The controller sends {HEAD ~ the file update information} to {TAIL ~ the agent}, and the agent requests the controller to retrieve file updates 
to
The controller sends { HEAD ~ the file update information } to { TAIL ~ the agent } , and the agent requests the controller to retrieve file updates
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH