Internet-Draft SCHC over Sigfox LPWAN February 2022
Zuniga, et al. Expires 25 August 2022 [Page]
Workgroup:
lpwan Working Group
Internet-Draft:
draft-ietf-lpwan-schc-over-sigfox-09
Published:
Intended Status:
Standards Track
Expires:
Authors:
JC. Zuniga
C. Gomez
Universitat Politècnica de Catalunya
S. Aguilar
Universitat Politècnica de Catalunya
L. Toutain
IMT-Atlantique
S. Cespedes
NIC Labs, Universidad de Chile
D. Wistuba
NIC Labs, Universidad de Chile
J. Boite
SIGFOX

SCHC over Sigfox LPWAN

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 25 August 2022.

Table of Contents

1. Introduction

The Generic Framework for Static Context Header Compression and Fragmentation (SCHC) specification [RFC8724] describes two mechanisms: i) a frame fragmentation and loss recovery functionality, and ii) an application header compression scheme. Either can be used on top of all the four LWPAN 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

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 the predictability of data 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.

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 (RG) 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   |    .
            |  (RG)   | === |    (NGW)     | === |C/D & F/R|.....
            +---------+     +--------------+     +---------+

Figure 1: Network Architecture

In the case of the global Sigfox Network, RGs (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 Sigfox base stations in the world, providing hence a global single star network topology.

The Device sends application flows 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.

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).

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 UL ACK-on-Error fragmentation mode, a DL 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 DL transmission opportunity, then no DL transmission takes place at that opportunity and after a timeout the UL transmissions continue. Intermediate SCHC fragments with FCN different from All-0 or All-1 MUST NOT use the DL 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.

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 UL 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 3.6.1.5 is RECOMMENDED.

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

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.

3.7.1.1. 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.


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

Figure 3: 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.

3.7.1.2. All-1 SCHC Fragment

Figure 4 shows an example of the All-1 message. The All-1 message MUST contain the last tile of the SCHC Packet. The last tile MUST be of at least 1 byte (one L2 word). Padding MUST NOT be added, as the resulting size is L2-word-multiple.


           |---  SCHC Fragment Header ---|
           + --------------------------- + ------------ +
           | RuleID |   W    | FCN=ALL-1 |    Payload   |
           + ------ + ------ + --------- + ------------ +
           | 3 bits | 2 bits |  3 bits   | 8 to 88 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, M, and N values). The All-1 MUST have the last tile of the SCHC Packet, which MUST be of at least 1 byte. 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.

3.7.1.3. SCHC ACK Format

Figure 5 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 5: 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 6. The window numbered 00, if present in the SCHC Compound ACK, MUST be placed between the Rule ID and the C bit to avoid confusion with padding bits. As padding is needed for the SCHC Compound ACK, padding bits MUST be 0 to make subsequent window numbers and bitmaps distinguishable.


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

     On top are noted the window number of the corresponding bitmap.
     Losses are found in windows x,...,x+i.

Figure 6: SCHC Compound ACK message format

The following figures show examples of the SCHC Compound ACK message format, when used on SCHC over Sigfox.


   |---- SCHC ACK Header ----|- W=00 -|----- W=01 ------|
   + ----------------------- + ------ + ------ + ------ + ------- +
   | RuleID | W=b'00 | C=b'0 | Bitmap | W=b'01 | Bitmap | b'0-pad |
   + ------ + ------ + ----- + ------ + ------ + ------ + ------- +
   | 3 bits | 2 bits | 1 bit | 7 bits | 2 bits | 7 bits | 42 bits |

        Losses are found in windows 00 and 01.

Figure 7: SCHC Compound ACK example 1

   |---- SCHC ACK Header ----|- W=01 -|----- W=11 ------|
   + ----------------------- + ------ + ------ + ------ + ------- +
   | RuleID | W=b'01 | C=b'0 | Bitmap | W=b'11 | Bitmap | b'0-pad |
   + ------ + ------ + ----- + ------ + ------ + ------ + ------- +
   | 3 bits | 2 bits | 1 bit | 7 bits | 2 bits | 7 bits | 42 bits |

        Losses are found in windows 01 and 11.

Figure 8: SCHC Compound ACK example 2

   |---- SCHC ACK Header ----|- W=00 -|----- W=10 ------|
   + ----------------------- + ------ + ------ + ------ + ------- +
   | RuleID | W=b'00 | C=b'0 | Bitmap | W=b'10 | Bitmap | b'0-pad |
   + ------ + ------ + ----- + ------ + ------ + ------ + ------- +
   | 3 bits | 2 bits | 1 bit | 7 bits | 2 bits | 7 bits | 42 bits |

        Losses are found in windows 00 and 10.

Figure 9: SCHC Compound ACK example 3

Figure 10 shows the SCHC Compound ACK message format when losses are found in all windows. The window numbers and its corresponding bitmaps are ordered from window numbered 00 to 11, notifying all four possible windows.


  |- SCHC ACK Header -|W=b'00|-- W=b'01 ---|
  +-------------------+------+ ---- +------+
  |RuleID|W=b'00|C=b'0|Bitmap|W=b'01|Bitmap| ...
  +------+------+-----+------+------+------+
  |3 bits|2 bits|1 bit|7 bits|2 bits|7 bits|


             |--- W=b'10 --|--- W=b'11 --|
             |------+------+------+------+-------+
         ... |W=b'10|Bitmap|W=b'11|Bitmap|b'0-pad|
             |------+------+------+------+-------+
             |2 bits|7 bits|2 bits|7 bits|24 bits|

        Losses are found in windows 00, 01, 10 and 11.

Figure 10: SCHC Compound ACK example 4

|- SCHC ACK Header -|W=b'00|-- W=b'01 ---|--- W=b'10 --|
+-------------------+------+------+------+------+------+-------+
|RuleID|W=b'00|C=b'0|Bitmap|W=b'01|Bitmap|W=b'10|Bitmap|b'0-pad|
+------+------+-----+------+------+------+------+------+-------+
|3 bits|2 bits|1 bit|7 bits|2 bits|7 bits|2 bits|7 bits|33 bits|

        Losses are found in windows 00, 01 and 10.

Figure 11: SCHC Compound ACK example 5
3.7.1.4. SCHC Sender-Abort Message format

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

Figure 12: SCHC Sender-Abort message format
3.7.1.5. SCHC Receiver-Abort Message format

                |- Receiver-Abort Header -|
                + ----------------------- + ------- +
                | RuleID | W=b'11 | C=b'1 | b'1-pad |
                + ------ + ------ + ----- + ------- +
                | 3 bits | 2 bits | 1 bit | 58 bits |

Figure 13: SCHC Receiver-Abort message format
3.7.2.1. Regular SCHC Fragment

Figure 14 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 14: 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 (Network SCHC). As per [RFC8724], the All-0 message is distinguishable from the SCHC ACK REQ (All-1 message).

3.7.2.2. All-1 SCHC Fragment

Figure 15 shows an example of the All-1 message. The All-1 message MUST contain the last tile of the SCHC Packet.


              |---  SCHC Fragment Header ---|
              + --------------------------- + ------------ +
              | RuleID |   W    | FCN=ALL-1 |    Payload   |
              + ------ + ------ + --------- + ------------ +
              | 8 bits | 3 bits |  5 bits   | 8 to 80 bits |

Figure 15: 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.

3.7.2.3. SCHC ACK Format

Figure 16 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 16: 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 17. The SCHC Compound ACK can report up to 3 windows with losses. The window number (W) and its corresponding bitmap MUST be ordered from the lowest-numbered window number to the highest-numbered window. If window numbered 000 is present in the SCHC Compound ACK, the window number 000 MUST be placed between the Rule ID and C bit to avoid confusion with padding bits.

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=b'x |...|--- W=b'x+i ---|
  +-------------------+-------+...+-------+-------+-------+
  |RuleID|W=b'x |C=b'0|Bitmap |...|W=b'x+i|Bitmap |b'0-pad|
  +------+------+-----+-------+...|-------+-------+-------+
  |8 bits|3 bits|1 bit|31 bits|   | 3 bits|31 bits|

        On top are noted the window number
                of the corresponding bitmap.
        Losses are found in windows x,...,x+i.

Figure 17: SCHC Compound ACK message format
3.7.2.4. SCHC Sender-Abort Messages

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

Figure 18: SCHC Sender-Abort message format
3.7.2.5. SCHC Receiver-Abort Message

              |-- Receiver-Abort Header -|
              + ------------------------ + ------- +
              | RuleID | W=b'111 | C=b'1 | b'1-pad |
              + ------ + ------- + ----- + ------- +
              | 8 bits |  3 bits | 1 bit | 52 bits |

Figure 19: 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.
  • The MAX_ACK_REQUEST counter MUST be reset when a SCHC ACK is successfully received.

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.
  • A SCHC-Receiver Abort can be triggered when the number of ACK attempts is not strictly less than MAX_ACK_REQUESTS. In the case of SCHC/Sigfox, a SCHC-Receiver Abort MUST be sent if the number of repeated SCHC ACKs sent in a row (i.e., synchronized with the ACK REQ case, and with identical bitmaps) is greater than or equal to MAX_ACK_REQUESTS.
  • 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.

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.

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 (0110), Seq=1-------->|
          |-------FCN=5 (0101), Seq=2-------->|
          |-------FCN=4 (0100), Seq=3-------->|
          |-------FCN=3 (0011), Seq=4-------->|
          |-------FCN=2 (0010), Seq=5-------->|
          |-------FCN=1 (0001), Seq=6-------->|
          |-------FCN=15 (1111), Seq=7------->| All fragments received
        (End)


Figure 20: UL 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).

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 21: UL No-ACK Losses (scenario 1)

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 DL ACK can be sent opportunistically.

Case No losses

The downlink flag must be enabled in the sender UL message to allow a DL message from the receiver. The DL 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
          |<------ ACK, W=1, C=1 ------| C=1
        (End)


Figure 22: UL ACK-on-Error No-Losses

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.



        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----->|
DL Enable |-----W=0, FCN=0, Seq=7----->| Missing Fragments W=0 => FCN=5, Seq=2 and FCN=2, Seq=5
          |<------ ACK, W=0, C=0 ------| Bitmap:1011011
          |-----W=0, FCN=5, Seq=8----->|
          |-----W=0, FCN=2, Seq=9----->|
          |-----W=1, FCN=6, Seq=10---->|
          |-----W=1, FCN=5, Seq=11---->|
          |-----W=1, FCN=4, Seq=12---->|
DL Enable |-----W=1, FCN=7, Seq=13---->| All fragments received
          |<------ ACK, W=1, C=1 ------| C=1
        (End)


Figure 23: UL 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.



        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---->|
DL Enable |-----W=1, FCN=7, Seq=11---->| Missing Fragment W=0, FCN=0, Seq=7
          |<------ ACK, W=0, C=0 ------| Bitmap:1111110
          |-----W=0, FCN=0, Seq=13---->| All fragments received
DL Enable |-----W=1, FCN=7, Seq=14---->|
          |<------ ACK, W=1, C=1 ------| C=1
        (End)


Figure 24: UL 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---->|
DL Enable |-----W=1, FCN=7, Seq=11---->| Missing Fragment W=0 => FCN= 5, 3 and 0
          |<------ ACK, W=0, C=0 ------| Bitmap:1010110
          |-----W=0, FCN=5, Seq=13---->|
          |-----W=0, FCN=3, Seq=14---->|
          |-----W=0, FCN=0, Seq=15---->| All fragments received
DL Enable |-----W=1, FCN=7, Seq=16---->|
          |<------ ACK, W=1, C=1 ------| C=1
        (End)


Figure 25: UL 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 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 (110), Seq=1----->|
          |-----W=0, FCN=5 (101), Seq=2--X-->|
          |-----W=0, FCN=4 (100), Seq=3----->|
          |-----W=0, FCN=3 (011), Seq=4--X-->|
          |-----W=0, FCN=2 (010), Seq=5----->|
          |-----W=0, FCN=1 (001), Seq=6----->|
DL enable |-----W=0, FCN=0 (000), Seq=7--X-->|
     (no ACK)
          |-----W=1, FCN=6 (110), Seq=8--X-->|
          |-----W=1, FCN=5 (101), Seq=9----->|
          |-----W=1, FCN=4 (011), Seq=10-X-->|
DL enable |-----W=1, FCN=7 (111), Seq=11---->| Missing Fragment W=0 => FCN= 5, 3 and 0, W=1 => FCN= 6 and 4
          |<--- Compound ACK, W=0,1, C=0 ----| Bitmap W=0:1010110, Bitmap W=1:0100001
          |-----W=0, FCN=5 (101), Seq=13---->|
          |-----W=0, FCN=3 (011), Seq=14---->|
          |-----W=0, FCN=0 (000), Seq=15---->|
          |-----W=1, FCN=6 (110), Seq=16---->|
          |-----W=1, FCN=4 (011), Seq=17---->| All fragments received
DL enable |-----W=1, FCN=7 (111), Seq=18---->|
          |<--------- ACK, W=1, C=1 ---------| C=1
        (End)


Figure 26: UL ACK-on-Error All-0 and other Fragments Lost on First and Second Windows (1)

Similar case as above, but with less fragments in the second window (W=1)



        Sender                            Receiver
          |-----W=0, FCN=6 (110), Seq=1----->|
          |-----W=0, FCN=5 (101), Seq=2--X-->|
          |-----W=0, FCN=4 (100), Seq=3----->|
          |-----W=0, FCN=3 (011), Seq=4--X-->|
          |-----W=0, FCN=2 (010), Seq=5----->|
          |-----W=0, FCN=1 (001), Seq=6----->|
DL enable |-----W=0, FCN=0 (000), Seq=7--X-->|
       (no ACK)
          |-----W=1, FCN=6 (110), Seq=8--X-->|
DL enable |-----W=1, FCN=7 (111), Seq=9----->| Missing Fragment W=0 => FCN= 5, 3 and 0, W=1 => FCN= 6 and 4
          |<--- Compound ACK, W=0,1, C=0 ----| Bitmap W=0:1010110, Bitmap W=1:0000001
          |-----W=0, FCN=5 (101), Seq=10---->|
          |-----W=0, FCN=3 (011), Seq=11---->|
          |-----W=0, FCN=0 (000), Seq=12---->|
          |-----W=1, FCN=6 (110), Seq=13---->| All fragments received
DL enable |-----W=1, FCN=7 (111), Seq=14---->|
          |<--------- ACK, W=1, C=1 ---------| C=1
        (End)


Figure 27: UL 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.



       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
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=13---->| RESEND ACK
          |<------ ACK, W=1, C=1 ------| C=1
        (End)


Figure 28: UL 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 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 (110), Seq=1----->|
          |-----W=0, FCN=5 (101), Seq=2--X-->|
          |-----W=0, FCN=4 (100), Seq=3----->|
          |-----W=0, FCN=3 (011), Seq=4--X-->|
          |-----W=0, FCN=2 (010), Seq=5----->|
          |-----W=0, FCN=1 (001), Seq=6----->|
DL enable |-----W=0, FCN=0 (000), Seq=7----->| Waits for next DL opportunity
       (no ACK)
          |-----W=1, FCN=6 (110), Seq=8--X-->|
DL enable |-----W=1, FCN=7 (111), Seq=9----->| Missing Fragment W=0 => FCN= 5, 3 and 0, W=1 => FCN= 6 and 4
          |<--- Compound ACK, W=0,1, C=0 ----| Bitmap W=0:1010110, Bitmap W=1:0000001
          |-----W=0, FCN=5 (101), Seq=10---->|
          |-----W=0, FCN=3 (011), Seq=11---->|
          |-----W=1, FCN=6 (110), Seq=12---->| All fragments received
DL enable |-----W=1, FCN=7 (111), Seq=13---->|
          |<--------- ACK, W=1, C=1 ---------| C=1
        (End)


Figure 29: UL 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
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=14---->| RESEND ACK  (1)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=15---->| RESEND ACK  (2)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=16---->| RESEND ACK  (3)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=17---->| RESEND ACK  (4)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=18---->| RESEND ACK  (5)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |----Sender-Abort, Seq=19--->| exit with error condition
        (End)


Figure 30: UL 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 DL 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 31: UL 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. 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 Clement Mannequin, Rafael Vidal, Julien Boite, Renaud Marty, and Antonis Platis for their useful comments and implementation design considerations.

7. References

7.1. 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-00, , <http://www.ietf.org/internet-drafts/draft-ietf-lpwan-schc-compound-ack-00.txt>.
[RFC8376]
Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) Overview", RFC 8376, DOI 10.17487/RFC8376, , <https://www.rfc-editor.org/info/rfc8376>.
[RFC8724]
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC. Zuniga, "SCHC: Generic Framework for Static Context Header Compression and Fragmentation", RFC 8724, DOI 10.17487/RFC8724, , <https://www.rfc-editor.org/info/rfc8724>.

7.2. Informative References

[sigfox-callbacks]
Sigfox, "Sigfox Callbacks", <https://support.sigfox.com/docs/callbacks-documentation>.
[sigfox-spec]
Sigfox, "Sigfox Radio Specifications", <https://build.sigfox.com/sigfox-device-radio-specifications>.

Authors' Addresses

Juan Carlos Zúñiga
Montreal QC
Canada
Carles Gomez
Universitat Politècnica de Catalunya
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Sergio Aguilar
Universitat Politècnica de Catalunya
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Laurent Toutain
IMT-Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Sandra Cespedes
NIC Labs, Universidad de Chile
Av. Almte. Blanco Encalada 1975
Santiago
Chile
Diego Wistuba
NIC Labs, Universidad de Chile
Av. Almte. Blanco Encalada 1975
Santiago
Chile
Julien Boite
SIGFOX
Labege
France