The usual way to ensure reliable delivery is to provide the （71） with some feedback about what is happening at the other end of the line. Typically, the protocol calls for the receiver to send back special （72） frame bearing positive or negative （73） about the incoming frames. If the sender receives a positive acknowledgement about a frame, it knows the the frame has arrived safely. On the other hand, a negative acknowledgement means that something has gone wrong, and the frame must be transmitted again.An additional complication comes from the possibility that hardware troubles may cause a frame to （74） completely. In this case, the receiver will not react at all, since it has no any reason to react. It should be clear that a protocol in which the sender transmits a frame and then waits for an acknowledgement, positive or negative, will hang forever if a frame is ever lost due to, for example, （75） hardware.。
A．data
B．controll
C．request
D．session
参考答案：B
解析：解析： 大致意思是：通信双方之间需要某种方式传递信息，以表示发给对方的数据是否正确收到。在发送过程中，数据可能错误，也可能因为各种各样的原因（例如硬件故障）导致接收方无法收到，造成发送方等待超时。 学好英语，一切都很简单。