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2 6TiSCH T. Watteyne, Ed.
3 Internet-Draft Linear Technology
4 Intended status: Informational MR. Palattella
5 Expires: May 25, 2014 University of Luxembourg
6 LA. Grieco
7 Politecnico di Bari
8 November 21, 2013
10 Using IEEE802.15.4e TSCH in an LLN context:
11 Overview, Problem Statement and Goals
12 draft-ietf-6tisch-tsch-00
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 May 25, 2014.
38 Copyright Notice
40 Copyright (c) 2013 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 . . . . . . . . . . . . . . . . . . . . . . . . 2
56 2. TSCH in the LLN Context . . . . . . . . . . . . . . . . . . . 4
57 3. Problems and Goals . . . . . . . . . . . . . . . . . . . . . 5
58 3.1. Network Formation . . . . . . . . . . . . . . . . . . . . 6
59 3.2. Network Maintenance . . . . . . . . . . . . . . . . . . . 6
60 3.3. Multi-Hop Topology . . . . . . . . . . . . . . . . . . . 7
61 3.4. Routing and Timing Parents . . . . . . . . . . . . . . . 7
62 3.5. Resource Management . . . . . . . . . . . . . . . . . . . 7
63 3.6. Dataflow Control . . . . . . . . . . . . . . . . . . . . 8
64 3.7. Deterministic Behavior . . . . . . . . . . . . . . . . . 8
65 3.8. Scheduling Mechanisms . . . . . . . . . . . . . . . . . . 8
66 3.9. Secure Communication . . . . . . . . . . . . . . . . . . 9
67 4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
68 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
69 5.1. Normative References . . . . . . . . . . . . . . . . . . 9
70 5.2. Informative References . . . . . . . . . . . . . . . . . 9
71 5.3. External Informative References . . . . . . . . . . . . . 12
72 Appendix A. TSCH Protocol Highlights . . . . . . . . . . . . . . 14
73 A.1. Timeslots . . . . . . . . . . . . . . . . . . . . . . . . 14
74 A.2. Slotframes . . . . . . . . . . . . . . . . . . . . . . . 15
75 A.3. Node TSCH Schedule . . . . . . . . . . . . . . . . . . . 15
76 A.4. Cells and Bundles . . . . . . . . . . . . . . . . . . . . 15
77 A.5. Dedicated vs. Shared Cells . . . . . . . . . . . . . . . 16
78 A.6. Absolute Slot Number . . . . . . . . . . . . . . . . . . 16
79 A.7. Channel Hopping . . . . . . . . . . . . . . . . . . . . . 16
80 A.8. Time Synchronization . . . . . . . . . . . . . . . . . . 17
81 A.9. Power Consumption . . . . . . . . . . . . . . . . . . . . 18
82 A.10. Network TSCH Schedule . . . . . . . . . . . . . . . . . . 18
83 A.11. Join Process . . . . . . . . . . . . . . . . . . . . . . 18
84 A.12. Information Elements . . . . . . . . . . . . . . . . . . 19
85 A.13. Extensibility . . . . . . . . . . . . . . . . . . . . . . 19
86 Appendix B. TSCH Gotchas . . . . . . . . . . . . . . . . . . . . 19
87 B.1. Collision Free Communication . . . . . . . . . . . . . . 19
88 B.2. Multi-Channel vs. Channel Hopping . . . . . . . . . . . . 19
89 B.3. Cost of (continuous) Synchronization . . . . . . . . . . 20
90 B.4. Topology Stability . . . . . . . . . . . . . . . . . . . 20
91 B.5. Multiple Concurrent Slotframes . . . . . . . . . . . . . 20
92 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
94 1. Introduction
95 The IEEE802.15.4e standard [IEEE802154e] was published in 2012 as an
96 amendment to the Medium Access Control (MAC) protocol defined by the
97 IEEE802.15.4-2011 [IEEE802154] standard. The Timeslotted Channel
98 Hopping (TSCH) mode of IEEE802.15.4e is the object of this document.
100 This document describes the main issues arising from the adoption of
101 the IEEE802.15.4e TSCH in the LLN context, following the terminology
102 defined in [I-D.palattella-6tisch-terminology].
104 TSCH was designed to "allow IEEE802.15.4 devices to support a wide
105 range of industrial applications" [IEEE802154e]. At its core is a
106 medium access technique which uses time synchronization to achieve
107 ultra low-power operation and channel hopping to enable high
108 reliability. This is very different from the "legacy" IEEE802.15.4
109 MAC protocol, and is therefore better described as a "redesign".
110 TSCH does not amend the physical layer; i.e., it can operate on any
111 IEEE802.15.4-compliant hardware.
113 IEEE802.15.4e can be seen as the latest generation of ultra-lower
114 power and reliable networking solutions for LLNs. [RFC5673]
115 discusses industrial applications, and highlights the harsh operating
116 conditions as well as the stringent reliability, availability, and
117 security requirements for an LLN to operate in an industrial
118 environment. Commercial networking solutions are available today in
119 which motes consume 10's of micro-amps on average [CurrentCalculator]
120 with end-to-end packet delivery ratios over 99.999%
121 [doherty07channel].
123 IEEE802.15.4e TSCH focuses on the MAC layer only. This clean
124 layering allows for TSCH to fit under an IPv6 enabled protocol stack
125 for LLNs, running 6LoWPAN [RFC6282], RPL [RFC6550] and CoAP
126 [I-D.ietf-core-coap].
128 Bringing industrial-like performance into the LLN stack developed by
129 the 6LoWPAN, ROLL and CORE working groups opens up new application
130 domains for these networks. Sensors deployed in smart cities
131 [RFC5548] will be able to be installed for years without needing
132 battery replacement. "Umbrella" networks will interconnect smart
133 elements from different entities in smart buildings [RFC5867]. Peel-
134 and-stick switches will obsolete the need for costly conduits for
135 lighting solutions in smart homes [RFC5826].
137 While [IEEE802154e] defines the mechanisms for a TSCH mote to
138 communicate, it does not define the policies to build and maintain
139 the communication schedule, match that schedule to the multi-hop
140 paths maintained by RPL, adapt the resources allocated between
141 neighbor nodes to the data traffic flows, enforce a differentiated
142 treatment for data generated at the application layer and signalling
143 messages needed by 6LoWPAN and RPL to discover neighbors, react to
144 topology changes, self-configure IP addresses, or manage keying
145 material.
147 In other words, IEEE802.15.4e TSCH is designed to allow optimizations
148 and strong customizations, simplifying the merging of TSCH with a
149 protocol stack based on IPv6, 6LoWPAN, and RPL.
151 2. TSCH in the LLN Context
153 In many cases, to map the services required by the IP layer to the
154 services provided by the link layer, an adaptation layer is used
155 [palattella12standardized]. The 6LoWPAN working group started
156 working in 2007 on specifications for transmitting IPv6 packets over
157 IEEE802.15.4 networks [RFC4919]. Typically, low-power WPANs are
158 characterized by small packet sizes, support for addresses with
159 different lengths, low bandwidth, star and mesh topologies, battery
160 powered devices, low cost, large number of devices, unknown node
161 positions, high unreliability, and periods during which communication
162 interfaces are turned off to save energy. Given these features, it
163 is clear that the adoption of IPv6 on top of a Low-Power WPAN is not
164 straightforward, but poses strong requirements for the optimization
165 of this adaptation layer. For instance, due to the IPv6 default
166 minimum MTU size (1280 bytes), an un-fragmented IPv6 packet is too
167 large to fit in an IEEE802.15.4 frame. Moreover, the overhead due to
168 the 40-byte long IPv6 header wastes the scarce bandwidth available at
169 the PHY layer [RFC4944]. For these reasons, the 6LoWPAN working
170 group has defined an effective adaptation layer [RFC6568]. Further
171 issues encompass the auto-configuration of IPv6 addresses
172 [RFC2464][RFC6755], the compliance with the recommendation on
173 supporting link-layer subnet broadcast in shared networks [RFC3819],
174 the reduction of routing and management overhead [RFC6606], the
175 adoption of lightweight application protocols (or novel data encoding
176 techniques), and the support for security mechanisms (confidentiality
177 and integrity protection, device bootstrapping, key establishment,
178 and management).
180 These features can run on top of TSCH. There are, however, important
181 issues to solve, as highlighted in Section 3.
183 Routing issues are challenging for 6LoWPAN, given the low-power and
184 lossy radio-links, the battery-powered nodes, the multi-hop mesh
185 topologies, and the frequent topology changes due to mobility.
186 Successful solutions take into account the specific application
187 requirements, along with IPv6 behavior and 6LoWPAN mechanisms
188 [palattella12standardized]. The ROLL working group has defined RPL
189 in [RFC6550]. RPL can support a wide variety of link layers,
190 including ones that are constrained, potentially lossy, or typically
191 utilized in conjunction with host or router devices with very limited
192 resources, as in building/home automation [RFC5867][RFC5826],
193 industrial environments [RFC5673], and urban applications [RFC5548].
194 RPL is able to quickly build up network routes, distribute routing
195 knowledge among nodes, and adapt to a changing topology. In a
196 typical setting, motes are connected through multi-hop paths to a
197 small set of root devices, which are usually responsible for data
198 collection and coordination. For each of them, a Destination
199 Oriented Directed Acyclic Graph (DODAG) is created by accounting for
200 link costs, node attributes/status information, and an Objective
201 Function, which maps the optimization requirements of the target
202 scenario. The topology is set up based on a Rank metric, which
203 encodes the distance of each node with respect to its reference root,
204 as specified by the Objective Function. Regardless of the way it is
205 computed, the Rank monotonically decreases along the DODAG towards
206 the destination, building a gradient. RPL encompasses different
207 kinds of traffic and signalling information. Multipoint-to-Point
208 (MP2P) is the dominant traffic in LLN applications. Data is routed
209 towards nodes with some application relevance, such as the LLN
210 gateway to the larger Internet, or to the core of private IP
211 networks. In general, these destinations are the DODAG roots and act
212 as data collection points for distributed monitoring applications.
213 Point-to-Multipoint (P2MP) data streams are used for actuation
214 purposes, where messages are sent from DODAG roots to destination
215 nodes. Point-to-Point (P2P) traffic allows communication between two
216 devices belonging to the same LLN, such as a sensor and an actuator.
217 A packet flows from the source to the common ancestor of those two
218 communicating devices, then downward towards the destination. RPL
219 therefore has to discover both upward routes (i.e. from nodes to
220 DODAG roots) in order to enable MP2P and P2P flows, and downward
221 routes (i.e. from DODAG roots to nodes) to support P2MP and P2P
222 traffic.
224 Section 3 highlights the challenges that need to be addressed to use
225 RPL on top of TSCH.
227 Several open-source initiatives have emerged around TSCH. The
228 OpenWSN project [OpenWSN][OpenWSNETT] is an open-source
229 implementation of a standards-based protocol stack, which aims at
230 evaluating the applicability of TSCH to different applications. This
231 implementation was used as the foundation for an IP for Smart Objects
232 Alliance (IPSO) [IPSO] interoperability event in 2011. In the
233 absence of a standardized scheduling mechanism for TSCH, a "slotted
234 Aloha" schedule was used.
236 3. Problems and Goals
237 As highlighted in Appendix A, TSCH is different for traditional low-
238 power MAC protocols because of its scheduled nature. TSCH defines
239 the mechanisms to execute a communication schedule, yet it is the
240 entity that sets up that schedule which controls the topology of the
241 network. This scheduling entity also controls the resources
242 allocated to each link in that topology.
244 How this entity should operate is out of scope of TSCH. The
245 remainder of this section highlights the problems this entity needs
246 to address. For simplicity, we will refer to this entity by the
247 generic name "6TiSCH". Note that the 6top sublayer, currently being
248 defined in [I-D.wang-6tsch-6top], can be seen as an embodiment of
249 this generic "6TiSCH".
251 Some of the issues 6TiSCH needs to target might overlap with the
252 scope of other protocols (e.g., 6LoWPAN, RPL, and RSVP). In this
253 case, it is entailed that 6TiSCH will profit from the services
254 provided by other protocols to pursue these objectives.
256 3.1. Network Formation
258 6TiSCH needs to control the way the network is formed, including how
259 new motes join, and how already joined motes advertise the presence
260 of the network. 6TiSCH needs to:
262 1. Define the Information Elements to include in the Enhanced
263 Beacons advertising the presence of the network.
265 2. For a new mote, define rules to process and filter received
266 Enhanced Beacons.
268 3. Define the joining procedure. This includes a mechanism to
269 assign a unique 16-bit address to a mote, and the management of
270 initial keying material.
272 4. Define a mechanism to secure the joining process and the
273 subsequent optional process of scheduling more communication
274 links.
276 3.2. Network Maintenance
278 Once a network is formed, 6TiSCH needs to maintain the network's
279 health, allowing for motes to stay synchronized. 6TiSCH needs to:
281 1. Manage each mote's time source neighbor.
283 2. Define a mechanism for a mote to update the join priority it
284 announces in its Enhanced Beacon.
286 3. Schedule transmissions of Enhanced Beacons to advertise the
287 presence of the network.
289 3.3. Multi-Hop Topology
291 RPL, given a weighted connectivity graph, determines multi-hop
292 routes. 6TiSCH needs to:
294 1. Define a mechanism to gather topological information, which it
295 can then feed to RPL.
297 2. Ensure that the TSCH schedule contains links along the multi-hop
298 routes identified by RPL.
300 3. Where applicable, maintain independent sets of links to transport
301 independent flows of data.
303 3.4. Routing and Timing Parents
305 At all times, a TSCH mote needs to have a time source neighbor it can
306 synchronize to. 6TiSCH therefore needs to assign a time source
307 neighbor to allow for correct operation of the TSCH network. A time
308 source neighbors could, or not, be taken from the RPL routing parent
309 set.
311 3.5. Resource Management
313 A link in a TSCH schedule is a "unit" of resource. The number of
314 links to assign between neighbor motes needs to be appropriate for
315 the size of the traffic flow. 6TiSCH needs to:
317 1. Define rules on when to create or delete a slotframe.
319 2. Define rules to determine the length of a slotframe, and the
320 trigger to modify the length of a slotframe.
322 3. Define rules on when to add or delete links in a particular
323 slotframe.
325 4. Define a mechanism for neighbor nodes to exchange information
326 about their schedule and, if applicable, negotiate the addition/
327 deletion of links.
329 5. Allow for an entity (e.g., a set of devices, a distributed
330 protocol, a PCE, etc.) to take control of the schedule.
332 6. Define a set of metrics to evaluate the trade-off between
333 latency, bandwidth and energy consumption achieved by a
334 particular schedule.
336 3.6. Dataflow Control
338 TSCH defines mechanisms for a mote to signal it cannot accept an
339 incoming packet. It does not, however, define the policy which
340 determines when to stop accepting packets. 6TiSCH needs to:
342 1. Define a queueing policy for incoming and outgoing packets.
344 2. Manage the buffer space, and indicate to TSCH when to stop
345 accepting incoming packets.
347 3. Handle transmissions that have failed. A transmission is
348 declared failed when TSCH has retransmitted the packet multiple
349 times, without receiving an acknowledgement. This covers both
350 dedicated and shared links.
352 3.7. Deterministic Behavior
354 As highlighted in [RFC5673], in some applications, data is generated
355 periodically and has a well understood data bandwidth requirement,
356 which is deterministic and predictable. 6TiSCH needs to:
358 1. Ensure timely delivery of such data.
360 2. Provide a mechanism for such deterministic flows to coexist with
361 bursty or infrequent traffic flows of different priorities.
363 3.8. Scheduling Mechanisms
365 Several scheduling mechanisms can be envisioned, and possibly coexist
366 in the same network. For example,
367 [I-D.phinney-roll-rpl-industrial-applicability] describe how the
368 allocation of bandwidth can be optimized by an external Path
369 Computation Element (PCE). Alternatively, two neighbor nodes can
370 adapt the number of cells autonomously by monitoring the amount of
371 traffic, and negotiating the allocation to extra cell when needed.
372 This mechanism can be used to establish multi-hop paths in a fashion
373 similar to RSVP. 6TiSCH needs to:
375 1. Provide a mechanism for two 6TiSCH devices to negotiate the
376 allocation and deallocation of cells between them.
378 2. Provide a mechanism for device to monitor and manage the 6TiSCH
379 capabilities of a node several hops away.
381 3. Define an mechanism for these different scheduling mechanisms to
382 coexist in the same network.
384 3.9. Secure Communication
386 Given some keying material, TSCH defines mechanisms to encrypt and
387 authenticate MAC frames. It does not define how this keying material
388 is generated. 6TiSCH needs to:
390 1. Define the keying material and authentication mechanism needed by
391 a new mote to join an existing network.
393 2. Define a mechanism to allow for the secure transfer of
394 application data between neighbor motes.
396 3. Define a mechanism to allow for the secure transfer of signalling
397 data between motes and 6TiSCH.
399 4. Acknowledgements
401 Special thanks to Jonathan Simon for his review and valuable
402 comments. Thanks to the IoT6 European Project (STREP) of the 7th
403 Framework Program (Grant 288445).
405 5. References
407 5.1. Normative References
409 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
410 Requirement Levels", BCP 14, RFC 2119, March 1997.
412 5.2. Informative References
414 [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
415 Networks", RFC 2464, December 1998.
417 [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
418 Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
419 Wood, "Advice for Internet Subnetwork Designers", BCP 89,
420 RFC 3819, July 2004.
422 [RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
423 over Low-Power Wireless Personal Area Networks (6LoWPANs):
424 Overview, Assumptions, Problem Statement, and Goals", RFC
425 4919, August 2007.
427 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
428 "Transmission of IPv6 Packets over IEEE 802.15.4
429 Networks", RFC 4944, September 2007.
431 [RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
432 "Routing Requirements for Urban Low-Power and Lossy
433 Networks", RFC 5548, May 2009.
435 [RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
436 Routing Requirements in Low-Power and Lossy Networks", RFC
437 5826, April 2010.
439 [RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
440 "Building Automation Routing Requirements in Low-Power and
441 Lossy Networks", RFC 5867, June 2010.
443 [RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
444 "Industrial Routing Requirements in Low-Power and Lossy
445 Networks", RFC 5673, October 2009.
447 [RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
448 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
449 September 2011.
451 [RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
452 Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
453 Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
454 Lossy Networks", RFC 6550, March 2012.
456 [RFC6568] Kim, E., Kaspar, D., and JP. Vasseur, "Design and
457 Application Spaces for IPv6 over Low-Power Wireless
458 Personal Area Networks (6LoWPANs)", RFC 6568, April 2012.
460 [RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
461 Statement and Requirements for IPv6 over Low-Power
462 Wireless Personal Area Network (6LoWPAN) Routing", RFC
463 6606, May 2012.
465 [RFC6755] Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
466 for OAuth", RFC 6755, October 2012.
468 [I-D.wang-6tsch-6top]
469 Wang, Q., Vilajosana, X., and T. Watteyne, "6TSCH
470 Operation Sublayer (6top)", draft-wang-6tsch-6top-00 (work
471 in progress), July 2013.
473 [I-D.palattella-6tisch-terminology]
474 Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
475 "Terminology in IPv6 over the TSCH mode of IEEE
476 802.15.4e", draft-palattella-6tisch-terminology-00 (work
477 in progress), October 2013.
479 [I-D.thubert-roll-forwarding-frags]
480 Thubert, P. and J. Hui, "LLN Fragment Forwarding and
481 Recovery", draft-thubert-roll-forwarding-frags-02 (work in
482 progress), September 2013.
484 [I-D.tsao-roll-security-framework]
485 Tsao, T., Alexander, R., Daza, V., and A. Lozano, "A
486 Security Framework for Routing over Low Power and Lossy
487 Networks", draft-tsao-roll-security-framework-02 (work in
488 progress), March 2010.
490 [I-D.thubert-roll-asymlink]
491 Thubert, P., "RPL adaptation for asymmetrical links",
492 draft-thubert-roll-asymlink-02 (work in progress),
493 December 2011.
495 [I-D.ietf-roll-terminology]
496 Vasseur, J., "Terms used in Routing for Low power And
497 Lossy Networks", draft-ietf-roll-terminology-13 (work in
498 progress), October 2013.
500 [I-D.ietf-roll-p2p-rpl]
501 Goyal, M., Baccelli, E., Philipp, M., Brandt, A., and J.
502 Martocci, "Reactive Discovery of Point-to-Point Routes in
503 Low Power and Lossy Networks", draft-ietf-roll-p2p-rpl-17
504 (work in progress), March 2013.
506 [I-D.ietf-roll-trickle-mcast]
507 Hui, J. and R. Kelsey, "Multicast Protocol for Low power
508 and Lossy Networks (MPL)", draft-ietf-roll-trickle-
509 mcast-05 (work in progress), August 2013.
511 [I-D.thubert-6lowpan-backbone-router]
512 Thubert, P., "6LoWPAN Backbone Router", draft-thubert-
513 6lowpan-backbone-router-03 (work in progress), February
514 2013.
516 [I-D.sarikaya-core-sbootstrapping]
517 Sarikaya, B., Ohba, Y., Moskowitz, R., Cao, Z., and R.
518 Cragie, "Security Bootstrapping Solution for Resource-
519 Constrained Devices", draft-sarikaya-core-
520 sbootstrapping-04 (work in progress), April 2012.
522 [I-D.gilger-smart-object-security-workshop]
523 Gilger, J. and H. Tschofenig, "Report from the 'Smart
524 Object Security Workshop', 23rd March 2012, Paris,
525 France", draft-gilger-smart-object-security-workshop-00
526 (work in progress), October 2012.
528 [I-D.phinney-roll-rpl-industrial-applicability]
529 Phinney, T., Thubert, P., and R. Assimiti, "RPL
530 applicability in industrial networks", draft-phinney-roll-
531 rpl-industrial-applicability-02 (work in progress),
532 February 2013.
534 [I-D.ietf-core-coap]
535 Shelby, Z., Hartke, K., and C. Bormann, "Constrained
536 Application Protocol (CoAP)", draft-ietf-core-coap-18
537 (work in progress), June 2013.
539 5.3. External Informative References
541 [IEEE802154e]
542 IEEE standard for Information Technology, "IEEE std.
543 802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
544 Networks (LR-WPANs) Amendament 1: MAC sublayer", April
545 2012.
547 [IEEE802154]
548 IEEE standard for Information Technology, "IEEE std.
549 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
550 and Physical Layer (PHY) Specifications for Low-Rate
551 Wireless Personal Area Networks", June 2011.
553 [OpenWSN] , "Berkeley's OpenWSN Project Homepage",
554 .
556 [OpenWSNETT]
557 Watteyne, T., Vilajosana, X., Kerkez, B., Chraim, F.,
558 Weekly, K., Wang, Q., Glaser, S., and K. Pister, "OpenWSN:
559 a standards-based low-power wireless development
560 environment", Transactions on Emerging Telecommunications
561 Technologies 2012, August 2012, .
564 [IPSO] , "IP for Smart Objects Alliance Homepage",
565 .
567 [CurrentCalculator]
568 Linear Technology, "Application Note: Using the Current
569 Calculator to Estimate Mote Power", August 2012, .
573 [doherty07channel]
574 Doherty, L., Lindsay, W., and J. Simon, "Channel-Specific
575 Wireless Sensor Network Path Data", IEEE International
576 Conference on Computer Communications and Networks (ICCCN)
577 2008, 2007.
579 [tinka10decentralized]
580 Tinka, A., Watteyne, T., and K. Pister, "A Decentralized
581 Scheduling Algorithm for Time Synchronized Channel
582 Hopping", Ad Hoc Networks 2010, 2010, < http://
583 robotics.eecs.berkeley.edu/~pister/publications/2008/
584 TSMP%20DSN08.pdf>.
586 [watteyne09reliability]
587 Watteyne, T., Mehta, A., and K. Pister, "Reliability
588 Through Frequency Diversity: Why Channel Hopping Makes
589 Sense", International Conference on Performance Evaluation
590 of Wireless Ad Hoc, Sensor, and Ubiquitous Networks (PE-
591 WASUN) 2009, Oct. 2009, .
594 [kerkez09feasibility]
595 Kerkez, B., Watteyne, T., and M. Magliocco, "Feasibility
596 analysis of controller design for adaptive channel
597 hopping", International Workshop on Performance
598 Methodologies and Tools for Wireless Sensor Networks
599 (WSNPERF) 2009, Oct. 2009, .
603 [TASA-PIMRC]
604 Palattella, MR., Accettura, N., Dohler, M., Grieco, LA.,
605 and G. Boggia, "Traffic Aware Scheduling Algorithm for
606 Multi-Hop IEEE 802.15.4e Networks", IEEE PIMRC 2012, Sept.
607 2012, < http://www.cttc.es/resources/doc/120531-submitted-
608 tasa-25511.pdf>.
610 [TASA-SENSORS]
611 Palattella, MR., Accettura, N., Dohler, M., Grieco, LA.,
612 and G. Boggia, "Traffic-Aware Time-Critical Scheduling In
613 Heavily Duty-Cycled IEEE 802.15.4e For An Industrial IoT",
614 IEEE SENSORS 2012, Oct. 2012, < http://www.cttc.es/
615 resources/doc/120821-sensors2012-4396981770946977737.pdf>.
617 [TASA-WCNC]
618 Accettura, N., Palattella, MR., Dohler, M., Grieco, LA.,
619 and G. Boggia, "Standardized Power-Efficient and Internet-
620 Enabled Communication Stack for Capillary M2M Networks",
621 IEEE WCNC 2012, Apr. 2012, < http://www.cttc.es/resources/
622 doc/120109-1569521283-submitted-58230.pdf>.
624 [palattella12standardized]
625 Palattella, MR., Accettura, N., Vilajosana, X., Watteyne,
626 T., Grieco, LA., Boggia, G., and M. Dohler, "Standardized
627 Protocol Stack For The Internet Of (Important) Things",
628 IEEE Communications Surveys and Tutorials 2012, Dec. 2012,
629 < http://www.cttc.es/resources/doc/121025
630 -completestackforiot-clean-4818610916636121981.pdf>.
632 [PANA] Kanda, M., Ohba, Y., Das, S., and S. Chasko, "PANA
633 applicability in constrained environments", Febr. 2012,
634 .
637 Appendix A. TSCH Protocol Highlights
639 This appendix gives an overview of the key features of the
640 IEEE802.15.4e Timeslotted Channel Hopping (TSCH) amendment. It makes
641 no attempt at repeating the standard, but rather focuses on the
642 following:
644 o Concepts which are sufficiently different from traditional
645 IEEE802.15.4 networking that they may need to be defined and
646 presented precisely.
648 o Techniques and ideas which are part of IEEE802.15.4e and which
649 might be useful for the work of the 6TiSCH WG.
651 A.1. Timeslots
653 All motes in a TSCH network are synchronized. Time is sliced up into
654 timeslots. A timeslot is long enough for a MAC frame of maximum size
655 to be sent from mote A to mote B, and for mote B to reply with an
656 acknowledgement (ACK) frame indicating successful reception.
658 The duration of a timeslot is not defined by the standard. With
659 IEEE802.15.4-compliant radios operating in the 2.4GHz frequency band,
660 a maximum-length frame of 127 bytes takes about 4ms to transmit; a
661 shorter ACK takes about 1ms. With a 10ms slot (a typical duration),
662 this leaves 5ms to radio turnaround, packet processing and security
663 operations.
665 A.2. Slotframes
667 Timeslots are grouped into one of more slotframes. A slotframe
668 continuously repeats over time. TSCH does not impose a slotframe
669 size. Depending on the application needs, these can range from 10s
670 to 1000s of timeslots. The shorter the slotframe, the more often a
671 timeslot repeats, resulting in more available bandwidth, but also in
672 a higher power consumption.
674 A.3. Node TSCH Schedule
676 A TSCH schedule instructs each mote what to do in each timeslot:
677 transmit, receive or sleep. The schedule indicates, for each
678 scheduled (transmit or receive) cell a channelOffset and the address
679 of the neighbor to communicate with.
681 Once a mote obtains its schedule, it executes it:
683 o For each transmit cell, the mote checks whether there is a packet
684 in the outgoing buffer which matches the neighbor written in the
685 schedule information for that timeslot. If there is none, the
686 mote keeps its radio off for the duration of the timeslot. If
687 there is one, the mote can ask for the neighbor to acknowledge it,
688 in which case it has to listen for the acknowledgement after
689 transmitting.
691 o For each receive cell, the mote listens for possible incoming
692 packets. If none is received after some listening period, it
693 shuts down its radio. If a packet is received, addressed to the
694 mote, and passes security checks, the mote can send back an
695 acknowledgement.
697 How the schedule is built, updated and maintained, and by which
698 entity, is outside of the scope of the IEEE802.15.4e standard.
700 A.4. Cells and Bundles
702 Assuming the schedule is well built, if mote A is scheduled to
703 transmit to mote B at slotOffset 5 and channelOffset 11, mote B will
704 be scheduled to receive from mote A at the same slotOffset and
705 channelOffset.
707 A single element of the schedule characterized by a slotOffset and
708 channelOffset, and reserved for mote A to transmit to mote B (or for
709 mote B to receive from mote A) within a given slotframe, is called a
710 "scheduled cell".
712 If there is a lot of data flowing from mote A to mote B, the schedule
713 might contain multiple cells from A to B, at different times.
714 Multiple cells scheduled to the same neighbor can be equivalent, i.e.
715 the MAC layer sends the packet on whichever of these cells happens to
716 show up first after the packet was put in the MAC queue. The union
717 of all cells between two neighbors, A and B, is called a "bundle".
718 Since the slotframe repeats over time (and the length of the
719 slotframe is typically constant), each cell gives a "quantum" of
720 bandwidth to a given neighbor. Modifying the number of equivalent
721 cells in a bundle modifies the amount of resources allocated between
722 two neighbors.
724 A.5. Dedicated vs. Shared Cells
726 By default, each scheduled transmit cell within the TSCH schedule is
727 dedicated, i.e., reserved only for mote A to transmit to mote B.
728 IEEE802.15.4e allows also to mark a cell as shared. In a shared
729 cell, multiple motes can transmit at the same time, on the same
730 frequency. To avoid contention, TSCH defines a back-off algorithm
731 for shared cells.
733 A scheduled cell can be marked as both transmitting and receiving.
734 In this case, a mote transmits if it has an appropriate packet in its
735 output buffer, or listens otherwise. Marking a cell as
736 [transmit,shared,receive] results in slotted-Aloha behavior.
738 A.6. Absolute Slot Number
740 TSCH defines a timeslot counter called Absolute Slot Number (ASN).
741 When a new network is created, the ASN is initialized to 0; from then
742 on, it increments by 1 at each timeslot. In detail:
744 ASN = (k*S+t)
746 where k is the slotframe cycle (i.e., the number of slotframe
747 repetitions since the network was started), S the slotframe size and
748 t the slotOffset. A mote learns the current ASN when it joins the
749 network. Since motes are synchronized, they all know the current
750 value of the ASN, at any time. The ASN is encoded as a 5-byte
751 number: this allows it to increment for hundreds of years (the exact
752 value depends on the duration of a timeslot) without wrapping. The
753 ASN is used to calculate the frequency to communicate on, and can be
754 used for security-related operations.
756 A.7. Channel Hopping
758 For each scheduled cell, the schedule specifies a slotOffset and a
759 channelOffset. In a well-built schedule, when mote A has a transmit
760 cell to mote B on channelOffset 5, mote B has a receive cell from
761 mote A on the same channelOffset. The channelOffset is translated by
762 both nodes into a frequency using the following function:
764 frequency = F {(ASN + channelOffset) mod nFreq}
766 The function F consists of a look-up table containing the set of
767 available channels. The value nFreq (the number of available
768 frequencies) is the size of this look-up table. There are as many
769 channelOffset values as there are frequencies available (e.g. 16 when
770 using IEEE802.15.4-compliant radios at 2.4GHz, when all channels are
771 used). Since both motes have the same channelOffset written in their
772 schedule for that scheduled cell, and the same ASN counter, they
773 compute the same frequency. At the next iteration (cycle) of the
774 slotframe, however, while the channelOffset is the same, the ASN has
775 changed, resulting in the computation of a different frequency.
777 This results in "channel hopping": even with a static schedule, pairs
778 of neighbors "hop" between the different frequencies when
779 communicating. Channel hopping is a technique known to efficiently
780 combat multi-path fading and external interference.
782 A.8. Time Synchronization
784 Because of the slotted nature of communication in a TSCH network,
785 motes have to maintain tight synchronization. All motes are assumed
786 to be equipped with clocks to keep track of time. Yet, because
787 clocks in different motes drift with respect to one another, neighbor
788 motes need to periodically re-synchronize.
790 Each mote needs to periodically synchronize its network clock to
791 another mote, and it also provides its network time to its neighbors.
792 It is up to the entity that manages the schedule to assign an
793 adequate time source neighbor to each mote, i.e., to indicate in the
794 schedule which of neighbor is its "time source neighbor". While
795 setting the time source neighbor, it is important to avoid
796 synchronization loops, which could result in the formation of
797 independent clusters of motes.
799 TSCH adds timing information in all packets that are exchanged (both
800 data and ACK frames). This means that neighbor motes can
801 resynchronize to one another whenever they exchange data. In detail,
802 in the IEEE 802.15.4e standard two methods are defined for allowing a
803 device to synchronize in a TSCH network: (i) Acknowledgement-Based
804 and (ii) Frame-Based synchronization. In both cases, the receiver
805 calculates the difference in time between the expected time of frame
806 arrival and its actual arrival. In Acknowledgement-Based
807 synchronization, the receiver provides such information to the sender
808 mote in its acknowledgement. Thus, in this case, it is the sender
809 mote that synchronizes to the clock of the receiver. In Frame-Based
810 synchronization, the receiver uses the computed delta for adjusting
811 its own clock. Therefore, it is the receiver mote that synchronizes
812 to the clock of the sender.
814 Different synchronization policies are possible. Motes can keep
815 synchronization exclusively by exchanging EBs. Motes can also keep
816 synchronized by periodically sending valid frames to a time source
817 neighbor and use the acknowledgement to resynchronize. Both method
818 (or a combination thereof) are valid synchronization policies; which
819 one to use depends on network requirements.
821 A.9. Power Consumption
823 There are only a handful of activities a mote can perform during a
824 timeslot: transmit, receive, or sleep. Each of these operations has
825 some energy cost associated to them, the exact value depending on the
826 the hardware used. Given the schedule of a mote, it is
827 straightforward to calculate the expected average power consumption
828 of that mote.
830 A.10. Network TSCH Schedule
832 The schedule defines entirely the synchronization and communication
833 between motes. By adding/removing cells between neighbors, one can
834 adapt a schedule to the needs of the application. Intuitive examples
835 are:
837 o Make the schedule "sparse" for applications where motes need to
838 consume as little energy as possible, at the price of reduced
839 bandwidth.
841 o Make the schedule "dense" for applications where motes generate a
842 lot of data, at the price of increased power consumption.
844 o Add more cells along a multi-hop route over which many packets
845 flow.
847 A.11. Join Process
849 Motes already part of the network can periodically send Enhanced
850 Beacon (EB) frames to announce the presence the network. These
851 contain information about the size of the timeslot used in the
852 network, the current ASN, information about the slotframes and
853 timeslots the beaconing mote is listening on, and a 1-byte join
854 priority. Because of the channel hopping nature of TSCH, these EB
855 frames are sent on all frequencies.
857 A mote wishing to join the network listens for EBs. Using the ASN
858 and the other timing information of the EB, the new mote synchronizes
859 to the network. Using the slotframe and link information from the
860 EB, it knows how to contact the network.
862 The IEEE802.15.4e TSCH standard does not define the steps beyond this
863 network "bootstrap".
865 A.12. Information Elements
867 TSCH introduces the concept of Information Elements (IEs). An
868 information element is a list of Type-Length-Value containers placed
869 at the end of the MAC header. A small number of types are defined
870 for TSCH (e.g., the ASN in the EB is contained in an IE), and an
871 unmanaged range is available for extensions.
873 A data bit in the MAC header indicates whether the frame contains
874 IEs. IEs are grouped into Header IEs, consumed by the MAC layer and
875 therefore typically invisible to the next higher layer, and Payload
876 IEs, which are passed untouched to the next higher layer, possibly
877 followed by regular payload. Payload IEs can therefore be used for
878 the next higher layers of two neighbor motes to exchange information.
880 A.13. Extensibility
882 The TSCH standard is designed to be extensible. It introduces the
883 mechanisms as "building block" (e.g., cells, bundles, slotframes,
884 etc.), but leaves entire freedom to the upper layer to assemble
885 those. The MAC protocol can be extended by defining new Header IEs.
886 An intermediate layer can be defined to manage the MAC layer by
887 defining new Payload IEs.
889 Appendix B. TSCH Gotchas
891 This section lists features of TSCH which we believe are important
892 and beneficial to the work of 6TiSCH.
894 B.1. Collision Free Communication
896 TSCH allows one to design a schedule which yields collision-free
897 communication. This is done by building the schedule with dedicated
898 cells in such a way that at most one node can communicate with a
899 specific neighbor in each slotOffset/channelOffset cell. Multiple
900 pairs of neighbor motes can exchange data at the same time, but on
901 different frequencies.
903 B.2. Multi-Channel vs. Channel Hopping
904 A TSCH schedule looks like a matrix of width "slotframe size", S, and
905 of height "number of frequencies", nFreq. For a scheduling
906 algorithm, these can be considered atomic "units" to schedule. In
907 particular, because of the channel hopping nature of TSCH, the
908 scheduling algorithm should not worry about the actual frequency
909 communication happens on, since it changes at each slotframe
910 iteration.
912 B.3. Cost of (continuous) Synchronization
914 When there is traffic in the network, motes which are communicating
915 implicitly re-synchronize using the data frames they exchange. In
916 the absence of data traffic, motes are required to synchronize to
917 their time source neighbor(s) periodically not to drift in time. If
918 they have not been communicating for some time (typically 30s), motes
919 can exchange an dummy data frame to re-synchronize. The frequency at
920 which such messages need to be transmitted depends on the stability
921 of the clock source, and on how "early" each mote starts listening
922 for data (the "guard time"). Theoretically, with a 10ppm clock and a
923 1ms guard time, this period can be 100s. Assuming this exchange
924 causes the mote's radio to be on for 5ms, this yields a radio duty
925 cycle needed to keep synchronized of 5ms/100s=0.005%. While TSCH does
926 requires motes to resynchronize periodically, the cost of doing so is
927 very low.
929 B.4. Topology Stability
931 The channel hopping nature of TSCH causes links to be very "stable".
932 Wireless phenomena such as multi-path fading and external
933 interference impact a wireless link between two motes differently on
934 each frequency. If a transmission from mote A to mote B fails,
935 retransmitting on a different frequency has a higher likelihood of
936 succeeding that retransmitting on the same frequency. As a result,
937 even when some frequencies are "behaving bad", channel hopping
938 "smoothens" the contribution of each frequency, resulting in more
939 stable links, and therefore a more stable topology.
941 B.5. Multiple Concurrent Slotframes
943 The TSCH standard allows for multiple slotframes to coexist in a
944 mote's schedule. It is possible that at some timeslot, a mote has
945 multiple activities scheduled (e.g. transmit to mote B on slotframe
946 2, receive from mote C on slotframe 1). To handle this situation,
947 the TSCH standard defines the following precedence rules:
949 1. Transmissions take precedence over receptions;
950 2. Lower slotframe identifiers take precedence over higher slotframe
951 identifiers.
953 In the example above, the mote would transmit to mote B on slotframe
954 2.
956 Authors' Addresses
958 Thomas Watteyne (editor)
959 Linear Technology
960 30695 Huntwood Avenue
961 Hayward, CA 94544
962 USA
964 Phone: +1 (510) 400-2978
965 Email: twatteyne@linear.com
967 Maria Rita Palattella
968 University of Luxembourg
969 Interdisciplinary Centre for Security, Reliability and Trust
970 4, rue Alphonse Weicker
971 Luxembourg L-2721
972 LUXEMBOURG
974 Phone: +352 46 66 44 5841
975 Email: maria-rita.palattella@uni.lu
977 Luigi Alfredo Grieco
978 Politecnico di Bari
979 Department of Electrical and Information Engineering
980 Via Orabona 4
981 Bari 70125
982 Italy
984 Phone: +39 08 05 96 3911
985 Email: a.grieco@poliba.it