I don't know .. but ideally is if nrf24l01 set ack with payload .. and this payload will contain random number or rtc time .. encrypted with xtea . all node or repeater if receive ack then got this number or rtc time .. and use in next packet .. nrf24l01 chip can make 50%work alone .. node if send data packet then wait for hardware ack . and this ack can contain payload .. this payload will be starting point ..gw will generate actual payload for ack sequence .. It's just idea ..