68.A transport layer protocol has several responsibilities. One is to create a process-to-process (program-to-program) communication; TCP uses port ( ) to accomplish this. Another responsibility of a transport layer protocol is to create a ( ) and error-control mechanism at the transport level. TCP uses a sliding ( ) protocol to achieve flow control. It uses the acknowledgment packet, time-out, and retransmission to achieve ( ) control. The transport layer is also responsible for providing a connection mechanism for the application program. The application program sends ( ) 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.