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Keranen 7 Ericsson 8 March 31, 2013 10 Terminology for Constrained Node Networks 11 draft-ietf-lwig-terminology-03 13 Abstract 15 The Internet Protocol Suite is increasingly used on small devices 16 with severe constraints, creating constrained node networks. This 17 document provides a number of basic terms that have turned out to be 18 useful in the standardization work for constrained environments. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on October 02, 2013. 37 Copyright Notice 39 Copyright (c) 2013 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2.1. Constrained Nodes . . . . . . . . . . . . . . . . . . . . 3 57 2.2. Constrained Networks . . . . . . . . . . . . . . . . . . 4 58 2.2.1. Challenged Networks . . . . . . . . . . . . . . . . . 5 59 2.3. Constrained Node Networks . . . . . . . . . . . . . . . . 5 60 2.3.1. LLN ("low-power lossy network") . . . . . . . . . . . 5 61 2.3.2. LoWPAN, 6LoWPAN . . . . . . . . . . . . . . . . . . . 6 62 3. Classes of Constrained Devices . . . . . . . . . . . . . . . 7 63 4. Power Terminology . . . . . . . . . . . . . . . . . . . . . . 9 64 4.1. Scaling Properties . . . . . . . . . . . . . . . . . . . 9 65 4.2. Classes of Energy Limitation . . . . . . . . . . . . . . 9 66 4.3. Strategies of Using Power for Communication . . . . . . . 10 67 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12 68 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 69 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 70 8. Informative References . . . . . . . . . . . . . . . . . . . 12 71 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 73 1. Introduction 75 Small devices with limited CPU, memory, and power resources, so 76 called constrained devices (also known as sensor, smart object, or 77 smart device) can constitute a network, becoming "constrained nodes" 78 in that network. Such a network may itself exhibit constraints, e.g. 79 with unreliable or lossy channels, limited and unpredictable 80 bandwidth, and a highly dynamic topology. 82 Constrained devices might be in charge of gathering information in 83 diverse settings including natural ecosystems, buildings, and 84 factories and sending the information to one or more server stations. 85 Constrained devices may work under severe resource constraints such 86 as limited battery and computing power, little memory and 87 insufficient wireless bandwidth, and communication capabilities. 88 Other entities on the network, e.g., a base station or controlling 89 server, might have more computational and communication resources and 90 could support the interaction between the constrained devices and 91 applications in more traditional networks. 93 Today diverse sizes of constrained devices with different resources 94 and capabilities are becoming connected. Mobile personal gadgets, 95 building-automation devices, cellular phones, Machine-to-machine 96 (M2M) devices, etc. benefit from interacting with other "things" 97 nearby or somewhere in the Internet. With this, the Internet of 98 Things (IoT) becomes a reality, built up out of uniquely identifiable 99 and addressable objects (things). And over the next decade, this 100 could grow to large numbers [fifty-billion] of Internet-connected 101 constrained devices, greatly increasing the Internet's size and 102 scope. 104 The present document provides a number of basic terms that have 105 turned out to be useful in the standardization work for constrained 106 environments. The intention is not to exhaustively cover the field, 107 but to make sure a few core terms are used consistently between 108 different groups cooperating in this space. 110 2. Terminology 112 The main focus of this field of work appears to be _scaling_: 114 o Scaling up Internet technologies to a large number [fifty-billion] 115 of inexpensive nodes, while 117 o scaling down the characteristics of each of these nodes and of the 118 networks being built out of them, to make this scaling up 119 econmically and physically viable. 121 The need for scaling down the characteristics of nodes leads to 122 _constrained nodes_. 124 2.1. Constrained Nodes 126 The term "constrained node" is best defined by contrasting the 127 characteristics of a constrained node with certain widely held 128 expectations on more familiar Internet nodes: 130 Constrained Node: A node where some of the characteristics that are 131 otherwise pretty much taken for granted for Internet nodes in 2013 132 are not attainable, often due to cost constraints and/or physical 133 constraints on characteristics such as size, weight, and available 134 power. 136 While this is less than satisfying as a rigorous definition, it is 137 grounded in the state of the art and clearly sets apart constrained 138 nodes from server systems, desktop or laptop computers, powerful 139 mobile devices such as smartphones etc. There may be many design 140 considerations that lead to these constraints, including cost, size, 141 weight, and other scaling factors. 143 (An alternative name, when the properties as a network node are not 144 in focus, is "constrained device".) 145 There are multiple facets to the constraints on nodes, often applying 146 in combination, e.g.: 148 o constraints on the maximum code complexity (ROM/Flash); 150 o constraints on the size of state and buffers (RAM); 152 o constraints on the available power. 154 Section 3 defines a small number of interesting classes ("class-N" 155 for N=0,1,2) of constrained nodes focusing on relevant combinations 156 of the first two constraints. With respect to available power, 157 [RFC6606] distinguishes "power-affluent" nodes (mains-powered or 158 regularly recharged) from "power-constrained nodes" that draw their 159 power from primary batteries or by using energy harvesting. 161 The use of constrained nodes in networks often also leads to 162 constraints on the networks themselves. However, there may also be 163 constraints on networks that are largely independent from those of 164 the nodes. We therefore distinguish _constrained networks_ and 165 _constrained node networks_. 167 2.2. Constrained Networks 169 We define "constrained network" in a similar way: 171 Constrained Network: A network where some of the characteristics 172 pretty much taken for granted for Internet link layers in 2013 are 173 not attainable. 175 Again, there may be several reasons for this: 177 o cost constraints on the network, 179 o constraints of the nodes (for constrained node networks), 181 o physical constraints (e.g., power constraints, media constraints 182 such as underwater operation, limited spectrum for very high 183 density, electromagnetic compatibility), 185 o regulatory constraints, such as very limited spectrum availability 186 (including limits on effective radiated power and duty cycle), or 187 explosion safety. 189 Constraints may include: 191 o low achievable bit rate (including limits on duty cycle), 192 o high packet loss, packet loss (delivery rate) variability, 194 o severe penalties for using larger packets (e.g., high packet loss 195 due to link layer fragmentation), 197 o lack of (or severe constraints on) advanced services such as IP 198 multicast. 200 2.2.1. Challenged Networks 202 A constrained network is not necessarily a _challenged_ network 203 [FALL]: 205 Challenged Network: A network that has serious trouble maintaining 206 what an application would today expect of the end-to-end IP model, 207 e.g., by: 209 o not being able to offer end-to-end IP connectivity at all; 211 o exhibiting serious interruptions in end-to-end IP connectivity; 213 o exhibiting delay well beyond the MSL defined by TCP. 215 All challenged networks are constrained networks in some sense, but 216 not all constrained networks are challenged networks. There is no 217 well-defined boundary between the two, though. Delay-Tolerant 218 Networking (DTN) has been designed to cope with challenged networks 219 [RFC4838]. 221 2.3. Constrained Node Networks 223 Constrained Node Network: A network whose characteristics are 224 influenced by being composed of a significant portion of 225 constrained nodes. 227 A constrained node network always is a constrained network because of 228 the network constraints stemming from the node constraints, but may 229 also have other constraints that already make it a constrained 230 network. 232 2.3.1. LLN ("low-power lossy network") 234 A related term that has been used recently is "low-power lossy 235 network" (LLN). The ROLL working group currently is struggling with 236 its definition [I-D.ietf-roll-terminology]: 238 LLN: Low power and Lossy networks (LLNs) are typically composed of 239 many embedded devices with limited power, memory, and processing 240 resources interconnected by a variety of links, such as IEEE 241 802.15.4 or Low Power WiFi. There is a wide scope of application 242 areas for LLNs, including industrial monitoring, building 243 automation (HVAC, lighting, access control, fire), connected home, 244 healthcare, environmental monitoring, urban sensor networks, 245 energy management, assets tracking and refrigeration.. [sic] 247 It is not clear that "LLN" is much more specific than "interesting" 248 or "the network characteristics that RPL has been designed for". 249 LLNs do appear to have significant loss at the physical layer, with 250 significant variability of the delivery rate, and some short-term 251 unreliability, coupled with some medium term stability that makes it 252 worthwhile to construct medium-term stable directed acyclic graphs 253 for routing and do measurements on the edges such as ETX [RFC6551]. 254 Actual "low power" does not seem to be required for an LLN 255 [I-D.hui-vasseur-roll-rpl-deployment], and the positions on scaling 256 of LLNs appear to vary widely [I-D.clausen-lln-rpl-experiences]. 258 Also, LLNs seem to be composed of constrained nodes; otherwise 259 operation modes such as RPL's "non-storing mode" would not be 260 sensible. So an LLN seems to be a constrained node network with 261 certain constraints on the network as well. 263 2.3.2. LoWPAN, 6LoWPAN 265 One interesting class of a constrained network often used as a 266 constrained node network is the "LoWPAN" [RFC4919], a term inspired 267 from the name of the IEEE 802.15.4 working group (low-rate wireless 268 personal area networks (LR-WPANs)). The expansion of that acronym, 269 "Low-Power Wireless Personal Area Network" contains a hard to justify 270 "Personal" that is due to IEEE politics more than due to an 271 orientation of LoWPANs around a single person. Actually, LoWPANs 272 have been suggested for urban monitoring, control of large buildings, 273 and industrial control applications, so the "Personal" can only be 274 considered a vestige. Maybe the term is best read as "Low-Power 275 Wireless Area Networks" (LoWPANs) [WEI]. Originally focused on IEEE 276 802.15.4, "LoWPAN" (or when used for IPv6, "6LoWPAN") is now also 277 being used for networks built from similarly constrained link layer 278 technologies [I-D.ietf-6lowpan-btle] 279 [I-D.mariager-6lowpan-v6over-dect-ule] [I-D.brandt-6man-lowpanz]. 281 3. Classes of Constrained Devices 283 Despite the overwhelming variety of Internet-connected devices that 284 can be envisioned, it may be worthwhile to have some succinct 285 terminology for different classes of constrained devices. In this 286 document, the class designations in Table 1 may be used as rough 287 indications of device capabilities: 289 +-------------+-----------------------+-------------------------+ 290 | Name | data size (e.g., RAM) | code size (e.g., Flash) | 291 +-------------+-----------------------+-------------------------+ 292 | Class 0, C0 | << 10 KiB | << 100 KiB | 293 | | | | 294 | Class 1, C1 | ~ 10 KiB | ~ 100 KiB | 295 | | | | 296 | Class 2, C2 | ~ 50 KiB | ~ 250 KiB | 297 +-------------+-----------------------+-------------------------+ 299 Table 1: Classes of Constrained Devices 301 As of the writing of this document, these characteristics correspond 302 to distinguishable clusters of commercially available chips and 303 design cores for constrained devices. While it is expected that the 304 boundaries of these classes will move over time, Moore's law tends to 305 be less effective in the embedded space than in personal computing 306 devices: Gains made available by increases in transistor count and 307 density are more likely to be invested in reductions of cost and 308 power requirements than into continual increases in computing power. 310 Class 0 devices are very constrained sensor-like motes. Most likely 311 they will not be able to communicate directly with the Internet in a 312 secure manner. Class 0 devices will participate in Internet 313 communications with the help of larger devices acting as proxies, 314 gateways or servers. Class 0 devices generally cannot be secured or 315 managed comprehensively in the traditional sense. They will most 316 likely be preconfigured (and will be reconfigured rarely, if at all), 317 with a very small data set. For management purposes, they could 318 answer keepalive signals and send on/off or basic health indications. 320 Class 1 devices cannot easily talk to other Internet nodes employing 321 a full protocol stack such as using HTTP, TLS and related security 322 protocols and XML-based data representations. However, they have 323 enough power to use a protocol stack specifically designed for 324 constrained nodes (e.g., CoAP over UDP) and participate in meaningful 325 conversations without the help of a gateway node. In particular, 326 they can provide support for the security functions required on a 327 large network. Therefore, they can be integrated as fully developed 328 peers into an IP network, but they need to be parsimonious with state 329 memory, code space, and often power expenditure for protocol and 330 application usage. 332 Class 2 can already support mostly the same protocol stacks as used 333 on notebooks or servers. However, even these devices can benefit 334 from lightweight and energy-efficient protocols and from consuming 335 less bandwidth. Furthermore, using fewer resources for networking 336 leaves more resources available to applications. Thus, using the 337 protocol stacks defined for very constrained devices also on Class 2 338 devices might reduce development costs and increase the 339 interoperability. 341 Constrained devices with capabilities significantly beyond Class 2 342 devices exist. They are less demanding from a standards development 343 point of view as they can largely use existing protocols unchanged. 344 The present document therefore does not make any attempt to define 345 classes beyond Class 2. These devices can still be constrained by a 346 limited energy supply. 348 With respect to examining the capabilities of constrained nodes, 349 particularly for Class 1 devices, it is important to understand what 350 type of applications they are able to run and which protocol 351 mechanisms would be most suitable. Because of memory and other 352 limitations, each specific Class 1 device might be able to support 353 only a few selected functions needed for its intended operation. In 354 other words, the set of functions that can actually be supported is 355 not static per device type: devices with similar constraints might 356 choose to support different functions. Even though Class 2 devices 357 have some more functionality available and may be able to provide a 358 more complete set of functions, they still need to be assessed for 359 the type of applications they will be running and the protocol 360 functions they would need. To be able to derive any requirements, 361 the use cases and the involvement of the devices in the application 362 and the operational scenario need to be analyzed. Use cases may 363 combine constrained devices of multiple classes as well as more 364 traditional Internet nodes. 366 4. Power Terminology 368 Devices not only differ in their computing capabilities, but also in 369 available electrical power and/or energy. While it is harder to find 370 recognizable clusters in this space, it is still useful to introduce 371 some common terminology. 373 4.1. Scaling Properties 375 The power and/or energy available to a device may vastly differ, from 376 kilowatts to microwatts, from essentially unlimited to hundreds of 377 microjoules. 379 Instead of defining classes or clusters, we propose simply stating, 380 in SI units, an approximate value for one or both of the quantities 381 listed in Table 2: 383 +--------+---------------------------------------------+------------+ 384 | Name | Definition | SI Unit | 385 +--------+---------------------------------------------+------------+ 386 | Ps | Sustainable average power available for the | W (Watt) | 387 | | device over the time it is functioning | | 388 | | | | 389 | Et | Total electrical energy available before | J (Joule) | 390 | | the energy source is exhausted | | 391 +--------+---------------------------------------------+------------+ 393 Table 2: Quantities Relevant to Power and Energy 395 The value of Et may need to be interpreted in conjunction with an 396 indication over which period of time the value is given; see the next 397 subsection. 399 4.2. Classes of Energy Limitation 401 As discussed above, some devices are limited in available energy as 402 opposed to (or in addition to) being limited in available power. 403 Where no relevant limitations exist with respect to energy, the 404 device is classified as E3. The energy limitation may be in total 405 energy available in the usable lifetime of the device (e.g. a device 406 with a non-replaceable primary battery, which is discarded when this 407 battery is exhausted), classified as E2. Where the relevant 408 limitation is for a specific period, this is classified as E1, e.g. 409 a limited amount of energy available for the night with a solar- 410 powered device, or for the period between recharges with a device 411 that is manually connected to a charger, or by a periodic (primary) 412 battery replacement interval. Finally, there may be a limited amount 413 of energy available for a specific event, e.g. for a button press in 414 an energy harvesting light switch; this is classified as E0. Note 415 that many E1 devices in a sense also are E2, as the rechargeable 416 battery has a limited number of useful recharging cycles. 418 In summary, we distinguish (Table 3): 420 +------+------------------------------+-----------------------------+ 421 | Name | Type of energy limitation | Example Power Source | 422 +------+------------------------------+-----------------------------+ 423 | E0 | Event energy-limited | Event-based harvesting | 424 | | | | 425 | E1 | Period energy-limited | Battery that is | 426 | | | periodically recharged or | 427 | | | replaced | 428 | | | | 429 | E2 | Lifetime energy-limited | Non-replaceable primary | 430 | | | battery | 431 | | | | 432 | E3 | No direct quantitative | Mains powered | 433 | | limitations to available | | 434 | | energy | | 435 +------+------------------------------+-----------------------------+ 437 Table 3: Classes of Energy Limitation 439 4.3. Strategies of Using Power for Communication 441 Especially when wireless transmission is used, the radio often 442 consumes a big portion of the total energy consumed by the device. 443 Design parameters such as the available spectrum, the desired range, 444 and the bitrate aimed for, influence the power consumed during 445 transmission and reception; the duration of transmission and 446 reception (including potential reception) influence the total energy 447 consumption. 449 Based on the type of the energy source (e.g., battery or mains power) 450 and how often device needs to communicate, it may use different kinds 451 of strategies for power usage and network attachment. 453 The general strategies for power usage can be described as follows: 455 Always-on: This strategy is most applicable if there is no reason 456 for extreme measures for power saving. The device can stay on in 457 the usual manner all the time. It may be useful to employ power- 458 friendly hardware or limit the number of wireless transmissions, 459 CPU speeds, and other aspects for general power saving and cooling 460 needs, but the device can be connected to the network all the 461 time. 463 Always-off: Under this strategy, the device sleeps such long periods 464 at a time that once it wakes up, it makes sense for it to not 465 pretend that it has been connected to the network during sleep: 466 The device re-attaches to the network as it is woken up. The main 467 optimization goal is to minimize the effort during such re- 468 attachment process and any resulting application communications. 470 If the device sleeps for long periods of time, and needs to 471 communicate infrequently, the relative increase in energy 472 expenditure during reattachment may be acceptable. 474 Low-power: This strategy is most applicable to devices that need to 475 operate on a very small amount of power, but still need to be able 476 to communicate on a relatively frequent basis. This implies that 477 extremely low power solutions needs to be used for the hardware, 478 chosen link layer mechanisms, and so on. Typically, given the 479 small amount of time between transmissions, despite their sleep 480 state these devices retain some form of network attachment to the 481 network. Techniques used for minimizing power usage for the 482 network communications include minimizing any work from re- 483 establishing communications after waking up, tuning the frequency 484 of communications, and other parameters appropriately. 486 In summary, we distinguish (Table 4): 488 +------+------------+----------------------------------------------+ 489 | Name | Strategy | Ability to communicate | 490 +------+------------+----------------------------------------------+ 491 | S0 | Always-off | Re-attach when required | 492 | | | | 493 | S1 | Low-power | Appears connected, perhaps with high latency | 494 | | | | 495 | S2 | Always-on | Always connected | 496 +------+------------+----------------------------------------------+ 498 Table 4: Strategies of Using Power for Communication 500 5. Security Considerations 502 This draft introduces common terminology that does not raise any new 503 security issue. 505 6. IANA Considerations 507 This document has no actions for IANA. 509 7. Acknowledgements 511 Dominique Barthel and Peter van der Stok provided useful comments; 512 Charles Palmer provided a full editorial review. 514 Peter van der Stok insisted that we should have power terminology, 515 hence Section 4. The text for Section 4.3 is mostly lifted from 516 [I-D.arkko-lwig-cellular] and has been adapted for this document. 518 8. Informative References 520 [FALL] Fall, K., "A Delay-Tolerant Network Architecture for 521 Challenged Internets", SIGCOMM 2003, 2003. 523 [I-D.arkko-lwig-cellular] 524 Arkko, J., Eriksson, A., and A. Keraenen, "Building Power- 525 Efficient CoAP Devices for Cellular Networks", draft- 526 arkko-lwig-cellular-00 (work in progress), February 2013. 528 [I-D.brandt-6man-lowpanz] 529 Brandt, A. and J. Buron, "Transmission of IPv6 packets 530 over ITU-T G.9959 Networks", draft-brandt-6man-lowpanz-00 531 (work in progress), February 2013. 533 [I-D.clausen-lln-rpl-experiences] 534 Clausen, T., Verdiere, A., Yi, J., Herberg, U., and Y. 535 Igarashi, "Observations of RPL: IPv6 Routing Protocol for 536 Low power and Lossy Networks", draft-clausen-lln-rpl- 537 experiences-06 (work in progress), February 2013. 539 [I-D.hui-vasseur-roll-rpl-deployment] 540 Vasseur, J., Hui, J., Dasgupta, S., and G. Yoon, "RPL 541 deployment experience in large scale networks", draft-hui- 542 vasseur-roll-rpl-deployment-01 (work in progress), July 543 2012. 545 [I-D.ietf-6lowpan-btle] 546 Nieminen, J., Savolainen, T., Isomaki, M., Patil, B., 547 Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets 548 over BLUETOOTH Low Energy", draft-ietf-6lowpan-btle-12 549 (work in progress), February 2013. 551 [I-D.ietf-roll-terminology] 552 Vasseur, J., "Terminology in Low power And Lossy 553 Networks", draft-ietf-roll-terminology-12 (work in 554 progress), March 2013. 556 [I-D.mariager-6lowpan-v6over-dect-ule] 557 Mariager, P. and J. Petersen, "Transmission of IPv6 558 Packets over DECT Ultra Low Energy", draft-mariager- 559 6lowpan-v6over-dect-ule-02 (work in progress), May 2012. 561 [RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst, 562 R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant 563 Networking Architecture", RFC 4838, April 2007. 565 [RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6 566 over Low-Power Wireless Personal Area Networks (6LoWPANs): 567 Overview, Assumptions, Problem Statement, and Goals", RFC 568 4919, August 2007. 570 [RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D. 571 Barthel, "Routing Metrics Used for Path Calculation in 572 Low-Power and Lossy Networks", RFC 6551, March 2012. 574 [RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem 575 Statement and Requirements for IPv6 over Low-Power 576 Wireless Personal Area Network (6LoWPAN) Routing", RFC 577 6606, May 2012. 579 [WEI] Shelby, Z. and C. Bormann, "6LoWPAN: the Wireless Embedded 580 Internet", ISBN 9780470747995, 2009. 582 [fifty-billion] 583 Ericsson, "More Than 50 Billion Connected Devices", 584 Ericsson White Paper 284 23-3149 Uen, February 2011, 585 . 588 Authors' Addresses 589 Carsten Bormann 590 Universitaet Bremen TZI 591 Postfach 330440 592 D-28359 Bremen 593 Germany 595 Phone: +49-421-218-63921 596 Email: cabo@tzi.org 598 Mehmet Ersue 599 Nokia Siemens Networks 600 St.-Martinstrasse 76 601 81541 Munich 602 Germany 604 Phone: +49 172 8432301 605 Email: mehmet.ersue@nsn.com 607 Ari Keranen 608 Ericsson 609 Hirsalantie 11 610 02420 Jorvas 611 Finland 613 Email: ari.keranen@ericsson.com