Static Context Header Compression and Fragmentation (SCHC) over LoRaWAN
RFC 9011

Document Type RFC - Proposed Standard (April 2021; No errata)
Authors Olivier Gimenez  , Ivaylo Petrov 
Last updated 2021-04-22
Replaces draft-petrov-lpwan-ipv6-schc-over-lorawan
Stream Internet Engineering Task Force (IETF)
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Internet Engineering Task Force (IETF)                   O. Gimenez, Ed.
Request for Comments: 9011                                       Semtech
Category: Standards Track                                 I. Petrov, Ed.
ISSN: 2070-1721                                                   Acklio
                                                              April 2021

Static Context Header Compression and Fragmentation (SCHC) over LoRaWAN

Abstract

   The Static Context Header Compression and fragmentation (SCHC)
   specification (RFC 8724) describes generic header compression and
   fragmentation techniques for Low-Power Wide Area Network (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 defines a profile of SCHC (RFC 8724) for use in LoRaWAN
   networks and provides elements such as efficient parameterization and
   modes of operation.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9011.

Copyright Notice

   Copyright (c) 2021 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
   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
   2.  Terminology
   3.  SCHC Overview
   4.  LoRaWAN Architecture
     4.1.  Device Classes (A, B, C) and Interactions
     4.2.  Device Addressing
     4.3.  General Frame Types
     4.4.  LoRaWAN MAC Frames
     4.5.  LoRaWAN FPort
     4.6.  LoRaWAN Empty Frame
     4.7.  Unicast and Multicast Technology
   5.  SCHC over LoRaWAN
     5.1.  LoRaWAN FPort and RuleID
     5.2.  RuleID Management
     5.3.  Interface IDentifier (IID) Computation
     5.4.  Padding
     5.5.  Decompression
     5.6.  Fragmentation
       5.6.1.  DTag
       5.6.2.  Uplink Fragmentation: From Device to SCHC Gateway
       5.6.3.  Downlink Fragmentation: From SCHC Gateway to Device
     5.7.  SCHC Fragment Format
       5.7.1.  All-0 SCHC Fragment
       5.7.2.  All-1 SCHC Fragment
       5.7.3.  Delay after Each LoRaWAN Frame to Respect Local
               Regulation
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Examples
     A.1.  Uplink - Compression Example - No Fragmentation
     A.2.  Uplink - Compression and Fragmentation Example
     A.3.  Downlink
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

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

   SCHC provides a generic framework that enables those devices to
   communicate on IP 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 parameters and modes of operation when
   SCHC is used over LoRaWAN networks.  The LoRaWAN protocol is
   specified by the LoRa Alliance in [LORAWAN-SPEC].

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 abbreviations used in this
   document.  For all other definitions, please look up the SCHC
   specification [RFC8724].

      |  Note: The SCHC acronym is pronounced like "sheek" in English
      |  (or "chic" in French).  Therefore, this document writes "a SCHC
      |  Packet" instead of "an SCHC Packet".

   AppKey:  Application Key. An AES-128 root key specific to each
      device.

   AppSKey:  Application Session Key. An AES-128 key derived from the
      AppKey for each new session.  It is used to encrypt the payload
      field of a LoRaWAN applicative frame.

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

      Statically:  by the device manufacturer in "Activation-by-
         Personalization" mode, or

      Dynamically:  after a LoRaWAN "Join Procedure" by the Network
         Gateway in "Over-the-Air-Activation" mode.

   DevEUI:  Device Extended Unique Identifier, 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.

   Downlink:  A LoRaWAN term for a frame transmitted by the network and
      received by the device.

   EUI:  Extended Unique Identifier

   FRMPayload:  Application data in a LoRaWAN frame

   IID:  Interface Identifier

   LoRaWAN:  LoRaWAN is a wireless technology based on Industrial,
      Scientific, and Medical (ISM) radio bands that is used for long-
      range, low-power, low-data-rate applications developed by the LoRa
      Alliance, a membership consortium: <https://www.lora-
      alliance.org>.

   MSB:  Most Significant Byte

   NGW:  Network Gateway

   OUI:  Organizationally Unique Identifier.  IEEE-assigned prefix for
      EUI.

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

   RGW:  Radio Gateway

   RX:  A device's reception window.

   RX1/RX2:  LoRaWAN class A devices open two RX windows following an
      uplink, called "RX1" and "RX2".

   SCHC C/D:  SCHC Compression/Decompression

   SCHC F/R:  SCHC Fragmentation/Reassembly

   SCHC gateway:  The LoRaWAN Application Server that manages
      translation between an IPv6 network and the Network Gateway
      (LoRaWAN Network Server).

   Tile:  A piece of a fragmented packet as described in Section 8.2.2.1
      of [RFC8724].

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

3.  SCHC 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 is part
       of the "context".  This component is called the "SCHC
       Compression/Decompression" (SCHC C/D).

   2.  Fragmentation mechanisms to allow SCHC Packet transportation on a
       small, and potentially variable, MTU.  This component is called
       the "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 |
                                  +----+    +----+
  |<- - - - LoRaWAN - - ->|

                          Figure 1: Architecture

   Figure 1 represents the architecture for compression/decompression;
   it is based on the terminology from [RFC8376].  The device is sending
   application flows using IPv6 or IPv6/UDP protocols.  These flows
   might be compressed by a SCHC C/D to reduce header size, and
   fragmented by the SCHC F/R.  The resulting information is sent on a
   Layer 2 (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 a SCHC C/D for
   decompression.  The SCHC C/D shares the same rules with the device.
   The SCHC C/D and SCHC F/R can be located on the 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 SCHC C/D.  The SCHC C/D and SCHC 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 SCHC F/R process is bidirectional, so the same
   principles can be applied to the other direction.

   In a LoRaWAN network, the RGW is called a "Gateway", the NGW is a
   "Network Server", and the SCHC C/D and SCHC F/R are one or more
   "Application Servers".  Application servers can be provided by the
   NGW 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 |
                                      +-----------+
 |<- - - - - LoRaWAN - - - ->|

             Figure 2: SCHC Architecture Mapped to LoRaWAN

4.  LoRaWAN Architecture

   An overview of the LoRaWAN protocol and architecture [LORAWAN-SPEC]
   is described in [RFC8376].  The mapping between the LPWAN
   architecture entities as described in [RFC8724] and the ones in
   [LORAWAN-SPEC] is as follows:

   *  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.

   *  The RGW is the endpoint of the constrained link.  This entity maps
      to the LoRaWAN Gateway.

   *  The NGW is the interconnection node between the Radio Gateway and
      the SCHC gateway (LoRaWAN Application Server).  This entity maps
      to the LoRaWAN Network Server.

   *  The SCHC C/D and SCHC F/R are handled by the LoRaWAN Application
      Server.

   *  The LPWAN-AAA Server is the LoRaWAN Join Server.  Its role is to
      manage and deliver security keys in a secure way so that the
      devices root key is never exposed.

                                         (LPWAN-AAA Server)
    ()   ()   ()       |                      +------+
     ()  () () ()     / \       +---------+   | Join |
    () () () () ()   /   \======|    ^    |===|Server|  +-----------+
     () ()  ()      |           | <--|--> |   +------+  |Application|
    () ()  ()  ()  / \==========|    v    |=============|  Server   |
     ()  ()  ()   /   \         +---------+             +-----------+
    End devices  Gateways     Network Server          (SCHC C/D and F/R)
     (devices)    (RGW)            (NGW)

                     Figure 3: LPWAN Architecture

      |  Note: Figure 3 terms are from LoRaWAN, with [RFC8376]
      |  terminology in brackets.

   The SCHC C/D and SCHC F/R are performed on the LoRaWAN end device and
   the Application Server (called the SCHC gateway).  While the point-
   to-point link between the device and the Application Server
   constitutes a single IP hop, the ultimate endpoint 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 Medium Access Control (MAC) layer supports three classes
   of devices named A, B, and C.  All devices implement Class A, and
   some devices may implement Class B or Class C.  Class B and Class C
   are mutually exclusive.

   Class A:  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
      downlink in one of the two 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.  Class A is the lowest power
      consumption class.

   Class B:  Class B devices implement all the functionalities of Class
      A devices but also schedule periodic listen windows.  Therefore,
      as opposed to 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:

      High periodicity:  Downlinks from the NGW will be sent faster but
         the device wakes up more often and power consumption is
         increased.

      Low periodicity:  Downlinks from the NGW will have higher latency
         but lower power consumption.

   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.  For example, a 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.

 +--------+         +---------+        +---------+          +----------+
 | 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
   frames:

   Confirmed frame:  The sender asks the receiver to acknowledge the
      frame.

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

   As SCHC defines its own acknowledgment mechanisms, SCHC does not
   require the use of LoRaWAN Confirmed frames (FType = 0b100 as per
   [LORAWAN-SPEC]).

4.4.  LoRaWAN MAC Frames

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

   JoinRequest:  This frame 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.

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

   Data:  This refers to MAC and application data.  Application data is
      protected with AES-128 encryption.  MAC-related data is 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.

4.6.  LoRaWAN Empty Frame

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

4.7.  Unicast and Multicast Technology

   LoRaWAN technology supports unicast downlinks but also multicast; a
   multicast packet sent over a LoRaWAN radio link can be received by
   several devices.  It is useful to address many devices with the same
   content: either a large binary file (firmware upgrade) or the 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].  The 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: The LoRa Alliance defined
      |  [LORAWAN-REMOTE-MULTICAST-SET] as the RECOMMENDED way to set up
      |  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 recompose the SCHC Message.

   | FPort | LoRaWAN payload  |
   + ------------------------ +
   |       SCHC Message       |

                     Figure 5: SCHC Message in LoRaWAN

      |  Note: The SCHC Message is any datagram sent by the SCHC C/D or
      |  F/R layers.

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

   All RuleIDs can use arbitrary values inside the FPort range allowed
   by the LoRaWAN specification [LORAWAN-SPEC] 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.  RuleID Management

   The RuleID MUST be 8 bits and 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
   [LORAWAN-SPEC]; it implies that the 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:

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

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

   *  RuleID = 22 (8-bit) for which SCHC compression was not possible
      (i.e., no matching compression Rule was found), as described in
      Section 6 of [RFC8724].

   The FPortUp value MUST be different from the FPortDown value.  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 frames using the FPortDown port are the fragmentation
   SCHC control messages of a downlink-fragmented datagram (for example,
   SCHC ACKs).  Similarly, the only downlink frames using the FPortUp
   port are the fragmentation SCHC control messages of an uplink-
   fragmented datagram.

   An application can have multiple fragmented 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-fragmented parameters but SHALL implement at least the
   parameters defined in this document.

   The mechanism for context distribution across devices and gateways is
   outside the scope of this document.

5.3.  Interface IDentifier (IID) Computation

   In order to mitigate the risks described in [RFC8064] and [RFC8065],
   implementations MUST implement the following algorithm and SHOULD use
   it.

   1.  key = LoRaWAN AppSKey

   2.  cmac = aes128_cmac(key, DevEUI)

   3.  IID = cmac[0..7]

   The aes128_cmac algorithm is described in [RFC4493].  It has been
   chosen as it is already used by devices for the 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
   concerns, for example, location tracking or correlation over time.
   Join periodicity is defined at the application level.

   Address-scan risk is mitigated thanks to the entropy added to the IID
   by the inclusion of AppSKey.

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

   Example with:

   *  DevEUI: 0x1122334455667788

   *  AppSKey: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB

   1. key: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB
   2. cmac: 0x4E822D9775B2649928F82066AF804FEC
   3. IID: 0x4E822D9775B26499

                    Figure 6: Example of IID Computation

   There is a small probability of IID collision in a LoRaWAN network.
   If this occurs, the IID can be changed by rekeying the device at the
   L2 level (i.e., triggering a LoRaWAN join).  The way the device is
   rekeyed is out of scope of this document and left to the
   implementation.

      |  Note: Implementations also using another IID source MUST ensure
      |  that the same IID is shared between the device and the SCHC
      |  gateway in the compression and decompression of the IPv6
      |  address of the device.

5.4.  Padding

   All padding bits MUST be 0.

5.5.  Decompression

   The 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 fragmented
   datagrams in the same direction (uplink or downlink).

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

5.6.1.  DTag

   Section 8.2.4 of [RFC8724] describes the possibility to interleave
   several fragmented SCHC datagrams for the same RuleID.  This is not
   used in the SCHC-over-LoRaWAN profile.  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 fragmented
      |  datagrams with more than one SCHC gateway thanks to distinct
      |  sets of FPorts, cf. Section 5.2.

5.6.2.  Uplink Fragmentation: From Device to SCHC Gateway

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

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

   SCHC header size:  2 bytes (the FPort byte + 1 additional byte).

   RuleID:  8 bits stored in the LoRaWAN FPort (cf.  Section 5.2).

   DTag:  Size T = 0 bits, not used (cf.  Section 5.6.1).

   Window index:  4 windows are used, encoded on M = 2 bits.

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

   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.
      Implementations 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 during the same session.

      *  If the last tile is in an All-1 SCHC Message, the current L2
         MTU MUST be big enough to fit the All-1 header and the last
         tile.

   Penultimate tile:  MUST be equal to the regular size.

   RCS:  Use the recommended calculation algorithm in Section 8.2.3 of
      [RFC8724], Integrity Checking.

   Tile:  Size is 10 bytes.

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

   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.

   MAX_ACK_REQUESTS:  8.  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.

   In addition to the per-rule context parameters specified in
   [RFC8724], for uplink rules, an additional context parameter is
   added: whether or not to ack after each window.  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:

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

   *  The 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 a SCHC ACK REQ up to
      MAX_ACK_REQUESTS times, as described previously.

   This will avoid useless uplinks if the device has lost network
   coverage.

   For non-battery powered devices, the SCHC receiver MAY also choose to
   send a SCHC ACK only at the end of all windows.  This will reduce
   downlink load on the LoRaWAN network by reducing the number of
   downlinks.

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

5.6.2.1.  Regular Fragments

   Figure 7 is an example of a regular fragment for all fragments except
   the last one.  SCHC Header Size is 16 Bits, including the LoRaWAN
   FPort.

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

                Figure 7: All Fragments Except the Last One.

5.6.2.2.  Last Fragment (All-1)

   Following figures are examples of All-1 messages.  Figure 8 is
   without the last tile, Figure 9 is with the last tile.

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

               Figure 8: All-1 SCHC Message without Last Tile

 | FPort  | LoRaWAN payload                                            |
 + ------ + ---------------------------------------------------------- +
 | RuleID |   W    | FCN=All-1 |  RCS    |  Last tile   | Opt. padding |
 + ------ + ------ + --------- + ------- + ------------ + ------------ +
 | 8 bits | 2 bits |  6 bits   | 32 bits | 1 to 80 bits | 0 to 7 bits  |

              Figure 9: All-1 SCHC Message with Last Tile

5.6.2.3.  SCHC ACK

   | FPort  | LoRaWAN payload           |
   + ------ + --------------------------+
   | RuleID |   W   | C = 1 |  padding  |
   |        |       |       | (b'00000) |
   + ------ + ----- + ----- + --------- +
   | 8 bits | 2 bit | 1 bit |  5 bits   |

               Figure 10: SCHC ACK Format - Correct RCS Check

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

              Figure 11: SCHC ACK Format - Incorrect RCS Check

      |  Note: Because of the bitmap compression mechanism and L2 byte
      |  alignment, only the following discrete values are possible for
      |  the compressed bitmap size: 5, 13, 21, 29, 37, 45, 53, 61, 62,
      |  and 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 12: 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 13: SCHC ACK REQ Format

5.6.3.  Downlink Fragmentation: From SCHC Gateway to Device

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

   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 for the SCHC
            gateway and REQUIRED for the device.

   RuleID:  8 bits stored in the LoRaWAN FPort (cf.  Section 5.2).

   DTag:  Size T = 0 bit, not used (cf.  Section 5.6.1).

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

   RCS:  Use the recommended calculation algorithm in Section 8.2.3 of
      [RFC8724], Integrity Checking.

   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.

   The following parameters apply to ACK-Always (Unicast) only:

   Retransmission timer:  See Section 5.6.3.5.

   MAX_ACK_REQUESTS:  8.

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

   As only one tile is used, its size can change for each downlink and
   will be the currently 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 (e.g., start a 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.  The RECOMMENDED
   uplink is a LoRaWAN empty frame as defined in Section 4.6.  As this
   uplink is sent only to open an RX window, any LoRaWAN uplink frame
   from the device MAY reset this counter.

      |  Note: The FPending bit included in the LoRaWAN protocol SHOULD
      |  NOT be used for the 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

   Figure 14 is an example of a regular fragment for all fragments
   except the last one.  SCHC Header Size is 10 Bits, including the
   LoRaWAN FPort.

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

                 Figure 14: All Fragments but the Last One.

5.6.3.2.  Last Fragment (All-1)

   | FPort  | LoRaWAN payload                                         |
   + ------ + --------------------------- + ------------------------- +
   | RuleID | W     | FCN = b'1 |   RCS   |   Payload   | Opt padding |
   + ------ + ----- + --------- + ------- + ----------- + ----------- +
   | 8 bits | 1 bit | 1 bit     | 32 bits | 6 to X bits | 0 to 7 bits |

              Figure 15: 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 16: SCHC ACK Format - Correct RCS Check

   | FPort  | LoRaWAN payload                                   |
   + ------ + ------------------------------------------------- +
   | RuleID | W     | C = b'0 | Bitmap = b'1 | Padding b'000000 |
   + ------ + ----- + ------- + ------------ + ---------------- +
   | 8 bits | 1 bit | 1 bit   |    1 bit     |      5 bits      |

              Figure 17: SCHC ACK Format - Incorrect RCS Check

5.6.3.4.  Receiver-Abort

   Figure 18 is an example of a Receiver-Abort packet, following an
   All-1 SCHC Fragment with incorrect RCS.

   | 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 18: Receiver-Abort Packet

5.6.3.5.  Downlink Retransmission Timer

   Class A, Class B, and Class C devices do not manage retransmissions
   and timers 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
   (for example, 1 packet a day in normal operation), that duration may
   be extended; it is application specific.

   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 the window size
   is 1).  Following an All-0 SCHC Fragment, 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 fragmented
   datagram, the SCHC layer should immediately queue for transmission
   those SCHC ACK messages if no SCHC downlink has been received during
   the RX1 and RX2 windows.  The LoRaWAN layer will respect the
   applicable local spectrum regulation.

      |  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, and 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 interval of 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 the
   opening of a downlink opportunity to any downlink with higher
   priority than the SCHC ACK REQ message.

      |  Note: The device MUST keep this SCHC ACK message in memory
      |  until it receives a downlink SCHC Fragmentation Message (with
      |  FPort == FPortDown) that is not a SCHC ACK REQ; this indicates
      |  that the SCHC gateway has received the SCHC 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 values are:

   Class B:  3 times the ping slot periodicity.

   Class C:  30 seconds.

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

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", the following condition being met: SCHC header is 2
   bytes.

   *Downlink fragmentation (ACK-Always)*:

   As per [RFC8724], SCHC All-1 MUST contain the last tile, and
   implementations MUST ensure that SCHC 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", the following condition being met: RCS is 4
   bytes.

5.7.3.  Delay after Each LoRaWAN Frame to Respect Local Regulation

   This profile does not define a delay to be added after each LoRaWAN
   frame; local regulation compliance is expected to be enforced by the
   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 beyond those already identified in
   [RFC8724].  Moreover, SCHC data (LoRaWAN payload) are protected at
   the LoRaWAN level by an AES-128 encryption with a session key shared
   by the device and the SCHC gateway.  These session keys are renewed
   at each LoRaWAN session (i.e., each join or rejoin to the LoRaWAN
   network).

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

   [LORAWAN-SPEC]
              LoRa Alliance, "LoRaWAN 1.0.4 Specification Package",
              <https://lora-alliance.org/resource_hub/lorawan-104-
              specification-package/>.

   [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>.

   [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>.

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zúñiga, "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>.

8.2.  Informative References

   [LORAWAN-REMOTE-MULTICAST-SET]
              LoRa Alliance, "LoRaWAN Remote Multicast Setup
              Specification v1.0.0", <https://lora-
              alliance.org/resource_hub/lorawan-remote-multicast-setup-
              specification-v1-0-0/>.

   [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>.

   [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>.

Appendix A.  Examples

   In the following examples, "applicative data" refers to the IPv6
   payload sent by the application to the SCHC layer.

A.1.  Uplink - Compression Example - No Fragmentation

   This example represents an applicative data going through SCHC over
   LoRaWAN; no fragmentation required.

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

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

                  Figure 19: Uplink Example: SCHC Message

   The current LoRaWAN MTU is 51 bytes, although 2-byte FOpts are used
   by the 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 20: Uplink Example: LoRaWAN Packet

A.2.  Uplink - Compression and Fragmentation Example

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

   An applicative data of 300 bytes is passed to the SCHC compression
   layer.  Rule 1 is used by the 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 21: Uplink Example: SCHC Message

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

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

                Figure 22: Uplink Example: LoRaWAN Packet 1

   The tile content is described in Figure 23

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

               Figure 23: Uplink Example: First Tile Content

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

   Next transmission MTU is 242 bytes, 4-byte 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 24: 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 25: 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 26: Uplink Example: LoRaWAN Packet 4 - All-1 SCHC 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:

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

           Figure 27: Uplink Example: LoRaWAN Packet 5 - SCHC ACK

A.3.  Downlink

   An applicative data of 155 bytes is passed to the SCHC compression
   layer.  Rule 1 is used by the 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 28: Downlink Example: SCHC Message

   The current LoRaWAN MTU is 51 bytes; no FOpts are used by the LoRaWAN
   protocol: 51 bytes are available for SCHC payload + FPort field; the
   applicative data 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 29: Downlink Example: LoRaWAN Packet 1 - SCHC Fragment 1

   The tile content is described in Figure 30

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

              Figure 30: Downlink Example: First Tile Content

   The receiver answers with a 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 31: 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 32: Downlink Example: LoRaWAN Packet 3 - SCHC Fragment 2

   The receiver answers with a 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 33: Downlink Example: LoRaWAN Packet 4 - SCHC ACK

   The last downlink is sent, no FOpts:

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

   Figure 34: Downlink Example: LoRaWAN Packet 5 - All-1 SCHC Message

   The receiver answers to the sender with a 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 35: Downlink Example: LoRaWAN Packet 6 - SCHC ACK

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

   LoRaWAN is a registered trademark of the LoRa Alliance.

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
   Chirp Foundation

   Email: nicolas.sornin@chirpfoundation.org

   Alper Yegin
   Actility

   Email: alper.yegin@actility.com

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-Sévigné Cedex
   France

   Email: ivaylo@ackl.io