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Perkins 5 Expires: March 16, 2015 Futurewei 6 September 12, 2014 8 Multi-hop Ad Hoc Wireless Communication 9 draft-baccelli-manet-multihop-communication-04 11 Abstract 13 This document describes characteristics of communication between 14 interfaces in a multi-hop ad hoc wireless network, that protocol 15 engineers and system analysts should be aware of when designing 16 solutions for ad hoc networks at the IP layer. 18 Status of This Memo 20 This Internet-Draft is submitted to IETF in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on March 16, 2015. 35 Copyright Notice 37 Copyright (c) 2014 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. 47 Table of Contents 49 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 50 2. Multi-hop Ad Hoc Wireless Networks . . . . . . . . . . . . . 2 51 3. Common Packet Transmission Characteristics in Multi-hop Ad 52 Hoc Wireless Networks . . . . . . . . . . . . . . . . . . . . 3 53 3.1. Asymmetry, Time-Variation, and Non-Transitivity . . . . . 3 54 3.2. Radio Range and Wireless Irregularities . . . . . . . . . 4 55 4. Alternative Terminology . . . . . . . . . . . . . . . . . . . 7 56 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 57 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 58 7. Informative References . . . . . . . . . . . . . . . . . . . 8 59 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10 61 1. Introduction 63 Experience gathered with ad hoc routing protocol development, 64 deployment and operation, shows that wireless communication presents 65 specific challenges [RFC2501] [DoD01], which Internet protocol 66 designers should be aware of, when designing solutions for ad hoc 67 networks at the IP layer. This document briefly describes these 68 challenges. 70 2. Multi-hop Ad Hoc Wireless Networks 72 For the purposes of this document, a multi-hop ad hoc wireless 73 network will be considered to be a collection of devices that each 74 have a radio transceiver (i.e., wireless network interface), and that 75 are moreover configured to self-organize and provide store-and- 76 forward functionality as needed to enable communications. This 77 document focuses on the characteristics of communications through 78 such a network interface. 80 Although the characteristics of packet transmission over multi-hop ad 81 hoc wireless networks, described below, are not the typical 82 characteristics expected by IP [RFC6250], it is desirable and 83 possible to run IP over such networks, as demonstrated in certain 84 deployments currently in operation, such as Freifunk [FREIFUNK], and 85 Funkfeuer [FUNKFEUER]. These deployments use routers running IP 86 protocols e.g., OLSR (Optimized Link State Routing [RFC3626]) on top 87 of IEEE 802.11 in ad hoc mode with the same ESSID (Extended Service 88 Set Identification) at the link layer. Multi-hop ad hoc wireless 89 networks may also run on link layers other than IEEE 802.11, and may 90 use routing protocols other than OLSR (for instance, AODV [RFC3561], 91 TBRPF [RFC3684], DSR [RFC4728], or OSPF-MPR [RFC5449]). 93 Note that in contrast, devices communicating via an IEEE 802.11 94 access point in infrastructure mode do not form a multi-hop ad hoc 95 wireless network, since the central role of the access point is 96 predetermined, and devices other than the access point do not 97 generally provide store-and-forward functionality. 99 3. Common Packet Transmission Characteristics in Multi-hop Ad Hoc 100 Wireless Networks 102 In the following, we will consider several devices in a multi-hop ad 103 hoc wireless network N. Each device will be considered only through 104 its own wireless interface to network N. For conciseness and 105 readability, this document uses the expressions "device A" (or simply 106 "A") as a synonym for "the wireless interface of device A to network 107 N". 109 Let A and B be two devices in network N. Suppose that, when device A 110 transmits an IP packet through its interface on network N, that 111 packet is correctly and directly received by device B without 112 requiring storage and/or forwarding by any other device. We will 113 then say that B can "detect" A. Note that therefore, when B detects 114 A, an IP packet transmitted by A will be rigorously identical to the 115 corresponding IP packet received by B. 117 Let S be the set of devices that detect device A through its wireless 118 interface on network N. The following section gathers common 119 characteristics concerning packet transmission over such networks, 120 which were observed through experience with MANET routing protocol 121 development (for instance, OLSR[RFC3626], AODV[RFC3561], 122 TBRPF[RFC3684], DSR[RFC4728], and OSPF-MPR[RFC5449]), as well as 123 deployment and operation (Freifunk[FREIFUNK], Funkfeuer[FUNKFEUER]). 125 3.1. Asymmetry, Time-Variation, and Non-Transitivity 127 First, even though a device C in set S can (by definition) detect 128 device A, there is no guarantee that C can, conversely, send IP 129 packets directly to A. In other words, even though C can detect A 130 (since it is a member of set S), there is no guarantee that A can 131 detect C. Thus, multi-hop ad hoc wireless communications may be 132 "asymmetric". Such cases are common. 134 Second, there is no guarantee that, as a set, S is at all stable, 135 i.e. the membership of set S may in fact change at any rate, at any 136 time. Thus, multi-hop ad hoc wireless communications may be "time- 137 variant". Time variation is often observed in multi-hop ad hoc 138 wireless networks due to variability of the wireless medium, and to 139 device mobility. 141 Now, conversely, let V be the set of devices which A detects. 142 Suppose that A is communicating at time t0 through its interface on 143 network N. As a consequence of time variation and asymmetry, we 144 observe that A: 146 1. cannot assume that S = V, 148 2. cannot assume that S and/or V are unchanged at time t1 later than 149 t0. 151 Furthermore, transitivity is not guaranteed over multi-hop ad hoc 152 wireless networks. Indeed, let's assume that, through their 153 respective interfaces within network N: 155 1. device B and device A can detect one another (i.e. B is a member 156 of sets S and V), and, 158 2. device A and device C can also detect one another (i.e. C is a 159 also a member of sets S and V). 161 These assumptions do not imply that B can detect C, nor that C can 162 detect B (through their interface on network N). Such "non- 163 transitivity" is common on multi-hop ad hoc wireless networks. 165 In a nutshell: multi-hop ad hoc wireless communications can be 166 asymmetric, non-transitive, and time-varying. 168 3.2. Radio Range and Wireless Irregularities 170 Section 3.1 presents an abstract description of some common 171 characteristics concerning packet transmission over multi-hop ad hoc 172 wireless networks. This section describes practical examples, which 173 illustrate the characteristics listed in Section 3.1 as well as other 174 common effects. 176 Wireless communications are subject to limitations to the distance 177 across which they may be established. The range-limitation factor 178 creates specific problems on multi-hop ad hoc wireless networks. In 179 this context, the radio ranges of several devices often partially 180 overlap. Such partial overlap causes communication to be non- 181 transitive and/or asymmetric, as described in Section 3.1. Moreover, 182 the range may vary from one device to another, depending on location 183 and environmental factors. This is in addition to the time variation 184 of range and signal strength caused by variability in the local 185 environment. 187 For example, as depicted in Figure 1, it may happen that a device B 188 detects a device A which transmits at high power, whereas B transmits 189 at lower power. In such cases, B detects A, but A cannot detect B. 191 This examplifies the asymmetry in multi-hop ad hoc wireless 192 communications as defined in Section 3.1. 194 Radio Ranges for Devices A and B 196 <~~~~~~~~~~~~~+~~~~~~~~~~~~~> 197 | <~~~~~~+~~~~~~> 198 +--|--+ +--|--+ 199 | A |======>| B | 200 +-----+ +-----+ 202 Figure 1: Asymmetric wireless communication example. Device A can communicate with 203 device B, but B cannot communicate with A. 205 Another example, depicted in Figure 2, is known as the "Hidden 206 Terminal" problem. Even though the devices all have equal power for 207 their radio transmissions, they cannot all detect one another. In 208 the figure, devices A and B can detect one another, and devices A and 209 C can also detect one another. On the other hand, B and C cannot 210 detect one another. When B and C simultaneously try to communicate 211 with A, their radio signals may collide. Device A may receive 212 incoherent noise, and may even be unable to determine the source of 213 the noise. The hidden terminal problem illustrates the property of 214 non-transitivity in multi-hop ad hoc wireless communications as 215 described in Section 3.1. 217 Radio Ranges for Devices A, B, C 219 <~~~~~~~~~~~~~+~~~~~~~~~~~~~> <~~~~~~~~~~~~~+~~~~~~~~~~~~~> 220 |<~~~~~~~~~~~~~+~~~~~~~~~~~~~>| 221 +--|--+ +--|--+ +--|--+ 222 | B |=======>| A |<=======| C | 223 +-----+ +-----+ +-----+ 225 Figure 2: The hidden terminal problem. Devices C and B 226 try to communicate with device A at the same time, 227 and their radio signals collide. 229 Another situation, shown in Figure 3, is known as the "Exposed 230 Terminal" problem. In the figure, device A and device B can detect 231 each other, and A is transmitting packets to B, thus A cannot detect 232 device C -- but C can detect A. As shown in Figure 3, during the on- 233 going transmission of A, device C cannot reliably communicate with 234 device D because of interference within C's radio range due to A 's 235 transmissions. Device C is then said to be "exposed", because it is 236 exposed to co-channel interference from A and is thereby prevented 237 from reliably exchanging protocol messages with D -- even though 238 these transmissions would not interfere with the reception of data 239 sent from A destined to B. 241 Radio Ranges for Devices A, B, C, D 243 <~~~~~~~~~~~~+~~~~~~~~~~~~> <~~~~~~~~~~+~~~~~~~~~~~> 244 |<~~~~~~~~~~~~+~~~~~~~~~~~~>|<~~~~~~~~~~~~+~~~~~~~~~> 245 +--|--+ +--|--+ +--|--+ +--|--+ 246 | B |<======| A | | C |======>| D | 247 +-----+ +-----+ +-----+ +-----+ 249 Figure 3: The exposed terminal problem. When device A is communicating 250 with device B, and device C is "exposed". 252 Hidden and exposed terminal situations are often observed in multi- 253 hop ad hoc wireless networks. Asymmetry issues with wireless 254 communication may also arise for reasons other than power inequality 255 (e.g., multipath interference). Such problems are often resolved by 256 specific mechanisms below the IP layer, for example, CSMA/CA, which 257 ensures transmission in periods perceived to be unoccupied by other 258 transmissions. However, depending on the link layer technology in 259 use and the position of the devices, such problems may affect the IP 260 layer due to range-limitation and partial overlap . 262 Besides radio range limitations, wireless communications are affected 263 by irregularities in the shape of the geographical area over which 264 devices may effectively communicate (see for instance [MC03], 265 [MI03]). For example, even omnidirectional wireless transmission is 266 typically non-isotropic (i.e. non-circular). Signal strength often 267 suffers frequent and significant variations, which are not a simple 268 function of distance. Instead, it is a complex function of the 269 environment including obstacles, weather conditions, interference, 270 and other factors that change over time. Because wireless 271 communications have to encounter different terrain, path, 272 obstructions, atmospheric conditions and other phenomena, analytical 273 formulation of signal strength is considered intractable [VTC99], and 274 the radio engineering community has thus developed numerous radio 275 propagation models, relying on median values observed in specific 276 environments [SAR03]. 278 The above irregularities also cause communications on multi-hop ad 279 hoc wireless networks to be non-transitive, asymmetric, or time- 280 varying, as described in Section 3.1, and may impact protocols at the 281 IP layer and above. There may be no indication to the IP layer when 282 a previously established communication channel becomes unusable; 283 "link down" triggers are generally absent in multi-hop ad hoc 284 wireless networks, since the absence of detectable radio energy 285 (e.g., in carrier waves) may simply indicate that neighboring devices 286 are not currently transmitting. Such an absence of detectable radio 287 energy does not therefore indicate whether or not transmissions have 288 failed to reach the intended destination. 290 4. Alternative Terminology 292 Many terms have been used in the past to describe the relationship of 293 devices in a multi-hop ad hoc wireless network based on their ability 294 to send or receive packets to/from each other. The terms used in 295 this document have been selected because the authors believe they are 296 unambiguous, with respect to the goal of this document (see 297 Section 1). 299 Nevertheless, here are some other terms that describe the same 300 relationship between devices in multi-hop ad hoc wireless networks. 301 In the following, let network N be, again, a multi-hop ad hoc 302 wireless network. Let the set S be, as before, the set of devices 303 that can directly receive packets transmitted by device A through its 304 interface on network N. In other words, any device B belonging to S 305 can detect packets transmitted by A. Then, due to the asymmetric 306 nature of wireless communications: 308 - We may say that device A "reaches" device B. In this 309 terminology, there is no guarantee that B reaches A, even if A 310 reaches B. 312 - We may say that device B "hears" device A. In this terminology, 313 there is no guarantee that A hears B, even if B hears A. 315 - We may say that device A "has a link" to device B. In this 316 terminology, there is no guarantee that B has a link to A, even if 317 A has a link to B. 319 - We may say that device B "is adjacent to" device A. In this 320 terminology, there is no guarantee that A is adjacent to B, even 321 if B is adjacent to A. 323 - We may say that device B "is downstream from" device A. In this 324 terminology, there is no guarantee that A is downstream from B, 325 even if B is downstream from A. 327 - We may say that device B "is a neighbor of" device A. In this 328 terminology, there is no guarantee that A is a neighbor of B, even 329 if B a neighbor of A. As it happens, a terminology based on 330 "neighborhood" is quite confusing for multi-hop wireless 331 communications. For example, when B can detect A, but A cannot 332 detect B, it is not clear whether B should be considered a 333 neighbor of A at all, since A would not necessarily be aware that 334 B was a neighbor, as it cannot detect B. It is thus best to avoid 335 the "neighbor" terminology, except for when some level of symmetry 336 has been verified. 338 This list of alternative terminologies is given here for illustrative 339 purposes only, and is not suggested to be complete or even 340 representative of the breadth of terminologies that have been used in 341 various ways to explain the properties mentioned in Section 3. 343 5. Security Considerations 345 This document does not have any security considerations. 347 6. IANA Considerations 349 This document does not have any IANA actions. 351 7. Informative References 353 [RFC2501] Corson, M. and J. Macker, "Mobile Ad hoc Networking 354 (MANET): Routing Protocol Performance Issues and 355 Evaluation Considerations", RFC 2501, January 1999. 357 [RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On- 358 Demand Distance Vector (AODV) Routing", RFC 3561, July 359 2003. 361 [RFC3626] Clausen, T. and P. Jacquet, "Optimized Link State Routing 362 Protocol (OLSR)", RFC 3626, October 2003. 364 [RFC3684] Ogier, R., Templin, F., and M. Lewis, "Topology 365 Dissemination Based on Reverse-Path Forwarding (TBRPF)", 366 RFC 3684, February 2004. 368 [RFC4728] Johnson, D., Hu, Y., and D. Maltz, "The Dynamic Source 369 Routing Protocol (DSR) for Mobile Ad Hoc Networks for 370 IPv4", RFC 4728, February 2007. 372 [RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903, June 373 2007. 375 [RFC5449] Baccelli, E., Jacquet, P., Nguyen, D., and T. Clausen, 376 "OSPF Multipoint Relay (MPR) Extension for Ad Hoc 377 Networks", RFC 5449, February 2009. 379 [RFC5889] Baccelli, E. and M. Townsley, "IP Addressing Model in Ad 380 Hoc Networks", RFC 5889, September 2010. 382 [RFC6250] Thaler, D., "Evolution of the IP Model", RFC 6250, May 383 2011. 385 [DoD01] Freebersyser, J. and B. Leiner, "A DoD perspective on 386 mobile ad hoc networks", Addison Wesley C. E. Perkins, 387 Ed., 2001, pp. 29--51, 2001. 389 [FUNKFEUER] 390 "Austria Wireless Community Network, 391 http://www.funkfeuer.at", 2013. 393 [MC03] Corson, S. and J. Macker, "Mobile Ad hoc Networking: 394 Routing Technology for Dynamic, Wireless Networks", IEEE 395 Press Mobile Ad hoc Networking, Chapter 9, 2003. 397 [SAR03] Sarkar, T., Ji, Z., Kim, K., Medour, A., and M. Salazar- 398 Palma, "A Survey of Various Propagation Models for Mobile 399 Communication", IEEE Press Antennas and Propagation 400 Magazine, Vol. 45, No. 3, 2003. 402 [VTC99] Kim, D., Chang, Y., and J. Lee, "Pilot power control and 403 service coverage support in CDMA mobile systems", IEEE 404 Press Proceedings of the IEEE Vehicular Technology 405 Conference (VTC), pp.1464-1468, 1999. 407 [MI03] Kotz, D., Newport, C., and C. Elliott, "The Mistaken 408 Axioms of Wireless-Network Research", Dartmouth College 409 Computer Science Technical Report TR2003-467, 2003. 411 [FREIFUNK] 412 "Freifunk Wireless Community Networks, 413 http://www.freifunk.net", 2013. 415 Appendix A. Acknowledgements 417 This document stems from discussions with the following people, in 418 alphabetical order: Jari Arkko, Teco Boot, Carlos Jesus Bernardos 419 Cano, Ian Chakeres, Thomas Clausen, Robert Cragie, Christopher 420 Dearlove, Ralph Droms, Brian Haberman, Ulrich Herberg, Paul Lambert, 421 Kenichi Mase, Thomas Narten, Erik Nordmark, Alexandru Petrescu, Stan 422 Ratliff, Zach Shelby, Shubhranshu Singh, Fred Templin, Dave Thaler, 423 Mark Townsley, Ronald Velt in't, and Seung Yi. 425 Authors' Addresses 427 Emmanuel Baccelli 428 INRIA 430 EMail: Emmanuel.Baccelli@inria.fr 431 URI: http://www.emmanuelbaccelli.org/ 433 Charles E. Perkins 434 Futurewei 436 Phone: +1-408-330-4586 437 EMail: charlie.perkins@huawei.com