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Kelsey 5 Expires: August 29, 2013 Silicon Labs 6 February 25, 2013 8 Multicast Protocol for Low power and Lossy Networks (MPL) 9 draft-ietf-roll-trickle-mcast-04 11 Abstract 13 This document specifies the Multicast Protocol for Low power and 14 Lossy Networks (MPL) that provides IPv6 multicast forwarding in 15 constrained networks. MPL avoids the need to construct or maintain 16 any multicast forwarding topology, disseminating messages to all MPL 17 Forwarders in an MPL Domain. MPL uses the Trickle algorithm to 18 manage message transmissions for both control and data-plane 19 messages. Different Trickle parameter configurations allow MPL to 20 trade between dissemination latency and transmission efficiency. 22 Status of this Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on August 29, 2013. 39 Copyright Notice 41 Copyright (c) 2013 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 58 3. Applicability Statement . . . . . . . . . . . . . . . . . . . 6 59 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 7 60 4.1. Information Base Overview . . . . . . . . . . . . . . . . 7 61 4.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 7 62 4.3. Signaling Overview . . . . . . . . . . . . . . . . . . . . 9 63 5. MPL Parameters and Constants . . . . . . . . . . . . . . . . . 10 64 5.1. MPL Multicast Addresses . . . . . . . . . . . . . . . . . 10 65 5.2. MPL Message Types . . . . . . . . . . . . . . . . . . . . 10 66 5.3. MPL Seed Identifiers . . . . . . . . . . . . . . . . . . . 10 67 5.4. MPL Forwarder Parameters . . . . . . . . . . . . . . . . . 10 68 5.5. MPL Trickle Parameters . . . . . . . . . . . . . . . . . . 11 69 6. Protocol Message Formats . . . . . . . . . . . . . . . . . . . 13 70 6.1. MPL Option . . . . . . . . . . . . . . . . . . . . . . . . 13 71 6.2. MPL Control Message . . . . . . . . . . . . . . . . . . . 14 72 6.3. MPL Seed Info . . . . . . . . . . . . . . . . . . . . . . 15 73 7. Information Base . . . . . . . . . . . . . . . . . . . . . . . 17 74 7.1. Local Interface Set . . . . . . . . . . . . . . . . . . . 17 75 7.2. Domain Set . . . . . . . . . . . . . . . . . . . . . . . . 17 76 7.3. Seed Set . . . . . . . . . . . . . . . . . . . . . . . . . 17 77 7.4. Buffered Message Set . . . . . . . . . . . . . . . . . . . 17 78 8. MPL Domains . . . . . . . . . . . . . . . . . . . . . . . . . 19 79 9. MPL Seed Sequence Numbers . . . . . . . . . . . . . . . . . . 20 80 10. MPL Data Messages . . . . . . . . . . . . . . . . . . . . . . 21 81 10.1. MPL Data Message Generation . . . . . . . . . . . . . . . 21 82 10.2. MPL Data Message Transmission . . . . . . . . . . . . . . 21 83 10.3. MPL Data Message Processing . . . . . . . . . . . . . . . 22 84 11. MPL Control Messages . . . . . . . . . . . . . . . . . . . . . 24 85 11.1. MPL Control Message Generation . . . . . . . . . . . . . . 24 86 11.2. MPL Control Message Transmission . . . . . . . . . . . . . 24 87 11.3. MPL Control Message Processing . . . . . . . . . . . . . . 25 88 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27 89 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 90 13.1. MPL Option Type . . . . . . . . . . . . . . . . . . . . . 28 91 13.2. MPL ICMPv6 Type . . . . . . . . . . . . . . . . . . . . . 28 92 13.3. Well-known Multicast Addresses . . . . . . . . . . . . . . 28 93 14. Security Considerations . . . . . . . . . . . . . . . . . . . 29 94 15. Normative References . . . . . . . . . . . . . . . . . . . . . 30 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31 97 1. Introduction 99 Low power and Lossy Networks typically operate with strict resource 100 constraints in communication, computation, memory, and energy. Such 101 resource constraints may preclude the use of existing IPv6 multicast 102 routing and forwarding mechanisms. Traditional IP multicast delivery 103 typically relies on topology maintenance mechanisms to discover and 104 maintain routes to all subscribers of a multicast group. However, 105 maintaining such topologies in LLNs is costly and may not be feasible 106 given the available resources. 108 Memory constraints may limit devices to maintaining links/routes to 109 one or a few neighbors. For this reason, the Routing Protocol for 110 LLNs (RPL) specifies both storing and non-storing modes [RFC6550]. 111 The latter allows RPL routers to maintain only one or a few default 112 routes towards a LLN Border Router (LBR) and use source routing to 113 forward messages away from the LBR. For the same reasons, a LLN 114 device may not be able to maintain a multicast routing topology when 115 operating with limited memory. 117 Furthermore, the dynamic properties of wireless networks can make the 118 cost of maintaining a multicast routing topology prohibitively 119 expensive. In wireless environments, topology maintenance may 120 involve selecting a connected dominating set used to forward 121 multicast messages to all nodes in an administrative domain. 122 However, existing mechanisms often require two-hop topology 123 information and the cost of maintaining such information grows 124 polynomially with network density. 126 This document specifies the Multicast Protocol for Low power and 127 Lossy Networks (MPL), which provides IPv6 multicast forwarding in 128 constrained networks. MPL avoids the need to construct or maintain 129 any multicast routing topology, disseminating multicast messages to 130 all MPL Forwarders in an MPL Domain. By using the Trickle algorithm 131 [RFC6206], MPL requires only small, constant state for each MPL 132 device that initiates disseminations. The Trickle algorithm also 133 allows MPL to be density-aware, allowing the communication rate to 134 scale logarithmically with density. 136 2. Terminology 138 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 139 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 140 "OPTIONAL" in this document are to be interpreted as described in 141 [RFC2119]. 143 The following terms are used throughout this document: 145 MPL Forwarder - A router that implements this protocol. An MPL 146 Forwarder is equipped with at least one MPL 147 Interface. 149 MPL Interface - An MPL Forwarder's attachment to a 150 communications medium, over which it transmits 151 and receives MPL Data Messages and MPL Control 152 Messages according to this specification. An MPL 153 Interface is assigned one or more unicast 154 addresses and is subscribed to one or more MPL 155 Domain Addresses. 157 MPL Domain Address - A multicast address that identifies the set of 158 MPL Interfaces within an MPL Domain. MPL Data 159 Messages disseminated in an MPL Domain have the 160 associated MPL Domain Address as their 161 destination address. 163 MPL Domain - A scope zone, as defined in [RFC4007], in which 164 MPL Interfaces subscribe to the same MPL Domain 165 Address and participate in disseminating MPL Data 166 Messages. 168 MPL Data Message - A multicast message that is used to communicate 169 a multicast payload between MPL Forwarders within 170 an MPL domain. An MPL Data Message contains an 171 MPL Option in the IPv6 header and has as its 172 destination address the MPL Domain Address 173 corresponding to the MPL Domain. 175 MPL Control Message - A link-local multicast message that is used to 176 communicate information about recently received 177 MPL Data Messages to neighboring MPL Forwarders. 179 MPL Seed - An MPL Forwarder that generates MPL Data 180 Messages and serves as an entry point into an MPL 181 Domain. 183 MPL Seed Identifier - An unsigned integer that uniquely identifies an 184 MPL Seed within an MPL Domain. 186 3. Applicability Statement 188 This protocol is an IPv6 multicast forwarding protocol for Low-Power 189 and Lossy Networks. By implementing a controlled dissemination using 190 the Trickle algorithm, this protocol is designed for networks that 191 communicate using low-power and lossy links with widely varying 192 topologies in both the space and time dimensions. 194 4. Protocol Overview 196 The goal of MPL is to deliver multicast messages to all interfaces 197 that subscribe to the multicast messages' destination address within 198 an MPL Domain. 200 4.1. Information Base Overview 202 A node records necessary protocol state in the following information 203 sets: 205 o The Local Interface Set records the set of local MPL Interfaces 206 and the unicast addresses assigned to those MPL Interfaces. 208 o The Domain Set records the set of MPL Domain Addresses and the 209 local MPL Interfaces that subscribe to those addresses. 211 o A Seed Set records information about received MPL Data Messages 212 received from an MPL Seed within an MPL Domain. Each MPL Domain 213 has an associated Seed Set. A Seed Set maintains the minimum 214 sequence number for MPL Data Messages that the MPL Forwarder is 215 willing to receive or has buffered in its Buffered Message Set 216 from an MPL Seed. MPL uses Seed Sets and Buffered Message Sets to 217 determine when to accept an MPL Data Message, process its payload, 218 and retransmit it. 220 o A Buffered Message Set records recently received MPL Data Messages 221 from an MPL Seed within an MPL Domain. Each MPL Domain has an 222 associated Buffered Message Set. MPL Data Messages resident in a 223 Buffered Message Set have sequence numbers that are greater than 224 or equal to the minimum threshold maintained in the corresponding 225 Seed Set. MPL uses Buffered Message Sets to store MPL Data 226 Messages that may be transmitted by the MPL Forwarder for 227 forwarding. 229 4.2. Overview 231 MPL achieves its goal by implementing a controlled flood that 232 attempts to disseminate the multicast data message to all interfaces 233 within an MPL Domain. MPL performs the following tasks to 234 disseminate a multicast message: 236 o When having a multicast message to forward into an MPL Domain, the 237 MPL Seed generates an MPL Data Message that includes the MPL 238 Domain Address as the IPv6 Destination Address, the MPL Seed 239 Identifier, a newly generated sequence number, and the multicast 240 message. If the multicast destination address is not the MPL 241 Domain Address, IP-in-IP [RFC2473] is used to encapsulate the 242 multicast message in an MPL Data Message, preserving the original 243 IPv6 Destination Address. 245 o Upon receiving an MPL Data Message, the MPL Forwarder extracts the 246 MPL Seed and sequence number and determines whether or not the MPL 247 Data Message was previously received using the MPL Domain's Seed 248 Set and Buffered Message Set. 250 * If the sequence number is less than the lower-bound sequence 251 number maintained in the Seed Set or a message with the same 252 sequence number exists within the Buffered Message Set, the MPL 253 Forwarder marks the MPL Data Message as old. 255 * Otherwise, the MPL Forwarder marks the MPL Data Message as new. 257 o For each newly received MPL Data Message, an MPL Forwarder updates 258 the Seed Set, adds the MPL Data Message into the Buffered Message 259 Set, processes its payload, and multicasts the MPL Data Message a 260 number of times on all MPL Interfaces participating in the same 261 MPL Domain to forward the message. 263 o Each MPL Forwarder may periodically link-local multicast MPL 264 Control Messages on MPL Interfaces to communicate information 265 contained in an MPL Domain's Seed Set and Buffered Message Set. 267 o Upon receiving an MPL Control Message, an MPL Forwarder determines 268 whether there are any new MPL Data Messages that have yet to be 269 received by the MPL Control Message's source and multicasts those 270 MPL Data Messages. 272 MPL's configuration parameters allow two forwarding strategies for 273 disseminating MPL Data Messages. 275 Proactive Forwarding - With proactive forwarding, an MPL Forwarder 276 schedules transmissions of MPL Data Messages using the Trickle 277 algorithm, without any prior indication that neighboring nodes 278 have yet to receive the message. After transmitting the MPL Data 279 Message a limited number of times, the MPL Forwarder may terminate 280 proactive forwarding for the MPL Data Message message. 282 Reactive Forwarding - With reactive forwarding, an MPL Forwarder 283 link-local multicasts MPL Control Messages using the Trickle 284 algorithm [RFC6206]. MPL Forwarders use MPL Control Messages to 285 discover new MPL Data Messages that have not yet been received. 286 When discovering that a neighboring MPL Forwarder has not yet 287 received an MPL Data Message, the MPL Forwarder schedules those 288 MPL Data Messages for transmission using the Trickle algorithm. 290 4.3. Signaling Overview 292 This protocol generates and processes the following messages: 294 MPL Data Message - Generated by an MPL Seed to deliver a multicast 295 message across an MPL Domain. The MPL Data Message's source is an 296 address in the Local Interface Set of the MPL Seed that generated 297 the message and is valid within the MPL Domain. The MPL Data 298 Message's destination is the MPL Domain Address corresponding to 299 the MPL Domain. An MPL Data Message contains: 301 * The Seed Identifier of the MPL Seed that generated the MPL Data 302 Message. 304 * The sequence number of the MPL Seed that generated the MPL Data 305 Message. 307 * The original multicast message. 309 MPL Control Message - Generated by an MPL Forwarder to communicate 310 information contained in an MPL Domain's Seed Set and Buffered 311 Message Set to neighboring MPL Forwarders. An MPL Control Message 312 contains a list of tuples for each entry in the Seed Set. Each 313 tuple contains: 315 * The minimum sequence number maintained in the Seed Set for the 316 MPL Seed. 318 * A bit-vector indicating the sequence numbers of MPL Data 319 Messages resident in the Buffered Message Set for the MPL Seed, 320 where the first bit represents a sequence number equal to the 321 minimum threshold maintained in the Seed Set. 323 * The length of the bit-vector. 325 5. MPL Parameters and Constants 327 This section describes various program and networking parameters and 328 constants used by MPL. 330 5.1. MPL Multicast Addresses 332 MPL makes use of MPL Domain Addresses to identify MPL Interfaces of 333 an MPL Domain. By default, MPL Forwarders subscribe to the 334 ALL_MPL_FORWARDERS multicast address with a scope value of 3 (subnet- 335 local). 337 For each MPL Domain Address that an MPL Interface subscribes to, the 338 MPL Interface MUST also subscribe to the MPL Domain Address with a 339 scope value of 2 (link-local) when reactive forwarding is in use. 340 MPL Forwarders use the link-scoped MPL Domain Address to communicate 341 MPL Control Messages to neighboring (i.e. on-link) MPL Forwarders. 343 5.2. MPL Message Types 345 MPL defines an IPv6 Option for carrying an MPL Seed Identifier and a 346 sequence number within an MPL Data Message. The IPv6 Option Type has 347 value MPL_OPT_TYPE. 349 MPL defines an ICMPv6 Message (MPL Control Message) for communicating 350 information contained in an MPL Domain's Seed Set and Buffered 351 Message Set to neighboring MPL Forwarders. The MPL Control Message 352 has ICMPv6 Type MPL_ICMP_TYPE. 354 5.3. MPL Seed Identifiers 356 MPL uses MPL Seed Identifiers to uniquely identify MPL Seeds within 357 an MPL Domain. For each MPL Domain that the MPL Forwarder serves as 358 an MPL Seed, the MPL Forwarder MUST have an associated MPL Seed 359 Identifier. An MPL Forwarder MAY use the same MPL Seed Identifier 360 across multiple MPL Domains, but the MPL Seed Identifier MUST be 361 unique within each MPL Domain. The mechanism for assigning and 362 verifying uniqueness of MPL Seed Identifiers is not specified in this 363 document. 365 5.4. MPL Forwarder Parameters 367 PROACTIVE_FORWARDING A boolean value that indicates whether the MPL 368 Forwarder should schedule MPL Data Message transmissions after 369 receiving them for the first time. PROACTIVE_FORWARDING has a 370 default value of TRUE. 372 SEED_SET_ENTRY_LIFETIME The minimum lifetime for an entry in the 373 Seed Set. SEED_SET_ENTRY_LIFETIME has a default value of 30 374 minutes. 376 It is RECOMMENDED that all MPL Forwarders use the same values for the 377 MPL Forwarder Parameters above for a given MPL Domain. The mechanism 378 for setting the MPL Forwarder Parameters is not specified within this 379 document. 381 5.5. MPL Trickle Parameters 383 As specified in [RFC6206], a Trickle timer runs for a defined 384 interval and has three configuration parameters: the minimum interval 385 size Imin, the maximum interval size Imax, and a redundancy constant 386 k. 388 This specification defines a fourth Trickle configuration parameter, 389 TimerExpirations, which indicates the number of Trickle timer 390 expiration events that occur before terminating the Trickle 391 algorithm. 393 Each MPL Forwarder uses the following Trickle parameters for MPL Data 394 Message and MPL Control Message transmissions. 396 DATA_MESSAGE_IMIN The minimum Trickle timer interval, as defined in 397 [RFC6206], for MPL Data Message transmissions. DATA_MESSAGE_IMIN 398 has a default value of 10 times the worst-case link-layer latency. 400 DATA MESSAGE_IMAX The maximum Trickle timer interval, as defined in 401 [RFC6206], for MPL Data Message transmissions. DATA_MESSAGE_IMAX 402 has a default value equal to DATA_MESSAGE_IMIN. 404 DATA_MESSAGE_K The redundancy constant, as defined in [RFC6206], for 405 MPL Data Message transmissions. DATA_MESSAGE_K has a default 406 value of 5. 408 DATA_MESSAGE_TIMER_EXPIRATIONS The number of Trickle timer 409 expirations that occur before terminating the Trickle algorithm 410 for MPL Data Message transmissions. 411 DATA_MESSAGE_TIMER_EXPIRATIONS has a default value of 3. 413 CONTROL_MESSAGE_IMIN The minimum Trickle timer interval, as defined 414 in [RFC6206], for MPL Control Message transmissions. 415 CONTROL_MESSAGE_IMIN has a default value of 10 times the worst- 416 case link-layer latency. 418 CONTROL_MESSAGE_IMAX The maximum Trickle timer interval, as defined 419 in [RFC6206], for MPL Control Message transmissions. 420 CONTROL_MESSAGE_IMAX has a default value of 5 minutes. 422 CONTROL_MESSAGE_K The redundancy constant, as defined in [RFC6206], 423 for MPL Control Message transmissions. CONTROL_MESSAGE_K has a 424 default value of 1. 426 CONTROL_MESSAGE_TIMER_EXPIRATIONS The number of Trickle timer 427 expirations that occur before terminating the Trickle algorithm 428 for MPL Control Message transmissions. 429 CONTROL_MESSAGE_TIMER_EXPIRATIONS has a default value of 10. 431 Following [RFC6206], it is RECOMMENDED that all MPL Forwarders use 432 the same values for the Trickle Parameters above for a given MPL 433 Domain. The mechanism for setting the Trickle Parameters is not 434 specified within this document. 436 6. Protocol Message Formats 438 The protocol messages generated and processed by an MPL Forwarder are 439 described in this section. 441 6.1. MPL Option 443 The MPL Option is carried in MPL Data Messages in an IPv6 Hop-by-Hop 444 Options header, immediately following the IPv6 header. The MPL 445 Option has the following format: 447 0 1 2 3 448 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 1 449 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 450 | Option Type | Opt Data Len | 451 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 452 | S |M|V| rsv | sequence | seed-id (optional) | 453 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 455 Option Type MPL_OPT_TYPE 457 Opt Data Len Length of the Option Data field in octets. 459 S 2-bit unsigned integer. Identifies the length of 460 seed-id. 0 indicates that the seed-id is the IPv6 461 Source Address and not included in the MPL 462 Option. 1 indicates that the seed-id is a 16-bit 463 unsigned integer. 2 indicates that the seed-id is 464 a 64-bit unsigned integer. 3 indicates that the 465 seed-id is a 128-bit unsigned integer. 467 M 1-bit flag. 1 indicates that the value in 468 sequence is known to be the largest sequence 469 number that was received from the MPL Seed. 471 V 1-bit flag. 0 indicates that the MPL Option 472 conforms to this specification. MPL Data 473 Messages with an MPL Option in which this flag is 474 1 MUST be dropped. 476 rsv 4-bit reserved field. MUST be set to 0 on 477 transmission and ignored on reception. 479 sequence 8-bit unsigned integer. Identifies relative 480 ordering of MPL Data Messages from the MPL Seed 481 identified by seed-id. 483 seed-id Uniquely identifies the MPL Seed that initiated 484 dissemination of the MPL Data Message. The size 485 of seed-id is indicated by the S field. 487 The Option Data (in particular the M flag) of the MPL Option is 488 updated by MPL Forwarders as the MPL Data Message is forwarded. 489 Nodes that do not understand the MPL Option MUST discard the MPL Data 490 Message. Thus, according to [RFC2460] the three high order bits of 491 the Option Type are set to '011'. The Option Data length is 492 variable. 494 The seed-id uniquely identifies an MPL Seed. When seed-id is 128 495 bits (S=3), the MPL seed MAY use an IPv6 address assigned to one of 496 its interfaces that is unique within the MPL Domain. Managing MPL 497 Seed Identifiers is not within scope of this document. 499 The sequence field establishes a total ordering of MPL Data Messages 500 generated by an MPL Seed for an MPL Domain. The MPL Seed MUST 501 increment the sequence field's value on each new MPL Data Message 502 that it generates for an MPL Domain. Implementations MUST follow the 503 Serial Number Arithmetic as defined in [RFC1982] when incrementing a 504 sequence value or comparing two sequence values. 506 Future updates to this specification may define additional fields 507 following the seed-id field. 509 6.2. MPL Control Message 511 An MPL Forwarder uses ICMPv6 messages to communicate information 512 contained in an MPL Domain's Seed Set and Buffered Message Set to 513 neighboring MPL Forwarders. The MPL Control Message has the 514 following format: 516 0 1 2 3 517 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 1 518 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 519 | Type | Code | Checksum | 520 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 521 | | 522 . MPL Seed Info[1..n] . 523 . . 524 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 526 IP Fields: 528 Source Address An IPv6 address in the AddressSet of the 529 corresponding MPL Interface and MUST be valid 530 within the MPL Domain. 532 Destination Address The link-scoped MPL Domain Address corresponding 533 to the MPL Domain. 535 Hop Limit 255 537 ICMPv6 Fields: 539 Type MPL_ICMP_TYPE 541 Code 0 543 Checksum The ICMP checksum. See [RFC4443]. 545 MPL Seed Info[0..n] List of zero or more MPL Seed Info entries. 547 The MPL Control Message indicates the sequence numbers of MPL Data 548 Messages that are within the MPL Domain's Buffered Message Set. The 549 MPL Control Message also indicates the sequence numbers of MPL Data 550 Messages that an MPL Forwarder is willing to receive. The MPL 551 Control Message allows neighboring MPL Forwarders to determine 552 whether there are any new MPL Data Messages to exchange. 554 6.3. MPL Seed Info 556 An MPL Seed Info encodes the minimum sequence number for an MPL Seed 557 maintained in the MPL Domain's Seed Set. The MPL Seed Info also 558 indicates the sequence numbers of MPL Data Messages generated by the 559 MPL Seed that are stored within the MPL Domain's Buffered Message 560 Set. The MPL Seed Info has the following format: 562 0 1 2 3 563 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 1 564 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 565 | min-seqno | bm-len | S | seed-id (0/2/8/16 octets) | 566 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 567 | | 568 . buffered-mpl-messages (variable length) . 569 . . 570 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 572 min-seqno 8-bit unsigned integer. The lower-bound sequence 573 number for the MPL Seed. 575 bm-len 6-bit unsigned integer. The size of buffered- 576 mpl-messages in octets. 578 S 2-bit unsigned integer. Identifies the length of 579 seed-id. 0 indicates that the seed-id value is 580 the IPv6 Source Address and not included in the 581 MPL Seed Info. 1 indicates that the seed-id value 582 is a 16-bit unsigned integer. 2 indicates that 583 the seed-id value is a 64-bit unsigned integer. 3 584 indicates that the seed-id is a 128-bit unsigned 585 integer. 587 seed-id Variable-length unsigned integer. Indicates the 588 MPL Seed associated with this MPL Seed Info. 590 buffered-mpl-messages Variable-length bit vector. Identifies the 591 sequence numbers of MPL Data Messages maintained 592 in the corresponding Buffered Message Set for the 593 MPL Seed. The i'th bit represents a sequence 594 number of min-seqno + i. '0' indicates that the 595 corresponding MPL Data Message does not exist in 596 the Buffered Message Set. '1' indicates that the 597 corresponding MPL Data Message does exist in the 598 Buffered Message Set. 600 The MPL Seed Info does not have any octet alignment requirement. 602 7. Information Base 604 7.1. Local Interface Set 606 The Local Interface Set records the local MPL Interfaces of an MPL 607 Forwarder. The Local Interface Set consists of Local Interface 608 Tuples, one per MPL Interface: (AddressSet). 610 AddressSet - a set of unicast addresses assigned to the MPL 611 Interface. 613 7.2. Domain Set 615 The Domain Set records the MPL Interfaces that subscribe to each MPL 616 Domain Address. The Domain Set consists of MPL Domain Tuples, one 617 per MPL Domain: (MPLInterfaceSet). 619 MPLInterfaceSet - a set of MPL Interfaces that subscribe to the MPL 620 Domain Address that identifies the MPL Domain. 622 7.3. Seed Set 624 A Seed Set records a sliding window used to determine the sequence 625 numbers of MPL Data Messages that an MPL Forwarder is willing to 626 accept generated by the MPL Seed. An MPL Forwarder maintains a Seed 627 Set for each MPL Domain that it participates in. A Seed Set consists 628 of MPL Seed Tuples: (SeedID, MinSequence, Lifetime). 630 SeedID - the identifier for the MPL Seed. 632 MinSequence - a lower-bound sequence number that represents the 633 sequence number of the oldest MPL Data Message the MPL Forwarder 634 is willing to receive or transmit. An MPL Forwarder MUST ignore 635 any MPL Data Message that has sequence value less than than 636 MinSequence. 638 Lifetime - indicates the minimum remaining lifetime of the Seed Set 639 entry. An MPL Forwarder MUST NOT free a Seed Set entry before the 640 remaining lifetime expires. 642 7.4. Buffered Message Set 644 A Buffered Message Set records recently received MPL Data Messages 645 from an MPL Seed within an MPL Domain. An MPL Forwarder uses a 646 Buffered Message Set to buffer MPL Data Messages while the MPL 647 Forwarder is forwarding the MPL Data Messages. An MPL Forwarder 648 maintains a Buffered Message Set for each MPL Domain that it 649 participates in. A Buffered Message Set consists of Buffered Message 650 Tuples: (SeedID, SequenceNumber, DataMessage). 652 SeedID - the identifier for the MPL Seed that generated the MPL Data 653 Message. 655 SequenceNumber - the sequence number for the MPL Data Message. 657 DataMessage - the MPL Data Message. 659 All MPL Data Messages within a Buffered Message Set MUST have a 660 sequence number greater than or equal to MinSequence for the 661 corresponding SeedID. When increasing MinSequence for an MPL Seed, 662 the MPL Forwarder MUST delete any MPL Data Messages from the 663 corresponding Buffered Message Set that have sequence numbers less 664 than MinSequence. 666 8. MPL Domains 668 An MPL Domain is a scope zone, as defined in [RFC4007], in which MPL 669 Interfaces subscribe to the same MPL Domain Address and participate 670 in disseminating MPL Data Messages. 672 By default, an MPL Forwarder SHOULD participate in an MPL Domain 673 identified by the ALL_MPL_FORWARDERS multicast address with a scope 674 value of 3 (subnet-local). 676 An MPL Forwarder MAY participate in additional MPL Domains identified 677 by other multicast addresses. An MPL Interface MUST subscribe to the 678 MPL Domain Addresses for the MPL Domains that it participates in. 679 The assignment of other multicast addresses is out of scope. 681 For each MPL Domain Address that an MPL Interface subscribes to, the 682 MPL Interface MUST also subscribe to the same MPL Domain Address with 683 a scope value of 2 (link-local) when reactive forwarding is in use 684 (i.e. when communicating MPL Control Messages). 686 9. MPL Seed Sequence Numbers 688 Each MPL Seed maintains a sequence number for each MPL Domain that it 689 serves. The sequence numbers are included in MPL Data Messages 690 generated by the MPL Seed. The MPL Seed MUST increment the sequence 691 number for each MPL Data Message that it generates for an MPL Domain. 692 Implementations MUST follow the Serial Number Arithmetic as defined 693 in [RFC1982] when incrementing a sequence value or comparing two 694 sequence values. This sequence number is used to establish a total 695 ordering of MPL Data Messages generated by an MPL Seed for an MPL 696 Domain. 698 10. MPL Data Messages 700 10.1. MPL Data Message Generation 702 MPL Data Messages are generated by MPL Seeds when these messages 703 enter the MPL Domain. All MPL Data messages have the following 704 properties: 706 o The IPv6 Source Address MUST be an address in the AddressSet of a 707 corresponding MPL Interface and MUST be valid within the MPL 708 Domain. 710 o The IPv6 Destination Address MUST be set to the MPL Domain Address 711 corresponding to the MPL Domain. 713 o An MPL Data Message MUST contain an MPL Option in its IPv6 Header 714 to identify the MPL Seed that generated the message and the 715 ordering relative to other MPL Data Messages generated by the MPL 716 Seed. 718 When the source address is in the AddressList of an MPL Interface 719 corresponding to the MPL Domain Address and the destination address 720 is the MPL Domain Address, the application message and the MPL Data 721 Message MAY be identical. In other words, the MPL Data Message may 722 contain a single IPv6 header that includes the MPL Option. 724 Otherwise, IPv6-in-IPv6 encapsulation MUST be used to satisfy the MPL 725 Data Message requirements listed above [RFC2473]. The complete IPv6- 726 in-IPv6 message forms an MPL Data Message. The outer IPv6 header 727 conforms to the MPL Data Message requirements listed above. The 728 encapsulated IPv6 datagram encodes the multicast data message that is 729 communicated beyond the MPL Domain. 731 10.2. MPL Data Message Transmission 733 An MPL Forwarder manages transmission of MPL Data Messages in its 734 Buffered Message Sets using the Trickle algorithm [RFC6206]. An MPL 735 Forwarder MUST use a separate Trickle timer for each MPL Data Message 736 that it is actively forwarding. In accordance with Section 5 of RFC 737 6206 [RFC6206], this document defines the following: 739 o This document defines a "consistent" transmission as receiving an 740 MPL Data Message that has the same MPL Domain Address, seed-id, 741 and sequence value as the MPL Data Message managed by the Trickle 742 timer. 744 o This document defines an "inconsistent" transmission as receiving 745 an MPL Data Message that has the same MPL Domain Address, seed-id 746 value, and the M flag set, but has a sequence value less than MPL 747 Data Message managed by the Trickle timer. 749 o This document does not define any external "events". 751 o This document defines MPL Data Messages as Trickle messages. 753 o The actions outside the Trickle algorithm that the protocol takes 754 involve managing the MPL Domain's Seed Set and Buffered Message 755 Set. 757 As specified in [RFC6206], a Trickle timer has three variables: the 758 current interval size I, a time within the current interval t, and a 759 counter c. MPL defines a fourth variable, e, which counts the number 760 of Trickle timer expiration events since the Trickle timer was last 761 reset. 763 After DATA_MESSAGE_TIMER_EXPIRATIONS Trickle timer events, the MPL 764 Forwarder MUST disable the Trickle timer. When a buffered MPL Data 765 Message does not have an associated Trickle timer, the MPL Forwarder 766 MAY delete the message from the Buffered Message Set by advancing 767 MinSequence of the corresponding MPL Seed in the Seed Set. When the 768 MPL Forwarder no longer buffers any messages for an MPL Seed, the MPL 769 Forwarder MUST NOT increment MinSequence for that MPL Seed. 771 When transmitting an MPL Data Message, the MPL Forwarder MUST either 772 set the M flag to zero or set it to a level that indicates whether or 773 not the message's sequence number is the largest value that has been 774 received from the MPL Seed. 776 10.3. MPL Data Message Processing 778 Upon receiving an MPL Data Message, the MPL Forwarder first processes 779 the MPL Option and updates the Trickle timer associated with the MPL 780 Data Message if one exists. 782 Upon receiving an MPL Data Message, an MPL Forwarder MUST perform one 783 of the following actions: 785 o Accept the message and enter the MPL Data Message in the MPL 786 Domain's Buffered Message Set. 788 o Accept the message and update the corresponding MinSequence in the 789 MPL Domain's Seed Set to 1 greater than the message's sequence 790 number. 792 o Discard the message without any change to the MPL Information 793 Base. 795 If a Seed Set entry exists for the MPL Seed, the MPL Forwarder MUST 796 discard the MPL Data Message if its sequence number is less than 797 MinSequence or exists in the Buffered Message Set. 799 If a Seed Set entry does not exist for the MPL Seed, the MPL 800 Forwarder MUST create a new entry for the MPL Seed before accepting 801 the MPL Data Message. 803 If memory is limited, an MPL Forwarder SHOULD reclaim memory 804 resources by: 806 o Incrementing MinSequence entries in a Seed Set and deleting MPL 807 Data Messages in the corresponding Buffered Message Set that fall 808 below the MinSequence value. 810 o Deleting other Seed Set entries that have expired and the 811 corresponding MPL Data Messages in the Buffered Message Set. 813 If the MPL Forwarder accepts the MPL Data Message, the MPL Forwarder 814 MUST perform the following actions: 816 o Reset the Lifetime of the corresponding Seed Set entry to 817 SEED_SET_ENTRY_LIFETIME. 819 o If PROACTIVE_FORWARDING is true, the MPL Forwarder MUST initialize 820 and start a Trickle timer for the MPL Data Message. 822 o If the MPL Control Message Trickle timer is not running and 823 CONTROL_MESSAGE_TIMER_EXPIRATIONS is non-zero, the MPL Forwarder 824 MUST initialize and start the MPL Control Message Trickle timer. 826 o If the MPL Control Message Trickle timer is running, the MPL 827 Forwarder MUST reset the MPL Control Message Trickle timer. 829 11. MPL Control Messages 831 11.1. MPL Control Message Generation 833 An MPL Forwarder generates MPL Control Messages to communicate an MPL 834 Domain's Seed Set and Buffered Message Set to neighboring MPL 835 Forwarders. Each MPL Control Message is generated according to 836 Section 6.2, with an MPL Seed Info for each entry in the MPL Domain's 837 Seed Set. Each MPL Seed Info entry has the following content: 839 o S set to the size of the seed-id field in the MPL Seed Info entry. 841 o min-seqno set to MinSequence of the MPL Seed. 843 o bm-len set to the size of buffered-mpl-messages in octets. 845 o seed-id set to the MPL seed identifier. 847 o buffered-mpl-messages with each bit representing whether or not an 848 MPL Data Message with the corresponding sequence number exists in 849 the Buffered Message Set. The i'th bit represents a sequence 850 number of min-seqno + i. '0' indicates that the corresponding MPL 851 Data Message does not exist in the Buffered Message Set. '1' 852 indicates that the corresponding MPL Data Message does exist in 853 the Buffered Message Set. 855 11.2. MPL Control Message Transmission 857 An MPL Forwarder transmits MPL Control Messages using the Trickle 858 algorithm. An MPL Forwarder maintains a single Trickle timer for 859 each MPL Domain. When CONTROL_MESSAGE_TIMER_EXPIRATIONS is 0, the 860 MPL Forwarder does not execute the Trickle algorithm and does not 861 transmit MPL Control Messages. In accordance with Section 5 of RFC 862 6206 [RFC6206], this document defines the following: 864 o This document defines a "consistent" transmission as receiving an 865 MPL Control Message that indicates neither the receiving nor 866 transmitting node has any new MPL Data Messages to offer. 868 o This document defines an "inconsistent" transmission as receiving 869 an MPL Control Message that indicates either the receiving or 870 transmitting node has at least one new MPL Data Message to offer. 872 o This document defines an "event" as increasing MinSequence of any 873 entry in the corresponding Seed Set or adding a message to the 874 corresponding Buffered Message Set. 876 o This document defines an MPL Control Message as a Trickle message. 878 As specified in [RFC6206], a Trickle timer has three variables: the 879 current interval size I, a time within the current interval t, and a 880 counter c. MPL defines a fourth variable, e, which counts the number 881 of Trickle timer expiration events since the Trickle timer was last 882 reset. After CONTROL_MESSAGE_TIMER_EXPIRATIONS Trickle timer events, 883 the MPL Forwarder MUST disable the Trickle timer. 885 11.3. MPL Control Message Processing 887 An MPL Forwarder processes each MPL Control Message that it receives 888 to determine if it has any new MPL Data Messages to receive or offer. 890 An MPL Forwarder determines if a new MPL Data Message has not been 891 received from a neighboring node if any of the following conditions 892 hold true: 894 o The MPL Control Message includes an MPL Seed that does not exist 895 in the MPL Domain's Seed Set. 897 o The MPL Control Message indicates that the neighbor has an MPL 898 Data Message in its Buffered Message Set with sequence number 899 greater than MinSequence (i.e. the i-th bit is set to 1 and min- 900 seqno + i > MinSequence) and is not included in the MPL Domain's 901 Buffered Message Set. 903 When an MPL Forwarder determines that it has not yet received an MPL 904 Data Message buffered by a neighboring device, the MPL Forwarder MUST 905 reset its Trickle timer associated with MPL Control Message 906 transmissions. If an MPL Control Message Trickle timer is not 907 running, the MPL Forwarder MUST initialize and start a new Trickle 908 timer. 910 An MPL Forwarder determines if an MPL Data Message in the Buffered 911 Message Set has not yet been received by a neighboring MPL Forwarder 912 if any of the following conditions hold true: 914 o The MPL Control Message does not include an MPL Seed for the MPL 915 Data Message. 917 o The MPL Data Message's sequence number is greater than or equal to 918 min-seqno and not included in the neighbor's corresponding 919 Buffered Message Set (i.e. the MPL Data Message's sequence number 920 does not have a corresponding bit in buffered-mpl-messages set to 921 1). 923 When an MPL Forwarder determines that it has at least one MPL Data 924 Message in its corresponding Buffered Message Set that has not yet 925 been received by a neighbor, the MPL Forwarder MUST reset the MPL 926 Control Message Trickle timer. Additionally, for each of those 927 entries in the Buffered Message Set, the MPL Forwarder MUST reset the 928 Trickle timer and reset e to 0. If a Trickle timer is not associated 929 with the MPL Data Message, the MPL Forwarder MUST initialize and 930 start a new Trickle timer. 932 12. Acknowledgements 934 The authors would like to acknowledge the helpful comments of Robert 935 Cragie, Esko Dijk, Ralph Droms, Paul Duffy, Ulrich Herberg, Owen 936 Kirby, Joseph Reddy, Don Sturek, Dario Tedeschi, and Peter van der 937 Stok, which greatly improved the document. 939 13. IANA Considerations 941 This document defines one IPv6 Option, a type that must be allocated 942 from the IPv6 "Destination Options and Hop-by-Hop Options" registry 943 of [RFC2780]. 945 This document defines one ICMPv6 Message, a type that must be 946 allocated from the "ICMPv6 "type" Numbers" registry of [RFC4443]. 948 This document registers two well-known multicast addresses from the 949 IPv6 multicast address space. 951 13.1. MPL Option Type 953 IANA is requested to allocate an IPv6 Option Type from the IPv6 954 "Destination Options and Hop-by-Hop Options" registry of [RFC2780], 955 as specified in Table 1 below: 957 +--------------+-----+-----+--------------+-------------+-----------+ 958 | Mnemonic | act | chg | rest | Description | Reference | 959 +--------------+-----+-----+--------------+-------------+-----------+ 960 | MPL_OPT_TYPE | 01 | 1 | TBD | MPL Option | This | 961 | | | | (suggested | | Document | 962 | | | | value 01101) | | | 963 +--------------+-----+-----+--------------+-------------+-----------+ 965 Table 1: IPv6 Option Type Allocation 967 13.2. MPL ICMPv6 Type 969 IANA is requested to allocate an ICMPv6 Type from the "ICMPv6 "type" 970 Numbers" registry of [RFC4443], as specified in Table 2 below: 972 +---------------+------+---------------------+---------------+ 973 | Mnemonic | Type | Name | Reference | 974 +---------------+------+---------------------+---------------+ 975 | MPL_ICMP_TYPE | TBD | MPL Control Message | This Document | 976 +---------------+------+---------------------+---------------+ 978 Table 2: IPv6 Option Type Allocation 980 13.3. Well-known Multicast Addresses 982 IANA is requested to allocate an IPv6 multicast address, with Group 983 ID in the range [0x01,0xFF] for 6LoWPAN compression [RFC6282], 984 "ALL_MPL_FORWARDERS" from the "Variable Scope Multicast Addresses" 985 sub-registry of the "INTERNET PROTOCOL VERSION 6 MULTICAST ADDRESSES" 986 registry. 988 14. Security Considerations 990 MPL uses sequence numbers to maintain a total ordering of MPL Data 991 Messages from an MPL Seed. The use of sequence numbers allows a 992 denial-of-service attack where an attacker can spoof a message with a 993 sufficiently large sequence number to: (i) flush messages from the 994 Buffered Message List and (ii) increase the MinSequence value for an 995 MPL Seed in the corresponding Seed Set. The former side effect allows 996 an attacker to halt the forwarding process of any MPL Data Messages 997 being disseminated. The latter side effect allows an attacker to 998 prevent MPL Forwarders from accepting new MPL Data Messages that an 999 MPL Seed generates while the sequence number is less than 1000 MinSequence. 1002 More generally, the basic ability to inject messages into a Low-power 1003 and Lossy Network can be used as a denial-of-service attack 1004 regardless of what forwarding protocol is used. For these reasons, 1005 Low-power and Lossy Networks typically employ link-layer security 1006 mechanisms to disable an attacker's ability to inject messages. 1008 To prevent attackers from injecting packets through an MPL Forwarder, 1009 the MPL Forwarder MUST NOT accept or forward MPL Data Messages from a 1010 communication interface that does not subscribe to the MPL Domain 1011 Address identified in message's destination address. 1013 MPL uses the Trickle algorithm to manage message transmissions and 1014 the security considerations described in [RFC6206] apply. 1016 15. Normative References 1018 [RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982, 1019 August 1996. 1021 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1022 Requirement Levels", BCP 14, RFC 2119, March 1997. 1024 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 1025 (IPv6) Specification", RFC 2460, December 1998. 1027 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in 1028 IPv6 Specification", RFC 2473, December 1998. 1030 [RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For 1031 Values In the Internet Protocol and Related Headers", 1032 BCP 37, RFC 2780, March 2000. 1034 [RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and 1035 B. Zill, "IPv6 Scoped Address Architecture", RFC 4007, 1036 March 2005. 1038 [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control 1039 Message Protocol (ICMPv6) for the Internet Protocol 1040 Version 6 (IPv6) Specification", RFC 4443, March 2006. 1042 [RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko, 1043 "The Trickle Algorithm", RFC 6206, March 2011. 1045 [RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6 1046 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 1047 September 2011. 1049 [RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., 1050 Levis, P., Pister, K., Struik, R., Vasseur, JP., and R. 1051 Alexander, "RPL: IPv6 Routing Protocol for Low-Power and 1052 Lossy Networks", RFC 6550, March 2012. 1054 Authors' Addresses 1056 Jonathan W. Hui 1057 Cisco 1058 170 West Tasman Drive 1059 San Jose, California 95134 1060 USA 1062 Phone: +408 424 1547 1063 Email: jonhui@cisco.com 1065 Richard Kelsey 1066 Silicon Labs 1067 25 Thomson Place 1068 Boston, Massachusetts 02210 1069 USA 1071 Phone: +617 951 1225 1072 Email: richard.kelsey@silabs.com