Three-time handshake:
First handshake: The client sends a SYN packet (SYN=X) to the server and enters the Syn_send state, waiting for the server to confirm;
Second handshake: The server receives the SYN packet, it must confirm the customer's SYN (ACK=X+1), and also send itself a SYN packet (syn=y), that is, the Syn+ack packet, when the server enters the SYN_RECV state;
Third handshake: The client receives the server's Syn+ack packet, sends the acknowledgment packet ack (ACK=Y+1) to the server, the packet is sent, the client and the server enter the established state, and the handshake is completed three times.
The data is not included in the packets that are delivered during the handshake, and the client and server formally begin transmitting the data after the three handshake is complete. Ideally, once a TCP connection is established, the TCP connection is maintained until either side of the communication actively closes the connection.
Four-time handshake
Similar to the "three-time handshake" that establishes a connection, disconnecting a TCP connection requires a "four-time handshake".
First wave: The active shutdown sends a FIN, which is used to close the active side to the passive closed side of the data transfer, that is, the active shutdown to tell the passive shut-off party: I will no longer send you the data (of course, the data sent before the FIN packet, if not received the corresponding ACK acknowledgement message, The active shutdown will still re-send the data), but the active shut-off party can also accept the data at this time.
Second wave: The passive closing party receives the fin packet, sends an ACK to the other, confirms that the serial number is received ordinal +1 (same as SYN, one fin occupies a serial number).
Third wave: The passive shut-off side sends a fin, used to close the passive shut-off side to the active shutdown of the data transfer, that is, to tell the active shut-off party, my data is sent out, will not send you data. Wave for the fourth time: the active closing party receives fin, sends an ACK to the passive closing party, confirms the serial number to receive the serial number +1, to this point, completes four times the wave.
The whole process of the TCP three handshake and four waves
