path: root/doc/rfc/rfc9442.txt

Internet Engineering Task Force (IETF)                        JC. Zúñiga
Request for Comments: 9442                                              
Category: Standards Track                                       C. Gomez
ISSN: 2070-1721                                               S. Aguilar
                                    Universitat Politècnica de Catalunya
                                                              L. Toutain
                                                          IMT-Atlantique
                                                             S. Céspedes
                                                    Concordia University
                                                              D. Wistuba
                                          NIC Labs, Universidad de Chile
                                                                J. Boite
                                                         Unabiz (Sigfox)
                                                               July 2023


Static Context Header Compression (SCHC) over Sigfox Low-Power Wide Area
                            Network (LPWAN)

Abstract

   The Static Context Header Compression (SCHC) and fragmentation
   specification (RFC 8724) describes a generic framework for
   application header compression and fragmentation modes designed for
   Low-Power Wide Area Network (LPWAN) technologies.  This document
   defines a profile of SCHC over Sigfox LPWAN and provides optimal
   parameter values 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/rfc9442.

Copyright Notice

   Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  SCHC over Sigfox
     3.1.  Network Architecture
     3.2.  Uplink
     3.3.  Downlink
       3.3.1.  SCHC ACK on Downlink
     3.4.  SCHC Rules
     3.5.  Fragmentation
       3.5.1.  Uplink Fragmentation
       3.5.2.  Downlink Fragmentation
     3.6.  SCHC over Sigfox F/R Message Formats
       3.6.1.  Uplink No-ACK Mode: Single-Byte SCHC Header
       3.6.2.  Uplink ACK-on-Error Mode: Single-Byte SCHC Header
       3.6.3.  Uplink ACK-on-Error Mode: Two-Byte SCHC Header Option 1
       3.6.4.  Uplink ACK-on-Error Mode: Two-Byte SCHC Header Option 2
       3.6.5.  Downlink ACK-Always Mode: Single-Byte SCHC Header
     3.7.  Padding
   4.  Fragmentation Rules Examples
     4.1.  Uplink Fragmentation Rules Examples
     4.2.  Downlink Fragmentation Rules Example
   5.  Fragmentation Sequence Examples
     5.1.  Uplink No-ACK Examples
     5.2.  Uplink ACK-on-Error Examples: Single-Byte SCHC Header
     5.3.  SCHC Abort Examples
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Generic Framework for Static Context Header Compression (SCHC)
   and Fragmentation specification [RFC8724] can be used in conjunction
   with any of the four LPWAN technologies described in [RFC8376].
   These LPWANs have similar characteristics, such as star-oriented
   topologies, network architecture, connected devices with built-in
   applications, etc.

   SCHC offers a considerable degree of flexibility to accommodate all
   these LPWAN technologies.  Even though there are a great number of
   similarities between them, some differences exist with respect to the
   transmission characteristics, payload sizes, etc.  Hence, there are
   optimal parameters and modes of operation that can be used when SCHC
   is used in conjunction with a specific LPWAN technology.

   Sigfox is an LPWAN technology that offers energy-efficient
   connectivity for devices at a very low cost.  Complete Sigfox
   documentation can be found in [sigfox-docs].  Sigfox aims to provide
   a very wide area network composed of Base Stations that receive short
   Uplink messages (up to 12 bytes in size) sent by devices over the
   long-range Sigfox radio protocol, as described in [RFC8376].  Base
   Stations then forward messages to the Sigfox Cloud infrastructure for
   further processing (e.g., to offer geolocation services) and final
   delivery to the customer.  Base Stations also relay Downlink messages
   (with a fixed 8-byte size) sent by the Sigfox Cloud to the devices,
   i.e., Downlink messages are being generated when devices explicitly
   request these messages with a flag in an Uplink message.  With SCHC
   functionalities, the Sigfox network offers more reliable
   communications (including recovery of lost messages) and is able to
   convey extended-size payloads (allowing for fragmentation/reassembly
   of messages) [sigfox-spec].

   This document describes the parameters, settings, and modes of
   operation to be used when SCHC is implemented over a Sigfox LPWAN.
   The set of parameters forms a "SCHC over Sigfox Profile".  The SCHC
   over Sigfox Profile is applicable to the Sigfox Radio specification
   versions up to v1.6/March 2022 [sigfox-spec] (support for future
   versions would have to be assessed).

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.

   It is assumed that the reader is familiar with the terms and
   mechanisms defined in [RFC8376] and [RFC8724].  Also, it is assumed
   that the reader is familiar with Sigfox terminology [sigfox-spec].

3.  SCHC over Sigfox

   The Generic SCHC Framework described in [RFC8724] takes advantage of
   previous knowledge of traffic flows existing in LPWAN applications to
   avoid context synchronization.

   Contexts need to be stored and pre-configured on both ends.  This can
   be done either by using a provisioning protocol, by out-of-band
   means, or by pre-provisioning them (e.g., at manufacturing time).
   For example, the context exchange can be done by using the Network
   Configuration Protocol (NETCONF) [RFC6241] with Secure Shell (SSH),
   RESTCONF [RFC8040] with secure HTTP methods, and CoAP Management
   Interface (CORECONF) [CORE-COMI] with the Constrained Application
   Protocol (CoAP) [RFC7252] as provisioning protocols.  The contexts
   can be encoded in XML under NETCONF, in JSON [RFC8259] under
   RESTCONF, and in Concise Binary Object Representation (CBOR)
   [RFC8949] under CORECONF.  The way contexts are configured and stored
   on both ends is out of the scope of this document.

3.1.  Network Architecture

   Figure 1 represents the architecture for Compression/Decompression
   (C/D) and Fragmentation/Reassembly (F/R) based on the terminology
   defined in [RFC8376], where the Radio Gateway (RGW) is a Sigfox Base
   Station and the Network Gateway (NGW) is the Sigfox cloud-based
   Network.

   Sigfox Device                                           Application
 +----------------+                                     +--------------+
 | APP1 APP2 APP3 |                                     |APP1 APP2 APP3|
 +----------------+                                     +--------------+
 |   UDP  |       |                                     |     |  UDP   |
 |  IPv6  |       |                                     |     | IPv6   |
 +--------+       |                                     |     +--------+
 | SCHC C/D & F/R |                                     |              |
 |                |                                     |              |
 +-------+--------+                                     +--------+-----+
         $                                                       .
         $   +---------+     +--------------+     +---------+    .
         $   |         |     |   Network    |     | Network |    .
         +~~ |Sigfox BS|     |   Gateway    |     |  SCHC   |    .
             |  (RGW)  | === |    (NGW)     | ... |C/D & F/R|.....
             |         |     | Sigfox Cloud |     |         |   IP-based
             +---------+     +--------------+     +---------+   Network
 ------- Uplink message ------>
                                        <------- Downlink message ------
 Legend:
 $, ~ : Radio link
 = : Internal Sigfox Network
 . : External IP-based Network

                     Figure 1: Network Architecture

   In the case of the global Sigfox network, RGWs (or Base Stations) are
   distributed over multiple countries wherever the Sigfox LPWAN service
   is provided.  The NGW (or cloud-based Sigfox Core Network) is a
   single entity that connects to all RGWs (Sigfox Base Stations) in the
   world, hence providing a global single star Network topology.

   The Sigfox Device sends application packets that are compressed and/
   or fragmented by a SCHC C/D + F/R to reduce header size and/or
   fragment the packet.  The resulting SCHC message is sent over a layer
   two (L2) Sigfox frame to the Sigfox Base Stations, which then forward
   the SCHC message to the NGW.  The NGW then delivers the SCHC message
   and associated gathered metadata to the Network SCHC C/D + F/R.

   The Sigfox cloud-based Network communicates with the Network SCHC C/D
   + F/R for compression/decompression and/or for fragmentation/
   reassembly.  The Network SCHC C/D + F/R shares the same set of Rules
   as the device SCHC C/D + F/R.  The Network SCHC C/D + F/R can be
   collocated with the NGW or it could be located in a different place,
   as long as a tunnel or secured communication is established between
   the NGW and the SCHC C/D + F/R functions.  After decompression and/or
   reassembly, the packet can be forwarded over the Internet to one (or
   several) LPWAN Application Server(s) (App(s)).

   The SCHC C/D + F/R processes are bidirectional, so the same
   principles are applicable on both Uplink (UL) and Downlink (DL).

3.2.  Uplink

   Uplink Sigfox transmissions occur in repetitions over different times
   and frequencies.  Besides time and frequency diversities, the Sigfox
   network also provides spatial diversity, as potentially an Uplink
   message will be received by several Base Stations.  The Uplink
   message application payload size can be up to 12 bytes.

   Since all messages are self-contained and Base Stations forward all
   these messages back to the same Sigfox network, multiple input copies
   can be combined at the NGW, providing for extra reliability based on
   the triple diversity (i.e., time, space, and frequency).

   A detailed description of the Sigfox radio protocol can be found in
   [sigfox-spec].

   Messages sent from the device to the Network are delivered by the
   Sigfox cloud-based Network to the Network SCHC C/D + F/R through a
   callback/API with the following information:

   *  Device ID

   *  Message Sequence Number

   *  Message Payload

   *  Message Timestamp

   *  Device Geolocation (optional)

   *  Received Signal Strength Indicator (RSSI) (optional)

   *  Device Temperature (optional)

   *  Device Battery Voltage (optional)

   The Device ID is a globally unique identifier assigned to the device,
   which is included in the Sigfox header of every message.  The Message
   Sequence Number is a monotonically increasing number identifying the
   specific transmission of this Uplink message, and it is also part of
   the Sigfox header.  The Message Payload corresponds to the payload
   that the device has sent in the Uplink transmission.  Battery
   Voltage, Device Temperature, and RSSI values are sent in the
   confirmation control message, which is mandatorily sent by the device
   after the successful reception of a Downlink message (see
   [sigfox-callbacks], Section 5.2).

   The Message Timestamp, Device Geolocation, RSSI, Device Temperature,
   and Device Battery Voltage are metadata parameters provided by the
   Network.

   A detailed description of the Sigfox callbacks/APIs can be found in
   [sigfox-callbacks].

   Only messages that have passed the L2 Cyclic Redundancy Check (CRC)
   at Network reception are delivered by the Sigfox network to the
   Network SCHC C/D + F/R.

   The L2 Word size used by Sigfox is 1 byte (8 bits).

   Figure 2 shows a SCHC message sent over Sigfox, where the SCHC
   message could be a full SCHC Packet (e.g., compressed) or a SCHC
   Fragment (e.g., a piece of a bigger SCHC Packet).

                    | Sigfox Header | Sigfox Payload  |
                    +---------------+---------------- +
                                    |   SCHC Message  |

                      Figure 2: SCHC Message in Sigfox

3.3.  Downlink

   Downlink transmissions are device-driven and can only take place
   following an Uplink communication that indicates Downlink
   communication can be performed.  Hence, a Sigfox Device explicitly
   indicates its intention to receive a Downlink message (with a size of
   8 bytes) using a Downlink request flag when sending the preceding
   Uplink message to the Network.  The Downlink request flag is part of
   the Sigfox protocol headers.  After completing the Uplink
   transmission, the device opens a fixed window for Downlink reception.
   The delay and duration of the reception opportunity window have fixed
   values.  If there is a Downlink message to be sent for this given
   device (e.g., either a response to the Uplink message or queued
   information waiting to be transmitted), the Network transmits this
   message to the device during the reception window.  If no message is
   received by the device after the reception opportunity window has
   elapsed, the device closes the reception window opportunity and gets
   back to the normal mode (e.g., continue Uplink transmissions, sleep,
   standby, etc.).

   When a Downlink message is sent to a device, a reception
   acknowledgement is generated by the device, sent back to the Network
   through the Sigfox radio protocol, and reported in the Sigfox network
   backend.

   A detailed description of the Sigfox radio protocol can be found in
   [sigfox-spec], and a detailed description of the Sigfox callbacks/
   APIs can be found in [sigfox-callbacks].  A Downlink request flag can
   be included in the information exchange between the Sigfox network
   and Network SCHC.

3.3.1.  SCHC ACK on Downlink

   As explained previously, Downlink transmissions are driven by devices
   and can only take place following a specific Uplink transmission that
   indicates and allows a following Downlink opportunity.  For this
   reason, when SCHC bidirectional services are used (e.g., ACK-on-Error
   fragmentation mode), the SCHC protocol implementation needs to
   consider the times when a Downlink message (e.g., SCHC
   Acknowledgement (ACK)) can be sent and/or received.

   For the Uplink ACK-on-Error fragmentation mode, a Downlink
   opportunity MUST be indicated by the last fragment of every window,
   which is signalled by a specific value of the Fragment Compressed
   Number (FCN) value, i.e., FCN = All-0 or FCN = All-1.  The FCN is the
   tile index in a specific window.  The combination of the FCN and the
   window number uniquely identifies a SCHC Fragment, as explained in
   [RFC8724].  The device sends the fragments in sequence and, after
   transmitting FCN = All-0 or FCN = All-1, it opens up a reception
   opportunity.  The Network SCHC can then decide to respond at that
   opportunity (or wait for a further one) with a SCHC ACK, indicating
   that there are missing fragments from the current or previous
   windows.  If there is no SCHC ACK to be sent, or if the Network
   decides to wait for a further Downlink transmission opportunity, then
   no Downlink transmission takes place at that opportunity and the
   Uplink transmissions continue after a timeout.  Intermediate SCHC
   Fragments with FCNs that are different from All-0 or All-1 MUST NOT
   use the Downlink request flag to request a SCHC ACK.

3.4.  SCHC Rules

   The RuleID MUST be included in the SCHC header.  The total number of
   Rules to be used directly affects the RuleID field size, and
   therefore the total size of the fragmentation header.  For this
   reason, it is RECOMMENDED to keep the number of Rules that are
   defined for a specific device to the minimum possible.  Large RuleID
   sizes (and thus larger fragmentation headers) are acceptable for
   devices without significant energy constraints (e.g., a sensor that
   is powered by the electricity grid).

   RuleIDs can be used to differentiate data traffic classes (e.g., QoS,
   control vs. data, etc.) and data sessions.  They can also be used to
   interleave simultaneous fragmentation sessions between a device and
   the Network.

3.5.  Fragmentation

   The SCHC specification [RFC8724] defines a generic fragmentation
   functionality that allows sending data packets or files larger than
   the maximum size of a Sigfox payload.  The functionality also defines
   a mechanism to reliably send multiple messages by allowing to
   selectively resend any lost fragments.

   The SCHC fragmentation supports several modes of operation.  These
   modes have different advantages and disadvantages, depending on the
   specifics of the underlying LPWAN technology and application use
   case.  This section describes how the SCHC fragmentation
   functionality should optimally be implemented when used over a Sigfox
   LPWAN for the most typical use case applications.

   As described in Section 8.2.3 of [RFC8724], the integrity of the
   fragmentation-reassembly process of a SCHC Packet MUST be checked at
   the receiver end.  Since only Uplink/Downlink messages/fragments that
   have passed the Sigfox CRC-check are delivered to the Network/Sigfox
   Device SCHC C/D + F/R, integrity can be guaranteed when no
   consecutive messages are missing from the sequence and all FCN
   bitmaps are complete.  With this functionality in mind, and in order
   to save protocol and processing overhead, the use of a Reassembly
   Check Sequence (RCS), as described in Section 3.5.1.5, MUST be used.

3.5.1.  Uplink Fragmentation

   Sigfox Uplink transmissions are completely asynchronous and take
   place in any random frequency of the allowed Uplink bandwidth
   allocation.  In addition, devices may go to deep sleep mode and then
   wake up and transmit whenever there is a need to send information to
   the Network, as there is no need to perform any Network attachment,
   synchronization, or other procedures before transmitting a data
   packet.

   Since Uplink transmissions are asynchronous, a SCHC Fragment can be
   transmitted at any given time by the device.  Sigfox Uplink messages
   are fixed in size, and as described in [RFC8376], they can carry a
   payload of 0-12 bytes (0-96 bits).  Hence, a single SCHC Tile size,
   per fragmentation mode, can be defined so that every Sigfox message
   always carries one SCHC Tile.

   When the ACK-on-Error mode is used for Uplink fragmentation, the SCHC
   Compound ACK defined in [RFC9441] MUST be used in the Downlink
   responses.

3.5.1.1.  SCHC Sender-Abort

   As defined in [RFC8724], a SCHC Sender-Abort can be triggered when
   the number of SCHC ACK REQ attempts is greater than or equal to
   MAX_ACK_REQUESTS.  In the case of SCHC over Sigfox, a SCHC Sender-
   Abort MUST be sent if the number of repeated All-1s sent in sequence,
   without a Compound ACK reception in between, is greater than or equal
   to MAX_ACK_REQUESTS.

3.5.1.2.  SCHC Receiver-Abort

   As defined in [RFC8724], a SCHC Receiver-Abort is triggered when the
   receiver has no RuleID and DTag pairs available for a new session.
   In the case of this profile, a SCHC Receiver-Abort MUST be sent if,
   for a single device, all the RuleIDs are being processed by the
   receiver (i.e., have an active session) at a certain time and a new
   one is requested or if the RuleID of the fragment is not valid.

   A SCHC Receiver-Abort MUST be triggered when the Inactivity Timer
   expires.

   MAX_ACK_REQUESTS can be increased when facing high error rates.

   Although a SCHC Receiver-Abort can be triggered at any point in time,
   a SCHC Receiver-Abort Downlink message MUST only be sent when there
   is a Downlink transmission opportunity.

3.5.1.3.  Single-Byte SCHC Header for Uplink Fragmentation

3.5.1.3.1.  Uplink No-ACK Mode: Single-Byte SCHC Header

   Single-byte SCHC Header No-ACK mode MUST be used for transmitting
   short, non-critical packets that require fragmentation and do not
   require full reliability.  This mode can be used by Uplink-only
   devices that do not support Downlink communications or by
   bidirectional devices when they send non-critical data.  Note that
   sending non-critical data by using a reliable fragmentation mode
   (which is only possible for bidirectional devices) may incur
   unnecessary overhead.

   Since there are no multiple windows in the No-ACK mode, the W bit is
   not present.  However, it MUST use the FCN field to indicate the size
   of the data packet.  In this sense, the data packet would need to be
   split into X fragments and, similarly to the other fragmentation
   modes, the first transmitted fragment would need to be marked with
   FCN = X-1.  Consecutive fragments MUST be marked with decreasing FCN
   values, having the last fragment marked with FCN = (All-1).  Hence,
   even though the No-ACK mode does not allow recovering missing
   fragments, it allows implicitly indicating the size of the expected
   packet to the Network and hence detects whether all fragments have
   been received or not at the receiver side.  In case the FCN field is
   not used to indicate the size of the data packet, the Network can
   detect whether all fragments have been received or not by using the
   integrity check.

   When using the Single-byte SCHC Header for Uplink fragmentation, the
   fragmentation header MUST be 8 bits in size and is composed as
   follows:

   *  RuleID size: 3 bits

   *  DTag size (T): 0 bits

   *  Fragment Compressed Number (FCN) size (N): 5 bits

   Other F/R parameters MUST be configured as follows:

   *  As per [RFC8724], in the No-ACK mode, the W (window) field is not
      present.

   *  Regular tile size: 11 bytes

   *  All-1 tile size: 0 to 10 bytes

   *  Inactivity Timer: Application-dependent.  The default value is 12
      hours.

   *  RCS size: 5 bits

   The maximum SCHC Packet size is 340 bytes.

   Section 3.6.1 presents SCHC Fragment format examples, and Section 5.1
   provides fragmentation examples, using Single-byte SCHC Header No-ACK
   mode.

3.5.1.3.2.  Uplink ACK-on-Error Mode: Single-Byte SCHC Header

   ACK-on-Error with a single-byte header MUST be used for short- to
   medium-sized packets that need to be sent reliably.  ACK-on-Error is
   optimal for reliable SCHC Packet transmission over Sigfox
   transmissions, since it leads to a reduced number of ACKs in the
   lower-capacity Downlink channel.  Also, Downlink messages can be sent
   asynchronously and opportunistically.  In contrast, ACK-Always would
   not minimize the number of ACKs, and No-ACK would not allow reliable
   transmission.

   Allowing transmission of packets/files up to 300 bytes long, the SCHC
   Uplink fragmentation header size is 8 bits in size and is composed as
   follows:

   *  RuleID size: 3 bits

   *  DTag size (T): 0 bits

   *  Window index (W) size (M): 2 bits

   *  Fragment Compressed Number (FCN) size (N): 3 bits

   Other F/R parameters MUST be configured as follows:

   *  MAX_ACK_REQUESTS: 5

   *  WINDOW_SIZE: 7 (i.e., the maximum FCN value is 0b110)

   *  Regular tile size: 11 bytes

   *  All-1 tile size: 0 to 10 bytes

   *  Retransmission Timer: Application-dependent.  The default value is
      12 hours.

   *  Inactivity Timer: Application-dependent.  The default value is 12
      hours.

   *  RCS size: 3 bits

   Section 3.6.2 presents SCHC Fragment format examples, and Section 5.2
   provides fragmentation examples, using ACK-on-Error with a single-
   byte header.

3.5.1.4.  Two-Byte SCHC Header for Uplink Fragmentation

   ACK-on-Error with a two-byte header MUST be used for medium- to
   large-sized packets that need to be sent reliably.  ACK-on-Error is
   optimal for reliable SCHC Packet transmission over Sigfox, since it
   leads to a reduced number of ACKs in the lower-capacity Downlink
   channel.  Also, Downlink messages can be sent asynchronously and
   opportunistically.  In contrast, ACK-Always would not minimize the
   number of ACKs, and No-ACK would not allow reliable transmission.

3.5.1.4.1.  Uplink ACK-on-Error Mode: Two-Byte SCHC Header Option 1

   In order to allow transmission of medium to large packets/files up to
   480 bytes long, the SCHC Uplink fragmentation header size is 16 bits
   in size and is composed as follows:

   *  RuleID size: 6 bits

   *  DTag size (T): 0 bits

   *  Window index (W) size (M): 2 bits

   *  Fragment Compressed Number (FCN) size (N): 4 bits

   *  RCS size: 4 bits

   Other F/R parameters MUST be configured as follows:

   *  MAX_ACK_REQUESTS: 5

   *  WINDOW_SIZE: 12 (with a maximum value of FCN=0b1011)

   *  Regular tile size: 10 bytes

   *  All-1 tile size: 1 to 10 bytes

   *  Retransmission Timer: Application-dependent.  The default value is
      12 hours.

   *  Inactivity Timer: Application-dependent.  The default value is 12
      hours.

   Note that WINDOW_SIZE is limited to 12.  This is because 4 windows (M
   = 2) with bitmaps of size 12 can be fitted in a single SCHC Compound
   ACK.

   Section 3.6.3 presents SCHC Fragment format examples, using ACK-on-
   Error with two-byte header Option 1.

3.5.1.4.2.  Uplink ACK-on-Error Mode: Two-Byte SCHC Header Option 2

   In order to allow transmission of very large packets/files up to 2400
   bytes long, the SCHC Uplink fragmentation header size is 16 bits in
   size and is composed as follows:

   *  RuleID size: 8 bits

   *  DTag size (T): 0 bits

   *  Window index (W) size (M): 3 bits

   *  Fragment Compressed Number (FCN) size (N): 5 bits

   *  RCS size: 5 bits

   Other F/R parameters MUST be configured as follows:

   *  MAX_ACK_REQUESTS: 5

   *  WINDOW_SIZE: 31 (with a maximum value of FCN=0b11110)

   *  Regular tile size: 10 bytes

   *  All-1 tile size: 0 to 9 bytes

   *  Retransmission Timer: Application-dependent.  The default value is
      12 hours.

   *  Inactivity Timer: Application-dependent.  The default value is 12
      hours.

   Section 3.6.4 presents SCHC Fragment format examples, using ACK-on-
   Error with two-byte header Option 2.

3.5.1.5.  All-1 SCHC Fragment and RCS Behavior

   For ACK-on-Error, as defined in [RFC8724], it is expected that the
   last SCHC Fragment of the last window will always be delivered with
   an All-1 FCN.  Since this last window may not be full (i.e., it may
   be composed of fewer than WINDOW_SIZE fragments), an All-1 fragment
   may follow a value of FCN higher than 1 (0b01).  In this case, the
   receiver cannot determine from the FCN values alone whether there are
   or are not any missing fragments right before the All-1 fragment.

   For Rules where the number of fragments in the last window is
   unknown, an RCS field MUST be used, indicating the number of
   fragments in the last window, including the All-1.  With this RCS
   value, the receiver can detect if there are missing fragments before
   the All-1 and hence construct the corresponding SCHC ACK Bitmap
   accordingly and send it in response to the All-1.

3.5.2.  Downlink Fragmentation

   In some LPWAN technologies, as part of energy-saving techniques,
   Downlink transmission is only possible immediately after an Uplink
   transmission.  This allows the device to go in a very deep sleep mode
   and preserve battery without the need to listen to any information
   from the Network.  This is the case for Sigfox-enabled devices, which
   can only listen to Downlink communications after performing an Uplink
   transmission and requesting a Downlink.

   When there are fragments to be transmitted in the Downlink, an Uplink
   message is required to trigger the Downlink communication.  In order
   to avoid a potentially high delay for fragmented datagram
   transmission in the Downlink, the fragment receiver MAY perform an
   Uplink transmission as soon as possible after reception of a Downlink
   fragment that is not the last one.  Such an Uplink transmission MAY
   be triggered by sending a SCHC message, such as a SCHC ACK.  However,
   other data messages can equally be used to trigger Downlink
   communications.  The fragment receiver MUST send an Uplink
   transmission (e.g., empty message) and request a Downlink every 24
   hours when no SCHC session is started.  Whether this Uplink
   transmission is used (and the transmission rate, if used) depends on
   application-specific requirements.

   Sigfox Downlink messages are fixed in size, and as described in
   [RFC8376] they can carry a payload of 0-8 bytes (0-64 bits).  Hence,
   a single SCHC Tile size per mode can be defined so that every Sigfox
   message always carries one SCHC Tile.

   For reliable Downlink fragment transmission, the ACK-Always mode
   SHOULD be used.  Note that ACK-on-Error does not guarantee Uplink
   feedback (since no SCHC ACK will be sent when no errors occur in a
   window), and No-ACK would not allow reliable transmission.

   The SCHC Downlink fragmentation header size is 8 bits in size and is
   composed as follows:

   *  RuleID size: 3 bits

   *  DTag size (T): 0 bits

   *  Window index (W) size (M): 0 bits

   *  Fragment Compressed Number (FCN) size (N): 5 bits

   Other F/R parameters MUST be configured as follows:

   *  MAX_ACK_REQUESTS: 5

   *  WINDOW_SIZE: 31 (with a maximum value of FCN=0b11110)

   *  Regular tile size: 7 bytes

   *  All-1 tile size: 0 to 6 bytes

   *  Retransmission Timer: Application-dependent.  The default value is
      12 hours.

   *  Inactivity Timer: Application-dependent.  The default value is 12
      hours.

   *  RCS size: 5 bits

3.6.  SCHC over Sigfox F/R Message Formats

   This section depicts the different formats of SCHC Fragment, SCHC ACK
   (including the SCHC Compound ACK defined in [RFC9441]), and SCHC
   Abort used in SCHC over Sigfox.

3.6.1.  Uplink No-ACK Mode: Single-Byte SCHC Header

3.6.1.1.  Regular SCHC Fragment

   Figure 3 shows an example of a Regular SCHC Fragment for all
   fragments except the last one.  As tiles are 11 bytes in size,
   padding MUST NOT be added.  The penultimate tile of a SCHC Packet is
   of regular size.

                   |- SCHC Fragment Header -|
                   +------------------------+---------+
                   |   RuleID   |    FCN    | Payload |
                   +------------+-----------+---------+
                   |   3 bits   |  5 bits   | 88 bits |

      Figure 3: Regular SCHC Fragment Format for All Fragments except
                                the Last One

3.6.1.2.  All-1 SCHC Fragment

   Figure 4 shows an example of the All-1 message.  The All-1 message
   MAY contain the last tile of the SCHC Packet.  Padding MUST NOT be
   added, as the resulting size is a multiple of an L2 Word.

   The All-1 messages Fragment Header includes a 5-bit RCS, and 3 bits
   are added as padding to complete 2 bytes.  The payload size of the
   All-1 message ranges from 0 to 80 bits.

          |--------  SCHC Fragment Header -------|
          +--------------------------------------+--------------+
          | RuleID | FCN=ALL-1 |  RCS   |  b'000 |   Payload    |
          +--------+-----------+--------+--------+--------------+
          | 3 bits |  5 bits   | 5 bits | 3 bits | 0 to 80 bits |

           Figure 4: All-1 SCHC Message Format with the Last Tile

   As per [RFC8724], the All-1 must be distinguishable from a SCHC
   Sender-Abort message (with the same RuleID and N values).  The All-1
   MAY have the last tile of the SCHC Packet.  The SCHC Sender-Abort
   message header size is 1 byte with no padding bits.

   For the All-1 message to be distinguishable from the Sender-Abort
   message, the Sender-Abort message MUST be 1 byte (only header with no
   padding).  This way, the minimum size of the All-1 is 2 bytes, and
   the Sender-Abort message is 1 byte.

3.6.1.3.  SCHC Sender-Abort Message Format

                               Sender-Abort
                          |------ Header ------|
                          +--------------------+
                          | RuleID | FCN=ALL-1 |
                          +--------+-----------+
                          | 3 bits |  5 bits   |

                 Figure 5: SCHC Sender-Abort Message Format

3.6.2.  Uplink ACK-on-Error Mode: Single-Byte SCHC Header

3.6.2.1.  Regular SCHC Fragment

   Figure 6 shows an example of a Regular SCHC Fragment for all
   fragments except the last one.  As tiles are 11 bytes in size,
   padding MUST NOT be added.

                  |-- SCHC Fragment Header --|
                  +--------------------------+---------+
                  | RuleID |   W    |  FCN   | Payload |
                  +--------+--------+--------+---------+
                  | 3 bits | 2 bits | 3 bits | 88 bits |

      Figure 6: Regular SCHC Fragment Format for All Fragments except
                                the Last One

   The SCHC ACK REQ MUST NOT be used, instead the All-1 SCHC Fragment
   MUST be used to request a SCHC ACK from the receiver (Network SCHC).
   As per [RFC8724], the All-0 message is distinguishable from the SCHC
   ACK REQ (All-1 message).  The penultimate tile of a SCHC Packet is of
   regular size.

3.6.2.2.  All-1 SCHC Fragment

   Figure 7 shows an example of the All-1 message.  The All-1 message
   MAY contain the last tile of the SCHC Packet.  Padding MUST NOT be
   added, as the resulting size is L2-word-multiple.

     |-------------  SCHC Fragment Header -----------|
     +-----------------------------------------------+--------------+
     | RuleID |   W    | FCN=ALL-1 |  RCS   |b'00000 |   Payload    |
     +--------+--------+-----------+--------+--------+--------------+
     | 3 bits | 2 bits |  3 bits   | 3 bits | 5 bits | 0 to 80 bits |

           Figure 7: All-1 SCHC Message Format with the Last Tile

   As per [RFC8724], the All-1 must be distinguishable from a SCHC
   Sender-Abort message (with same RuleID, M, and N values).  The All-1
   MAY have the last tile of the SCHC Packet.  The SCHC Sender-Abort
   message header size is 1 byte with no padding bits.

   For the All-1 message to be distinguishable from the Sender-Abort
   message, the Sender-Abort message MUST be 1 byte (only header with no
   padding).  This way, the minimum size of the All-1 is 2 bytes, and
   the Sender-Abort message is 1 byte.

3.6.2.3.  SCHC ACK Format

   Figure 8 shows the SCHC ACK format when all fragments have been
   correctly received (C=1).  Padding MUST be added to complete the
   64-bit Sigfox Downlink frame payload size.

                   |---- SCHC ACK Header ----|
                   +-------------------------+---------+
                   | RuleID |    W   | C=b'1 | b'0-pad |
                   +--------+--------+-------+---------+
                   | 3 bits | 2 bits | 1 bit | 58 bits |

                 Figure 8: SCHC Success ACK Message Format

   In case SCHC Fragment losses are found in any of the windows of the
   SCHC Packet (C=0), the SCHC Compound ACK defined in [RFC9441] MUST be
   used.  The SCHC Compound ACK message format is shown in Figure 9.

 |--- SCHC ACK Header ---|- W=w1 -|...|----- W=wi ------|
 +------+--------+-------+--------+...+--------+--------+------+-------+
 |RuleID| W=b'w1 | C=b'0 | Bitmap |...| W=b'wi | Bitmap | b'00 |b'0-pad|
 +------+--------+-------+--------+...+--------+--------+------+-------+
 |3 bits| 2 bits | 1 bit | 7 bits |...| 2 bits | 7 bits |2 bits|

               Figure 9: SCHC Compound ACK Message Format

   Losses are found in windows W = w1,...,wi, where w1 < w2 <...< wi.

3.6.2.4.  SCHC Sender-Abort Message Format

                      |---- Sender-Abort Header ----|
                      +-----------------------------+
                      | RuleID | W=b'11 | FCN=ALL-1 |
                      +--------+--------+-----------+
                      | 3 bits | 2 bits |  3 bits   |

                Figure 10: SCHC Sender-Abort Message Format

3.6.2.5.  SCHC Receiver-Abort Message Format

      |- Receiver-Abort Header -|
      +---------------------------------+-----------------+---------+
      | RuleID | W=b'11 | C=b'1 |  b'11 |  0xFF (all 1's) | b'0-pad |
      +--------+--------+-------+-------+-----------------+---------+
      | 3 bits | 2 bits | 1 bit | 2 bit |  8 bit          | 48 bits |
                next L2 Word boundary ->| <-- L2 Word --> |

               Figure 11: SCHC Receiver-Abort Message Format

3.6.3.  Uplink ACK-on-Error Mode: Two-Byte SCHC Header Option 1

3.6.3.1.  Regular SCHC Fragment

   Figure 12 shows an example of a Regular SCHC Fragment for all
   fragments except the last one.  The penultimate tile of a SCHC Packet
   is of the regular size.

              |------- SCHC Fragment Header ------|
              +-----------------------------------+---------+
              | RuleID |    W   |  FCN   | b'0000 | Payload |
              +--------+--------+--------+--------+---------+
              | 6 bits | 2 bits | 4 bits | 4 bits | 80 bits |

      Figure 12: Regular SCHC Fragment Format for All Fragments except
                                the Last One

   The SCHC ACK REQ MUST NOT be used, instead the All-1 SCHC Fragment
   MUST be used to request a SCHC ACK from the receiver (Network SCHC).
   As per [RFC8724], the All-0 message is distinguishable from the SCHC
   ACK REQ (All-1 message).

3.6.3.2.  All-1 SCHC Fragment

   Figure 13 shows an example of the All-1 message.  The All-1 message
   MUST contain the last tile of the SCHC Packet.

   The All-1 message Fragment Header contains an RCS of 4 bits to
   complete the two-byte size.  The size of the last tile ranges from 8
   to 80 bits.

          |--------- SCHC Fragment Header -------|
          +--------------------------------------+--------------+
          | RuleID |    W   | FCN=ALL-1 |  RCS   |    Payload   |
          +--------+--------+-----------+--------+--------------+
          | 6 bits | 2 bits |  4 bits   | 4 bits | 8 to 80 bits |

          Figure 13: All-1 SCHC Message Format with the Last Tile

   As per [RFC8724], the All-1 must be distinguishable from the SCHC
   Sender-Abort message (with same RuleID, M, and N values).  The All-1
   MUST have the last tile of the SCHC Packet that MUST be at least 1
   byte.  The SCHC Sender-Abort message header size is 2 bytes with no
   padding bits.

   For the All-1 message to be distinguishable from the Sender-Abort
   message, the Sender-Abort message MUST be 2 bytes (only header with
   no padding).  This way, the minimum size of the All-1 is 3 bytes, and
   the Sender-Abort message is 2 bytes.

3.6.3.3.  SCHC ACK Format

   Figure 14 shows the SCHC ACK format when all fragments have been
   correctly received (C=1).  Padding MUST be added to complete the
   64-bit Sigfox Downlink frame payload size.

                   |---- SCHC ACK Header ----|
                   +-------------------------+---------+
                   | RuleID |    W   | C=b'1 | b'0-pad |
                   +--------+--------+-------+---------+
                   | 6 bits | 2 bits | 1 bit | 55 bits |

                 Figure 14: SCHC Success ACK Message Format

   The SCHC Compound ACK message MUST be used in case SCHC Fragment
   losses are found in any window of the SCHC Packet (C=0).  The SCHC
   Compound ACK message format is shown in Figure 15.  The SCHC Compound
   ACK can report up to 4 windows with losses, as shown in Figure 16.

   When sent in the Downlink, the SCHC Compound ACK MUST be 0 padded
   (padding bits must be 0) to complement the 64 bits required by the
   Sigfox payload.

   |--- SCHC ACK Header ---|- W=w1 -|...|---- W=wi -----|
   +--------+------+-------+--------+...+------+--------+------+-------+
   | RuleID |W=b'w1| C=b'0 | Bitmap |...|W=b'wi| Bitmap | b'00 |b'0-pad|
   +--------+------+-------+--------+...+------+--------+------+-------+
   | 6 bits |2 bits| 1 bit | 12 bits|...|2 bits| 12 bits|2 bits|

                Figure 15: SCHC Compound ACK Message Format

   Losses are found in windows W = w1,...,wi, where w1 < w2 <...< wi.

            |- SCHC ACK Header -|- W=0 -|      |- W=1 -|...
            +------+------+-----+-------+------+-------+...
            |RuleID|W=b'00|C=b'0|Bitmap |W=b'01|Bitmap |...
            +------+------+-----+-------+------+-------+...
            |6 bits|2 bits|1 bit|12 bits|2 bits|12 bits|...

                        ...       |- W=2 -|      |- W=3 -|
                        ...+------+-------+------+-------+---+
                        ...|W=b'10|Bitmap |W=b'11|Bitmap |b'0|
                        ...+------+-------+------+-------+---+
                        ...|2 bits|12 bits|2 bits|12 bits|

      Figure 16: SCHC Compound ACK Message Format Example with Losses
                               in All Windows

   Losses are found in windows W = w1,...,wi, where w1 < w2 <...< wi.

3.6.3.4.  SCHC Sender-Abort Message Format

                      |---- Sender-Abort Header ----|
                      +-----------------------------+
                      | RuleID |   W    | FCN=ALL-1 |
                      +--------+--------+-----------+
                      | 6 bits | 2 bits |  4 bits   |

                Figure 17: SCHC Sender-Abort Message Format

3.6.3.5.  SCHC Receiver-Abort Message Format

      |- Receiver-Abort Header -|
      +---------------------------------+-----------------+---------+
      | RuleID | W=b'11 | C=b'1 |  0x7F |  0xFF (all 1's) | b'0-pad |
      +--------+--------+-------+-------+-----------------+---------+
      | 6 bits | 2 bits | 1 bit | 7 bit |  8 bit          | 40 bits |
                next L2 Word boundary ->| <-- L2 Word --> |

               Figure 18: SCHC Receiver-Abort Message Format

3.6.4.  Uplink ACK-on-Error Mode: Two-Byte SCHC Header Option 2

3.6.4.1.  Regular SCHC Fragment

   Figure 19 shows an example of a Regular SCHC Fragment for all
   fragments except the last one.  The penultimate tile of a SCHC Packet
   is of the regular size.

                  |-- SCHC Fragment Header --|
                  +--------------------------+---------+
                  | RuleID |   W    | FCN    | Payload |
                  +--------+--------+--------+---------+
                  | 8 bits | 3 bits | 5 bits | 80 bits |

      Figure 19: Regular SCHC Fragment Format for All Fragments except
                                the Last One

   The SCHC ACK REQ MUST NOT be used, instead the All-1 SCHC Fragment
   MUST be used to request a SCHC ACK from the receiver (Network SCHC).
   As per [RFC8724], the All-0 message is distinguishable from the SCHC
   ACK REQ (All-1 message).

3.6.4.2.  All-1 SCHC Fragment

   Figure 20 shows an example of the All-1 message.  The All-1 message
   MAY contain the last tile of the SCHC Packet.

   The All-1 message Fragment Header contains an RCS of 5 bits and 3
   padding bits to complete a 3-byte Fragment Header.  The size of the
   last tile, if present, ranges from 8 to 72 bits.

     |-------------- SCHC Fragment Header -----------|
     +-----------------------------------------------+--------------+
     | RuleID |    W   | FCN=ALL-1 |  RCS   | b'000  |    Payload   |
     +--------+--------+-----------+--------+--------+--------------+
     | 8 bits | 3 bits |  5 bits   | 5 bits | 3 bits | 8 to 72 bits |

          Figure 20: All-1 SCHC Message Format with the Last Tile

   As per [RFC8724], the All-1 must be distinguishable from the SCHC
   Sender-Abort message (with same RuleID, M, and N values).  The SCHC
   Sender-Abort message header size is 2 bytes with no padding bits.

   For the All-1 message to be distinguishable from the Sender-Abort
   message, the Sender-Abort message MUST be 2 bytes (only header with
   no padding).  This way, the minimum size of the All-1 is 3 bytes, and
   the Sender-Abort message is 2 bytes.

3.6.4.3.  SCHC ACK Format

   Figure 21 shows the SCHC ACK format when all fragments have been
   correctly received (C=1).  Padding MUST be added to complete the
   64-bit Sigfox Downlink frame payload size.

                   |---- SCHC ACK Header ----|
                   +-------------------------+---------+
                   | RuleID |    W   | C=b'1 | b'0-pad |
                   +--------+--------+-------+---------+
                   | 8 bits | 3 bits | 1 bit | 52 bits |

                 Figure 21: SCHC Success ACK Message Format

   The SCHC Compound ACK message MUST be used in case SCHC Fragment
   losses are found in any window of the SCHC Packet (C=0).  The SCHC
   Compound ACK message format is shown in Figure 22.  The SCHC Compound
   ACK can report up to 3 windows with losses.

   When sent in the Downlink, the SCHC Compound ACK MUST be 0 padded
   (padding bits must be 0) to complement the 64 bits required by the
   Sigfox payload.

    |-- SCHC ACK Header --|- W=w1 -|...|---- W=wi -----|
    +------+------+-------+--------+...+------+--------+------+-------+
    |RuleID|W=b'w1| C=b'0 | Bitmap |...|W=b'wi| Bitmap | 000  |b'0-pad|
    +------+------+-------+--------+...+------+--------+------+-------+
    |8 bits|3 bits| 1 bit | 31 bits|...|3 bits| 31 bits|3 bits|

                Figure 22: SCHC Compound ACK Message Format

   Losses are found in windows W = w1,...,wi, where w1 < w2 <...< wi.

3.6.4.4.  SCHC Sender-Abort Message Format

                      |---- Sender-Abort Header ----|
                      +-----------------------------+
                      | RuleID |   W    | FCN=ALL-1 |
                      +--------+--------+-----------+
                      | 8 bits | 3 bits |  5 bits   |

                Figure 23: SCHC Sender-Abort Message Format

3.6.4.5.  SCHC Receiver-Abort Message Format

     |-- Receiver-Abort Header -|
     +-----------------------------------+-----------------+---------+
     | RuleID | W=b'111 | C=b'1 | b'1111 |  0xFF (all 1's) | b'0-pad |
     +--------+---------+-------+--------+-----------------+---------+
     | 8 bits |  3 bits | 1 bit | 4 bit  |  8 bit          | 40 bits |
                 next L2 Word boundary ->| <-- L2 Word --> |

               Figure 24: SCHC Receiver-Abort Message Format

3.6.5.  Downlink ACK-Always Mode: Single-Byte SCHC Header

3.6.5.1.  Regular SCHC Fragment

   Figure 25 shows an example of a Regular SCHC Fragment for all
   fragments except the last one.  The penultimate tile of a SCHC Packet
   is of the regular size.

                          SCHC Fragment
                       |--    Header   --|
                       +-----------------+---------+
                       | RuleID |  FCN   | Payload |
                       +--------+--------+---------+
                       | 3 bits | 5 bits | 56 bits |

      Figure 25: Regular SCHC Fragment Format for All Fragments except
                                the Last One

   The SCHC ACK MUST NOT be used, instead the All-1 SCHC Fragment MUST
   be used to request a SCHC ACK from the receiver.  As per [RFC8724],
   the All-0 message is distinguishable from the SCHC ACK REQ (All-1
   message).

3.6.5.2.  All-1 SCHC Fragment

   Figure 26 shows an example of the All-1 message.  The All-1 message
   MAY contain the last tile of the SCHC Packet.

   The All-1 message Fragment Header contains an RCS of 5 bits and 3
   padding bits to complete a 2-byte Fragment Header.  The size of the
   last tile, if present, ranges from 8 to 48 bits.

          |--------- SCHC Fragment Header -------|
          +--------------------------------------+--------------+
          | RuleID | FCN=ALL-1 |  RCS   | b'000  |    Payload   |
          +--------+-----------+--------+--------+--------------+
          | 3 bits |  5 bits   | 5 bits | 3 bits | 0 to 48 bits |

          Figure 26: All-1 SCHC Message Format with the Last Tile

   As per [RFC8724], the All-1 must be distinguishable from the SCHC
   Sender-Abort message (with same RuleID and N values).  The SCHC
   Sender-Abort message header size is 1 byte with no padding bits.

   For the All-1 message to be distinguishable from the Sender-Abort
   message, the Sender-Abort message MUST be 1 byte (only header with no
   padding).  This way, the minimum size of the All-1 is 2 bytes, and
   the Sender-Abort message is 1 bytes.

3.6.5.3.  SCHC ACK Format

   Figure 27 shows the SCHC ACK format when all fragments have been
   correctly received (C=1).  Padding MUST be added to complete 2 bytes.

                            SCHC ACK
                       |--   Header   --|
                       +----------------+---------+
                       | RuleID | C=b'1 | b'0-pad |
                       +--------+-------+---------+
                       | 3 bits | 1 bit |  4 bits |

                 Figure 27: SCHC Success ACK Message Format

   The SCHC ACK message format is shown in Figure 28.

                   |---- SCHC ACK Header ----|
                   +--------+-------+--------+---------+
                   | RuleID | C=b'0 | Bitmap | b'0-pad |
                   +--------+-------+--------+---------+
                   | 3 bits | 1 bit | 31 bits|  5 bits |

                Figure 28: SCHC Compound ACK Message Format

3.6.5.4.  SCHC Sender-Abort Message Format

                               Sender-Abort
                          |----   Header   ----|
                          +--------------------+
                          | RuleID | FCN=ALL-1 |
                          +--------+-----------+
                          | 3 bits |  5 bits   |

                Figure 29: SCHC Sender-Abort Message Format

3.6.5.5.  SCHC Receiver-Abort Message Format

                 Receiver-Abort
               |---  Header  ---|
               +----------------+--------+-----------------+
               | RuleID | C=b'1 | b'1111 |  0xFF (all 1's) |
               +--------+-------+--------+-----------------+
               | 3 bits | 1 bit | 4 bit  |  8 bit          |

               Figure 30: SCHC Receiver-Abort Message Format

3.7.  Padding

   The Sigfox payload fields have different characteristics in Uplink
   and Downlink.

   Uplink messages can contain a payload size from 0 to 12 bytes.  The
   Sigfox radio protocol allows sending zero bits, one single bit of
   information for binary applications (e.g., status), or an integer
   number of bytes.  Therefore, for 2 or more bits of payload, it is
   required to add padding to the next integer number of bytes.  The
   reason for this flexibility is to optimize transmission time and
   hence save battery consumption at the device.

   On the other hand, Downlink frames have a fixed length.  The payload
   length MUST be 64 bits (i.e., 8 bytes).  Hence, if less information
   bits are to be transmitted, padding MUST be used with bits equal to
   0.  The receiver MUST remove the added padding bits before the SCHC
   reassembly process.

4.  Fragmentation Rules Examples

   This section provides an example of RuleID configuration for
   interoperability between the F/R modes presented in this document.
   Note that the RuleID space for Uplink F/R is different than the one
   for Downlink F/R; therefore, this section is divided in two
   subsections: Rules for Uplink fragmentation and Rules for Downlink
   fragmentation.

   For Uplink F/R, multiple header lengths were described in
   Section 3.5.  All of them are part of the SCHC over Sigfox Profile
   and offer not only low protocol overhead for small payloads (single
   byte header) but also extensibility to transport larger payloads with
   more overhead (2-byte header, Options 1 and 2).  The usage of the
   RuleID space for each header length is an implementation choice, but
   we provide an example of it in the following section.  This
   illustrates implementation choices made in order to 1) identify the
   different header length and 2) finally parse the RuleID field to
   identify the RuleID value and execute the associated treatment.

4.1.  Uplink Fragmentation Rules Examples

   The RuleID field for Uplink F/R modes has different sizes depending
   on the header length.  In order to identify the header length and
   then the value of the RuleID, the RuleID field is interpreted as
   follows:

   *  The RuleID field is the first one to be parsed in the SCHC header,
      starting from the leftmost bits.

   *  For Single-byte SCHC Header F/R modes, a RuleID field of 3 bits is
      expected:

      -  If the first 3 leftmost bits have a value different than
         0b'111, then it signals a Single-byte SCHC Header F/R mode.

      -  If their value is 0b'111, then it signals a Two-byte SCHC
         Header F/R mode.

   *  For Single-byte SCHC Header F/R modes:

      -  There are 7 RuleIDs available (with values from 0b'000-0b'110);
         the RuleID with value 0b'111 is reserved to indicate a Two-byte
         SCHC Header.

      -  This set of Rules is called "standard rules", and it is used to
         implement Single-byte SCHC Header modes.

      -  Each RuleID is associated with a set of properties defining if
         Uplink F/R is used and which Uplink F/R mode is used.  As an
         example, the RuleID 0b'000 is mapped onto Uplink No-ACK Mode:
         Single-byte SCHC Header, and the RuleIDs 0b'001 and 0b'002 are
         mapped onto Uplink ACK-on-Error mode: Single-byte SCHC Header
         (2 RuleIDs to allow for SCHC Packet interleaving).

   *  For Two-byte SCHC Header F/R modes, at least 6 bits for the RuleID
      field are expected:

      -  The 3 first leftmost bits are always 0b'111.

         o  If the following 3 bits have a different value than 0b'111,
            then it signals the Two-byte SCHC Header Option 1.

         o  If the following 3 bits are 0b'111, then it signals the Two-
            byte SCHC Header Option 2.

      -  For the Two-byte SCHC Header Option 1, there are 7 RuleIDs
         available (0b'111000-0b'111110), 0b'111111 being reserved to
         indicate the Two-byte SCHC Header Option 2.  This set of Rules
         is called "extended rules", and it is used to implement the
         Uplink ACK-on-Error mode: Two-byte SCHC Header Option 1.

      -  For the Two-byte SCHC Header Option 2, there are 2 additional
         bits to parse as the RuleID, so 4 RuleIDs are available
         (0b'11111100-0b'11111111).  This set of Rules is used to cover
         specific cases that previous RuleIDs do not cover.  As an
         example, RuleID 0b'00111111 is used to transport uncompressed
         IPv6 packets using the Uplink ACK-on-Error mode: Two-byte SCHC
         Header Option 2.

4.2.  Downlink Fragmentation Rules Example

   For the Downlink ACK-Always Mode: Single-byte SCHC Header, RuleIDs
   can get values in ranges from 0b'000 to 0b'111.

5.  Fragmentation Sequence Examples

   In this section, some sequence diagrams depict message exchanges for
   different fragmentation modes and use cases are shown.  In the
   examples, 'Seq' indicates the Sigfox Sequence Number of the frame
   carrying a fragment.

5.1.  Uplink No-ACK Examples

   The FCN field indicates the size of the data packet.  The first
   fragment is marked with FCN = X-1, where X is the number of fragments
   the message is split into.  All fragments are marked with decreasing
   FCN values.  The last packet fragment is marked with FCN = All-1
   (1111).

   *Case No Losses - All fragments are sent and received successfully.*

          Sender                     Receiver
            |-------FCN=6,Seq=1-------->|
            |-------FCN=5,Seq=2-------->|
            |-------FCN=4,Seq=3-------->|
            |-------FCN=3,Seq=4-------->|
            |-------FCN=2,Seq=5-------->|
            |-------FCN=1,Seq=6-------->|
            |-------FCN=15,Seq=7------->| All fragments received
          (End)

                     Figure 31: Uplink No-ACK No-Losses

   When the first SCHC Fragment is received, the receiver can calculate
   the total number of SCHC Fragments that the SCHC Packet is composed
   of.  For example, if the first fragment is numbered with FCN=6, the
   receiver can expect six more messages/fragments (i.e., with FCN going
   from 5 downwards and the last fragment with an FCN equal to 15).

   *Case Losses on Any Fragment except the First*

   Sender                     Receiver
     |-------FCN=6,Seq=1-------->|
     |-------FCN=5,Seq=2----X    |
     |-------FCN=4,Seq=3-------->|
     |-------FCN=3,Seq=4-------->|
     |-------FCN=2,Seq=5-------->|
     |-------FCN=1,Seq=6-------->|
     |-------FCN=15,Seq=7------->| Missing Fragment Unable to reassemble
   (End)

                Figure 32: Uplink No-ACK Losses (Scenario 1)

5.2.  Uplink ACK-on-Error Examples: Single-Byte SCHC Header

   The Single-byte SCHC Header ACK-on-Error mode allows sending up to 28
   fragments and packet sizes up to 300 bytes.  The SCHC Fragments may
   be delivered asynchronously, and Downlink ACK can be sent
   opportunistically.

   *Case No Losses*

   The Downlink flag must be enabled in the sender Uplink message to
   allow a Downlink message from the receiver.  The Downlink Enable in
   the figures shows where the sender MUST enable the Downlink and wait
   for an ACK.

               Sender                    Receiver
                 |-----W=0,FCN=6,Seq=1----->|
                 |-----W=0,FCN=5,Seq=2----->|
                 |-----W=0,FCN=4,Seq=3----->|
                 |-----W=0,FCN=3,Seq=4----->|
                 |-----W=0,FCN=2,Seq=5----->|
                 |-----W=0,FCN=1,Seq=6----->|
       DL Enable |-----W=0,FCN=0,Seq=7----->|
             (no ACK)
                 |-----W=1,FCN=6,Seq=8----->|
                 |-----W=1,FCN=5,Seq=9----->|
                 |-----W=1,FCN=4,Seq=10---->|
       DL Enable |-----W=1,FCN=7,Seq=11---->| All fragments received
                 |<- Compound ACK,W=1,C=1 --| C=1
               (End)

                  Figure 33: Uplink ACK-on-Error No-Losses

   *Case Fragment Losses in the First Window*

   In this case, fragments are lost in the first window (W=0).  After
   the first All-0 message arrives, the receiver leverages the
   opportunity and sends a SCHC ACK with the corresponding bitmap and
   C=0.

   After the loss fragments from the first window (W=0) are resent, the
   sender continues transmitting the fragments of the following window
   (W=1) without opening a reception opportunity.  Finally, the All-1
   fragment is sent, the Downlink is enabled, and the SCHC ACK is
   received with C=1.  Note that the SCHC Compound ACK also uses a
   Sequence Number.

          Sender                    Receiver
            |-----W=0,FCN=6,Seq=1----->|
            |-----W=0,FCN=5,Seq=2--X   |
            |-----W=0,FCN=4,Seq=3----->|
            |-----W=0,FCN=3,Seq=4----->|
            |-----W=0,FCN=2,Seq=5--X   |                    __
            |-----W=0,FCN=1,Seq=6----->|                   | W=0
  DL Enable |-----W=0,FCN=0,Seq=7----->| Missing Fragments<- FCN=5,Seq=2
            |<- Compound ACK,W=0,C=0 --| Bitmap:1011011    | FCN=2,Seq=5
            |-----W=0,FCN=5,Seq=9----->|                    --
            |-----W=0,FCN=2,Seq=10---->|
            |-----W=1,FCN=6,Seq=11---->|
            |-----W=1,FCN=5,Seq=12---->|
            |-----W=1,FCN=4,Seq=13---->|
  DL Enable |-----W=1,FCN=7,Seq=14---->| All fragments received
            |<-Compound ACK,W=1,C=1 ---| C=1
          (End)

        Figure 34: Uplink ACK-on-Error Losses in the First Window

   *Case Fragment All-0 Lost in the First Window (W=0)*

   In this example, the All-0 of the first window (W=0) is lost.
   Therefore, the receiver waits for the next All-0 message of
   intermediate windows or All-1 message of last window to generate the
   corresponding SCHC ACK, which indicates that the All-0 of window 0 is
   absent.

   The sender resends the missing All-0 messages (with any other missing
   fragment from window 0) without opening a reception opportunity.

           Sender                    Receiver
             |-----W=0,FCN=6,Seq=1----->|
             |-----W=0,FCN=5,Seq=2----->|
             |-----W=0,FCN=4,Seq=3----->|
             |-----W=0,FCN=3,Seq=4----->|
             |-----W=0,FCN=2,Seq=5----->|
             |-----W=0,FCN=1,Seq=6----->| DL Enable
             |-----W=0,FCN=0,Seq=7--X   |
         (no ACK)
             |-----W=1,FCN=6,Seq=8----->|
             |-----W=1,FCN=5,Seq=9----->|                    __
             |-----W=1,FCN=4,Seq=10---->|                   |W=0
   DL Enable |-----W=1,FCN=7,Seq=11---->| Missing Fragment<- FCN=0,Seq=7
             |<-Compound ACK,W=0,C=0 ---| Bitmap:1111110    |__
             |-----W=0,FCN=0,Seq=13---->| All fragments received
   DL Enable |-----W=1,FCN=7,Seq=14---->|
             |<-Compound ACK,W=1,C=1 ---| C=1
           (End)

       Figure 35: Uplink ACK-on-Error All-0 Lost in the First Window

   In the following diagram, besides the All-0, there are other fragment
   losses in the first window (W=0).

           Sender                    Receiver
             |-----W=0,FCN=6,Seq=1----->|
             |-----W=0,FCN=5,Seq=2--X   |
             |-----W=0,FCN=4,Seq=3----->|
             |-----W=0,FCN=3,Seq=4--X   |
             |-----W=0,FCN=2,Seq=5----->|
             |-----W=0,FCN=1,Seq=6----->|
   DL Enable |-----W=0,FCN=0,Seq=7--X   |
         (no ACK)
             |-----W=1,FCN=6,Seq=8----->|
             |-----W=1,FCN=5,Seq=9----->|                    __
             |-----W=1,FCN=4,Seq=10---->|                   |W=0
   DL Enable |-----W=1,FCN=7,Seq=11---->| Missing Fragment<- FCN=5,Seq=2
             |<--Compound ACK,W=0,C=0 --| Bitmap:1010110    |FCN=3,Seq=4
             |-----W=0,FCN=5,Seq=13---->|                   |FCN=0,Seq=7
             |-----W=0,FCN=3,Seq=14---->|                    --
             |-----W=0,FCN=0,Seq=15---->| All fragments received
   DL Enable |-----W=1,FCN=7,Seq=16---->|
             |<-Compound ACK,W=1,C=1 ---| C=1
           (End)

      Figure 36: Uplink ACK-on-Error All-0 and Other Fragments Lost in
                              the First Window

   In the next examples, there are fragment losses in both the first
   (W=0) and second (W=1) windows.  The retransmission cycles after the
   All-1 is sent (i.e., not in intermediate windows) MUST always finish
   with an All-1, as it serves as an ACK Request message to confirm the
   correct reception of the retransmitted fragments.

          Sender                    Receiver
            |-----W=0,FCN=6,Seq=1----->|
            |-----W=0,FCN=5,Seq=2--X   |
            |-----W=0,FCN=4,Seq=3----->|
            |-----W=0,FCN=3,Seq=4--X   |                    __
            |-----W=0,FCN=2,Seq=5----->|                   |W=0
            |-----W=0,FCN=1,Seq=6----->|                   |FCN=5,Seq=2
  DL Enable |-----W=0,FCN=0,Seq=7--X   |                   |FCN=3,Seq=4
       (no ACK)                                            |FCN=0,Seq=7
            |-----W=1,FCN=6,Seq=8--X   |                   |W=1
            |-----W=1,FCN=5,Seq=9----->|                   |FCN=6,Seq=8
            |-----W=1,FCN=4,Seq=10-X   |                   |FCN=4,Seq=10
  DL Enable |-----W=1,FCN=7,Seq=11---->| Missing Fragment<-|__
            |<-Compound ACK,W=0,1,C=0--| Bitmap W=0:1010110
            |-----W=0,FCN=5,Seq=13---->|        W=1:0100001
            |-----W=0,FCN=3,Seq=14---->|
            |-----W=0,FCN=0,Seq=15---->|
            |-----W=1,FCN=6,Seq=16---->|
            |-----W=1,FCN=4,Seq=17---->| All fragments received
  DL Enable |-----W=1,FCN=7,Seq=18---->|
            |<-Compound ACK,W=1,C=1----| C=1
          (End)

     Figure 37: Uplink ACK-on-Error All-0 and Other Fragments Lost in
                     the First and Second Windows (1)

   The figure below is a similar case as above but with fewer fragments
   in the second window (W=1).

          Sender                    Receiver
            |-----W=0,FCN=6,Seq=1----->|
            |-----W=0,FCN=5,Seq=2--X   |
            |-----W=0,FCN=4,Seq=3----->|
            |-----W=0,FCN=3,Seq=4--X   |
            |-----W=0,FCN=2,Seq=5----->|                     __
            |-----W=0,FCN=1,Seq=6----->|                    |W=0
  DL Enable |-----W=0,FCN=0,Seq=7--X   |                    |FCN=5,Seq=2
         (no ACK)                                           |FCN=3,Seq=4
            |-----W=1,FCN=6,Seq=8--X   |                    |FCN=0,Seq=7
  DL Enable |-----W=1,FCN=7,Seq=9----->| Missing Fragment--> W=1
            |<-Compound ACK,W=0,1, C=0-| Bitmap W=0:1010110,|FCN=6,Seq=8
            |-----W=0,FCN=5,Seq=11---->|        W=1:0000001 |__
            |-----W=0,FCN=3,Seq=12---->|
            |-----W=0,FCN=0,Seq=13---->|
            |-----W=1,FCN=6,Seq=14---->| All fragments received
  DL Enable |-----W=1,FCN=7,Seq=15---->|
            |<-Compound ACK, W=1,C=1---| C=1
          (End)

     Figure 38: Uplink ACK-on-Error All-0 and Other Fragments Lost in
                     the First and Second Windows (2)

   *Case SCHC ACK is Lost*

   SCHC over Sigfox does not implement the SCHC ACK REQ message.
   Instead, it uses the SCHC All-1 message to request a SCHC ACK when
   required.

              Sender                     Receiver
                 |-----W=0,FCN=6,Seq=1----->|
                 |-----W=0,FCN=5,Seq=2----->|
                 |-----W=0,FCN=4,Seq=3----->|
                 |-----W=0,FCN=3,Seq=4----->|
                 |-----W=0,FCN=2,Seq=5----->|
                 |-----W=0,FCN=1,Seq=6----->|
       DL Enable |-----W=0,FCN=0,Seq=7----->|
             (no ACK)
                 |-----W=1,FCN=6,Seq=8----->|
                 |-----W=1,FCN=5,Seq=9----->|
                 |-----W=1,FCN=4,Seq=10---->|
       DL Enable |-----W=1,FCN=7,Seq=11---->| All fragments received
                 | X--Compound ACK,W=1,C=1 -| C=1
       DL Enable |-----W=1,FCN=7,Seq=13---->| RESEND ACK
                 |<-Compound ACK,W=1,C=1 ---| C=1
               (End)

                  Figure 39: Uplink ACK-on-Error ACK Lost

   *Case SCHC Compound ACK at the End*

   In this example, SCHC Fragment losses are found in both windows 0 and
   1.  However, the sender does not send a SCHC Compound ACK after the
   All-0 of window 0.  Instead, it sends a SCHC Compound ACK indicating
   fragment losses on both windows.

          Sender                    Receiver
            |-----W=0,FCN=6,Seq=1----->|
            |-----W=0,FCN=5,Seq=2--X   |
            |-----W=0,FCN=4,Seq=3----->|
            |-----W=0,FCN=3,Seq=4--X   |
            |-----W=0,FCN=2,Seq=5----->|
            |-----W=0,FCN=1,Seq=6----->|                     __
  DL Enable |-----W=0,FCN=0,Seq=7----->| Waits for          |W=0
         (no ACK)                       next DL opportunity |FCN=5,Seq=2
            |-----W=1,FCN=6,Seq=8--X   |                    |FCN=3,Seq=4
  DL Enable |-----W=1,FCN=7,Seq=9----->| Missing Fragment<-- W=1
            |<-Compound ACK,W=0,1, C=0-| Bitmap W=0:1010110 |FCN=6,Seq=8
            |-----W=0,FCN=5,Seq=11---->|        W=1:0000001  --
            |-----W=0,FCN=3,Seq=12---->|
            |-----W=1,FCN=6,Seq=13---->| All fragments received
  DL Enable |-----W=1,FCN=7,Seq=14---->|
            |<-Compound ACK, W=1, C=1 -| C=1
          (End)

      Figure 40: Uplink ACK-on-Error Fragments Lost in the First and
                   Second Windows with One Compound ACK

   The number of times the same SCHC ACK message will be retransmitted
   is determined by the MAX_ACK_REQUESTS.

5.3.  SCHC Abort Examples

   *Case SCHC Sender-Abort*

   The sender may need to send a Sender-Abort to stop the current
   communication.  For example, this may happen if the All-1 has been
   sent MAX_ACK_REQUESTS times.

             Sender                    Receiver
               |-----W=0,FCN=6,Seq=1----->|
               |-----W=0,FCN=5,Seq=2----->|
               |-----W=0,FCN=4,Seq=3----->|
               |-----W=0,FCN=3,Seq=4----->|
               |-----W=0,FCN=2,Seq=5----->|
               |-----W=0,FCN=1,Seq=6----->|
     DL Enable |-----W=0,FCN=0,Seq=7----->|
           (no ACK)
               |-----W=1,FCN=6,Seq=8----->|
               |-----W=1,FCN=5,Seq=9----->|
               |-----W=1,FCN=4,Seq=10---->|
     DL Enable |-----W=1,FCN=7,Seq=11---->| All fragments received
               | X--Compound ACK,W=1,C=1 -| C=1
     DL Enable |-----W=1,FCN=7,Seq=13---->| RESEND ACK  (1)
               | X--Compound ACK,W=1,C=1 -| C=1
     DL Enable |-----W=1,FCN=7,Seq=15---->| RESEND ACK  (2)
               | X--Compound ACK,W=1,C=1 -| C=1
     DL Enable |-----W=1,FCN=7,Seq=17---->| RESEND ACK  (3)
               | X--Compound ACK,W=1,C=1 -| C=1
     DL Enable |-----W=1,FCN=7,Seq=18---->| RESEND ACK  (4)
               | X--Compound ACK,W=1,C=1 -| C=1
     DL Enable |-----W=1,FCN=7,Seq=19---->| RESEND ACK  (5)
               | X--Compound ACK,W=1,C=1 -| C=1
     DL Enable |----Sender-Abort,Seq=20-->| exit with error condition
             (End)

                Figure 41: Uplink ACK-on-Error Sender-Abort

   *Case Receiver-Abort*

   The receiver may need to send a Receiver-Abort to stop the current
   communication.  This message can only be sent after a Downlink
   Enable.

                  Sender                    Receiver
                    |-----W=0,FCN=6,Seq=1----->|
                    |-----W=0,FCN=5,Seq=2----->|
                    |-----W=0,FCN=4,Seq=3----->|
                    |-----W=0,FCN=3,Seq=4----->|
                    |-----W=0,FCN=2,Seq=5----->|
                    |-----W=0,FCN=1,Seq=6----->|
          DL Enable |-----W=0,FCN=0,Seq=7----->|
                    |<------  RECV ABORT ------| under-resourced
                 (Error)

               Figure 42: Uplink ACK-on-Error Receiver-Abort

6.  Security Considerations

   The radio protocol authenticates and ensures the integrity of each
   message.  This is achieved by using a unique Device ID and an AES-
   128-based message authentication code, ensuring that the message has
   been generated and sent by the device (see [sigfox-spec],
   Section 3.8) or Network (see [sigfox-spec], Section 4.3) with the ID
   claimed in the message [sigfox-spec].

   Application data may or may not be encrypted at the application
   layer, depending on the criticality of the use case.  This
   flexibility allows a balance between cost and effort versus risk.
   AES-128 in counter mode is used for encryption.  Cryptographic keys
   are independent for each device.  These keys are associated with the
   Device ID, and separate integrity and encryption keys are pre-
   provisioned.  An encryption key is only provisioned if
   confidentiality is to be used (see [sigfox-spec], Section 5.3; note
   that further documentation is available at Sigfox upon request).

   The radio protocol has protections against replay attacks, and the
   cloud-based core Network provides firewall protection against
   undesired incoming communications [sigfox-spec].

   The previously described security mechanisms do not guarantee end-to-
   end security between the device SCHC C/D + F/R and the Network SCHC
   C/D + F/R; potential security threats described in [RFC8724] are
   applicable to the profile specified in this document.

   In some circumstances, sending device location information is privacy
   sensitive.  The Device Geolocation parameter provided by the Network
   is optional; therefore, it can be omitted to protect this aspect of
   the device privacy.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  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>.

   [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.
              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>.

   [RFC9441]  Zúñiga, JC., Gomez, C., Aguilar, S., Toutain, L.,
              Céspedes, S., and D. Wistuba, "Static Context Header
              Compression (SCHC) Compound Acknowledgement (ACK)",
              RFC 9441, DOI 10.17487/RFC9441, July 2023,
              <https://www.rfc-editor.org/info/rfc9441>.

   [sigfox-spec]
              Sigfox, "Sigfox Device Radio Specifications",
              <https://build.sigfox.com/sigfox-device-radio-
              specifications>.

8.2.  Informative References

   [CORE-COMI]
              Veillette, M., Ed., van der Stok, P., Ed., Pelov, A.,
              Bierman, A., and C. Bormann, Ed., "CoAP Management
              Interface (CORECONF)", Work in Progress, Internet-Draft,
              draft-ietf-core-comi-12, 13 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              comi-12>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [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>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [sigfox-callbacks]
              Sigfox, "Sigfox Callbacks",
              <https://support.sigfox.com/docs/callbacks-documentation>.

   [sigfox-docs]
              Sigfox, "Sigfox Documentation",
              <https://support.sigfox.com/docs>.

Acknowledgements

   Carles Gomez has been funded in part by the Spanish Government
   through the TEC2016-79988-P grant and the PID2019-106808RA-I00 grant
   (funded by MCIN / AEI / 10.13039/501100011033) and by Secretaria
   d'Universitats i Recerca del Departament d'Empresa i Coneixement de
   la Generalitat de Catalunya through 2017 grant SGR 376 and 2021 grant
   SGR 00330.

   Sergio Aguilar has been funded by the ERDF and the Spanish Government
   through project TEC2016-79988-P and project PID2019-106808RA-I00,
   AEI/FEDER, EU (funded by MCIN / AEI / 10.13039/501100011033).

   Sandra Cespedes has been funded in part by the ANID Chile Project
   FONDECYT Regular 1201893 and Basal Project FB0008.

   Diego Wistuba has been funded by the ANID Chile Project FONDECYT
   Regular 1201893.

   The authors would like to thank Ana Minaburo, Clement Mannequin,
   Rafael Vidal, Julien Boite, Renaud Marty, and Antonis Platis for
   their useful comments and implementation design considerations.

Authors' Addresses

   Juan Carlos Zúñiga
   Montreal QC
   Canada
   Email: j.c.zuniga@ieee.org


   Carles Gomez
   Universitat Politècnica de Catalunya
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain
   Email: carles.gomez@upc.edu


   Sergio Aguilar
   Universitat Politècnica de Catalunya
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain
   Email: sergio.aguilar.romero@upc.edu


   Laurent Toutain
   IMT-Atlantique
   CS 17607
   2 rue de la Chataigneraie
   35576 Cesson-Sevigne Cedex
   France
   Email: Laurent.Toutain@imt-atlantique.fr


   Sandra Céspedes
   Concordia University
   1455 De Maisonneuve Blvd. W.
   Montreal QC H3G 1M8
   Canada
   Email: sandra.cespedes@concordia.ca


   Diego Wistuba
   NIC Labs, Universidad de Chile
   Av. Almte. Blanco Encalada 1975
   Santiago
   Chile
   Email: research@witu.cl


   Julien Boite
   Unabiz (Sigfox)
   Labege
   France
   Email: juboite@free.fr
   URI:   https://www.sigfox.com/