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 protocolis 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（3）处应选择（）
A.path
B.window
C.frame
D.diagram
参考答案：B
解析：
传输层协议有多种作用。其一是产生进程（或程序）之间的通信，TCP使用端口号来实现这个功能。传输层协议的另一个作用是形成传输层的流量和差错控制机制，TCP使用滑动窗口协议来进行流量控制，并且使用应答分组、超时机制和重传技术来实现差错控制功能。传输层还要为应用程序建立连接，应用程序向传输层发送数据流，发送方的传输层就与接收方的传输层建立对应的连接。