RFCOMM is a Bluetooth protocol based on L2CAP. The protocol overhead for RFCOMM is between 4 and 5 bytes for small packets. For every 127 bytes of data, the header increases in size by 1 byte. So the overall protocol overhead is about 8 to 9 bytes for data less than 127 bytes (4 bytes from L2CAP and 4 to 5 bytes from RFCOMM).