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