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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 lpwan Working Group O. Gimenez, Ed. 3 Internet-Draft Semtech 4 Intended status: Standards Track I. Petrov, Ed. 5 Expires: April 18, 2021 Acklio 6 October 15, 2020 8 Static Context Header Compression (SCHC) over LoRaWAN 9 draft-ietf-lpwan-schc-over-lorawan-11 11 Abstract 13 The Static Context Header Compression (SCHC) specification describes 14 generic header compression and fragmentation techniques for Low Power 15 Wide Area Networks (LPWAN) technologies. SCHC is a generic mechanism 16 designed for great flexibility so that it can be adapted for any of 17 the LPWAN technologies. 19 This document specifies a profile of RFC8724 to use SCHC in LoRaWAN 20 networks, and provides elements such as efficient parameterization 21 and modes of operation. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on April 18, 2021. 40 Copyright Notice 42 Copyright (c) 2020 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (https://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 58 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 3. Static Context Header Compression Overview . . . . . . . . . 4 60 4. LoRaWAN Architecture . . . . . . . . . . . . . . . . . . . . 6 61 4.1. Device classes (A, B, C) and interactions . . . . . . . . 7 62 4.2. Device addressing . . . . . . . . . . . . . . . . . . . . 8 63 4.3. General Frame Types . . . . . . . . . . . . . . . . . . . 8 64 4.4. LoRaWAN MAC Frames . . . . . . . . . . . . . . . . . . . 9 65 4.5. LoRaWAN FPort . . . . . . . . . . . . . . . . . . . . . . 9 66 4.6. LoRaWAN empty frame . . . . . . . . . . . . . . . . . . . 9 67 4.7. Unicast and multicast technology . . . . . . . . . . . . 9 68 5. SCHC-over-LoRaWAN . . . . . . . . . . . . . . . . . . . . . . 10 69 5.1. LoRaWAN FPort and RuleID . . . . . . . . . . . . . . . . 10 70 5.2. Rule ID management . . . . . . . . . . . . . . . . . . . 10 71 5.3. Interface IDentifier (IID) computation . . . . . . . . . 11 72 5.4. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 12 73 5.5. Decompression . . . . . . . . . . . . . . . . . . . . . . 12 74 5.6. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 12 75 5.6.1. DTag . . . . . . . . . . . . . . . . . . . . . . . . 13 76 5.6.2. Uplink fragmentation: From device to SCHC gateway . . 13 77 5.6.3. Downlink fragmentation: From SCHC gateway to device . 16 78 5.7. SCHC Fragment Format . . . . . . . . . . . . . . . . . . 19 79 5.7.1. All-0 SCHC fragment . . . . . . . . . . . . . . . . . 19 80 5.7.2. All-1 SCHC fragment . . . . . . . . . . . . . . . . . 20 81 5.7.3. Delay after each LoRaWAN frame to respect local 82 regulation . . . . . . . . . . . . . . . . . . . . . 20 83 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 84 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 85 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20 86 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 20 87 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 88 10.1. Normative References . . . . . . . . . . . . . . . . . . 21 89 10.2. Informative References . . . . . . . . . . . . . . . . . 22 90 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 22 91 A.1. Uplink - Compression example - No fragmentation . . . . . 22 92 A.2. Uplink - Compression and fragmentation example . . . . . 23 93 A.3. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 25 94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 96 1. Introduction 98 SCHC specification [RFC8724] describes generic header compression and 99 fragmentation techniques that can be used on all LPWAN technologies 100 defined in [RFC8376]. Even though those technologies share a great 101 number of common features like star-oriented topologies, network 102 architecture, devices with mostly quite predictable communications, 103 etc; they do have some slight differences with respect to payload 104 sizes, reactiveness, etc. 106 SCHC provides a generic framework that enables those devices to 107 communicate on IP networks. However, for efficient performance, some 108 parameters and modes of operation need to be set appropriately for 109 each of the LPWAN technologies. 111 This document describes the parameters and modes of operation when 112 SCHC is used over LoRaWAN networks. 114 2. Terminology 116 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 117 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 118 "OPTIONAL" in this document are to be interpreted as described in BCP 119 14 [RFC2119] [RFC8174] when, and only when, they appear in all 120 capitals, as shown here. 122 This section defines the terminology and acronyms used in this 123 document. For all other definitions, please look up the SCHC 124 specification [RFC8724]. 126 o DevEUI: an IEEE EUI-64 identifier used to identify the device 127 during the procedure while joining the network (Join Procedure). 128 It is assigned by the manufacturer or the device owner and 129 provisioned on the Network Gateway. 131 o DevAddr: a 32-bit non-unique identifier assigned to a device 132 either: 134 * Statically: by the device manufacturer in _Activation by 135 Personalization_ mode. 137 * Dynamically: after a Join Procedure by the Network Gateway in 138 _Over The Air Activation_ mode. 140 o Downlink: LoRaWAN term for a frame transmitted by the network and 141 received by the device. 143 o FRMPayload: Application data in a LoRaWAN frame. 145 o OUI: Organisation Unique Identifier. IEEE assigned prefix for 146 EUI. 148 o RCS: Reassembly Check Sequence. Used to verify the integrity of 149 the fragmentation-reassembly process. 151 o SCHC gateway: It corresponds to the LoRaWAN Application Server. 152 It manages translation between IPv6 network and the Network 153 Gateway (LoRaWAN Network Server). 155 o Tile: Piece of a fragmented packet as described in [RFC8724] 156 section 8.2.2.1 158 o Uplink: LoRaWAN term for a frame transmitted by the device and 159 received by the network. 161 3. Static Context Header Compression Overview 163 This section contains a short overview of SCHC. For a detailed 164 description, refer to the full specification [RFC8724]. 166 It defines: 168 1. Compression mechanisms to avoid transporting information known by 169 both sender and receiver over the air. Known information is part 170 of the "context". This component is called SCHC Compressor/ 171 Decompressor (SCHC C/D). 173 2. Fragmentation mechanisms to allow SCHC Packet transportation on 174 small, and potentially variable, MTU. This component is called 175 SCHC Fragmentation/Reassembly (SCHC F/R). 177 Context exchange or pre-provisioning is out of scope of this 178 document. 180 Device App 181 +----------------+ +----+ +----+ +----+ 182 | App1 App2 App3 | |App1| |App2| |App3| 183 | | | | | | | | 184 | UDP | |UDP | |UDP | |UDP | 185 | IPv6 | |IPv6| |IPv6| |IPv6| 186 | | | | | | | | 187 |SCHC C/D and F/R| | | | | | | 188 +--------+-------+ +----+ +----+ +----+ 189 | +---+ +----+ +----+ +----+ . . . 190 +~ |RGW| === |NGW | == |SCHC| == |SCHC|...... Internet .... 191 +---+ +----+ |F/R | |C/D | 192 +----+ +----+ 194 Figure 1: Architecture 196 Figure 1 represents the architecture for compression/decompression, 197 it is based on [RFC8376] terminology. The device is sending 198 applications flows using IPv6 or IPv6/UDP protocols. These flows 199 might be compressed by a Static Context Header Compression 200 Compressor/Decompressor (SCHC C/D) to reduce headers size and 201 fragmented by the SCHC Fragmentation/Reassembly (SCHC F/R). The 202 resulting information is sent on a layer two (L2) frame to an LPWAN 203 Radio Gateway (RGW) that forwards the frame to a Network Gateway 204 (NGW). The NGW sends the data to a SCHC F/R for reassembly, if 205 required, then to SCHC C/D for decompression. The SCHC C/D shares 206 the same rules with the device. The SCHC C/D and F/R can be located 207 on the Network Gateway (NGW) or in another place as long as a 208 communication is established between the NGW and the SCHC F/R, then 209 SCHC F/R and C/D. The SCHC C/D and F/R in the device and the SCHC 210 gateway MUST share the same set of rules. After decompression, the 211 packet can be sent on the Internet to one or several LPWAN 212 Application Servers (App). 214 The SCHC C/D and F/R process is bidirectional, so the same principles 215 can be applied to the other direction. 217 In a LoRaWAN network, the RGW is called a Gateway, the NGW is Network 218 Server, and the SCHC C/D and F/R are an Application Server. It can 219 be provided by the Network Gateway or any third party software. 220 Figure 1 can be mapped in LoRaWAN terminology to: 222 End Device App 223 +--------------+ +----+ +----+ +----+ 224 |App1 App2 App3| |App1| |App2| |App3| 225 | | | | | | | | 226 | UDP | |UDP | |UDP | |UDP | 227 | IPv6 | |IPv6| |IPv6| |IPv6| 228 | | | | | | | | 229 |SCHC C/D & F/R| | | | | | | 230 +-------+------+ +----+ +----+ +----+ 231 | +-------+ +-------+ +-----------+ . . . 232 +~ |Gateway| === |Network| == |Application|..... Internet .... 233 +-------+ |server | |server | 234 +-------+ | F/R - C/D | 235 +-----------+ 237 Figure 2: SCHC Architecture mapped to LoRaWAN 239 4. LoRaWAN Architecture 241 An overview of LoRaWAN [lora-alliance-spec] protocol and architecture 242 is described in [RFC8376]. The mapping between the LPWAN 243 architecture entities as described in [RFC8724] and the ones in 244 [lora-alliance-spec] is as follows: 246 o Devices are LoRaWAN End Devices (e.g. sensors, actuators, etc.). 247 There can be a very high density of devices per radio gateway 248 (LoRaWAN gateway). This entity maps to the LoRaWAN end-device. 250 o The Radio Gateway (RGW), which is the endpoint of the constrained 251 link. This entity maps to the LoRaWAN Gateway. 253 o The Network Gateway (NGW) is the interconnection node between the 254 Radio Gateway and the SCHC gateway (LoRaWAN Application server). 255 This entity maps to the LoRaWAN Network Server. 257 o SCHC C/D and F/R are handled by LoRaWAN Application Server; ie the 258 LoRaWAN application server will do the SCHC C/D and F/R. 260 o The LPWAN-AAA Server is the LoRaWAN Join Server. Its role is to 261 manage and deliver security keys in a secure way, so that the devices 262 root key is never exposed. 264 (LPWAN-AAA Server) 265 () () () | +------+ 266 () () () () / \ +---------+ | Join | 267 () () () () () / \======| ^ |===|Server| +-----------+ 268 () () () | | <--|--> | +------+ |Application| 269 () () () () / \==========| v |=============| Server | 270 () () () / \ +---------+ +-----------+ 271 End-devices Gateways Network Server (SCHC C/D and F/R) 272 (devices) (RGW) (NGW) 274 Figure 3: LPWAN Architecture 276 _Note_: Figure 3 terms are from LoRaWAN, with [RFC8376] terminology 277 in brackets. 279 SCHC Compressor/Decompressor (SCHC C/D) and SCHC Fragmentation/ 280 Reassembly (SCHC F/R) are performed on the LoRaWAN end-device and the 281 Application Server (called SCHC gateway). While the point-to-point 282 link between the device and the Application Server constitutes a 283 single IP hop, the ultimate end-point of the IP communication may be 284 an Internet node beyond the Application Server. In other words, the 285 LoRaWAN Application Server (SCHC gateway) acts as the first hop IP 286 router for the device. The Application Server and Network Server may 287 be co-located, which effectively turns the Network/Application Server 288 into the first hop IP router. 290 4.1. Device classes (A, B, C) and interactions 292 The LoRaWAN MAC layer supports 3 classes of devices named A, B and C. 293 All devices implement the Class A, some devices may implement Class B 294 or Class C. Class B and Class C are mutually exclusive. 296 o Class A: The Class A is the simplest class of devices. The device 297 is allowed to transmit at any time, randomly selecting a 298 communication channel. The Network Gateway may reply with a 299 downlink in one of the 2 receive windows immediately following the 300 uplinks. Therefore, the Network Gateway cannot initiate a 301 downlink, it has to wait for the next uplink from the device to 302 get a downlink opportunity. The Class A is the lowest power 303 consumption class. 305 o Class B: Class B devices implement all the functionalities of 306 Class A devices, but also schedule periodic listen windows. 307 Therefore, opposed to the Class A devices, Class B devices can 308 receive downlinks that are initiated by the Network Gateway and 309 not following an uplink. There is a trade-off between the 310 periodicity of those scheduled Class B listen windows and the 311 power consumption of the device: if the periodicity is high 312 downlinks from the NGW will be sent faster, but the device wakes 313 up more often: it will have higher power consumption. 315 o Class C: Class C devices implement all the functionalities of 316 Class A devices, but keep their receiver open whenever they are 317 not transmitting. Class C devices can receive downlinks at any 318 time at the expense of a higher power consumption. Battery- 319 powered devices can only operate in Class C for a limited amount 320 of time (for example for a firmware upgrade over-the-air). Most 321 of the Class C devices are grid powered (for example Smart Plugs). 323 4.2. Device addressing 325 LoRaWAN end-devices use a 32-bit network address (devAddr) to 326 communicate with the Network Gateway over-the-air, this address might 327 not be unique in a LoRaWAN network; devices using the same devAddr 328 are distinguished by the Network Gateway based on the cryptographic 329 signature appended to every LoRaWAN frame. 331 To communicate with the SCHC gateway, the Network Gateway MUST 332 identify the devices by a unique 64-bit device identifier called the 333 DevEUI. 335 The DevEUI is assigned to the device during the manufacturing process 336 by the device's manufacturer. It is built like an Ethernet MAC 337 address by concatenating the manufacturer's IEEE OUI field with a 338 vendor unique number. e.g.: 24-bit OUI is concatenated with a 40-bit 339 serial number. The Network Gateway translates the devAddr into a 340 DevEUI in the uplink direction and reciprocally on the downlink 341 direction. 343 +--------+ +---------+ +---------+ +----------+ 344 | Device | <=====> | Network | <====> | SCHC | <======> | Internet | 345 | | devAddr | Gateway | DevEUI | Gateway | IPv6/UDP | | 346 +--------+ +---------+ +---------+ +----------+ 348 Figure 4: LoRaWAN addresses 350 4.3. General Frame Types 352 LoRaWAN implements the possibility to send confirmed or unconfirmed 353 frames: 355 o Confirmed frame: The sender asks the receiver to acknowledge the 356 frame. 358 o Unconfirmed frame: The sender does not ask the receiver to 359 acknowledge the frame. 361 As SCHC defines its own acknowledgment mechanisms, SCHC does not 362 require to use LoRaWAN Confirmed frames. 364 4.4. LoRaWAN MAC Frames 366 In addition to regular data frames, LoRaWAN implements JoinRequest 367 and JoinAccept frame types, which are used by a device to join a 368 network: 370 o JoinRequest: This frame is used by a device to join a network. It 371 contains the device's unique identifier DevEUI and a random nonce 372 that will be used for session key derivation. 374 o JoinAccept: To on-board a device, the Network Gateway responds to 375 the JoinRequest issued by a device with a JoinAccept frame. That 376 frame is encrypted with the device's AppKey and contains (amongst 377 other fields) the network's major settings and a random nonce used 378 to derive the session keys. 380 o Data: MAC and application data. Application data are protected 381 with AES-128 encryption, MAC related data are AES-128 encrypted 382 with another key. 384 4.5. LoRaWAN FPort 386 The LoRaWAN MAC layer features a frame port field in all frames. 387 This field (FPort) is 8 bits long and the values from 1 to 223 can be 388 used. It allows LoRaWAN networks and applications to identify data. 390 4.6. LoRaWAN empty frame 392 A LoRaWAN empty frame is a LoRaWAN frame without FPort (cf 393 Section 5.1) and FRMPayload. 395 4.7. Unicast and multicast technology 397 LoRaWAN technology supports unicast downlinks, but also multicast: a 398 packet sent over LoRaWAN radio link can be received by several 399 devices. It is useful to address many devices with same content, 400 either a large binary file (firmware upgrade), or same command (e.g: 401 lighting control). As IPv6 is also a multicast technology this 402 feature can be used to address a group of devices. 404 _Note 1_: IPv6 multicast addresses must be defined as per [RFC4291]. 405 LoRaWAN multicast group definition in a Network Gateway and the 406 relation between those groups and IPv6 groupID are out of scope of 407 this document. 409 _Note 2_: LoRa Alliance defined [lora-alliance-remote-multicast-set] 410 as the RECOMMENDED way to setup multicast groups on devices and 411 create a synchronized reception window. 413 5. SCHC-over-LoRaWAN 415 5.1. LoRaWAN FPort and RuleID 417 The FPort field is part of the SCHC Message, as shown in Figure 5. 418 The SCHC C/D and the SCHC F/R SHALL concatenate the FPort field with 419 the LoRaWAN payload to recompose the SCHC Message. 421 | FPort | LoRaWAN payload | 422 + ------------------------ + 423 | SCHC packet | 425 Figure 5: SCHC Message in LoRaWAN 427 A fragmented datagram with application payload transferred from 428 device to Network Gateway, is called uplink fragmented datagram. It 429 uses an FPort for data uplink and its associated SCHC control 430 downlinks, named FPortUp in this document. The other way, a 431 fragmented datagram with application payload transferred from Network 432 Gateway to device, is called downlink fragmented datagram. It uses 433 another FPort for data downlink and its associated SCHC control 434 uplinks, named FPortDown in this document. 436 All RuleID can use arbitrary values inside the FPort range allowed by 437 LoRaWAN specification and MUST be shared by the device and SCHC 438 gateway prior to the communication with the selected rule. The 439 uplink and downlink fragmentation FPorts MUST be different. 441 5.2. Rule ID management 443 RuleID MUST be 8 bits, encoded in the LoRaWAN FPort as described in 444 Section 5.1. LoRaWAN supports up to 223 application FPorts in the 445 range [1;223] as defined in section 4.3.2 of [lora-alliance-spec], it 446 implies that RuleID MSB SHOULD be inside this range. An application 447 can send non SCHC traffic by using FPort values different from the 448 ones used for SCHC. 450 In order to improve interoperability, RECOMMENDED fragmentation 451 RuleID values are: 453 o RuleID = 20 (8-bit) for uplink fragmentation, named FPortUp. 455 o RuleID = 21 (8-bit) for downlink fragmentation, named FPortDown. 457 o RuleID = 22 (8-bit) for which SCHC compression was not possible 458 (i.e., no matching compression Rule was found), as described in 459 [RFC8724] section 6. 461 FPortUp value MUST be different from FPortDown. The remaining 462 RuleIDs are available for compression. RuleIDs are shared between 463 uplink and downlink sessions. A RuleID not in the set(s) of FPortUp 464 or FPortDown means that the fragmentation is not used, thus, on 465 reception, the SCHC Message MUST be sent to the SCHC C/D layer. 467 The only uplink frames using the FPortDown port are the fragmentation 468 SCHC control messages of a downlink fragmented datagram (for example, 469 SCHC ACKs). Similarly, the only downlink frames using the FPortUp 470 port are the fragmentation SCHC control messages of an uplink 471 fragmented datagram. 473 An application can have multiple fragmented datagrams between a 474 device and one or several SCHC gateways. A set of FPort values is 475 REQUIRED for each SCHC gateway instance the device is required to 476 communicate with. The application can use additional uplinks or 477 downlink fragmented parameters but SHALL implement at least the 478 parameters defined in this document. 480 The mechanism for context distribution across devices and gateways is 481 outside the scope of this document. 483 5.3. Interface IDentifier (IID) computation 485 In order to mitigate the risks described in [RFC8064] and [RFC8065], 486 IID MUST be created regarding the following algorithm: 488 1. key = LoRaWAN AppSKey 490 2. cmac = aes128_cmac(key, DevEUI) 492 3. IID = cmac[0..7] 494 aes128_cmac algorithm is described in [RFC4493]. It has been chosen 495 as it is already used by devices for LoRaWAN protocol. 497 As AppSKey is renewed each time a device joins or rejoins a LoRaWAN 498 network, the IID will change over time; this mitigates privacy, 499 location tracking and correlation over time risks. Join periodicity 500 is defined at the application level. 502 Address scan risk is mitigated thanks to AES-128, which provides 503 enough entropy bits of the IID. 505 Using this algorithm will also ensure that there is no correlation 506 between the hardware identifier (IEEE-64 DevEUI) and the IID, so an 507 attacker cannot use manufacturer OUI to target devices. 509 Example with: 511 o DevEUI: 0x1122334455667788 513 o appSKey: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB 515 1. key: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB 516 2. cmac: 0xBA59F4B196C6C3432D9383C145AD412A 517 3. IID: 0xBA59F4B196C6C343 519 Figure 6: Example of IID computation. 521 There is a small probability of IID collision in a LoRaWAN network. 522 If this occurs, the IID can be changed by rekeying the device at L2 523 level (ie: trigger a LoRaWAN join). The way the device is rekeyed is 524 out of scope of this document and left to the implementation. 526 5.4. Padding 528 All padding bits MUST be 0. 530 5.5. Decompression 532 SCHC C/D MUST concatenate FPort and LoRaWAN payload to retrieve the 533 SCHC Packet as per Section 5.1. 535 RuleIDs matching FPortUp and FPortDown are reserved for SCHC 536 Fragmentation. 538 5.6. Fragmentation 540 The L2 Word Size used by LoRaWAN is 1 byte (8 bits). The SCHC 541 fragmentation over LoRaWAN uses the ACK-on-Error mode for uplink 542 fragmentation and Ack-Always mode for downlink fragmentation. A 543 LoRaWAN device cannot support simultaneous interleaved fragmented 544 datagrams in the same direction (uplink or downlink). 546 The fragmentation parameters are different for uplink and downlink 547 fragmented datagrams and are successively described in the next 548 sections. 550 5.6.1. DTag 552 [RFC8724] section 8.2.4 describes the possibility to interleave 553 several fragmented SCHC datagrams for the same RuleID. This is not 554 used in SCHC over LoRaWAN profile. A device cannot interleave 555 several fragmented SCHC datagrams on the same FPort. This field is 556 not used and its size is 0. 558 Note: The device can still have several parallel fragmented datagrams 559 with more than one SCHC gateway thanks to distinct sets of FPorts, cf 560 Section 5.2. 562 5.6.2. Uplink fragmentation: From device to SCHC gateway 564 In this case, the device is the fragment transmitter, and the SCHC 565 gateway the fragment receiver. A single fragmentation rule is 566 defined. SCHC F/R MUST concatenate FPort and LoRaWAN payload to 567 retrieve the SCHC Packet, as per Section 5.1. 569 o SCHC header size is two bytes (the FPort byte + 1 additional 570 byte). 572 o RuleID: 8 bits stored in LoRaWAN FPort. 574 o SCHC fragmentation reliability mode: "ACK-on-Error". 576 o DTag: Size is 0 bit, not used. 578 o FCN: The FCN field is encoded on N = 6 bits, so WINDOW_SIZE = 63 579 tiles are allowed in a window. 581 o Window index: encoded on W = 2 bits. So 4 windows are available. 583 o RCS: Use recommended calculation algorithm in [RFC8724]. 585 o MAX_ACK_REQUESTS: 8. 587 o Tile: size is 10 bytes. 589 o Retransmission timer: Set by the implementation depending on the 590 application requirements. 592 o Inactivity timer: The SCHC gateway implements an "inactivity 593 timer". The default RECOMMENDED duration of this timer is 12 594 hours; this value is mainly driven by application requirements and 595 MAY be changed by the application. 597 o Penultimate tile MUST be equal to the regular size. 599 o Last tile: it can be carried in a Regular SCHC Fragment, alone in 600 an All-1 SCHC Fragment or with any of these two methods. 601 Implementation must ensure that: 603 * The sender MUST ascertain that the receiver will not receive 604 the last tile through both a Regular SCHC Fragment and an All-1 605 SCHC Fragment during the same session. 607 * If the last tile is in All-1 SCHC message: current L2 MTU MUST 608 be big enough to fit the All-1 header and the last tile. 610 With this set of parameters, the SCHC fragment header is 16 bits, 611 including FPort; payload overhead will be 8 bits as FPort is already 612 a part of LoRaWAN payload. MTU is: _4 windows * 63 tiles * 10 bytes 613 per tile = 2520 bytes_ 615 For battery powered devices, it is RECOMMENDED to use the ACK 616 mechanism at the end of each window instead of waiting until the end 617 of all windows: 619 o the SCHC receiver SHOULD send a SCHC ACK after every window even 620 if there is no missing tile. 622 o the SCHC sender SHOULD wait for the SCHC ACK from the SCHC 623 receiver before sending tiles from the next window. If the SCHC 624 ACK is not received, it SHOULD send a SCHC ACK REQ up to 625 MAX_ACK_REQUESTS times, as described previously. 627 For non-battery powered devices, the SCHC receiver MAY also choose to 628 send a SCHC ACK only at the end of all windows. This will reduce 629 downlink load on the LoRaWAN network, by reducing the number of 630 downlinks. 632 SCHC implementations MUST be compatible with both behaviors, and this 633 selection is part of the rule context. 635 5.6.2.1. Regular fragments 637 | FPort | LoRaWAN payload | 638 + ------ + ------------------------- + 639 | RuleID | W | FCN | Payload | 640 + ------ + ------ + ------ + ------- + 641 | 8 bits | 2 bits | 6 bits | | 643 Figure 7: All fragments except the last one. SCHC header size is 16 644 bits, including LoRaWAN FPort. 646 5.6.2.2. Last fragment (All-1) 648 | FPort | LoRaWAN payload | 649 + ------ + ---------------------------- + 650 | RuleID | W | FCN=All-1 | RCS | 651 + ------ + ------ + --------- + ------- + 652 | 8 bits | 2 bits | 6 bits | 32 bits | 654 Figure 8: All-1 SCHC Message: the last fragment without last tile. 656 | FPort | LoRaWAN payload | 657 + ------ + ------------------------------------------- + 658 | RuleID | W | FCN=All-1 | RCS | Last tile | 659 + ------ + ------ + --------- + ------- + ------------ + 660 | 8 bits | 2 bits | 6 bits | 32 bits | 1 to 80 bits | 662 Figure 9: All-1 SCHC Message: the last fragment with last tile. 664 5.6.2.3. SCHC ACK 666 | FPort | LoRaWAN payload | 667 + ------ + --------------------------------- + ---------------- + 668 | RuleID | W | C | Compressed bitmap | Optional padding | 669 | | | | (C = 0) | (b'0...0) | 670 + ------ + ----- + ----- + ----------------- + ---------------- + 671 | 8 bits | 2 bit | 1 bit | 5 to 63 bits | 0, 6 or 7 bits | 673 Figure 10: SCHC ACK format, failed RCS check. 675 Note: Because of the bitmap compression mechanism and L2 byte 676 alignment, only the following discrete values are possible for the 677 compressed bitmap size: 5, 13, 21, 29, 37, 45, 53, 61, 62 and 63. 678 Bitmaps of 63 bits will require 6 bits of padding. 680 5.6.2.4. Receiver-Abort 682 | FPort | LoRaWAN payload | 683 + ------ + -------------------------------------------- + 684 | RuleID | W = b'11 | C = 1 | b'11111 | 0xFF (all 1's) | 685 + ------ + -------- + ------+-------- + ----------------+ 686 | 8 bits | 2 bits | 1 bit | 5 bits | 8 bits | 687 next L2 Word boundary ->| <-- L2 Word --> | 689 Figure 11: Receiver-Abort format. 691 5.6.2.5. SCHC acknowledge request 693 | FPort | LoRaWAN payload | 694 +------- +------------------------- + 695 | RuleID | W | FCN = b'000000 | 696 + ------ + ------ + --------------- + 697 | 8 bits | 2 bits | 6 bits | 699 Figure 12: SCHC ACK REQ format. 701 5.6.3. Downlink fragmentation: From SCHC gateway to device 703 In this case, the device is the fragmentation receiver, and the SCHC 704 gateway the fragmentation transmitter. The following fields are 705 common to all devices. SCHC F/R MUST concatenate FPort and LoRaWAN 706 payload to retrieve the SCHC Packet as described in Section 5.1. 708 o SCHC fragmentation reliability mode: 710 * Unicast downlinks: ACK-Always. 712 * Multicast downlinks: No-ACK, reliability has to be ensured by 713 the upper layer. This feature is OPTIONAL and may not be 714 implemented by SCHC gateway. 716 o RuleID: 8 bits stored in LoRaWAN FPort. 718 o Window index (unicast only): encoded on W=1 bit, as per [RFC8724]. 720 o DTag: Size is 0 bit, not used. 722 o FCN: The FCN field is encoded on N=1 bit, so WINDOW_SIZE = 1 tile. 724 o RCS: Use recommended calculation algorithm in [RFC8724]. 726 o MAX_ACK_REQUESTS: 8. 728 o Retransmission timer: See Section 5.6.3.5. 730 o Inactivity timer: The default RECOMMENDED duration of this timer 731 is 12 hours; this value is mainly driven by application 732 requirements and MAY be changed by the application. 734 As only 1 tile is used, its size can change for each downlink, and 735 will be the currently available MTU. 737 Class A devices can only receive during an RX slot, following the 738 transmission of an uplink. Therefore the SCHC gateway cannot 739 initiate communication (e.g., start a new SCHC session). In order to 740 create a downlink opportunity it is RECOMMENDED for Class A devices 741 to send an uplink every 24 hours when no SCHC session is started, 742 this is application specific and can be disabled. The RECOMMENDED 743 uplink is a LoRaWAN empty frame as defined Section 4.6. As this 744 uplink is to open an RX window, any LoRaWAN uplink frame from the 745 device MAY reset this counter. 747 _Note_: The Fpending bit included in LoRaWAN protocol SHOULD NOT be 748 used for SCHC-over-LoRaWAN protocol. It might be set by the Network 749 Gateway for other purposes but not SCHC needs. 751 5.6.3.1. Regular fragments 753 | FPort | LoRaWAN payload | 754 + ------ + ------------------------------------ + 755 | RuleID | W | FCN = b'0 | Payload | 756 + ------ + ----- + --------- + ---------------- + 757 | 8 bits | 1 bit | 1 bit | X bytes + 6 bits | 759 Figure 13: All fragments but the last one. Header size 10 bits, 760 including LoRaWAN FPort. 762 5.6.3.2. Last fragment (All-1) 764 | FPort | LoRaWAN payload | 765 + ------ + --------------------------- + ----------------- + 766 | RuleID | W | FCN = b'1 | RCS | Payload | 767 + ------ + ----- + --------- + ------- + ----------------- + 768 | 8 bits | 1 bit | 1 bit | 32 bits | 6 bits to X bytes | 770 Figure 14: All-1 SCHC Message: the last fragment. 772 5.6.3.3. SCHC ACK 774 | FPort | LoRaWAN payload | 775 + ------ + ---------------------------------- + 776 | RuleID | W | C = b'1 | Padding b'000000 | 777 + ------ + ----- + ------- + ---------------- + 778 | 8 bits | 1 bit | 1 bit | 6 bits | 780 Figure 15: SCHC ACK format, RCS is correct. 782 5.6.3.4. Receiver-Abort 784 | FPort | LoRaWAN payload | 785 + ------ + ---------------------------------------------- + 786 | RuleID | W = b'1 | C = b'1 | b'111111 | 0xFF (all 1's) | 787 + ------ + ------- + ------- + -------- + --------------- + 788 | 8 bits | 1 bit | 1 bits | 6 bits | 8 bits | 789 next L2 Word boundary ->| <-- L2 Word --> | 791 Figure 16: Receiver-Abort packet (following an All-1 SCHC Fragment 792 with incorrect RCS). 794 5.6.3.5. Downlink retransmission timer 796 Class A and Class B or Class C devices do not manage retransmissions 797 and timers the same way. 799 5.6.3.5.1. Class A devices 801 Class A devices can only receive in an RX slot following the 802 transmission of an uplink. 804 The SCHC gateway implements an inactivity timer with a RECOMMENDED 805 duration of 36 hours. For devices with very low transmission rates 806 (example 1 packet a day in normal operation), that duration may be 807 extended: it is application specific. 809 RETRANSMISSION_TIMER is application specific and its RECOMMENDED 810 value is INACTIVITY_TIMER/(MAX_ACK_REQUESTS + 1). 812 *SCHC All-0 (FCN=0)* All fragments but the last have an FCN=0 813 (because window size is 1). Following it, the device MUST transmit 814 the SCHC ACK message. It MUST transmit up to MAX_ACK_REQUESTS SCHC 815 ACK messages before aborting. In order to progress the fragmented 816 datagram, the SCHC layer should immediately queue for transmission 817 those SCHC ACK if no SCHC downlink have been received during RX1 and 818 RX2 window. LoRaWAN layer will respect the applicable local spectrum 819 regulation. 821 _Note_: The ACK bitmap is 1 bit long and is always 1. 823 *SCHC All-1 (FCN=1)* SCHC All-1 is the last fragment of a datagram, 824 the corresponding SCHC ACK message might be lost; therefore the SCHC 825 gateway MUST request a retransmission of this ACK when the 826 retransmission timer expires. To open a downlink opportunity the 827 device MUST transmit an uplink every 828 RETRANSMISSION_TIMER/(MAX_ACK_REQUESTS * 829 SCHC_ACK_REQ_DN_OPPORTUNITY). The format of this uplink is 830 application specific. It is RECOMMENDED for a device to send an 831 empty frame (see Section 4.6) but it is application specific and will 832 be used by the NGW to transmit a potential SCHC ACK REQ. 833 SCHC_ACK_REQ_DN_OPPORTUNITY is application specific and its 834 recommended value <<<<<<< HEAD is 2, it MUST be greater than 1. This 835 allows for more downlink opportunities than required by SCHC control 836 traffic, leaving opportunity for any other downlink with higher 837 priority than SCHC ACK REQ message. ======= is 2. It MUST be 838 greater than 1. This allows to open a downlink opportunity to any 839 downlink with higher priority than the SCHC ACK REQ message. >>>>>>> 840 0b92a54cc8a5551fcf289f74e1ae3637a2f1c217 842 _Note_: The device MUST keep this SCHC ACK message in memory until it 843 receives a downlink SCHC Fragmentation Message (with FPort == 844 FPortDown) that is not a SCHC ACK REQ: it indicates that the SCHC 845 gateway has received the SCHC ACK message. 847 5.6.3.6. Class B or Class C devices 849 Class B devices can receive in scheduled RX slots or in RX slots 850 following the transmission of an uplink. Class C devices are almost 851 in constant reception. 853 RECOMMENDED retransmission timer value: 855 o Class B: 3 times the ping slot periodicity. 857 o Class C: 30 seconds. 859 The RECOMMENDED inactivity timer value is 12 hours for both Class B 860 and Class C devices. 862 5.7. SCHC Fragment Format 864 5.7.1. All-0 SCHC fragment 866 *Uplink fragmentation (Ack-On-Error)*: 868 All-0 is distinguishable from a SCHC ACK REQ as [RFC8724] states 869 _This condition is also met if the SCHC Fragment Header is a multiple 870 of L2 Words_; this condition met: SCHC header is 2 bytes. 872 *Downlink fragmentation (Ack-always)*: 874 As per [RFC8724] the SCHC All-1 MUST contain the last tile, 875 implementation must ensure that SCHC All-0 message Payload will be at 876 least the size of an L2 Word. 878 5.7.2. All-1 SCHC fragment 880 All-1 is distinguishable from a SCHC Sender-Abort as [RFC8724] states 881 _This condition is met if the RCS is present and is at least the size 882 of an L2 Word_; this condition met: RCS is 4 bytes. 884 5.7.3. Delay after each LoRaWAN frame to respect local regulation 886 This profile does not define a delay to be added after each LoRaWAN 887 frame, local regulation compliance is expected to be enforced by 888 LoRaWAN stack. 890 6. Security Considerations 892 This document is only providing parameters that are expected to be 893 best suited for LoRaWAN networks for [RFC8724]. IID security is 894 discussed in Section 5.3. As such, this document does not contribute 895 to any new security issues beyond those already identified in 896 [RFC8724]. Moreover, SCHC data (LoRaWAN payload) are protected at 897 the LoRaWAN level by an AES-128 encryption with a session key shared 898 by the device and the SCHC gateway. These session keys are renewed 899 at each LoRaWAN session (ie: each join or rejoin to the LoRaWAN 900 network) 902 7. IANA Considerations 904 This document has no IANA actions. 906 Acknowledgements 908 Thanks to all those listed in the Contributors section for the 909 excellent text, insightful discussions, reviews and suggestions, and 910 also to (in alphabetical order) Dominique Barthel, Arunprabhu 911 Kandasamy, Rodrigo Munoz, Alexander Pelov, Pascal Thubert, Laurent 912 Toutain for useful design considerations, reviews and comments. 914 Contributors 916 Contributors ordered by family name. 918 Vincent Audebert 919 EDF R&D 920 Email: vincent.audebert@edf.fr 921 Julien Catalano 922 Kerlink 923 Email: j.catalano@kerlink.fr 925 Michael Coracin 926 Semtech 927 Email: mcoracin@semtech.com 929 Marc Le Gourrierec 930 Sagemcom 931 Email: marc.legourrierec@sagemcom.com 933 Nicolas Sornin 934 Semtech 935 Email: nsornin@semtech.com 937 Alper Yegin 938 Actility 939 Email: alper.yegin@actility.com 941 10. References 943 10.1. Normative References 945 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 946 Requirement Levels", BCP 14, RFC 2119, 947 DOI 10.17487/RFC2119, March 1997, 948 . 950 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 951 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 952 2006, . 954 [RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu, 955 "Recommendation on Stable IPv6 Interface Identifiers", 956 RFC 8064, DOI 10.17487/RFC8064, February 2017, 957 . 959 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 960 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 961 May 2017, . 963 [RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC. 964 Zuniga, "SCHC: Generic Framework for Static Context Header 965 Compression and Fragmentation", RFC 8724, 966 DOI 10.17487/RFC8724, April 2020, 967 . 969 10.2. Informative References 971 [lora-alliance-remote-multicast-set] 972 Alliance, L., "LoRaWAN Remote Multicast Setup 973 Specification Version 1.0.0", . 977 [lora-alliance-spec] 978 Alliance, L., "LoRaWAN Specification Version V1.0.3", 979 . 982 [RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The 983 AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June 984 2006, . 986 [RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation- 987 Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065, 988 February 2017, . 990 [RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) 991 Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018, 992 . 994 Appendix A. Examples 996 In following examples "applicative payload" refers to the IPv6 997 payload sent by the application to the SCHC layer. 999 A.1. Uplink - Compression example - No fragmentation 1001 This example represents an applicative payload going through SCHC 1002 over LoRaWAN, no fragmentation required 1004 An applicative payload of 78 bytes is passed to SCHC compression 1005 layer. Rule 1 is used by SCHC C/D layer, allowing to compress it to 1006 40 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 37 bytes 1007 payload. 1009 | RuleID | Compression residue | Payload | Padding=b'000 | 1010 + ------ + ------------------- + --------- + ------------- + 1011 | 1 | 21 bits | 37 bytes | 3 bits | 1013 Figure 17: Uplink example: SCHC Message 1015 The current LoRaWAN MTU is 51 bytes, although 2 bytes FOpts are used 1016 by LoRaWAN protocol: 49 bytes are available for SCHC payload; no need 1017 for fragmentation. The payload will be transmitted through FPort = 1018 1. 1020 | LoRaWAN Header | LoRaWAN payload (40 bytes) | 1021 + ------------------------- + --------------------------------------- + 1022 | | FOpts | RuleID=1 | Compression | Payload | Padding=b'000 | 1023 | | | | residue | | | 1024 + ---- + ------- + -------- + ----------- + --------- + ------------- + 1025 | XXXX | 2 bytes | 1 byte | 21 bits | 37 bytes | 3 bits | 1027 Figure 18: Uplink example: LoRaWAN packet 1029 A.2. Uplink - Compression and fragmentation example 1031 This example represents an applicative payload going through SCHC, 1032 with fragmentation. 1034 An applicative payload of 478 bytes is passed to SCHC compression 1035 layer. Rule 1 is used by SCHC C/D layer, allowing to compress it to 1036 282 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 279 bytes 1037 payload. 1039 | RuleID | Compression residue | Payload | 1040 + ------ + ------------------- + --------- + 1041 | 1 | 21 bits | 279 bytes | 1043 Figure 19: Uplink example: SCHC Message 1045 The current LoRaWAN MTU is 11 bytes, 0 bytes FOpts are used by 1046 LoRaWAN protocol: 11 bytes are available for SCHC payload + 1 byte 1047 FPort field. SCHC header is 2 bytes (including FPort) so 1 tile is 1048 sent in first fragment. 1050 | LoRaWAN Header | LoRaWAN payload (11 bytes) | 1051 + -------------------------- + -------------------------- + 1052 | | RuleID=20 | W | FCN | 1 tile | 1053 + -------------- + --------- + ----- + ------ + --------- + 1054 | XXXX | 1 byte | 0 0 | 62 | 10 bytes | 1056 Figure 20: Uplink example: LoRaWAN packet 1 1058 Content of the tile is: 1059 | RuleID | Compression residue | Payload | 1060 + ------ + ------------------- + ----------------- + 1061 | 1 | 21 bits | 6 bytes + 3 bits | 1063 Figure 21: Uplink example: LoRaWAN packet 1 - Tile content 1065 Next transmission MTU is 11 bytes, although 2 bytes FOpts are used by 1066 LoRaWAN protocol: 9 bytes are available for SCHC payload + 1 byte 1067 FPort field, a tile does not fit inside so LoRaWAN stack will send 1068 only FOpts. 1070 Next transmission MTU is 242 bytes, 4 bytes FOpts. 23 tiles are 1071 transmitted: 1073 | LoRaWAN Header | LoRaWAN payload (231 bytes) | 1074 + --------------------------------------+ --------------------------- + 1075 | | FOpts | RuleID=20 | W | FCN | 23 tiles | 1076 + -------------- + ------- + ---------- + ----- + ----- + ----------- + 1077 | XXXX | 4 bytes | 1 byte | 0 0 | 61 | 230 bytes | 1079 Figure 22: Uplink example: LoRaWAN packet 2 1081 Next transmission MTU is 242 bytes, no FOpts. All 5 remaining tiles 1082 are transmitted, the last tile is only 2 bytes + 5 bits. Padding is 1083 added for the remaining 3 bits. 1085 | LoRaWAN Header | LoRaWAN payload (44 bytes) | 1086 + ---- + ---------- + ----------------------------------------------- + 1087 | | RuleID=20 | W | FCN | 5 tiles | Padding=b'000 | 1088 + ---- + ---------- + ----- + ----- + --------------- + ------------- + 1089 | XXXX | 1 byte | 0 0 | 38 | 42 bytes+5 bits | 3 bits | 1091 Figure 23: Uplink example: LoRaWAN packet 3 1093 Then All-1 message can be transmitted: 1095 | LoRaWAN Header | LoRaWAN payload (44 bytes) | 1096 + ---- + -----------+ -------------------------- + 1097 | | RuleID=20 | W | FCN | RCS | 1098 + ---- + ---------- + ----- + ----- + ---------- + 1099 | XXXX | 1 byte | 0 0 | 63 | 4 bytes | 1101 Figure 24: Uplink example: LoRaWAN packet 4 - All-1 SCHC message 1103 All packets have been received by the SCHC gateway, computed RCS is 1104 correct so the following ACK is sent to the device by the SCHC 1105 receiver: 1107 | LoRaWAN Header | LoRaWAN payload | 1108 + -------------- + --------- + ------------------- + 1109 | | RuleID=20 | W | C | Padding | 1110 + -------------- + --------- + ----- + - + ------- + 1111 | XXXX | 1 byte | 0 0 | 1 | 5 bits | 1113 Figure 25: Uplink example: LoRaWAN packet 5 - SCHC ACK 1115 A.3. Downlink 1117 An applicative payload of 443 bytes is passed to SCHC compression 1118 layer. Rule 1 is used by SCHC C/D layer, allowing to compress it to 1119 130 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 127 bytes 1120 payload. 1122 | RuleID | Compression residue | Payload | 1123 + ------ + ------------------- + --------- + 1124 | 1 | 21 bits | 127 bytes | 1126 Figure 26: Downlink example: SCHC Message 1128 The current LoRaWAN MTU is 51 bytes, no FOpts are used by LoRaWAN 1129 protocol: 51 bytes are available for SCHC payload + FPort field => it 1130 has to be fragmented. 1132 | LoRaWAN Header | LoRaWAN payload (51 bytes) | 1133 + ---- + ---------- + -------------------------------------- + 1134 | | RuleID=21 | W = 0 | FCN = 0 | 1 tile | 1135 + ---- + ---------- + ------ + ------- + ------------------- + 1136 | XXXX | 1 byte | 1 bit | 1 bit | 50 bytes and 6 bits | 1138 Figure 27: Downlink example: LoRaWAN packet 1 - SCHC Fragment 1 1140 Content of the tile is: 1142 | RuleID | Compression residue | Payload | 1143 + ------ + ------------------- + ------------------ + 1144 | 1 | 21 bits | 48 bytes and 1 bit | 1146 Figure 28: Downlink example: LoRaWAN packet 1: Tile content 1148 The receiver answers with a SCHC ACK: 1150 | LoRaWAN Header | LoRaWAN payload | 1151 + ---- + --------- + -------------------------------- + 1152 | | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 | 1153 + ---- + --------- + ----- + ----- + ---------------- + 1154 | XXXX | 1 byte | 1 bit | 1 bit | 6 bits | 1156 Figure 29: Downlink example: LoRaWAN packet 2 - SCHC ACK 1158 The second downlink is sent, two FOpts: 1160 | LoRaWAN Header | LoRaWAN payload (49 bytes) | 1161 + --------------------------- + ------------------------------------- + 1162 | | FOpts | RuleID=21 | W = 1 | FCN = 0 | 1 tile | 1163 + ---- + ------- + ---------- + ----- + ------- + ------------------- + 1164 | XXXX | 2 bytes | 1 byte | 1 bit | 1 bit | 48 bytes and 6 bits | 1166 Figure 30: Downlink example: LoRaWAN packet 3 - SCHC Fragment 2 1168 The receiver answers with an SCHC ACK: 1170 | LoRaWAN Header | LoRaWAN payload | 1171 + ---- + --------- + -------------------------------- + 1172 | | RuleID=21 | W = 1 | C = 1 | Padding=b'000000 | 1173 + ---- + --------- + ----- + ----- + ---------------- + 1174 | XXXX | 1 byte | 1 bit | 1 bit | 6 bits | 1176 Figure 31: Downlink example: LoRaWAN packet 4 - SCHC ACK 1178 The last downlink is sent, no FOpts: 1180 | LoRaWAN Header | LoRaWAN payload (37 bytes) | 1181 + ---- + ------- + --------------------------------------------------- + 1182 | | RuleID | W | FCN | RCS | 1 tile | Padding | 1183 | | 21 | 0 | 1 | | | b'00000 | 1184 + ---- + ------- + ----- + ----- + ------- + --------------- + ------- + 1185 | XXXX | 1 byte | 1 bit | 1 bit | 4 bytes | 31 bytes+1 bits | 5 bits | 1187 Figure 32: Downlink example: LoRaWAN packet 5 - All-1 SCHC message 1189 The receiver answers to the sender with an SCHC ACK: 1191 | LoRaWAN Header | LoRaWAN payload | 1192 + ---- + --------- + -------------------------------- + 1193 | | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 | 1194 + ---- + --------- + ----- + ----- + ---------------- + 1195 | XXXX | 1 byte | 1 bit | 1 bit | 6 bits | 1197 Figure 33: Downlink example: LoRaWAN packet 6 - SCHC ACK 1199 Authors' Addresses 1201 Olivier Gimenez (editor) 1202 Semtech 1203 14 Chemin des Clos 1204 Meylan 1205 France 1207 Email: ogimenez@semtech.com 1209 Ivaylo Petrov (editor) 1210 Acklio 1211 1137A Avenue des Champs Blancs 1212 35510 Cesson-Sevigne Cedex 1213 France 1215 Email: ivaylo@ackl.io