Unicast messages are sent to a single end-device and multicast messages are sent to multiple end-devices. All devices of a multicast group must share the same multicast address and associated encryption keys. All end-devices must listen to the multicast channel in order to get multicast messages (end-devices must be C-class). The best use-cases for Multicast is Lighting domain and Firmware update.

There is no ACK for Multicast messages. So we don't know if the multicast message has been received by all end-devices.

The mandatory information is :

• a Cluster name
• a Multicast address: DevAddr, a 32 bit network address (must be unique and not belong to an existing end-device)
  
• the Application session key (AppSKey) and Network session key (NWkSKey) used for frames encryption, 32 digits hexadecimal string.
• radio settings: a fixed frequency in MHz (float32)and a DataRate (a string formatted as “SFxxBWyyy” with xx from 7 to 12 and yyy = 125/250/500 for LoRa modulation or yyy=5000 for FSK modulation)
• a frame counter: FCntDown
• the LoRaWAN MAC version: for Europe 868 MHz, select 1.0.2 MAC version and Regional parameters B revision to be compatible with WMC 2.x

The multicast address and associated network session key and application session key must come from the application layer.
Class-C Multicast downlinks SHALL NOT carry MAC commands.

Yes. The same multicast adrress can be used by several Multicast groups.

Yes. A end-device can belong to several multicast groups.

LNS selects gateways according to the dutycycle and the coverage of each gateway.

In all cases, a gateway cannot be selected if its duty cycle is over the maximum limit or if downlink is not allowed by its owner.

If a end-device has never transmitted data, it won't be taken into account for the choice of gateways used for multicast DataDown.

LNS selects gateways according to their SNR (Signal/Noise ratio). The gateway with the higher SNR (lowest noise) is choosen in priority.

For the moment, this case is not managed.