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

4.1. Device Classes (A, B, C) and Interactions

The LoRaWAN Medium Access Control (MAC) layer supports three classes of devices named A, B, and C. All devices implement Class A, and some devices may implement Class B or Class C. Class B and Class C are mutually exclusive.¶

Class A:

Class A is the simplest class of devices. The device is allowed to transmit at any time, randomly selecting a communication channel. The Network Gateway may reply with a downlink in one of the two receive windows immediately following the uplinks. Therefore, the Network Gateway cannot initiate a downlink; it has to wait for the next uplink from the device to get a downlink opportunity. Class A is the lowest power consumption class.¶

Class B:

Class B devices implement all the functionalities of Class A devices but also schedule periodic listen windows. Therefore, as opposed to Class A devices, Class B devices can receive downlinks that are initiated by the Network Gateway and not following an uplink. There is a trade-off between the periodicity of those scheduled Class B listen windows and the power consumption of the device:¶

High periodicity:

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

Low periodicity:

Downlinks from the NGW will have higher latency but lower power consumption.¶

Class C:

Class C devices implement all the functionalities of Class A devices but keep their receiver open whenever they are not transmitting. Class C devices can receive downlinks at any time at the expense of a higher power consumption. Battery-powered devices can only operate in Class C for a limited amount of time (for example, for a firmware upgrade over-the-air). Most of the Class C devices are grid powered (for example, Smart Plugs).¶

4.2. Device Addressing

LoRaWAN end devices use a 32-bit network address (DevAddr) to communicate with the Network Gateway over the air; this address might not be unique in a LoRaWAN network. Devices using the same DevAddr are distinguished by the Network Gateway based on the cryptographic signature appended to every LoRaWAN frame.¶

To communicate with the SCHC gateway, the Network Gateway MUST identify the devices by a unique 64-bit device identifier called the "DevEUI".¶

The DevEUI is assigned to the device during the manufacturing process by the device's manufacturer. It is built like an Ethernet MAC address by concatenating the manufacturer's IEEE OUI field with a vendor unique number. For example, a 24-bit OUI is concatenated with a 40-bit serial number. The Network Gateway translates the DevAddr into a DevEUI in the uplink direction and reciprocally on the downlink direction.¶

+--------+         +---------+        +---------+          +----------+
| Device | <=====> | Network | <====> | SCHC    | <======> | Internet |
|        | DevAddr | Gateway | DevEUI | Gateway | IPv6/UDP |          |
+--------+         +---------+        +---------+          +----------+

4.3. General Frame Types

LoRaWAN implements the possibility to send confirmed or unconfirmed frames:¶

Confirmed frame:

The sender asks the receiver to acknowledge the frame.¶

Unconfirmed frame:

The sender does not ask the receiver to acknowledge the frame.¶

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

4.4. LoRaWAN MAC Frames

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

JoinRequest:

This frame is used by a device to join a network. It contains the device's unique identifier DevEUI and a random nonce that will be used for session key derivation.¶

JoinAccept:

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

Data:

This refers to MAC and application data. Application data is protected with AES-128 encryption. MAC-related data is AES-128 encrypted with another key.¶

4.5. LoRaWAN FPort

The LoRaWAN MAC layer features a frame port field in all frames. This field (FPort) is 8 bits long and the values from 1 to 223 can be used. It allows LoRaWAN networks and applications to identify data.¶

4.6. LoRaWAN Empty Frame

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

4.7. Unicast and Multicast Technology

LoRaWAN technology supports unicast downlinks but also multicast; a multicast packet sent over a LoRaWAN radio link can be received by several devices. It is useful to address many devices with the same content: either a large binary file (firmware upgrade) or the same command (e.g., lighting control). As IPv6 is also a multicast technology, this feature can be used to address a group of devices.¶

5.1. LoRaWAN FPort and RuleID

The FPort field is part of the SCHC Message, as shown in Figure 5. The SCHC C/D and the SCHC F/R SHALL concatenate the FPort field with the LoRaWAN payload to recompose the SCHC Message.¶

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

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

All RuleIDs can use arbitrary values inside the FPort range allowed by the LoRaWAN specification [LORAWAN-SPEC] and MUST be shared by the device and SCHC gateway prior to the communication with the selected rule. The uplink and downlink fragmentation FPorts MUST be different.¶

5.2. RuleID Management

The RuleID MUST be 8 bits and encoded in the LoRaWAN FPort as described in Section 5.1. LoRaWAN supports up to 223 application FPorts in the range [1..223] as defined in Section 4.3.2 of [LORAWAN-SPEC]; it implies that the RuleID MSB SHOULD be inside this range. An application can send non-SCHC traffic by using FPort values different from the ones used for SCHC.¶

In order to improve interoperability, RECOMMENDED fragmentation RuleID values are:¶

The FPortUp value MUST be different from the FPortDown value. The remaining RuleIDs are available for compression. RuleIDs are shared between uplink and downlink sessions. A RuleID not in the set(s) of FPortUp or FPortDown means that the fragmentation is not used; thus, on reception, the SCHC Message MUST be sent to the SCHC C/D layer.¶

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

An application can have multiple fragmented datagrams between a device and one or several SCHC gateways. A set of FPort values is REQUIRED for each SCHC gateway instance the device is required to communicate with. The application can use additional uplinks or downlink-fragmented parameters but SHALL implement at least the parameters defined in this document.¶

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

5.3. Interface IDentifier (IID) Computation

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

The aes128_cmac algorithm is described in [RFC4493]. It has been chosen as it is already used by devices for the LoRaWAN protocol.¶

As AppSKey is renewed each time a device joins or rejoins a LoRaWAN network, the IID will change over time; this mitigates privacy concerns, for example, location tracking or correlation over time. Join periodicity is defined at the application level.¶

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

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

Example with:¶

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

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

5.4. Padding

All padding bits MUST be 0.¶

5.5. Decompression

The SCHC C/D MUST concatenate FPort and LoRaWAN payload to retrieve the SCHC Packet as per Section 5.1.¶

RuleIDs matching FPortUp and FPortDown are reserved for SCHC fragmentation.¶

5.6. Fragmentation

The L2 Word Size used by LoRaWAN is 1 byte (8 bits). The SCHC fragmentation over LoRaWAN uses the ACK-on-Error mode for uplink fragmentation and ACK-Always mode for downlink fragmentation. A LoRaWAN device cannot support simultaneous interleaved fragmented datagrams in the same direction (uplink or downlink).¶

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

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

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

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

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

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

| FPort  | LoRaWAN payload                              |
+ ------ + -------------------------------------------- +
| RuleID | W = b'11 | C = 1 | b'11111 | 0xFF (all 1's)  |
+ ------ + -------- + ------+-------- + ----------------+
| 8 bits |  2 bits  | 1 bit | 5 bits  | 8 bits          |
              next L2 Word boundary ->| <-- L2 Word --> |

| FPort  | LoRaWAN payload          |
+------- +------------------------- +
| RuleID | W      | FCN = b'000000  |
+ ------ + ------ + --------------- +
| 8 bits | 2 bits | 6 bits          |

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

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

| FPort  | LoRaWAN payload                    |
+ ------ + ---------------------------------- +
| RuleID | W     | C = b'1 | Padding b'000000 |
+ ------ + ----- + ------- + ---------------- +
| 8 bits | 1 bit | 1 bit   | 6 bits           |

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

| FPort  | LoRaWAN payload                                |
+ ------ + ---------------------------------------------- +
| RuleID | W = b'1 | C = b'1 | b'111111 | 0xFF (all 1's)  |
+ ------ + ------- + ------- + -------- + --------------- +
| 8 bits | 1 bit   | 1 bits  | 6 bits   | 8 bits          |
                next L2 Word boundary ->| <-- L2 Word --> |

5.7. SCHC Fragment Format

A.1. Uplink - Compression Example - No Fragmentation

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

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

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

The current LoRaWAN MTU is 51 bytes, although 2-byte FOpts are used by the LoRaWAN protocol: 49 bytes are available for SCHC payload; no need for fragmentation. The payload will be transmitted through FPort = 1.¶

| LoRaWAN Header            | LoRaWAN payload (40 bytes)              |
+ ------------------------- + --------------------------------------- +
|      |  FOpts  | RuleID=1 | Compression | Payload   | Padding=b'000 |
|      |         |          | residue     |           |               |
+ ---- + ------- + -------- + ----------- + --------- + ------------- +
| XXXX | 2 bytes | 1 byte   | 21 bits     |  37 bytes |    3 bits     |

A.2. Uplink - Compression and Fragmentation Example

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

An applicative data of 300 bytes is passed to the SCHC compression layer. Rule 1 is used by the SCHC C/D layer, allowing to compress it to 282 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 279 bytes payload.¶

| RuleID | Compression residue |  Payload  |
+ ------ + ------------------- + --------- +
|   1    |       21 bits       | 279 bytes |

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

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

The tile content is described in Figure 23¶

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

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

Next transmission MTU is 242 bytes, 4-byte FOpts. 23 tiles are transmitted:¶

| LoRaWAN Header                        | LoRaWAN payload (231 bytes) |
+ --------------------------------------+ --------------------------- +
|                |  FOpts  | RuleID=20  |   W   |  FCN  |  23 tiles   |
+ -------------- + ------- + ---------- + ----- + ----- + ----------- +
|       XXXX     | 4 bytes |  1 byte    | 0   0 |   61  | 230 bytes   |

Next transmission MTU is 242 bytes, no FOpts. All 5 remaining tiles are transmitted, the last tile is only 2 bytes + 5 bits. Padding is added for the remaining 3 bits.¶

| LoRaWAN Header    | LoRaWAN payload (44 bytes)                      |
+ ---- + ---------- + ----------------------------------------------- +
|      | RuleID=20  |   W   |  FCN  |    5 tiles      | Padding=b'000 |
+ ---- + ---------- + ----- + ----- + --------------- + ------------- +
| XXXX | 1 byte     | 0   0 |  38   | 42 bytes+5 bits |    3 bits     |

Then All-1 message can be transmitted:¶

| LoRaWAN Header    | LoRaWAN payload (44 bytes) |
+ ---- + -----------+ -------------------------- +
|      | RuleID=20  |   W   |  FCN  |     RCS    |
+ ---- + ---------- + ----- + ----- + ---------- +
| XXXX | 1 byte     | 0   0 |   63  |  4 bytes   |

All packets have been received by the SCHC gateway, computed RCS is correct so the following ACK is sent to the device by the SCHC receiver:¶

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

A.3. Downlink

An applicative data of 155 bytes is passed to the SCHC compression layer. Rule 1 is used by the SCHC C/D layer, allowing to compress it to 130 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 127 bytes payload.¶

| RuleID | Compression residue |  Payload  |
+ ------ + ------------------- + --------- +
|   1    |       21 bits       | 127 bytes |

The current LoRaWAN MTU is 51 bytes; no FOpts are used by the LoRaWAN protocol: 51 bytes are available for SCHC payload + FPort field; the applicative data has to be fragmented.¶

| LoRaWAN Header    | LoRaWAN payload (51 bytes)             |
+ ---- + ---------- + -------------------------------------- +
|      | RuleID=21  |  W = 0 | FCN = 0 |       1 tile        |
+ ---- + ---------- + ------ + ------- + ------------------- +
| XXXX | 1 byte     |  1 bit |  1 bit  | 50 bytes and 6 bits |

The tile content is described in Figure 30¶

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

The receiver answers with a SCHC ACK:¶

| LoRaWAN Header   | LoRaWAN payload                  |
+ ---- + --------- + -------------------------------- +
|      | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 |
+ ---- + --------- + ----- + ----- + ---------------- +
| XXXX |  1 byte   | 1 bit | 1 bit |     6 bits       |

The second downlink is sent, two FOpts:¶

| LoRaWAN Header              |  LoRaWAN payload (49 bytes)           |
+ --------------------------- + ------------------------------------- +
|      |  FOpts  | RuleID=21  | W = 1 | FCN = 0 |        1 tile       |
+ ---- + ------- + ---------- + ----- + ------- + ------------------- +
| XXXX | 2 bytes | 1 byte     | 1 bit |  1 bit  | 48 bytes and 6 bits |

The receiver answers with a SCHC ACK:¶

| LoRaWAN Header   | LoRaWAN payload                  |
+ ---- + --------- + -------------------------------- +
|      | RuleID=21 | W = 1 | C = 1 | Padding=b'000000 |
+ ---- + --------- + ----- + ----- + ---------------- +
| XXXX |  1 byte   | 1 bit | 1 bit |     6 bits       |

The last downlink is sent, no FOpts:¶

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

The receiver answers to the sender with a SCHC ACK:¶

| LoRaWAN Header   | LoRaWAN payload                  |
+ ---- + --------- + -------------------------------- +
|      | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 |
+ ---- + --------- + ----- + ----- + ---------------- +
| XXXX |  1 byte   | 1 bit | 1 bit |     6 bits       |