Internet Engineering Task Force (IETF) JC. Zúñiga

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
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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: [email protected]

Carles Gomez
Universitat Politècnica de Catalunya
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: [email protected]

Sergio Aguilar
Universitat Politècnica de Catalunya
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: [email protected]

Laurent Toutain
IMT-Atlantique
CS 17607
2 rue de la Chataigneraie
35576 Cesson-Sevigne Cedex
France
Email: [email protected]

Sandra Céspedes
Concordia University
1455 De Maisonneuve Blvd. W.
Montreal QC H3G 1M8
Canada
Email: [email protected]

Diego Wistuba
NIC Labs, Universidad de Chile
Av. Almte. Blanco Encalada 1975
Santiago
Chile
Email: [email protected]

Julien Boite
Unabiz (Sigfox)
Labege
France
Email: [email protected]
URI: https://www.sigfox.com/