A brief introduction to the working principle of three-way handshake and four-time closing of TCP

Three-way handshake:

Client A relaxes a connection request message to server B, B sends back a confirmation message after receiving it, A sends a confirmation message after receiving it, and B formally establishes the connection after receiving it. Why does client A need to send another confirmation message later? This is to prevent the last invalid connection request message sent by A from being sent to B suddenly, when B needs to send back a confirmation message, but A knows that this is an invalid connection request message. it does not send a confirmation message, and B does not wait for a period of time without receiving the confirmation, thus saving the resources of server B.

Shut down four times:

Why does it take four times for TCP to shut down? Because TCP connection communication is a two-way process, including client A sending data to server B, and server B sending data to client A. Client A sends disconnection message, server B sends back confirmation message after receiving, A no longer sends data after receiving, but it is only in a semi-closed state at this time, server B can still send data; when B sends disconnection message after sending data, A sends back confirmation message after receiving, and B really completes the release of TCP connection after receiving this confirmation message.