lpwan Working Group JC. Zuniga Internet-Draft Intended status: Standards Track C. Gomez Expires: 2 February 2023 S. Aguilar Universitat Politecnica de Catalunya L. Toutain IMT-Atlantique S. Cespedes Concordia University D. Wistuba NIC Labs, Universidad de Chile J. Boite SIGFOX 1 August 2022 SCHC over Sigfox LPWAN draft-ietf-lpwan-schc-over-sigfox-12 Abstract The Generic Framework for Static Context Header Compression and Fragmentation (SCHC) specification describes two mechanisms: i) an application header compression scheme, and ii) a frame fragmentation and loss recovery functionality. SCHC offers a great level of flexibility that can be tailored for different Low Power Wide Area Network (LPWAN) technologies. The present document provides the optimal parameters and modes of operation when SCHC is implemented over a Sigfox LPWAN. This set of parameters are also known as a "SCHC over Sigfox profile." Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 2 February 2023. Zuniga, et al. Expires 2 February 2023 [Page 1] Internet-Draft SCHC over Sigfox LPWAN August 2022 Copyright Notice Copyright (c) 2022 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. SCHC over Sigfox . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Network Architecture . . . . . . . . . . . . . . . . . . 4 3.2. Uplink . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.3. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 6 3.4. SCHC-ACK on Downlink . . . . . . . . . . . . . . . . . . 7 3.5. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 7 3.6. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 8 3.6.1. Uplink Fragmentation . . . . . . . . . . . . . . . . 8 3.6.2. Downlink Fragmentation . . . . . . . . . . . . . . . 12 3.7. SCHC-over-Sigfox F/R Message Formats . . . . . . . . . . 14 3.7.1. Uplink No-ACK Mode: Single-byte SCHC Header . . . . . 14 3.7.2. Uplink ACK-on-Error Mode: Single-byte SCHC Header . . 15 3.7.3. Uplink ACK-on-Error Mode: Two-byte SCHC Header Option 1 . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.7.4. Uplink ACK-on-Error Mode: Two-byte SCHC Header Option 2 . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.7.5. Downlink ACK-Always Mode: Single-byte SCHC Header . . 22 3.8. SCHC-Sender Abort . . . . . . . . . . . . . . . . . . . . 24 3.9. SCHC-Receiver Abort . . . . . . . . . . . . . . . . . . . 24 3.10. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 24 4. Fragmentation Sequence Examples . . . . . . . . . . . . . . . 25 4.1. Uplink No-ACK Examples . . . . . . . . . . . . . . . . . 25 4.2. Uplink ACK-on-Error Examples: Single-byte SCHC Header . . 26 4.3. SCHC Abort Examples . . . . . . . . . . . . . . . . . . . 32 5. Security considerations . . . . . . . . . . . . . . . . . . . 33 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 8.1. Normative References . . . . . . . . . . . . . . . . . . 34 8.2. Informative References . . . . . . . . . . . . . . . . . 35 Zuniga, et al. Expires 2 February 2023 [Page 2] Internet-Draft SCHC over Sigfox LPWAN August 2022 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35 1. Introduction The Generic Framework for Static Context Header Compression and Fragmentation (SCHC) specification [RFC8724] can be used on top of all the four LPWAN technologies defined in [RFC8376]. These LPWANs have similar characteristics such as star-oriented topologies, network architecture, connected devices with built-in applications, etc. SCHC offers a great level 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 on top of a specific LPWAN technology. This document describes the recommended parameters, settings, and modes of operation to be used when SCHC is implemented over a Sigfox LPWAN. This set of parameters are also known as a "SCHC over Sigfox profile" or simply "SCHC/Sigfox." 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 in [RFC8724]. 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). The way contexts are configured and stored on both ends is out of the scope of this document. Zuniga, et al. Expires 2 February 2023 [Page 3] Internet-Draft SCHC over Sigfox LPWAN August 2022 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 | . +~~ |Sigfox BS| |Sigfox Network| | SCHC | . | (RGW) | === | (NGW) | === |C/D & F/R|..... +---------+ +--------------+ +---------+ Legend: $, ~ : Radio link = : Internal Sigfox Network . : External 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, providing hence a global single star network topology. The Sigfox Device sends application packets that are compressed and/ or fragmented by a SCHC Compressor/Decompressor (SCHC C/D + F/R) to reduce headers 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 Network Gateway (NGW). The NGW then delivers the SCHC Message and associated gathered metadata to the Network SCHC C/D + F/R. Zuniga, et al. Expires 2 February 2023 [Page 4] Internet-Draft SCHC over Sigfox LPWAN August 2022 The Sigfox Network (NGW) 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 Dev 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). 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 space diversity, as potentially an Uplink message will be received by several base stations. 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 network (NGW) 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) * RSSI (optional) * Device Temperature (optional) * Device Battery Voltage (optional) Zuniga, et al. Expires 2 February 2023 [Page 5] Internet-Draft SCHC over Sigfox LPWAN August 2022 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. 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. +---------------+-----------------+ | Sigfox Header | Sigfox payload | +---------------+---------------- + | SCHC message | +-----------------+ Figure 2: SCHC Message in Sigfox 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). 3.3. Downlink Downlink transmissions are Device-driven and can only take place following an Uplink communication that so indicates. Hence, a Sigfox Device explicitly indicates its intention to receive a Downlink message 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, stand-by, etc.) Zuniga, et al. Expires 2 February 2023 [Page 6] Internet-Draft SCHC over Sigfox LPWAN August 2022 When a Downlink message is sent to a Device, a reception acknowledgement is generated by the Device and 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]. Downlink request flag can be included in the information exchange between the Sigfox Network and Network SCHC. 3.4. SCHC-ACK on Downlink As explained previously, Downlink transmissions are Device-driven and can only take place following a specific Uplink transmission that indicates and allows a following Downlink opportunity. For this reason, when SCHC bi-directional 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-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 (i.e. FCN = All-0, or FCN = All-1). The Device sends the fragments in sequence and, after transmitting the 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 in case 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 after a timeout the Uplink transmissions continue. Intermediate SCHC fragments with FCN different from All-0 or All-1 MUST NOT use the Downlink request flag to request a SCHC-ACK. 3.5. SCHC Rules The RuleID MUST be included in the SCHC header. The total number of rules to be used affects directly the Rule ID 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. 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. Zuniga, et al. Expires 2 February 2023 [Page 7] Internet-Draft SCHC over Sigfox LPWAN August 2022 3.6. 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 send reliably multiple messages, by allowing to resend selectively 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 receive end. Since only Uplink messages/fragments that have passed the Sigfox CRC-check are delivered to the Network 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 3.6.1.3 is RECOMMENDED. The L2 Word Size used by Sigfox is 1 byte (8 bits). 3.6.1. 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. Data packets are self-contained (aka "message in a bottle") with all the required information for the network to process them accordingly. Hence, there is no need to perform any network attachment, synchronization, or other procedure 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 0-12 bytes payload. Hence, a single SCHC Tile size per fragmentation mode can be defined so that every Sigfox message always carries one SCHC Tile. Zuniga, et al. Expires 2 February 2023 [Page 8] Internet-Draft SCHC over Sigfox LPWAN August 2022 When the ACK-on-Error mode is used for Uplink fragmentation, the SCHC Compound ACK defined in [I-D.ietf-lpwan-schc-compound-ack]) MUST be used in the Downlink responses. 3.6.1.1. Single-byte SCHC Header for Uplink Fragmentation 3.6.1.1.1. Uplink No-ACK Mode: Single-byte SCHC Header Single-byte SCHC Header No-ACK mode is RECOMMENDED to 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. Since there are no multiple windows in the No-ACK mode, the W bit is not present. However, it is RECOMMENDED to use the FCN field to indicate the size of the data packet. In this sense, the data packet would need to be splitted 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 indicating implicitly the size of the expected packet to the Network and hence detect at the receiver side whether all fragments have been received or not. The RECOMMENDED Fragmentation Header size is 8 bits, and it is composed as follows: * RuleID size: 3 bits * DTag size (T): 0 bits * Fragment Compressed Number (FCN) size (N): 5 bits * 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 RECOMMENDED value is 12 hours. * RCS size: 5 bits The maximum SCHC Packet size is of 340 bytes. Zuniga, et al. Expires 2 February 2023 [Page 9] Internet-Draft SCHC over Sigfox LPWAN August 2022 3.6.1.1.2. Uplink ACK-on-Error Mode: Single-byte SCHC Header ACK-on-Error with single-byte header is RECOMMENDED for short to medium size packets that need to be sent reliably. ACK-on-Error is optimal for 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. Allowing transmission of packets/files up to 300 bytes long, the SCHC Uplink Fragmentation Header size is RECOMMENDED to be 8 bits in size and is composed as follows: * Rule ID size: 3 bits * DTag size (T): 0 bits * Window index (W) size (M): 2 bits * Fragment Compressed Number (FCN) size (N): 3 bits * MAX_ACK_REQUESTS: 5 * WINDOW_SIZE: 7 (with a maximum value of FCN=0b110) * Regular tile size: 11 bytes * All-1 tile size: 0 to 10 bytes * Retransmission Timer: Application-dependent. The RECOMMENDED value is 12 hours. * Inactivity Timer: Application-dependent. The RECOMMENDED value is 12 hours. * RCS size: 3 bits 3.6.1.2. Two-byte SCHC Header for Uplink Fragmentation ACK-on-Error with two-byte header is RECOMMENDED for medium-large size packets that need to be sent reliably. ACK-on-Error is optimal for 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. Zuniga, et al. Expires 2 February 2023 [Page 10] Internet-Draft SCHC over Sigfox LPWAN August 2022 3.6.1.2.1. Uplink ACK-on-Error Mode: Two-byte SCHC Header Option 1 In order to allow transmission of medium-large packets/files up to 480 bytes long, the SCHC Uplink Fragmentation Header size is RECOMMENDED to be 16 bits in size and composed as follows: * Rule ID size is: 6 bits * DTag size (T) is: 0 bits * Window index (W) size (M): 2 bits * Fragment Compressed Number (FCN) size (N): 4 bits. * 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 RECOMMENDED value is 12 hours. * Inactivity Timer: Application-dependent. The RECOMMENDED value is 12 hours. * RCS size: 4 bits Note that WINDOW_SIZE is limited to 12. This because, 4 windows (M = 2) with bitmaps of size 12 can be fitted in a single SCHC Compound ACK. 3.6.1.2.2. 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 RECOMMENDED to be 16 bits in size and composed as follows: * Rule ID size is: 8 bits * DTag size (T) is: 0 bits * Window index (W) size (M): 3 bits * Fragment Compressed Number (FCN) size (N): 5 bits. Zuniga, et al. Expires 2 February 2023 [Page 11] Internet-Draft SCHC over Sigfox LPWAN August 2022 * 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 RECOMMENDED value is 12 hours. * Inactivity Timer: Application-dependent. The RECOMMENDED value is 12 hours. * RCS size: 5 bits 3.6.1.3. All-1 and RCS behaviour 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 comprised of less 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 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. 3.6.2. 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 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 Zuniga, et al. Expires 2 February 2023 [Page 12] Internet-Draft SCHC over Sigfox LPWAN August 2022 fragment that is not the last one. Such 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. It is RECOMMENDED for the fragment receiver to send an Uplink transmission (e.g., empty message) and request a Downlink every 24 hours when no SCHC session is started. The use or not of this Uplink transmission will depend on application specific requirements. Sigfox Downlink messages are fixed in size, and as described in [RFC8376] they can carry up to 8 bytes payload. 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 is RECOMMENDED. The SCHC Downlink Fragmentation Header size is RECOMMENDED to be 8 bits in size and is composed as follows: * RuleID size: 3 bits * DTag size (T): 0 bits * Window index (W) size (M) is: 0 bits * Fragment Compressed Number (FCN) size (N): 5 bits * 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 RECOMMENDED value is 12 hours. * Inactivity Timer: Application-dependent. The RECOMMENDED value is 12 hours. * RCS size: 5 bits Zuniga, et al. Expires 2 February 2023 [Page 13] Internet-Draft SCHC over Sigfox LPWAN August 2022 3.7. 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 [I-D.ietf-lpwan-schc-compound-ack]), and SCHC Abort used in SCHC over Sigfox. 3.7.1. Uplink No-ACK Mode: Single-byte SCHC Header 3.7.1.1. Regular SCHC Fragment Figure 3 shows an example of a regular SCHC fragment for all fragments except the last one. As tiles are of 11 bytes, 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 3.7.1.2. 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 L2-word-multiple. The All-1 messages Fragment Header includes a 5-bit RCS, and 3 bits are added as padding to complete two 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 last tile As per [RFC8724] the All-1 must be distinguishable from a SCHC Sender-Abort message (with same Rule ID, and N values). The All-1 MAY have the last tile of the SCHC Packet. The SCHC Sender-Abort message header size is of 1 byte, with no padding bits. Zuniga, et al. Expires 2 February 2023 [Page 14] Internet-Draft SCHC over Sigfox LPWAN August 2022 For the All-1 message to be distinguishable from the Sender-Abort message, the Sender-Abort message MUST be of 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. 3.7.1.3. SCHC Sender-Abort Message format Sender-Abort |------ Header ------| +--------------------+ | RuleID | FCN=ALL-1 | +--------+-----------+ | 3 bits | 5 bits | Figure 5: SCHC Sender-Abort message format 3.7.2. Uplink ACK-on-Error Mode: Single-byte SCHC Header 3.7.2.1. Regular SCHC Fragment Figure 6 shows an example of a regular SCHC fragment for all fragments except the last one. As tiles are of 11 bytes, 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 use of SCHC ACK REQ is NOT RECOMMENDED, instead the All-1 SCHC Fragment SHOULD 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. 3.7.2.2. 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. Zuniga, et al. Expires 2 February 2023 [Page 15] Internet-Draft SCHC over Sigfox LPWAN August 2022 |------------- 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 last tile As per [RFC8724] the All-1 must be distinguishable from a SCHC Sender-Abort message (with same Rule ID, M, and N values). The All-1 MAY have the last tile of the SCHC Packet. The SCHC Sender-Abort message header size is of 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 of 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. 3.7.2.3. 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 [I-D.ietf-lpwan-schc-compound-ack] 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| Losses are found in windows W = w1,...,wi; where w1| <-- L2 Word --> | Figure 11: SCHC Receiver-Abort message format 3.7.3. Uplink ACK-on-Error Mode: Two-byte SCHC Header Option 1 3.7.3.1. 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 use of SCHC ACK REQ is NOT RECOMMENDED, instead the All-1 SCHC Fragment SHOULD 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). 3.7.3.2. 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. Zuniga, et al. Expires 2 February 2023 [Page 17] Internet-Draft SCHC over Sigfox LPWAN August 2022 The All-1 message Fragment Header contains a 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 last tile As per [RFC8724] the All-1 must be distinguishable from the a SCHC Sender-Abort message (with same Rule ID, M and N values). The All-1 MUST have the last tile of the SCHC Packet, that MUST be of at least 1 byte. The SCHC Sender-Abort message header size is of 2 byte, with no padding bits. For the All-1 message to be distinguishable from the Sender-Abort message, the Sender-Abort message MUST be of 2 byte (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. 3.7.3.3. 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. Zuniga, et al. Expires 2 February 2023 [Page 18] Internet-Draft SCHC over Sigfox LPWAN August 2022 |--- 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| Losses are found in windows W = w1,...,wi; where w1| <-- L2 Word --> | Figure 18: SCHC Receiver-Abort message format Zuniga, et al. Expires 2 February 2023 [Page 19] Internet-Draft SCHC over Sigfox LPWAN August 2022 3.7.4. Uplink ACK-on-Error Mode: Two-byte SCHC Header Option 2 3.7.4.1. 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 use of SCHC ACK REQ is NOT RECOMMENDED, instead the All-1 SCHC Fragment SHOULD 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). 3.7.4.2. 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 last tile As per [RFC8724] the All-1 must be distinguishable from the a SCHC Sender-Abort message (with same Rule ID, M and N values). The SCHC Sender-Abort message header size is of 2 byte, with no padding bits. For the All-1 message to be distinguishable from the Sender-Abort message, the Sender-Abort message MUST be of 2 byte (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. Zuniga, et al. Expires 2 February 2023 [Page 20] Internet-Draft SCHC over Sigfox LPWAN August 2022 3.7.4.3. 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| Losses are found in windows W = w1,...,wi; where w1| <-- L2 Word --> | Figure 24: SCHC Receiver-Abort message format 3.7.5. Downlink ACK-Always Mode: Single-byte SCHC Header 3.7.5.1. 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 use of SCHC ACK is NOT RECOMMENDED, instead the All-1 SCHC Fragment SHOULD 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). 3.7.5.2. 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 last tile Zuniga, et al. Expires 2 February 2023 [Page 22] Internet-Draft SCHC over Sigfox LPWAN August 2022 As per [RFC8724] the All-1 must be distinguishable from the a SCHC Sender-Abort message (with same Rule ID and N values). The SCHC Sender-Abort message header size is of 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 of 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. 3.7.5.3. 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 3.7.5.4. SCHC Sender-Abort Messages Sender-Abort |---- Header ----| +--------------------+ | RuleID | FCN=ALL-1 | +--------+-----------+ | 3 bits | 5 bits | Figure 29: SCHC Sender-Abort message format 3.7.5.5. SCHC Receiver-Abort Message Zuniga, et al. Expires 2 February 2023 [Page 23] Internet-Draft SCHC over Sigfox LPWAN August 2022 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 3.8. 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/Sigfox, a SCHC-Sender Abort MUST be sent if the number of repeated All-1s (i.e., with the same bitmap) sent in sequence is greater than or equal to MAX_ACK_REQUESTS. 3.9. 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 SCHC/Sigfox 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 increase 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. 3.10. Padding The Sigfox payload fields have different characteristics in Uplink and Downlink. Zuniga, et al. Expires 2 February 2023 [Page 24] Internet-Draft SCHC over Sigfox LPWAN August 2022 Uplink frames 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. Downlink frames on the other hand 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. 4. Fragmentation Sequence Examples In this section, some sequence diagrams depicting messages 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. 4.1. 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. Last packet fragment is marked with the 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=31,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 a FCN equal to 15). Zuniga, et al. Expires 2 February 2023 [Page 25] Internet-Draft SCHC over Sigfox LPWAN August 2022 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=31,Seq=7------->| Missing Fragment Unable to reassemble (End) Figure 32: Uplink No-ACK Losses (scenario 1) 4.2. 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 should 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 Zuniga, et al. Expires 2 February 2023 [Page 26] Internet-Draft SCHC over Sigfox LPWAN August 2022 Case Fragment losses in 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 on First Window Case Fragment All-0 lost in 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, notifying the absence of the All-0 of window 0. The sender resends the missing All-0 messages (with any other missing fragment from window 0) without opening a reception opportunity. Zuniga, et al. Expires 2 February 2023 [Page 27] Internet-Draft SCHC over Sigfox LPWAN August 2022 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 on 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) Zuniga, et al. Expires 2 February 2023 [Page 28] Internet-Draft SCHC over Sigfox LPWAN August 2022 Figure 36: Uplink ACK-on-Error All-0 and other Fragments Lost on 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) should 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<-|__ |<-Compoud 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---->| |<-Compoud ACK,W=1,C=1 ----| C=1 (End) Figure 37: Uplink ACK-on-Error All-0 and other Fragments Lost on First and Second Windows (1) Similar case as above, but with fewer fragments in the second window (W=1) Zuniga, et al. Expires 2 February 2023 [Page 29] Internet-Draft SCHC over Sigfox LPWAN August 2022 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 on 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. Zuniga, et al. Expires 2 February 2023 [Page 30] Internet-Draft SCHC over Sigfox LPWAN August 2022 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 -X-| 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 window 0 and 1. However, the sender does not send a SCHC ACK after the All-0 of window 0. Instead, it sends a SCHC Compound ACK notifying losses of 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) Zuniga, et al. Expires 2 February 2023 [Page 31] Internet-Draft SCHC over Sigfox LPWAN August 2022 Figure 40: Uplink ACK-on-Error Fragments Lost on 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. 4.3. SCHC Abort Examples Case SCHC Sender-Abort The sender may need to send a Sender-Abort to stop the current communication. This may happen, for example, 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 |<-Compound ACK,W=1,C=1 -X-| C=1 DL Enable |-----W=1,FCN=7,Seq=13---->| RESEND ACK (1) |<-Compound ACK,W=1,C=1 -X-| C=1 DL Enable |-----W=1,FCN=7,Seq=15---->| RESEND ACK (2) |<-Compound ACK,W=1,C=1 -X-| C=1 DL Enable |-----W=1,FCN=7,Seq=17---->| RESEND ACK (3) |<-Compound ACK,W=1,C=1 -X-| C=1 DL Enable |-----W=1,FCN=7,Seq=18---->| RESEND ACK (4) |<-Compound ACK,W=1,C=1 -X-| C=1 DL Enable |-----W=1,FCN=7,Seq=19---->| RESEND ACK (5) |<-Compound ACK,W=1,C=1 -X-| C=1 DL Enable |----Sender-Abort,Seq=20-->| exit with error condition (End) Figure 41: Uplink ACK-on-Error Sender-Abort Case Receiver-Abort Zuniga, et al. Expires 2 February 2023 [Page 32] Internet-Draft SCHC over Sigfox LPWAN August 2022 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 5. 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 with the ID claimed in the message. Application data can be encrypted at the application level or not, depending on the criticality of the use case. This flexibility allows providing a balance between cost and effort vs. 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 confidentiality keys are pre- provisioned. A confidentiality key is only provisioned if confidentiality is to be used. The radio protocol has protections against reply attacks, and the cloud-based core network provides firewalling protection against undesired incoming communications. 6. IANA Considerations This document has no IANA actions. Zuniga, et al. Expires 2 February 2023 [Page 33] Internet-Draft SCHC over Sigfox LPWAN August 2022 7. Acknowledgements Carles Gomez has been funded in part by the Spanish Government through the Jose Castillejo CAS15/00336 grant, the TEC2016-79988-P grant, and the PID2019-106808RA-I00 grant, and by Secretaria d'Universitats i Recerca del Departament d'Empresa i Coneixement de la Generalitat de Catalunya 2017 through grant SGR 376. 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. 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. 8. References 8.1. Normative References [I-D.ietf-lpwan-schc-compound-ack] Zuniga, JC., Gomez, C., Aguilar, S., Toutain, L., Cespedes, S., and D. Wistuba, "SCHC Compound ACK", Work in Progress, Internet-Draft, draft-ietf-lpwan-schc-compound- ack-05, July 2021, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", DOI 10.17487/RFC2119, BCP 14, RFC 2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", DOI 10.17487/RFC8174, RFC 8174, BCP 14, May 2017, . [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, . Zuniga, et al. Expires 2 February 2023 [Page 34] Internet-Draft SCHC over Sigfox LPWAN August 2022 8.2. Informative References [RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018, . [sigfox-callbacks] Sigfox, "Sigfox Callbacks", . [sigfox-spec] Sigfox, "Sigfox Radio Specifications", . Authors' Addresses Juan Carlos Zuniga Montreal QC Canada Email: j.c.zuniga@ieee.org Carles Gomez Universitat Politecnica de Catalunya C/Esteve Terradas, 7 08860 Castelldefels Spain Email: carlesgo@entel.upc.edu Sergio Aguilar Universitat Politecnica de Catalunya C/Esteve Terradas, 7 08860 Castelldefels Spain Email: sergio.aguilar.romero@upc.edu Laurent Toutain IMT-Atlantique 2 rue de la Chataigneraie CS 17607 35576 Cesson-Sevigne Cedex France Email: Laurent.Toutain@imt-atlantique.fr Zuniga, et al. Expires 2 February 2023 [Page 35] Internet-Draft SCHC over Sigfox LPWAN August 2022 Sandra Cespedes 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: wistuba@niclabs.cl Julien Boite SIGFOX Labege France Email: julien.boite@sigfox.com URI: http://www.sigfox.com/ Zuniga, et al. Expires 2 February 2023 [Page 36]