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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 6lo P. Thubert, Ed. 3 Internet-Draft Cisco Systems 4 Updates: 6775, 8505 (if approved) C.E. Perkins 5 Intended status: Standards Track Blue Meadow Networking 6 Expires: 3 September 2020 E. Levy-Abegnoli 7 Cisco Systems 8 2 March 2020 10 IPv6 Backbone Router 11 draft-ietf-6lo-backbone-router-18 13 Abstract 15 This document updates RFC 6775 and RFC 8505 in order to enable proxy 16 services for IPv6 Neighbor Discovery by Routing Registrars called 17 Backbone Routers. Backbone Routers are placed along the wireless 18 edge of a Backbone, and federate multiple wireless links to form a 19 single Multi-Link Subnet. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at https://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on 3 September 2020. 38 Copyright Notice 40 Copyright (c) 2020 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 45 license-info) in effect on the date of publication of this document. 46 Please review these documents carefully, as they describe your rights 47 and restrictions with respect to this document. Code Components 48 extracted from this document must include Simplified BSD License text 49 as described in Section 4.e of the Trust Legal Provisions and are 50 provided without warranty as described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 56 2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 5 57 2.2. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 5 58 2.3. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 6 59 2.4. References . . . . . . . . . . . . . . . . . . . . . . . 7 60 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 7 61 3.1. Updating RFC 6775 and RFC 8505 . . . . . . . . . . . . . 10 62 3.2. Access Link . . . . . . . . . . . . . . . . . . . . . . . 11 63 3.3. Route-Over Mesh . . . . . . . . . . . . . . . . . . . . . 13 64 3.4. The Binding Table . . . . . . . . . . . . . . . . . . . . 14 65 3.5. Primary and Secondary 6BBRs . . . . . . . . . . . . . . . 15 66 3.6. Using Optimistic DAD . . . . . . . . . . . . . . . . . . 16 67 4. Multi-Link Subnet Considerations . . . . . . . . . . . . . . 16 68 5. Optional 6LBR serving the Multi-Link Subnet . . . . . . . . . 17 69 6. Using IPv6 ND Over the Backbone Link . . . . . . . . . . . . 18 70 7. Routing Proxy Operations . . . . . . . . . . . . . . . . . . 20 71 8. Bridging Proxy Operations . . . . . . . . . . . . . . . . . . 21 72 9. Creating and Maintaining a Binding . . . . . . . . . . . . . 22 73 9.1. Operations on a Binding in Tentative State . . . . . . . 23 74 9.2. Operations on a Binding in Reachable State . . . . . . . 24 75 9.3. Operations on a Binding in Stale State . . . . . . . . . 25 76 10. Registering Node Considerations . . . . . . . . . . . . . . . 26 77 11. Security Considerations . . . . . . . . . . . . . . . . . . . 27 78 12. Protocol Constants . . . . . . . . . . . . . . . . . . . . . 27 79 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 80 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28 81 15. Normative References . . . . . . . . . . . . . . . . . . . . 28 82 16. Informative References . . . . . . . . . . . . . . . . . . . 29 83 Appendix A. Possible Future Extensions . . . . . . . . . . . . . 32 84 Appendix B. Applicability and Requirements Served . . . . . . . 33 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 87 1. Introduction 89 IEEE STD. 802.1 [IEEEstd8021] Ethernet Bridging provides an efficient 90 and reliable broadcast service for wired networks; applications and 91 protocols have been built that heavily depend on that feature for 92 their core operation. Unfortunately, Low-Power Lossy Networks (LLNs) 93 and local wireless networks generally do not provide the broadcast 94 capabilities of Ethernet Bridging in an economical fashion. 96 As a result, protocols designed for bridged networks that rely on 97 multicast and broadcast often exhibit disappointing behaviours when 98 employed unmodified on a local wireless medium (see 99 [I-D.ietf-mboned-ieee802-mcast-problems]). 101 Wi-Fi [IEEEstd80211] Access Points (APs) deployed in an Extended 102 Service Set (ESS) act as Ethernet Bridges [IEEEstd8021], with the 103 property that the bridging state is established at the time of 104 association. This ensures connectivity to the end node (the Wi-Fi 105 STA) and protects the wireless medium against broadcast-intensive 106 Transparent Bridging reactive Lookups. In other words, the 107 association process is used to register the MAC Address of the STA to 108 the AP. The AP subsequently proxies the bridging operation and does 109 not need to forward the broadcast Lookups over the radio. 111 In the same way as Transparent Bridging, IPv6 [RFC8200] Neighbor 112 Discovery [RFC4861] [RFC4862] Protocol (IPv6 ND) is a reactive 113 protocol, based on multicast transmissions to locate an on-link 114 correspondent and ensure the uniqueness of an IPv6 address. The 115 mechanism for Duplicate Address Detection (DAD) [RFC4862] was 116 designed for the efficient broadcast operation of Ethernet Bridging. 117 Since broadcast can be unreliable over wireless media, DAD often 118 fails to discover duplications [I-D.yourtchenko-6man-dad-issues]. In 119 practice, the fact that IPv6 addresses very rarely conflict is mostly 120 attributable to the entropy of the 64-bit Interface IDs as opposed to 121 the succesful operation of the IPv6 ND duplicate address detection 122 and resolution mechanisms. 124 The IPv6 ND Neighbor Solicitation (NS) [RFC4861] message is used for 125 DAD and address Lookup when a node moves, or wakes up and reconnects 126 to the wireless network. The NS message is targeted to a Solicited- 127 Node Multicast Address (SNMA) [RFC4291] and should in theory only 128 reach a very small group of nodes. But in reality, IPv6 multicast 129 messages are typically broadcast on the wireless medium, and so they 130 are processed by most of the wireless nodes over the subnet (e.g., 131 the ESS fabric) regardless of how few of the nodes are subscribed to 132 the SNMA. As a result, IPv6 ND address Lookups and DADs over a large 133 wireless and/or a LowPower Lossy Network (LLN) can consume enough 134 bandwidth to cause a substantial degradation to the unicast traffic 135 service. 137 Because IPv6 ND messages sent to the SNMA group are broadcast at the 138 radio MAC Layer, wireless nodes that do not belong to the SNMA group 139 still have to keep their radio turned on to listen to multicast NS 140 messages, which is a waste of energy for them. In order to reduce 141 their power consumption, certain battery-operated devices such as IoT 142 sensors and smartphones ignore some of the broadcasts, making IPv6 ND 143 operations even less reliable. 145 These problems can be alleviated by reducing the IPv6 ND broadcasts 146 over wireless access links. This has been done by splitting the 147 broadcast domains and routing between subnets, at the extreme by 148 assigning a /64 prefix to each wireless node (see [RFC8273]). But 149 deploying a single large subnet can still be attractive to avoid 150 renumbering in situations that involve large numbers of devices and 151 mobility within a bounded area. 153 A way to reduce the propagation of IPv6 ND broadcast in the wireless 154 domain while preserving a large single subnet is to form a Multi-Link 155 Subnet (MLSN). Each Link in the MLSN, including the backbone, is its 156 own broadcast domain. A key property of MLSNs is that link-local 157 unicast traffic, link-scope multicast, and traffic with a hop limit 158 of 1 will not transit to nodes in the same subnet on a different 159 link, something that may produce unexpected behavior in software that 160 expects a subnet to be entirely contained within a single link. 162 This specification considers a special type of MLSN with a central 163 backbone that federates edge (LLN) links, each Link providing its own 164 protection against rogue access and tempering or replaying packets. 165 In that particular topology, ND proxies can be placed at the boundary 166 of the edge links and the backbone to handle IPv6 ND on behalf of 167 Registered Nodes and forward IPv6 packets back and forth. The ND 168 proxy enables the continuity of IPv6 ND operations beyond the 169 backbone, and enables communication using Global or Unique Local 170 Addresses between any pair of nodes in the MLSN. 172 The 6LoWPAN Backbone Router (6BBR) is a Routing Registrar [RFC8505] 173 that provides proxy-ND services. A 6BBR acting as a Bridging Proxy 174 provides a proxy-ND function with Layer-2 continuity and can be 175 collocated with a Wi-Fi Access Point (AP) as prescribed by IEEE Std 176 802.11 [IEEEstd80211]. A 6BBR acting as a Routing Proxy is 177 applicable to any type of LLN, including LLNs that cannot be bridged 178 onto the backbone, such as IEEE Std 802.15.4 [IEEEstd802154]. 180 Knowledge of which address to proxy for can be obtained by snooping 181 the IPV6 ND protocol (see [I-D.bi-savi-wlan]), but it has been found 182 to be unreliable. An IPv6 address may not be discovered immediately 183 due to a packet loss, or if a "silent" node is not currently using 184 one of its addresses. A change of state (e.g., due to movement) may 185 be missed or misordered, leading to unreliable connectivity and 186 incomplete knowledge of the state of the network. 188 With this specification, the address to be proxied is signaled 189 explicitly through a registration process. A 6LoWPAN node (6LN) 190 registers all its IPv6 Addresses using NS messages with an Extended 191 Address Registration Option (EARO) as specified in [RFC8505] to a 192 6LoWPAN Router (6LR) to which it is directly attached. If the 6LR is 193 a 6BBR then the 6LN is both the Registered Node and the Registering 194 Node. If not, then the 6LoWPAN Border Router (6LBR) that serves the 195 LLN proxies the registration to the 6BBR. In that case, the 6LN is 196 the Registered Node and the 6LBR is the Registering Node. The 6BBR 197 performs IPv6 Neighbor Discovery (IPv6 ND) operations on its Backbone 198 interface on behalf of the 6LNs that have registered addresses on its 199 LLN interfaces without the need of a broadcast over the wireless 200 medium. 202 A Registering Node that resides on the backbone does not register to 203 the SNMA groups associated to its Registered Addresses and defers to 204 the 6BBR to answer or preferably forward to it as unicast the 205 corresponding multicast packets. 207 2. Terminology 209 2.1. BCP 14 211 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 212 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 213 "OPTIONAL" in this document are to be interpreted as described in BCP 214 14 [RFC2119] [RFC8174] when, and only when, they appear in all 215 capitals, as shown here. 217 2.2. New Terms 219 This document introduces the following terminology: 221 Federated: A subnet that comprises a Backbone and one or more 222 (wireless) access links, is said to be federated into one Multi- 223 Link Subnet. The proxy-ND operation of 6BBRs over the Backbone 224 extends IPv6 ND operation over the access links. 226 Sleeping Proxy: A 6BBR acts as a Sleeping Proxy if it answers IPv6 227 ND Neighbor Solicitations over the Backbone on behalf of the 228 Registering Node that is in a sleep state and cannot answer in due 229 time. 231 Routing Proxy: A Routing Proxy provides IPv6 ND proxy functions and 232 enables the MLSN operation over federated links that may not be 233 compatible for bridging. The Routing Proxy advertises its own MAC 234 Address as the Target Link Layer Address (TLLA) in the proxied NAs 235 over the Backbone, and routes at the Network Layer between the 236 federated links. 238 Bridging Proxy: A Bridging Proxy provides IPv6 ND proxy functions 239 while preserving forwarding continuity at the MAC Layer. In that 240 case, the MAC Address and the mobility of the Registering Node is 241 visible across the bridged Backbone. The Bridging Proxy 242 advertises the MAC Address of the Registering Node as the TLLA in 243 the proxied NAs over the Backbone, and proxies ND for all unicast 244 addresses including Link-Local Addresses. Instead of replying on 245 behalf of the Registering Node, a Bridging Proxy will preferably 246 forward the NS Lookup and NUD messages that target the Registered 247 Address to the Registering Node as unicast frames and let it 248 respond in its own. 250 Binding Table: The Binding Table is an abstract database that is 251 maintained by the 6BBR to store the state associated with its 252 registrations. 254 Binding: A Binding is an abstract state associated to one 255 registration, in other words one entry in the Binding Table. 257 2.3. Abbreviations 259 This document uses the following abbreviations: 261 6BBR: 6LoWPAN Backbone Router 262 6LBR: 6LoWPAN Border Router 263 6LN: 6LoWPAN Node 264 6LR: 6LoWPAN Router 265 ARO: Address Registration Option 266 DAC: Duplicate Address Confirmation 267 DAD: Duplicate Address Detection 268 DAR: Duplicate Address Request 269 EARO: Extended Address Registration Option 270 EDAC: Extended Duplicate Address Confirmation 271 EDAR: Extended Duplicate Address Request 272 DODAG: Destination-Oriented Directed Acyclic Graph 273 ID: Identifier 274 LLN: Low-Power and Lossy Network 275 NA: Neighbor Advertisement 276 MAC: Medium Access Control 277 NCE: Neighbor Cache Entry 278 ND: Neighbor Discovery 279 NDP: Neighbor Discovery Protocol 280 NS: Neighbor Solicitation 281 NS(DAD): NDP NS message used for the purpose of duplication 282 avoidance (multicast) 283 NS(Lookup): NDP NS message used for the purpose of address 284 resolution (multicast) 285 NS(NUD): NDP NS message used for the purpose of unreachability 286 detection (unicast) 287 NUD: Neighbor Unreachability Detection 288 ROVR: Registration Ownership Verifier 289 RPL: IPv6 Routing Protocol for LLNs 290 RA: Router Advertisement 291 RS: Router Solicitation 292 SNMA: Solicited-Node Multicast Address 293 LLA: Link Layer Address (aka MAC address) 294 SLLA: Source Link Layer Address 295 TLLA: Target Link Layer Address 296 TID: Transaction ID 298 2.4. References 300 In this document, readers will encounter terms and concepts that are 301 discussed in the following documents: 303 Classical IPv6 ND: "Neighbor Discovery for IP version 6" [RFC4861], 304 "IPv6 Stateless Address Autoconfiguration" [RFC4862] and 305 "Optimistic Duplicate Address Detection" [RFC4429], 307 IPv6 ND over multiple links: "Neighbor Discovery Proxies (proxy-ND)" 308 [RFC4389] and "Multi-Link Subnet Issues" [RFC4903], 310 6LoWPAN: "Problem Statement and Requirements for IPv6 over Low-Power 311 Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606], and 313 6LoWPAN ND: Neighbor Discovery Optimization for Low-Power and Lossy 314 Networks [RFC6775] and "Registration Extensions for 6LoWPAN 315 Neighbor Discovery" [RFC8505]. 317 3. Overview 319 This section and its subsections present a non-normative high level 320 view of the operation of the 6BBR. The following sections cover the 321 normative part. Figure 1 illustrates a backbone link that federates 322 a collection of LLNs as a single IPv6 Subnet, with a number of 6BBRs 323 providing proxy-ND services to their attached LLNs. 325 | 326 +-----+ +-----+ +-----+ IPv6 327 (default) | | (Optional) | | | | Node 328 Router | | 6LBR | | | | or 329 +-----+ +-----+ +-----+ 6LN 330 | Backbone side | | 331 ----+-------+-----------------+---+-------------+----+----- 332 | | | 333 +------+ +------+ +------+ 334 | 6BBR | | 6BBR | | 6BBR | 335 | | | | | | 336 +------+ +------+ +------+ 337 o Wireless side o o o o o o 338 o o o o o o o o o o o o o o o o o o o o 339 o o o o o o o o o o o o o o o o o o o 340 o o o o o o o o o LLN o o o o o o o o o 341 o o o o o o o o o o o o o o 342 o o o 344 Figure 1: Backbone Link and Backbone Routers 346 The LLN may be a hub-and-spoke access link such as (Low-Power) IEEE 347 STD. 802.11 (Wi-Fi) [IEEEstd80211] and IEEE STD. 802.15.1 (Bluetooth) 348 [IEEEstd802151], or a Mesh-Under or a Route-Over network [RFC8505]. 349 The proxy state can be distributed across multiple 6BBRs attached to 350 the same Backbone. 352 The main features of a 6BBR are as follows: 354 * Multi-Link-subnet functions (provided by the 6BBR on the backbone) 355 performed on behalf of Registered Nodes, and 357 * Routing registrar services that reduce multicast within the LLN: 359 - Binding Table management 360 - failover, e.g., due to mobility 362 Each Backbone Router (6BBR) maintains a data structure for its 363 Registered Addresses called a Binding Table. The abstract data that 364 is stored in the Binding Table includes the Registered Address, 365 anchor information on the Registering Node such as connecting 366 interface, Link-Local Address and Link-Layer Address of the 367 Registering Node on that interface, the EARO including ROVR and TID, 368 a state that can be either Reachable, Tentative, or Stale, and other 369 information such as a trust level that may be configured, e.g., to 370 protect a server. The combined Binding Tables of all the 6BBRs on a 371 backbone form a distributed database of Registered Nodes that reside 372 in the LLNs or on the IPv6 Backbone. 374 Unless otherwise configured, a 6BBR does the following: 376 * Create a new entry in a Binding Table for a new Registered Address 377 and ensure that the Address is not duplicated over the Backbone. 379 * Advertise a Registered Address over the Backbone using an NA 380 message, either unsolicited or as a response to a NS message. 381 This includes joining the multicast group associated to the SNMA 382 derived from the Registered Address as specified in section 7.2.1. 383 of [RFC4861] over the Backbone. 385 * The 6BBR MAY respond immediately as a Proxy in lieu of the 386 Registering Node, e.g., if the Registering Node has a sleeping 387 cycle that the 6BBR does not want to interrupt, or if the 6BBR has 388 a recent state that is deemed fresh enough to permit the proxied 389 response. It is preferred, though, that the 6BBR checks whether 390 the Registering Node is still responsive on the Registered 391 Address. To that effect: 393 - as a Bridging Proxy: 394 the 6BBR forwards the multicast DAD and Address Lookup messages 395 as a unicast MAC-Layer frames to the MAC address of the 396 Registering Node that matches the Target in the ND message, and 397 forwards as is the unicast Neighbor Unreachability Detection 398 (NUD) messages, so as to let the Registering Node answer with 399 the ND Message and options that it sees fit; 400 - as a Routing Proxy: 401 the 6BBR checks the liveliness of the Registering Node, e.g., 402 using a NUD verification, before answering on its behalf. 404 * Deliver packets arriving from the LLN, using Neighbor Solicitation 405 messages to look up the destination over the Backbone. 407 * Forward or bridge packets between the LLN and the Backbone. 409 * Verify liveness for a registration, when needed. 411 The first of these functions enables the 6BBR to fulfill its role as 412 a Routing Registrar for each of its attached LLNs. The remaining 413 functions fulfill the role of the 6BBRs as the border routers that 414 federate the Multi-link IPv6 subnet. 416 The operation of IPv6 ND and of proxy-ND are not mutually exclusive 417 on the Backbone, meaning that nodes attached to the Backbone and 418 using IPv6 ND can transparently interact with 6LNs that rely on a 419 6BBR to proxy ND for them, whether the 6LNs are reachable over an LLN 420 or directly attached to the Backbone. 422 The [RFC8505] registration mechanism used to learn addresses to be 423 proxied may co-exist in a 6BBR with a proprietary snooping or the 424 traditional bridging functionality of an Access Point, in order to 425 support legacy LLN nodes that do not support this specification. 427 The registration to a proxy service uses an NS/NA exchange with EARO. 428 The 6BBR operation resembles that of a Mobile IPv6 (MIPv6) [RFC6275] 429 Home Agent (HA). The combination of a 6BBR and a MIPv6 HA enables 430 full mobility support for 6LNs, inside and outside the links that 431 form the subnet. 433 The 6BBRs performs IPv6 ND functions over the backbone as follows: 435 * The EARO [RFC8505] is used in the IPv6 ND exchanges over the 436 Backbone between the 6BBRs to help distinguish duplication from 437 movement. Extended Duplicate Address Messages (EDAR and EDAC) may 438 also be used to communicate with a 6LBR, if one is present. 439 Address duplication is detected using the ROVR field. Conflicting 440 registrations to different 6BBRs for the same Registered Address 441 are resolved using the TID field which forms an order of 442 registrations. 444 * The Link Layer Address (LLA) that the 6BBR advertises for the 445 Registered Address on behalf of the Registered Node over the 446 Backbone can belong to the Registering Node; in that case, the 447 6BBR (acting as a Bridging Proxy (see Section 8)) bridges the 448 unicast packets. Alternatively, the LLA can be that of the 6BBR 449 on the Backbone interface, in which case the 6BBR (acting as a 450 Routing Proxy (see Section 7)) receives the unicast packets at 451 Layer 3 and routes over. 453 3.1. Updating RFC 6775 and RFC 8505 455 This specification adds the EARO as a possible option in RS, NS(DAD) 456 and NA messages over the backbone. This document specifies the use 457 of those ND messages by 6BBRs over the backbone, at a high level in 458 Section 6 and in more detail in Section 9. 460 Note: [RFC8505] requires that the registration NS(EARO) contains an 461 Source Link Layer Address Option (SLLAO). [RFC4862] requires that 462 the NS(DAD) is sent from the unspecified address for which there 463 cannot be a SLLAO. Consequently, an NS(DAD) cannot be confused with 464 a registration. 466 This specification allows to deploy a 6LBR on the backbone where EDAR 467 and EDAC messages coexist with classical ND. It also adds the 468 capability to insert IPv6 ND options in the EDAR and EDAC messages. 469 A 6BBR acting as a 6LR for the Registered Address can insert an SLLAO 470 in the EDAR to the 6LBR in order to avoid a Lookup back. This 471 enables the 6LBR to store the MAC address associated to the 472 Registered Address on a Link and to serve as a mapping server as 473 described in [I-D.thubert-6lo-unicast-lookup]. 475 This specification allows for an address to be registered to more 476 than one 6BBR. Consequently a 6LBR that is deployed on the backbone 477 MUST be capable of maintaining state for each of the 6BBR having 478 registered with the same TID and same ROVR. 480 3.2. Access Link 482 The simplest Multi-Link Subnet topology from the Layer 3 perspective 483 occurs when the wireless network appears as a single hop hub-and- 484 spoke network as shown in Figure 2. The Layer 2 operation may 485 effectively be hub-and-spoke (e.g., Wi-Fi) or Mesh-Under, with a 486 Layer 2 protocol handling the complex topology. 488 | 489 +-----+ +-----+ +-----+ IPv6 490 (default) | | (Optional) | | | | Node 491 Router | | 6LBR | | | | or 492 +-----+ +-----+ +-----+ 6LN 493 | Backbone side | | 494 ----+-------+-----------------+---+-------------+----+----- 495 | | | 496 +------+ +------+ +------+ 497 | 6BBR | | 6BBR | | 6BBR | 498 | 6LR | | 6LR | | 6LR | 499 +------+ +------+ +------+ 500 (6LN) (6LN) (6LN) (6LN) (6LN) (6LN) (6LN) (6LN) 502 Figure 2: Access Link Use case 504 Figure 3 illustrates a flow where 6LN forms an IPv6 Address and 505 registers it to a 6BBR acting as a 6LR [RFC8505]. The 6BBR applies 506 ODAD (see Section 3.6) to the registered address to enable 507 connectivity while the message flow is still in progress. 509 6LN(STA) 6BBR(AP) 6LBR default GW 510 | | | | 511 | LLN Access Link | IPv6 Backbone (e.g., Ethernet) | 512 | | | | 513 | RS(multicast) | | | 514 |---------------->| | | 515 | RA(PIO, Unicast)| | | 516 |<----------------| | | 517 | NS(EARO) | | | 518 |---------------->| | | 519 | | Extended DAR | | 520 | |--------------->| | 521 | | Extended DAC | | 522 | |<---------------| | 523 | | | 524 | | NS-DAD(EARO, multicast) | 525 | |--------> | 526 | |----------------------------------->| 527 | | | 528 | | RS(no SLLAO, for ODAD) | 529 | |----------------------------------->| 530 | | if (no fresher Binding) NS(Lookup) | 531 | | <----------------| 532 | |<-----------------------------------| 533 | | NA(SLLAO, not(O), EARO) | 534 | |----------------------------------->| 535 | | RA(unicast) | 536 | |<-----------------------------------| 537 | | | 538 | IPv6 Packets in optimistic mode | 539 |<---------------------------------------------------->| 540 | | | 541 | | 542 | NA(EARO) | 543 |<----------------| 544 | | 546 Figure 3: Initial Registration Flow to a 6BBR acting as Routing Proxy 548 In this example, a 6LBR is deployed on the backbone link to serve the 549 whole subnet, and EDAR / EDAC messages are used in combination with 550 DAD to enable coexistence with IPv6 ND over the backbone. 552 The RS sent initially by the 6LN (e.g., a Wi-Fi STA) is transmitted 553 as a multicast but since it is intercepted by the 6BBR, it is never 554 effectively broadcast. The multiple arrows associated to the ND 555 messages on the Backbone denote a real Layer 2 broadcast. 557 3.3. Route-Over Mesh 559 A more complex Multi-Link Subnet topology occurs when the wireless 560 network appears as a Layer 3 Mesh network as shown in Figure 4. A 561 so-called Route-Over routing protocol exposes routes between 6LRs 562 towards both 6LRs and 6LNs, and a 6LBR acts as Root of the Layer 3 563 Mesh network and proxy-registers the LLN addresses to the 6BBR. 565 | 566 +-----+ +-----+ +-----+ IPv6 567 (default) | | (Optional) | | | | Node 568 Router | | 6LBR | | | | or 569 +-----+ +-----+ +-----+ 6LN 570 | Backbone side | | 571 ----+-------+-----------------+---+-------------+----+----- 572 | | | 573 +------+ +------+ +------+ 574 | 6BBR | | 6BBR | | 6BBR | 575 +------+ +------+ +------+ 576 | | | 577 +------+ +------+ +------+ 578 | 6LBR | | 6LBR | | 6LBR | 579 +------+ +------+ +------+ 580 (6LN) (6LR) (6LN) (6LR) (6LN) (6LR) (6LR) (6LR)(6LN) 581 (6LN)(6LR) (6LR) (6LN) (6LN) (6LR)(6LN) (6LR) (6LR) (6LR) (6LN) 582 (6LR)(6LR) (6LR) (6LR) (6LR)(6LN) (6LR) (6LR)(6LR) 583 (6LR) (6LR) (6LR) (6LR) (6LN)(6LR) (6LR) (6LR) (6LR) (6LR) 584 (6LN) (6LN)(6LN) (6LN) (6LN) (6LN) (6LN) (6LN) (6LN) (6LN) 586 Figure 4: Route-Over Mesh Use case 588 Figure 5 illustrates IPv6 signaling that enables a 6LN (the 589 Registered Node) to form a Global or a Unique-Local Address and 590 register it to the 6LBR that serves its LLN using [RFC8505] using a 591 neighboring 6LR as relay. The 6LBR (the Registering Node) then 592 proxies the [RFC8505] registration to the 6BBR to obtain proxy-ND 593 services from the 6BBR. 595 The RS sent initially by the 6LN is a transmitted as a multicast and 596 contained within 1-hop broadcast range where hopefully a 6LR is 597 found. The 6LR is expected to be already connected to the LLN and 598 capable to reach the 6LBR, possibly multiple hops away, using unicast 599 messages. 601 6LoWPAN Node 6LR 6LBR 6BBR 602 (mesh leaf) (mesh router) (mesh root) 603 | | | | 604 | 6LoWPAN ND |6LoWPAN ND | 6LoWPAN ND | IPv6 ND 605 | LLN link |Route-Over mesh|Ethernet/serial| Backbone 606 | | |/Internal call | 607 | IPv6 ND RS | | | 608 |-------------->| | | 609 |-----------> | | | 610 |------------------> | | 611 | IPv6 ND RA | | | 612 |<--------------| | | 613 | | | | 614 | NS(EARO) | | | 615 |-------------->| | | 616 | 6LoWPAN ND | Extended DAR | | 617 | |-------------->| | 618 | | | NS(EARO) | 619 | | |-------------->| 620 | | | (proxied) | NS-DAD 621 | | | |------> 622 | | | | (EARO) 623 | | | | 624 | | | NA(EARO) | 625 | | |<--------------| 626 | | Extended DAC | | 627 | |<--------------| | 628 | NA(EARO) | | | 629 |<--------------| | | 630 | | | | 632 Figure 5: Initial Registration Flow over Route-Over Mesh 634 As a non-normative example of a Route-Over Mesh, the 6TiSCH 635 architecture [I-D.ietf-6tisch-architecture] suggests using the RPL 636 [RFC6550] routing protocol and collocating the RPL root with a 6LBR 637 that serves the LLN. The 6LBR is also either collocated with or 638 directly connected to the 6BBR over an IPv6 Link. 640 3.4. The Binding Table 642 Addresses in an LLN that are reachable from the Backbone by way of 643 the 6BBR function must be registered to that 6BBR, using an NS(EARO) 644 with the R flag set [RFC8505]. The 6BBR answers with an NA(EARO) and 645 maintains a state for the registration in an abstract Binding Table. 647 An entry in the Binding Table is called a "Binding". A Binding may 648 be in Tentative, Reachable or Stale state. 650 The 6BBR uses a combination of [RFC8505] and IPv6 ND over the 651 Backbone to advertise the registration and avoid a duplication. 652 Conflicting registrations are solved by the 6BBRs, transparently to 653 the Registering Nodes. 655 Only one 6LN may register a given Address, but the Address may be 656 registered to Multiple 6BBRs for higher availability. 658 Over the LLN, Binding Table management is as follows: 660 * De-registrations (newer TID, same ROVR, null Lifetime) are 661 accepted with a status of 4 ("Removed"); the entry is deleted; 663 * Newer registrations (newer TID, same ROVR, non-null Lifetime) are 664 accepted with a status of 0 (Success); the Binding is updated with 665 the new TID, the Registration Lifetime and the Registering Node; 666 in Tentative state the EDAC response is held and may be 667 overwritten; in other states the Registration Lifetime timer is 668 restarted and the entry is placed in Reachable state. 670 * Identical registrations (same TID, same ROVR) from the same 671 Registering Node are accepted with a status of 0 (Success). In 672 Tentative state, the response is held and may be overwritten, but 673 the response is eventually produced, carrying the result of the 674 DAD process; 676 * Older registrations (older TID, same ROVR) from the same 677 Registering Node are discarded; 679 * Identical and older registrations (not-newer TID, same ROVR) from 680 a different Registering Node are rejected with a status of 3 681 (Moved); this may be rate limited to avoid undue interference; 683 * Any registration for the same address but with a different ROVR is 684 rejected with a status of 1 (Duplicate). 686 The operation of the Binding Table is specified in detail in 687 Section 9. 689 3.5. Primary and Secondary 6BBRs 691 The same address may be successfully registered to more than one 692 6BBR, in which case the Registering Node uses the same EARO in all 693 the parallel registrations. To allow for this, ND(DAD) and NA 694 messages with an EARO that indicate an identical Binding in another 695 6BBR (same Registered address, same TID, same ROVR) are silently 696 ignored but for the purpose of selecting the primary 6BBR for that 697 registration. 699 A 6BBR may be either primary or secondary. The primary is the 6BBR 700 that has the highest EUI-64 Address of all the 6BBRs that share a 701 registration for the same Registered Address, with the same ROVR and 702 same Transaction ID, the EUI-64 Address being considered as an 703 unsigned 64bit integer. A given 6BBR can be primary for a given 704 Address and secondary for another Address, regardless of whether or 705 not the Addresses belong to the same 6LN. 707 In the following sections, is is expected that an NA is sent over the 708 backbone only if the node is primary or does not support the concept 709 of primary. More than one 6BBR claiming or defending an address 710 generates unwanted traffic but no reachability issue since all 6BBRs 711 provide reachability from the Backbone to the 6LN. 713 3.6. Using Optimistic DAD 715 Optimistic Duplicate Address Detection [RFC4429] (ODAD) specifies how 716 an IPv6 Address can be used before completion of Duplicate Address 717 Detection (DAD). ODAD guarantees that this behavior will not cause 718 harm if the new Address is a duplicate. 720 Support for ODAD avoids delays in installing the Neighbor Cache Entry 721 (NCE) in the 6BBRs and the default router, enabling immediate 722 connectivity to the registered node. As shown in Figure 3, if the 723 6BBR is aware of the Link-Layer Address (LLA) of a router, then the 724 6BBR sends a Router Solicitation (RS), using the Registered Address 725 as the IP Source Address, to the known router(s). The RS is sent 726 without a Source LLA Option (SLLAO), to avoid invalidating a 727 preexisting NCE in the router. 729 Following ODAD, the router may then send a unicast RA to the 730 Registered Address, and it may resolve that Address using an 731 NS(Lookup) message. In response, the 6BBR sends an NA with an EARO 732 and the Override flag [RFC4861] that is not set. The router can then 733 determine the freshest EARO in case of conflicting NA(EARO) messages, 734 using the method described in section 5.2.1 of [RFC8505]. If the 735 NA(EARO) is the freshest answer, the default router creates a Binding 736 with the SLLAO of the 6BBR (in Routing Proxy mode) or that of the 737 Registering Node (in Bridging Proxy mode) so that traffic from/to the 738 Registered Address can flow immediately. 740 4. Multi-Link Subnet Considerations 742 The Backbone and the federated LLN Links are considered as different 743 links in the Multi-Link Subnet, even if multiple LLNs are attached to 744 the same 6BBR. ND messages are link-scoped and are not forwarded by 745 the 6BBR between the backbone and the LLNs though some packets may be 746 reinjected in Bridging Proxy mode (see Section 8). 748 Legacy nodes located on the backbone expect that the subnet is 749 deployed within a single link and that there is a common Maximum 750 Transmission Unit (MTU) for intra-subnet communication, the Link MTU. 751 They will not perform the IPv6 Path MTU Discovery [RFC8201] for a 752 destination within the subnet. For that reason, the MTU MUST have 753 the same value on the Backbone and all federated LLNs in the MLSN. 754 As a consequence, the 6BBR MUST use the same MTU value in RAs over 755 the Backbone and in the RAs that it transmits towards the LLN links. 757 5. Optional 6LBR serving the Multi-Link Subnet 759 A 6LBR can be deployed to serve the whole MLSN. It may be attached 760 to the backbone, in which case it can be discovered by its capability 761 advertisement (see section 4.3. of [RFC8505]) in RA messages. 763 When a 6LBR is present, the 6BBR uses an EDAR/EDAC message exchange 764 with the 6LBR to check if the new registration corresponds to a 765 duplication or a movement. This is done prior to the NS(DAD) 766 process, which may be avoided if the 6LBR already maintains a 767 conflicting state for the Registered Address. 769 If this registration is duplicate or not the freshest, then the 6LBR 770 replies with an EDAC message with a status code of 1 ("Duplicate 771 Address") or 3 ("Moved"), respectively. If this registration is the 772 freshest, then the 6LBR replies with a status code of 0. In that 773 case, if this registration is fresher than an existing registration 774 for another 6BBR, then the 6LBR also sends an asynchronous EDAC with 775 a status of 4 ("Removed") to that other 6BBR. 777 The EDAR message SHOULD carry the SLLAO used in NS messages by the 778 6BBR for that Binding, and the EDAC message SHOULD carry the Target 779 Link Layer Address Option (TLLAO) associated with the currently 780 accepted registration. This enables a 6BBR to locate the new 781 position of a mobile 6LN in the case of a Routing Proxy operation, 782 and opens the capability for the 6LBR to serve as a mapping server in 783 the future. 785 Note that if Link-Local addresses are registered, then the scope of 786 uniqueness on which the address duplication is checked is the total 787 collection of links that the 6LBR serves as opposed to the sole link 788 on which the Link-Local address is assigned. 790 6. Using IPv6 ND Over the Backbone Link 792 On the Backbone side, the 6BBR MUST join the SNMA group corresponding 793 to a Registered Address as soon as it creates a Binding for that 794 Address, and maintain that SNMA membership as long as it maintains 795 the registration. The 6BBR uses either the SNMA or plain unicast to 796 defend the Registered Addresses in its Binding Table over the 797 Backbone (as specified in [RFC4862]). The 6BBR advertises and 798 defends the Registered Addresses over the Backbone Link using RS, 799 NS(DAD) and NA messages with the Registered Address as the Source or 800 Target address. 802 The 6BBR MUST place an EARO in the IPv6 ND messages that it generates 803 on behalf of the Registered Node. Note that an NS(DAD) does not 804 contain an SLLAO and cannot be confused with a proxy registration 805 such as performed by a 6LBR. 807 IPv6 ND operates as follows on the backbone: 809 * Section 7.2.8 of [RFC4861] specifies that an NA message generated 810 as a proxy does not have the Override flag set in order to ensure 811 that if the real owner is present on the link, its own NA will 812 take precedence, and that this NA does not update the NCE for the 813 real owner if one exists. 815 * A node that receives multiple NA messages updates an existing NCE 816 only if the Override flag is set; otherwise the node will probe 817 the cached address. 819 * When an NS(DAD) is received for a tentative address, which means 820 that two nodes form the same address at nearly the same time, 821 section 5.4.3 of [RFC4862] cannot detect which node first claimed 822 the address and the address is abandoned. 824 * In any case, [RFC4862] indicates that a node never responds to a 825 Neighbor Solicitation for a tentative address. 827 This specification adds information about proxied addresses that 828 helps sort out a duplication (different ROVR) from a movement (same 829 ROVR, different TID), and in the latter case the older registration 830 from the fresher one (by comparing TIDs). 832 When a Registering Node moves from one 6BBR to the next, the new 6BBR 833 sends NA messages over the backbone to update existing NCEs. A node 834 that supports this specification and that receives multiple NA 835 messages with an EARO option and the same ROVR MUST favor the NA with 836 the freshest EARO over the others. 838 The 6BBR MAY set the Override flag in the NA messages if it does not 839 compete with the Registering Node for the NCE in backbone nodes. 840 This is assured if the Registering Node is attached via an interface 841 that cannot be bridged onto the backbone, making it impossible for 842 the Registering Node to defend its own addresses there. This may 843 also be signaled by the Registering Node through a protocol extension 844 that is not in scope for this specification. 846 When the Binding is in Tentative state, the 6BBR acts as follows: 848 * an NS(DAD) that indicates a duplication can still not be asserted 849 for first come, but the situation can be avoided using a 6LBR on 850 the backbone that will serialize the order of appearance of the 851 address and ensure first-come/first-serve. 853 * an NS or an NA that denotes an older registration for the same 854 Registered Node is not interpreted as a duplication as specified 855 in section 5.4.3 and 5.4.4 of [RFC4862], respectively. 857 When the Binding is no longer in Tentative state, the 6BBR acts as 858 follows: 860 * an NS or an NA with an EARO that denotes a duplicate registration 861 (different ROVR) is answered with an NA message that carries an 862 EARO with a status of 1 (Duplicate), unless the received message 863 is an NA that carries an EARO with a status of 1. 865 In any state, the 6BBR acts as follows: 867 * an NS or an NA with an EARO that denotes an older registration 868 (same ROVR) is answered with an NA message that carries an EARO 869 with a status of 3 (Moved) to ensure that the stale state is 870 removed rapidly. 872 This behavior is specified in more detail in Section 9. 874 This specification enables proxy operation for the IPv6 ND resolution 875 of LLN devices and a prefix that is used across a Multi-Link Subnet 876 MAY be advertised as on-link over the Backbone. This is done for 877 backward compatibility with existing IPv6 hosts by setting the L flag 878 in the Prefix Information Option (PIO) of RA messages [RFC4861]. 880 For movement involving a slow reattachment, the NUD procedure defined 881 in [RFC4861] may time out too quickly. Nodes on the backbone SHOULD 882 support [RFC7048] whenever possible. 884 7. Routing Proxy Operations 886 A Routing Proxy provides IPv6 ND proxy functions for Global and 887 Unique Local addresses between the LLN and the backbone, but not for 888 Link-Local addresses. It operates as an IPv6 border router and 889 provides a full Link-Layer isolation. 891 In this mode, it is not required that the MAC addresses of the 6LNs 892 are visible at Layer 2 over the Backbone. It is thus useful when the 893 messaging over the Backbone that is associated to wireless mobility 894 becomes expensive, e.g., when the Layer 2 topology is virtualized 895 over a wide area IP underlay. 897 This mode is definitely required when the LLN uses a MAC address 898 format that is different from that on the Backbone (e.g., EUI-64 vs. 899 EUI-48). Since a 6LN may not be able to resolve an arbitrary 900 destination in the MLSN directly, a prefix that is used across a MLSN 901 MUST NOT be advertised as on-link in RA messages sent towards the 902 LLN. 904 In order to maintain IP connectivity, the 6BBR installs a connected 905 Host route to the Registered Address on the LLN interface, via the 906 Registering Node as identified by the Source Address and the SLLA 907 option in the NS(EARO) messages. 909 When operating as a Routing Proxy, the 6BBR MUST use its Layer 2 910 Address on its Backbone Interface in the SLLAO of the RS messages and 911 the TLLAO of the NA messages that it generates to advertise the 912 Registered Addresses. 914 For each Registered Address, multiple peers on the Backbone may have 915 resolved the Address with the 6BBR MAC Address, maintaining that 916 mapping in their Neighbor Cache. The 6BBR SHOULD maintain a list of 917 the peers on the Backbone which have associated its MAC Address with 918 the Registered Address. If that Registered Address moves to another 919 6BBR, the previous 6BBR SHOULD unicast a gratuitous NA to each such 920 peer, to supply the LLA of the new 6BBR in the TLLA option for the 921 Address. A 6BBR that does not maintain this list MAY multicast a 922 gratuitous NA message; this NA will possibly hit all the nodes on the 923 Backbone, whether or not they maintain an NCE for the Registered 924 Address. In either case, the 6BBR MAY set the Override flag if it is 925 known that the Registered Node cannot attach to the backbone, so as 926 to avoid interruptions and save probing flows in the future. 928 If a correspondent fails to receive the gratuitous NA, it will keep 929 sending traffic to a 6BBR to which the node was previously 930 registered. Since the previous 6BBR removed its Host route to the 931 Registered Address, it will look up the address over the backbone, 932 resolve the address with the LLA of the new 6BBR, and forward the 933 packet to the correct 6BBR. The previous 6BBR SHOULD also issue a 934 redirect message [RFC4861] to update the cache of the correspondent. 936 8. Bridging Proxy Operations 938 A Bridging Proxy provides IPv6 ND proxy functions between the LLN and 939 the backbone while preserving the forwarding continuity at the MAC 940 Layer. It acts as a Layer 2 Bridge for all types of unicast packets 941 including link-scoped, and appears as an IPv6 Host on the Backbone. 943 The Bridging Proxy registers any Binding including for a Link-Local 944 address to the 6LBR (if present) and defends it over the backbone in 945 IPv6 ND procedures. 947 To achieve this, the Bridging Proxy intercepts the IPv6 ND messages 948 and may reinject them on the other side, respond directly or drop 949 them. For instance, an ND(Lookup) from the backbone that matches a 950 Binding can be responded directly, or turned into a unicast on the 951 LLN side to let the 6LN respond. 953 As a Bridging Proxy, the 6BBR MUST use the Registering Node's Layer 2 954 Address in the SLLAO of the NS/RS messages and the TLLAO of the NA 955 messages that it generates to advertise the Registered Addresses. 956 The Registering Node's Layer 2 address is found in the SLLA of the 957 registration NS(EARO), and maintained in the Binding Table. 959 The Multi-Link Subnet prefix SHOULD NOT be advertised as on-link in 960 RA messages sent towards the LLN. If a destination address is seen 961 as on-link, then a 6LN may use NS(Lookup) messages to resolve that 962 address. In that case, the 6BBR MUST either answer the NS(Lookup) 963 message directly or reinject the message on the backbone, either as a 964 Layer 2 unicast or a multicast. 966 If the Registering Node owns the Registered Address, meaning that the 967 Registering Node is the Registered Node, then its mobility does not 968 impact existing NCEs over the Backbone. In a network where proxy 969 registrations are used, meaning that the Registering Node acts on 970 behalf of the Registered Node, if the Registered Node selects a new 971 Registering Node then the existing NCEs across the Backbone pointing 972 at the old Registering Node must be updated. In that case, the 6BBR 973 SHOULD attempt to fix the existing NCEs across the Backbone pointing 974 at other 6BBRs using NA messages as described in Section 7. 976 This method can fail if the multicast message is not received; one or 977 more correspondent nodes on the Backbone might maintain an stale NCE, 978 and packets to the Registered Address may be lost. When this 979 condition happens, it is eventually discovered and resolved using NUD 980 as defined in [RFC4861]. 982 9. Creating and Maintaining a Binding 984 Upon receiving a registration for a new Address (i.e., an NS(EARO) 985 with the R flag set), the 6BBR creates a Binding and operates as a 986 6LR according to [RFC8505], interacting with the 6LBR if one is 987 present. 989 An implementation of a Routing Proxy that creates a Binding MUST also 990 create an associated Host route pointing to the registering node in 991 the LLN interface from which the registration was received. 993 Acting as a 6BBR, the 6LR operation is modified as follows: 995 * Acting as Bridging Proxy the 6LR MUST proxy ND over the backbone 996 for registered Link-Local addresses. 998 * EDAR and EDAC messages SHOULD carry a SLLAO and a TLLAO, 999 respectively. 1001 * An EDAC message with a status of 9 (6LBR Registry Saturated) is 1002 assimilated as a status of 0 if a following DAD process protects 1003 the address against duplication. 1005 This specification enables nodes on a Backbone Link to co-exist along 1006 with nodes implementing IPv6 ND [RFC4861] as well as other non- 1007 normative specifications such as [I-D.bi-savi-wlan]. It is possible 1008 that not all IPv6 addresses on the Backbone are registered and known 1009 to the 6LBR, and an EDAR/EDAC echange with the 6LBR might succeed 1010 even for a duplicate address. Consequently the 6BBR still needs to 1011 perform IPv6 ND DAD over the backbone after an EDAC with a status 1012 code of 0 or 9. 1014 For the DAD operation, the Binding is placed in Tentative state for a 1015 duration of TENTATIVE_DURATION (Section 12), and an NS(DAD) message 1016 is sent as a multicast message over the Backbone to the SNMA 1017 associated with the registered Address [RFC4862]. The EARO from the 1018 registration MUST be placed unchanged in the NS(DAD) message. 1020 If a registration is received for an existing Binding with a non-null 1021 Registration Lifetime and the registration is fresher (same ROVR, 1022 fresher TID), then the Binding is updated, with the new Registration 1023 Lifetime, TID, and possibly Registering Node. In Tentative state 1024 (see Section 9.1), the current DAD operation continues unaltered. In 1025 other states (see Section 9.2 and Section 9.3 ), the Binding is 1026 placed in Reachable state for the Registration Lifetime, and the 6BBR 1027 returns an NA(EARO) to the Registering Node with a status of 0 1028 (Success). 1030 Upon a registration that is identical (same ROVR, TID, and 1031 Registering Node), the 6BBR does not alter its current state. In 1032 Reachable State it returns an NA(EARO) back to the Registering Node 1033 with a status of 0 (Success). A registration that is not as fresh 1034 (same ROVR, older TID) is ignored. 1036 If a registration is received for an existing Binding and a 1037 registration Lifetime of zero, then the Binding is removed, and the 1038 6BBR returns an NA(EARO) back to the Registering Node with a status 1039 of 0 (Success). An implementation of a Routing Proxy that removes a 1040 binding MUST remove the associated Host route pointing on the 1041 registering node. 1043 The old 6BBR removes its Binding Table entry and notifies the 1044 Registering Node with a status of 3 (Moved) if a new 6BBR claims a 1045 fresher registration (same ROVR, fresher TID) for the same address. 1046 The old 6BBR MAY preserve a temporary state in order to forward 1047 packets in flight. The state may for instance be a NCE formed based 1048 on a received NA message. It may also be a Binding Table entry in 1049 Stale state and pointing at the new 6BBR on the backbone, or any 1050 other abstract cache entry that can be used to resolve the Link-Layer 1051 Address of the new 6BBR. The old 6BBR SHOULD also use REDIRECT 1052 messages as specified in [RFC4861] to update the correspondents for 1053 the Registered Address, pointing to the new 6BBR. 1055 9.1. Operations on a Binding in Tentative State 1057 The Tentative state covers a DAD period over the backbone during 1058 which an address being registered is checked for duplication using 1059 procedures defined in [RFC4862]. 1061 For a Binding in Tentative state: 1063 * The Binding MUST be removed if an NA message is received over the 1064 Backbone for the Registered Address with no EARO, or containing an 1065 EARO that indicates an existing registration owned by a different 1066 Registering Node (different ROVR). In that case, an NA is sent 1067 back to the Registering Node with a status of 1 (Duplicate) to 1068 indicate that the binding has been rejected. This behavior might 1069 be overridden by policy, in particular if the registration is 1070 trusted, e.g., based on the validation of the ROVR field (see 1071 [I-D.ietf-6lo-ap-nd]). 1073 * The Binding MUST be removed if an NS(DAD) message is received over 1074 the Backbone for the Registered Address with no EARO, or 1075 containing an EARO with a different ROVR that indicates a 1076 tentative registration by a different Registering Node. In that 1077 case, an NA is sent back to the Registering Node with a status of 1078 1 (Duplicate). This behavior might be overridden by policy, in 1079 particular if the registration is trusted, e.g., based on the 1080 validation of the ROVR field (see [I-D.ietf-6lo-ap-nd]). 1082 * The Binding MUST be removed if an NA or an NS(DAD) message is 1083 received over the Backbone for the Registered Address containing 1084 an EARO with a that indicates a fresher registration ([RFC8505]) 1085 for the same Registering Node (same ROVR). In that case, an NA 1086 MUST be sent back to the Registering Node with a status of 3 1087 (Moved). 1089 * The Binding MUST be kept unchanged if an NA or an NS(DAD) message 1090 is received over the Backbone for the Registered Address 1091 containing an EARO with a that indicates an older registration 1092 ([RFC8505]) for the same Registering Node (same ROVR). The 1093 message is answered with an NA that carries an EARO with a status 1094 of 3 (Moved) and the Override flag not set. This behavior might 1095 be overridden by policy, in particular if the registration is not 1096 trusted. 1098 * Other NS(DAD) and NA messages from the Backbone are ignored. 1100 * NS(Lookup) and NS(NUD) messages SHOULD be optimistically answered 1101 with an NA message containing an EARO with a status of 0 and the 1102 Override flag not set (see Section 3.6). If optimistic DAD is 1103 disabled, then they SHOULD be queued to be answered when the 1104 Binding goes to Reachable state. 1106 When the TENTATIVE_DURATION (Section 12) timer elapses, the Binding 1107 is placed in Reachable state for the Registration Lifetime, and the 1108 6BBR returns an NA(EARO) to the Registering Node with a status of 0 1109 (Success). 1111 The 6BBR also attempts to take over any existing Binding from other 1112 6BBRs and to update existing NCEs in backbone nodes. This is done by 1113 sending an NA message with an EARO and the Override flag not set over 1114 the backbone (see Section 7 and Section 8). 1116 9.2. Operations on a Binding in Reachable State 1118 The Reachable state covers an active registration after a successful 1119 DAD process. 1121 If the Registration Lifetime is of a long duration, an implementation 1122 might be configured to reassess the availability of the Registering 1123 Node at a lower period, using a NUD procedure as specified in 1124 [RFC7048]. If the NUD procedure fails, the Binding SHOULD be placed 1125 in Stale state immediately. 1127 For a Binding in Reachable state: 1129 * The Binding MUST be removed if an NA or an NS(DAD) message is 1130 received over the Backbone for the Registered Address containing 1131 an EARO that indicates a fresher registration ([RFC8505]) for the 1132 same Registered Node (i.e., same ROVR but fresher TID). A status 1133 of 4 (Removed) is returned in an asynchronous NA(EARO) to the 1134 Registering Node. Based on configuration, an implementation may 1135 delay this operation by a timer with a short setting, e.g., a few 1136 seconds to a minute, in order to a allow for a parallel 1137 registration to reach this node, in which case the NA might be 1138 ignored. 1140 * NS(DAD) and NA messages containing an EARO that indicates a 1141 registration for the same Registered Node that is not as fresh as 1142 this binding MUST be answered with an NA message containing an 1143 EARO with a status of 3 (Moved). 1145 * An NS(DAD) with no EARO or with an EARO that indicates a duplicate 1146 registration (i.e., different ROVR) MUST be answered with an NA 1147 message containing an EARO with a status of 1 (Duplicate) and the 1148 Override flag not set, unless the received message is an NA that 1149 carries an EARO with a status of 1, in which case the node 1150 refrains from answering. 1152 * Other NS(DAD) and NA messages from the Backbone are ignored. 1154 * NS(Lookup) and NS(NUD) messages SHOULD be answered with an NA 1155 message containing an EARO with a status of 0 and the Override 1156 flag not set. The 6BBR MAY check whether the Registering Node is 1157 still available using a NUD procedure over the LLN prior to 1158 answering; this behaviour depends on the use case and is subject 1159 to configuration. 1161 When the Registration Lifetime timer elapses, the Binding is placed 1162 in Stale state for a duration of STALE_DURATION (Section 12). 1164 9.3. Operations on a Binding in Stale State 1166 The Stale state enables tracking of the Backbone peers that have a 1167 NCE pointing to this 6BBR in case the Registered Address shows up 1168 later. 1170 If the Registered Address is claimed by another 6LN on the Backbone, 1171 with an NS(DAD) or an NA, the 6BBR does not defend the Address. 1173 For a Binding in Stale state: 1175 * The Binding MUST be removed if an NA or an NS(DAD) message is 1176 received over the Backbone for the Registered Address containing 1177 no EARO or an EARO that indicates either a fresher registration 1178 for the same Registered Node or a duplicate registration. A 1179 status of 4 (Removed) MAY be returned in an asynchronous NA(EARO) 1180 to the Registering Node. 1182 * NS(DAD) and NA messages containing an EARO that indicates a 1183 registration for the same Registered Node that is not as fresh as 1184 this MUST be answered with an NA message containing an EARO with a 1185 status of 3 (Moved). 1187 * If the 6BBR receives an NS(Lookup) or an NS(NUD) message for the 1188 Registered Address, the 6BBR MUST attempt a NUD procedure as 1189 specified in [RFC7048] to the Registering Node, targeting the 1190 Registered Address, prior to answering. If the NUD procedure 1191 succeeds, the operation in Reachable state applies. If the NUD 1192 fails, the 6BBR refrains from answering. 1194 * Other NS(DAD) and NA messages from the Backbone are ignored. 1196 When the STALE_DURATION (Section 12) timer elapses, the Binding MUST 1197 be removed. 1199 10. Registering Node Considerations 1201 A Registering Node MUST implement [RFC8505] in order to interact with 1202 a 6BBR (which acts as a routing registrar). Following [RFC8505], the 1203 Registering Node signals that it requires IPv6 proxy-ND services from 1204 a 6BBR by registering the corresponding IPv6 Address using an 1205 NS(EARO) message with the R flag set. 1207 The Registering Node may be the 6LN owning the IPv6 Address, or a 1208 6LBR that performs the registration on its behalf in a Route-Over 1209 mesh. 1211 A 6LN MUST register all of its IPv6 Addresses to its 6LR, which is 1212 the 6BBR when they are connected at Layer 2. Failure to register an 1213 address may result in the address being unreachable by other parties. 1214 This would happen for instance if the 6BBR propagates the NS(Lookup) 1215 from the backbone only to the LLN nodes that do not register their 1216 addresses. 1218 The Registering Node MUST refrain from using multicast NS(Lookup) 1219 when the destination is not known as on-link, e.g., if the prefix is 1220 advertised in a PIO with the L flag that is not set. In that case, 1221 the Registering Node sends its packets directly to its 6LR. 1223 The Registering Node SHOULD also follow BCP 202 [RFC7772] in order to 1224 limit the use of multicast RAs. It SHOULD also implement Simple 1225 Procedures for Detecting Network Attachment in IPv6 [RFC6059] (DNA 1226 procedures) to detect movements, and support Packet-Loss Resiliency 1227 for Router Solicitations [RFC7559] in order to improve reliability 1228 for the unicast RS messages. 1230 11. Security Considerations 1232 This specification applies to LLNs and a backbone in which the 1233 individual links are protected against rogue access, e.g., by 1234 authenticating a node that attaches to the network and encrypting at 1235 the MAC layer the transmissions that may be overheard. In 1236 particular, the LLN MAC is required to provide secure unicast to/from 1237 the Backbone Router and secure Broadcast from the Backbone Router in 1238 a way that prevents tampering with or replaying the RA messages. 1240 [I-D.ietf-6lo-ap-nd] guarantees the ownership of a registered address 1241 based on a proof-of-ownership encoded in the ROVR field and protects 1242 against address theft and impersonation inside the LLN, because the 1243 6LR can challenge the Registered Node for a proof-of-ownership. This 1244 method does not extend over the backbone since the 6BBR cannot 1245 provide the proof-of-ownership. A possible attack over the backbone 1246 can be done by sending an NS with an EARO and expecting the NA(EARO) 1247 back to contain the TID and ROVR fields of the existing state. With 1248 that information, the attacker can easily increase the TID and take 1249 over the Binding. 1251 12. Protocol Constants 1253 This Specification uses the following constants: 1255 TENTATIVE_DURATION: 800 milliseconds 1257 STALE_DURATION: see below 1259 In LLNs with long-lived Addresses such as LPWANs, STALE_DURATION 1260 SHOULD be configured with a relatively long value to cover an 1261 interval when the address may be reused, and before it is safe to 1262 expect that the address was definitively released. A good default 1263 value can be 24 hours. In LLNs where addresses are renewed rapidly, 1264 e.g., for privacy reasons, STALE_DURATION SHOULD be configured with a 1265 relatively shorter value, by default 5 minutes. 1267 13. IANA Considerations 1269 This document has no request to IANA. 1271 14. Acknowledgments 1273 Many thanks to Dorothy Stanley, Thomas Watteyne and Jerome Henry for 1274 their various contributions. Also many thanks to Timothy Winters and 1275 Erik Nordmark for their help, review and support in preparation to 1276 the IESG cycle, and to Kyle Rose, Elwyn Davies and Dominique Barthel 1277 for their useful contributions through the IETF last call and IESG 1278 process. 1280 15. Normative References 1282 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1283 Requirement Levels", BCP 14, RFC 2119, 1284 DOI 10.17487/RFC2119, March 1997, 1285 . 1287 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 1288 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 1289 2006, . 1291 [RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD) 1292 for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006, 1293 . 1295 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1296 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1297 DOI 10.17487/RFC4861, September 2007, 1298 . 1300 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1301 Address Autoconfiguration", RFC 4862, 1302 DOI 10.17487/RFC4862, September 2007, 1303 . 1305 [RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for 1306 Detecting Network Attachment in IPv6", RFC 6059, 1307 DOI 10.17487/RFC6059, November 2010, 1308 . 1310 [RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. 1311 Bormann, "Neighbor Discovery Optimization for IPv6 over 1312 Low-Power Wireless Personal Area Networks (6LoWPANs)", 1313 RFC 6775, DOI 10.17487/RFC6775, November 2012, 1314 . 1316 [RFC7048] Nordmark, E. and I. Gashinsky, "Neighbor Unreachability 1317 Detection Is Too Impatient", RFC 7048, 1318 DOI 10.17487/RFC7048, January 2014, 1319 . 1321 [RFC7559] Krishnan, S., Anipko, D., and D. Thaler, "Packet-Loss 1322 Resiliency for Router Solicitations", RFC 7559, 1323 DOI 10.17487/RFC7559, May 2015, 1324 . 1326 [RFC7772] Yourtchenko, A. and L. Colitti, "Reducing Energy 1327 Consumption of Router Advertisements", BCP 202, RFC 7772, 1328 DOI 10.17487/RFC7772, February 2016, 1329 . 1331 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1332 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1333 May 2017, . 1335 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 1336 (IPv6) Specification", STD 86, RFC 8200, 1337 DOI 10.17487/RFC8200, July 2017, 1338 . 1340 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 1341 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 1342 DOI 10.17487/RFC8201, July 2017, 1343 . 1345 [RFC8505] Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C. 1346 Perkins, "Registration Extensions for IPv6 over Low-Power 1347 Wireless Personal Area Network (6LoWPAN) Neighbor 1348 Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018, 1349 . 1351 16. Informative References 1353 [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery 1354 Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April 1355 2006, . 1357 [RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903, 1358 DOI 10.17487/RFC4903, June 2007, 1359 . 1361 [RFC5415] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley, 1362 Ed., "Control And Provisioning of Wireless Access Points 1363 (CAPWAP) Protocol Specification", RFC 5415, 1364 DOI 10.17487/RFC5415, March 2009, 1365 . 1367 [RFC5568] Koodli, R., Ed., "Mobile IPv6 Fast Handovers", RFC 5568, 1368 DOI 10.17487/RFC5568, July 2009, 1369 . 1371 [RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem 1372 Statement and Requirements for IPv6 over Low-Power 1373 Wireless Personal Area Network (6LoWPAN) Routing", 1374 RFC 6606, DOI 10.17487/RFC6606, May 2012, 1375 . 1377 [RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility 1378 Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July 1379 2011, . 1381 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 1382 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 1383 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 1384 Low-Power and Lossy Networks", RFC 6550, 1385 DOI 10.17487/RFC6550, March 2012, 1386 . 1388 [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The 1389 Locator/ID Separation Protocol (LISP)", RFC 6830, 1390 DOI 10.17487/RFC6830, January 2013, 1391 . 1393 [RFC8273] Brzozowski, J. and G. Van de Velde, "Unique IPv6 Prefix 1394 per Host", RFC 8273, DOI 10.17487/RFC8273, December 2017, 1395 . 1397 [I-D.yourtchenko-6man-dad-issues] 1398 Yourtchenko, A. and E. Nordmark, "A survey of issues 1399 related to IPv6 Duplicate Address Detection", Work in 1400 Progress, Internet-Draft, draft-yourtchenko-6man-dad- 1401 issues-01, 3 March 2015, . 1404 [I-D.nordmark-6man-dad-approaches] 1405 Nordmark, E., "Possible approaches to make DAD more robust 1406 and/or efficient", Work in Progress, Internet-Draft, 1407 draft-nordmark-6man-dad-approaches-02, 19 October 2015, 1408 . 1411 [I-D.ietf-6man-rs-refresh] 1412 Nordmark, E., Yourtchenko, A., and S. Krishnan, "IPv6 1413 Neighbor Discovery Optional RS/RA Refresh", Work in 1414 Progress, Internet-Draft, draft-ietf-6man-rs-refresh-02, 1415 31 October 2016, . 1418 [I-D.ietf-6lo-ap-nd] 1419 Thubert, P., Sarikaya, B., Sethi, M., and R. Struik, 1420 "Address Protected Neighbor Discovery for Low-power and 1421 Lossy Networks", Work in Progress, Internet-Draft, draft- 1422 ietf-6lo-ap-nd-19, 6 February 2020, 1423 . 1425 [I-D.ietf-6tisch-architecture] 1426 Thubert, P., "An Architecture for IPv6 over the TSCH mode 1427 of IEEE 802.15.4", Work in Progress, Internet-Draft, 1428 draft-ietf-6tisch-architecture-28, 29 October 2019, 1429 . 1432 [I-D.ietf-mboned-ieee802-mcast-problems] 1433 Perkins, C., McBride, M., Stanley, D., Kumari, W., and J. 1434 Zuniga, "Multicast Considerations over IEEE 802 Wireless 1435 Media", Work in Progress, Internet-Draft, draft-ietf- 1436 mboned-ieee802-mcast-problems-11, 11 December 2019, 1437 . 1440 [I-D.bi-savi-wlan] 1441 Bi, J., Wu, J., Wang, Y., and T. Lin, "A SAVI Solution for 1442 WLAN", Work in Progress, Internet-Draft, draft-bi-savi- 1443 wlan-18, 17 November 2019, 1444 . 1446 [I-D.thubert-6lo-unicast-lookup] 1447 Thubert, P. and E. Levy-Abegnoli, "IPv6 Neighbor Discovery 1448 Unicast Lookup", Work in Progress, Internet-Draft, draft- 1449 thubert-6lo-unicast-lookup-00, 25 January 2019, 1450 . 1453 [IEEEstd8021] 1454 IEEE standard for Information Technology, "IEEE Standard 1455 for Information technology -- Telecommunications and 1456 information exchange between systems Local and 1457 metropolitan area networks Part 1: Bridging and 1458 Architecture". 1460 [IEEEstd80211] 1461 IEEE standard for Information Technology, "IEEE Standard 1462 for Information technology -- Telecommunications and 1463 information exchange between systems Local and 1464 metropolitan area networks-- Specific requirements Part 1465 11: Wireless LAN Medium Access Control (MAC) and Physical 1466 Layer (PHY) Specifications". 1468 [IEEEstd802151] 1469 IEEE standard for Information Technology, "IEEE Standard 1470 for Information Technology - Telecommunications and 1471 Information Exchange Between Systems - Local and 1472 Metropolitan Area Networks - Specific Requirements. - Part 1473 15.1: Wireless Medium Access Control (MAC) and Physical 1474 Layer (PHY) Specifications for Wireless Personal Area 1475 Networks (WPANs)". 1477 [IEEEstd802154] 1478 IEEE standard for Information Technology, "IEEE Standard 1479 for Local and metropolitan area networks -- Part 15.4: 1480 Low-Rate Wireless Personal Area Networks (LR-WPANs)". 1482 Appendix A. Possible Future Extensions 1484 With the current specification, the 6LBR is not leveraged to avoid 1485 multicast NS(Lookup) on the Backbone. This could be done by adding a 1486 lookup procedure in the EDAR/EDAC exchange. 1488 By default the specification does not have a fine-grained trust 1489 model: all nodes that can authenticate to the LLN MAC or attach to 1490 the backbone are equally trusted. It would be desirable to provide a 1491 stronger authorization model, e.g., whereby nodes that associate 1492 their address with a proof-of-ownership [I-D.ietf-6lo-ap-nd] should 1493 be more trusted than nodes that do not. Such a trust model and 1494 related signaling could be added in the future to override the 1495 default operation and favor trusted nodes. 1497 Future documents may extend this specification by allowing the 6BBR 1498 to redistribute Host routes in routing protocols that would operate 1499 over the Backbone, or in MIPv6 [RFC6275], or FMIP [RFC5568], or the 1500 Locator/ID Separation Protocol (LISP) [RFC6830] to support mobility 1501 on behalf of the 6LNs, etc... LISP may also be used to provide an 1502 equivalent to the EDAR/EDAC exchange using a Map Server / Map 1503 Resolver as a replacement to the 6LBR. 1505 Appendix B. Applicability and Requirements Served 1507 This document specifies proxy-ND functions that can be used to 1508 federate an IPv6 Backbone Link and multiple IPv6 LLNs into a single 1509 Multi-Link Subnet. The proxy-ND functions enable IPv6 ND services 1510 for Duplicate Address Detection (DAD) and Address Lookup that do not 1511 require broadcasts over the LLNs. 1513 The term LLN is used to cover multiple types of WLANs and WPANs, 1514 including (Low-Power) Wi-Fi, BLUETOOTH(R) Low Energy, IEEE STD 1515 802.11ah and IEEE STD.802.15.4 wireless meshes, covering the types of 1516 networks listed in Appendix B.3 of [RFC8505] "Requirements Related to 1517 Various Low-Power Link Types". 1519 Each LLN in the subnet is attached to an IPv6 Backbone Router (6BBR). 1520 The Backbone Routers interconnect the LLNs and advertise the 1521 Addresses of the 6LNs over the Backbone Link using proxy-ND 1522 operations. 1524 This specification updates IPv6 ND over the Backbone to distinguish 1525 Address movement from duplication and eliminate stale state in the 1526 Backbone routers and Backbone nodes once a 6LN has roamed. This way, 1527 mobile nodes may roam rapidly from one 6BBR to the next and 1528 requirements in Appendix B.1 of [RFC8505] "Requirements Related to 1529 Mobility" are met. 1531 A 6LN can register its IPv6 Addresses and thereby obtain proxy-ND 1532 services over the Backbone, meeting the requirements expressed in 1533 Appendix B.4 of [RFC8505], "Requirements Related to Proxy 1534 Operations". 1536 The negative impact of the IPv6 ND-related broadcasts can be limited 1537 to one of the federated links, enabling the number of 6LNs to grow. 1538 The Routing Proxy operation avoids the need to expose the MAC 1539 addresses of the 6LNs onto the backbone, keeping the Layer 2 topology 1540 simple and stable. This meets the requirements in Appendix B.6 of 1541 [RFC8505] "Requirements Related to Scalability", as long has the 1542 6BBRs are dimensioned for the number of registrations that each needs 1543 to support. 1545 In the case of a Wi-Fi access link, a 6BBR may be collocated with the 1546 Access Point (AP), or with a Fabric Edge (FE) or a CAPWAP [RFC5415] 1547 Wireless LAN Controller (WLC). In those cases, the wireless client 1548 (STA) is the 6LN that makes use of [RFC8505] to register its IPv6 1549 Address(es) to the 6BBR acting as Routing Registrar. The 6LBR can be 1550 centralized and either connected to the Backbone Link or reachable 1551 over IP. The 6BBR proxy-ND operations eliminate the need for 1552 wireless nodes to respond synchronously when a Lookup is performed 1553 for their IPv6 Addresses. This provides the function of a Sleep 1554 Proxy for ND [I-D.nordmark-6man-dad-approaches]. 1556 For the TimeSlotted Channel Hopping (TSCH) mode of [IEEEstd802154], 1557 the 6TiSCH architecture [I-D.ietf-6tisch-architecture] describes how 1558 a 6LoWPAN ND host could connect to the Internet via a RPL mesh 1559 Network, but doing so requires extensions to the 6LOWPAN ND protocol 1560 to support mobility and reachability in a secure and manageable 1561 environment. The extensions detailed in this document also work for 1562 the 6TiSCH architecture, serving the requirements listed in 1563 Appendix B.2 of [RFC8505] "Requirements Related to Routing 1564 Protocols". 1566 The registration mechanism may be seen as a more reliable alternate 1567 to snooping [I-D.bi-savi-wlan]. It can be noted that registration 1568 and snooping are not mutually exclusive. Snooping may be used in 1569 conjunction with the registration for nodes that do not register 1570 their IPv6 Addresses. The 6BBR assumes that if a node registers at 1571 least one IPv6 Address to it, then the node registers all of its 1572 Addresses to the 6BBR. With this assumption, the 6BBR can possibly 1573 cancel all undesirable multicast NS messages that would otherwise 1574 have been delivered to that node. 1576 Scalability of the Multi-Link Subnet [RFC4903] requires avoidance of 1577 multicast/broadcast operations as much as possible even on the 1578 Backbone [I-D.ietf-mboned-ieee802-mcast-problems]. Although hosts 1579 can connect to the Backbone using IPv6 ND operations, multicast RAs 1580 can be saved by using [I-D.ietf-6man-rs-refresh], which also requires 1581 the support of [RFC7559]. 1583 Authors' Addresses 1585 Pascal Thubert (editor) 1586 Cisco Systems, Inc 1587 Building D 1588 45 Allee des Ormes - BP1200 1589 06254 MOUGINS - Sophia Antipolis 1590 France 1592 Phone: +33 497 23 26 34 1593 Email: pthubert@cisco.com 1595 Charles E. Perkins 1596 Blue Meadow Networking 1597 Saratoga, 95070 1598 United States of America 1599 Email: charliep@computer.org 1601 Eric Levy-Abegnoli 1602 Cisco Systems, Inc 1603 Building D 1604 45 Allee des Ormes - BP1200 1605 06254 MOUGINS - Sophia Antipolis 1606 France 1608 Phone: +33 497 23 26 20 1609 Email: elevyabe@cisco.com