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Versions: (draft-petrov-lpwan-ipv6-schc-over-lorawan) 00 01 02 03 04 05 09

lpwan Working Group                                      O. Gimenez, Ed.
Internet-Draft                                                   Semtech
Intended status: Informational                            I. Petrov, Ed.
Expires: March 15, 2021                                           Acklio
                                                      September 11, 2020


         Static Context Header Compression (SCHC) over LoRaWAN
               draft-ietf-lpwan-schc-over-lorawan-09

Abstract

   The Static Context Header Compression (SCHC) specification describes
   generic header compression and fragmentation techniques for Low Power
   Wide Area Networks (LPWAN) technologies.  SCHC is a generic mechanism
   designed for great flexibility so that it can be adapted for any of
   the LPWAN technologies.

   This document provides the adaptation of SCHC for use in LoRaWAN
   networks, and provides elements such as efficient parameterization
   and modes of operation.  This is called a profile.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 15, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Static Context Header Compression Overview  . . . . . . . . .   4
   4.  LoRaWAN Architecture  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Device classes (A, B, C) and interactions . . . . . . . .   6
     4.2.  Device addressing . . . . . . . . . . . . . . . . . . . .   7
     4.3.  General Frame Types . . . . . . . . . . . . . . . . . . .   8
     4.4.  LoRaWAN MAC Frames  . . . . . . . . . . . . . . . . . . .   8
     4.5.  LoRaWAN FPort . . . . . . . . . . . . . . . . . . . . . .   8
     4.6.  LoRaWAN empty frame . . . . . . . . . . . . . . . . . . .   9
     4.7.  Unicast and multicast technology  . . . . . . . . . . . .   9
   5.  SCHC-over-LoRaWAN . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  LoRaWAN FPort and RuleID  . . . . . . . . . . . . . . . .   9
     5.2.  Rule ID management  . . . . . . . . . . . . . . . . . . .  10
     5.3.  Interface IDentifier (IID) computation  . . . . . . . . .  11
     5.4.  Padding . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.5.  Decompression . . . . . . . . . . . . . . . . . . . . . .  12
     5.6.  Fragmentation . . . . . . . . . . . . . . . . . . . . . .  12
       5.6.1.  DTag  . . . . . . . . . . . . . . . . . . . . . . . .  12
       5.6.2.  Uplink fragmentation: From device to SCHC gateway . .  12
       5.6.3.  Downlink fragmentation: From SCHC gateway to device .  15
     5.7.  SCHC Fragment Format  . . . . . . . . . . . . . . . . . .  19
       5.7.1.  All-0 SCHC fragment . . . . . . . . . . . . . . . . .  19
       5.7.2.  All-1 SCHC fragment . . . . . . . . . . . . . . . . .  19
       5.7.3.  Delay after each message to respect local regulation   19
   6.  Security considerations . . . . . . . . . . . . . . . . . . .  19
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     10.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  21
     A.1.  Uplink - Compression example - No fragmentation . . . . .  21
     A.2.  Uplink - Compression and fragmentation example  . . . . .  22
     A.3.  Downlink  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26






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1.  Introduction

   SCHC specification [RFC8724] describes generic header compression and
   fragmentation techniques that can be used on all LPWAN technologies
   defined in [RFC8376].  Even though those technologies share a great
   number of common features like star-oriented topologies, network
   architecture, devices with mostly quite predictable communications,
   etc; they do have some slight differences in respect to payload
   sizes, reactiveness, etc.

   SCHC provides a generic framework that enables those devices to
   communicate with other Internet networks.  However, for efficient
   performance, some parameters and modes of operation need to be set
   appropriately for each of the LPWAN technologies.

   This document describes the efficient parameters and modes of
   operation when SCHC is used over LoRaWAN networks.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This section defines the terminology and acronyms used in this
   document.  For all other definitions, please look up the SCHC
   specification [RFC8724].

   o  DevEUI: an IEEE EUI-64 identifier used to identify the device
      during the procedure while joining the network (Join Procedure).
      It is assigned by the manufacturer or the device owner and
      provisioned on the Network Gateway.

   o  DevAddr: a 32-bit non-unique identifier assigned to a device
      either:

      *  Statically: by the device manufacturer in _Activation by
         Personalization_ mode.

      *  Dynamically: after a Join Procedure by the Network Gateway in
         _Over The Air Activation_ mode.

   o  Downlink: LoRaWAN term for a message transmitted by the network
      and received by the device.





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   o  OUI: Organisation Unique Identifier.  IEEE assigned prefix for
      EUI.

   o  RCS: Reassembly Check Sequence.  Used to verify the integrity of
      the fragmentation-reassembly process.

   o  SCHC gateway: It corresponds to the LoRaWAN Application Server.
      It manages translation between IPv6 network and the Network
      Gateway (LoRaWAN Network Server).

   o  Uplink: LoRaWAN term for a message transmitted by the device and
      received by the network.

3.  Static Context Header Compression Overview

   This section contains a short overview of SCHC.  For a detailed
   description, refer to the full specification [RFC8724].

   It defines:

   1.  Compression mechanisms to avoid transporting information known by
       both sender and receiver over the air.  Known information are
       part of the "context".  This component is called SCHC Compressor/
       Decompressor (SCHC C/D)

   2.  Fragmentation mechanisms to allow SCHC Packet transportation on
       small, and potentially variable, MTU.  This component called SCHC
       Fragmentation/Reassembly (SCHC F/R)

   Context exchange or pre-provisioning is out of scope of this
   document.

      Device                                                App
  +----------------+                                +----+ +----+ +----+
  | App1 App2 App3 |                                |App1| |App2| |App3|
  |                |                                |    | |    | |    |
  |       UDP      |                                |UDP | |UDP | |UDP |
  |      IPv6      |                                |IPv6| |IPv6| |IPv6|
  |                |                                |    | |    | |    |
  |SCHC C/D and F/R|                                |    | |    | |    |
  +--------+-------+                                +----+ +----+ +----+
           |  +---+     +----+    +----+    +----+     .      .      .
           +~ |RGW| === |NGW | == |SCHC| == |SCHC|...... Internet ....
              +---+     +----+    |F/R |    |C/D |
                                  +----+    +----+

                          Figure 1: Architecture




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   Figure 1 represents the architecture for compression/decompression,
   it is based on [RFC8376] terminology.  The device is sending
   applications flows using IPv6 or IPv6/UDP protocols.  These flows
   might be compressed by a Static Context Header Compression
   Compressor/Decompressor (SCHC C/D) to reduce headers size and
   fragmented by the SCHC Fragmentation/Reassembly (SCHC F/R).  The
   resulting information is sent on a layer two (L2) frame to an LPWAN
   Radio Gateway (RGW) that forwards the frame to a Network Gateway
   (NGW).  The NGW sends the data to a SCHC F/R for reassembly, if
   required, then to SCHC C/D for decompression.  The SCHC C/D shares
   the same rules with the device.  The SCHC C/D and F/R can be located
   on the Network Gateway (NGW) or in another place as long as a
   communication is established between the NGW and the SCHC F/R, then
   SCHC F/R and C/D.  The SCHC C/D and F/R in the device and the SCHC
   gateway MUST share the same set of rules.  After decompression, the
   packet can be sent on the Internet to one or several LPWAN
   Application Servers (App).

   The SCHC C/D and F/R process is bidirectional, so the same principles
   can be applied in the other direction.

   In a LoRaWAN network, the RG is called a Gateway, the NGW is Network
   Server, and the SCHC C/D and F/R are an Application Server.  It can
   be provided by the Network Gateway or any third party software.
   Figure 1 can be mapped in LoRaWAN terminology to:

   End Device                                               App
+--------------+                                    +----+ +----+ +----+
|App1 App2 App3|                                    |App1| |App2| |App3|
|              |                                    |    | |    | |    |
|      UDP     |                                    |UDP | |UDP | |UDP |
|     IPv6     |                                    |IPv6| |IPv6| |IPv6|
|              |                                    |    | |    | |    |
|SCHC C/D & F/R|                                    |    | |    | |    |
+-------+------+                                    +----+ +----+ +----+
        |  +-------+     +-------+    +-----------+    .      .      .
        +~ |Gateway| === |Network| == |Application|..... Internet ....
           +-------+     |server |    |server     |
                         +-------+    | F/R - C/D |
                                      +-----------+

               Figure 2: SCHC Architecture mapped to LoRaWAN

4.  LoRaWAN Architecture

   An overview of LoRaWAN [lora-alliance-spec] protocol and architecture
   is described in [RFC8376].  The mapping between the LPWAN




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   architecture entities as described in [RFC8724] and the ones in
   [lora-alliance-spec] is as follows:

   o Devices are LoRaWAN End Devices (e.g. sensors, actuators, etc.).
   There can be a very high density of devices per radio gateway
   (LoRaWAN gateway).  This entity maps to the LoRaWAN end-device.

   o The Radio Gateway (RGW), which is the endpoint of the constrained
   link.  This entity maps to the LoRaWAN Gateway.

   o The Network Gateway (NGW) is the interconnection node between the
   Radio Gateway and the Internet.  This entity maps to the LoRaWAN
   Network Server.

   o SCHC C/D and F/R are LoRaWAN Application Server; ie the LoRaWAN
   application server will do the SCHC C/D and F/R.

       ()   ()   ()       |                      +------+
        ()  () () ()     / \       +---------+   | Join |
       () () () () ()   /   \======|    ^    |===|Server|  +-----------+
        () ()  ()      |           | <--|--> |   +------+  |Application|
       () ()  ()  ()  / \==========|    v    |=============|  Server   |
        ()  ()  ()   /   \         +---------+             +-----------+
       End Devices  Gateways     Network Server


                       Figure 3: LPWAN Architecture

   SCHC Compressor/Decompressor (SCHC C/D) and SCHC Fragmentation/
   Reassembly (SCHC F/R) are performed on the LoRaWAN end-device and the
   Application Server (called SCHC gateway).  While the point-to-point
   link between the device and the Application Server constitutes single
   IP hop, the ultimate end-point of the IP communication may be an
   Internet node beyond the Application Server.  In other words, the
   LoRaWAN Application Server (SCHC gateway) acts as the first hop IP
   router for the device.  The Application Server and Network Server may
   be co-located, which effectively turns the Network/Application Server
   into the first hop IP router.

4.1.  Device classes (A, B, C) and interactions

   The LoRaWAN MAC layer supports 3 classes of devices named A, B and C.
   All devices implement the Class A, some devices may implement Class B
   or Class C.  Class B and Class C are mutually exclusive.

   o  Class A: The Class A is the simplest class of devices.  The device
      is allowed to transmit at any time, randomly selecting a
      communication channel.  The Network Gateway may reply with a



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      downlink in one of the 2 receive windows immediately following the
      uplinks.  Therefore, the Network Gateway cannot initiate a
      downlink, it has to wait for the next uplink from the device to
      get a downlink opportunity.  The Class A is the lowest power
      consumption class.

   o  Class B: Class B devices implement all the functionalities of
      Class A devices, but also schedule periodic listen windows.
      Therefore, opposed to the Class A devices, Class B devices can
      receive downlinks that are initiated by the Network Gateway and
      not following an uplink.  There is a trade-off between the
      periodicity of those scheduled Class B listen windows and the
      power consumption of the device.  The lower the downlink latency,
      the higher the power consumption.

   o  Class C: Class C devices implement all the functionalities of
      Class A devices, but keep their receiver open whenever they are
      not transmitting.  Class C devices can receive downlinks at any
      time at the expense of a higher power consumption.  Battery-
      powered devices can only operate in Class C for a limited amount
      of time (for example for a firmware upgrade over-the-air).  Most
      of the Class C devices are grid powered (for example Smart Plugs).

4.2.  Device addressing

   LoRaWAN end-devices use a 32-bit network address (devAddr) to
   communicate with the Network Gateway over-the-air, this address might
   not be unique in a LoRaWAN network; devices using the same devAddr
   are distinguished by the Network Gateway based on the cryptographic
   signature appended to every LoRaWAN frame.

   To communicate with the SCHC gateway the Network Gateway MUST
   identify the devices by a unique 64-bit device identifier called the
   DevEUI.

   The DevEUI is assigned to the device during the manufacturing process
   by the device's manufacturer.  It is built like an Ethernet MAC
   address by concatenating the manufacturer's IEEE OUI field with a
   vendor unique number.  e.g.: 24-bit OUI is concatenated with a 40-bit
   serial number.  The Network Gateway translates the devAddr into a
   DevEUI in the uplink direction and reciprocally on the downlink
   direction.









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+--------+         +---------+        +---------+            +----------+
| Device | <=====> | Network | <====> | SCHC    | <========> | Internet |
|        | devAddr | Gateway | DevEUI | Gateway |  IPv6/UDP  |          |
+--------+         +---------+        +---------+            +----------+


                        Figure 4: LoRaWAN addresses

4.3.  General Frame Types

   LoRaWAN implements the possibility to send confirmed or unconfirmed
   messages:

   o  Confirmed message: The sender asks the receiver to acknowledge the
      message.

   o  Unconfirmed message: The sender does not ask the receiver to
      acknowledge the message.

   As SCHC defines its own acknowledgment mechanisms, SCHC does not
   require to use LoRaWAN Confirmed messages.

4.4.  LoRaWAN MAC Frames

   In addition to regular data frames LoRaWAN implements JoinRequest and
   JoinAccept frame types, used by a device to join a network:

   o  JoinRequest: This message is used by a device to join a network.
      It contains the device's unique identifier DevEUI and a random
      nonce that will be used for session key derivation.

   o  JoinAccept: To on-board a device, the Network Gateway responds to
      the JoinRequest issued by a device with a JoinAccept message.
      That message is encrypted with the device's AppKey and contains
      (amongst other fields) the major network's settings and a random
      nonce used to derive the session keys.

   o  Data: MAC and application data.  Application data are protected
      with AES-128 encryption, MAC related data are AES-128 encrypted
      with another key.

4.5.  LoRaWAN FPort

   The LoRaWAN MAC layer features a frame port field in all frames.
   This field (FPort) is 8 bits long and the values from 1 to 223 can be
   used.  It allows LoRaWAN networks and applications to identify data.





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4.6.  LoRaWAN empty frame

   A LoRaWAN empty frame is a LoRaWAN message without FPort (cf
   Section 5.1) and FRMPayload.

4.7.  Unicast and multicast technology

   LoRaWAN technology supports unicast downlinks, but also multicast: a
   packet send over LoRaWAN radio link can be received by several
   devices.  It is useful to address many devices with same content,
   either a large binary file (firmware upgrade), or same command (e.g:
   lighting control).  As IPv6 is also a multicast technology this
   feature can be used to address a group of devices.

   _Note 1_: IPv6 multicast addresses must be defined as per [RFC4291].
   LoRaWAN multicast group definition in a Network Gateway and the
   relation between those groups and IPv6 groupID are out of scope of
   this document.

   _Note 2_: LoRa Alliance defined [lora-alliance-remote-multicast-set]
   as RECOMMENDED way to setup multicast groups on devices and create a
   synchronized reception window.

5.  SCHC-over-LoRaWAN

5.1.  LoRaWAN FPort and RuleID

   The FPort field is part of the SCHC Message, as shown in Figure 5.
   The SCHC C/D and the SCHC F/R SHALL concatenate the FPort field with
   the LoRaWAN payload to retrieve their payload as it is used as a part
   of the RuleID field.

   | FPort | LoRaWAN payload  |
   + ------------------------ +
   |       SCHC packet        |


                     Figure 5: SCHC Message in LoRaWAN

   A fragmentation datagram with application payload transferred from
   device to Network Gateway, is called uplink fragmentation datagram.
   It uses an FPort for data uplink and its associated SCHC control
   downlinks, named FPortUp in this document.  The other way, a
   fragmentation datagram with application payload transferred from
   Network Gateway to device, is called downlink fragmentation datagram.
   It uses another FPort for data downlink and its associated SCHC
   control uplinks, named FPortDown in this document.




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   All RuleID can use arbitrary values inside the FPort range allowed by
   LoRaWAN specification and MUST be shared by the device and SCHC
   gateway prior to the communication with the selected rule.  The
   uplink and downlink fragmentation FPorts MUST be different.

5.2.  Rule ID management

   RuleID MUST be 8 bits, encoded in the LoRaWAN FPort as described in
   Section 5.1.  LoRaWAN supports up to 223 application FPorts in the
   range [1;223] as defined in section 4.3.2 of [lora-alliance-spec], it
   implies that RuleID MSB SHOULD be inside this range.  An application
   can send non SCHC traffic by using FPort values different from the
   ones used for SCHC.

   In order to improve interoperability RECOMMENDED fragmentation RuleID
   values are:

   o  RuleID = 20 (8-bit) for uplink fragmentation, named FPortUp.

   o  RuleID = 21 (8-bit) for downlink fragmentation, named FPortDown.

   o  RuleID = 22 (8-bit) for which SCHC compression was not possible
      (no matching rule was found).

   The remaining RuleIDs are available for compression.  RuleIDs are
   shared between uplink and downlink sessions.  A RuleID not in the
   set(s) of FPortUp or FPortDown means that the fragmentation is not
   used, thus, on reception, the SCHC Message MUST be sent to the SCHC
   C/D layer.

   The only uplink messages using the FPortDown port are the
   fragmentation SCHC control messages of a downlink fragmentation
   datagram (for example, SCHC ACKs).  Similarly, the only downlink
   messages using the FPortUp port are the fragmentation SCHC control
   messages of an uplink fragmentation datagram.

   An application can have multiple fragmentation datagrams between a
   device and one or several SCHC gateways.  A set of FPort values is
   REQUIRED for each SCHC gateway instance the device is required to
   communicate with.  The application can use additional uplinks or
   downlink fragmentation parameters but SHALL implement at least the
   parameters defined in this document.

   The mechanism for sharing those RuleID values is outside the scope of
   this document.






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5.3.  Interface IDentifier (IID) computation

   In order to mitigate risks described in [RFC8064] and [RFC8065] IID
   MUST be created regarding the following algorithm:

   1.  key = LoRaWAN AppSKey

   2.  cmac = aes128_cmac(key, DevEUI)

   3.  IID = cmac[0..7]

   aes128_cmac algorithm is described in [RFC4493].  It has been chosen
   as it is already used by devices for LoRaWAN protocol.

   As AppSKey is renewed each time a device joins or rejoins a LoRaWAN
   network, the IID will change over time; this mitigates privacy,
   location tracking and correlation over time risks.  Join periodicity
   is defined at the application level.

   Address scan risk is mitigated thanks to AES-128, which provides
   enough entropy bits of the IID.

   Using this algorithm will also ensure that there is no correlation
   between the hardware identifier (IEEE-64 DevEUI) and the IID, so an
   attacker cannot use manufacturer OUI to target devices.

   Example with:

   o  DevEUI: 0x1122334455667788

   o  appSKey: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB

   1. key: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB
   2. cmac: 0xBA59F4B196C6C3432D9383C145AD412A
   3. IID: 0xBA59F4B196C6C343

                   Figure 6: Example of IID computation.

   There is a small probability of IID collision in a LoRaWAN network,
   if such event occurs the IID can be changed by rekeying the device on
   L2 level (ie: trigger a LoRaWAN join).  The way the device is rekeyed
   is out of scope of this document and left to the implementation.

5.4.  Padding

   All padding bits MUST be 0.





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5.5.  Decompression

   SCHC C/D MUST concatenate FPort and LoRaWAN payload to retrieve the
   SCHC Packet as per Section 5.1.

   RuleIDs matching FPortUp and FPortDown are reserved for SCHC
   Fragmentation.

5.6.  Fragmentation

   The L2 Word Size used by LoRaWAN is 1 byte (8 bits).  The SCHC
   fragmentation over LoRaWAN uses the ACK-on-Error mode for uplink
   fragmentation and Ack-Always mode for downlink fragmentation.  A
   LoRaWAN device cannot support simultaneous interleaved fragmentation
   datagrams in the same direction (uplink or downlink).

   The fragmentation parameters are different for uplink and downlink
   fragmentation datagrams and are successively described in the next
   sections.

5.6.1.  DTag

   A Device cannot interleave several fragmented SCHC datagrams on the
   same FPort.  This field is not used and its size is 0.

   Note: The device can still have several parallel fragmentation
   datagrams with one or more SCHC gateway(s) thanks to distinct sets of
   FPorts, cf Section 5.2

5.6.2.  Uplink fragmentation: From device to SCHC gateway

   In that case the device is the fragmentation transmitter, and the
   SCHC gateway the fragmentation receiver.  A single fragmentation rule
   is defined.  SCHC F/R MUST concatenate FPort and LoRaWAN payload to
   retrieve the SCHC Packet, as per Section 5.1.

   o  SCHC header size is two bytes (the FPort byte + 1 additional
      byte).

   o  RuleID: 8 bits stored in LoRaWAN FPort.

   o  SCHC fragmentation reliability mode: "ACK-on-Error".

   o  DTag: Size is 0 bit, not used.

   o  FCN: The FCN field is encoded on N = 6 bits, so WINDOW_SIZE = 63
      tiles are allowed in a window.




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   o  Window index: encoded on W = 2 bits.  So 4 windows are available.

   o  RCS: Use recommended calculation algorithm in [RFC8724].

   o  MAX_ACK_REQUESTS: 8.

   o  Tile: size is 10 bytes.

   o  Retransmission timer: Set by the implementation depending on the
      application requirements.

   o  Inactivity timer: The SCHC gateway implements an "inactivity
      timer".  The default RECOMMENDED duration of this timer is 12
      hours; this value is mainly driven by application requirements and
      MAY be changed by the application.

   o  Penultimate tile MUST be equal to the regular size.

   o  Last tile: it can be carried in a Regular SCHC Fragment, alone in
      an All-1 SCHC Fragment or with any of these two methods.
      Implementation must ensure that:

      *  The sender MUST ascertain that the receiver will not receive
         the last tile through both a Regular SCHC Fragment and an All-1
         SCHC Fragment.

      *  If last tile is in All-1 message: current L2 MTU MUST be big
         enough to fit the All-1 and the last tile.

   With this set of parameters, the SCHC fragment header is 16 bits,
   including FPort; payload overhead will be 8 bits as FPort is already
   a part of LoRaWAN payload.  MTU is: _4 windows * 63 tiles * 10 bytes
   per tile = 2520 bytes_

   For battery powered devices, it is RECOMMENDED to use the ACK
   mechanism at the end of each window instead of waiting until the end
   of all windows:

   o  SCHC receiver SHOULD send a SCHC ACK after every window even if
      there is no missing tile.

   o  SCHC sender SHOULD wait for the SCHC ACK from the SCHC receiver
      before sending tiles from the next window.  If the SCHC ACK is not
      received, it SHOULD send an SCHC ACK REQ up to MAX_ACK_REQUESTS,
      time as described previously.

   For non-battery powered devices, SCHC receiver MAY also choose to
   send a SCHC ACK only at the end of all windows.  It will reduce



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   downlink load on the LoRaWAN network, by reducing the number of
   downlinks.

   SCHC implementations MUST be compatible with both behavior, and
   selection is a part of the rule context.

5.6.2.1.  Regular fragments

   | FPort  |  LoRaWAN payload          |
   + ------ + ------------------------- +
   | RuleID |   W    | FCN    | Payload |
   + ------ + ------ + ------ + ------- +
   | 8 bits | 2 bits | 6 bits |         |


   Figure 7: All fragments except the last one.  SCHC header size is 16
                      bits, including LoRaWAN FPort.

5.6.2.2.  Last fragment (All-1)

   | FPort  | LoRaWAN payload              |
   + ------ + ---------------------------- +
   | RuleID |   W    | FCN=All-1 |  RCS    |
   + ------ + ------ + --------- + ------- +
   | 8 bits | 2 bits | 6 bits    | 32 bits |


    Figure 8: All-1 SCHC Message: the last fragment without last tile.

   | FPort  | LoRaWAN payload                             |
   + ------ + ------------------------------------------- +
   | RuleID |   W    | FCN=All-1 |  RCS    |  Last tile   |
   + ------ + ------ + --------- + ------- + ------------ +
   | 8 bits | 2 bits | 6 bits    | 32 bits | 1 to 80 bits |


      Figure 9: All-1 SCHC Message: the last fragment with last tile.

5.6.2.3.  SCHC ACK

| FPort  | LoRaWAN payload                                                      |
+ ------ + -------------------------------------------------------------------- +
| RuleID |   W   |   C   | Compressed bitmap(C = 0) | Optional padding(b'0...0) |
+ ------ + ----- + ----- + ------------------------ + ------------------------- +
| 8 bits | 2 bit | 1 bit |       5 to 63 bits       |      0, 6 or 7 bits       |


               Figure 10: SCHC ACK format, failed RCS check.



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   Note: Because of the bitmap compression mechanism and L2 byte
   alignment only few discrete values are possible: 5, 13, 21, 29, 37,
   45, 53, 61, 62, 63.  Bitmaps of 63 bits will require 6 bits of
   padding.

5.6.2.4.  Receiver-Abort

   | FPort  | LoRaWAN payload                              |
   + ------ + -------------------------------------------- +
   | RuleID | W = b'11 | C = 1 | b'11111 | 0xFF (all 1's)  |
   + ------ + -------- + ------+-------- + ----------------+
   | 8 bits |  2 bits  | 1 bit | 5 bits  | 8 bits          |
                 next L2 Word boundary ->| <-- L2 Word --> |


                     Figure 11: Receiver-Abort format.

5.6.2.5.  SCHC acknowledge request

   | FPort  | LoRaWAN payload          |
   +------- +------------------------- +
   | RuleID | W      | FCN = b'000000  |
   + ------ + ------ + --------------- +
   | 8 bits | 2 bits | 6 bits          |



                      Figure 12: SCHC ACK REQ format.

5.6.3.  Downlink fragmentation: From SCHC gateway to device

   In that case the device is the fragmentation receiver, and the SCHC
   gateway the fragmentation transmitter.  The following fields are
   common to all devices.  SCHC F/R MUST concatenate FPort and LoRaWAN
   payload to retrieve the SCHC Packet as described in Section 5.1.

   o  SCHC fragmentation reliability mode:

      *  Unicast downlinks: ACK-Always.

      *  Multicast downlinks: No-ACK, reliability has to be ensured by
         the upper layer.  This feature is OPTIONAL and may not be
         implemented by SCHC gateway.

   o  RuleID: 8 bits stored in LoRaWAN FPort.

   o  Window index (unicast only): encoded on W=1 bit, as per [RFC8724].




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   o  DTag: Size is 0 bit, not used.

   o  FCN: The FCN field is encoded on N=1 bit, so WINDOW_SIZE = 1 tile.

   o  RCS: Use recommended calculation algorithm in [RFC8724].

   o  MAX_ACK_REQUESTS: 8.

   o  Retransmission timer: See Section 5.6.3.5.

   o  Inactivity timer: The default RECOMMENDED duration of this timer
      is 12 hours; this value is mainly driven by application
      requirements and MAY be changed by the application.

   As only 1 tile is used, its size can change for each downlink, and
   will be maximum available MTU.

   Class A devices can only receive during an RX slot, following the
   transmission of an uplink.  Therefore the SCHC gateway cannot
   initiate communication (ex: new SCHC session); in order to create a
   downlink opportunity it is RECOMMENDED for Class A devices to send an
   uplink every 24 hours when no SCHC session is started, this is
   application specific and can be disabled.  RECOMMENDED uplink is a
   LoRaWAN empty frame as defined Section 4.6.  As this uplink is to
   open an RX window any applicative uplink MAY reset this counter.

   _Note_: The Fpending bit included in LoRaWAN protocol SHOULD NOT be
   used for SCHC-over-LoRaWAN protocol.  It might be set by the Network
   Gateway for other purposes but not SCHC needs.

5.6.3.1.  Regular fragments

   | FPort  | LoRaWAN payload                      |
   + ------ + ------------------------------------ +
   | RuleID | W     | FCN = b'0 | Payload          |
   + ------ + ----- + --------- + ---------------- +
   | 8 bits | 1 bit | 1 bit     | X bytes + 6 bits |


     Figure 13: All fragments but the last one.  Header size 10 bits,
                         including LoraWAN FPort.

5.6.3.2.  Last fragment (All-1)








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   | FPort  | LoRaWAN payload                                 |
   + ------ + --------------------------- + ----------------- +
   | RuleID | W     | FCN = b'1 | RCS     |      Payload      |
   + ------ + ----- + --------- + ------- + ----------------- +
   | 8 bits | 1 bit | 1 bit     | 32 bits | 6 bits to X bytes |


             Figure 14: All-1 SCHC Message: the last fragment.

5.6.3.3.  SCHC ACK

   | FPort  | LoRaWAN payload                    |
   + ------ + ---------------------------------- +
   | RuleID | W     | C = b'1 | Padding b'000000 |
   + ------ + ----- + ------- + ---------------- +
   | 8 bits | 1 bit | 1 bit   | 6 bits           |


                Figure 15: SCHC ACK format, RCS is correct.

5.6.3.4.  Receiver-Abort

   | FPort  | LoRaWAN payload                                |
   + ------ + ---------------------------------------------- +
   | RuleID | W = b'1 | C = b'1 | b'111111 | 0xFF (all 1's)  |
   + ------ + ------- + ------- + -------- + --------------- +
   | 8 bits | 1 bit   | 1 bits  | 6 bits   | 8 bits          |
                   next L2 Word boundary ->| <-- L2 Word --> |



    Figure 16: Receiver-Abort packet (following an All-1 SCHC Fragment
                           with incorrect RCS).

5.6.3.5.  Downlink retransmission timer

   Class A and Class B or Class C devices do not manage retransmissions
   and timers in the same way.

5.6.3.5.1.  Class A devices

   Class A devices can only receive in an RX slot following the
   transmission of an uplink.

   The SCHC gateway implements an inactivity timer with a RECOMMENDED
   duration of 36 hours.  For devices with very low transmission rates
   (example 1 packet a day in normal operation), that duration may be
   extended: it is application specific.



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   RETRANSMISSION_TIMER is application specific and its RECOMMENDED
   value is INACTIVITY_TIMER/(MAX_ACK_REQUESTS + 1).

   *SCHC All-0 (FCN=0)* All fragments but the last have an FCN=0
   (because window size is 1).  Following it the device MUST transmit
   the SCHC ACK message.  It MUST transmit up to MAX_ACK_REQUESTS SCHC
   ACK messages before aborting.  In order to progress the fragmentation
   datagram, the SCHC layer should immediately queue for transmission
   those SCHC ACK if no SCHC downlink have been received during RX1 and
   RX2 window.  LoRaWAN layer will respect the regulation if required.

   _Note_: The ACK bitmap is 1 bit long and is always 1.

   *SCHC All-1 (FCN=1)* SCHC All-1 is the last fragment of a datagram,
   the corresponding SCHC ACK message might be lost; therefore the SCHC
   gateway MUST request a retransmission of this ACK when the
   retransmission timer expires.  To open a downlink opportunity the
   device MUST transmit an uplink every
   RETRANSMISSION_TIMER/(MAX_ACK_REQUESTS *
   SCHC_ACK_REQ_DN_OPPORTUNITY).  The format of this uplink is
   application specific.  It is RECOMMENDED for a device to send an
   empty frame (see Section 4.6) but it is application specific and will
   be used by the NGW to transmit a potential SCHC ACK REQ.
   SCHC_ACK_REQ_DN_OPPORTUNITY is application specific and its
   recommended value is 2, it MUST be greater than 1.  This allows to
   open downlink opportunity to other eventual downlink with higher
   priority than SCHC ACK REQ message.

   _Note_: The device MUST keep this SCHC ACK message in memory until it
   receives a downlink, on SCHC FPortDown different from an SCHC ACK
   REQ: it indicates that the SCHC gateway has received the ACK message.

5.6.3.6.  Class B or Class C devices

   Class B devices can receive in scheduled RX slots or in RX slots
   following the transmission of an uplink.  Class C devices are almost
   in constant reception.

   RECOMMENDED retransmission timer value:

   o  Class B: 3 times the ping slot periodicity.

   o  Class C: 30 seconds.

   The RECOMMENDED inactivity timer value is 12 hours for both Class B
   and Class C devices.





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5.7.  SCHC Fragment Format

5.7.1.  All-0 SCHC fragment

   *Uplink fragmentation (Ack-On-Error)*:

   All-0 is distinguishable from a SCHC ACK REQ as [RFC8724] states
   _This condition is also met if the SCHC Fragment Header is a multiple
   of L2 Words_; this condition met: SCHC header is 2 bytes.

   *Downlink fragmentation (Ack-always)*:

   As per [RFC8724] the SCHC All-1 MUST contain the last tile,
   implementation must ensure that All-0 message Payload will be at
   least the size of an L2 Word.

5.7.2.  All-1 SCHC fragment

   All-1 is distinguishable from a SCHC Sender-Abort as [RFC8724] states
   _This condition is met if the RCS is present and is at least the size
   of an L2 Word_; this condition met: RCS is 4 bytes.

5.7.3.  Delay after each message to respect local regulation

   This profile does not define a delay to be added after each SCHC
   message, local regulation compliance is expected to be enforced by
   LoRaWAN stack.

6.  Security considerations

   This document is only providing parameters that are expected to be
   best suited for LoRaWAN networks for [RFC8724].  IID security is
   discussed in Section 5.3.  As such, this document does not contribute
   to any new security issues in addition to those identified in
   [RFC8724].  Moreover, SCHC data (LoRaWAN payload) are protected on
   LoRaWAN level by an AES-128 encryption with key shared by device and
   SCHC gateway.  Those keys are renewed at each LoRaWAN session (ie:
   each join or rejoin to the LoRaWAN network)

7.  IANA Considerations

   This document has no IANA actions.

Acknowledgements

   Thanks to all those listed in the Contributors section for the
   excellent text, insightful discussions, reviews and suggestions, and
   also to (in alphabetical order) Dominique Barthel, Arunprabhu



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   Kandasamy, Rodrigo Munoz, Alexander Pelov, Pascal Thubert, Laurent
   Toutain for useful design considerations, reviews and comments.

Contributors

   Contributors ordered by family name.

   Vincent Audebert
   EDF R&D
   Email: vincent.audebert@edf.fr

   Julien Catalano
   Kerlink
   Email: j.catalano@kerlink.fr

   Michael Coracin
   Semtech
   Email: mcoracin@semtech.com

   Marc Le Gourrierec
   Sagemcom
   Email: marc.legourrierec@sagemcom.com

   Nicolas Sornin
   Semtech
   Email: nsornin@semtech.com

   Alper Yegin
   Actility
   Email: alper.yegin@actility.com

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4493]  Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
              AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
              2006, <https://www.rfc-editor.org/info/rfc4493>.




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   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,
              <https://www.rfc-editor.org/info/rfc8064>.

   [RFC8065]  Thaler, D., "Privacy Considerations for IPv6 Adaptation-
              Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
              February 2017, <https://www.rfc-editor.org/info/rfc8065>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
              <https://www.rfc-editor.org/info/rfc8376>.

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,
              <https://www.rfc-editor.org/info/rfc8724>.

10.2.  Informative References

   [lora-alliance-remote-multicast-set]
              Alliance, L., "LoRaWAN Remote Multicast Setup
              Specification Version 1.0.0", <https://lora-
              alliance.org/sites/default/files/2018-09/
              remote_multicast_setup_v1.0.0.pdf>.

   [lora-alliance-spec]
              Alliance, L., "LoRaWAN Specification Version V1.0.3",
              <https://lora-alliance.org/sites/default/files/2018-07/
              lorawan1.0.3.pdf>.

Appendix A.  Examples

A.1.  Uplink - Compression example - No fragmentation

   This example represents an applicative payload going through SCHC
   over LoRaWAN, no fragmentation required

   An applicative payload of 78 bytes is passed to SCHC compression
   layer.  Rule 1 is used by SCHC C/D layer, allowing to compress it to
   40 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 37 bytes
   payload.




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   | RuleID | Compression residue |  Payload  | Padding=b'000 |
   + ------ + ------------------- + --------- + ------------- +
   |   1    |       21 bits       |  37 bytes |    3 bits     |

                  Figure 17: Uplink example: SCHC Message

   The current LoRaWAN MTU is 51 bytes, although 2 bytes FOpts are used
   by LoRaWAN protocol: 49 bytes are available for SCHC payload; no need
   for fragmentation.  The payload will be transmitted through FPort =
   1.

 | LoRaWAN Header            | LoRaWAN payload (40 bytes)              |
 + ------------------------- + --------------------------------------- +
 |      |  FOpts  | RuleID=1 | Compression | Payload   | Padding=b'000 |
 |      |         |          | residue     |           |               |
 + ---- + ------- + -------- + ----------- + --------- + ------------- +
 | XXXX | 2 bytes | 1 byte   | 21 bits     |  37 bytes |    3 bits     |

                 Figure 18: Uplink example: LoRaWAN packet

A.2.  Uplink - Compression and fragmentation example

   This example represents an applicative payload going through SCHC,
   with fragmentation.

   An applicative payload of 478 bytes is passed to SCHC compression
   layer.  Rule 1 is used by SCHC C/D layer, allowing to compress it to
   282 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 279 bytes
   payload.

   | RuleID | Compression residue |  Payload  |
   + ------ + ------------------- + --------- +
   |   1    |       21 bits       | 279 bytes |

                  Figure 19: Uplink example: SCHC Message

   The current LoRaWAN MTU is 11 bytes, 0 bytes FOpts are used by
   LoRaWAN protocol: 11 bytes are available for SCHC payload + 1 byte
   FPort field.  SCHC header is 2 bytes (including FPort) so 1 tile is
   sent in first fragment.

   | LoRaWAN Header             | LoRaWAN payload (11 bytes) |
   + -------------------------- + -------------------------- +
   |                | RuleID=20 |   W   |  FCN   |  1 tile   |
   + -------------- + --------- + ----- + ------ + --------- +
   |       XXXX     | 1 byte    | 0   0 |   62   | 10 bytes  |

                Figure 20: Uplink example: LoRaWAN packet 1



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   Content of the tile is:
   | RuleID | Compression residue |  Payload          |
   + ------ + ------------------- + ----------------- +
   |   1    |       21 bits       |  6 bytes + 3 bits |

        Figure 21: Uplink example: LoRaWAN packet 1 - Tile content

   Next transmission MTU is 11 bytes, although 2 bytes FOpts are used by
   LoRaWAN protocol: 9 bytes are available for SCHC payload + 1 byte
   FPort field, a tile does not fit inside so LoRaWAN stack will send
   only FOpts.

   Next transmission MTU is 242 bytes, 4 bytes FOpts. 23 tiles are
   transmitted:

 | LoRaWAN Header                        | LoRaWAN payload (231 bytes) |
 + --------------------------------------+ --------------------------- +
 |                |  FOpts  | RuleID=20  |   W   |  FCN  |  23 tiles   |
 + -------------- + ------- + ---------- + ----- + ----- + ----------- +
 |       XXXX     | 4 bytes |  1 byte    | 0   0 |   61  | 230 bytes   |

                Figure 22: Uplink example: LoRaWAN packet 2

   Next transmission MTU is 242 bytes, no FOpts.  All 5 remaining tiles
   are transmitted, the last tile is only 2 bytes + 5 bits.  Padding is
   added for the remaining 3 bits.

| LoRaWAN Header    | LoRaWAN payload (44 bytes)                        |
+ ---- + -----------+ ------------------------------------------------- +
|      | RuleID=20  |   W   |  FCN  |     5 tiles       | Padding=b'000 |
+ ---- + ---------- + ----- + ----- + ----------------- + ------------- +
| XXXX | 1 byte     | 0   0 |   38  | 42 bytes + 5 bits |    3 bits     |

                Figure 23: Uplink example: LoRaWAN packet 3

   Then All-1 message can be transmitted:

   | LoRaWAN Header    | LoRaWAN payload (44 bytes) |
   + ---- + -----------+ -------------------------- +
   |      | RuleID=20  |   W   |  FCN  |     RCS    |
   + ---- + ---------- + ----- + ----- + ---------- +
   | XXXX | 1 byte     | 0   0 |   63  |  4 bytes   |

        Figure 24: Uplink example: LoRaWAN packet 4 - All-1 message

   All packets have been received by the SCHC gateway, computed RCS is
   correct so the following ACK is sent to the device by the SCHC
   receiver:



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   | LoRaWAN Header             | LoRaWAN payload     |
   + -------------- + --------- + ------------------- +
   |                | RuleID=20 |   W   | C | Padding |
   + -------------- + --------- + ----- + - + ------- +
   |       XXXX     | 1 byte    | 0   0 | 1 | 5 bits  |

          Figure 25: Uplink example: LoRaWAN packet 5 - SCHC ACK

A.3.  Downlink

   An applicative payload of 443 bytes is passed to SCHC compression
   layer.  Rule 1 is used by SCHC C/D layer, allowing to compress it to
   130 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 127 bytes
   payload.

   | RuleID | Compression residue |  Payload  |
   + ------ + ------------------- + --------- +
   |   1    |       21 bits       | 127 bytes |

                 Figure 26: Downlink example: SCHC Message

   The current LoRaWAN MTU is 51 bytes, no FOpts are used by LoRaWAN
   protocol: 51 bytes are available for SCHC payload + FPort field => it
   has to be fragmented.

   | LoRaWAN Header    | LoRaWAN payload (51 bytes)             |
   + ---- + ---------- + -------------------------------------- +
   |      | RuleID=21  |  W = 0 | FCN = 0 |       1 tile        |
   + ---- + ---------- + ------ + ------- + ------------------- +
   | XXXX | 1 byte     |  1 bit |  1 bit  | 50 bytes and 6 bits |

      Figure 27: Downlink example: LoRaWAN packet 1 - SCHC Fragment 1

   Content of the tile is:

   | RuleID | Compression residue |        Payload     |
   + ------ + ------------------- + ------------------ +
   |   1    |       21 bits       | 48 bytes and 1 bit |

        Figure 28: Downlink example: LoRaWAN packet 1: Tile content

   The receiver answers with a SCHC ACK:









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   | LoRaWAN Header   | LoRaWAN payload                  |
   + ---- + --------- + -------------------------------- +
   |      | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 |
   + ---- + --------- + ----- + ----- + ---------------- +
   | XXXX |  1 byte   | 1 bit | 1 bit |     6 bits       |

         Figure 29: Downlink example: LoRaWAN packet 2 - SCHC ACK

   The second downlink is sent, two FOpts:

 | LoRaWAN Header              |  LoRaWAN payload (49 bytes)           |
 + --------------------------- + ------------------------------------- +
 |      |  FOpts  | RuleID=21  | W = 1 | FCN = 0 |        1 tile       |
 + ---- + ------- + ---------- + ----- + ------- + ------------------- +
 | XXXX | 2 bytes | 1 byte     | 1 bit |  1 bit  | 48 bytes and 6 bits |

      Figure 30: Downlink example: LoRaWAN packet 3 - SCHC Fragment 2

   The receiver answers with an SCHC ACK:

   | LoRaWAN Header   | LoRaWAN payload                  |
   + ---- + --------- + -------------------------------- +
   |      | RuleID=21 | W = 1 | C = 1 | Padding=b'000000 |
   + ---- + --------- + ----- + ----- + ---------------- +
   | XXXX |  1 byte   | 1 bit | 1 bit |     6 bits       |

         Figure 31: Downlink example: LoRaWAN packet 4 - SCHC ACK

   The last downlink is sent, no FOpts:

| LoRaWAN Header   | LoRaWAN payload (37 bytes)                                        |
+ ---- + --------- + ----------------------------------------------------------------- +
|      | RuleID=21 |  W = 0  | FCN = 1 |   RCS   |      1 tile       | Padding=b'00000 |
+ ---- + --------- + ------- + ------- + ------- + ----------------- + --------------- +
| XXXX | 1 byte    |  1 bit  |  1 bit  | 4 bytes | 31 bytes + 1 bits |     5 bits      |

       Figure 32: Downlink example: LoRaWAN packet 5 - All-1 message

   The receiver answers to the sender with an SCHC ACK:

   | LoRaWAN Header   | LoRaWAN payload                  |
   + ---- + --------- + -------------------------------- +
   |      | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 |
   + ---- + --------- + ----- + ----- + ---------------- +
   | XXXX |  1 byte   | 1 bit | 1 bit |     6 bits       |

         Figure 33: Downlink example: LoRaWAN packet 6 - SCHC ACK




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Authors' Addresses

   Olivier Gimenez (editor)
   Semtech
   14 Chemin des Clos
   Meylan
   France

   Email: ogimenez@semtech.com


   Ivaylo Petrov (editor)
   Acklio
   1137A Avenue des Champs Blancs
   35510 Cesson-Sevigne Cedex
   France

   Email: ivaylo@ackl.io

































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