tunneling headers in the payload packets. 6792 These rules are necessary to remove the vulnerabilities associated 6793 with their unrestricted use. The effect of the rules is discussed in 6794 Section 15.7, Section 15.8, and Section 15.9. 6796 Denial-of-Service threats against Mobile IPv6 security mechanisms 6797 themselves concern mainly the Binding Update procedures with 6798 correspondent nodes. The protocol has been designed to limit the 6799 effects of such attacks, as will be described in Section 15.4.5. 6801 15.3. Binding Updates to Home Agent 6803 Signaling between the mobile node and the home agent requires message 6804 integrity. This is necessary to assure the home agent that a Binding 6805 Update is from a legitimate mobile node. In addition, correct 6806 ordering and anti-replay protection are optionally needed. 6808 IPsec ESP protects the integrity of the Binding Updates and Binding 6809 Acknowledgements by securing mobility messages between the mobile 6810 node and the home agent. 6812 IPsec can provide anti-replay protection only if dynamic keying is 6813 used (which may not always be the case). IPsec does not guarantee 6814 correct ordering of packets, only that they have not been replayed. 6815 Because of this, sequence numbers within the Mobile IPv6 messages are 6816 used to ensure correct ordering (see Section 5.1). However, if the 6817 16 bit Mobile IPv6 sequence number space is cycled through, or the 6818 home agent reboots and loses its state regarding the sequence 6819 numbers, replay and reordering attacks become possible. The use of 6820 dynamic keying, IPsec anti-replay protection, and the Mobile IPv6 6821 sequence numbers can together prevent such attacks. It is also 6822 recommended that use of non-volatile storage be considered for home 6823 agents, to avoid losing their state. 6825 A sliding window scheme is used for the sequence numbers. The 6826 protection against replays and reordering attacks without a key 6827 management mechanism works when the attacker remembers up to a 6828 maximum of 2**15 Binding Updates. 6830 The above mechanisms do not show that the care-of address given in 6831 the Binding Update is correct. This opens the possibility for 6832 Denial-of-Service attacks against third parties. However, since the 6833 mobile node and home agent have a security association, the home 6834 agent can always identify an ill-behaving mobile node. This allows 6835 the home agent operator to discontinue the mobile node's service, and 6836 possibly take further actions based on the business relationship with 6837 the mobile node's owner. 6839 Note that the use of a single pair of manually keyed security 6840 associations conflicts with the generation of a new home address [23] 6841 for the mobile node, or with the adoption of a new home subnet 6842 prefix. This is because IPsec security associations are bound to the 6843 used addresses. While certificate-based automatic keying alleviates 6844 this problem to an extent, it is still necessary to ensure that a 6845 given mobile node cannot send Binding Updates for the address of 6846 another mobile node. In general, this leads to the inclusion of home 6847 addresses in certificates in the Subject AltName field. This again 6848 limits the introduction of new addresses without either manual or 6849 automatic procedures to establish new certificates. Therefore, this 6850 specification restricts the generation of new home addresses (for any 6851 reason) to those situations where a security association or 6852 certificate for the new address already exists. (Appendix A.4 lists 6853 the improvement of security for new addresses as one of the future 6854 developments for Mobile IPv6.) 6856 Support for IKE has been specified as optional. The following should 6857 be observed about the use of manual keying: 6859 o As discussed above, with manually keyed IPsec, only a limited form 6860 of protection exists against replay and reordering attacks. A 6861 vulnerability exists if either the sequence number space is cycled 6862 through, or if the home agent reboots and forgets its sequence 6863 numbers (and uses volatile memory to store the sequence numbers). 6865 Assuming the mobile node moves continuously every 10 minutes, it 6866 takes roughly 455 days before the sequence number space has been 6867 cycled through. Typical movement patterns rarely reach this high 6868 frequency today. 6870 o A mobile node and its home agent belong to the same domain. If 6871 this were not the case, manual keying would not be possible [41], 6872 but in Mobile IPv6 only these two parties need to know the 6873 manually configured keys. Similarly, we note that Mobile IPv6 6874 employs standard block ciphers in IPsec, and is not vulnerable to 6875 problems associated with stream ciphers and manual keying. 6877 o It is expected that the owner of the mobile node and the 6878 administrator of the home agent agree on the used keys and other 6879 parameters with some off-line mechanism. 6881 The use of IKEv1 with Mobile IPv6 is documented in more detail in 6882 [15]. The following should be observed from the use of IKEv1: 6884 o It is necessary to prevent a mobile node from claiming another 6885 mobile node's home address. The home agent must verify that the 6886 mobile node trying to negotiate the SA for a particular home 6887 address is authorized for that home address. This implies that 6888 even with the use of IKE, a policy entry needs to be configured 6889 for each home address served by the home agent. 6891 It may be possible to include home addresses in the Subject 6892 AltName field of certificate to avoid this. However, 6893 implementations are not guaranteed to support the use of a 6894 particular IP address (care-of address) while another address 6895 (home address) appears in the certificate. In any case, even this 6896 approach would require user-specific tasks in the certificate 6897 authority. 6899 o If preshared secret authentication is used, IKEv1 main mode cannot 6900 be used. Aggressive mode or group preshared secrets need to be 6901 used with corresponding security implications instead. 6903 Note that, like many other issues, this is a general IKEv1 issue 6904 related to the ability to use different IP addresses, and not 6905 specifically related to Mobile IPv6. For further information, see 6906 Section 4.4 in [15]. 6908 o Due to the problems outlined in Section 11.3.2, IKE phase 1 6909 between the mobile node and its home agent is established using 6910 the mobile node's current care-of address. This implies that when 6911 the mobile node moves to a new location, it may have to re- 6912 establish phase 1. A Key Management Mobility Capability (K) flag 6913 is provided for implementations that can update the IKE phase 1 6914 endpoints without re-establishing phase 1, but the support for 6915 this behavior is optional. 6917 o When certificates are used, IKE fragmentation can occur as 6918 discussed in Section 7 in [15]. 6920 o Nevertheless, even if per-mobile node configuration is required 6921 with IKE, an important benefit of IKE is that it automates the 6922 negotiation of cryptographic parameters, including the SPIs, 6923 cryptographic algorithms, and so on. Thus, less configuration 6924 information is needed. 6926 o The frequency of movements in some link layers or deployment 6927 scenarios may be high enough to make replay and reordering attacks 6928 possible, if only manual keying is used. IKE SHOULD be used in 6929 such cases. Potentially vulnerable scenarios involve continuous 6930 movement through small cells, or uncontrolled alternation between 6931 available network attachment points. 6933 o Similarly, in some deployment scenarios the number of mobile nodes 6934 may be very large. In these cases, it can be necessary to use 6935 automatic mechanisms to reduce the management effort in the 6936 administration of cryptographic parameters, even if some per- 6937 mobile node configuration is always needed. IKE SHOULD also be 6938 used in such cases. 6940 o Other automatic key management mechanisms exist beyond IKEv1, but 6941 this document does not address the issues related to them. We 6942 note, however, that most of the above discussion applies to IKEv2 6943 [43] as well, at least as it is currently specified. 6945 15.4. Binding Updates to Correspondent Nodes 6947 The motivation for designing the return routability procedure was to 6948 have sufficient support for Mobile IPv6, without creating significant 6949 new security problems. The goal for this procedure was not to 6950 protect against attacks that were already possible before the 6951 introduction of Mobile IPv6. 6953 The next sections will describe the security properties of the used 6954 method, both from the point of view of possible on-path attackers who 6955 can see those cryptographic values that have been sent in the clear 6956 (Section 15.4.2 and Section 15.4.3) and from the point of view of 6957 other attackers (Section 15.4.6). 6959 15.4.1. Overview 6961 The chosen infrastructureless method verifies that the mobile node is 6962 "live" (that is, it responds to probes) at its home and care-of 6963 addresses. Section 5.2 describes the return routability procedure in 6964 detail. The procedure uses the following principles: 6966 o A message exchange verifies that the mobile node is reachable at 6967 its addresses, i.e., is at least able to transmit and receive 6968 traffic at both the home and care-of addresses. 6970 o The eventual Binding Update is cryptographically bound to the 6971 tokens supplied in the exchanged messages. 6973 o Symmetric exchanges are employed to avoid the use of this protocol 6974 in reflection attacks. In a symmetric exchange, the responses are 6975 always sent to the same address the request was sent from. 6977 o The correspondent node operates in a stateless manner until it 6978 receives a fully authorized Binding Update. 6980 o Some additional protection is provided by encrypting the tunnels 6981 between the mobile node and home agent with IPsec ESP. As the 6982 tunnel also transports the nonce exchanges, the ability of 6983 attackers to see these nonces is limited. For instance, this 6984 prevents attacks from being launched from the mobile node's 6985 current foreign link, even when no link-layer confidentiality is 6986 available. 6988 The resulting level of security is in theory the same even without 6989 this additional protection: the return routability tokens are 6990 still exposed only to one path within the whole Internet. 6991 However, the mobile nodes are often found on an insecure link, 6992 such as a public access Wireless LAN. Thus, in many cases, this 6993 addition makes a practical difference. 6995 For further information about the design rationale of the return 6996 routability procedure, see [31] [36] [35] [42]. The mechanisms used 6997 have been adopted from these documents. 6999 15.4.2. Achieved Security Properties 7001 The return routability procedure protects Binding Updates against all 7002 attackers who are unable to monitor the path between the home agent 7003 and the correspondent node. The procedure does not defend against 7004 attackers who can monitor this path. Note that such attackers are in 7005 any case able to mount an active attack against the mobile node when 7006 it is at its home location. The possibility of such attacks is not 7007 an impediment to the deployment of Mobile IPv6 because these attacks 7008 are possible regardless of whether or not Mobile IPv6 is in use. 7010 This procedure also protects against Denial-of-Service attacks in 7011 which the attacker pretends to be mobile, but uses the victim's 7012 address as the care-of address. This would cause the correspondent 7013 node to send the victim some unexpected traffic. This procedure 7014 defends against these attacks by requiring at least the passive 7015 presence of the attacker at the care-of address or on the path from 7016 the correspondent to the care-of address. Normally, this will be the 7017 mobile node. 7019 15.4.3. Comparison to Regular IPv6 Communications 7021 This section discusses the protection offered by the return 7022 routability method by comparing it to the security of regular IPv6 7023 communications. We will divide vulnerabilities into three classes: 7024 (1) those related to attackers on the local network of the mobile 7025 node, home agent, or the correspondent node, (2) those related to 7026 attackers on the path between the home network and the correspondent 7027 node, and (3) off-path attackers, i.e., the rest of the Internet. 7029 We will now discuss the vulnerabilities of regular IPv6 7030 communications. The on-link vulnerabilities of IPv6 communications 7031 include Denial-of-Service, Masquerading, Man-in-the-Middle, 7032 Eavesdropping, and other attacks. These attacks can be launched 7033 through spoofing Router Discovery, Neighbor Discovery and other IPv6 7034 mechanisms. Some of these attacks can be prevented with the use of 7035 cryptographic protection in the packets. 7037 A similar situation exists with on-path attackers. That is, without 7038 cryptographic protection, the traffic is completely vulnerable. 7040 Assuming that attackers have not penetrated the security of the 7041 Internet routing protocols, attacks are much harder to launch from 7042 off-path locations. Attacks that can be launched from these 7043 locations are mainly Denial-of-Service attacks, such as flooding 7044 and/or reflection attacks. It is not possible for an off-path 7045 attacker to become a Man-in-the-Middle. 7047 Next, we will consider the vulnerabilities that exist when IPv6 is 7048 used together with Mobile IPv6 and the return routability procedure. 7049 On the local link, the vulnerabilities are the same as those in IPv6, 7050 but Masquerade and Man-in-the-Middle attacks can now also be launched 7051 against future communications, and not just against current 7052 communications. If a Binding Update was sent while the attacker was 7053 present on the link, its effects remain for the lifetime of the 7054 binding. This happens even if the attacker moves away from the link. 7055 In contrast, an attacker who uses only plain IPv6 generally has to 7056 stay on the link in order to continue the attack. Note that in order 7057 to launch these new attacks, the IP address of the victim must be 7058 known. This makes this attack feasible, mainly in the context of 7059 well-known interface IDs, such as those already appearing in the 7060 traffic on the link or registered in the DNS. 7062 On-path attackers can exploit similar vulnerabilities as in regular 7063 IPv6. There are some minor differences, however. Masquerade, Man- 7064 in-the-Middle, and Denial-of-Service attacks can be launched with 7065 just the interception of a few packets, whereas in regular IPv6 it is 7066 necessary to intercept every packet. The effect of the attacks is 7067 the same regardless of the method, however. In any case, the most 7068 difficult task an attacker faces in these attacks is getting on the 7069 right path. 7071 The vulnerabilities for off-path attackers are the same as in regular 7072 IPv6. Those nodes that are not on the path between the home agent 7073 and the correspondent node will not be able to receive the home 7074 address probe messages. 7076 In conclusion, we can state the following main results from this 7077 comparison: 7079 o Return routability prevents any off-path attacks beyond those that 7080 are already possible in regular IPv6. This is the most important 7081 result, preventing attackers on the Internet from exploiting any 7082 vulnerabilities. 7084 o Vulnerabilities to attackers on the home agent link, the 7085 correspondent node link, and the path between them are roughly the 7086 same as in regular IPv6. 7088 o However, one difference is that in basic IPv6 an on-path attacker 7089 must be constantly present on the link or the path, whereas with 7090 Mobile IPv6, an attacker can leave a binding behind after moving 7091 away. 7093 For this reason, this specification limits the creation of 7094 bindings to at most MAX_TOKEN_LIFETIME seconds after the last 7095 routability check has been performed, and limits the duration of a 7096 binding to at most MAX_RR_BINDING_LIFETIME seconds. With these 7097 limitations, attackers cannot take any practical advantages of 7098 this vulnerability. 7100 o There are some other minor differences, such as an effect to the 7101 Denial-of-Service vulnerabilities. These can be considered to be 7102 insignificant. 7104 o The path between the home agent and a correspondent node is 7105 typically easiest to attack on the links at either end, in 7106 particular if these links are publicly accessible wireless LANs. 7108 Attacks against the routers or switches on the path are typically 7109 harder to accomplish. The security on layer 2 of the links plays 7110 then a major role in the resulting overall network security. 7111 Similarly, security of IPv6 Neighbor and Router Discovery on these 7112 links has a large impact. If these were secured using some new 7113 technology in the future, this could change the situation 7114 regarding the easiest point of attack. 7116 For a more in-depth discussion of these issues, see [42]. 7118 15.4.4. Replay Attacks 7120 The return routability procedure also protects the participants 7121 against replayed Binding Updates. The attacker is unable replay the 7122 same message due to the sequence number which is a part of the 7123 Binding Update. It is also unable to modify the Binding Update since 7124 the MAC verification would fail after such a modification. 7126 Care must be taken when removing bindings at the correspondent node, 7127 however. If a binding is removed while the nonce used in its 7128 creation is still valid, an attacker could replay the old Binding 7129 Update. Rules outlined in Section 5.2.8 ensure that this cannot 7130 happen. 7132 15.4.5. Denial-of-Service Attacks 7134 The return routability procedure has protection against resource 7135 exhaustion Denial-of-Service attacks. The correspondent nodes do not 7136 retain any state about individual mobile nodes until an authentic 7137 Binding Update arrives. This is achieved through the construct of 7138 keygen tokens from the nonces and node keys that are not specific to 7139 individual mobile nodes. The keygen tokens can be reconstructed by 7140 the correspondent node, based on the home and care-of address 7141 information that arrives with the Binding Update. This means that 7142 the correspondent nodes are safe against memory exhaustion attacks 7143 except where on-path attackers are concerned. Due to the use of 7144 symmetric cryptography, the correspondent nodes are relatively safe 7145 against CPU resource exhaustion attacks as well. 7147 Nevertheless, as [31] describes, there are situations in which it is 7148 impossible for the mobile and correspondent nodes to determine if 7149 they actually need a binding or whether they just have been fooled 7150 into believing so by an attacker. Therefore, it is necessary to 7151 consider situations where such attacks are being made. 7153 Even if route optimization is a very important optimization, it is 7154 still only an optimization. A mobile node can communicate with a 7155 correspondent node even if the correspondent refuses to accept any 7156 Binding Updates. However, performance will suffer because packets 7157 from the correspondent node to the mobile node will be routed via the 7158 mobile's home agent rather than a more direct route. A correspondent 7159 node can protect itself against some of these resource exhaustion 7160 attacks as follows. If the correspondent node is flooded with a 7161 large number of Binding Updates that fail the cryptographic integrity 7162 checks, it can stop processing Binding Updates. If a correspondent 7163 node finds that it is spending more resources on checking bogus 7164 Binding Updates than it is likely to save by accepting genuine 7165 Binding Updates, then it may silently discard some or all Binding 7166 Updates without performing any cryptographic operations. 7168 Layers above IP can usually provide additional information to help 7169 decide if there is a need to establish a binding with a specific 7170 peer. For example, TCP knows if the node has a queue of data that it 7171 is trying to send to a peer. An implementation of this specification 7172 is not required to make use of information from higher protocol 7173 layers, but some implementations are likely to be able to manage 7174 resources more effectively by making use of such information. 7176 We also require that all implementations be capable of 7177 administratively disabling route optimization. 7179 15.4.6. Key Lengths 7181 Attackers can try to break the return routability procedure in many 7182 ways. Section 15.4.2 discusses the situation where the attacker can 7183 see the cryptographic values sent in the clear, and Section 15.4.3 7184 discusses the impact this has on IPv6 communications. This section 7185 discusses whether attackers can guess the correct values without 7186 seeing them. 7188 While the return routability procedure is in progress, 64 bit cookies 7189 are used to protect spoofed responses. This is believed to be 7190 sufficient, given that to blindly spoof a response a very large 7191 number of messages would have to be sent before success would be 7192 probable. 7194 The tokens used in the return routability procedure provide together 7195 128 bits of information. This information is used internally as 7196 input to a hash function to produce a 160 bit quantity suitable for 7197 producing the keyed hash in the Binding Update using the HMAC_SHA1 7198 algorithm. The final keyed hash length is 96 bits. The limiting 7199 factors in this case are the input token lengths and the final keyed 7200 hash length. The internal hash function application does not reduce 7201 the entropy. 7203 The 96 bit final keyed hash is of typical size and is believed to be 7204 secure. The 128 bit input from the tokens is broken in two pieces, 7205 the home keygen token and the care-of keygen token. An attacker can 7206 try to guess the correct cookie value, but again this would require a 7207 large number of messages (an the average 2**63 messages for one or 7208 2**127 for two). Furthermore, given that the cookies are valid only 7209 for a short period of time, the attack has to keep a high constant 7210 message rate to achieve a lasting effect. This does not appear 7211 practical. 7213 When the mobile node is returning home, it is allowed to use just the 7214 home keygen token of 64 bits. This is less than 128 bits, but 7215 attacking it blindly would still require a large number of messages 7216 to be sent. If the attacker is on the path and capable of seeing the 7217 Binding Update, it could conceivably break the keyed hash with brute 7218 force. However, in this case the attacker has to be on the path, 7219 which appears to offer easier ways for denial-of-service than 7220 preventing route optimization. 7222 15.5. Dynamic Home Agent Address Discovery 7224 The dynamic home agent address discovery function could be used to 7225 learn the addresses of home agents in the home network. 7227 The ability to learn addresses of nodes may be useful to attackers 7228 because brute-force scanning of the address space is not practical 7229 with IPv6. Thus, they could benefit from any means which make 7230 mapping the networks easier. For example, if a security threat 7231 targeted at routers or even home agents is discovered, having a 7232 simple ICMP mechanism to easily find out possible targets may prove 7233 to be an additional (though minor) security risk. 7235 This document does not define any authentication mechanism for 7236 dynamic home agent address discovery messages. Therefore the home 7237 agent cannot verify the home address of the mobile node that 7238 requested the list of home agents. 7240 Apart from discovering the address(es) of home agents, attackers will 7241 not be able to learn much from this information, and mobile nodes 7242 cannot be tricked into using wrong home agents, as all other 7243 communication with the home agents is secure. 7245 In cases where additional security is needed, one may consider 7246 instead the use of MIPv6 bootstrapping [25], (based on DNS SRV 7247 Resource Records [12]) in conjunction with security mechanisms 7248 suggested in these specifications. In that solution, security is 7249 provided by the DNSSEC [16] framework. The needed pre-configured 7250 data on the mobile node for this mechanism is the domain name of the 7251 mobile service provider, which is marginally better than the home 7252 subnet prefix. For the security, a trust anchor which dominates the 7253 domain is needed. 7255 15.6. Mobile Prefix Discovery 7257 The mobile prefix discovery function may leak interesting information 7258 about network topology and prefix lifetimes to eavesdroppers; for 7259 this reason, requests for this information has to be authenticated. 7260 Responses and unsolicited prefix information needs to be 7261 authenticated to prevent the mobile nodes from being tricked into 7262 believing false information about the prefixes and possibly 7263 preventing communications with the existing addresses. Optionally, 7264 encryption may be applied to prevent leakage of the prefix 7265 information. 7267 15.7. Tunneling via the Home Agent 7269 Tunnels between the mobile node and the home agent can be protected 7270 by ensuring proper use of source addresses, and optional 7271 cryptographic protection. These procedures are discussed in 7272 Section 5.5. 7274 Binding Updates to the home agents are secure. When receiving 7275 tunneled traffic, the home agent verifies that the outer IP address 7276 corresponds to the current location of the mobile node. This acts as 7277 a weak form of protection against spoofing packets that appear to 7278 come from the mobile node. This is particularly useful, if no end- 7279 to-end security is being applied between the mobile and correspondent 7280 nodes. The outer IP address check prevents attacks where the 7281 attacker is controlled by ingress filtering. It also prevents 7282 attacks when the attacker does not know the current care-of address 7283 of the mobile node. Attackers who know the care-of address and are 7284 not controlled by ingress filtering could still send traffic through 7285 the home agent. This includes attackers on the same local link as 7286 the mobile node is currently on. But such attackers could send 7287 packets that appear to come from the mobile node without attacking 7288 the tunnel; the attacker could simply send packets with the source 7289 address set to the mobile node's home address. However, this attack 7290 does not work if the final destination of the packet is in the home 7291 network, and some form of perimeter defense is being applied for 7292 packets sent to those destinations. In such cases it is recommended 7293 that either end-to-end security or additional tunnel protection be 7294 applied, as is usual in remote access situations. 7296 Home agents and mobile nodes may use IPsec ESP to protect payload 7297 packets tunneled between themselves. This is useful for protecting 7298 communications against attackers on the path of the tunnel. 7300 When site local home addresses are used, reverse tunneling can be 7301 used to send site local traffic from another location. 7302 Administrators should be aware of this when allowing such home 7303 addresses. In particular, the outer IP address check described above 7304 is not sufficient against all attackers. The use of encrypted 7305 tunnels is particularly useful for these kinds of home addresses. 7307 15.8. Home Address Option 7309 When the mobile node sends packets directly to the correspondent 7310 node, the Source Address field of the packet's IPv6 header is the 7311 care-of address. Therefore, ingress filtering [30] works in the 7312 usual manner even for mobile nodes, as the Source Address is 7313 topologically correct. The Home Address option is used to inform the 7314 correspondent node of the mobile node's home address. 7316 However, the care-of address in the Source Address field does not 7317 survive in replies sent by the correspondent node unless it has a 7318 binding for this mobile node. Also, not all attacker tracing 7319 mechanisms work when packets are being reflected through 7320 correspondent nodes using the Home Address option. For these 7321 reasons, this specification restricts the use of the Home Address 7322 option. It may only be used when a binding has already been 7323 established with the participation of the node at the home address, 7324 as described in Section 5.5 and Section 6.3. This prevents 7325 reflection attacks through the use of the Home Address option. It 7326 also ensures that the correspondent nodes reply to the same address 7327 that the mobile node sends traffic from. 7329 No special authentication of the Home Address option is required 7330 beyond the above, but note that if the IPv6 header of a packet is 7331 covered by IPsec Authentication Header, then that authentication 7332 covers the Home Address option as well. Thus, even when 7333 authentication is used in the IPv6 header, the security of the Source 7334 Address field in the IPv6 header is not compromised by the presence 7335 of a Home Address option. Without authentication of the packet, any 7336 field in the IPv6 header, including the Source Address field or any 7337 other part of the packet and the Home Address option can be forged or 7338 modified in transit. In this case, the contents of the Home Address 7339 option is no more suspect than any other part of the packet. 7341 15.9. Type 2 Routing Header 7343 The definition of the type 2 routing header is described in 7344 Section 6.4. This definition and the associated processing rules 7345 have been chosen so that the header cannot be used for what is 7346 traditionally viewed as source routing. In particular, the Home 7347 Address in the routing header will always have to be assigned to the 7348 home address of the receiving node; otherwise the packet will be 7349 dropped. 7351 Generally, source routing has a number of security concerns. These 7352 include the automatic reversal of unauthenticated source routes 7353 (which is an issue for IPv4, but not for IPv6). Another concern is 7354 the ability to use source routing to "jump" between nodes inside, as 7355 well as outside a firewall. These security concerns are not issues 7356 in Mobile IPv6, due to the rules mentioned above. 7358 In essence the semantics of the type 2 routing header is the same as 7359 a special form of IP-in-IP tunneling where the inner and outer source 7360 addresses are the same. 7362 This implies that a device which implements the filtering of packets 7363 should be able to distinguish between a type 2 routing header and 7364 other routing headers, as required in Section 8.3. This is necessary 7365 in order to allow Mobile IPv6 traffic while still having the option 7366 of filtering out other uses of routing headers. 7368 16. Contributors 7370 Tuomas Aura, Mike Roe, Greg O'Shea, Pekka Nikander, Erik Nordmark, 7371 and Michael Thomas worked on the return routability protocols 7372 eventually led to the procedures used in this protocol. The 7373 procedures described in [36] were adopted in the protocol. 7375 Significant contributions were made by members of the Mobile IPv6 7376 Security Design Team, including (in alphabetical order) Gabriel 7377 Montenegro, Erik Nordmark and Pekka Nikander. 7379 17. Acknowledgements 7381 We would like to thank the members of the Mobile IP and IPng Working 7382 Groups for their comments and suggestions on this work. We would 7383 particularly like to thank (in alphabetical order) Fred Baker, Josh 7384 Broch, Samita Chakrabarti, Robert Chalmers, Noel Chiappa, Greg Daley, 7385 Vijay Devarapalli, Rich Draves, Francis Dupont, Thomas Eklund, Jun- 7386 Ichiro Itojun Hagino, Brian Haley, Marc Hasson, John Ioannidis, James 7387 Kempf, Rajeev Koodli, Krishna Kumar, T.J. Kniveton, Joe Lau, Jiwoong 7388 Lee, Aime Le Rouzic, Vesa-Matti Mantyla, Kevin Miles, Glenn Morrow, 7389 Thomas Narten, Karen Nielsen, Simon Nybroe, David Oran, Brett 7390 Pentland, Lars Henrik Petander, Basavaraj Patil, Mohan Parthasarathy, 7391 Alexandru Petrescu, Mattias Petterson, Ken Powell, Phil Roberts, Ed 7392 Remmell, Patrice Romand, Luis A. Sanchez, Jeff Schiller, Pekka 7393 Savola, Arvind Sevalkar, Keiichi Shima, Tom Soderlund, Hesham 7394 Soliman, Jim Solomon, Tapio Suihko, Dave Thaler, Benny Van Houdt, 7395 Jon-Olov Vatn, Carl E. Williams, Vladislav Yasevich, Alper Yegin, and 7396 Xinhua Zhao, for their detailed reviews of earlier versions of this 7397 document. Their suggestions have helped to improve both the design 7398 and presentation of the protocol. 7400 We would also like to thank the participants of the Mobile IPv6 7401 testing event (1999), implementors who participated in Mobile IPv6 7402 interoperability testing at Connectathons (2000, 2001, 2002, and 7403 2003), and the participants at the ETSI interoperability testing 7404 (2000, 2002). Finally, we would like to thank the TAHI project who 7405 has provided test suites for Mobile IPv6. 7407 18. References 7409 18.1. Normative References 7411 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 7412 Levels", BCP 14, RFC 2119, March 1997. 7414 [2] Kent, S. and R. Atkinson, "Security Architecture for the 7415 Internet Protocol", RFC 2401, November 1998. 7417 [3] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, 7418 November 1998. 7420 [4] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload 7421 (ESP)", RFC 2406, November 1998. 7423 [5] Piper, D., "The Internet IP Security Domain of Interpretation 7424 for ISAKMP", RFC 2407, November 1998. 7426 [6] Maughan, D., Schneider, M., and M. Schertler, "Internet 7427 Security Association and Key Management Protocol (ISAKMP)", 7428 RFC 2408, November 1998. 7430 [7] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", 7431 RFC 2409, November 1998. 7433 [8] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) 7434 Specification", RFC 2460, December 1998. 7436 [9] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 7437 Specification", RFC 2473, December 1998. 7439 [10] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast 7440 Addresses", RFC 2526, March 1999. 7442 [11] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener 7443 Discovery (MLD) for IPv6", RFC 2710, October 1999. 7445 [12] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 7446 specifying the location of services (DNS SRV)", RFC 2782, 7447 February 2000. 7449 [13] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On- 7450 line Database", RFC 3232, January 2002. 7452 [14] National Institute of Standards and Technology, "Secure Hash 7453 Standard", FIPS PUB 180-1, April 1995, 7454 . 7456 [15] Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to 7457 Protect Mobile IPv6 Signaling Between Mobile Nodes and Home 7458 Agents", RFC 3776, June 2004. 7460 [16] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, 7461 "DNS Security Introduction and Requirements", RFC 4033, 7462 March 2005. 7464 [17] Eastlake, D., Schiller, J., and S. Crocker, "Randomness 7465 Requirements for Security", BCP 106, RFC 4086, June 2005. 7467 [18] Hinden, R. and S. Deering, "IP Version 6 Addressing 7468 Architecture", RFC 4291, February 2006. 7470 [19] Conta, A., Deering, S., and M. Gupta, "Internet Control Message 7471 Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) 7472 Specification", RFC 4443, March 2006. 7474 [20] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 7475 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 7476 September 2007. 7478 [21] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address 7479 Autoconfiguration", RFC 4862, September 2007. 7481 [22] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast 7482 Addresses", RFC 4193, October 2005. 7484 [23] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions 7485 for Stateless Address Autoconfiguration in IPv6", RFC 4941, 7486 September 2007. 7488 [24] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6 Socket 7489 API for Source Address Selection", RFC 5014, September 2007. 7491 [25] Giaretta, G., Kempf, J., and V. Devarapalli, "Mobile IPv6 7492 Bootstrapping in Split Scenario", RFC 5026, October 2007. 7494 [26] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA 7495 Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 7497 18.2. Informative References 7499 [27] Perkins, C., "IP Encapsulation within IP", RFC 2003, 7500 October 1996. 7502 [28] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, 7503 October 1996. 7505 [29] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing 7506 for Message Authentication", RFC 2104, February 1997. 7508 [30] Ferguson, P. and D. Senie, "Network Ingress Filtering: 7509 Defeating Denial of Service Attacks which employ IP Source 7510 Address Spoofing", BCP 38, RFC 2827, May 2000. 7512 [31] Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses", 7513 draft-aura-mipv6-bu-attacks-01 (work in progress), March 2002. 7515 [32] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. 7516 Carney, "Dynamic Host Configuration Protocol for IPv6 7517 (DHCPv6)", RFC 3315, July 2003. 7519 [33] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, 7520 August 2002. 7522 [34] Draves, R., "Default Address Selection for Internet Protocol 7523 version 6 (IPv6)", RFC 3484, February 2003. 7525 [35] Nordmark, E., "Securing MIPv6 BUs using return routability 7526 (BU3WAY)", draft-nordmark-mobileip-bu3way-00 (work in 7527 progress), November 2001. 7529 [36] Roe, M., "Authentication of Mobile IPv6 Binding Updates and 7530 Acknowledgments", draft-roe-mobileip-updateauth-02 (work in 7531 progress), March 2002. 7533 [37] Chowdhury, K. and A. Yegin, "MIP6-bootstrapping for the 7534 Integrated Scenario", 7535 draft-ietf-mip6-bootstrapping-integrated-dhc-06 (work in 7536 progress), April 2008. 7538 [38] Savola, P., "Use of /127 Prefix Length Between Routers 7539 Considered Harmful", RFC 3627, September 2003. 7541 [39] Savola, P., "Security of IPv6 Routing Header and Home Address 7542 Options", draft-savola-ipv6-rh-ha-security-02 (work in 7543 progress), March 2002. 7545 [40] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 7546 (MLDv2) for IPv6", RFC 3810, June 2004. 7548 [41] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key 7549 Management", BCP 107, RFC 4107, June 2005. 7551 [42] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E. 7552 Nordmark, "Mobile IP Version 6 Route Optimization Security 7553 Design Background", RFC 4225, December 2005. 7555 [43] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", 7556 RFC 4306, December 2005. 7558 Appendix A. Future Extensions 7560 A.1. Piggybacking 7562 This document does not specify how to piggyback payload packets on 7563 the binding related messages. However, it is envisioned that this 7564 can be specified in a separate document when issues such as the 7565 interaction between piggybacking and IPsec are fully resolved (see 7566 also Appendix A.3). The return routability messages can indicate 7567 support for piggybacking with a new mobility option. 7569 A.2. Triangular Routing 7571 Due to the concerns about opening reflection attacks with the Home 7572 Address destination option, this specification requires that this 7573 option be verified against the Binding Cache, i.e., there must be a 7574 Binding Cache entry for the Home Address and Care-of Address. 7576 Future extensions may be specified that allow the use of unverified 7577 Home Address destination options in ways that do not introduce 7578 security issues. 7580 A.3. New Authorization Methods 7582 While the return routability procedure provides a good level of 7583 security, there exist methods that have even higher levels of 7584 security. Secondly, as discussed in Section 15.4, future 7585 enhancements of IPv6 security may cause a need to also improve the 7586 security of the return routability procedure. Using IPsec as the 7587 sole method for authorizing Binding Updates to correspondent nodes is 7588 also possible. The protection of the Mobility Header for this 7589 purpose is easy, though one must ensure that the IPsec SA was created 7590 with appropriate authorization to use the home address referenced in 7591 the Binding Update. For instance, a certificate used by IKE to 7592 create the security association might contain the home address. A 7593 future specification may specify how this is done. 7595 A.4. Dynamically Generated Home Addresses 7597 A future version of this specification may include functionality that 7598 allows the generation of new home addresses without requiring pre- 7599 arranged security associations or certificates even for the new 7600 addresses. 7602 A.5. Remote Home Address Configuration 7604 The method for initializing a mobile node's home address upon 7605 power-up or after an extended period of being disconnected from the 7606 network is beyond the scope of this specification. Whatever 7607 procedure is used should result in the mobile node having the same 7608 stateless or stateful (e.g., DHCPv6) home address autoconfiguration 7609 information it would have if it were attached to the home network. 7610 Due to the possibility that the home network could be renumbered 7611 while the mobile node is disconnected, a robust mobile node would not 7612 rely solely on storing these addresses locally. 7614 Such a mobile node could be initialized by using the following 7615 procedure: 7617 1. Generate a care-of address. 7619 2. Query DNS for an anycast address associated with the FQDN of the 7620 home agent(s). 7622 3. Perform home agent address discovery, and select a home agent. 7624 4. Configure one home address based on the selected home agent's 7625 subnet prefix and the interface identifier of the mobile node. 7627 5. Create security associations and security policy database entries 7628 for protecting the traffic between the selected home address and 7629 home agent. 7631 6. Perform a home registration on the selected home agent. 7633 7. Perform mobile prefix discovery. 7635 8. Make a decision if further home addresses need to be configured. 7637 This procedure is restricted to those situations where the home 7638 prefix is 64 bits and the mobile node knows its own interface 7639 identifier, which is also 64 bits. 7641 A.6. Neighbor Discovery Extensions 7643 Future specifications may improve the efficiency of Neighbor 7644 Discovery tasks, which could be helpful for fast movements. One 7645 factor is currently being looked at: the delays caused by the 7646 Duplicate Address Detection mechanism. Currently, Duplicate Address 7647 Detection needs to be performed for every new care-of address as the 7648 mobile node moves, and for the mobile node's link-local address on 7649 every new link. In particular, the need and the trade-offs of re- 7650 performing Duplicate Address Detection for the link-local address 7651 every time the mobile node moves on to new links will need to be 7652 examined. Improvements in this area are, however, generally 7653 applicable and progress independently from the Mobile IPv6 7654 specification. 7656 Future functional improvements may also be relevant for Mobile IPv6 7657 and other applications. For instance, mechanisms that would allow 7658 recovery from a Duplicate Address Detection collision would be useful 7659 for link-local, care-of, and home addresses. 7661 Authors' Addresses 7663 David B. Johnson 7664 Rice University 7665 Dept. of Computer Science, MS 132 7666 6100 Main Street 7667 Houston TX 77005-1892 7668 USA 7670 Email: dbj@cs.rice.edu 7672 Charles E. Perkins 7673 WiChorus Inc. 7674 3590 N. 1st Street, Suite 300 7675 San Jose CA 95134 7676 USA 7678 Email: charliep@computer.org 7680 Jari Arkko 7681 Ericsson 7682 Jorvas 02420 7683 Finland 7685 Email: jari.arkko@ericsson.com 7687 Full Copyright Statement 7689 Copyright (C) The IETF Trust (2008). 7691 This document is subject to the rights, licenses and restrictions 7692 contained in BCP 78, and except as set forth therein, the authors 7693 retain all their rights. 7695 This document and the information contained herein are provided on an 7696 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 7697 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 7698 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 7699 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 7700 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 7701 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 7703 Intellectual Property 7705 The IETF takes no position regarding the validity or scope of any 7706 Intellectual Property Rights or other rights that might be claimed to 7707 pertain to the implementation or use of the technology described in 7708 this document or the extent to which any license under such rights 7709 might or might not be available; nor does it represent that it has 7710 made any independent effort to identify any such rights. Information 7711 on the procedures with respect to rights in RFC documents can be 7712 found in BCP 78 and BCP 79. 7714 Copies of IPR disclosures made to the IETF Secretariat and any 7715 assurances of licenses to be made available, or the result of an 7716 attempt made to obtain a general license or permission for the use of 7717 such proprietary rights by implementers or users of this 7718 specification can be obtained from the IETF on-line IPR repository at 7719 http://www.ietf.org/ipr. 7721 The IETF invites any interested party to bring to its attention any 7722 copyrights, patents or patent applications, or other proprietary 7723 rights that may cover technology that may be required to implement 7724 this standard. Please address the information to the IETF at 7725 ietf-ipr@ietf.org.