TCP handshake
First handshake: When establishing a connection, the client sends an SYN package (SYN = J) to the server, and enter the SYN_SEND state, waiting for the server to confirm;
Second Handshake: The server receives the SYN package, must confirm the customer's SYN (ACK = J 1), and also send an SYN package (SYN = K), the SYN ACK package, at which time the server enters the SYN_RECV status;
The third handshake: The client receives the server's SYN ACK package, send a confirmation package to the server (Ack = K 1), this package is sent, the client and the server enters the Established state, complete three handshakes.
