【71-75】the usual way to ensure reliable delivery is to provide the （71）with some feedbackaboutwhat is happening at the other end of the line. typically， the protocol calls for thereceivertosend back special （72） frame bearing positive or negative （73）about the incomingframes.if the sender receives a positive acknowledgement about a frame， it knows the the framehasarrivedsafely. on the other hand， a negative acknowledgement means that something has gone wrong，andtheframe must be transmitted again. an additional complication comes from the possibility that hardware troubles may causeaframeto （74） completely. in this case， the receiver will not react at all， since it has noanyreasonto react. it should be clear that a protocol in which the sender transmits a frame andthenwaitsfor an acknowledgement， positive or negative， will hang forever if a frame is ever lostdueto，forexample， （75） hardware.
