i have a really simple question relating to 6LoWPAN, (i am new to this field so ignore any mistakes i make in terminology) in 6LoWPAN fragmentation there is no reliability because it uses UDP at transport layer i.e unlike TCP there is no ACK mechanism. is there any way that the source can know about the fragment loss on the way to the destination device? or any other device (intermediate or destination) can inform the source about it?