receiving the failure message and deciding whether to return back to {TAIL ~ an IP multicast access} of {HEAD ~ a unicast mode} 
receiving the failure message and deciding whether to return back to { TAIL ~ an IP multicast access } of { HEAD ~ a unicast mode }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD