WisBlock LoRaWan DevNonce

The LoRaWan specification indicates that a device can only use a dev-nonce once. If the device powers down and needs to re-establish a connection to a LoRa Gateway it either needs to start at 1 and step through each value until one that has not been used is found, or it needs to store the last dev-nonce value that was used and increment that value. It is this latter more efficient approach that is implemented here.

The LoRaWan 1.04 specification chapter 6.2.5 “Join-Request Frame” says “A DevNonce value shall never be reused for a given JoinEUI value. If the end-device can be power-cycled, then DevNonce shall be persisted (e.g. stored in non-volatile memory). Resetting DevNonce without changing JoinEUI will cause the Join Server to discard Join-Requests of the end-device. For each end-device the Join Server keeps track of the last DevNonce value used by the end-device and ignores Join-Requests if DevNonce is not incremented“.

The structure of the lora-mac code allows for board specific customizations by implementing hardware device files including eeprom-board.c and rtc-board.c. I have implemented this in the rtc-board file and I extended the rtc-board.h template to include interfaces for RtcEEPROMWrite and RtcEEPROMRead.

The DevNonce value is generated in the LoRaMacCrypto.c file in the LoRaMacCryptoPrepareJoinRequest function. Here I retrieve the two bytes having the last DevNonce value used, increment it and store the two bytes back into RTC EEPROM. Full source code is on my GitHub repository here.