Three handshakes and four disconnects of TCP

TCP The purpose of the three handshakes is to establish a connection between the client and the server , It is convenient for a series of data transmission in the future .

Three handshakes

The first handshake ： The client sends a message to the server with the ID SYN A connectable request packet （ The package serial number is randomly generated , namely seq number=x）, Request a connection to the server .

The second handshake ： The server confirms that it has received the SYN After the message , Return to your own SYN message （seq number=y One of the serial numbers SYN Connection request package ） In response , And send a message with ACK Identify an answer packet （ack=y+1）.

The third handshake ： The client received a message from the server SYN+ACK After the packet , Will send a message with ACK Identification confirmation package （ack=y+1）, After sending , The client and server enter ESTABLISHED state , That is, the connection is successful .

Four disconnects

Disconnect for the first time ： After the data that the client needs to transmit to the server is transmitted , The client will send a message with FIN Identified packets （FIN=1,seq number=x）, Request to disconnect from the server .

Second disconnection ： The server received a message with FIN After the identified packet , Return to the client with ACK Identified packets , namely ACK message （ACK=1,ack=x+1）, It indicates that the server has received the message from the client , At this time, the server stops accepting connections , That is, the server is CLOSE_WAIT state .

Disconnect for the third time ： The server will send a message with FIN Identified packets （FIN=1,seq number=y） , And send a message with ACK Identification confirmation package （ACK=1,ack=x+1） This means that the data that the server needs to transmit to the client has been transmitted , Used to stop the server from sending data to the client .

Disconnect for the fourth time ： Client received FIN After the identified packet , There will still be one ACK The identified message is used as a reply （ACK=1,ack=y+1）, At this time, the client stops accepting connections , That is, the client is CLOSE_WAIT state .