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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 6TiSCH T. Watteyne, Ed. 3 Internet-Draft Linear Technology 4 Intended status: Informational MR. Palattella 5 Expires: July 12, 2015 University of Luxembourg 6 LA. Grieco 7 Politecnico di Bari 8 January 8, 2015 10 Using IEEE802.15.4e TSCH in an IoT context: 11 Overview, Problem Statement and Goals 12 draft-ietf-6tisch-tsch-05 14 Abstract 16 This document describes the environment, problem statement, and goals 17 for using the IEEE802.15.4e TSCH MAC protocol in the context of LLNs. 18 The set of goals enumerated in this document form an initial set 19 only. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on July 12, 2015. 38 Copyright Notice 40 Copyright (c) 2015 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4 57 3. TSCH in the LLN Context . . . . . . . . . . . . . . . . . . . 4 58 4. Problems and Goals . . . . . . . . . . . . . . . . . . . . . 6 59 4.1. Network Formation . . . . . . . . . . . . . . . . . . . . 7 60 4.2. Network Maintenance . . . . . . . . . . . . . . . . . . . 7 61 4.3. Multi-Hop Topology . . . . . . . . . . . . . . . . . . . 7 62 4.4. Routing and Timing Parents . . . . . . . . . . . . . . . 8 63 4.5. Resource Management . . . . . . . . . . . . . . . . . . . 8 64 4.6. Dataflow Control . . . . . . . . . . . . . . . . . . . . 8 65 4.7. Deterministic Behavior . . . . . . . . . . . . . . . . . 8 66 4.8. Scheduling Mechanisms . . . . . . . . . . . . . . . . . . 9 67 4.9. Secure Communication . . . . . . . . . . . . . . . . . . 9 68 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 69 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 70 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 71 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 72 8.1. Normative References . . . . . . . . . . . . . . . . . . 10 73 8.2. Informative References . . . . . . . . . . . . . . . . . 10 74 8.3. External Informative References . . . . . . . . . . . . . 12 75 Appendix A. TSCH Protocol Highlights . . . . . . . . . . . . . . 13 76 A.1. Timeslots . . . . . . . . . . . . . . . . . . . . . . . . 13 77 A.2. Slotframes . . . . . . . . . . . . . . . . . . . . . . . 14 78 A.3. Node TSCH Schedule . . . . . . . . . . . . . . . . . . . 14 79 A.4. Cells and Bundles . . . . . . . . . . . . . . . . . . . . 14 80 A.5. Dedicated vs. Shared Cells . . . . . . . . . . . . . . . 15 81 A.6. Absolute Slot Number . . . . . . . . . . . . . . . . . . 15 82 A.7. Channel Hopping . . . . . . . . . . . . . . . . . . . . . 16 83 A.8. Time Synchronization . . . . . . . . . . . . . . . . . . 16 84 A.9. Power Consumption . . . . . . . . . . . . . . . . . . . . 17 85 A.10. Network TSCH Schedule . . . . . . . . . . . . . . . . . . 17 86 A.11. Join Process . . . . . . . . . . . . . . . . . . . . . . 18 87 A.12. Information Elements . . . . . . . . . . . . . . . . . . 18 88 A.13. Extensibility . . . . . . . . . . . . . . . . . . . . . . 18 89 Appendix B. TSCH Gotchas . . . . . . . . . . . . . . . . . . . . 19 90 B.1. Collision Free Communication . . . . . . . . . . . . . . 19 91 B.2. Multi-Channel vs. Channel Hopping . . . . . . . . . . . . 19 92 B.3. Cost of (continuous) Synchronization . . . . . . . . . . 19 93 B.4. Topology Stability . . . . . . . . . . . . . . . . . . . 20 94 B.5. Multiple Concurrent Slotframes . . . . . . . . . . . . . 20 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 97 1. Introduction 99 IEEE802.15.4e [IEEE802154e] was published in 2012 as an amendment to 100 the Medium Access Control (MAC) protocol defined by the 101 IEEE802.15.4-2011 [IEEE802154] standard. IEEE802.15.4e will be 102 rolled into the next revision of IEEE802.15.4, scheduled to be 103 published in 2015. The Timeslotted Channel Hopping (TSCH) mode of 104 IEEE802.15.4e is the object of this document. 106 This document describes the main issues arising from the adoption of 107 the IEEE802.15.4e TSCH in the LLN context, following the terminology 108 defined in [I-D.ietf-6tisch-terminology]. 110 TSCH was designed to allow IEEE802.15.4 devices to support a wide 111 range of applications including, but not limited to, industrial ones 112 [IEEE802154e]. At its core is a medium access technique which uses 113 time synchronization to achieve ultra low-power operation and channel 114 hopping to enable high reliability. Synchronization accuracy impacts 115 power consumption, and can vary from micro-seconds to milli-seconds 116 depending on the solution. This is very different from the "legacy" 117 IEEE802.15.4 MAC protocol, and is therefore better described as a 118 "redesign". TSCH does not amend the physical layer; i.e., it can 119 operate on any IEEE802.15.4-compliant hardware. 121 IEEE802.15.4e is the latest generation of ultra-lower power and 122 reliable networking solutions for LLNs. [RFC5673] discusses 123 industrial applications, and highlights the harsh operating 124 conditions as well as the stringent reliability, availability, and 125 security requirements for an LLN to operate in an industrial 126 environment. In these environments, vast deployment environments 127 with large (metallic) equipment cause multi-path fading and 128 interference to thwart any attempt of a single-channel solution to be 129 reliable; the channel agility of TSCH is the key to its ultra high 130 reliability. Commercial networking solutions are available today in 131 which nodes consume 10's of micro-amps on average [CurrentCalculator] 132 with end-to-end packet delivery ratios over 99.999% 133 [doherty07channel]. 135 IEEE802.15.4e has been designed for low-power constrained devices, 136 often called "motes". Several terms are used in the IETF to refer to 137 those devices, including "LLN nodes" [RFC7102] and "constrained 138 nodes" [RFC7228]. In this document, we use the generic (and shorter) 139 term "node", used as a synonym for "LLN node", "constrained node" or 140 "mote". 142 Bringing industrial-like performance into the LLN stack developed by 143 Internet of Things (IoT) related IETF working groups such as 6Lo, 144 ROLL and CoRE opens up new application domains for these networks. 146 Sensors deployed in smart cities [RFC5548] will be able to be 147 installed for years without needing battery replacement. "Umbrella" 148 networks will interconnect smart elements from different entities in 149 smart buildings [RFC5867]. Peel-and-stick switches will obsolete the 150 need for costly conduits for lighting solutions in smart homes 151 [RFC5826]. 153 IEEE802.15.4e TSCH focuses on the MAC layer only. This clean 154 layering allows for TSCH to fit under an IPv6 enabled protocol stack 155 for LLNs, running 6LoWPAN [RFC6282], IPv6 Routing Protocol for Low 156 power and Lossy Networks (RPL) [RFC6550] and the Constrained 157 Application Protocol (CoAP) [RFC7252]. What is missing is a Logical 158 Link Control (LLC) layer between the IP abstraction of a link and the 159 TSCH MAC, which is in charge of scheduling a timeslot for a given 160 packet coming down the stack from the upper layer. 162 While [IEEE802154e] defines the mechanisms for a TSCH node to 163 communicate, it does not define the policies to build and maintain 164 the communication schedule, match that schedule to the multi-hop 165 paths maintained by RPL, adapt the resources allocated between 166 neighbor nodes to the data traffic flows, enforce a differentiated 167 treatment for data generated at the application layer and signaling 168 messages needed by 6LoWPAN and RPL to discover neighbors, react to 169 topology changes, self-configure IP addresses, or manage keying 170 material. 172 In other words, IEEE802.15.4e TSCH is designed to allow optimizations 173 and strong customizations, simplifying the merging of TSCH with a 174 protocol stack based on IPv6, 6LoWPAN, and RPL. 176 2. Requirements Language 178 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 179 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 180 document are to be interpreted as described in RFC 2119 [RFC2119]. 182 3. TSCH in the LLN Context 184 To map the services required by the IP layer to the services provided 185 by the link layer, an adaptation layer is used 186 [palattella12standardized]. The 6LoWPAN working group started 187 working in 2007 on specifications for transmitting IPv6 packets over 188 IEEE802.15.4 networks [RFC4919]. Low-power WPANs are characterized 189 by small packet sizes, support for addresses with different lengths, 190 low bandwidth, star and mesh topologies, battery powered devices, low 191 cost, large number of devices, unknown node positions, high 192 unreliability, and periods during which communication interfaces are 193 turned off to save energy. Given these features, it is clear that 194 the adoption of IPv6 on top of a Low-Power WPAN is not 195 straightforward, but poses strong requirements for the optimization 196 of this adaptation layer. 198 For instance, due to the IPv6 default minimum MTU size (1280 bytes), 199 an un-fragmented IPv6 packet is too large to fit in an IEEE802.15.4 200 frame. Moreover, the overhead due to the 40-byte long IPv6 header 201 wastes the scarce bandwidth available at the PHY layer [RFC4944]. 202 For these reasons, the 6LoWPAN working group has defined an effective 203 adaptation layer [RFC6282]. Further issues encompass the auto- 204 configuration of IPv6 addresses [RFC2460][RFC4862], the compliance 205 with the recommendation on supporting link-layer subnet broadcast in 206 shared networks [RFC3819], the reduction of routing and management 207 overhead [RFC6606], the adoption of lightweight application protocols 208 (or novel data encoding techniques), and the support for security 209 mechanisms (confidentiality and integrity protection, device 210 bootstrapping, key establishment, and management). 212 These features can run on top of TSCH. There are, however, important 213 issues to solve, as highlighted in Section 4. 215 Routing issues are challenging for 6LoWPAN, given the low-power and 216 lossy radio links, the battery-powered nodes, the multi-hop mesh 217 topologies, and the frequent topology changes due to mobility. 218 Successful solutions take into account the specific application 219 requirements, along with IPv6 behavior and 6LoWPAN mechanisms 220 [palattella12standardized]. The ROLL working group has defined RPL 221 in [RFC6550]. RPL can support a wide variety of link layers, 222 including ones that are constrained, potentially lossy, or typically 223 utilized in conjunction with host or router devices with very limited 224 resources, as in building/home automation [RFC5867][RFC5826], 225 industrial environments [RFC5673], and urban applications [RFC5548]. 226 RPL is able to quickly build up network routes, distribute routing 227 knowledge among nodes, and adapt to a changing topology. In a 228 typical setting, nodes are connected through multi-hop paths to a 229 small set of root devices, which are usually responsible for data 230 collection and coordination. For each of them, a Destination 231 Oriented Directed Acyclic Graph (DODAG) is created by accounting for 232 link costs, node attributes/status information, and an Objective 233 Function, which maps the optimization requirements of the target 234 scenario. 236 The topology is set up based on a Rank metric, which encodes the 237 distance of each node with respect to its reference root, as 238 specified by the Objective Function. Regardless of the way it is 239 computed, the Rank monotonically decreases along the DODAG towards 240 the root, building a gradient. RPL encompasses different kinds of 241 traffic and signaling information. Multipoint-to-Point (MP2P) is the 242 dominant traffic in LLN applications. Data is routed towards nodes 243 with some application relevance, such as the LLN gateway to the 244 larger Internet, or to the core of private IP networks. In general, 245 these destinations are the DODAG roots and act as data collection 246 points for distributed monitoring applications. Point-to-Multipoint 247 (P2MP) data streams are used for actuation purposes, where messages 248 are sent from DODAG roots to destination nodes. Point-to-Point (P2P) 249 traffic allows communication between two devices belonging to the 250 same LLN, such as a sensor and an actuator. A packet flows from the 251 source to the common ancestor of those two communicating devices, 252 then downward towards the destination. RPL therefore has to discover 253 both upward routes (i.e. from nodes to DODAG roots) in order to 254 enable MP2P and P2P flows, and downward routes (i.e. from DODAG roots 255 to nodes) to support P2MP and P2P traffic. 257 Section 4 highlights the challenges that need to be addressed to use 258 RPL on top of TSCH. 260 Several open-source initiatives have emerged around TSCH. The 261 OpenWSN project [OpenWSN][OpenWSNETT] is an open-source 262 implementation of a standards-based protocol stack, which aims at 263 evaluating the applicability of TSCH to different applications. This 264 implementation was used as the foundation for an IP for Smart Objects 265 Alliance (IPSO) [IPSO] interoperability event in 2011. In the 266 absence of a standardized scheduling mechanism for TSCH, a "slotted 267 Aloha" schedule was used. 269 4. Problems and Goals 271 As highlighted in Appendix A, TSCH differs from traditional low-power 272 MAC protocols because of its scheduled nature. TSCH defines the 273 mechanisms to execute a communication schedule, yet it is the entity 274 that sets up that schedule which controls the topology of the 275 network. This scheduling entity also controls the resources 276 allocated to each link in that topology. 278 How this entity should operate is out of scope of TSCH. The 279 remainder of this section highlights the problems this entity needs 280 to address. For simplicity, we refer to this entity by the generic 281 name "LLC". Note that the 6top sublayer, currently being defined in 282 [I-D.wang-6tisch-6top-sublayer], can be seen as an embodiment of this 283 generic "LLC". 285 Some of the issues the LLC needs to target might overlap with the 286 scope of other protocols (e.g., 6LoWPAN, RPL, and RSVP). In this 287 case, it is entailed that the LLC will profit from the services 288 provided by other protocols to pursue these objectives. 290 4.1. Network Formation 292 The LLC needs to control the way the network is formed, including how 293 new nodes join, and how already joined nodes advertise the presence 294 of the network. The LLC needs to: 296 1. Define the Information Elements included in the Enhanced Beacons 297 advertising the presence of the network. 299 2. For a new node, define rules to process and filter received 300 Enhanced Beacons. 302 3. Define the joining procedure. This might include a mechanism to 303 assign a unique 16-bit address to a node, and the management of 304 initial keying material. 306 4. Define a mechanism to secure the joining process and the 307 subsequent optional process of scheduling more communication 308 cells. 310 4.2. Network Maintenance 312 Once a network is formed, the LLC needs to maintain the network's 313 health, allowing for nodes to stay synchronized. The LLC needs to: 315 1. Manage each node's time source neighbor. 317 2. Define a mechanism for a node to update the join priority it 318 announces in its Enhanced Beacon. 320 3. Schedule transmissions of Enhanced Beacons to advertise the 321 presence of the network. 323 4.3. Multi-Hop Topology 325 RPL, given a weighted connectivity graph, determines multi-hop 326 routes. The LLC needs to: 328 1. Define a mechanism to gather topological information, node and 329 link state, which it can then feed to RPL. 331 2. Ensure that the TSCH schedule contains cells along the multi-hop 332 routes identified by RPL. 334 3. Where applicable, maintain independent sets of cells to transport 335 independent flows of data. 337 4.4. Routing and Timing Parents 339 At all times, a TSCH node needs to have a time source neighbor it can 340 synchronize to. The LLC therefore needs to assign a time source 341 neighbor to allow for correct operation of the TSCH network. A time 342 source neighbors could, or not, be taken from the RPL routing parent 343 set. 345 4.5. Resource Management 347 A cell in a TSCH schedule is an atomic "unit" of resource. The 348 number of cells to assign between neighbor nodes needs to be 349 appropriate for the size of the traffic flow. The LLC needs to: 351 1. Define a mechanism for neighbor nodes to exchange information 352 about their schedule and, if applicable, negotiate the addition/ 353 deletion of cells. 355 2. Allow for an entity (e.g., a set of devices, a distributed 356 protocol, a PCE, etc.) to take control of the schedule. 358 4.6. Dataflow Control 360 TSCH defines mechanisms for a node to signal it cannot accept an 361 incoming packet. It does not, however, define the policy which 362 determines when to stop accepting packets. The LLC needs to: 364 1. Define a queuing policy for incoming and outgoing packets. 366 2. Manage the buffer space, and indicate to TSCH when to stop 367 accepting incoming packets. 369 3. Handle transmissions that have failed. A transmission is 370 declared failed when TSCH has retransmitted the packet multiple 371 times, without receiving an acknowledgment. This covers both 372 dedicated and shared cells. 374 4.7. Deterministic Behavior 376 As highlighted in [RFC5673], in some applications, data is generated 377 periodically and has a well understood data bandwidth requirement, 378 which is deterministic and predictable. The LLC needs to: 380 1. Ensure timely delivery of such data. 382 2. Provide a mechanism for such deterministic flows to coexist with 383 bursty or infrequent traffic flows of different priorities. 385 4.8. Scheduling Mechanisms 387 Several scheduling mechanisms can be envisioned, and possibly coexist 388 in the same network. For example, 389 [I-D.phinney-roll-rpl-industrial-applicability] describes how the 390 allocation of bandwidth can be optimized by an external Path 391 Computation Element (PCE). Another centralized (PCE-based) traffic- 392 aware scheduling algorithm is defined in [TASA-PIMRC]. 393 Alternatively, two neighbor nodes can adapt the number of cells 394 autonomously by monitoring the amount of traffic, and negotiating the 395 allocation to extra cell when needed. An example of decentralized 396 algorithm is provided in [tinka10decentralized]. This mechanism can 397 be used to establish multi-hop paths in a fashion similar to RSVP. 398 The LLC needs to: 400 1. Provide a mechanism for two 6TiSCH devices to negotiate the 401 allocation and deallocation of cells between them. 403 2. Provide a mechanism for device to monitor and manage the 6TiSCH 404 capabilities of a node several hops away. 406 3. Define an mechanism for these different scheduling mechanisms to 407 coexist in the same network. 409 4.9. Secure Communication 411 Given some keying material, TSCH defines mechanisms to encrypt and 412 authenticate MAC frames. It does not define how this keying material 413 is generated. The LLC needs to: 415 1. Define the keying material and authentication mechanism needed by 416 a new node to join an existing network. 418 2. Define a mechanism to allow for the secure transfer of 419 application data between neighbor nodes. 421 3. Define a mechanism to allow for the secure transfer of signaling 422 data between nodes and the LLC. 424 5. IANA Considerations 426 This memo includes no request to IANA. 428 6. Security Considerations 430 This memo is an informational overview of existing standards, and 431 does define any new mechanisms or protocols. 433 It does describe the need for the 6TiSCH WG to define a secure 434 solution. In particular, Section 4.1 describes security in the join 435 process. Section 4.9 discusses data frame protection. 437 7. Acknowledgments 439 Special thanks to Dominique Barthel, Patricia Brett, Guillaume 440 Gaillard, Pat Kinney, Ines Robles, Timothy J. Salo, Jonathan Simon, 441 Rene Struik, Xavi Vilajosana for reviewing the document and providing 442 valuable feedback. Thanks to the IoT6 European Project (STREP) of 443 the 7th Framework Program (Grant 288445). 445 8. References 447 8.1. Normative References 449 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 450 Requirement Levels", BCP 14, RFC 2119, March 1997. 452 8.2. Informative References 454 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 455 Application Protocol (CoAP)", RFC 7252, June 2014. 457 [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for 458 Constrained-Node Networks", RFC 7228, May 2014. 460 [RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and 461 Lossy Networks", RFC 7102, January 2014. 463 [RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem 464 Statement and Requirements for IPv6 over Low-Power 465 Wireless Personal Area Network (6LoWPAN) Routing", RFC 466 6606, May 2012. 468 [RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., 469 Levis, P., Pister, K., Struik, R., Vasseur, JP., and R. 470 Alexander, "RPL: IPv6 Routing Protocol for Low-Power and 471 Lossy Networks", RFC 6550, March 2012. 473 [RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6 474 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 475 September 2011. 477 [RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen, 478 "Building Automation Routing Requirements in Low-Power and 479 Lossy Networks", RFC 5867, June 2010. 481 [RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation 482 Routing Requirements in Low-Power and Lossy Networks", RFC 483 5826, April 2010. 485 [RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney, 486 "Industrial Routing Requirements in Low-Power and Lossy 487 Networks", RFC 5673, October 2009. 489 [RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel, 490 "Routing Requirements for Urban Low-Power and Lossy 491 Networks", RFC 5548, May 2009. 493 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 494 "Transmission of IPv6 Packets over IEEE 802.15.4 495 Networks", RFC 4944, September 2007. 497 [RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6 498 over Low-Power Wireless Personal Area Networks (6LoWPANs): 499 Overview, Assumptions, Problem Statement, and Goals", RFC 500 4919, August 2007. 502 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 503 Address Autoconfiguration", RFC 4862, September 2007. 505 [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., 506 Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. 507 Wood, "Advice for Internet Subnetwork Designers", BCP 89, 508 RFC 3819, July 2004. 510 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 511 (IPv6) Specification", RFC 2460, December 1998. 513 [I-D.ietf-6tisch-terminology] 514 Palattella, M., Thubert, P., Watteyne, T., and Q. Wang, 515 "Terminology in IPv6 over the TSCH mode of IEEE 516 802.15.4e", draft-ietf-6tisch-terminology-02 (work in 517 progress), July 2014. 519 [I-D.wang-6tisch-6top-sublayer] 520 Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH 521 Operation Sublayer (6top)", draft-wang-6tisch-6top- 522 sublayer-01 (work in progress), July 2014. 524 [I-D.phinney-roll-rpl-industrial-applicability] 525 Phinney, T., Thubert, P., and R. Assimiti, "RPL 526 applicability in industrial networks", draft-phinney-roll- 527 rpl-industrial-applicability-02 (work in progress), 528 February 2013. 530 8.3. External Informative References 532 [IEEE802154e] 533 IEEE standard for Information Technology, "IEEE std. 534 802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area 535 Networks (LR-WPANs) Amendment 1: MAC sublayer", April 536 2012. 538 [IEEE802154] 539 IEEE standard for Information Technology, "IEEE std. 540 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) 541 and Physical Layer (PHY) Specifications for Low-Rate 542 Wireless Personal Area Networks", June 2011. 544 [OpenWSN] "Berkeley's OpenWSN Project Homepage", 545 . 547 [OpenWSNETT] 548 Watteyne, T., Vilajosana, X., Kerkez, B., Chraim, F., 549 Weekly, K., Wang, Q., Glaser, S., and K. Pister, "OpenWSN: 550 a Standards-Based Low-Power Wireless Development 551 Environment", Transactions on Emerging Telecommunications 552 Technologies , August 2012. 554 [IPSO] "IP for Smart Objects Alliance Homepage", 555 . 557 [CurrentCalculator] 558 Linear Technology, "Application Note: Using the Current 559 Calculator to Estimate Mote Power", August 2012, 560 . 564 [doherty07channel] 565 Doherty, L., Lindsay, W., and J. Simon, "Channel-Specific 566 Wireless Sensor Network Path Data", IEEE International 567 Conference on Computer Communications and Networks (ICCCN) 568 2008, 2007. 570 [tinka10decentralized] 571 Tinka, A., Watteyne, T., and K. Pister, "A Decentralized 572 Scheduling Algorithm for Time Synchronized Channel 573 Hopping", Ad Hoc Networks 2010, 2010, < 574 http://robotics.eecs.berkeley.edu/~pister/ 575 publications/2008/TSMP%20DSN08.pdf>. 577 [watteyne09reliability] 578 Watteyne, T., Mehta, A., and K. Pister, "Reliability 579 Through Frequency Diversity: Why Channel Hopping Makes 580 Sense", International Conference on Performance Evaluation 581 of Wireless Ad Hoc, Sensor, and Ubiquitous Networks (PE- 582 WASUN) 2009, Oct. 2009, . 585 [TASA-PIMRC] 586 Palattella, MR., Accettura, N., Dohler, M., Grieco, LA., 587 and G. Boggia, "Traffic Aware Scheduling Algorithm for 588 Multi-Hop IEEE 802.15.4e Networks", IEEE PIMRC 2012, Sept. 589 2012, < http://www.cttc.es/resources/ 590 doc/120531-submitted-tasa-25511.pdf>. 592 [palattella12standardized] 593 Palattella, MR., Accettura, N., Vilajosana, X., Watteyne, 594 T., Grieco, LA., Boggia, G., and M. Dohler, "Standardized 595 Protocol Stack For The Internet Of (Important) Things", 596 IEEE Communications Surveys and Tutorials 2012, Dec. 2012, 597 < http://www.cttc.es/resources/doc/121025- 598 completestackforiot-clean-4818610916636121981.pdf>. 600 Appendix A. TSCH Protocol Highlights 602 This appendix gives an overview of the key features of the 603 IEEE802.15.4e Timeslotted Channel Hopping (TSCH) amendment. It makes 604 no attempt at repeating the standard, but rather focuses on the 605 following: 607 o Concepts which are sufficiently different from traditional 608 IEEE802.15.4 networking that they may need to be defined and 609 presented precisely. 611 o Techniques and ideas which are part of IEEE802.15.4e and which 612 might be useful for the work of the 6TiSCH WG. 614 A.1. Timeslots 616 All nodes in a TSCH network are synchronized. Time is sliced up into 617 timeslots. A timeslot is long enough for a MAC frame of maximum size 618 to be sent from node A to node B, and for node B to reply with an 619 acknowledgment (ACK) frame indicating successful reception. 621 The duration of a timeslot is not defined by the standard. With 622 IEEE802.15.4-compliant radios operating in the 2.4GHz frequency band, 623 a maximum-length frame of 127 bytes takes about 4ms to transmit; a 624 shorter ACK takes about 1ms. With a 10ms slot (a typical duration), 625 this leaves 5ms to radio turnaround, packet processing and security 626 operations. 628 A.2. Slotframes 630 Timeslots are grouped into one of more slotframes. A slotframe 631 continuously repeats over time. TSCH does not impose a slotframe 632 size. Depending on the application needs, these can range from 10s 633 to 1000s of timeslots. The shorter the slotframe, the more often a 634 timeslot repeats, resulting in more available bandwidth, but also in 635 a higher power consumption. 637 A.3. Node TSCH Schedule 639 A TSCH schedule instructs each node what to do in each timeslot: 640 transmit, receive or sleep. The schedule indicates, for each 641 scheduled (transmit or receive) cell, a channelOffset and the address 642 of the neighbor to communicate with. 644 Once a node obtains its schedule, it executes it: 646 o For each transmit cell, the node checks whether there is a packet 647 in the outgoing buffer which matches the neighbor written in the 648 schedule information for that timeslot. If there is none, the 649 node keeps its radio off for the duration of the timeslot. If 650 there is one, the node can ask for the neighbor to acknowledge it, 651 in which case it has to listen for the acknowledgment after 652 transmitting. 654 o For each receive cell, the node listens for possible incoming 655 packets. If none is received after some listening period, it 656 shuts down its radio. If a packet is received, addressed to the 657 node, and passes security checks, the node can send back an 658 acknowledgment. 660 How the schedule is built, updated and maintained, and by which 661 entity, is outside of the scope of the IEEE802.15.4e standard. 663 A.4. Cells and Bundles 665 Assuming the schedule is well built, if node A is scheduled to 666 transmit to node B at slotOffset 5 and channelOffset 11, node B will 667 be scheduled to receive from node A at the same slotOffset and 668 channelOffset. 670 A single element of the schedule characterized by a slotOffset and 671 channelOffset, and reserved for node A to transmit to node B (or for 672 node B to receive from node A) within a given slotframe, is called a 673 "scheduled cell". 675 If there is a lot of data flowing from node A to node B, the schedule 676 might contain multiple cells from A to B, at different times. 677 Multiple cells scheduled to the same neighbor can be equivalent, i.e. 678 the MAC layer sends the packet on whichever of these cells shows up 679 first after the packet was put in the MAC queue. The union of all 680 cells between two neighbors, A and B, is called a "bundle". Since 681 the slotframe repeats over time (and the length of the slotframe is 682 typically constant), each cell gives a "quantum" of bandwidth to a 683 given neighbor. Modifying the number of equivalent cells in a bundle 684 modifies the amount of resources allocated between two neighbors. 686 A.5. Dedicated vs. Shared Cells 688 By default, each scheduled transmit cell within the TSCH schedule is 689 dedicated, i.e., reserved only for node A to transmit to node B. 690 IEEE802.15.4e allows also to mark a cell as shared. In a shared 691 cell, multiple nodes can transmit at the same time, on the same 692 frequency. To avoid contention, TSCH defines a back-off algorithm 693 for shared cells. 695 A scheduled cell can be marked as both transmitting and receiving. 696 In this case, a node transmits if it has an appropriate packet in its 697 output buffer, or listens otherwise. Marking a cell as 698 [transmit,receive,shared] results in slotted-Aloha behavior. 700 A.6. Absolute Slot Number 702 TSCH defines a timeslot counter called Absolute Slot Number (ASN). 703 When a new network is created, the ASN is initialized to 0; from then 704 on, it increments by 1 at each timeslot. In detail: 706 ASN = (k*S+t) 708 where k is the slotframe cycle (i.e., the number of slotframe 709 repetitions since the network was started), S the slotframe size and 710 t the slotOffset. A node learns the current ASN when it joins the 711 network. Since nodes are synchronized, they all know the current 712 value of the ASN, at any time. The ASN is encoded as a 5-byte 713 number: this allows it to increment for hundreds of years (the exact 714 value depends on the duration of a timeslot) without wrapping over. 715 The ASN is used to calculate the frequency to communicate on, and can 716 be used for security-related operations. 718 A.7. Channel Hopping 720 For each scheduled cell, the schedule specifies a slotOffset and a 721 channelOffset. In a well-built schedule, when node A has a transmit 722 cell to node B on channelOffset 5, node B has a receive cell from 723 node A on the same channelOffset. The channelOffset is translated by 724 both nodes into a frequency using the following function: 726 frequency = F {(ASN + channelOffset) mod nFreq} 728 The function F consists of a look-up table containing the set of 729 available channels. The value nFreq (the number of available 730 frequencies) is the size of this look-up table. There are as many 731 channelOffset values as there are frequencies available (e.g. 16 when 732 using IEEE802.15.4-compliant radios at 2.4GHz, when all channels are 733 used). Since both nodes have the same channelOffset written in their 734 schedule for that scheduled cell, and the same ASN counter, they 735 compute the same frequency. At the next iteration (cycle) of the 736 slotframe, however, while the channelOffset is the same, the ASN has 737 changed, resulting in the computation of a different frequency. 739 This results in "channel hopping": even with a static schedule, pairs 740 of neighbors "hop" between the different frequencies when 741 communicating. A way of ensuring communication happens on all 742 available frequencies is to set the number of timeslots in a 743 slotframe to a prime number. Channel hopping is a technique known to 744 efficiently combat multi-path fading and external interference 745 [watteyne09reliability]. 747 A.8. Time Synchronization 749 Because of the slotted nature of communication in a TSCH network, 750 nodes have to maintain tight synchronization. All nodes are assumed 751 to be equipped with clocks to keep track of time. Yet, because 752 clocks in different nodes drift with respect to one another, neighbor 753 nodes need to periodically re-synchronize. 755 Each node needs to periodically synchronize its network clock to 756 another node, and it also provides its network time to its neighbors. 757 It is up to the entity that manages the schedule to assign an 758 adequate time source neighbor to each node, i.e., to indicate in the 759 schedule which of neighbor is its "time source neighbor". While 760 setting the time source neighbor, it is important to avoid 761 synchronization loops, which could result in the formation of 762 independent clusters of synchronized nodes. 764 TSCH adds timing information in all packets that are exchanged (both 765 data and ACK frames). This means that neighbor nodes can 766 resynchronize to one another whenever they exchange data. In detail, 767 two methods are defined in IEEE802.15.4e-2012 for allowing a device 768 to synchronize in a TSCH network: (i) Acknowledgment-Based and (ii) 769 Frame-Based synchronization. In both cases, the receiver calculates 770 the difference in time between the expected time of frame arrival and 771 its actual arrival. In Acknowledgment-Based synchronization, the 772 receiver provides such information to the sender node in its 773 acknowledgment. In this case, it is the sender node that 774 synchronizes to the clock of the receiver. In Frame-Based 775 synchronization, the receiver uses the computed delta for adjusting 776 its own clock. In this case, it is the receiver node that 777 synchronizes to the clock of the sender. 779 Different synchronization policies are possible. Nodes can keep 780 synchronization exclusively by exchanging EBs. Nodes can also keep 781 synchronized by periodically sending valid frames to a time source 782 neighbor and use the acknowledgment to resynchronize. Both method 783 (or a combination thereof) are valid synchronization policies; which 784 one to use depends on network requirements. 786 A.9. Power Consumption 788 There are only a handful of activities a node can perform during a 789 timeslot: transmit, receive, or sleep. Each of these operations has 790 some energy cost associated to them, the exact value depends on the 791 the hardware used. Given the schedule of a node, it is 792 straightforward to calculate the expected average power consumption 793 of that node. 795 A.10. Network TSCH Schedule 797 The schedule entirely defines the synchronization and communication 798 between nodes. By adding/removing cells between neighbors, one can 799 adapt a schedule to the needs of the application. Intuitive examples 800 are: 802 o Make the schedule "sparse" for applications where nodes need to 803 consume as little energy as possible, at the price of reduced 804 bandwidth. 806 o Make the schedule "dense" for applications where nodes generate a 807 lot of data, at the price of increased power consumption. 809 o Add more cells along a multi-hop route over which many packets 810 flow. 812 A.11. Join Process 814 Nodes already part of the network can periodically send Enhanced 815 Beacon (EB) frames to announce the presence of the network. These 816 contain information about the size of the timeslot used in the 817 network, the current ASN, information about the slotframes and 818 timeslots the beaconing node is listening on, and a 1-byte join 819 priority. The join priority field gives information to make a better 820 decision of which node to join. Even if a node is configured to send 821 all EB frames on the same channel offset, because of the channel 822 hopping nature of TSCH described in Appendix A.7, this channel offset 823 translates into a different frequency at different slotframe cycles. 824 As a result, EB frames are sent on all frequencies. 826 A node wishing to join the network listens for EBs. Since EBs are 827 sent on all frequencies, the joining node can listen on any frequency 828 until it hears an EB. What frequency it listens on is 829 implementation-specific. Once it has received one or more EBs, the 830 new node enables the TSCH mode and uses the ASN and the other timing 831 information from the EB to synchronize to the network. Using the 832 slotframe and cell information from the EB, it knows how to contact 833 other nodes in the network. 835 The IEEE802.15.4e TSCH standard does not define the steps beyond this 836 network "bootstrap". 838 A.12. Information Elements 840 TSCH introduces the concept of Information Elements (IEs). An 841 information element is a list of Type-Length-Value containers placed 842 at the end of the MAC header. A small number of types are defined 843 for TSCH (e.g., the ASN in the EB is contained in an IE), and an 844 unmanaged range is available for extensions. 846 A data bit in the MAC header indicates whether the frame contains 847 IEs. IEs are grouped into Header IEs, consumed by the MAC layer and 848 therefore typically invisible to the next higher layer, and Payload 849 IEs, which are passed untouched to the next higher layer, possibly 850 followed by regular payload. Payload IEs can therefore be used for 851 the next higher layers of two neighbor nodes to exchange information. 853 A.13. Extensibility 855 The TSCH standard is designed to be extensible. It introduces the 856 mechanisms as "building block" (e.g., cells, bundles, slotframes, 857 etc.), but leaves entire freedom to the upper layer to assemble 858 those. The MAC protocol can be extended by defining new Header IEs. 860 An intermediate layer can be defined to manage the MAC layer by 861 defining new Payload IEs. 863 Appendix B. TSCH Gotchas 865 This section lists features of TSCH which we believe are important 866 and beneficial to the work of 6TiSCH. 868 B.1. Collision Free Communication 870 TSCH allows one to design a schedule which yields collision-free 871 communication. This is done by building the schedule with dedicated 872 cells in such a way that at most one node communicates with a 873 specific neighbor in each slotOffset/channelOffset cell. Multiple 874 pairs of neighbor nodes can exchange data at the same time, but on 875 different frequencies. 877 B.2. Multi-Channel vs. Channel Hopping 879 A TSCH schedule looks like a matrix of width "slotframe size", S, and 880 of height "number of frequencies", nFreq. For a scheduling 881 algorithm, cells can be considered atomic "units" to schedule. In 882 particular, because of the channel hopping nature of TSCH, the 883 scheduling algorithm should not worry about the actual frequency 884 communication happens on, since it changes at each slotframe 885 iteration. 887 B.3. Cost of (continuous) Synchronization 889 When there is traffic in the network, nodes which are communicating 890 implicitly re-synchronize using the data frames they exchange. In 891 the absence of data traffic, nodes are required to synchronize to 892 their time source neighbor(s) periodically not to drift in time. If 893 they have not been communicating for some time (typically 30s), nodes 894 can exchange an dummy data frame to re-synchronize. The frequency at 895 which such messages need to be transmitted depends on the stability 896 of the clock source, and on how "early" each node starts listening 897 for data (the "guard time"). Theoretically, with a 10ppm clock and a 898 1ms guard time, this period can be 100s. Assuming this exchange 899 causes the node's radio to be on for 5ms, this yields a radio duty 900 cycle needed to keep synchronized of 5ms/100s=0.005%. While TSCH does 901 requires nodes to resynchronize periodically, the cost of doing so is 902 very low. 904 B.4. Topology Stability 906 The channel hopping nature of TSCH causes links to be very "stable". 907 Wireless phenomena such as multi-path fading and external 908 interference impact a wireless link between two nodes differently on 909 each frequency. If a transmission from node A to node B fails, 910 retransmitting on a different frequency has a higher likelihood of 911 succeeding that retransmitting on the same frequency. As a result, 912 even when some frequencies are "behaving bad", channel hopping 913 "smoothens" the contribution of each frequency, resulting in more 914 stable links, and therefore a more stable topology. 916 B.5. Multiple Concurrent Slotframes 918 The TSCH standard allows for multiple slotframes to coexist in a 919 node's schedule. It is possible that, at some timeslot, a node has 920 multiple activities scheduled (e.g. transmit to node B on slotframe 921 2, receive from node C on slotframe 1). To handle this situation, 922 the TSCH standard defines the following precedence rules: 924 1. Transmissions take precedence over receptions; 926 2. Lower slotframe identifiers take precedence over higher slotframe 927 identifiers. 929 In the example above, the node would transmit to node B on slotframe 930 2. 932 Authors' Addresses 934 Thomas Watteyne (editor) 935 Linear Technology 936 32990 Alvarado-Niles Road, Suite 910 937 Union City, CA 94587 938 USA 940 Phone: +1 (510) 400-2978 941 Email: twatteyne@linear.com 942 Maria Rita Palattella 943 University of Luxembourg 944 Interdisciplinary Centre for Security, Reliability and Trust 945 4, rue Alphonse Weicker 946 Luxembourg L-2721 947 LUXEMBOURG 949 Phone: +352 46 66 44 5841 950 Email: maria-rita.palattella@uni.lu 952 Luigi Alfredo Grieco 953 Politecnico di Bari 954 Department of Electrical and Information Engineering 955 Via Orabona 4 956 Bari 70125 957 Italy 959 Phone: +39 08 05 96 3911 960 Email: a.grieco@poliba.it