Patent ID: 8925068

Claim:
A method of preventing a denial of service (DoS) attack using transmission control protocol (TCP) state transition by monitoring flow of packets transmitted between a client and a server using TCP, the method comprising the steps of: when the server receives a connection request (SYN) packet of the client, switching a TCP state of the server from a listening (LISTEN) state to a connection-request-received (SYN_RCVD) state; and after the server sends a connection-request acknowledgment (SYN/ACK — 1) packet to the client, switching the TCP state of the server from the SYN_RCVD state to an acknowledgment-waiting (ACK_WAIT) state; wherein, if the server receives the same SYN packet again in the ACK_WAIT state, the TCP state of the server is switched from the ACK_WAIT state directly to a closed (CLOSED) state and safely terminating a TCP connection, wherein, if the server receives a finish (FIN) packet in the ACK_WAIT state, the server sends an acknowledgement (ACK — 3) packet corresponding to the FIN packet to the client, and then the TCP state of the server is switched from the ACK_WAIT state to a preparing-for-close (CLOSING) state, and wherein if the server does not receive an packets during a previously set timeout period in the CLOSING state, the TCP state of the server is switched from the CLOSING state directly to the CLOSED state, and the TCP connection is safely terminated.