Static Context Header Compression (SCHC) over LoRaWAN

4.1. Device classes (A, B, C) and interactions

The LoRaWAN MAC layer supports 3 classes of devices named A, B and C. All devices implement the Class A, some devices may implement Class B or Class C. Class B and Class C are mutually exclusive.¶

• Class A: The 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 2 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. The 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, opposed to the 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: if the periodicity is high downlinks from the NGW will be sent faster, but the device wakes up more often: it will have higher 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. e.g.: 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 (MType=0b100 as per [lora-alliance-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 on-board 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 (amongst other fields) the network's major settings and a random nonce used to derive the session keys.¶
• Data: MAC and application data. Application data are protected with AES-128 encryption. MAC related data are 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 packet sent over LoRaWAN radio link can be received by several devices. It is useful to address many devices with same content, either a large binary file (firmware upgrade), or same command (e.g: lighting control). As IPv6 is also a multicast technology this feature can be used to address a group of devices.¶

Note 1: IPv6 multicast addresses must be defined as per [RFC4291]. LoRaWAN multicast group definition in a Network Gateway and the relation between those groups and IPv6 groupID are out of scope of this document.¶

Note 2: LoRa Alliance defined [lora-alliance-remote-multicast-set] as the RECOMMENDED way to setup multicast groups on devices and create a synchronized reception window.¶

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       |

Note: SCHC Message is any datagram sent by SCHC C/D or F/R layers.¶

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 downlink fragmented datagram. It uses another FPort for data downlink and its associated SCHC control uplinks, named FPortDown in this document.¶

All RuleID can use arbitrary values inside the FPort range allowed by LoRaWAN specification 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. Rule ID management

RuleID MUST be 8 bits, 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 [lora-alliance-spec], it implies that 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:¶

• RuleID = 20 (8-bit) for uplink fragmentation, named FPortUp.¶
• RuleID = 21 (8-bit) for downlink fragmentation, named FPortDown.¶
• RuleID = 22 (8-bit) for which SCHC compression was not possible (i.e., no matching compression Rule was found), as described in [RFC8724] section 6.¶

FPortUp value MUST be different from FPortDown. 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], implementation MUST implement the following algorithm and SHOULD use it.¶

1. key = LoRaWAN AppSKey¶
2. cmac = aes128_cmac(key, DevEUI)¶
3. IID = cmac[0..7]¶

aes128_cmac algorithm is described in [RFC4493]. It has been chosen as it is already used by devices for 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, location tracking and correlation over time risks. Join periodicity is defined at the application level.¶

Address scan risk is mitigated thanks to AES-128, which provides enough entropy bits of the IID.¶

Using this algorithm will also ensure that there is no correlation between the hardware identifier (IEEE-64 DevEUI) and the IID, so an attacker cannot use manufacturer OUI to target devices.¶

Example with:¶

• DevEUI: 0x1122334455667788¶
• appSKey: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB¶

1. key: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB
2. cmac: 0xBA59F4B196C6C3432D9383C145AD412A
3. IID: 0xBA59F4B196C6C343

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 L2 level (ie: trigger a LoRaWAN join). The way the device is rekeyed is out of scope of this document and left to the implementation.¶

Note: Implementation also using another IID source MUST ensure that the same IID is shared between the device and the SCHC gateway in the compression and decompression of the IPv6 address of the device.¶

5.4. Padding

All padding bits MUST be 0.¶

5.5. Decompression

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 SCHC compression layer. Rule 1 is used by 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 bytes FOpts are used by 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 SCHC compression layer. Rule 1 is used by 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 bytes FOpts are used by LoRaWAN protocol: 11 bytes are available for SCHC payload + 1 byte FPort field. SCHC header is 2 bytes (including FPort) so 1 tile is sent in first fragment.¶

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

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

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

Next transmission MTU is 242 bytes, 4 bytes 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 SCHC compression layer. Rule 1 is used by 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 LoRaWAN protocol: 51 bytes are available for SCHC payload + FPort field => it 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 |

Content of the tile is:¶

| 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 an 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 bits | 5 bits  |

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

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