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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 6TiSCH P. Thubert, Ed. 3 Internet-Draft Cisco 4 Intended status: Standards Track November 10, 2018 5 Expires: May 14, 2019 7 An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4 8 draft-ietf-6tisch-architecture-17 10 Abstract 12 This document describes a network architecture that provides low- 13 latency, low-jitter and high-reliability packet delivery. It 14 combines a high speed powered backbone and subnetworks using IEEE 15 802.15.4 time-slotted channel hopping (TSCH) to meet the requirements 16 of LowPower wireless deterministic applications. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at https://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on May 14, 2019. 35 Copyright Notice 37 Copyright (c) 2018 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (https://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 53 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 54 2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 4 55 2.2. 6TiSCH Terminology . . . . . . . . . . . . . . . . . . . 4 56 2.3. References . . . . . . . . . . . . . . . . . . . . . . . 9 57 2.4. Subset of a 6LoWPAN Glossary . . . . . . . . . . . . . . 10 58 3. High Level Architecture . . . . . . . . . . . . . . . . . . . 11 59 3.1. 6TiSCH Stack . . . . . . . . . . . . . . . . . . . . . . 11 60 3.2. TSCH: A Deterministic MAC Layer . . . . . . . . . . . . . 13 61 3.3. Scheduling TSCH . . . . . . . . . . . . . . . . . . . . . 14 62 3.4. Routing and Forwarding Over TSCH . . . . . . . . . . . . 15 63 3.5. A Non-Broadcast Multi-Access Radio Mesh Network . . . . . 17 64 3.6. A Multi-Link Subnet Model . . . . . . . . . . . . . . . . 18 65 3.7. Join Process and Registration . . . . . . . . . . . . . . 20 66 4. Architecture Components . . . . . . . . . . . . . . . . . . . 22 67 4.1. 6LoWPAN (and RPL) . . . . . . . . . . . . . . . . . . . . 22 68 4.1.1. RPL Leaf Support in 6LoWPAN ND . . . . . . . . . . . 22 69 4.1.2. RPL Root And 6LBR . . . . . . . . . . . . . . . . . . 23 70 4.2. TSCH and 6top . . . . . . . . . . . . . . . . . . . . . . 24 71 4.2.1. 6top . . . . . . . . . . . . . . . . . . . . . . . . 24 72 4.2.2. Scheduling Functions and the 6P protocol . . . . . . 24 73 4.2.3. 6top and RPL Objective Function operations . . . . . 25 74 4.2.4. Network Synchronization . . . . . . . . . . . . . . . 26 75 4.2.5. SlotFrames and Priorities . . . . . . . . . . . . . . 27 76 4.2.6. Distributing the reservation of cells . . . . . . . . 28 77 4.3. Communication Paradigms and Interaction Models . . . . . 30 78 4.4. Schedule Management Mechanisms . . . . . . . . . . . . . 31 79 4.4.1. Static Scheduling . . . . . . . . . . . . . . . . . . 31 80 4.4.2. Neighbor-to-neighbor Scheduling . . . . . . . . . . . 32 81 4.4.3. Remote Monitoring and Schedule Management . . . . . . 32 82 4.4.4. Hop-by-hop Scheduling . . . . . . . . . . . . . . . . 35 83 4.5. On Tracks . . . . . . . . . . . . . . . . . . . . . . . . 35 84 4.5.1. General Behavior of Tracks . . . . . . . . . . . . . 35 85 4.5.2. Serial Track . . . . . . . . . . . . . . . . . . . . 36 86 4.5.3. Complex Track with Replication and Elimination . . . 37 87 4.5.4. DetNet End-to-end Path . . . . . . . . . . . . . . . 37 88 4.5.5. Cell Reuse . . . . . . . . . . . . . . . . . . . . . 38 89 4.6. Forwarding Models . . . . . . . . . . . . . . . . . . . . 39 90 4.6.1. Track Forwarding . . . . . . . . . . . . . . . . . . 39 91 4.6.2. IPv6 Forwarding . . . . . . . . . . . . . . . . . . . 42 92 4.6.3. Fragment Forwarding . . . . . . . . . . . . . . . . . 42 93 4.7. Distributed vs. Centralized Routing . . . . . . . . . . . 44 94 4.7.1. Packet Marking and Handling . . . . . . . . . . . . . 44 95 4.7.2. Replication, Retries and Elimination . . . . . . . . 45 96 4.7.3. Differentiated Services Per-Hop-Behavior . . . . . . 46 97 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46 98 6. Security Considerations . . . . . . . . . . . . . . . . . . . 46 99 6.1. Join Process Highlights . . . . . . . . . . . . . . . . . 47 100 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 49 101 7.1. Contributors . . . . . . . . . . . . . . . . . . . . . . 49 102 7.2. Special Thanks . . . . . . . . . . . . . . . . . . . . . 50 103 7.3. And Do not Forget . . . . . . . . . . . . . . . . . . . . 50 104 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 51 105 8.1. Normative References . . . . . . . . . . . . . . . . . . 51 106 8.2. Informative References . . . . . . . . . . . . . . . . . 52 107 8.3. Other Informative References . . . . . . . . . . . . . . 58 108 Appendix A. Dependencies on Work In Progress . . . . . . . . . . 59 109 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 61 111 1. Introduction 113 Wireless Networks enable a wide variety of devices of any size to get 114 interconnected, often at a very low marginal cost per device, at any 115 distance ranging from Near Field to interplanetary, and in 116 circumstances where wiring may be impractical, for instance on fast- 117 moving or rotating devices. 119 In the other hand, Deterministic Networks enable traffic that is 120 highly sensitive to jitter, quite sensitive to latency, and with a 121 high degree of operational criticality so that loss should be 122 minimized at all times. Applications that need such networks are 123 presented in [I-D.ietf-detnet-use-cases]. They include Professional 124 Media and Operation Technology (OT) Industrial Automation Control 125 Systems (IACS). 127 The Medium access Control (MAC) of IEEE Std 802.15.4 [IEEE802154] has 128 evolved with the IEEE Std 802.15.4e timeslotted Channel Hopping 129 (TSCH) [RFC7554] mode to provide deterministic properties on wireless 130 networks. TSCH was initially introduced with the IEEE Std 802.15.4e 131 amendment [IEEE802154e] of the IEEE Std 802.15.4 standard and 132 constituted a part of the standard from that day. For all practical 133 purpose, this document is expected to be insensitive to the revisions 134 of the IEEE Std 802.15.4 standard, which is thus referenced undated. 136 Proven Deterministic Networking standards for use in Process Control, 137 including ISA100.11a [ISA100.11a] and WirelessHART [WirelessHART], 138 have demonstrated the capabilities of the IEEE Std 802.15.4 TSCH MAC 139 for high reliability against interference, low-power consumption on 140 well-known flows, and its applicability for Traffic Engineering (TE) 141 from a central controller. 143 In order to enable the convergence of IT and OT in LLN environments, 144 6TiSCH ports the IETF suite of protocols that are defined for such 145 environments over the TSCH MAC. 6TiSCH also provides large scaling 146 capabilities, which, in a number of scenarios, require the addition 147 of a high speed and reliable backbone and the use of IP version 6 148 (IPv6). The 6TiSCH Architecture introduces an IPv6 Multi-Link subnet 149 model that is composed of a federating backbone and a number of IEEE 150 Std 802.15.4 TSCH low-power wireless networks attached and 151 synchronized by Backbone Routers. 153 The architecture defines mechanisms to establish and maintain routing 154 and scheduling in a centralized, distributed, or mixed fashion, for 155 use in multiple OT environments. It is applicable in particular to 156 industrial control systems, building automation that leverage 157 distributed routing to address multipath over a large number of hops, 158 in-vehicle command and control that can be as demanding as industrial 159 applications, commercial automation and asset Tracking with mobile 160 scenarios, home automation and domotics which become more reliable 161 and thus provide a better user experience, and resource management 162 (energy, water, etc.). 164 2. Terminology 166 2.1. BCP 14 168 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 169 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 170 "OPTIONAL" in this document are to be interpreted as described in BCP 171 14 [RFC2119][RFC8174] when, and only when, they appear in all 172 capitals, as shown here. 174 2.2. 6TiSCH Terminology 176 The draft does not reuse terms from the IEEE Std 802.15.4 177 [IEEE802154] standard such as "path" or "link" which bear a meaning 178 that is quite different from classical IETF parlance. 180 This document adds the following terms: 182 6TiSCH (IPv6 over the TSCH mode of IEEE 802.15.4e): 6TiSCH defines 183 an adaptation sublayer for IPv6 over TSCH called 6top, a 184 set of protocols for setting up a TSCH schedule in 185 distributed approach, and a security solution. 6TiSCH may 186 be extended in the future for other MAC/PHY pairs 187 providing a service similar to TSCH. 189 6top (6TiSCH Operation Sublayer): The next highest layer of the IEEE 190 Std 802.15.4 TSCH medium access control layer. It 191 implements and terminates 6P, and contains at least one 192 SF. 194 6P (6top Protocol): Allows neighbor nodes to communicate to add/ 195 delete cells to one another in their TSCH schedule. 197 6P Transaction: Part of 6P, the action of two neighbors exchanging a 198 6P request message and the corresponding 6P response 199 message. 201 ASN (Absolute Slot Number): The total number of timeslots that have 202 elapsed since the PAN coordinator has started the TSCH 203 network. Incremented by one at each timeslot. It is 204 wide enough to not roll over in practice. 206 blacklist of frequencies: A set of frequencies which should not be 207 used for communication. 209 broadcast cell: A scheduled cell used for broadcast transmission. 211 bundle: A group of equivalent scheduled cells, i.e. cells 212 identified by different [slotOffset, channelOffset], 213 which are scheduled for a same purpose, with the same 214 neighbor, with the same flags, and the same slotframe. 215 The size of the bundle refers to the number of cells it 216 contains. For a given slotframe length, the size of the 217 bundle translates directly into bandwidth. A bundle is a 218 local abstraction that represents a half-duplex link for 219 either sending or receiving, with bandwidth that amounts 220 to the sum of the cells in the bundle. 222 CCA (Clear Channel Assessment): Mechanism defined in [IEEE802154], 223 section 6.2.5.2. In a TSCH network, CCA can be used to 224 detect other radio networks in vicinity. Nodes listen 225 the channel before sending, to detect other ongoing 226 transmissions. Because the network is synchronized, CCA 227 cannot be used to detect colliding transmission within 228 the same network. 230 cell: A single element in the TSCH schedule, identified by a 231 slotOffset, a channelOffset, a slotframeHandle. A cell 232 can be scheduled or unscheduled. 234 centralized cell reservation: A reservation of a cell done by a 235 centralized entity (e.g., a PCE) in the network. 237 centralized track reservation: A reservation of a track done by a 238 centralized entity (e.g., a PCE) in the network. 240 Channel Distribution/Usage (CDU) matrix: : Matrix of cells (i,j) 241 representing the spectrum (channel) distribution among 242 the different nodes in the 6TiSCH network. The CDU 243 matrix has width in timeslots, equal to the period of the 244 network scheduling operation, and height equal to the 245 number of available channels. Every cell (i,j) in the 246 CDU, identified by (slotOffset, channelOffset), belongs 247 to a specific chunk. It has to be noticed that such a 248 matrix which includes all the cells grouped in chunks, 249 belonging to different slotframes, is different from the 250 TSCH schedule. 252 channelOffset: Identifies a row in the TSCH schedule. The number of 253 available channelOffset values is equal to the number of 254 available frequencies. The channelOffset translates into 255 a frequency when the communication takes place, resulting 256 in channel hopping. 258 chunk: A well-known list of cells, distributed in time and 259 frequency, within a CDU matrix. A chunk represents a 260 portion of a CDU matrix. The partition of the CDU matrix 261 in chunks is globally known by all the nodes in the 262 network to support the appropriation process, which is a 263 negotiation between nodes within an interference domain. 264 A node that manages to appropriate a chunk gets to decide 265 which transmissions will occur over the cells in the 266 chunk within its interference domain (i.e., a parent node 267 will decide when the cells within the appropriated chunk 268 are used and by which node, among its children. 270 dedicated cell: A cell that is reserved for a given node to transmit 271 to a specific neighbor. 273 deterministic network: The generic concept of deterministic network 274 is defined in [I-D.ietf-detnet-architecture]. When 275 applied to 6TiSCH, it refers to the reservation of tracks 276 which guarantee an end-to-end latency and optimize the 277 PDR for well-characterized flows. 279 distributed cell reservation: A reservation of a cell done by one or 280 more in-network entities. 282 distributed track reservation: A reservation of a track done by one 283 or more in-network entities. 285 EB (Enhanced Beacon): A special frame defined used by a node, 286 including the JP, to announce the presence of the 287 network. It contains enough information for a pledge to 288 synchronize to the network. 290 hard cell: A scheduled cell which the 6top sublayer cannot relocate. 292 hopping sequence: Ordered sequence of frequencies, identified by a 293 Hopping_Sequence_ID, used for channel hopping when 294 translating the channel offset value into a frequency. 296 IE (Information Element): Type-Length-Value containers placed at the 297 end of the MAC header, used to pass data between layers 298 or devices. Some IE identifiers are managed by the IEEE 299 [IEEE802154]. Some IE identifiers are managed by the 300 IETF [I-D.kivinen-802-15-ie]. 302 join process: The overall process that includes the discovery of the 303 network by pledge(s) and the execution of the join 304 protocol. 306 join protocol: The protocol that allows the pledge to join the 307 network. The join protocol encompasses authentication, 308 authorization and parameter distribution. The join 309 protocol is executed between the pledge and the JRC. 311 joined node: The new device, after having completed the join 312 process, often just called a node. 314 JP (Join Proxy): Node already part of the 6TiSCH network that serves 315 as a relay to provide connectivity between the pledge and 316 the JRC. The JP announces the presence of the network by 317 regularly sending EB frames. 319 JRC (Join Registrar/Coordinator): Central entity responsible for the 320 authentication, authorization and configuration of the 321 pledge. 323 link: A communication facility or medium over which nodes can 324 communicate at the link layer, the layer immediately 325 below IP. The IETF parlance for the term "Link" is 326 adopted, as opposed to the IEEE Std 802.15.4 terminology. 328 pledge: A new device that attempts to join a 6TiSCH network. 330 (to) relocate a cell: The action operated by the 6top sublayer of 331 changing the slotOffset and/or channelOffset of a soft 332 cell. 334 (to) schedule a cell: The action of turning an unscheduled cell into 335 a scheduled cell. 337 scheduled cell: A cell which is assigned a neighbor MAC address 338 (broadcast address is also possible), and one or more of 339 the following flags: TX, RX, shared, timeskeeping. A 340 scheduled cell can be used by the IEEE Std 802.15.4 TSCH 341 implementation to communicate. A scheduled cell can 342 either be a hard or a soft cell. 344 SF (6top Scheduling Function): The cell management entity that adds 345 or deletes cells dynamically based on application 346 networking requirements. The cell negotiation with a 347 neighbor is done using 6P. 349 SFID (6top Scheduling Function Identifier): A 4-bit field 350 identifying an SF. 352 shared cell: A cell marked with both the "TX" and "shared" flags. 353 This cell can be used by more than one transmitter node. 354 A back-off algorithm is used to resolve contention. 356 slotframe: A collection of timeslots repeating in time, analogous to 357 a superframe in that it defines periods of communication 358 opportunities. It is characterized by a slotframe_ID, 359 and a slotframe_size. Multiple slotframes can coexist in 360 a node's schedule, i.e., a node can have multiple 361 activities scheduled in different slotframes, based on 362 the priority of its packets/traffic flows. The timeslots 363 in the Slotframe are indexed by the SlotOffset; the first 364 timeslot is at SlotOffset 0. 366 slotOffset: A column in the TSCH schedule, i.e. the number of 367 timeslots since the beginning of the current iteration of 368 the slotframe. 370 soft cell: A scheduled cell which the 6top sublayer can relocate. 372 time source neighbor: A neighbor that a node uses as its time 373 reference, and to which it needs to keep its clock 374 synchronized. 376 timeslot: A basic communication unit in TSCH which allows a 377 transmitter node to send a frame to a receiver neighbor, 378 and that receiver neighbor to optionally send back an 379 acknowledgment. 381 Track: A determined sequence of cells along a multi-hop path. 382 It is typically the result of a track reservation. The 383 node that initializes the process of establishing a track 384 is the owner of the track. The latter assigns a unique 385 identifier to the track, called TrackID. 387 TrackID: Unique identifier of a track. 389 TSCH: A medium access mode of the IEEE Std 802.15.4 390 [IEEE802154] standard which uses time synchronization to 391 achieve ultra low-power operation, and channel hopping to 392 enable high reliability. 394 TSCH Schedule: A matrix of cells, each cell indexed by a slotOffset 395 and a channelOffset. The TSCH schedule contains all the 396 scheduled cells from all slotframes and is sufficient to 397 qualify the communication in the TSCH network. The 398 number of channelOffset values (the "height" of the 399 matrix) is equal to the number of available frequencies. 401 Unscheduled Cell: A cell which is not used by the IEEE Std 802.15.4 402 TSCH implementation. 404 2.3. References 406 The draft uses domain-specific terminology defined or referenced in: 408 "Neighbor Discovery Optimization for Low-power and Lossy Networks" 409 [RFC6775], 411 "Registration Extensions for 6LoWPAN Neighbor Discovery" 412 [I-D.ietf-6lo-rfc6775-update], 414 "Terms Used in Routing for Low-Power and Lossy Networks (LLNs)" 415 [RFC7102], 417 "Objective Function Zero for the Routing Protocol for Low-Power 418 and Lossy Networks (RPL)" [RFC6552], and 420 "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks" 421 [RFC6550]. 423 Other terms in use in LLNs are found in "Terminology for Constrained- 424 Node Networks" [RFC7228]. 426 Readers are expected to be familiar with all the terms and concepts 427 that are discussed in 429 o "Neighbor Discovery for IP version 6" [RFC4861], 431 o "IPv6 Stateless Address Autoconfiguration" [RFC4862], 432 o "Problem Statement and Requirements for IPv6 over Low-Power 433 Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606]. 435 The draft also conforms to the terms and models described in 436 [RFC3444] and [RFC5889] and uses the vocabulary and the concepts 437 defined in [RFC4291] for the IPv6 Architecture and refers [RFC4080] 438 for reservation 440 In addition, readers would benefit from reading: 442 o "Multi-Link Subnet Issues" [RFC4903], 444 o "Mobility Support in IPv6" [RFC6275], 446 o "RPL applicability in industrial networks" 447 [I-D.ietf-roll-rpl-industrial-applicability], 449 o "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs): 450 Overview, Assumptions, Problem Statement, and Goals" [RFC4919]. 452 o "Optimistic Duplicate Address Detection" [RFC4429], 454 o "Neighbor Discovery Proxies (ND Proxy)" [RFC4389], 456 o "FCFS SAVI: First-Come, First-Served Source Address Validation 457 Improvement for Locally Assigned IPv6 Addresses" [RFC6620], and 459 o "Optimistic Duplicate Address Detection" [RFC4429] 461 prior to this specification for a clear understanding of the art in 462 ND-proxying and binding. 464 2.4. Subset of a 6LoWPAN Glossary 466 This document often uses the following acronyms: 468 6BBR: 6LoWPAN Backbone Router (router with a proxy ND function) 470 6LBR: 6LoWPAN Border Router (authoritative on DAD) 472 6LN: 6LoWPAN Node 474 6LR: 6LoWPAN Router (relay to the registration process) 476 6CIO: Capability Indication Option 478 (E)ARO: (Extended) Address Registration Option 479 (E)DAR: (Extended) Duplicate Address Request 481 (E)DAC: (Extended) Duplicate Address Confirmation 483 DAD: Duplicate Address Detection 485 DODAG: Destination-Oriented Directed Acyclic Graph 487 LLN: Low-Power and Lossy Network (a typical IoT network) 489 NA: Neighbor Advertisement 491 NCE: Neighbor Cache Entry 493 ND: Neighbor Discovery 495 NDP: Neighbor Discovery Protocol 497 NS: Neighbor Solicitation 499 ROVR: Registration Ownership Verifier (pronounced rover) 501 RPL: IPv6 Routing Protocol for LLNs (pronounced ripple) 503 RA: Router Advertisement 505 RS: Router Solicitation 507 TSCH: timeslotted Channel Hopping 509 TID: Transaction ID (a sequence counter in the EARO) 511 3. High Level Architecture 513 3.1. 6TiSCH Stack 515 The 6TiSCH architecture presents a reference stack that is 516 implemented and interop tested by a conjunction of opensource, IETF 517 and ETSI efforts. One goal is to help other bodies to adopt the 518 stack as a whole, making the effort to move to an IPv6-based IOT 519 stack easier. Now, for a particular environment, some of the choices 520 that are made in this architecture may not be relevant. For 521 instance, RPL is not required for star topologies and mesh-under 522 Layer-2 routed networks, and the 6LoWPAN compression may not be 523 sufficient for ultra-constrained cases such as some Low Power Wide 524 Area (LPWA) networks. In such cases, it is perfectly doable to adopt 525 a subset of the selection that is presented hereafter and then select 526 alternate components to complete the solution wherever needed. 528 The IETF proposes multiple techniques for implementing functions 529 related to routing, transport or security. In order to control the 530 complexity of the possible deployments and device interactions, and 531 to limit the size of the resulting object code, the architecture 532 limits the possible variations of the stack and recommends a number 533 of base elements for LLN applications. In particular, UDP [RFC0768] 534 [RFC8200] and the Constrained Application Protocol [RFC7252] (CoAP) 535 are used as the transport / binding of choice for applications and 536 management as opposed to TCP and HTTP. 538 The resulting protocol stack is represented below: 540 +-----+-----+-----+------+-------+-----+ 541 | CoAP/OSCORE | 6LoWPAN ND | RPL | 542 +-----+-----+-----+------+-------+-----+ 543 | UDP | ICMPv6 | 544 +-----+-----+-----+-----+-------+------+ 545 | IPv6 | 546 +--------------------------------------+----------------------+ 547 | 6LoWPAN HC / 6LoRH HC | Scheduling Functions | 548 +--------------------------------------+----------------------+ 549 | 6top (to be IEEE Std 802.15.12) inc. 6top protocol | 550 +-------------------------------------------------------------+ 551 | IEEE Std 802.15.4 TSCH | 552 +-------------------------------------------------------------+ 554 Figure 1: 6TiSCH Protocol Stack 556 RPL is the routing protocol of choice for LLNs. So far, there was no 557 identified need to define a 6TiSCH specific Objective Function. The 558 Minimal 6TiSCH Configuration [RFC8180] describes the operation of RPL 559 over a static schedule used in a slotted aloha fashion, whereby all 560 active slots may be used for emission or reception of both unicast 561 and multicast frames. 563 The 6LoWPAN Header Compression [RFC6282] is used to compress the IPv6 564 and UDP headers, whereas the 6LoWPAN Routing Header (6LoRH) [RFC8138] 565 is used to compress the RPL artifacts in the IPv6 data packets, 566 including the RPL Packet Information (RPI), the IP-in-IP 567 encapsulation to/from the RPL root, and the Source Route Header (SRH) 568 in non-storing mode. 570 The Datagram Transport Layer Security (DTLS) [RFC6347] sitting either 571 under CoAP or over CoAP so as to traverse proxies, as well as Object 572 Security for Constrained RESTful Environments (OSCORE) 573 [I-D.ietf-core-object-security], are examples of protocols that could 574 be used to protect application payload, and OSCORE is used in 575 particular by the "Minimal Security Framework for 6TiSCH" 576 [I-D.ietf-6tisch-minimal-security] for the the Join Process. 578 An overview of the the initial steps of a device in a network can be 579 found in Section 3.7; the security aspects of the join process are 580 further detailed in Section 6. 582 The 6TiSCH Operation sublayer (6top) is a sublayer of a Logical Link 583 Control (LLC) that provides the abstraction of an IP link over a TSCH 584 MAC and schedules packets over TSCH cells, as further discussed in 585 the next sections, providing in particular dynamic cell allocation 586 with the 6top Protocol (6P) [RFC8480]. 588 3.2. TSCH: A Deterministic MAC Layer 590 Though at a different time scale (several orders of magnitude), both 591 IEEE Std 802.1TSN and IEEE Std 802.15.4 TSCH standards provide 592 Deterministic capabilities to the point that a packet that pertains 593 to a certain flow may traverse a network from node to node following 594 a very precise schedule, as a train that enters and then leaves 595 intermediate stations at precise times along its path. With TSCH, 596 time is formatted into timeslots, and individual communication cells 597 are allocated to unicast or broadcast communication at the MAC level. 598 The time-slotted operation reduces collisions, saves energy, and 599 enables to more closely engineer the network for deterministic 600 properties. The channel hopping aspect is a simple and efficient 601 technique to combat multipath fading and co-channel interference. 603 6TiSCH builds on the IEEE Std 802.15.4 TSCH MAC and inherits its 604 advanced capabilities to enable them in multiple environments where 605 they can be leveraged to improve automated operations. The 6TiSCH 606 Architecture also inherits the capability to perform a centralized 607 route computation to achieve deterministic properties, though it 608 relies on the IETF DetNet Architecture 609 [I-D.ietf-detnet-architecture], and IETF components such as the Path 610 Computation Element (PCE) [PCE], for the protocol aspects. 612 On top of this inheritance, 6TiSCH adds capabilities for distributed 613 routing and scheduling operations based on the RPL routing protocol 614 and capabilities to negotiate schedule adjustments between peers. 615 These distributed routing and scheduling operations simplify the 616 deployment of TSCH networks and enable wireless solutions in a larger 617 variety of use cases from operational technology in general. 618 Examples of such use-cases in industrial environments include plant 619 setup and decommissioning, as well as monitoring of lots of lesser 620 importance measurements such as corrosion and events. RPL also 621 enables mobile use cases such as mobile workers and cranes, as 622 presented in [I-D.ietf-roll-rpl-industrial-applicability]. 624 3.3. Scheduling TSCH 626 A scheduling operation attributes cells in a Time-Division- 627 Multiplexing (TDM) / Frequency-Division Multiplexing (FDM) matrix 628 called the Channel distribution/usage (CDU) to either individual 629 transmissions or as multi-access shared resources (see the 630 Section 2.2 for more on these terms). Scheduling effectively enables 631 multiple communications at a same time in a same interference domain 632 using different channels; but a node equipped with a single radio can 633 only transmit or receive on one channel at any given point of time. 635 From the standpoint of a 6TiSCH node (at the MAC layer), its schedule 636 is the collection of the times at which it must wake up for 637 transmission, and the channels to which it should either send or 638 listen at those times. The schedule is expressed as one or more 639 slotframes that repeat over and over. Slotframes may collide and 640 require a device to wake up at a same time, in which case a priority 641 indicates which slotframe is actually activated. 643 The 6top sublayer hides the complexity of the schedule from the upper 644 layers. The Link that IP may utilize between the 6TiSCH node and a 645 peer may in fact be composed of a pair of cell bundles, one to 646 receive and one to transmit. Some of the cells may be shared, in 647 which case the 6top sublayer must perform some arbitration. 649 The 6TiSCH architecture identifies four ways a schedule can be 650 managed and CDU cells can be allocated: Static Scheduling, Neighbor- 651 to-Neighbor Scheduling, Remote Monitoring and Schedule Management, 652 and Hop-by-hop Scheduling. 654 Static Scheduling: This refers to the minimal 6TiSCH operation 655 whereby a static schedule is configured for the whole network for 656 use in a slotted-Aloha fashion. The static schedule is 657 distributed through the native methods in the TSCH MAC layer. 658 This operation leverages RPL to maintain a loopless graph for 659 routing and time distribution. It is specified in the Minimal 660 6TiSCH Configuration [RFC8180] specification. and does not 661 preclude other scheduling operations to co-exist on a same 6TiSCH 662 network. 664 Neighbor-to-Neighbor Scheduling: This refers to the dynamic 665 adaptation of the bandwidth of the Links that are used for IPv6 666 traffic between adjacent routers. Scheduling Functions such as 667 the "6TiSCH Minimal Scheduling Function (MSF)" 668 [I-D.ietf-6tisch-msf] influence the operation of the MAC layer to 669 add, update and remove cells in peers schedule, using 6P [RFC8480] 670 for the negotiation of the MAC resources. 672 Remote Monitoring and Schedule Management: This refers to the 673 central computation of a schedule and the capability to forward a 674 frame based on the cell of arrival. In that case, the related 675 portion of the device schedule as well as other device resources 676 are managed by an abstract Network Management Entity (NME), which 677 may cooperate with the PCE in order to minimize the interaction 678 with and the load on the constrained device. This model is the 679 TSCH adaption of the "DetNet Architecture" 680 [I-D.ietf-detnet-architecture], and it enables Traffic Engineering 681 with deterministic properties. 683 Hop-by-hop Scheduling: This refers to the possibility to reserves 684 cells along a path for a particular flow using a distributed 685 mechanism. 687 It is not expected that all use cases will require all those 688 mechanisms. Static Scheduling with minimal configuration one is the 689 only one that is expected in all implementations, since it provides a 690 simple and solid basis for convergecast routing and time 691 distribution. 693 A deeper dive in those mechanisms can be found in Section 4.4. 695 3.4. Routing and Forwarding Over TSCH 697 6TiSCH leverages the RPL routing protocol for interoperable 698 distributed routing operations. RPL is applicable to Static 699 Scheduling and Neighbor-to-Neighbor Scheduling. The architecture 700 also supports a centralized routing model for Remote Monitoring and 701 Schedule Management. It is expected that a routing protocol that is 702 more optimized for point-to-point routing than RPL [RFC6550], such as 703 the "Asymmetric AODV-P2P-RPL in Low-Power and Lossy Networks" 704 [I-D.ietf-roll-aodv-rpl] (AODV-RPL), which derives from the Ad Hoc 705 On-demand Distance Vector Routing (AODV) [I-D.ietf-manet-aodvv2] will 706 be selected for Hop-by-hop Scheduling. 708 The 6TiSCH architecture supports three different forwarding models, 709 the classical IPv6 Forwarding, where the node selects a feasible 710 successor at Layer-3 on a per packet basis and based on its routing 711 table, G-MPLS Track Forwarding, which switches a frame received at a 712 particular timeslot into another timeslot at Layer-2, and 6LoWPAN 713 Fragment Forwarding, which allows to forward individual 6loWPAN 714 fragments along the route set by the first fragment. 716 IPv6 Forwarding: This is the classical IP forwarding model, with a 717 Routing Information Based (RIB) that is installed by the RPL 718 routing protocol and used to select a feasible successor per 719 packet. The packet is placed on an outgoing Link, that the 6top 720 layer maps into a (Layer-3) bundle of cells, and scheduled for 721 transmission based on QoS parameters. On top of RPL, this model 722 also applies to any routing protocol which may be operated in the 723 6TiSCH network, and corresponds to all the distributed scheduling 724 models, Static, Neighbor-to-Neighbor and Hop-by-Hop Scheduling. 726 G-MPLS Track Forwarding: This model corresponds to the Remote 727 Monitoring and Schedule Management. In this model, A central 728 controller (hosting a PCE) computes and installs the schedules in 729 the devices per flow. The incoming (Layer-2) bundle of cells from 730 the previous node along the path determines the outgoing (Layer-2) 731 bundle towards the next hop for that flow as determined by the 732 PCE. The programmed sequence for bundles is called a Track and 733 can assume shapes that are more complex than a simple direct 734 sequence of nodes. 736 6LoWPAN Fragment Forwarding: This is an hybrid model that derives 737 from IPv6 forwarding for the case where packets must be fragmented 738 at the 6LoWPAN sublayer. The first fragment is forwarded like any 739 IPv6 packet and leaves a state in the intermediate hops to enable 740 forwarding of the next fragments that do not have a IP header 741 without the need to recompose the packet at every hop. 743 This can be broadly summarized in the following table: 745 +---------------------+------------+-----------------------------------+ 746 | Forwarding Model | Routing | Scheduling | 747 +=====================+============+===================================+ 748 | | | Static (Minimal Configuration) | 749 + classical IPv6 + RPL +-----------------------------------+ 750 | / | | Neighbor-to-Neighbor (SF+6P) | 751 + 6LoWPAN Fragment F. +------------+-----------------------------------+ 752 | |Reactive P2P| Hop-by-Hop (TBD) | 753 +---------------------+------------+-----------------------------------+ 754 |G-MPLS Track Fwrding | PCE |Remote Monitoring and Schedule Mgt | 755 +---------------------+------------+-----------------------------------+ 757 Figure 2: Routing, Forwarding and Scheduling 759 3.5. A Non-Broadcast Multi-Access Radio Mesh Network 761 A 6TiSCH network is an IPv6 [RFC8200] subnet which, in its basic 762 configuration, is a single Low Power Lossy Network (LLN) operating 763 over a synchronized TSCH-based mesh. 765 Inside a 6TiSCH LLN, nodes rely on 6LoWPAN Header Compression 766 (6LoWPAN HC) [RFC6282] to encode IPv6 packets. From the perspective 767 of the network layer, a single LLN interface (typically an IEEE Std 768 802.15.4-compliant radio) may be seen as a collection of Links with 769 different capabilities for unicast or multicast services. 771 6TiSCH nodes are not necessarily reachable from one another at 772 Layer-2 and an LLN may span over multiple links. This effectively 773 forms an homogeneous non-broadcast multi-access (NBMA) subnet, which 774 is beyond the scope of existing IPv6 ND methods. Extensions to IPv6 775 ND have to be introduced. 777 Within that subnet, neighbor devices are discovered with 6LoWPAN 778 Neighbor Discovery [RFC6775] (6LoWPAN ND), whereas RPL [RFC6550] 779 enables routing in the so called Route Over fashion, either in 780 storing (stateful) or non-storing (stateless, with routing headers) 781 mode. 783 ---+-------- ............ ------------ 784 | External Network | 785 | +-----+ 786 +-----+ | NME | 787 | | LLN Border | | 788 | | router +-----+ 789 +-----+ 790 o o o 791 o o o o o 792 o o 6LoWPAN + RPL o o 793 o o o o 794 o o 796 Figure 3: Basic Configuration of a 6TiSCH Network 798 6TiSCH nodes join the mesh by attaching to nodes that are already 799 members of the mesh. Some nodes act as routers for 6LoWPAN ND and 800 RPL operations, as detailed in Section 4.1. Security aspects of the 801 join process by which a device obtains access to the network are 802 discussed in Section 6. 804 With TSCH, devices are time-synchronized at the MAC level. The use 805 of a particular RPL Instance for time synchronization is discussed in 806 Section 4.2.4. With this mechanism, the time synchronization starts 807 at the RPL root and follows the RPL DODAGs with no timing loop. 809 RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs) 810 within Instances of the protocol, each Instance being associated with 811 an Objective Function (OF) to form a routing topology. A particular 812 6TiSCH node, the LLN Border Router (6LBR), acts as RPL root, 6LoWPAN 813 HC terminator, and Border Router for the LLN to the outside. The 814 6LBR is usually powered. More on RPL Instances can be found in 815 section 3.1 of RPL [RFC6550], in particular "3.1.2. RPL Identifiers" 816 and "3.1.3. Instances, DODAGs, and DODAG Versions". RPL adds 817 artifacts in the data packets that are compressed with a 6LoWPAN 818 addition 6LoRH [RFC8138]. 820 Additional routing and scheduling protocols may be deployed to 821 establish on-demand Peer-to-Peer routes with particular 822 characteristics inside the 6TiSCH network. This may be achieved in a 823 centralized fashion by a PCE [PCE] that programs both the routes and 824 the schedules inside the 6TiSCH nodes, or by in a distributed fashion 825 using a reactive routing protocol and a Hop-by-Hop scheduling 826 protocol. 828 A Backbone Router may be connected to the node that acts as RPL root 829 and / or 6LoWPAN 6LBR and provides connectivity to the larger campus 830 / factory plant network over a high speed backbone or a back-haul 831 link. A Backbone Router may perform proxy IPv6 Neighbor Discovery 832 (ND) [RFC4861] operations over the backbone on behalf of the 6TiSCH 833 nodes so they can share a same IPv6 subnet and appear to be connected 834 to the same backbone as classical devices. A Backbone Router may 835 alternatively redistribute the registration in a routing protocol 836 such as OSPF [RFC5340] or BGP [RFC2545], or inject them in a mobility 837 protocol such as MIPv6 [RFC6275], NEMO [RFC3963], or LISP [RFC6830]. 839 This architecture expects that a 6LoWPAN node can connect as a leaf 840 to a RPL network, where the leaf support is the minimal functionality 841 to connect as a host to a RPL network without the need to participate 842 to the full routing protocol. The architecture also expects that a 843 6LoWPAN node that is not aware at all of the RPL protocol may also 844 connect as a host but the specifications for this to happen are not 845 available at the time of this writing. 847 3.6. A Multi-Link Subnet Model 849 An extended configuration of the subnet comprises multiple LLNs. The 850 LLNs are interconnected and synchronized over a backbone, that can be 851 wired or wireless. The backbone can be a classical IPv6 network, 852 with Neighbor Discovery operating as defined in [RFC4861] and 854 [RFC4862]. This architecture requires work to standardize the the 855 registration of 6LoWPAN nodes to the Backbone Routers. 857 In the extended configuration, a Backbone Router (6BBR) operates as 858 described in [I-D.ietf-6lo-backbone-router]. The 6BBR performs ND 859 proxy operations between the registered devices and the classical ND 860 devices that are located over the backbone. 6TiSCH 6BBRs synchronize 861 with one another over the backbone, so as to ensure that the multiple 862 LLNs that form the IPv6 subnet stay tightly synchronized. 864 ---+-------- ............ ------------ 865 | External Network | 866 | +-----+ 867 | +-----+ | NME | 868 +-----+ | +-----+ | | 869 | | Router | | PCE | +-----+ 870 | | +--| | 871 +-----+ +-----+ 872 | | 873 | Subnet Backbone | 874 +--------------------+------------------+ 875 | | | 876 +-----+ +-----+ +-----+ 877 | | Backbone | | Backbone | | Backbone 878 o | | router | | router | | router 879 +-----+ +-----+ +-----+ 880 o o o o o 881 o o o o o o o o o o o 882 o o o LLN o o o o 883 o o o o o o o o o o o o 885 Figure 4: Extended Configuration of a 6TiSCH Network 887 As detailed in Section 4.1 the 6LoWPAN ND 6LBR and the root of the 888 RPL network need to be collocated and share information about the 889 devices that is learned through either protocol but not both. The 890 combined RPL root and 6LBR may be collocated with the 6BBR, or 891 directly attached to the 6BBR. In the latter case, it leverages the 892 extended registration process defined in 893 [I-D.ietf-6lo-backbone-router] to proxy the 6LoWPAN ND registration 894 to the 6BBR on behalf of the LLN nodes, so that the 6BBR may in turn 895 perform proxy classical ND operations over the backbone. 897 If the Backbone is Deterministic (such as defined by the Time 898 Sensitive Networking WG at IEEE), then the Backbone Router ensures 899 that the end-to-end deterministic behavior is maintained between the 900 LLN and the backbone. The DetNet Architecture 901 [I-D.ietf-detnet-architecture] studies Layer-3 aspects of 902 Deterministic Networks, and covers networks that span multiple 903 Layer-2 domains. 905 3.7. Join Process and Registration 907 As detailed in Section 6, a node that wishes to join the 6TiSCH 908 network with a preshared key (PSK) performs the role of the pledge in 909 the 6TiSCH Constrained Join Protocol (CoJP) 910 [I-D.ietf-6tisch-minimal-security] In order to join, the pledge is 911 helped by a Join Proxy (JP) that relays the link-scope 6JP Join 912 request over the IP network to the Join Registrar/Coordinator (JRC) 913 that can authenticate the pledge and validate that it is attached to 914 the appropriate network. As a result of this exchange the pledge is 915 in possession of a Link-Layer material including a key and a short 916 address, and all traffic is secured at the Link Layer . 918 Figure 5 illustrates that very initial step. 920 6LoWPAN Node 6LR 6LBR Join Registrar 921 (pledge) (Join Proxy) (root) /Coordinator (JRC) 922 | | | | 923 | 6LoWPAN ND |6LoWPAN ND+RPL | IPv6 network | 924 | LLN link |Route-Over mesh| (the Internet)| 925 | | | | 926 | Layer-2 | | | 927 |enhanced beacon| | | 928 |<--------------| | | 929 <-----------------| | | 930 | <------------| | | 931 | | | | 932 | 6JP Join Req | | | 933 | Link Local @ | | | 934 |-------------->| | | 935 | | 6JP Join Request | 936 | | Global Unicast @ | 937 | |------------------------------>| 938 | | | | 939 | | 6JP Join Response | 940 | | Global Unicast @ | 941 | |<------------------------------| 942 | 6JP Join Resp | | | 943 | Link Local @ | | | 944 |<--------------| | | 945 | | | | 947 Figure 5: (Re-)Registration Flow over Multi-Link Subnet 949 As detailed in Section 4.1, the combined 6LoWPAN ND 6LBR and root of 950 the RPL network learn information such as the device Unique ID (from 951 6LoWPAN ND) and the updated Sequence Number (from RPL), and perform 952 6LoWPAN ND proxy registration to the 6BBR of behalf of the LLN nodes. 954 Figure 6 illustrates the initial IPv6 signaling that eables a 6LN to 955 form a global address and register it to a 6LBR using 6LoWPAN ND 956 [I-D.ietf-6lo-rfc6775-update], is then carried over RPL to the RPL 957 root, and then to the 6BBR. 959 6LoWPAN Node 6LR 6LBR 6BBR 960 (RPL leaf) (router) (root) 961 | | | | 962 | 6LoWPAN ND |6LoWPAN ND+RPL | 6LoWPAN ND | IPv6 ND 963 | LLN link |Route-Over mesh| ant IPv6 link | Backbone 964 | | | | 965 | IPv6 ND RS | | | 966 |-------------->| | | 967 |-----------> | | | 968 |------------------> | | 969 | IPv6 ND RA | | | 970 |<--------------| | | 971 | | | | 972 | NS(EARO) | | | 973 |-------------->| | | 974 | 6LoWPAN ND | Extended DAR | | 975 | |-------------->| | 976 | | | NS(EARO) | 977 | | |-------------->| 978 | | | | DAD (once) 979 | | | |------> 980 | | | | 981 | | | NA(EARO) | 982 | | |<--------------| 983 | | Extended DAC | | 984 | |<--------------| | 985 | NA(EARO) | | | 986 |<--------------| | | 987 | | | | 989 Figure 6: Initial Registration Flow over Multi-Link Subnet 991 Figure 7 illustrates the repeating IPv6 signaling that enables a 6LN 992 to keep a global address alive and registered to its 6LBR using 993 6LoWPAN ND [I-D.ietf-6lo-rfc6775-update], using 6LoWPAN ND ot the 994 6LR, RPL to the RPL root, and then 6LoWPAN ND again to the 6BBR. 996 6LoWPAN Node 6LR 6LBR 6BBR 997 (RPL leaf) (router) (root) 998 | | | | 999 | 6LoWPAN ND |6LoWPAN ND+RPL | 6LoWPAN ND | IPv6 ND 1000 | LLN link |Route-Over mesh| ant IPv6 link | Backbone 1001 | | | | 1002 | | | | 1003 | | | | 1004 | NS(EARO) | | | 1005 |-------------->| | | 1006 | NA(EARO) | | | 1007 |<--------------| | | 1008 | | DAO | | 1009 | |-------------->| | 1010 | | DAO-ACK | | 1011 | |<--------------| | 1012 | | | NS(EARO) | 1013 | | |-------------->| 1014 | | | NA(EARO) | 1015 | | |<--------------| 1016 | | | | 1017 | | | | 1019 Figure 7: Next Registration Flow over Multi-Link Subnet 1021 As the network builds up, a node should start as a leaf to join the 1022 RPL network, and may later turn into both a RPL-capable router and a 1023 6LR, so as to accept leaf nodes to recursively join the network. 1025 4. Architecture Components 1027 4.1. 6LoWPAN (and RPL) 1029 4.1.1. RPL Leaf Support in 6LoWPAN ND 1031 RPL needs a set of information in order to advertise a leaf node 1032 through a DAO message and establish reachability. 1034 At the bare minimum the leaf device must provide a sequence number 1035 that matches the RPL specification in section 7. Section 5.3 of 1036 [I-D.ietf-6lo-backbone-router], on the Extended Address Registration 1037 Option (EARO), already incorporates that addition with a new field in 1038 the option called the Transaction ID. 1040 If for some reason the node is aware of RPL topologies, then 1041 providing the RPL InstanceID for the instances to which the node 1042 wishes to participate would be a welcome addition. In the absence of 1043 such information, the RPL router must infer the proper instanceID 1044 from external rules and policies. 1046 On the backbone, the InstanceID is expected to be mapped onto a an 1047 overlay that matches the instanceID, for instance a VLANID. 1049 This architecture leverages [I-D.ietf-6lo-backbone-router] that 1050 extends 6LoWPAN ND [RFC6775] to carry the counter as an abstract 1051 Transaction ID (TID). 1053 4.1.2. RPL Root And 6LBR 1055 With [RFC6775], information on the 6LBR is disseminated via an 1056 Authoritative Border Router Option (ABRO) in RA messages. The 1057 discovery and liveliness of the RPL root are obtained through the RPL 1058 protocol [RFC6550]. The capability to support the update to RFC6775 1059 [I-D.ietf-6lo-rfc6775-update] is indicated in the 6LoWPAN Capability 1060 Indication Option (6CIO). 1062 "Routing for RPL Leaves" [I-D.thubert-roll-unaware-leaves] details 1063 the basic interaction of 6LoWPAN ND and RPL and enables a plain 6LN 1064 that supports [I-D.ietf-6lo-rfc6775-update] to obtain return 1065 connectivity via the RPL network as a non-RPL-aware leaf. Though the 1066 above specification enables a model where the separation is possible, 1067 this architecture recommends to collocate the functions of 6LBR and 1068 RPL root. 1070 When 6LoWPAN ND is coupled with RPL, the 6LBR and RPL root 1071 functionalities are co-located in order that the address of the 6LBR 1072 be indicated by RPL DIO messages and to associate the unique ID from 1073 the DAR/DAC exchange with the state that is maintained by RPL. The 1074 DAR/DAC exchange becomes a preamble to the DAO messages that are used 1075 from then on to reconfirm the registration, thus eliminating a 1076 duplication of functionality between DAO and DAR messages. 1078 Even though the root of the RPL network is integrated with the 6LBR, 1079 it is logically separated from the Backbone Router (6BBR) that is 1080 used to connect the 6TiSCH LLN to the backbone. This way, the root 1081 has all information from 6LoWPAN ND and RPL about the LLN devices 1082 attached to it. 1084 This architecture also expects that the root of the RPL network 1085 (proxy-)registers the 6TiSCH nodes on their behalf to the 6BBR, for 1086 whatever operation the 6BBR performs on the backbone, such as ND 1087 proxy, or redistribution in a routing protocol. This relies on an 1088 extension of the 6LoWPAN ND registration described in 1089 [I-D.ietf-6lo-backbone-router]. 1091 This model supports the movement of a 6TiSCH device across the Multi- 1092 Link Subnet, and allows the proxy registration of 6TiSCH nodes deep 1093 into the 6TiSCH LLN by the 6LBR / RPL root. This requires an 1094 alteration from [RFC6775] whereby the Target Address of the NS 1095 message is registered as opposed to the Source, which, in the case of 1096 a proxy registration, is that of the 6LBR / RPL root itself. 1098 4.2. TSCH and 6top 1100 4.2.1. 6top 1102 6top is a logical link control sitting between the IP layer and the 1103 TSCH MAC layer, which provides the link abstraction that is required 1104 for IP operations. The 6top operations are specified in [RFC8480]. 1105 In particular, 6top provides a management interface that enables an 1106 external management entity to schedule cells and slotFrames, and 1107 allows the addition of complementary functionality, for instance to 1108 support a dynamic schedule management based on observed resource 1109 usage as discussed in Section 4.4.2. 1111 The 6top data model and management interfaces are further discussed 1112 in Section 4.4.3. 1114 4.2.1.1. Hard Cells 1116 The architecture defines "soft" cells and "hard" cells. "Hard" cells 1117 are owned and managed by an separate scheduling entity (e.g. a PCE) 1118 that specifies the slotOffset/channelOffset of the cells to be 1119 added/moved/deleted, in which case 6top can only act as instructed, 1120 and may not move hard cells in the TSCH schedule on its own. 1122 4.2.1.2. Soft Cells 1124 6top contains a monitoring process which monitors the performance of 1125 cells, and can move a cell in the TSCH schedule when it performs 1126 poorly. This is only applicable to cells which are marked as "soft". 1127 To reserve a soft cell, the higher layer does not indicate the exact 1128 slotOffset/channelOffset of the cell to add, but rather the resulting 1129 bandwidth and QoS requirements. When the monitoring process triggers 1130 a cell reallocation, the two neighbor devices communicating over this 1131 cell negotiate its new position in the TSCH schedule. 1133 4.2.2. Scheduling Functions and the 6P protocol 1135 In the case of soft cells, the cell management entity that controls 1136 the dynamic attribution of cells to adapt to the dynamics of variable 1137 rate flows is called a Scheduling Function (SF). There may be 1138 multiple SFs with more or less aggressive reaction to the dynamics of 1139 the network. The "6TiSCH Minimal Scheduling Function (MSF)" 1140 [I-D.ietf-6tisch-msf] provides a simple scheduling function that can 1141 be used by default by devices that support dynamic scheduling of soft 1142 cells. 1144 The SF may be seen as divided between an upper bandwidth adaptation 1145 logic that is not aware of the particular technology that is used to 1146 obtain and release bandwidth, and an underlying service that maps 1147 those needs in the actual technology, which means mapping the 1148 bandwidth onto cells in the case of TSCH. 1150 +------------------------+ +------------------------+ 1151 | Scheduling Function | | Scheduling Function | 1152 | Bandwidth adaptation | | Bandwidth adaptation | 1153 +------------------------+ +------------------------+ 1154 | Scheduling Function | | Scheduling Function | 1155 | TSCH mapping to cells | | TSCH mapping to cells | 1156 +------------------------+ +------------------------+ 1157 | 6top cells negotiation | <- 6P -> | 6top cells negotiation | 1158 +------------------------+ +------------------------+ 1159 Device A Device B 1161 Figure 8: SF/6P stack in 6top 1163 The SF relies on 6top services that implement the 6top Protocol (6P) 1164 [RFC8480] to negotiate the precise cells that will be allocated or 1165 freed based on the schedule of the peer. It may be for instance that 1166 a peer wants to use a particular time slot that is free in its 1167 schedule, but that timeslot is already in use by the other peer for a 1168 communication with a third party on a different cell. The 6P 1169 protocol enables the peers to find an agreement in a transactional 1170 manner that ensures the final consistency of the nodes state. 1172 4.2.3. 6top and RPL Objective Function operations 1174 An implementation of a RPL [RFC6550] Objective Function (OF), such as 1175 the RPL Objective Function Zero (OF0) [RFC6552] that is used in the 1176 Minimal 6TiSCH Configuration [RFC8180] to support RPL over a static 1177 schedule, may leverage, for its internal computation, the information 1178 maintained by 6top. 1180 Most OFs require metrics about reachability, such as the ETX. 6top 1181 creates and maintains an abstract neighbor table, and this state may 1182 be leveraged to feed an OF and/or store OF information as well. A 1183 neighbor table entry may contain a set of statistics with respect to 1184 that specific neighbor including the time when the last packet has 1185 been received from that neighbor, a set of cell quality metrics (e.g. 1187 RSSI or LQI), the number of packets sent to the neighbor or the 1188 number of packets received from it. This information can be obtained 1189 through 6top management APIs and used for instance to compute a Rank 1190 Increment that will determine the selection of the preferred parent. 1192 6top provides statistics about the underlying layer so the OF can be 1193 tuned to the nature of the TSCH MAC layer. 6top also enables the RPL 1194 OF to influence the MAC behaviour, for instance by configuring the 1195 periodicity of IEEE Std 802.15.4 Extended Beacons (EBs). By 1196 augmenting the EB periodicity, it is possible to change the network 1197 dynamics so as to improve the support of devices that may change 1198 their point of attachment in the 6TiSCH network. 1200 Some RPL control messages, such as the DODAG Information Object (DIO) 1201 are ICMPv6 messages that are broadcast to all neighbor nodes. With 1202 6TiSCH, the broadcast channel requirement is addressed by 6top by 1203 configuring TSCH to provide a broadcast channel, as opposed to, for 1204 instance, piggybacking the DIO messages in Enhance Beacons. 1205 Consideration was given towards finding a way to embed the Route 1206 Advertisements and the RPL DIO messages (both of which are multicast) 1207 into the IEEE Std 802.15.4 Enhanced Beacons. It was determined that 1208 this produced undue timer coupling among layers, that the resulting 1209 packet size was potentially too large, and required it is not yet 1210 clear that there is any need for Enhanced Beacons in a production 1211 network. 1213 4.2.4. Network Synchronization 1215 Nodes in a TSCH network must be time synchronized. A node keeps 1216 synchronized to its time source neighbor through a combination of 1217 frame-based and acknowledgment-based synchronization. In order to 1218 maximize battery life and network throughput, it is advisable that 1219 RPL ICMP discovery and maintenance traffic (governed by the trickle 1220 timer) be somehow coordinated with the transmission of time 1221 synchronization packets (especially with enhanced beacons). This 1222 could be achieved through an interaction of the 6top sublayer and the 1223 RPL objective Function, or could be controlled by a management 1224 entity. 1226 Time distribution requires a loop-free structure. Nodes taken in a 1227 synchronization loop will rapidly desynchronize from the network and 1228 become isolated. It is expected that a RPL DAG with a dedicated 1229 global Instance is deployed for the purpose of time synchronization. 1230 That Instance is referred to as the Time Synchronization Global 1231 Instance (TSGI). The TSGI can be operated in either of the 3 modes 1232 that are detailed in section 3.1.3 of RPL [RFC6550], "Instances, 1233 DODAGs, and DODAG Versions". Multiple uncoordinated DODAGs with 1234 independent roots may be used if all the roots share a common time 1235 source such as the Global Positioning System (GPS). In the absence 1236 of a common time source, the TSGI should form a single DODAG with a 1237 virtual root. A backbone network is then used to synchronize and 1238 coordinate RPL operations between the backbone routers that act as 1239 sinks for the LLN. Optionally, RPL's periodic operations may be used 1240 to transport the network synchronization. This may mean that 6top 1241 would need to trigger (override) the trickle timer if no other 1242 traffic has occurred for such a time that nodes may get out of 1243 synchronization. 1245 A node that has not joined the TSGI advertises a MAC level Join 1246 Priority of 0xFF to notify its neighbors that is not capable of 1247 serving as time parent. A node that has joined the TSGI advertises a 1248 MAC level Join Priority set to its DAGRank() in that Instance, where 1249 DAGRank() is the operation specified in section 3.5.1 of [RFC6550], 1250 "Rank Comparison". 1252 A root is configured or obtains by some external means the knowledge 1253 of the RPLInstanceID for the TSGI. The root advertises its DagRank 1254 in the TSGI, that must be less than 0xFF, as its Join Priority in its 1255 IEEE Std 802.15.4 Extended Beacons (EB). We'll note that the Join 1256 Priority is now specified between 0 and 0x3F leaving 2 bits in the 1257 octet unused in the IEEE Std 802.15.4e specification. After 1258 consultation with IEEE authors, it was asserted that 6TiSCH can make 1259 a full use of the octet to carry an integer value up to 0xFF. 1261 A node that reads a Join Priority of less than 0xFF should join the 1262 neighbor with the lesser Join Priority and use it as time parent. If 1263 the node is configured to serve as time parent, then the node should 1264 join the TSGI, obtain a Rank in that Instance and start advertising 1265 its own DagRank in the TSGI as its Join Priority in its EBs. 1267 4.2.5. SlotFrames and Priorities 1269 6TiSCH enables in essence the capability to use IPv6 over a MAC layer 1270 that enables to schedule the transmissions. In order to ensure that 1271 the medium is free of contending packets when time arrives for a 1272 scheduled transmission, a window of time is defined around the 1273 scheduled transmission time where the medium must be free of 1274 contending energy. 1276 One simple way to obtain such a window is to format time and 1277 frequencies in cells of transmission of equal duration. This is the 1278 method that is adopted in IEEE Std 802.15.4 TSCH as well as the Long 1279 Term Evolution (LTE) of cellular networks. 1281 In order to describe that formatting of time and frequencies, the 1282 6TiSCH architecture defines a global concept that is called a Channel 1283 Distribution and Usage (CDU) matrix; a CDU matrix is a matrix of 1284 cells with an height equal to the number of available channels 1285 (indexed by ChannelOffsets) and a width (in timeslots) that is the 1286 period of the network scheduling operation (indexed by slotOffsets) 1287 for that CDU matrix. The size of a cell is a timeslot duration, and 1288 values of 10 to 15 milliseconds are typical in 802.15.4 TSCH to 1289 accommodate for the transmission of a frame and an ack, including the 1290 security validation on the receive side which may take up to a few 1291 milliseconds on some device architecture. 1293 A CDU matrix iterates over and over with a well-known channel 1294 rotation called the hopping sequence. In a given network, there 1295 might be multiple CDU matrices that operate with different width, so 1296 they have different durations and represent different periodic 1297 operations. It is recommended that all CDU matrices in a 6TiSCH 1298 domain operate with the same cell duration and are aligned, so as to 1299 reduce the chances of interferences from slotted-aloha operations. 1300 The knowledge of the CDU matrices is shared between all the nodes and 1301 used in particular to define slotFrames. 1303 A slotFrame is a MAC-level abstraction that is common to all nodes 1304 and contains a series of timeslots of equal length and precedence. 1305 It is characterized by a slotFrame_ID, and a slotFrame_size. A 1306 slotFrame aligns to a CDU matrix for its parameters, such as number 1307 and duration of timeslots. 1309 Multiple slotFrames can coexist in a node schedule, i.e., a node can 1310 have multiple activities scheduled in different slotFrames, based on 1311 the precedence of the 6TiSCH topologies. The slotFrames may be 1312 aligned to different CDU matrices and thus have different width. 1313 There is typically one slotFrame for scheduled traffic that has the 1314 highest precedence and one or more slotFrame(s) for RPL traffic. The 1315 timeslots in the slotFrame are indexed by the SlotOffset; the first 1316 cell is at SlotOffset 0. 1318 When a packet is received from a higher layer for transmission, 6top 1319 inserts that packet in the outgoing queue which matches the packet 1320 best (Differentiated Services [RFC2474] can therefore be used). At 1321 each scheduled transmit slot, 6top looks for the frame in all the 1322 outgoing queues that best matches the cells. If a frame is found, it 1323 is given to the TSCH MAC for transmission. 1325 4.2.6. Distributing the reservation of cells 1327 6TiSCH expects a high degree of scalability together with a 1328 distributed routing functionality based on RPL. To achieve this 1329 goal, the spectrum must be allocated in a way that allows for spatial 1330 reuse between zones that will not interfere with one another. In a 1331 large and spatially distributed network, a 6TiSCH node is often in a 1332 good position to determine usage of spectrum in its vicinity. 1334 Use cases for distributed routing are often associated with a 1335 statistical distribution of best-effort traffic with variable needs 1336 for bandwidth on each individual link. With 6TiSCH, the abstraction 1337 of an IPv6 link is implemented as a pair of bundles of cells, one in 1338 each direction; the size of a bundle is optimal when both the energy 1339 wasted idle listening and the packet drops due to congestion loss are 1340 minimized. This can be maintained if the number of cells in a bundle 1341 is adapted dynamically, and with enough reactivity, to match the 1342 variations of best-effort traffic. In turn, the agility to fulfill 1343 the needs for additional cells improves when the number of 1344 interactions with other devices and the protocol latencies are 1345 minimized. 1347 6TiSCH limits that interaction to RPL parents that will only 1348 negotiate with other RPL parents, and performs that negotiation by 1349 groups of cells as opposed to individual cells. The 6TiSCH 1350 architecture allows RPL parents to adjust dynamically, and 1351 independently from the PCE, the amount of bandwidth that is used to 1352 communicate between themselves and their children, in both 1353 directions; to that effect, an allocation mechanism enables a RPL 1354 parent to obtain the exclusive use of a portion of a CDU matrix 1355 within its interference domain. Note that a PCE is expected to have 1356 precedence in the allocation, so that a RPL parent would only be able 1357 to obtain portions that are not in-use by the PCE. 1359 The 6TiSCH architecture introduces the concept of chunks Section 2.2) 1360 to operate such spectrum distribution for a whole group of cells at a 1361 time. The CDU matrix is formatted into a set of chunks, each of them 1362 identified uniquely by a chunk-ID. The knowledge of this formatting 1363 is shared between all the nodes in a 6TiSCH network. 6TiSCH also 1364 defines the process of chunk ownership appropriation whereby a RPL 1365 parent discovers a chunk that is not used in its interference domain 1366 (e.g lack of energy detected in reference cells in that chunk); then 1367 claims the chunk, and then defends it in case another RPL parent 1368 would attempt to appropriate it while it is in use. The chunk is the 1369 basic unit of ownership that is used in that process. 1371 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1372 chan.Off. 0 |chnkA|chnkP|chnk7|chnkO|chnk2|chnkK|chnk1| ... |chnkZ| 1373 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1374 chan.Off. 1 |chnkB|chnkQ|chnkA|chnkP|chnk3|chnkL|chnk2| ... |chnk1| 1375 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1376 ... 1377 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1378 chan.Off. 15 |chnkO|chnk6|chnkN|chnk1|chnkJ|chnkZ|chnkI| ... |chnkG| 1379 +-----+-----+-----+-----+-----+-----+-----+ +-----+ 1380 0 1 2 3 4 5 6 M 1382 Figure 9: CDU matrix Partitioning in Chunks 1384 As a result of the process of chunk ownership appropriation, the RPL 1385 parent has exclusive authority to decide which cell in the 1386 appropriated chunk can be used by which node in its interference 1387 domain. In other words, it is implicitly delegated the right to 1388 manage the portion of the CDU matrix that is represented by the 1389 chunk. The RPL parent may thus orchestrate which transmissions occur 1390 in any of the cells in the chunk, by allocating cells from the chunk 1391 to any form of communication (unicast, multicast) in any direction 1392 between itself and its children. Initially, those cells are added to 1393 the heap of free cells, then dynamically placed into existing 1394 bundles, in new bundles, or allocated opportunistically for one 1395 transmission. 1397 The appropriation of a chunk can also be requested explicitly by the 1398 PCE to any node. In that case, the node still may need to perform 1399 the appropriation process to validate that no other node has claimed 1400 that chunk already. After a successful appropriation, the PCE owns 1401 the cells in that chunk, and may use them as hard cells to set up 1402 Tracks. 1404 4.3. Communication Paradigms and Interaction Models 1406 Section 2.2 provides the terms of Communication Paradigms and 1407 Interaction Models, which can be placed in parallel to the 1408 Information Models and Data Models that are defined in [RFC3444]. 1410 A Communication Paradigms would be an abstract view of a protocol 1411 exchange, and would come with an Information Model for the 1412 information that is being exchanged. In contrast, an Interaction 1413 Models would be more refined and could point on standard operation 1414 such as a Representational state transfer (REST) "GET" operation and 1415 would match a Data Model for the data that is provided over the 1416 protocol exchange. 1418 Section 2.1.3 of [I-D.ietf-roll-rpl-industrial-applicability] and 1419 next sections discuss application-layer paradigms, such as Source- 1420 sink (SS) that is a Multipeer to Multipeer (MP2MP) model primarily 1421 used for alarms and alerts, Publish-subscribe (PS, or pub/sub) that 1422 is typically used for sensor data, as well as Peer-to-peer (P2P) and 1423 Peer-to-multipeer (P2MP) communications. Additional considerations 1424 on Duocast and its N-cast generalization are also provided. Those 1425 paradigms are frequently used in industrial automation, which is a 1426 major use case for IEEE Std 802.15.4 TSCH wireless networks with 1427 [ISA100.11a] and [WirelessHART], that provides a wireless access to 1428 [HART] applications and devices. 1430 This specification focuses on Communication Paradigms and Interaction 1431 Models for packet forwarding and TSCH resources (cells) management. 1432 Management mechanisms for the TSCH schedule at Link-layer (one-hop), 1433 Network-layer (multithop along a Track), and Application-layer 1434 (remote control) are discussed in Section 4.4. Link-layer frame 1435 forwarding interactions are discussed in Section 4.6, and Network- 1436 layer Packet routing is addressed in Section 4.7. 1438 4.4. Schedule Management Mechanisms 1440 6TiSCH uses 4 paradigms to manage the TSCH schedule of the LLN nodes: 1441 Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring 1442 and scheduling management, and Hop-by-hop scheduling. Multiple 1443 mechanisms are defined that implement the associated Interaction 1444 Models, and can be combined and used in the same LLN. Which 1445 mechanism(s) to use depends on application requirements. 1447 4.4.1. Static Scheduling 1449 In the simplest instantiation of a 6TiSCH network, a common fixed 1450 schedule may be shared by all nodes in the network. Cells are 1451 shared, and nodes contend for slot access in a slotted aloha manner. 1453 A static TSCH schedule can be used to bootstrap a network, as an 1454 initial phase during implementation, or as a fall-back mechanism in 1455 case of network malfunction. This schedule is pre-established, for 1456 instance decided by a network administrator based on operational 1457 needs. It can be pre-configured into the nodes, or, more commonly, 1458 learned by a node when joining the network using standard IEEE Std 1459 802.15.4 Information Elements (IE). Regardless, the schedule remains 1460 unchanged after the node has joined a network. RPL is used on the 1461 resulting network. This "minimal" scheduling mechanism that 1462 implements this paradigm is detailed in [RFC8180]. 1464 4.4.2. Neighbor-to-neighbor Scheduling 1466 In the simplest instantiation of a 6TiSCH network described in 1467 Section 4.4.1, nodes may expect a packet at any cell in the schedule 1468 and will waste energy idle listening. In a more complex 1469 instantiation of a 6TiSCH network, a matching portion of the schedule 1470 is established between peers to reflect the observed amount of 1471 transmissions between those nodes. The aggregation of the cells 1472 between a node and a peer forms a bundle that the 6top layer uses to 1473 implement the abstraction of a link for IP. The bandwidth on that 1474 link is proportional to the number of cells in the bundle. 1476 If the size of a bundle is configured to fit an average amount of 1477 bandwidth, peak traffic is dropped. If the size is configured to 1478 allow for peak emissions, energy is be wasted idle listening. 1480 The 6top Protocol [RFC8480] specifies the exchanges between neighbor 1481 nodes to reserve soft cells to transmit to one another. Because this 1482 reservation is done without global knowledge of the schedule of other 1483 nodes in the LLN, scheduling collisions are possible. An optional 1484 Scheduling Function (SF) such as MSF [I-D.ietf-6tisch-msf] is used to 1485 monitor bandwidth usage and perform requests for dynamic allocation 1486 by the 6top sublayer. The SF component is not part of the 6top 1487 sublayer. It may be collocated on the same device or may be 1488 partially or fully offloaded to an external system. 1490 Monitoring and relocation is done in the 6top layer. For the upper 1491 layer, the connection between two neighbor nodes appears as an number 1492 of cells. Depending on traffic requirements, the upper layer can 1493 request 6top to add or delete a number of cells scheduled to a 1494 particular neighbor, without being responsible for choosing the exact 1495 slotOffset/channelOffset of those cells. 1497 4.4.3. Remote Monitoring and Schedule Management 1499 The work at the 6TiSCH WG is focused on non-deterministic traffic and 1500 does not provide the generic data model that would be necessary to 1501 monitor and manage resources of the 6top sublayer. It is recognized 1502 that CoAP can be appropriate to interact with the 6top layer of a 1503 node that is multiple hops away across a 6TiSCH mesh. 1505 The entity issuing the CoAP requests can be a central scheduling 1506 entity (e.g. a PCE), a node multiple hops away with the authority to 1507 modify the TSCH schedule (e.g. the head of a local cluster), or a 1508 external device monitoring the overall state of the network (e.g. 1509 NME). It is also possible that a mapping entity on the backbone 1510 transforms a non-CoAP protocol such as PCEP into the RESTful 1511 interfaces that the 6TiSCH devices support. 1513 With respect to Centralized routing and scheduling, it is envisionned 1514 that the related component of the 6TiSCH Architecture would be an 1515 extension of the Deterministic Networking Architecture 1516 [I-D.ietf-detnet-architecture], which studies Layer-3 aspects of 1517 Deterministic Networks, and covers networks that span multiple 1518 Layer-2 domains. The DetNet architecture is a form of SDN 1519 Architecture and is composed of three planes, a (User) Application 1520 Plane, a Controller Plane (where the PCE operates), and a Network 1521 Plane which in our case is the 6TiSCH LLN. The generic SDN 1522 architecture is discussed in Software-Defined Networking (SDN): 1523 Layers and Architecture Terminology [RFC7426] and is represented 1524 below: 1526 SDN Layers and Architecture Terminology per RFC 7426 1528 o--------------------------------o 1529 | | 1530 | +-------------+ +----------+ | 1531 | | Application | | Service | | 1532 | +-------------+ +----------+ | 1533 | Application Plane | 1534 o---------------Y----------------o 1535 | 1536 *-----------------------------Y---------------------------------* 1537 | Network Services Abstraction Layer (NSAL) | 1538 *------Y------------------------------------------------Y-------* 1539 | | 1540 | Service Interface | 1541 | | 1542 o------Y------------------o o---------------------Y------o 1543 | | Control Plane | | Management Plane | | 1544 | +----Y----+ +-----+ | | +-----+ +----Y----+ | 1545 | | Service | | App | | | | App | | Service | | 1546 | +----Y----+ +--Y--+ | | +--Y--+ +----Y----+ | 1547 | | | | | | | | 1548 | *----Y-----------Y----* | | *---Y---------------Y----* | 1549 | | Control Abstraction | | | | Management Abstraction | | 1550 | | Layer (CAL) | | | | Layer (MAL) | | 1551 | *----------Y----------* | | *----------Y-------------* | 1552 | | | | | | 1553 o------------|------------o o------------|---------------o 1554 | | 1555 | CP | MP 1556 | Southbound | Southbound 1557 | Interface | Interface 1558 | | 1559 *------------Y---------------------------------Y----------------* 1560 | Device and resource Abstraction Layer (DAL) | 1561 *------------Y---------------------------------Y----------------* 1562 | | | | 1563 | o-------Y----------o +-----+ o--------Y----------o | 1564 | | Forwarding Plane | | App | | Operational Plane | | 1565 | o------------------o +-----+ o-------------------o | 1566 | Network Device | 1567 +---------------------------------------------------------------+ 1569 Figure 10 1571 The PCE establishes end-to-end Tracks of hard cells, which are 1572 described in more details in Section 4.6.1. The DetNet work is 1573 expected to enable end to end Deterministic Path across heterogeneous 1574 network (e.g. a 6TiSCH LLN and an Ethernet Backbone). This model 1575 fits the 6TiSCH extended configuration, whereby a 6BBR federates 1576 multiple 6TiSCH LLN in a single subnet over a backbone that can be, 1577 for instance, Ethernet or Wi-Fi. In that model, 6TiSCH 6BBRs 1578 synchronize with one another over the backbone, so as to ensure that 1579 the multiple LLNs that form the IPv6 subnet stay tightly 1580 synchronized. 1582 If the Backbone is Deterministic, then the Backbone Router ensures 1583 that the end-to-end deterministic behavior is maintained between the 1584 LLN and the backbone. It is the responsibility of the PCE to compute 1585 a deterministic path and to end across the TSCH network and an IEEE 1586 Std 802.1 TSN Ethernet backbone, and that of DetNet to enable end-to- 1587 end deterministic forwarding. 1589 4.4.4. Hop-by-hop Scheduling 1591 A node can reserve a Track (Section 4.5) to a destination node 1592 multiple hops away by installing soft cells at each intermediate 1593 node. This forms a Track of soft cells. It is the responsibility of 1594 the 6top sublayer of each node on the Track to monitor these soft 1595 cells and trigger relocation when needed. 1597 This hop-by-hop reservation mechanism is expected to be similar in 1598 essence to [RFC3209] and/or [RFC4080]/[RFC5974]. The protocol for a 1599 node to trigger hop-by-hop scheduling is not yet defined. 1601 4.5. On Tracks 1603 4.5.1. General Behavior of Tracks 1605 The architecture introduces the concept of a Track, which is a 1606 directed path from a source 6TiSCH node to a destination 6TiSCH node 1607 across a 6TiSCH LLN. A Track is the 6TiSCH instantiation of the 1608 concept of a Deterministic Path as described in 1609 [I-D.ietf-detnet-architecture]. Constrained resources such as memory 1610 buffers are reserved for that Track in intermediate 6TiSCH nodes to 1611 avoid loss related to limited capacity. A 6TiSCH node along a Track 1612 not only knows which bundles of cells it should use to receive 1613 packets from a previous hop, but also knows which bundle(s) it should 1614 use to send packets to its next hop along the Track. 1616 A Track is composed of bundles of cells with related schedules and 1617 logical relationships and that ensure that a packet that is injected 1618 in a Track will progress in due time all the way to destination. 1619 Multiple cells may be scheduled in a Track for the transmission of a 1620 single packet, in which case the normal operation of IEEE Std 1621 802.15.4 Automatic Repeat-reQuest (ARQ) can take place; the 1622 acknowledgment may be omitted in some cases, for instance if there is 1623 no scheduled cell for a possible retry. 1625 There are several benefits for using a Track to forward a packet from 1626 a source node to the destination node. 1628 1. Track forwarding, as further described in Section 4.6.1, is a 1629 Layer-2 forwarding scheme, which introduces less process delay 1630 and overhead than Layer-3 forwarding scheme. Therefore, LLN 1631 Devices can save more energy and resource, which is critical for 1632 resource constrained devices. 1634 2. Since channel resources, i.e. bundles of cells, have been 1635 reserved for communications between 6TiSCH nodes of each hop on 1636 the Track, the throughput and the maximum latency of the traffic 1637 along a Track are guaranteed and the jitter is maintained small. 1639 3. By knowing the scheduled time slots of incoming bundle(s) and 1640 outgoing bundle(s), 6TiSCH nodes on a Track could save more 1641 energy by staying in sleep state during in-active slots. 1643 4. Tracks are protected from interfering with one another if a cell 1644 belongs to at most one Track, and congestion loss is avoided if 1645 at most one packet can be presented to the MAC to use that cell. 1646 Tracks enhance the reliability of transmissions and thus further 1647 improve the energy consumption in LLN Devices by reducing the 1648 chances of retransmission. 1650 4.5.2. Serial Track 1652 A Serial (or simple) Track is the 6TiSCH version of a circuit; a 1653 bundle of cells that are programmed to receive (RX-cells) is uniquely 1654 paired to a bundle of cells that are set to transmit (TX-cells), 1655 representing a Layer-2 forwarding state which can be used regardless 1656 of the network layer protocol. 1658 A Serial Track is thus formed end-to-end as a succession of paired 1659 bundles, a receive bundle from the previous hop and a transmit bundle 1660 to the next hop along the Track. For a given iteration of the device 1661 schedule, the effective channel of the cell is obtained by adding a 1662 pseudo-random number to the channelOffset of the cell, which results 1663 in a rotation of the frequency that used for transmission. 1665 The bundles may be computed so as to accommodate both variable rates 1666 and retransmissions, so they might not be fully used at a given 1667 iteration of the schedule. 1669 4.5.3. Complex Track with Replication and Elimination 1671 As opposed to a Serial Track that is a sequence of nodes and links, a 1672 Complex Track is shaped as a directed acyclic graph towards a 1673 destination to support multi-path forwarding and route around 1674 failures. 1676 A Complex Track may also branch off and rejoin, for the purpose of 1677 the DetNet Packet Replication and Elimination (PRE), over non 1678 congruent branches. PRE may be used to complement Layer-2 ARQ to 1679 meet industrial expectations in Packet Delivery Ratio (PDR), in 1680 particular when the Track extends beyond the 6TiSCH network in a 1681 larger DetNet network. 1683 The art of Deterministic Networks already include PRE techniques. 1684 Example standards include the Parallel Redundancy Protocol (PRP) and 1685 the High-availability Seamless Redundancy (HSR) [IEC62439]. 1687 At each 6TiSCH hop along the Track, the PCE may schedule more than 1688 one timeslot for a packet, so as to support Layer-2 retries (ARQ). 1689 It is also possible that the field device only uses the second branch 1690 if sending over the first branch fails. 1692 In the art of TSCH, a path does not necessarily support PRE but it is 1693 almost systematically multi-path. This means that a Track is 1694 scheduled so as to ensure that each hop has at least two forwarding 1695 solutions, and the forwarding decision is to try the preferred one 1696 and use the other in case of Layer-2 transmission failure as detected 1697 by ARQ. 1699 4.5.4. DetNet End-to-end Path 1701 Ultimately, DetNet should enable to extend a Track beyond the 6TiSCH 1702 LLN. Figure 11 illustrates a Track that is laid out from a field 1703 device in a 6TiSCH network to an IoT gateway that is located on an 1704 802.1 Time-Sensitive Networking (TSN) backbone. 1706 +-=-=-+ 1707 | IoT | 1708 | G/W | 1709 +-=-=-+ 1710 ^ <=== Elimination 1711 | | 1712 Track branch | | 1713 +-=-=-=-+ +-=-=-=-=+ Subnet Backbone 1714 | | 1715 +-=|-=+ +-=|-=+ 1716 | | | Backbone | | | Backbone 1717 o | | | router | | | router 1718 +-=/-=+ +-=|-=+ 1719 o / o o-=-o-=-=/ o 1720 o o-=-o-=/ o o o o o 1721 o \ / o o LLN o 1722 o v <=== Replication 1723 o 1725 Figure 11: End-to-End deterministic Track 1727 The Replication function in the 6TiSCH Node sends a copy of each 1728 packet over two different branches, and the PCE schedules each hop of 1729 both branches so that the two copies arrive in due time at the 1730 gateway. In case of a loss on one branch, hopefully the other copy 1731 of the packet still makes it in due time. If two copies make it to 1732 the IoT gateway, the Elimination function in the gateway ignores the 1733 extra packet and presents only one copy to upper layers. 1735 4.5.5. Cell Reuse 1737 The 6TiSCH architecture provides means to avoid waste of cells as 1738 well as overflows in the transmit bundle pof a Track, as follows: 1740 In one hand, a TX-cell that is not needed for the current 1741 iteration may be reused opportunistically on a per-hop basis for 1742 routed packets. When all of the frame that were received for a 1743 given Track are effectively transmitted, any available TX-cell for 1744 that Track can be reused for upper layer traffic for which the 1745 next-hop router matches the next hop along the Track. In that 1746 case, the cell that is being used is effectively a TX-cell from 1747 the Track, but the short address for the destination is that of 1748 the next-hop router. It results that a frame that is received in 1749 a RX-cell of a Track with a destination MAC address set to this 1750 node as opposed to broadcast must be extracted from the Track and 1751 delivered to the upper layer (a frame with an unrecognized 1752 destination MAC address is dropped at the lower MAC layer and thus 1753 is not received at the 6top sublayer). 1755 On the other hand, it might happen that there are not enough TX- 1756 cells in the transmit bundle to accommodate the Track traffic, for 1757 instance if more retransmissions are needed than provisioned. In 1758 that case, the frame can be placed for transmission in the bundle 1759 that is used for Layer-3 traffic towards the next hop along the 1760 Track as long as it can be routed by the upper layer, that is, 1761 typically, if the frame transports an IPv6 packet. The MAC 1762 address should be set to the next-hop MAC address to avoid 1763 confusion. It results that a frame that is received over a 1764 Layer-3 bundle may be in fact associated to a Track. In a 1765 classical IP link such as an Ethernet, off-Track traffic is 1766 typically in excess over reservation to be routed along the non- 1767 reserved path based on its QoS setting. But with 6TiSCH, since 1768 the use of the Layer-3 bundle may be due to transmission failures, 1769 it makes sense for the receiver to recognize a frame that should 1770 be re-Tracked, and to place it back on the appropriate bundle if 1771 possible. A frame should be re-Tracked if the Per-Hop-Behavior 1772 group indicated in the Differentiated Services Field of the IPv6 1773 header is set to Deterministic Forwarding, as discussed in 1774 Section 4.7.1. A frame is re-Tracked by scheduling it for 1775 transmission over the transmit bundle associated to the Track, 1776 with the destination MAC address set to broadcast. 1778 4.6. Forwarding Models 1780 By forwarding, this specification means the per-packet operation that 1781 allows to deliver a packet to a next hop or an upper layer in this 1782 node. Forwarding is based on pre-existing state that was installed 1783 as a result of a routing computation Section 4.7. 6TiSCH supports 1784 three different forwarding model, G-MPLS Track Forwarding (TF), 1785 6LoWPAN Fragment Forwarding (FF) and IPv6 Forwarding (6F). 1787 4.6.1. Track Forwarding 1789 Forwarding along a Track can be seen as a Generalized Multi-protocol 1790 Label Switching (G-MPLS) operation in that the information used to 1791 switch a frame is not an explicit label, but rather related to other 1792 properties of the way the packet was received, a particular cell in 1793 the case of 6TiSCH. As a result, as long as the TSCH MAC (and 1794 Layer-2 security) accepts a frame, that frame can be switched 1795 regardless of the protocol, whether this is an IPv6 packet, a 6LoWPAN 1796 fragment, or a frame from an alternate protocol such as WirelessHART 1797 or ISA100.11a. 1799 A data frame that is forwarded along a Track normally has a 1800 destination MAC address that is set to broadcast - or a multicast 1801 address depending on MAC support. This way, the MAC layer in the 1802 intermediate nodes accepts the incoming frame and 6top switches it 1803 without incurring a change in the MAC header. In the case of IEEE 1804 Std 802.15.4, this means effectively broadcast, so that along the 1805 Track the short address for the destination of the frame is set to 1806 0xFFFF. 1808 There are 2 modes for a Track, transport mode and tunnel mode. 1810 4.6.1.1. Transport Mode 1812 In transport mode, the Protocol Data Unit (PDU) is associated with 1813 flow-dependant meta-data that refers uniquely to the Track, so the 1814 6top sublayer can place the frame in the appropriate cell without 1815 ambiguity. In the case of IPv6 traffic, this flow identification is 1816 transported in the Flow Label of the IPv6 header. Associated with 1817 the source IPv6 address, the Flow Label forms a globally unique 1818 identifier for that particular Track that is validated at egress 1819 before restoring the destination MAC address (DMAC) and punting to 1820 the upper layer. 1822 | ^ 1823 +--------------+ | | 1824 | IPv6 | | | 1825 +--------------+ | | 1826 | 6LoWPAN HC | | | 1827 +--------------+ ingress egress 1828 | 6top | sets +----+ +----+ restores 1829 +--------------+ dmac to | | | | dmac to 1830 | TSCH MAC | brdcst | | | | self 1831 +--------------+ | | | | | | 1832 | LLN PHY | +-------+ +--...-----+ +-------+ 1833 +--------------+ 1835 Track Forwarding, Transport Mode 1837 4.6.1.2. Tunnel Mode 1839 In tunnel mode, the frames originate from an arbitrary protocol over 1840 a compatible MAC that may or may not be synchronized with the 6TiSCH 1841 network. An example of this would be a router with a dual radio that 1842 is capable of receiving and sending WirelessHART or ISA100.11a frames 1843 with the second radio, by presenting itself as an access Point or a 1844 Backbone Router, respectively. 1846 In that mode, some entity (e.g. PCE) can coordinate with a 1847 WirelessHART Network Manager or an ISA100.11a System Manager to 1848 specify the flows that are to be transported transparently over the 1849 Track. 1851 +--------------+ 1852 | IPv6 | 1853 +--------------+ 1854 | 6LoWPAN HC | 1855 +--------------+ set restore 1856 | 6top | +dmac+ +dmac+ 1857 +--------------+ to|brdcst to|nexthop 1858 | TSCH MAC | | | | | 1859 +--------------+ | | | | 1860 | LLN PHY | +-------+ +--...-----+ +-------+ 1861 +--------------+ | ingress egress | 1862 | | 1863 +--------------+ | | 1864 | LLN PHY | | | 1865 +--------------+ | | 1866 | TSCH MAC | | | 1867 +--------------+ | dmac = | dmac = 1868 |ISA100/WiHART | | nexthop v nexthop 1869 +--------------+ 1871 Figure 12: Track Forwarding, Tunnel Mode 1873 In that case, the flow information that identifies the Track at the 1874 ingress 6TiSCH router is derived from the RX-cell. The dmac is set 1875 to this node but the flow information indicates that the frame must 1876 be tunneled over a particular Track so the frame is not passed to the 1877 upper layer. Instead, the dmac is forced to broadcast and the frame 1878 is passed to the 6top sublayer for switching. 1880 At the egress 6TiSCH router, the reverse operation occurs. Based on 1881 metadata associated to the Track, the frame is passed to the 1882 appropriate Link Layer with the destination MAC restored. 1884 4.6.1.3. Tunnel Metadata 1886 Metadata coming with the Track configuration is expected to provide 1887 the destination MAC address of the egress endpoint as well as the 1888 tunnel mode and specific data depending on the mode, for instance a 1889 service access point for frame delivery at egress. If the tunnel 1890 egress point does not have a MAC address that matches the 1891 configuration, the Track installation fails. 1893 In transport mode, if the final Layer-3 destination is the tunnel 1894 termination, then it is possible that the IPv6 address of the 1895 destination is compressed at the 6LoWPAN sublayer based on the MAC 1896 address. It is thus mandatory at the ingress point to validate that 1897 the MAC address that was used at the 6LoWPAN sublayer for compression 1898 matches that of the tunnel egress point. For that reason, the node 1899 that injects a packet on a Track checks that the destination is 1900 effectively that of the tunnel egress point before it overwrites it 1901 to broadcast. The 6top sublayer at the tunnel egress point reverts 1902 that operation to the MAC address obtained from the tunnel metadata. 1904 4.6.2. IPv6 Forwarding 1906 As the packets are routed at Layer-3, traditional QoS and Active 1907 Queue Management (AQM) operations are expected to prioritize flows; 1908 the application of Differentiated Services is further discussed in 1909 [I-D.svshah-tsvwg-lln-diffserv-recommendations]. 1911 | ^ 1912 +--------------+ | | 1913 | IPv6 | | +-QoS+ +-QoS+ | 1914 +--------------+ | | | | | | 1915 | 6LoWPAN HC | | | | | | | 1916 +--------------+ | | | | | | 1917 | 6top | | | | | | | 1918 +--------------+ | | | | | | 1919 | TSCH MAC | | | | | | | 1920 +--------------+ | | | | | | 1921 | LLN PHY | +-------+ +--...-----+ +-------+ 1922 +--------------+ 1924 Figure 13: IP Forwarding 1926 4.6.3. Fragment Forwarding 1928 Considering that 6LoWPAN packets can be as large as 1280 bytes (the 1929 IPv6 MTU), and that the non-storing mode of RPL implies Source 1930 Routing that requires space for routing headers, and that a IEEE Std 1931 802.15.4 frame with security may carry in the order of 80 bytes of 1932 effective payload, an IPv6 packet might be fragmented into more than 1933 16 fragments at the 6LoWPAN sublayer. 1935 This level of fragmentation is much higher than that traditionally 1936 experienced over the Internet with IPv4 fragments, where 1937 fragmentation is already known as harmful. 1939 In the case to a multihop route within a 6TiSCH network, Hop-by-Hop 1940 recomposition occurs at each hop in order to reform the packet and 1941 route it. This creates additional latency and forces intermediate 1942 nodes to store a portion of a packet for an undetermined time, thus 1943 impacting critical resources such as memory and battery. 1945 [I-D.ietf-6lo-minimal-fragment] describes a framework for forwarding 1946 fragments end-to-end across a 6TiSCH route-over mesh. Within that 1947 framework, [I-D.ietf-lwig-6lowpan-virtual-reassembly] details a 1948 virtual reassembly buffer mechanism whereby the datagram tag in the 1949 6LoWPAN Fragment is used as a label for switching at the 6LoWPAN 1950 sublayer. Building on this technique, 1951 [I-D.ietf-6lo-fragment-recovery] introduces a new format for 6LoWPAN 1952 fragments that enables the selective recovery of individual 1953 fragments, and allows for a degree of flow control based on an 1954 Explicit Congestion Notification. 1956 | ^ 1957 +--------------+ | | 1958 | IPv6 | | +----+ +----+ | 1959 +--------------+ | | | | | | 1960 | 6LoWPAN HC | | learn learn | 1961 +--------------+ | | | | | | 1962 | 6top | | | | | | | 1963 +--------------+ | | | | | | 1964 | TSCH MAC | | | | | | | 1965 +--------------+ | | | | | | 1966 | LLN PHY | +-------+ +--...-----+ +-------+ 1967 +--------------+ 1969 Figure 14: Forwarding First Fragment 1971 In that model, the first fragment is routed based on the IPv6 header 1972 that is present in that fragment. The 6LoWPAN sublayer learns the 1973 next hop selection, generates a new datagram tag for transmission to 1974 the next hop, and stores that information indexed by the incoming MAC 1975 address and datagram tag. The next fragments are then switched based 1976 on that stored state. 1978 | ^ 1979 +--------------+ | | 1980 | IPv6 | | | 1981 +--------------+ | | 1982 | 6LoWPAN HC | | replay replay | 1983 +--------------+ | | | | | | 1984 | 6top | | | | | | | 1985 +--------------+ | | | | | | 1986 | TSCH MAC | | | | | | | 1987 +--------------+ | | | | | | 1988 | LLN PHY | +-------+ +--...-----+ +-------+ 1989 +--------------+ 1991 Figure 15: Forwarding Next Fragment 1993 A bitmap and an ECN echo in the end-to-end acknowledgment enable the 1994 source to resend the missing fragments selectively. The first 1995 fragment may be resent to carve a new path in case of a path failure. 1996 The ECN echo set indicates that the number of outstanding fragments 1997 should be reduced. 1999 4.7. Distributed vs. Centralized Routing 2001 6TiSCH enables a mixed model of centralized routes and distributed 2002 routes. Centralized routes can for example be computed by a entity 2003 such as a PCE. Distributed routes are computed by RPL. 2005 Both methods may inject routes in the Routing Tables of the 6TiSCH 2006 routers. In either case, each route is associated with a 6TiSCH 2007 topology that can be a RPL Instance topology or a Track. The 6TiSCH 2008 topology is indexed by a Instance ID, in a format that reuses the 2009 RPLInstanceID as defined in RPL [RFC6550]. 2011 Both RPL and PCE rely on shared sources such as policies to define 2012 Global and Local RPLInstanceIDs that can be used by either method. 2013 It is possible for centralized and distributed routing to share a 2014 same topology. Generally they will operate in different slotFrames, 2015 and centralized routes will be used for scheduled traffic and will 2016 have precedence over distributed routes in case of conflict between 2017 the slotFrames. 2019 4.7.1. Packet Marking and Handling 2021 All packets inside a 6TiSCH domain must carry the Instance ID that 2022 identifies the 6TiSCH topology that is to be used for routing and 2023 forwarding that packet. The location of that information must be the 2024 same for all packets forwarded inside the domain. 2026 For packets that are routed by a PCE along a Track, the tuple formed 2027 by the IPv6 source address and a local RPLInstanceID in the packet 2028 identify uniquely the Track and associated transmit bundle. 2030 For packets that are routed by RPL, that information is the 2031 RPLInstanceID which is carried in the RPL Packet Information, as 2032 discussed in section 11.2 of [RFC6550], "Loop Avoidance and 2033 Detection". 2035 The RPL Packet Information (RPI) is carried in IPv6 packets as a RPL 2036 option in the IPv6 Hop-By-Hop Header [RFC6553]. 2038 A compression mechanism for the RPL packet artifacts that integrates 2039 the compression of IP-in-IP encapsulation and the Routing Header type 2040 3 [RFC6554] with that of the RPI in a 6LoWPAN dispatch/header type is 2041 specified in [RFC8025] and [RFC8138]. 2043 Either way, the method and format used for encoding the RPLInstanceID 2044 is generalized to all 6TiSCH topological Instances, which include 2045 both RPL Instances and Tracks. 2047 4.7.2. Replication, Retries and Elimination 2049 6TiSCH expects elimination and replication of packets along a complex 2050 Track, but has no position about how the sequence numbers would be 2051 tagged in the packet. 2053 As it goes, 6TiSCH expects that timeslots corresponding to copies of 2054 a same packet along a Track are correlated by configuration, and does 2055 not need to process the sequence numbers. 2057 The semantics of the configuration will enable correlated timeslots 2058 to be grouped for transmit (and respectively receive) with a 'OR' 2059 relations, and then a 'AND' relation would be configurable between 2060 groups. The semantics is that if the transmit (and respectively 2061 receive) operation succeeded in one timeslot in a 'OR' group, then 2062 all the other timeslots in the group are ignored. Now, if there are 2063 at least two groups, the 'AND' relation between the groups indicates 2064 that one operation must succeed in each of the groups. 2066 On the transmit side, timeslots provisioned for retries along a same 2067 branch of a Track are placed a same 'OR' group. The 'OR' relation 2068 indicates that if a transmission is acknowledged, then further 2069 transmissions should not be attempted for timeslots in that group. 2070 There are as many 'OR' groups as there are branches of the Track 2071 departing from this node. Different 'OR' groups are programmed for 2072 the purpose of replication, each group corresponding to one branch of 2073 the Track. The 'AND' relation between the groups indicates that 2074 transmission over any of branches must be attempted regardless of 2075 whether a transmission succeeded in another branch. It is also 2076 possible to place cells to different next-hop routers in a same 'OR' 2077 group. This allows to route along multi-path tracks, trying one 2078 next-hop and then another only if sending to the first fails. 2080 On the receive side, all timeslots are programmed in a same 'OR' 2081 group. Retries of a same copy as well as converging branches for 2082 elimination are converged, meaning that the first successful 2083 reception is enough and that all the other timeslots can be ignored. 2085 4.7.3. Differentiated Services Per-Hop-Behavior 2087 Additionally, an IP packet that is sent along a Track uses the 2088 Differentiated Services Per-Hop-Behavior Group called Deterministic 2089 Forwarding, as described in 2090 [I-D.svshah-tsvwg-deterministic-forwarding]. 2092 5. IANA Considerations 2094 This specification does not require IANA action. 2096 6. Security Considerations 2098 This architecture operates on IEEE Std 802.15.4 and expects Link- 2099 Layer security to be enabled at all times between connected devices, 2100 except for the very first step of the device join process, where a 2101 joining device may need some initial, unsecured exchanges so as to 2102 obtain its initial key material. 2104 The Minimal Security Framework for 6TiSCH 2105 [I-D.ietf-6tisch-minimal-security] describes the minimal mechanisms 2106 required to support secure enrollment of a pledge to a 6TiSCH network 2107 based on PSK. The specification enables to establish of Link-Layer 2108 keys, typically used in combination with a variation of Counter with 2109 CBC-MAC (CCM) [RFC3610], and set up a secure end-to-end session 2110 between the joining node (called the pledge) and the join registrar/ 2111 coordinator (JRC) in charge of authenticating the node via a Join 2112 Proxy (JP). It can also be used to obtain a Link Layer short address 2113 as a side effect. CoJP uses shared slots which are a constrained 2114 resource, so it is optimized to limit the number of messages to the 2115 strict minimum. As an example, Neighbor Discovery between the pledge 2116 and the JP can be skipped when the IPv6 Link Local addresses that are 2117 used derive from the node's EUI-64 address. 2119 The "6tisch Zero-Touch Secure Join protocol" 2120 [I-D.ietf-6tisch-dtsecurity-zerotouch-join] wraps the minimal 2121 security draft with a flow inspired from ANIMA "Bootstrapping Remote 2122 Secure Key Infrastructures (BRSKI)" 2123 [I-D.ietf-anima-bootstrapping-keyinfra]. 2125 6.1. Join Process Highlights 2127 The BRSKI architecture specifies three logical elements to describe 2128 the join process: 2130 Pledge: Node that wishes to become part of the network; 2132 Join Registrar/Coordinator (JRC) : An entity that arbitrates network 2133 access and hands out network parameters (such as keying 2134 material); 2136 Join Proxy (JP), a one-hop (radio) neighbor of the joining node that 2137 acts as proxy network node and may provide connectivity with 2138 the JRC. 2140 The join protocol consists of three major activities: 2142 Device Authentication: The Pledge and the JP mutually authenticate 2143 each other and establish a shared key, so as to ensure on-going 2144 authenticated communications. This may involve a server as a 2145 third party. 2147 Authorization: The JP decides on whether/how to authorize a Pledge 2148 (if denied, this may result in loss of bandwidth). Conversely, 2149 the Pledge decides on whether/how to authorize the network (if 2150 denied, it will not join the network). Authorization decisions 2151 may involve other nodes in the network. 2153 Configuration/Parameterization: The JP distributes configuration 2154 information to the Pledge, such as scheduling information, IP 2155 address assignment information, and network policies. This may 2156 originate from other network devices, for which the JP may act 2157 as proxy. This step may also include distribution of 2158 information from the Pledge to the JP and other nodes in the 2159 network and, more generally, synchronization of information 2160 between these entities. 2162 The device joining process is depicted in Figure 16, where it is 2163 assumed that devices have access to certificates and where entities 2164 have access to the root CA keys of their communicating parties 2165 (initial set-up requirement). Under these assumptions, the 2166 authentication step of the device joining process does not require 2167 online involvement of a third party. Mutual authentication is 2168 performed between the Pledge and the JP using their certificates, 2169 which also results in a shared key between these two entities. 2171 The JP assists the Pledge in mutual authentication with a remote 2172 server node (primarily via provision of a communication path with the 2173 server), which also results in a shared (end-to-end) key between 2174 those two entities. The server node may be a JRC that arbitrages the 2175 network authorization of the Pledge (where the JP will deny bandwidth 2176 if authorization is not successful); it may distribute network- 2177 specific configuration parameters (including network-wide keys) to 2178 the Pledge. In its turn, the Pledge may distribute and synchronize 2179 information (including, e.g., network statistics) to the server node 2180 and, if so desired, also to the JP. The actual decision of the 2181 Pledge to become part of the network may depend on authorization of 2182 the network itself. 2184 The server functionality is a role which may be implemented with one 2185 (centralized) or multiple devices (distributed). In either case, 2186 mutual authentication is established with each physical server entity 2187 with which a role is implemented. 2189 Note that in the above description, the JP does not solely act as a 2190 relay node, thereby allowing it to first filter traffic to be relayed 2191 based on cryptographic authentication criteria - this provides first- 2192 level access control and mitigates certain types of denial-of-service 2193 attacks on the network at large. 2195 Depending on more detailed insight in cost/benefit trade-offs, this 2196 process might be complemented by a more "relaxed" mechanism, where 2197 the JP acts as a relay node only. The final architecture will 2198 provide mechanisms to also cover cases where the initial set-up 2199 requirements are not met or where some other out-of-sync behavior 2200 occurs; it will also suggest some optimizations in case JRC-related 2201 information is already available with the JP (via caching of 2202 information). 2204 When a device rejoins the network in the same authorization domain, 2205 the authorization step could be omitted if the server distributes the 2206 authorization state for the device to the JP when the device 2207 initially joined the network. However, this generally still requires 2208 the exchange of updated configuration information, e.g., related to 2209 time schedules and bandwidth allocation. 2211 {joining node} {neighbor} {server, etc.} Example: 2212 +---------+ +---------+ +---------+ 2213 | Joining | | Join | +--| CA |certificate 2214 | Node | |Assistant| | +---------+ issuance 2215 +---------+ +---------+ | +---------+ 2216 | | +--|Authoriz.| membership 2217 |<----Beaconing------| | +---------+ test (JRC) 2218 | | | +---------+ 2219 |<--Authentication-->| +--| Routing | IP address 2220 | |<--Authorization-->| +--------- assignment 2221 |<-------------------| | +---------+ 2222 | | +--| Gateway | backbone, 2223 |------------------->| | +---------+ cloud 2224 | |<--Configuration-->| +---------+ 2225 |<-------------------| +--|Bandwidth| PCE 2226 +---------+ schedule 2227 . . . 2228 . . . 2230 Figure 16: Network joining, with only authorization by third party 2232 7. Acknowledgments 2234 7.1. Contributors 2236 The co-authors of this document are listed below: 2238 Robert Assimiti for his breakthrough work on RPL over TSCH and 2239 initial text and guidance; 2241 Kris Pister for creating it all and his continuing guidance through 2242 the elaboration of this design; 2244 Maria Rita Palattella for managing the Terminology document merged 2245 into this through the work of 6TiSCH; 2247 Michael Richardson for his leadership role in the Security Design 2248 Team and his contribution throughout this document; 2250 Rene Struik for the security section and his contribution to the 2251 Security Design Team; 2253 Malisa Vucinic for the work on the one-touch join process and his 2254 contribution to the Security Design Team; 2256 Xavier Vilajosana who lead the design of the minimal support with 2257 RPL and contributed deeply to the 6top design and the G-MPLS 2258 operation of Track switching; 2260 Qin Wang who lead the design of the 6top sublayer and contributed 2261 related text that was moved and/or adapted in this document; 2263 Thomas Watteyne for his contribution to the whole design, in 2264 particular on TSCH and security. 2266 7.2. Special Thanks 2268 Special thanks to Tero Kivinen, Jonathan Simon, Giuseppe Piro, Subir 2269 Das and Yoshihiro Ohba for their deep contribution to the initial 2270 security work, to Diego Dujovne for starting and leading the SF0 2271 effort and to Tengfei Chang for evolving it in the MSF. 2273 Special thanks also to Pat Kinney for his support in maintaining the 2274 connection active and the design in line with work happening at IEEE 2275 Std 802.15.4. 2277 Special thanks to Ted Lemon who was the INT Area A-D while this 2278 specification was developed for his great support and help 2279 throughout. 2281 Also special thanks to Ralph Droms who performed the first INT Area 2282 Directorate review, that was very deep and through and radically 2283 changed the orientations of this document. 2285 7.3. And Do not Forget 2287 This specification is the result of multiple interactions, in 2288 particular during the 6TiSCH (bi)Weekly Interim call, relayed through 2289 the 6TiSCH mailing list at the IETF. 2291 The authors wish to thank: Alaeddine Weslati, Chonggang Wang, 2292 Georgios Exarchakos, Zhuo Chen, Alfredo Grieco, Bert Greevenbosch, 2293 Cedric Adjih, Deji Chen, Martin Turon, Dominique Barthel, Elvis 2294 Vogli, Geraldine Texier, Malisa Vucinic, Guillaume Gaillard, Herman 2295 Storey, Kazushi Muraoka, Ken Bannister, Kuor Hsin Chang, Laurent 2296 Toutain, Maik Seewald, Maria Rita Palattella, Michael Behringer, 2297 Nancy Cam Winget, Nicola Accettura, Nicolas Montavont, Oleg Hahm, 2298 Patrick Wetterwald, Paul Duffy, Peter van der Stock, Rahul Sen, 2299 Pieter de Mil, Pouria Zand, Rouhollah Nabati, Rafa Marin-Lopez, 2300 Raghuram Sudhaakar, Sedat Gormus, Shitanshu Shah, Steve Simlo, 2301 Tengfei Chang, Tina Tsou, Tom Phinney, Xavier Lagrange, Ines Robles 2302 and Samita Chakrabarti for their participation and various 2303 contributions. 2305 8. References 2307 8.1. Normative References 2309 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 2310 DOI 10.17487/RFC0768, August 1980, 2311 . 2313 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2314 Requirement Levels", BCP 14, RFC 2119, 2315 DOI 10.17487/RFC2119, March 1997, 2316 . 2318 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 2319 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 2320 DOI 10.17487/RFC4861, September 2007, 2321 . 2323 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 2324 Address Autoconfiguration", RFC 4862, 2325 DOI 10.17487/RFC4862, September 2007, 2326 . 2328 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 2329 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 2330 DOI 10.17487/RFC6282, September 2011, 2331 . 2333 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 2334 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 2335 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 2336 Low-Power and Lossy Networks", RFC 6550, 2337 DOI 10.17487/RFC6550, March 2012, 2338 . 2340 [RFC6552] Thubert, P., Ed., "Objective Function Zero for the Routing 2341 Protocol for Low-Power and Lossy Networks (RPL)", 2342 RFC 6552, DOI 10.17487/RFC6552, March 2012, 2343 . 2345 [RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low- 2346 Power and Lossy Networks (RPL) Option for Carrying RPL 2347 Information in Data-Plane Datagrams", RFC 6553, 2348 DOI 10.17487/RFC6553, March 2012, 2349 . 2351 [RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6 2352 Routing Header for Source Routes with the Routing Protocol 2353 for Low-Power and Lossy Networks (RPL)", RFC 6554, 2354 DOI 10.17487/RFC6554, March 2012, 2355 . 2357 [RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. 2358 Bormann, "Neighbor Discovery Optimization for IPv6 over 2359 Low-Power Wireless Personal Area Networks (6LoWPANs)", 2360 RFC 6775, DOI 10.17487/RFC6775, November 2012, 2361 . 2363 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 2364 Application Protocol (CoAP)", RFC 7252, 2365 DOI 10.17487/RFC7252, June 2014, 2366 . 2368 [RFC8025] Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power 2369 Wireless Personal Area Network (6LoWPAN) Paging Dispatch", 2370 RFC 8025, DOI 10.17487/RFC8025, November 2016, 2371 . 2373 [RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie, 2374 "IPv6 over Low-Power Wireless Personal Area Network 2375 (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138, 2376 April 2017, . 2378 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2379 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2380 May 2017, . 2382 [RFC8180] Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal 2383 IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH) 2384 Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180, 2385 May 2017, . 2387 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 2388 (IPv6) Specification", STD 86, RFC 8200, 2389 DOI 10.17487/RFC8200, July 2017, 2390 . 2392 8.2. Informative References 2394 [I-D.ietf-6lo-ap-nd] 2395 Thubert, P., Sarikaya, B., Sethi, M., and R. Struik, 2396 "Address Protected Neighbor Discovery for Low-power and 2397 Lossy Networks", draft-ietf-6lo-ap-nd-08 (work in 2398 progress), October 2018. 2400 [I-D.ietf-6lo-backbone-router] 2401 Thubert, P. and C. Perkins, "IPv6 Backbone Router", draft- 2402 ietf-6lo-backbone-router-08 (work in progress), October 2403 2018. 2405 [I-D.ietf-6lo-fragment-recovery] 2406 Thubert, P., "6LoWPAN Selective Fragment Recovery", draft- 2407 ietf-6lo-fragment-recovery-00 (work in progress), 2408 September 2018. 2410 [I-D.ietf-6lo-minimal-fragment] 2411 Watteyne, T., Bormann, C., and P. Thubert, "LLN Minimal 2412 Fragment Forwarding", draft-ietf-6lo-minimal-fragment-00 2413 (work in progress), October 2018. 2415 [I-D.ietf-6lo-rfc6775-update] 2416 Thubert, P., Nordmark, E., Chakrabarti, S., and C. 2417 Perkins, "Registration Extensions for 6LoWPAN Neighbor 2418 Discovery", draft-ietf-6lo-rfc6775-update-21 (work in 2419 progress), June 2018. 2421 [I-D.ietf-6tisch-dtsecurity-zerotouch-join] 2422 Richardson, M., "6tisch Zero-Touch Secure Join protocol", 2423 draft-ietf-6tisch-dtsecurity-zerotouch-join-03 (work in 2424 progress), October 2018. 2426 [I-D.ietf-6tisch-minimal-security] 2427 Vucinic, M., Simon, J., Pister, K., and M. Richardson, 2428 "Minimal Security Framework for 6TiSCH", draft-ietf- 2429 6tisch-minimal-security-08 (work in progress), November 2430 2018. 2432 [I-D.ietf-6tisch-msf] 2433 Chang, T., Vucinic, M., Vilajosana, X., Duquennoy, S., and 2434 D. Dujovne, "6TiSCH Minimal Scheduling Function (MSF)", 2435 draft-ietf-6tisch-msf-01 (work in progress), October 2018. 2437 [I-D.ietf-anima-bootstrapping-keyinfra] 2438 Pritikin, M., Richardson, M., Behringer, M., Bjarnason, 2439 S., and K. Watsen, "Bootstrapping Remote Secure Key 2440 Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping- 2441 keyinfra-17 (work in progress), November 2018. 2443 [I-D.ietf-core-object-security] 2444 Selander, G., Mattsson, J., Palombini, F., and L. Seitz, 2445 "Object Security for Constrained RESTful Environments 2446 (OSCORE)", draft-ietf-core-object-security-15 (work in 2447 progress), August 2018. 2449 [I-D.ietf-detnet-architecture] 2450 Finn, N., Thubert, P., Varga, B., and J. Farkas, 2451 "Deterministic Networking Architecture", draft-ietf- 2452 detnet-architecture-09 (work in progress), October 2018. 2454 [I-D.ietf-detnet-use-cases] 2455 Grossman, E., "Deterministic Networking Use Cases", draft- 2456 ietf-detnet-use-cases-19 (work in progress), October 2018. 2458 [I-D.ietf-lwig-6lowpan-virtual-reassembly] 2459 Bormann, C. and T. Watteyne, "Virtual reassembly buffers 2460 in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-00 2461 (work in progress), July 2018. 2463 [I-D.ietf-manet-aodvv2] 2464 Perkins, C., Ratliff, S., Dowdell, J., Steenbrink, L., and 2465 V. Mercieca, "Ad Hoc On-demand Distance Vector Version 2 2466 (AODVv2) Routing", draft-ietf-manet-aodvv2-16 (work in 2467 progress), May 2016. 2469 [I-D.ietf-roll-aodv-rpl] 2470 Anamalamudi, S., Zhang, M., Perkins, C., Anand, S., and B. 2471 Liu, "Asymmetric AODV-P2P-RPL in Low-Power and Lossy 2472 Networks (LLNs)", draft-ietf-roll-aodv-rpl-05 (work in 2473 progress), October 2018. 2475 [I-D.ietf-roll-rpl-industrial-applicability] 2476 Phinney, T., Thubert, P., and R. Assimiti, "RPL 2477 applicability in industrial networks", draft-ietf-roll- 2478 rpl-industrial-applicability-02 (work in progress), 2479 October 2013. 2481 [I-D.kivinen-802-15-ie] 2482 Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information 2483 Element for IETF", draft-kivinen-802-15-ie-06 (work in 2484 progress), March 2017. 2486 [I-D.svshah-tsvwg-deterministic-forwarding] 2487 Shah, S. and P. Thubert, "Deterministic Forwarding PHB", 2488 draft-svshah-tsvwg-deterministic-forwarding-04 (work in 2489 progress), August 2015. 2491 [I-D.svshah-tsvwg-lln-diffserv-recommendations] 2492 Shah, S. and P. Thubert, "Differentiated Service Class 2493 Recommendations for LLN Traffic", draft-svshah-tsvwg-lln- 2494 diffserv-recommendations-04 (work in progress), February 2495 2015. 2497 [I-D.thubert-6lo-bier-dispatch] 2498 Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A 2499 6loRH for BitStrings", draft-thubert-6lo-bier-dispatch-05 2500 (work in progress), July 2018. 2502 [I-D.thubert-bier-replication-elimination] 2503 Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER- 2504 TE extensions for Packet Replication and Elimination 2505 Function (PREF) and OAM", draft-thubert-bier-replication- 2506 elimination-03 (work in progress), March 2018. 2508 [I-D.thubert-roll-unaware-leaves] 2509 Thubert, P., "Routing for RPL Leaves", draft-thubert-roll- 2510 unaware-leaves-05 (work in progress), May 2018. 2512 [I-D.wang-6tisch-6top-sublayer] 2513 Wang, Q. and X. Vilajosana, "6TiSCH Operation Sublayer 2514 (6top)", draft-wang-6tisch-6top-sublayer-04 (work in 2515 progress), November 2015. 2517 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 2518 "Definition of the Differentiated Services Field (DS 2519 Field) in the IPv4 and IPv6 Headers", RFC 2474, 2520 DOI 10.17487/RFC2474, December 1998, 2521 . 2523 [RFC2545] Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol 2524 Extensions for IPv6 Inter-Domain Routing", RFC 2545, 2525 DOI 10.17487/RFC2545, March 1999, 2526 . 2528 [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., 2529 and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP 2530 Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001, 2531 . 2533 [RFC3444] Pras, A. and J. Schoenwaelder, "On the Difference between 2534 Information Models and Data Models", RFC 3444, 2535 DOI 10.17487/RFC3444, January 2003, 2536 . 2538 [RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with 2539 CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September 2540 2003, . 2542 [RFC3963] Devarapalli, V., Wakikawa, R., Petrescu, A., and P. 2543 Thubert, "Network Mobility (NEMO) Basic Support Protocol", 2544 RFC 3963, DOI 10.17487/RFC3963, January 2005, 2545 . 2547 [RFC4080] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den 2548 Bosch, "Next Steps in Signaling (NSIS): Framework", 2549 RFC 4080, DOI 10.17487/RFC4080, June 2005, 2550 . 2552 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 2553 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2554 2006, . 2556 [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery 2557 Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April 2558 2006, . 2560 [RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD) 2561 for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006, 2562 . 2564 [RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903, 2565 DOI 10.17487/RFC4903, June 2007, 2566 . 2568 [RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6 2569 over Low-Power Wireless Personal Area Networks (6LoWPANs): 2570 Overview, Assumptions, Problem Statement, and Goals", 2571 RFC 4919, DOI 10.17487/RFC4919, August 2007, 2572 . 2574 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 2575 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 2576 . 2578 [RFC5889] Baccelli, E., Ed. and M. Townsley, Ed., "IP Addressing 2579 Model in Ad Hoc Networks", RFC 5889, DOI 10.17487/RFC5889, 2580 September 2010, . 2582 [RFC5974] Manner, J., Karagiannis, G., and A. McDonald, "NSIS 2583 Signaling Layer Protocol (NSLP) for Quality-of-Service 2584 Signaling", RFC 5974, DOI 10.17487/RFC5974, October 2010, 2585 . 2587 [RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility 2588 Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July 2589 2011, . 2591 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 2592 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 2593 January 2012, . 2595 [RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem 2596 Statement and Requirements for IPv6 over Low-Power 2597 Wireless Personal Area Network (6LoWPAN) Routing", 2598 RFC 6606, DOI 10.17487/RFC6606, May 2012, 2599 . 2601 [RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS 2602 SAVI: First-Come, First-Served Source Address Validation 2603 Improvement for Locally Assigned IPv6 Addresses", 2604 RFC 6620, DOI 10.17487/RFC6620, May 2012, 2605 . 2607 [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The 2608 Locator/ID Separation Protocol (LISP)", RFC 6830, 2609 DOI 10.17487/RFC6830, January 2013, 2610 . 2612 [RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and 2613 Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January 2614 2014, . 2616 [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for 2617 Constrained-Node Networks", RFC 7228, 2618 DOI 10.17487/RFC7228, May 2014, 2619 . 2621 [RFC7426] Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S., 2622 Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software- 2623 Defined Networking (SDN): Layers and Architecture 2624 Terminology", RFC 7426, DOI 10.17487/RFC7426, January 2625 2015, . 2627 [RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using 2628 IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the 2629 Internet of Things (IoT): Problem Statement", RFC 7554, 2630 DOI 10.17487/RFC7554, May 2015, 2631 . 2633 [RFC8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH 2634 Operation Sublayer (6top) Protocol (6P)", RFC 8480, 2635 DOI 10.17487/RFC8480, November 2018, 2636 . 2638 8.3. Other Informative References 2640 [ACE] IETF, "Authentication and Authorization for Constrained 2641 Environments", 2642 . 2644 [ANIMA] IETF, "Autonomic Networking Integrated Model and 2645 Approach", 2646 . 2648 [CCAMP] IETF, "Common Control and Measurement Plane", 2649 . 2651 [DETNET] IETF, "Deterministic Networking", 2652 . 2654 [DICE] IETF, "DTLS In Constrained Environments", 2655 . 2657 [HART] www.hartcomm.org, "Highway Addressable remote Transducer, 2658 a group of specifications for industrial process and 2659 control devices administered by the HART Foundation". 2661 [IEC62439] 2662 IEC, "Industrial communication networks - High 2663 availability automation networks - Part 3: Parallel 2664 Redundancy Protocol (PRP) and High-availability Seamless 2665 Redundancy (HSR) - IEC62439-3", 2012, 2666 . 2668 [IEEE802.1TSNTG] 2669 IEEE Standards Association, "IEEE 802.1 Time-Sensitive 2670 Networks Task Group", March 2013, 2671 . 2673 [IEEE802154] 2674 IEEE standard for Information Technology, "IEEE Std. 2675 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) 2676 and Physical Layer (PHY) Specifications for Low-Rate 2677 Wireless Personal Area Networks". 2679 [IEEE802154e] 2680 IEEE standard for Information Technology, "IEEE standard 2681 for Information Technology, IEEE Std. 802.15.4, Part. 2682 15.4: Wireless Medium Access Control (MAC) and Physical 2683 Layer (PHY) Specifications for Low-Rate Wireless Personal 2684 Area Networks, June 2011 as amended by IEEE Std. 2685 802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area 2686 Networks (LR-WPANs) Amendment 1: MAC sublayer", April 2687 2012. 2689 [ISA100] ISA/ANSI, "ISA100, Wireless Systems for Automation", 2690 . 2692 [ISA100.11a] 2693 ISA/ANSI, "Wireless Systems for Industrial Automation: 2694 Process Control and Related Applications - ISA100.11a-2011 2695 - IEC 62734", 2011, . 2698 [PCE] IETF, "Path Computation Element", 2699 . 2701 [TEAS] IETF, "Traffic Engineering Architecture and Signaling", 2702 . 2704 [WirelessHART] 2705 www.hartcomm.org, "Industrial Communication Networks - 2706 Wireless Communication Network and Communication Profiles 2707 - WirelessHART - IEC 62591", 2010. 2709 Appendix A. Dependencies on Work In Progress 2711 In order to control the complexity and the size of the 6TiSCH work, 2712 the architecture and the associated IETF work are staged and the WG 2713 is expected to recharter multiple times. This document is been 2714 incremented as the work progressed following the evolution of the WG 2715 charter and the availability of dependent work. The intent was to 2716 publish when the WG concludes on the covered items. 2718 At the time of publishing: 2720 o The need of a reactive routing protocol to establish on-demand 2721 constraint-optimized routes and a reservation protocol to 2722 establish Layer-3 Tracks is being discussed at 6TiSCH but not 2723 chartered for. 2725 o The operation of the Backbone Router 2726 [I-D.ietf-6lo-backbone-router] is stable but the RFC is not 2727 published yet. The protection of registered addresses against 2728 impersonation and take over will be guaranteed by Address 2729 Protected Neighbor Discovery for Low-power and Lossy Networks 2730 [I-D.ietf-6lo-ap-nd], which is not yet published either. 2732 o The work on centralized Track computation is deferred to a 2733 subsequent work, not necessarily at 6TiSCH. A Predicatable and 2734 Available Wireless (PAW) bar-BoF took place; PAW may form as a WG 2735 and take over that work. The 6TiSCH Architecture should thus 2736 inherit from the DetNet [I-D.ietf-detnet-architecture] 2737 architecture and thus depends on it. The Path Computation Element 2738 (PCE) should be a core component of that architecture. Around the 2739 PCE, a protocol such as an extension to a TEAS [TEAS] protocol 2740 will be required to expose the 6TiSCH node capabilities and the 2741 network peers to the PCE, and a protocol such as a lightweight 2742 PCEP or an adaptation of CCAMP [CCAMP] G-MPLS formats and 2743 procedures will be used to publish the Tracks, as computed by the 2744 PCE, to the 6TiSCH nodes. 2746 o BIER-TE-based OAM, Replication and Elimination 2747 [I-D.thubert-bier-replication-elimination] leverages Bit Index 2748 Explicit Replication - Traffic Engineering to control in the data 2749 plane the DetNet Replication and Elimination activities, and to 2750 provide traceability on links where replication and loss happen, 2751 in a manner that is abstract to the forwarding information, 2752 whereas a 6loRH for BitStrings [I-D.thubert-6lo-bier-dispatch] 2753 proposes a 6LoWPAN compression for the BIER Bitstring based on 2754 6LoWPAN Routing Header [RFC8138]. 2756 o The security model and in particular the join process depends on 2757 the ANIMA [ANIMA] Bootstrapping Remote Secure Key Infrastructures 2758 (BRSKI) [I-D.ietf-anima-bootstrapping-keyinfra] in order to enable 2759 zero-touch security provisionning; for highly constrained nodes, a 2760 minimal model based on pre-shared keys (PSK) is also available. 2762 o The current charter positions 6TiSCH on IEEE Std 802.15.4 only. 2763 Though most of the design should be portable on other link types, 2764 6TiSCH has a strong dependency on IEEE Std 802.15.4 and its 2765 evolution. The impact of changes to TSCH on this Architecture 2766 should be minimal to non-existent, but deeper work such as 6top 2767 and security may be impacted. A 6TiSCH Interest Group at the IEEE 2768 maintains the synchronization and helps foster work at the IEEE 2769 should 6TiSCH demand it. 2771 o Work is being proposed at IEEE (802.15.12 PAR) for an LLC that 2772 would logically include the 6top sublayer. The interaction with 2773 the 6top sublayer and the Scheduling Functions described in this 2774 document are yet to be defined. 2776 o ISA100 [ISA100] Common Network Management (CNM) is another 2777 external work of interest for 6TiSCH. The group, referred to as 2778 ISA100.20, defines a Common Network Management framework that 2779 should enable the management of resources that are controlled by 2780 heterogeneous protocols such as ISA100.11a [ISA100.11a], 2781 WirelessHART [WirelessHART], and 6TiSCH. Interestingly, the 2782 establishment of 6TiSCH Deterministic paths, called Tracks, are 2783 also in scope, and ISA100.20 is working on requirements for 2784 DetNet. 2786 Author's Address 2788 Pascal Thubert (editor) 2789 Cisco Systems, Inc 2790 Building D 2791 45 Allee des Ormes - BP1200 2792 MOUGINS - Sophia Antipolis 06254 2793 FRANCE 2795 Phone: +33 497 23 26 34 2796 Email: pthubert@cisco.com