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Petrov, Ed. 5 Expires: July 29, 2021 Acklio 6 January 25, 2021 8 Static Context Header Compression (SCHC) over LoRaWAN 9 draft-ietf-lpwan-schc-over-lorawan-14 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 20 LoRaWAN(R) networks, and provides elements such as efficient 21 parameterization 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 July 29, 2021. 40 Copyright Notice 42 Copyright (c) 2021 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 . . . . . . . . . . . . . . . . . . . . . . 13 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 . 17 78 5.7. SCHC Fragment Format . . . . . . . . . . . . . . . . . . 20 79 5.7.1. All-0 SCHC fragment . . . . . . . . . . . . . . . . . 20 80 5.7.2. All-1 SCHC fragment . . . . . . . . . . . . . . . . . 21 81 5.7.3. Delay after each LoRaWAN frame to respect local 82 regulation . . . . . . . . . . . . . . . . . . . . . 21 83 6. Security Considerations . . . . . . . . . . . . . . . . . . . 21 84 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 85 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 21 86 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 21 87 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 88 10.1. Normative References . . . . . . . . . . . . . . . . . . 22 89 10.2. Informative References . . . . . . . . . . . . . . . . . 23 90 10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 23 91 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 23 92 A.1. Uplink - Compression example - No fragmentation . . . . . 23 93 A.2. Uplink - Compression and fragmentation example . . . . . 24 94 A.3. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 26 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 97 1. Introduction 99 SCHC specification [RFC8724] describes generic header compression and 100 fragmentation techniques that can be used on all Low Power Wide Area 101 Networks (LPWAN) technologies defined in [RFC8376]. Even though 102 those technologies share a great number of common features like star- 103 oriented topologies, network architecture, devices with mostly quite 104 predictable communications, etc; they do have some slight differences 105 with respect to payload sizes, reactiveness, etc. 107 SCHC provides a generic framework that enables those devices to 108 communicate on IP networks. However, for efficient performance, some 109 parameters and modes of operation need to be set appropriately for 110 each of the LPWAN technologies. 112 This document describes the parameters and modes of operation when 113 SCHC is used over LoRaWAN networks. LoRaWAN protocol is specified by 114 the LoRa Alliance(R) in [lora-alliance-spec] 116 2. Terminology 118 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 119 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 120 "OPTIONAL" in this document are to be interpreted as described in BCP 121 14 [RFC2119] [RFC8174] when, and only when, they appear in all 122 capitals, as shown here. 124 This section defines the terminology and acronyms used in this 125 document. For all other definitions, please look up the SCHC 126 specification [RFC8724]. 128 o DevEUI: Device Extended Unique Identifier, an IEEE EUI-64 129 identifier used to identify the device during the procedure while 130 joining the network (Join Procedure). It is assigned by the 131 manufacturer or the device owner and provisioned on the Network 132 Gateway. 134 o DevAddr: a 32-bit non-unique identifier assigned to a device 135 either: 137 * Statically: by the device manufacturer in _Activation by 138 Personalization_ mode. 140 * Dynamically: after a Join Procedure by the Network Gateway in 141 _Over The Air Activation_ mode. 143 o Downlink: LoRaWAN term for a frame transmitted by the network and 144 received by the device. 146 o EUI: Extended Unique Identifier 148 o LoRaWAN: LoRaWAN is a wireless technology based on Industrial, 149 Scientific, and Medical (ISM) radio bands that is used for long- 150 range, low-power, low-data-rate applications developed by the LoRa 151 Alliance, a membership consortium: https://www.lora-alliance.org 152 [1]. 154 o FRMPayload: Application data in a LoRaWAN frame. 156 o MSB: Most Significant Byte 158 o OUI: Organisation Unique Identifier. IEEE assigned prefix for 159 EUI. 161 o RCS: Reassembly Check Sequence. Used to verify the integrity of 162 the fragmentation-reassembly process. 164 o RX: Device's reception window. 166 o RX1/RX2: LoRaWAN class A devices open two RX windows following an 167 uplink, called RX1 and RX2. 169 o SCHC gateway: The LoRaWAN Application Server that manages 170 translation between IPv6 network and the Network Gateway (LoRaWAN 171 Network Server). 173 o Tile: Piece of a fragmented packet as described in [RFC8724] 174 section 8.2.2.1 176 o Uplink: LoRaWAN term for a frame transmitted by the device and 177 received by the network. 179 3. Static Context Header Compression Overview 181 This section contains a short overview of SCHC. For a detailed 182 description, refer to the full specification [RFC8724]. 184 It defines: 186 1. Compression mechanisms to avoid transporting information known by 187 both sender and receiver over the air. Known information is part 188 of the "context". This component is called SCHC Compressor/ 189 Decompressor (SCHC C/D). 191 2. Fragmentation mechanisms to allow SCHC Packet transportation on 192 small, and potentially variable, MTU. This component is called 193 SCHC Fragmentation/Reassembly (SCHC F/R). 195 Context exchange or pre-provisioning is out of scope of this 196 document. 198 Device App 199 +----------------+ +----+ +----+ +----+ 200 | App1 App2 App3 | |App1| |App2| |App3| 201 | | | | | | | | 202 | UDP | |UDP | |UDP | |UDP | 203 | IPv6 | |IPv6| |IPv6| |IPv6| 204 | | | | | | | | 205 |SCHC C/D and F/R| | | | | | | 206 +--------+-------+ +----+ +----+ +----+ 207 | +---+ +----+ +----+ +----+ . . . 208 +~ |RGW| === |NGW | == |SCHC| == |SCHC|...... Internet .... 209 +---+ +----+ |F/R | |C/D | 210 +----+ +----+ 211 |<- - - - LoRaWAN - - ->| 213 Figure 1: Architecture 215 Figure 1 represents the architecture for compression/decompression, 216 it is based on [RFC8376] terminology. The device is sending 217 applications flows using IPv6 or IPv6/UDP protocols. These flows 218 might be compressed by a Static Context Header Compression 219 Compressor/Decompressor (SCHC C/D) to reduce headers size and 220 fragmented by the SCHC Fragmentation/Reassembly (SCHC F/R). The 221 resulting information is sent on a layer two (L2) frame to an LPWAN 222 Radio Gateway (RGW) that forwards the frame to a Network Gateway 223 (NGW). The NGW sends the data to a SCHC F/R for reassembly, if 224 required, then to SCHC C/D for decompression. The SCHC C/D shares 225 the same rules with the device. The SCHC C/D and F/R can be located 226 on the Network Gateway (NGW) or in another place as long as a 227 communication is established between the NGW and the SCHC F/R, then 228 SCHC F/R and C/D. The SCHC C/D and F/R in the device and the SCHC 229 gateway MUST share the same set of rules. After decompression, the 230 packet can be sent on the Internet to one or several LPWAN 231 Application Servers (App). 233 The SCHC C/D and F/R process is bidirectional, so the same principles 234 can be applied to the other direction. 236 In a LoRaWAN network, the RGW is called a Gateway, the NGW is Network 237 Server, and the SCHC C/D and F/R are an Application Server. It can 238 be provided by the Network Gateway or any third party software. 239 Figure 1 can be mapped in LoRaWAN terminology to: 241 End Device App 242 +--------------+ +----+ +----+ +----+ 243 |App1 App2 App3| |App1| |App2| |App3| 244 | | | | | | | | 245 | UDP | |UDP | |UDP | |UDP | 246 | IPv6 | |IPv6| |IPv6| |IPv6| 247 | | | | | | | | 248 |SCHC C/D & F/R| | | | | | | 249 +-------+------+ +----+ +----+ +----+ 250 | +-------+ +-------+ +-----------+ . . . 251 +~ |Gateway| === |Network| == |Application|..... Internet .... 252 +-------+ |server | |server | 253 +-------+ | F/R - C/D | 254 +-----------+ 255 |<- - - - - LoRaWAN - - - ->| 257 Figure 2: SCHC Architecture mapped to LoRaWAN 259 4. LoRaWAN Architecture 261 An overview of LoRaWAN [lora-alliance-spec] protocol and architecture 262 is described in [RFC8376]. The mapping between the LPWAN 263 architecture entities as described in [RFC8724] and the ones in 264 [lora-alliance-spec] is as follows: 266 o Devices are LoRaWAN End Devices (e.g. sensors, actuators, etc.). 267 There can be a very high density of devices per radio gateway 268 (LoRaWAN gateway). This entity maps to the LoRaWAN end-device. 270 o The Radio Gateway (RGW), which is the endpoint of the constrained 271 link. This entity maps to the LoRaWAN Gateway. 273 o The Network Gateway (NGW) is the interconnection node between the 274 Radio Gateway and the SCHC gateway (LoRaWAN Application server). 275 This entity maps to the LoRaWAN Network Server. 277 o SCHC C/D and F/R are handled by LoRaWAN Application Server; ie the 278 LoRaWAN application server will do the SCHC C/D and F/R. 280 o The LPWAN-AAA Server is the LoRaWAN Join Server. Its role is to 281 manage and deliver security keys in a secure way, so that the devices 282 root key is never exposed. 284 (LPWAN-AAA Server) 285 () () () | +------+ 286 () () () () / \ +---------+ | Join | 287 () () () () () / \======| ^ |===|Server| +-----------+ 288 () () () | | <--|--> | +------+ |Application| 289 () () () () / \==========| v |=============| Server | 290 () () () / \ +---------+ +-----------+ 291 End-devices Gateways Network Server (SCHC C/D and F/R) 292 (devices) (RGW) (NGW) 294 Figure 3: LPWAN Architecture 296 _Note_: Figure 3 terms are from LoRaWAN, with [RFC8376] terminology 297 in brackets. 299 SCHC Compressor/Decompressor (SCHC C/D) and SCHC Fragmentation/ 300 Reassembly (SCHC F/R) are performed on the LoRaWAN end-device and the 301 Application Server (called SCHC gateway). While the point-to-point 302 link between the device and the Application Server constitutes a 303 single IP hop, the ultimate end-point of the IP communication may be 304 an Internet node beyond the Application Server. In other words, the 305 LoRaWAN Application Server (SCHC gateway) acts as the first hop IP 306 router for the device. The Application Server and Network Server may 307 be co-located, which effectively turns the Network/Application Server 308 into the first hop IP router. 310 4.1. Device classes (A, B, C) and interactions 312 The LoRaWAN MAC layer supports 3 classes of devices named A, B and C. 313 All devices implement the Class A, some devices may implement Class B 314 or Class C. Class B and Class C are mutually exclusive. 316 o Class A: The Class A is the simplest class of devices. The device 317 is allowed to transmit at any time, randomly selecting a 318 communication channel. The Network Gateway may reply with a 319 downlink in one of the 2 receive windows immediately following the 320 uplinks. Therefore, the Network Gateway cannot initiate a 321 downlink, it has to wait for the next uplink from the device to 322 get a downlink opportunity. The Class A is the lowest power 323 consumption class. 325 o Class B: Class B devices implement all the functionalities of 326 Class A devices, but also schedule periodic listen windows. 327 Therefore, opposed to the Class A devices, Class B devices can 328 receive downlinks that are initiated by the Network Gateway and 329 not following an uplink. There is a trade-off between the 330 periodicity of those scheduled Class B listen windows and the 331 power consumption of the device: if the periodicity is high 332 downlinks from the NGW will be sent faster, but the device wakes 333 up more often: it will have higher power consumption. 335 o Class C: Class C devices implement all the functionalities of 336 Class A devices, but keep their receiver open whenever they are 337 not transmitting. Class C devices can receive downlinks at any 338 time at the expense of a higher power consumption. Battery- 339 powered devices can only operate in Class C for a limited amount 340 of time (for example for a firmware upgrade over-the-air). Most 341 of the Class C devices are grid powered (for example Smart Plugs). 343 4.2. Device addressing 345 LoRaWAN end-devices use a 32-bit network address (devAddr) to 346 communicate with the Network Gateway over-the-air, this address might 347 not be unique in a LoRaWAN network. Devices using the same devAddr 348 are distinguished by the Network Gateway based on the cryptographic 349 signature appended to every LoRaWAN frame. 351 To communicate with the SCHC gateway, the Network Gateway MUST 352 identify the devices by a unique 64-bit device identifier called the 353 DevEUI. 355 The DevEUI is assigned to the device during the manufacturing process 356 by the device's manufacturer. It is built like an Ethernet MAC 357 address by concatenating the manufacturer's IEEE OUI field with a 358 vendor unique number. e.g.: 24-bit OUI is concatenated with a 40-bit 359 serial number. The Network Gateway translates the devAddr into a 360 DevEUI in the uplink direction and reciprocally on the downlink 361 direction. 363 +--------+ +---------+ +---------+ +----------+ 364 | Device | <=====> | Network | <====> | SCHC | <======> | Internet | 365 | | devAddr | Gateway | DevEUI | Gateway | IPv6/UDP | | 366 +--------+ +---------+ +---------+ +----------+ 368 Figure 4: LoRaWAN addresses 370 4.3. General Frame Types 372 LoRaWAN implements the possibility to send confirmed or unconfirmed 373 frames: 375 o Confirmed frame: The sender asks the receiver to acknowledge the 376 frame. 378 o Unconfirmed frame: The sender does not ask the receiver to 379 acknowledge the frame. 381 As SCHC defines its own acknowledgment mechanisms, SCHC does not 382 require the use of LoRaWAN Confirmed frames (MType=0b100 as per 383 [lora-alliance-spec]) 385 4.4. LoRaWAN MAC Frames 387 In addition to regular data frames, LoRaWAN implements JoinRequest 388 and JoinAccept frame types, which are used by a device to join a 389 network: 391 o JoinRequest: This frame is used by a device to join a network. It 392 contains the device's unique identifier DevEUI and a random nonce 393 that will be used for session key derivation. 395 o JoinAccept: To on-board a device, the Network Gateway responds to 396 the JoinRequest issued by a device with a JoinAccept frame. That 397 frame is encrypted with the device's AppKey and contains (amongst 398 other fields) the network's major settings and a random nonce used 399 to derive the session keys. 401 o Data: MAC and application data. Application data are protected 402 with AES-128 encryption. MAC related data are AES-128 encrypted 403 with another key. 405 4.5. LoRaWAN FPort 407 The LoRaWAN MAC layer features a frame port field in all frames. 408 This field (FPort) is 8 bits long and the values from 1 to 223 can be 409 used. It allows LoRaWAN networks and applications to identify data. 411 4.6. LoRaWAN empty frame 413 A LoRaWAN empty frame is a LoRaWAN frame without FPort (cf 414 Section 5.1) and FRMPayload. 416 4.7. Unicast and multicast technology 418 LoRaWAN technology supports unicast downlinks, but also multicast: a 419 packet sent over LoRaWAN radio link can be received by several 420 devices. It is useful to address many devices with same content, 421 either a large binary file (firmware upgrade), or same command (e.g: 422 lighting control). As IPv6 is also a multicast technology this 423 feature can be used to address a group of devices. 425 _Note 1_: IPv6 multicast addresses must be defined as per [RFC4291]. 426 LoRaWAN multicast group definition in a Network Gateway and the 427 relation between those groups and IPv6 groupID are out of scope of 428 this document. 430 _Note 2_: LoRa Alliance defined [lora-alliance-remote-multicast-set] 431 as the RECOMMENDED way to setup multicast groups on devices and 432 create a synchronized reception window. 434 5. SCHC-over-LoRaWAN 436 5.1. LoRaWAN FPort and RuleID 438 The FPort field is part of the SCHC Message, as shown in Figure 5. 439 The SCHC C/D and the SCHC F/R SHALL concatenate the FPort field with 440 the LoRaWAN payload to recompose the SCHC Message. 442 | FPort | LoRaWAN payload | 443 + ------------------------ + 444 | SCHC Message | 446 Figure 5: SCHC Message in LoRaWAN 448 Note: SCHC Message is any datagram sent by SCHC C/D or F/R layers. 450 A fragmented datagram with application payload transferred from 451 device to Network Gateway, is called an uplink fragmented datagram. 452 It uses an FPort for data uplink and its associated SCHC control 453 downlinks, named FPortUp in this document. The other way, a 454 fragmented datagram with application payload transferred from Network 455 Gateway to device, is called downlink fragmented datagram. It uses 456 another FPort for data downlink and its associated SCHC control 457 uplinks, named FPortDown in this document. 459 All RuleID can use arbitrary values inside the FPort range allowed by 460 LoRaWAN specification and MUST be shared by the device and SCHC 461 gateway prior to the communication with the selected rule. The 462 uplink and downlink fragmentation FPorts MUST be different. 464 5.2. Rule ID management 466 RuleID MUST be 8 bits, encoded in the LoRaWAN FPort as described in 467 Section 5.1. LoRaWAN supports up to 223 application FPorts in the 468 range [1;223] as defined in section 4.3.2 of [lora-alliance-spec], it 469 implies that RuleID MSB SHOULD be inside this range. An application 470 can send non SCHC traffic by using FPort values different from the 471 ones used for SCHC. 473 In order to improve interoperability, RECOMMENDED fragmentation 474 RuleID values are: 476 o RuleID = 20 (8-bit) for uplink fragmentation, named FPortUp. 478 o RuleID = 21 (8-bit) for downlink fragmentation, named FPortDown. 480 o RuleID = 22 (8-bit) for which SCHC compression was not possible 481 (i.e., no matching compression Rule was found), as described in 482 [RFC8724] section 6. 484 FPortUp value MUST be different from FPortDown. The remaining 485 RuleIDs are available for compression. RuleIDs are shared between 486 uplink and downlink sessions. A RuleID not in the set(s) of FPortUp 487 or FPortDown means that the fragmentation is not used, thus, on 488 reception, the SCHC Message MUST be sent to the SCHC C/D layer. 490 The only uplink frames using the FPortDown port are the fragmentation 491 SCHC control messages of a downlink fragmented datagram (for example, 492 SCHC ACKs). Similarly, the only downlink frames using the FPortUp 493 port are the fragmentation SCHC control messages of an uplink 494 fragmented datagram. 496 An application can have multiple fragmented datagrams between a 497 device and one or several SCHC gateways. A set of FPort values is 498 REQUIRED for each SCHC gateway instance the device is required to 499 communicate with. The application can use additional uplinks or 500 downlink fragmented parameters but SHALL implement at least the 501 parameters defined in this document. 503 The mechanism for context distribution across devices and gateways is 504 outside the scope of this document. 506 5.3. Interface IDentifier (IID) computation 508 In order to mitigate the risks described in [RFC8064] and [RFC8065], 509 implementation MUST implement the following algorithm and SHOULD use 510 it. 512 1. key = LoRaWAN AppSKey 514 2. cmac = aes128_cmac(key, DevEUI) 516 3. IID = cmac[0..7] 518 aes128_cmac algorithm is described in [RFC4493]. It has been chosen 519 as it is already used by devices for LoRaWAN protocol. 521 As AppSKey is renewed each time a device joins or rejoins a LoRaWAN 522 network, the IID will change over time; this mitigates privacy, 523 location tracking and correlation over time risks. Join periodicity 524 is defined at the application level. 526 Address scan risk is mitigated thanks to AES-128, which provides 527 enough entropy bits of the IID. 529 Using this algorithm will also ensure that there is no correlation 530 between the hardware identifier (IEEE-64 DevEUI) and the IID, so an 531 attacker cannot use manufacturer OUI to target devices. 533 Example with: 535 o DevEUI: 0x1122334455667788 537 o appSKey: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB 539 1. key: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB 540 2. cmac: 0xBA59F4B196C6C3432D9383C145AD412A 541 3. IID: 0xBA59F4B196C6C343 543 Figure 6: Example of IID computation. 545 There is a small probability of IID collision in a LoRaWAN network. 546 If this occurs, the IID can be changed by rekeying the device at L2 547 level (ie: trigger a LoRaWAN join). The way the device is rekeyed is 548 out of scope of this document and left to the implementation. 550 Note: Implementation also using another IID source MUST ensure that 551 the same IID is shared between the device and the SCHC gateway in the 552 compression and decompression of the IPv6 address of the device. 554 5.4. Padding 556 All padding bits MUST be 0. 558 5.5. Decompression 560 SCHC C/D MUST concatenate FPort and LoRaWAN payload to retrieve the 561 SCHC Packet as per Section 5.1. 563 RuleIDs matching FPortUp and FPortDown are reserved for SCHC 564 Fragmentation. 566 5.6. Fragmentation 568 The L2 Word Size used by LoRaWAN is 1 byte (8 bits). The SCHC 569 fragmentation over LoRaWAN uses the ACK-on-Error mode for uplink 570 fragmentation and Ack-Always mode for downlink fragmentation. A 571 LoRaWAN device cannot support simultaneous interleaved fragmented 572 datagrams in the same direction (uplink or downlink). 574 The fragmentation parameters are different for uplink and downlink 575 fragmented datagrams and are successively described in the next 576 sections. 578 5.6.1. DTag 580 [RFC8724] section 8.2.4 describes the possibility to interleave 581 several fragmented SCHC datagrams for the same RuleID. This is not 582 used in SCHC over LoRaWAN profile. A device cannot interleave 583 several fragmented SCHC datagrams on the same FPort. This field is 584 not used and its size is 0. 586 Note: The device can still have several parallel fragmented datagrams 587 with more than one SCHC gateway thanks to distinct sets of FPorts, cf 588 Section 5.2. 590 5.6.2. Uplink fragmentation: From device to SCHC gateway 592 In this case, the device is the fragment transmitter, and the SCHC 593 gateway the fragment receiver. A single fragmentation rule is 594 defined. SCHC F/R MUST concatenate FPort and LoRaWAN payload to 595 retrieve the SCHC Packet, as per Section 5.1. 597 o SCHC fragmentation reliability mode: "ACK-on-Error". 599 o SCHC header size is two bytes (the FPort byte + 1 additional 600 byte). 602 o RuleID: 8 bits stored in LoRaWAN FPort. cf Section 5.2 604 o DTag: Size T=0 bit, not used. cf Section 5.6.1 606 o Window index: 4 windows are used, encoded on M = 2 bits 608 o FCN: The FCN field is encoded on N = 6 bits, so WINDOW_SIZE = 63 609 tiles are allowed in a window. 611 o Last tile: it can be carried in a Regular SCHC Fragment, alone in 612 an All-1 SCHC Fragment or with any of these two methods. 613 Implementation must ensure that: 615 * The sender MUST ascertain that the receiver will not receive 616 the last tile through both a Regular SCHC Fragment and an All-1 617 SCHC Fragment during the same session. 619 * If the last tile is in All-1 SCHC message: current L2 MTU MUST 620 be big enough to fit the All-1 header and the last tile. 622 o Penultimate tile MUST be equal to the regular size. 624 o RCS: Use recommended calculation algorithm in [RFC8724] (S.8.2.3. 625 Integrity Checking). 627 o Tile: size is 10 bytes. 629 o Retransmission timer: Set by the implementation depending on the 630 application requirements. The default RECOMMENDED duration of 631 this timer is 12 hours; this value is mainly driven by application 632 requirements and MAY be changed by the application. 634 o Inactivity timer: The SCHC gateway implements an "inactivity 635 timer". The default RECOMMENDED duration of this timer is 12 636 hours; this value is mainly driven by application requirements and 637 MAY be changed by the application. 639 o MAX_ACK_REQUESTS: 8. With this set of parameters, the SCHC 640 fragment header is 16 bits, including FPort; payload overhead will 641 be 8 bits as FPort is already a part of LoRaWAN payload. MTU is: 642 _4 windows * 63 tiles * 10 bytes per tile = 2520 bytes_ 644 In addition to the per-rule context parameters specified in 645 [RFC8724], for uplink rules, an additional context parameter is 646 added: whether or not to ack after each window. 647 For battery powered devices, it is RECOMMENDED to use the ACK 648 mechanism at the end of each window instead of waiting until the end 649 of all windows: 651 o The SCHC receiver SHOULD send a SCHC ACK after every window even 652 if there is no missing tile. 654 o The SCHC sender SHOULD wait for the SCHC ACK from the SCHC 655 receiver before sending tiles from the next window. If the SCHC 656 ACK is not received, it SHOULD send a SCHC ACK REQ up to 657 MAX_ACK_REQUESTS times, as described previously. 659 This will avoid useless uplinks if the device has lost network 660 coverage. 662 For non-battery powered devices, the SCHC receiver MAY also choose to 663 send a SCHC ACK only at the end of all windows. This will reduce 664 downlink load on the LoRaWAN network, by reducing the number of 665 downlinks. 667 SCHC implementations MUST be compatible with both behaviors, and this 668 selection is part of the rule context. 670 5.6.2.1. Regular fragments 672 | FPort | LoRaWAN payload | 673 + ------ + ------------------------- + 674 | RuleID | W | FCN | Payload | 675 + ------ + ------ + ------ + ------- + 676 | 8 bits | 2 bits | 6 bits | | 678 Figure 7: All fragments except the last one. SCHC header size is 16 679 bits, including LoRaWAN FPort. 681 5.6.2.2. Last fragment (All-1) 683 | FPort | LoRaWAN payload | 684 + ------ + ---------------------------- + 685 | RuleID | W | FCN=All-1 | RCS | 686 + ------ + ------ + --------- + ------- + 687 | 8 bits | 2 bits | 6 bits | 32 bits | 689 Figure 8: All-1 SCHC Message: the last fragment without last tile. 691 | FPort | LoRaWAN payload | 692 + ------ + ---------------------------------------------------------- + 693 | RuleID | W | FCN=All-1 | RCS | Last tile | Opt. padding | 694 + ------ + ------ + --------- + ------- + ------------ + ------------ + 695 | 8 bits | 2 bits | 6 bits | 32 bits | 1 to 80 bits | 0 to 7 bits | 697 Figure 9: All-1 SCHC Message: the last fragment with last tile. 699 5.6.2.3. SCHC ACK 700 | FPort | LoRaWAN payload | 701 + ------ + --------------------------+ 702 | RuleID | W | C = 1 | padding | 703 | | | | (b'00000) | 704 + ------ + ----- + ----- + --------- + 705 | 8 bits | 2 bit | 1 bit | 5 bits | 707 Figure 10: SCHC ACK format, correct RCS check. 709 | FPort | LoRaWAN payload | 710 + ------ + --------------------------------- + ---------------- + 711 | RuleID | W | C = 0 | Compressed bitmap | Optional padding | 712 | | | | (C = 0) | (b'0...0) | 713 + ------ + ----- + ----- + ----------------- + ---------------- + 714 | 8 bits | 2 bit | 1 bit | 5 to 63 bits | 0, 6 or 7 bits | 716 Figure 11: SCHC ACK format, failed RCS check. 718 Note: Because of the bitmap compression mechanism and L2 byte 719 alignment, only the following discrete values are possible for the 720 compressed bitmap size: 5, 13, 21, 29, 37, 45, 53, 61, 62 and 63. 721 Bitmaps of 63 bits will require 6 bits of padding. 723 5.6.2.4. Receiver-Abort 725 | FPort | LoRaWAN payload | 726 + ------ + -------------------------------------------- + 727 | RuleID | W = b'11 | C = 1 | b'11111 | 0xFF (all 1's) | 728 + ------ + -------- + ------+-------- + ----------------+ 729 | 8 bits | 2 bits | 1 bit | 5 bits | 8 bits | 730 next L2 Word boundary ->| <-- L2 Word --> | 732 Figure 12: Receiver-Abort format. 734 5.6.2.5. SCHC acknowledge request 736 | FPort | LoRaWAN payload | 737 +------- +------------------------- + 738 | RuleID | W | FCN = b'000000 | 739 + ------ + ------ + --------------- + 740 | 8 bits | 2 bits | 6 bits | 742 Figure 13: SCHC ACK REQ format. 744 5.6.3. Downlink fragmentation: From SCHC gateway to device 746 In this case, the device is the fragmentation receiver, and the SCHC 747 gateway the fragmentation transmitter. The following fields are 748 common to all devices. SCHC F/R MUST concatenate FPort and LoRaWAN 749 payload to retrieve the SCHC Packet as described in Section 5.1. 751 o SCHC fragmentation reliability mode: 753 * Unicast downlinks: ACK-Always. 755 * Multicast downlinks: No-ACK, reliability has to be ensured by 756 the upper layer. This feature is OPTIONAL and may not be 757 implemented by SCHC gateway. 759 o RuleID: 8 bits stored in LoRaWAN FPort. cf Section 5.2 761 o DTag: Size T=0 bit, not used. cf Section 5.6.1 763 o FCN: The FCN field is encoded on N=1 bit, so WINDOW_SIZE = 1 tile. 765 o RCS: Use recommended calculation algorithm in [RFC8724] (S.8.2.3. 766 Integrity Checking). 768 o Inactivity timer: The default RECOMMENDED duration of this timer 769 is 12 hours; this value is mainly driven by application 770 requirements and MAY be changed by the application. 772 The following parameters apply to ACK-Always (Unicast) only: 774 o Retransmission timer: See Section 5.6.3.5. 776 o MAX_ACK_REQUESTS: 8. 778 o Window index (unicast only): encoded on M=1 bit, as per [RFC8724]. 780 As only 1 tile is used, its size can change for each downlink, and 781 will be the currently available MTU. 783 Class A devices can only receive during an RX slot, following the 784 transmission of an uplink. Therefore the SCHC gateway cannot 785 initiate communication (e.g., start a new SCHC session). In order to 786 create a downlink opportunity it is RECOMMENDED for Class A devices 787 to send an uplink every 24 hours when no SCHC session is started, 788 this is application specific and can be disabled. The RECOMMENDED 789 uplink is a LoRaWAN empty frame as defined Section 4.6. As this 790 uplink is to open an RX window, any LoRaWAN uplink frame from the 791 device MAY reset this counter. 793 _Note_: The Fpending bit included in LoRaWAN protocol SHOULD NOT be 794 used for SCHC-over-LoRaWAN protocol. It might be set by the Network 795 Gateway for other purposes but not SCHC needs. 797 5.6.3.1. Regular fragments 799 | FPort | LoRaWAN payload | 800 + ------ + ------------------------------------ + 801 | RuleID | W | FCN = b'0 | Payload | 802 + ------ + ----- + --------- + ---------------- + 803 | 8 bits | 1 bit | 1 bit | X bytes + 6 bits | 805 Figure 14: All fragments but the last one. Header size 10 bits, 806 including LoRaWAN FPort. 808 5.6.3.2. Last fragment (All-1) 810 | FPort | LoRaWAN payload | 811 + ------ + --------------------------- + ------------------------- + 812 | RuleID | W | FCN = b'1 | RCS | Payload | Opt padding | 813 + ------ + ----- + --------- + ------- + ----------- + ----------- + 814 | 8 bits | 1 bit | 1 bit | 32 bits | 6 to X bits | 0 to 7 bits | 816 Figure 15: All-1 SCHC Message: the last fragment. 818 5.6.3.3. SCHC ACK 820 | FPort | LoRaWAN payload | 821 + ------ + ---------------------------------- + 822 | RuleID | W | C = b'1 | Padding b'000000 | 823 + ------ + ----- + ------- + ---------------- + 824 | 8 bits | 1 bit | 1 bit | 6 bits | 826 Figure 16: SCHC ACK format, RCS is correct. 828 | FPort | LoRaWAN payload | 829 + ------ + ------------------------------------------------- + 830 | RuleID | W | C = b'0 | Bitmap = b'1 | Padding b'000000 | 831 + ------ + ----- + ------- + ------------ + ---------------- + 832 | 8 bits | 1 bit | 1 bit | 1 bit | 5 bits | 834 Figure 17: SCHC ACK format, RCS is incorrect. 836 5.6.3.4. Receiver-Abort 838 | FPort | LoRaWAN payload | 839 + ------ + ---------------------------------------------- + 840 | RuleID | W = b'1 | C = b'1 | b'111111 | 0xFF (all 1's) | 841 + ------ + ------- + ------- + -------- + --------------- + 842 | 8 bits | 1 bit | 1 bits | 6 bits | 8 bits | 843 next L2 Word boundary ->| <-- L2 Word --> | 845 Figure 18: Receiver-Abort packet (following an All-1 SCHC Fragment 846 with incorrect RCS). 848 5.6.3.5. Downlink retransmission timer 850 Class A and Class B or Class C devices do not manage retransmissions 851 and timers the same way. 853 5.6.3.5.1. Class A devices 855 Class A devices can only receive in an RX slot following the 856 transmission of an uplink. 858 The SCHC gateway implements an inactivity timer with a RECOMMENDED 859 duration of 36 hours. For devices with very low transmission rates 860 (example 1 packet a day in normal operation), that duration may be 861 extended: it is application specific. 863 RETRANSMISSION_TIMER is application specific and its RECOMMENDED 864 value is INACTIVITY_TIMER/(MAX_ACK_REQUESTS + 1). 866 *SCHC All-0 (FCN=0)* 868 All fragments but the last have an FCN=0 (because window size is 1). 869 Following an All-0 SCHC Fragment, the device MUST transmit the SCHC 870 ACK message. It MUST transmit up to MAX_ACK_REQUESTS SCHC ACK 871 messages before aborting. In order to progress the fragmented 872 datagram, the SCHC layer should immediately queue for transmission 873 those SCHC ACK if no SCHC downlink have been received during RX1 and 874 RX2 window. LoRaWAN layer will respect the applicable local spectrum 875 regulation. 877 _Note_: The ACK bitmap is 1 bit long and is always 1. 879 *SCHC All-1 (FCN=1)* 880 SCHC All-1 is the last fragment of a datagram, the corresponding SCHC 881 ACK message might be lost; therefore the SCHC gateway MUST request a 882 retransmission of this ACK when the retransmission timer expires. To 883 open a downlink opportunity the device MUST transmit an uplink every 884 RETRANSMISSION_TIMER/(MAX_ACK_REQUESTS * 885 SCHC_ACK_REQ_DN_OPPORTUNITY). The format of this uplink is 886 application specific. It is RECOMMENDED for a device to send an 887 empty frame (see Section 4.6) but it is application specific and will 888 be used by the NGW to transmit a potential SCHC ACK REQ. 889 SCHC_ACK_REQ_DN_OPPORTUNITY is application specific and its 890 recommended value is 2. It MUST be greater than 1. This allows to 891 open a downlink opportunity to any downlink with higher priority than 892 the SCHC ACK REQ message. 894 _Note_: The device MUST keep this SCHC ACK message in memory until it 895 receives a downlink SCHC Fragmentation Message (with FPort == 896 FPortDown) that is not a SCHC ACK REQ: it indicates that the SCHC 897 gateway has received the SCHC ACK message. 899 5.6.3.6. Class B or Class C devices 901 Class B devices can receive in scheduled RX slots or in RX slots 902 following the transmission of an uplink. Class C devices are almost 903 in constant reception. 905 RECOMMENDED retransmission timer value: 907 o Class B: 3 times the ping slot periodicity. 909 o Class C: 30 seconds. 911 The RECOMMENDED inactivity timer value is 12 hours for both Class B 912 and Class C devices. 914 5.7. SCHC Fragment Format 916 5.7.1. All-0 SCHC fragment 918 *Uplink fragmentation (Ack-On-Error)*: 920 All-0 is distinguishable from a SCHC ACK REQ as [RFC8724] states 921 _This condition is also met if the SCHC Fragment Header is a multiple 922 of L2 Words_; this condition met: SCHC header is 2 bytes. 924 *Downlink fragmentation (Ack-always)*: 926 As per [RFC8724] the SCHC All-1 MUST contain the last tile, 927 implementation must ensure that SCHC All-0 message Payload will be at 928 least the size of an L2 Word. 930 5.7.2. All-1 SCHC fragment 932 All-1 is distinguishable from a SCHC Sender-Abort as [RFC8724] states 933 _This condition is met if the RCS is present and is at least the size 934 of an L2 Word_; this condition met: RCS is 4 bytes. 936 5.7.3. Delay after each LoRaWAN frame to respect local regulation 938 This profile does not define a delay to be added after each LoRaWAN 939 frame, local regulation compliance is expected to be enforced by 940 LoRaWAN stack. 942 6. Security Considerations 944 This document is only providing parameters that are expected to be 945 best suited for LoRaWAN networks for [RFC8724]. IID security is 946 discussed in Section 5.3. As such, this document does not contribute 947 to any new security issues beyond those already identified in 948 [RFC8724]. Moreover, SCHC data (LoRaWAN payload) are protected at 949 the LoRaWAN level by an AES-128 encryption with a session key shared 950 by the device and the SCHC gateway. These session keys are renewed 951 at each LoRaWAN session (ie: each join or rejoin to the LoRaWAN 952 network) 954 7. IANA Considerations 956 This document has no IANA actions. 958 Acknowledgements 960 Thanks to all those listed in the Contributors section for the 961 excellent text, insightful discussions, reviews and suggestions, and 962 also to (in alphabetical order) Dominique Barthel, Arunprabhu 963 Kandasamy, Rodrigo Munoz, Alexander Pelov, Pascal Thubert, Laurent 964 Toutain for useful design considerations, reviews and comments. 966 Contributors 968 Contributors ordered by family name. 970 Vincent Audebert 971 EDF R&D 972 Email: vincent.audebert@edf.fr 973 Julien Catalano 974 Kerlink 975 Email: j.catalano@kerlink.fr 977 Michael Coracin 978 Semtech 979 Email: mcoracin@semtech.com 981 Marc Le Gourrierec 982 Sagemcom 983 Email: marc.legourrierec@sagemcom.com 985 Nicolas Sornin 986 Semtech 987 Email: nsornin@semtech.com 989 Alper Yegin 990 Actility 991 Email: alper.yegin@actility.com 993 10. References 995 10.1. Normative References 997 [lora-alliance-spec] 998 Alliance, L., "LoRaWAN Specification Version V1.0.4", 999 . 1002 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1003 Requirement Levels", BCP 14, RFC 2119, 1004 DOI 10.17487/RFC2119, March 1997, 1005 . 1007 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 1008 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 1009 2006, . 1011 [RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The 1012 AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June 1013 2006, . 1015 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1016 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1017 May 2017, . 1019 [RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC. 1020 Zuniga, "SCHC: Generic Framework for Static Context Header 1021 Compression and Fragmentation", RFC 8724, 1022 DOI 10.17487/RFC8724, April 2020, 1023 . 1025 10.2. Informative References 1027 [lora-alliance-remote-multicast-set] 1028 Alliance, L., "LoRaWAN Remote Multicast Setup 1029 Specification Version 1.0.0", . 1033 [RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu, 1034 "Recommendation on Stable IPv6 Interface Identifiers", 1035 RFC 8064, DOI 10.17487/RFC8064, February 2017, 1036 . 1038 [RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation- 1039 Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065, 1040 February 2017, . 1042 [RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) 1043 Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018, 1044 . 1046 10.3. URIs 1048 [1] https://www.lora-alliance.org 1050 Appendix A. Examples 1052 In following examples "applicative data" refers to the IPv6 payload 1053 sent by the application to the SCHC layer. 1055 A.1. Uplink - Compression example - No fragmentation 1057 This example represents an applicative data going through SCHC over 1058 LoRaWAN, no fragmentation required 1060 An applicative data of 78 bytes is passed to SCHC compression layer. 1061 Rule 1 is used by SCHC C/D layer, allowing to compress it to 40 bytes 1062 and 5 bits: 1 byte RuleID, 21 bits residue + 37 bytes payload. 1064 | RuleID | Compression residue | Payload | Padding=b'000 | 1065 + ------ + ------------------- + --------- + ------------- + 1066 | 1 | 21 bits | 37 bytes | 3 bits | 1068 Figure 19: Uplink example: SCHC Message 1070 The current LoRaWAN MTU is 51 bytes, although 2 bytes FOpts are used 1071 by LoRaWAN protocol: 49 bytes are available for SCHC payload; no need 1072 for fragmentation. The payload will be transmitted through FPort = 1073 1. 1075 | LoRaWAN Header | LoRaWAN payload (40 bytes) | 1076 + ------------------------- + --------------------------------------- + 1077 | | FOpts | RuleID=1 | Compression | Payload | Padding=b'000 | 1078 | | | | residue | | | 1079 + ---- + ------- + -------- + ----------- + --------- + ------------- + 1080 | XXXX | 2 bytes | 1 byte | 21 bits | 37 bytes | 3 bits | 1082 Figure 20: Uplink example: LoRaWAN packet 1084 A.2. Uplink - Compression and fragmentation example 1086 This example represents an applicative data going through SCHC, with 1087 fragmentation. 1089 An applicative data of 300 bytes is passed to SCHC compression layer. 1090 Rule 1 is used by SCHC C/D layer, allowing to compress it to 282 1091 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 279 bytes payload. 1093 | RuleID | Compression residue | Payload | 1094 + ------ + ------------------- + --------- + 1095 | 1 | 21 bits | 279 bytes | 1097 Figure 21: Uplink example: SCHC Message 1099 The current LoRaWAN MTU is 11 bytes, 0 bytes FOpts are used by 1100 LoRaWAN protocol: 11 bytes are available for SCHC payload + 1 byte 1101 FPort field. SCHC header is 2 bytes (including FPort) so 1 tile is 1102 sent in first fragment. 1104 | LoRaWAN Header | LoRaWAN payload (11 bytes) | 1105 + -------------------------- + -------------------------- + 1106 | | RuleID=20 | W | FCN | 1 tile | 1107 + -------------- + --------- + ----- + ------ + --------- + 1108 | XXXX | 1 byte | 0 0 | 62 | 10 bytes | 1110 Figure 22: Uplink example: LoRaWAN packet 1 1112 Content of the tile is: 1113 | RuleID | Compression residue | Payload | 1114 + ------ + ------------------- + ----------------- + 1115 | 1 | 21 bits | 6 bytes + 3 bits | 1117 Figure 23: Uplink example: LoRaWAN packet 1 - Tile content 1119 Next transmission MTU is 11 bytes, although 2 bytes FOpts are used by 1120 LoRaWAN protocol: 9 bytes are available for SCHC payload + 1 byte 1121 FPort field, a tile does not fit inside so LoRaWAN stack will send 1122 only FOpts. 1124 Next transmission MTU is 242 bytes, 4 bytes FOpts. 23 tiles are 1125 transmitted: 1127 | LoRaWAN Header | LoRaWAN payload (231 bytes) | 1128 + --------------------------------------+ --------------------------- + 1129 | | FOpts | RuleID=20 | W | FCN | 23 tiles | 1130 + -------------- + ------- + ---------- + ----- + ----- + ----------- + 1131 | XXXX | 4 bytes | 1 byte | 0 0 | 61 | 230 bytes | 1133 Figure 24: Uplink example: LoRaWAN packet 2 1135 Next transmission MTU is 242 bytes, no FOpts. All 5 remaining tiles 1136 are transmitted, the last tile is only 2 bytes + 5 bits. Padding is 1137 added for the remaining 3 bits. 1139 | LoRaWAN Header | LoRaWAN payload (44 bytes) | 1140 + ---- + ---------- + ----------------------------------------------- + 1141 | | RuleID=20 | W | FCN | 5 tiles | Padding=b'000 | 1142 + ---- + ---------- + ----- + ----- + --------------- + ------------- + 1143 | XXXX | 1 byte | 0 0 | 38 | 42 bytes+5 bits | 3 bits | 1145 Figure 25: Uplink example: LoRaWAN packet 3 1147 Then All-1 message can be transmitted: 1149 | LoRaWAN Header | LoRaWAN payload (44 bytes) | 1150 + ---- + -----------+ -------------------------- + 1151 | | RuleID=20 | W | FCN | RCS | 1152 + ---- + ---------- + ----- + ----- + ---------- + 1153 | XXXX | 1 byte | 0 0 | 63 | 4 bytes | 1155 Figure 26: Uplink example: LoRaWAN packet 4 - All-1 SCHC message 1157 All packets have been received by the SCHC gateway, computed RCS is 1158 correct so the following ACK is sent to the device by the SCHC 1159 receiver: 1161 | LoRaWAN Header | LoRaWAN payload | 1162 + -------------- + --------- + ------------------- + 1163 | | RuleID=20 | W | C | Padding | 1164 + -------------- + --------- + ----- + - + ------- + 1165 | XXXX | 1 byte | 0 0 | 1 | 5 bits | 1167 Figure 27: Uplink example: LoRaWAN packet 5 - SCHC ACK 1169 A.3. Downlink 1171 An applicative data of 155 bytes is passed to SCHC compression layer. 1172 Rule 1 is used by SCHC C/D layer, allowing to compress it to 130 1173 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 127 bytes payload. 1175 | RuleID | Compression residue | Payload | 1176 + ------ + ------------------- + --------- + 1177 | 1 | 21 bits | 127 bytes | 1179 Figure 28: Downlink example: SCHC Message 1181 The current LoRaWAN MTU is 51 bytes, no FOpts are used by LoRaWAN 1182 protocol: 51 bytes are available for SCHC payload + FPort field => it 1183 has to be fragmented. 1185 | LoRaWAN Header | LoRaWAN payload (51 bytes) | 1186 + ---- + ---------- + -------------------------------------- + 1187 | | RuleID=21 | W = 0 | FCN = 0 | 1 tile | 1188 + ---- + ---------- + ------ + ------- + ------------------- + 1189 | XXXX | 1 byte | 1 bit | 1 bit | 50 bytes and 6 bits | 1191 Figure 29: Downlink example: LoRaWAN packet 1 - SCHC Fragment 1 1193 Content of the tile is: 1195 | RuleID | Compression residue | Payload | 1196 + ------ + ------------------- + ------------------ + 1197 | 1 | 21 bits | 48 bytes and 1 bit | 1199 Figure 30: Downlink example: LoRaWAN packet 1: Tile content 1201 The receiver answers with a SCHC ACK: 1203 | LoRaWAN Header | LoRaWAN payload | 1204 + ---- + --------- + -------------------------------- + 1205 | | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 | 1206 + ---- + --------- + ----- + ----- + ---------------- + 1207 | XXXX | 1 byte | 1 bit | 1 bit | 6 bits | 1209 Figure 31: Downlink example: LoRaWAN packet 2 - SCHC ACK 1211 The second downlink is sent, two FOpts: 1213 | LoRaWAN Header | LoRaWAN payload (49 bytes) | 1214 + --------------------------- + ------------------------------------- + 1215 | | FOpts | RuleID=21 | W = 1 | FCN = 0 | 1 tile | 1216 + ---- + ------- + ---------- + ----- + ------- + ------------------- + 1217 | XXXX | 2 bytes | 1 byte | 1 bit | 1 bit | 48 bytes and 6 bits | 1219 Figure 32: Downlink example: LoRaWAN packet 3 - SCHC Fragment 2 1221 The receiver answers with an SCHC ACK: 1223 | LoRaWAN Header | LoRaWAN payload | 1224 + ---- + --------- + -------------------------------- + 1225 | | RuleID=21 | W = 1 | C = 1 | Padding=b'000000 | 1226 + ---- + --------- + ----- + ----- + ---------------- + 1227 | XXXX | 1 byte | 1 bit | 1 bit | 6 bits | 1229 Figure 33: Downlink example: LoRaWAN packet 4 - SCHC ACK 1231 The last downlink is sent, no FOpts: 1233 | LoRaWAN Header | LoRaWAN payload (37 bytes) | 1234 + ---- + ------- + --------------------------------------------------- + 1235 | | RuleID | W | FCN | RCS | 1 tile | Padding | 1236 | | 21 | 0 | 1 | | | b'00000 | 1237 + ---- + ------- + ----- + ----- + ------- + --------------- + ------- + 1238 | XXXX | 1 byte | 1 bit | 1 bit | 4 bytes | 31 bytes+1 bits | 5 bits | 1240 Figure 34: Downlink example: LoRaWAN packet 5 - All-1 SCHC message 1242 The receiver answers to the sender with an SCHC ACK: 1244 | LoRaWAN Header | LoRaWAN payload | 1245 + ---- + --------- + -------------------------------- + 1246 | | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 | 1247 + ---- + --------- + ----- + ----- + ---------------- + 1248 | XXXX | 1 byte | 1 bit | 1 bit | 6 bits | 1250 Figure 35: Downlink example: LoRaWAN packet 6 - SCHC ACK 1252 Authors' Addresses 1254 Olivier Gimenez (editor) 1255 Semtech 1256 14 Chemin des Clos 1257 Meylan 1258 France 1260 Email: ogimenez@semtech.com 1262 Ivaylo Petrov (editor) 1263 Acklio 1264 1137A Avenue des Champs Blancs 1265 35510 Cesson-Sevigne Cedex 1266 France 1268 Email: ivaylo@ackl.io