I would like to ask what is the data protocol used over BT to send messages in Ruuvitag? Is it the OMA Lightweight M2M or another REST based protocol?
Data is in BLE advertisements in manufacturer specific format.
https://github.com/ruuvi/ruuvi-sensor-protocols/blob/master/dataformat_05.md is the most relevant one in the future, https://github.com/ruuvi/ruuvi-sensor-protocols/blob/master/dataformat_03.md is shipped right now.
You can find ready-made parsers at
https://github.com/lautis/ruuvitag-listener (only RAWv1 / format 3 AFAIK)