identifying {TAIL ~ outdated trackers}, using {HEAD ~ last update field}, and discarding all trackers that are identified as outdated 
identifying { TAIL ~ outdated trackers } , using { HEAD ~ last update field } , and discarding all trackers that are identified as outdated
OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH