QT TCP UDP network communication ＜ theory ＞

It takes about three or four blogs to talk about QT The content of network communication inside , Actually, it can't be said that QT Network communication , No matter what language , It's almost the same , The essence lies in the different ways of implementation , The mechanism of communication is the same

Chapter one ： Basic knowledge of network communication

Network communication overview ： Network programming is the foundation of the interconnection of all things , What's in use now TCP/IP agreement , It itself is divided into four layers ： application layer 、 Transport layer 、 The layer 、 Network interface layer . But as an ordinary programmer , We only use the transport layer and application layer when we write the program . Now common network communication protocols mainly include ：TCP UDP FTP HTTP, Whether in the window still Linux, The system provides a unified socket , That's what they say socket, stay QT in ,QT The network module has been encapsulated QtNetwor, The integration is very high , Basically meet the basic needs .

Transmission control protocol （transmission control protocol ,TCP） It's a reliable 、 Connection oriented 、 Transport protocols for data streams ,HTTP、FTP There are TCP Shadow ,TCP The protocol is suitable for continuous data transmission , It's also a transmission with guaranteed data security .

TCP working principle ：TCP Protocol can provide reliable communication connection , So that the byte stream sent by my computer can be sent to other specified computers on the network without error , Its reliability is mainly due to its request ---- The reply This set of safe mechanism , It's not a simple request and response , It's a complex connection process , Here's the picture ：

Three handshakes and four waves are more important , This mainly introduces the communication mechanism , About tcp Connect three handshakes and four waves, and add to other blogs next time .

Start the server first , Start the client after a while , It establishes a connection with this server after three handshakes . For some time after that , The client sends a request to the server , The server processes the request , And send back a response for the client . This process continues , Until the client sends a file terminator to the server , And close the client connection , Then the server also closes the connection on the server side , End running or wait for a new client connection .

Because of the complicated communication process , So the communication speed must be slower than UDP communication .

UDP：
User datagram protocol (User Data Protocol, UDP) It's a simple lightweight 、 unreliable 、 For datagram 、
Connectionless transport layer protocol , It can be used when reliability is not very important , Short message 、 Broadcast information, etc .
There are several situations suitable for application :
● Most of the network data is short message ;
● Have a large number of clients ;
● No special requirements for data security ;
● The burden of the Internet is very heavy , But it requires high response speed .

This is obvious , for instance ：QQ And wechat are fully in line with the above applications .

working principle ：UDP Client to UDP The server sends a certain length of request message , The limitation of message size is related to the protocol implementation of each system , But not beyond the lower level IP Under the agreement 64KB; UDP The server also responds in the form of a message . If the server does not receive this request , The client does not resend , So the transmission of message is unreliable .

Communication process ：

The client doesn't have to connect to the server , Only responsible for sending message data to the server , Of course, the server does not need to manage whether the client is connected , Just listen to the client .