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client closes socket:
clientSocket.close();
step1 :client sends TCP FIN control segment to server
step2: Server receives FIN,replies with ACK. Closes connection,sends FIN.
time wait 30 seconds(in order to receive the package which already send but unreceived)
TCP waits until it has been three duplicate ACKs before retransmitting the packet
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原文地址：http://www.cnblogs.com/mengfanrong/p/5156888.html
