A transport layer protocol has several responsibilities, One is to create a process-to-process (program-to-program) communication; TCP uses port (1) to accomplish this. Another responsibility of a transport layer protocol is to create a (2) and error-control mechanism at the transport level. TCP uses a sliding (3) protocol to achieve flow control. It uses the acknowledgment packet, time-out, and retransmission to achieve (4) control. The transport layer is also responsible for providing a connection mechanism for the application program. The application program sends (5) of data to the transport layer. It is the responsibility of the transport layer at the sending station to make a connection with the receiver.
空白（4）处应选择（）
A.packet
B.time
C.error
D.phase
参考答案：C
解析：
传输层协议有多个职责。一是创建进程到进程（程序到程序）的通信，由TCP使用端口号来完成；另一个职责是在传输层创建流和差错控制机制。TCP利用滑动窗口协议来实现流控制。它使用确认分组、超时、重传机制来完成差错控制。传输层也负责为应用程序提供连接机制，应用程序将数据流发送到传输层，发送端的传输层负责建立于接收端之间的连接。