TCP's three handshakes and four waves

    One .TCP：（Transmission Control Protocol, Transmission control protocol ） Is a reliable connection oriented transport protocol .

After completing the basic work of the transport layer , It also needs to further ensure the reliability of transmission , Data needs to be connected before transmission , Disconnect after data transmission .

    Two .TCP Datagram structure

 

TCP Identifier for ：

URG: Pointer to an emergency （urgent pointer）

ACK: Confirm that the serial number is valid .

PSH: Urge the receiver to deliver the message to the application layer as soon as possible .

RST: Reset connection

SYN: Create a new connection .

FIN: Disconnect a connection . 

    3、 ... and .TPC Three handshakes of

TCP When establishing a connection, three packets are transmitted , Commonly known as three handshakes

The first handshake ： Issued by client SYN The flag requests to establish a connection with the server .

The second handshake ： The server replies ACK Confirm the message to the client , confirm The receiving capacity of the server And The sending ability of the client No problem, and allocate resources for this link .     

The third handshake ： Again, it is sent by the client ACK Message reply server , confirm Client acceptance And The sending capacity of the server No problem and establish a connection .

    Four .TCP Four waves

    TCP The four waves are the four processes of disconnecting , It allows the computer to release resources that are no longer in use . If the connection does not disconnect normally , Not only will it cause data transmission errors , It also causes the socket to fail to close , Keep taking up resources , If the concurrency is high , Server pressure is worrying .

    Process Overview

 a( Data transmission completed , Request disconnect .)

 b( Disconnect request received , Please give me some time to prepare .）

b( I'm ready , You can disconnect .）

a( well , Thank You for Your Cooperation .）       

First wave ： The client sends one FIN The end flag is used to disconnect the data transmission from the client to the server .

Second wave ： Server received FIN After the end flag , send out ACK The confirmation message replies to the client .

Third wave ： Server send FIN End flag to client , Used to disconnect the data transmission from the server to the client .

Fourth wave ： Client received FIN After the end flag , send out ACK The confirmation message replies to the server .