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 totransmit at any time, randomly selecting a communication channel. The NetworkGateway may reply with a downlink in one of the two receive windows immediatelyfollowing the uplinks. Therefore, the Network Gateway cannot initiate adownlink; it has to wait for the next uplink from the device to get a downlinkopportunity. Class A is the lowest power consumption class.¶

Class B:

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

High periodicity:

Downlinks from the NGW will be sent faster but the device wakes up moreoften 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 butkeep their receiver open whenever they are not transmitting. Class C devicescan receive downlinks at any time at the expense of a higher powerconsumption. Battery-powered devices can only operate in Class C for a limitedamount of time (for example, for a firmware upgrade over-the-air). Most of theClass 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 GatewayMUST 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 requirethe 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 JoinAcceptframe 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'sunique identifier DevEUI and a random nonce that will be used for session keyderivation.¶

JoinAccept:

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

Data:

This refers to MAC and application data. Application data is protected with AES-128encryption. 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 allowsLoRaWAN 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.¶

Note 1: IPv6 multicast addresses must be defined as per[RFC4291]. The 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: The LoRa Alliance defined[LORAWAN-REMOTE-MULTICAST-SET] as theRECOMMENDED way to set up 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 inFigure 5. The SCHC C/D and the SCHC F/RSHALL concatenatethe FPort field with the LoRaWAN payload to recompose the SCHC Message.¶

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

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

A fragmented datagram with application payload transferred from device toNetwork Gateway is called an "uplink-fragmented datagram". It uses an FPort for data uplinkand its associated SCHC control downlinks, named "FPortUp" in this document. Theother way, a fragmented datagram with application payload transferred fromNetwork Gateway to device is called a "downlink-fragmented datagram". It uses anotherFPort 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] andMUST be shared by the device and SCHC gateway prior to the communication with the selected rule. The uplink and downlink fragmentation FPortsMUST be different.¶

5.2.RuleID Management

The RuleIDMUST be 8 bits and encoded in the LoRaWAN FPort as described inSection 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 MSBSHOULD 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 matchingcompression Rule was found), as described inSection 6 of [RFC8724].¶

The FPortUp valueMUST 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 MessageMUST 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 oneor several SCHC gateways. A set of FPort values isREQUIRED for each SCHC gatewayinstance the device is required to communicate with. The application can useadditional uplinks or downlink-fragmented parameters butSHALL implement atleast the parameters defined in this document.¶

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

5.3.Interface IDentifier (IID) Computation

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

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

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:¶

• DevEUI: 0x1122334455667788¶
• AppSKey: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB¶

1. key: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB2. cmac: 0x4E822D9775B2649928F82066AF804FEC3. 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.¶

Note: Implementations also using another IID sourceMUST 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 bitsMUST be 0.¶

5.5.Decompression

The SCHC C/DMUST concatenate FPort and LoRaWAN payload to retrieve the SCHC Packet as perSection 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, withfragmentation.¶

An applicative data of 300 bytes is passed to the SCHC compression layer. Rule 1is used by the SCHC C/D layer, allowing to compress it to 282 bytes and 5 bits: 1 byteRuleID, 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 inFigure 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 aretransmitted, the last tile is only 2 bytes + 5 bits. Padding is added forthe 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 iscorrect 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 1is used by the SCHC C/D layer, allowing to compress it to 130 bytes and 5 bits: 1 byteRuleID, 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 inFigure 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       |