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