1）p1模拟发送方：首先从界面读取待发送字符（每接受一个字符的输入），保存到文件file1.txt中，并启动计时器；　（2）p2模拟接收方，它从file1.txt中查找是否有新字符到来，并提供模拟界面给用户选择：　　1、 Ack-->接收该字符　　 2、 NAK--〉丢弃　　　3、无反应--〉导致超时　　 将用户选择的结果记录到file2.txt中；　　接收的字符保存到file3.txt 中；　（3）p1等待接收方应答：读取file2.txt 决定下面的操作：　1、如果是ACK，则继续接收用户输入； 2、如果是NAK，则重传该字符；　3、如果超时仍没有新的ACK或NAK，则重传该字符。(1) p1 simulate the sender: first, to be sent from the interface to read characters (each character to accept an input), save to file file1.txt, and start the timer; (2) P2 analog receiver, it looks from file1.txt whether there are new characters come and provide analog interface for the user to choose: 1, Ack--> receiver the 2 character, NAK-- "discard 3 and no response to a timeout --" will be the result of user selection to record file2.txt; save the received characters to file3.txt; (3) p1 receiver response: reading file2.txt decided to wait for the following operations: 1, if ACK, will continue to receive user input, if it is 2; NAK, the retransmission of the character; 3, if the timeout is still no new ACK or NAK, the retransmission of the character.)