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Dujovne 3 Internet-Draft Universidad Diego Portales 4 Intended status: Standards Track M. Richardson 5 Expires: 17 August 2020 Sandelman Software Works 6 14 February 2020 8 IEEE 802.15.4 Information Element encapsulation of 6TiSCH Join and 9 Enrollment Information 10 draft-ietf-6tisch-enrollment-enhanced-beacon-11 12 Abstract 14 In TSCH mode of IEEE STD 802.15.4, opportunities for broadcasts are 15 limited to specific times and specific channels. Nodes in a TSCH 16 network typically frequently transmit Enhanced Beacon (EB) frames to 17 announce the presence of the network. This document provides a 18 mechanism by which information critical for new nodes (pledges) and 19 long sleeping nodes may be carried within the Enhanced Beacon. 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 https://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 17 August 2020. 38 Copyright Notice 40 Copyright (c) 2020 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 (https://trustee.ietf.org/ 45 license-info) in effect on the date of publication of this document. 46 Please review these documents carefully, as they describe your rights 47 and restrictions with respect to this document. Code Components 48 extracted from this document must include Simplified BSD License text 49 as described in Section 4.e of the Trust Legal Provisions and are 50 provided without warranty as described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 1.1. Use of BCP 14 Terminology . . . . . . . . . . . . . . . . 2 56 1.2. Layer-2 Synchronization . . . . . . . . . . . . . . . . . 2 57 1.3. Layer-3 synchronization: IPv6 Router Solicitations and 58 Advertisements . . . . . . . . . . . . . . . . . . . . . 3 59 2. Protocol Definition . . . . . . . . . . . . . . . . . . . . . 4 60 3. Security Considerations . . . . . . . . . . . . . . . . . . . 6 61 4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 7 62 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 63 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 64 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 65 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 66 7.2. Informative References . . . . . . . . . . . . . . . . . 8 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 69 1. Introduction 71 [RFC7554] describes the use of the time-slotted channel hopping 72 (TSCH) mode of [ieee802154]. As further detailed in [RFC8180], an 73 Enhanced Beacon (EB) is transmitted during a slot designated as a 74 broadcast slot. 76 1.1. Use of BCP 14 Terminology 78 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 79 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 80 "OPTIONAL" in this document are to be interpreted as described in 81 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 82 capitals, as shown here. 84 Other terminology can be found in [I-D.ietf-6tisch-architecture] in 85 section 2.1. 87 1.2. Layer-2 Synchronization 89 As explained in section 6 of [RFC8180], the Enhanced Beacon (EB) has 90 a number of purposes: synchronization of ASN and Join Metric, 91 carrying timeslot template identifier, carrying the channel hopping 92 sequence identifier, and indicating the TSCH SlotFrame. 94 The EB is used by nodes already part of a TSCH network to announce 95 their existence. Receiving an EB allows a Joining Node (pledge) to 96 learn about the network and synchronize to it. The EB may also be 97 used as a means for a node already part of the network to re- 98 synchronize [RFC7554]. 100 There are a limited number of timeslots designated as broadcast slots 101 by each router in the network. Considering 10ms slots and a slot- 102 frame length of 100, these slots are rare and could result in only 1 103 slot per second for broadcasts, which needs to be used for the 104 beacon. Additional broadcasts for Router Advertisements, or Neighbor 105 Discovery could even more scarce. 107 1.3. Layer-3 synchronization: IPv6 Router Solicitations and 108 Advertisements 110 At layer 3, [RFC4861] defines a mechanism by which nodes learn about 111 routers by receiving multicast Router Advertisements (RA). If no RA 112 is received within a set time, then a Router Solicitation (RS) may be 113 transmitted as a multicast, to which an RA will be received, usually 114 unicast. 116 Although [RFC6775] reduces the amount of multicast necessary to do 117 address resolution via Neighbor Solicitation (NS) messages, it still 118 requires multicast of either RAs or RS. This is an expensive 119 operation for two reasons: First, there are few multicast timeslots 120 for unsolicited RAs; and second, if a pledge node does not receive an 121 RA, and decides to transmit an RS, a broadcast aloha slot is consumed 122 with unencrypted traffic. In this case, a unicast RS may be 123 transmitted in response. 125 This is a particularly acute issue for the join process for the 126 following reasons: 128 1. Use of a multicast slot by even a non-malicious unauthenticated 129 node for a Router Solicitation (RS) may overwhelm that time slot. 131 2. It may require many seconds of on-time before a new pledge 132 receives a Router Advertisement (RA) that it can use. 134 3. A new pledge may have to receive many Enhanced Beacons (EB) 135 before it can pick an appropriate network and/or closest Join 136 Assistant to attach to. If it must remain in the receive state 137 for an RA as well as find the Enhanced Beacon (EB), then the 138 process may take a very long time. 140 This document defines a new IETF IE subtype to provide join and 141 enrollment information to prospective pledges in a more efficient 142 way. 144 2. Protocol Definition 146 [RFC8137] creates a registry for new IETF IE subtypes. This document 147 allocates a new subtype. 149 The new IE subtype structure is as follows. As explained in 150 [RFC8137] the length of the Sub-Type Content can be calculated from 151 the container, so no length information is necessary. 153 1 2 3 154 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 155 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 156 | TBD-XXX |R|P| res | proxy prio | rank priority | 157 +-+-+-+-+-+-+-+-+-+-------------+-------------+-----------------+ 158 | pan priority | | 159 +---------------+ + 160 | Join Proxy lower-64 | 161 + (present if P=1) + 162 | | 163 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 164 | | | 165 +-+-+-+-+-+-+-+-+ + 166 | network ID | 167 + variable length, up to 16 bytes + 168 ~ ~ 169 + + 170 | | 171 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 172 | | 173 +-+-+-+-+-+-+-+-+ 175 Figure 1: IE subtype structure 177 R: The Router Advertisement R-flag is set if the sending node will 178 act as a Router for host-only nodes that need addressing via 179 unicast Router Solicitation messages. 181 In most cases, every node sending a beacon will set this flag, and 182 in a typical mesh, this will be every single node. When this bit 183 is not set, it indicates that this node may be under provisioned, 184 or may have no additional slots for additional nodes. This could 185 make this node more interesting to an attacker. 187 P: If the Proxy Address P-flag is set, then the Join Proxy lower-64 188 bit field is present. Otherwise, it is not provided. 190 This bit only indicates if another part of the structure is 191 present, and has little security or privacy impact. 193 proxy priority (proxy prio): This field indicates the willingness of 194 the sender to act as join proxy. Lower value indicates greater 195 willingness to act as a Join Proxy as described in 196 [I-D.ietf-6tisch-minimal-security]. Values range 0x00 (most 197 willing) to 0x7e (least willing). A priority of 0x7f indicates 198 that the announcer should never be considered as a viable 199 enrollment proxy. Only unenrolled pledges look at this value. 201 Lower values in this field indicate that the transmitter may have 202 more capacity to handle unencrypted traffic. A higher value may 203 indicate that the transmitter is low on neighbor cache entries, or 204 other resources. 206 rank priority: The rank "priority" is set by the 6LR which sent the 207 beacon and is an indication of how willing this 6LR is to serve as 208 an RPL parent within a particular network ID. This is a local 209 value to be determined in other work. It might be calculated from 210 RPL rank, and it may include some modifications based upon current 211 number of children, or number of neighbor cache entries available. 212 This value MUST be ignored by pledges, it is for enrolled devices 213 only. Lower values are better. 215 An attacker can use this value to determine which nodes are 216 potentially more interesting. Nodes which are less willing to be 217 parents likely have more traffic, and an attacker could use this 218 information to determine which nodes would be more interesting to 219 attack or disrupt. 221 pan priority: The pan priority is a value set by the DODAG root to 222 indicate the relative priority of this LLN compared to those with 223 different PANIDs. This value may be used as part of the 224 enrollment priority, but typically is used by devices which have 225 already enrolled, and need to determine which PAN to pick. 226 Unenrolled pledges MAY consider this value when selecting a PAN to 227 join. Enrolled devices MAY consider this value when looking for 228 an eligible parent device. 230 An attacker can use this value, along with the observed PANID in 231 the Beacon to determine which PANIDs have more network resources, 232 and may have more interesting traffic. 234 Join Proxy lower-64: If the P bit is set, then 64 bits (8 bytes) of 235 address are present. This field provides the suffix (IID) of the 236 Link-Local address of the Join Proxy. The associated prefix is 237 well-known as fe80::/64. If this field is not present, then IID 238 is derived from the layer-2 address of the sender. 240 This field communicates a lower-64 bits that should be used for 241 this nodes' layer-3 address, if it should not be derived from the 242 layer-2 address. Communication with the Join Proxy occurs in the 243 clear, this field avoids the need for an additional service 244 discovery process for the case where the L3 address is not derived 245 from the L2 address. An attacker will see both L2 and L3 246 addresses, so this field provides no new information. 248 network ID: This is a variable length field, up to 16-bytes in size 249 that uniquely identifies this network, potentially among many 250 networks that are operating in the same frequencies in overlapping 251 physical space. The length of this field can be calculated as 252 being whatever is left in the Information Element. 254 In a 6tisch network, where RPL [RFC6550] is used as the mesh 255 routing protocol, the network ID can be constructed from a SHA256 256 hash of the prefix (/64) of the network. That is just a 257 suggestion for a default value. In some LLNs where multiple 258 PANIDs may lead to the same management device (the JRC), then a 259 common value that is the same across all PANs MUST be configured. 261 If the the network ID is derived as suggested, then it will an 262 opaque, seemingly random value, and will reveal nothing in of 263 itself. An attacker can match this value across many 264 transmissions to map the extent of a network beyond what the PANID 265 might already provide. 267 3. Security Considerations 269 All of the contents of this Information Element are transmitted in 270 the clear. The content of the Enhanced Beacon is not encrypted. 271 This is a restriction in the cryptographic architecture of the 272 802.15.4 mechanism. In order to decrypt or do integrity checking of 273 layer-2 frames in TSCH, the TSCH Absolute Slot Number (ASN) is 274 needed. The Enhanced Beacon provides the ASN to new (and long- 275 sleeping) nodes. 277 The Enhanced Beacon is authenticated at the layer-2 level using 278 802.15.4 mechanisms using the network-wide keying material. Nodes 279 which are enrolled will have the network-wide keying material and can 280 validate the beacon. 282 Pledges which have not yet enrolled are unable to authenticate the 283 beacons, and will be forced to temporarily take the contents on 284 faith. After enrollment, a newly enrolled node will be able to 285 return to the beacon and validate it. 287 In addition to the enrollment and join information described in this 288 document, the Enhanced Beacon contains a description of the TSCH 289 schedule to be used by the transmitter of this packet. The schedule 290 can provide an attacker with a list of channels and frequencies on 291 which communication will occur. Knowledge of this can help an 292 attacker to more efficiently jam communications, although there is 293 future work being considered to make some of the schedule less 294 visible. Encrypting the schedule does not prevent an attacker from 295 jamming, but rather increases the energy cost of doing that jamming. 297 4. Privacy Considerations 299 The use of a network ID may reveal information about the network. 300 The use of a SHA256 hash of the DODAGID, rather than using the 301 DODAGID (which is usually derived from the LLN prefix) directly 302 provides some privacy for the the addresses used within the network. 303 The DODAGID is usually the IPv6 address of the root of the RPL mesh. 305 An interloper with a radio sniffer would be able to use the network 306 ID to map out the extent of the mesh network. 308 5. IANA Considerations 310 Allocate a new number TBD-XXX from Registry IETF Information Element 311 (IE) Sub-type ID, as defined by [RFC8137]. This entry should be 312 called 6tisch-Join-Info, and should refer to this document. 314 6. Acknowledgements 316 Thomas Watteyne provided extensive editorial comments on the 317 document. Carles Gomez Montenegro generated a detailed review of the 318 document at WGLC. Tim Evens provided a number of useful editorial 319 suggestions. 321 7. References 323 7.1. Normative References 325 [BCP14] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 326 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 327 May 2017, . 329 [I-D.ietf-6tisch-minimal-security] 330 Vucinic, M., Simon, J., Pister, K., and M. Richardson, 331 "Constrained Join Protocol (CoJP) for 6TiSCH", Work in 332 Progress, Internet-Draft, draft-ietf-6tisch-minimal- 333 security-15, 10 December 2019, . 337 [ieee802154] 338 IEEE standard for Information Technology, ., "IEEE Std. 339 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) 340 and Physical Layer (PHY) Specifications for Low-Rate 341 Wireless Personal Area Networks", 2015, 342 . 345 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 346 Requirement Levels", BCP 14, RFC 2119, 347 DOI 10.17487/RFC2119, March 1997, 348 . 350 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 351 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 352 DOI 10.17487/RFC4861, September 2007, 353 . 355 [RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. 356 Bormann, "Neighbor Discovery Optimization for IPv6 over 357 Low-Power Wireless Personal Area Networks (6LoWPANs)", 358 RFC 6775, DOI 10.17487/RFC6775, November 2012, 359 . 361 [RFC8137] Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information 362 Element for the IETF", RFC 8137, DOI 10.17487/RFC8137, May 363 2017, . 365 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 366 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 367 May 2017, . 369 7.2. Informative References 371 [I-D.ietf-6tisch-architecture] 372 Thubert, P., "An Architecture for IPv6 over the TSCH mode 373 of IEEE 802.15.4", Work in Progress, Internet-Draft, 374 draft-ietf-6tisch-architecture-28, 29 October 2019, 375 . 378 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 379 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 380 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 381 Low-Power and Lossy Networks", RFC 6550, 382 DOI 10.17487/RFC6550, March 2012, 383 . 385 [RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using 386 IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the 387 Internet of Things (IoT): Problem Statement", RFC 7554, 388 DOI 10.17487/RFC7554, May 2015, 389 . 391 [RFC8180] Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal 392 IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH) 393 Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180, 394 May 2017, . 396 Authors' Addresses 398 Diego Dujovne (editor) 399 Universidad Diego Portales 400 Escuela de Informatica y Telecomunicaciones, Av. Ejercito 441 401 Santiago, Region Metropolitana 402 Chile 404 Phone: +56 (2) 676-8121 405 Email: diego.dujovne@mail.udp.cl 407 Michael Richardson 408 Sandelman Software Works 410 Email: mcr+ietf@sandelman.ca