TCP/IP communications

TCP Communication Process

The APPLICATION program based on TCP protocol is divided into two parts: server and client. The working sequence of the two is:

(1) Start to listen to the server application bound to the specific port of the specific host address, listen to the client request; (2) Establish the connection the client application initiates the connection to the server application, and establishes the connection after three handshakes; (3) Data transmission parties use the Socket Socket ports created when the connection is created for data reading and writing; (4) Disconnection Communication ends, one party initiates a disconnection message, and after four waves, the connection is officially disconnected.

TCP three-way handshake

TCP uses a “three-way handshake” to establish a client-server connection. The specific process is as follows:

The Socket communication

After establishing a connection, the server and client in a TCP application create a pair of connected Socket objects, which are used to write and read data from each other.

Net. The Server object

Net. The Socket object

Creating a TCP Server

const net=require('net');
var server=net.createServer({
	allowHalfOpen:false.// Whether to allow half-connections. Default is false
  pauseOnConnect:false// Whether to suspend the Socket once the connection is established. The default value is false
},(socket)=>{	// Connect listener
  console.log("Received a client connection!")
  socket.on("data",(data)=>{	// Data is read
  	console.log("Read client data: %s",data)
  })
  socket.write('hello\r\n');
  socket.on('end', () = > {console.log("Connection disconnected!"); })}); server.on('error',(err)=>{
	throw err;
})
server.listen('8000', () = > {console.log('Listening on port 8000... ')})Copy the code

Connect requests can be processed using the createServer callback described above, or you can bind a listener function to the server to listen for the connect connection to be established before processing:

const net=require('net');
var server=net.createServer();// If the default configuration parameters do not need to be modified, you can omit them
server.on('connection',(socket)=>{
  server.getConnections((err,count) = >{
        if(err) throw err;
        console.log("Number of client requests being processed by the server: %d!",count)
  })
	console.log("Received a client connection request...");
})
server.on('error',(err)=>{
	throw err;
})
server.listen('8000', () = > {console.log('Listening on port 8000... ')})// Listening on port 8000...
// Received a client connection!
// The number of client requests being processed by the server is: 1!
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Suppose we only allow the TCP server to connect to a maximum of 2 requests:

const net=require('net');
var server=net.createServer();
server.setMaxListeners=2;		// Set the maximum number of connections the server can hold
server.on('connection',(socket)=>{
    server.getConnections((err,count) = >{
        if(err) throw err;
        console.log("Number of client requests being processed by the server: %d!",count)
    })
    console.log("Received a client connection request...");
})
server.on('error',(err)=>{
    throw err;
})
server.listen('8000', () = > {console.log('Listening on port 8000... ')})Copy the code

The first connection and the second connection request were successful, but the third connection request could not be connected.

We can also determine the number of connections and shut down the server when the number of connections reaches 3 or more:

const net=require('net');
var server=net.createServer();// If the default configuration parameters do not need to be modified, you can omit them
server.on('connection',(socket)=>{
    server.getConnections((err,count) = >{
        if(err) throw err;
        console.log("Number of client requests being processed by the server: %d!",count);
        if(count>2){ server.close(); }})console.log("Received a client connection request...");
})
server.on('error',(err)=>{
    throw err;
})
server.listen('8000', () = > {console.log('Listening on port 8000... ')})Copy the code

Sockte object attributes:

const net=require('net');
var server=net.createServer();// If the default configuration parameters do not need to be modified, you can omit them
server.on('connection',(serverSocket)=>{
    console.log("Received a client connection request...");
    console.log("Server port: %s",serverSocket.localPort);
    console.log("Server IP address: %s",serverSocket.localAddress);
    console.log("Client port: %s",serverSocket.remotePort);
    console.log("Client IP address: %s",serverSocket.remoteAddress);
})
server.listen('8000', () = > {console.log('Listening on port 8000... ')})// Listening on port 8000...
// Received a client connection request...
// Server port: 8000
// Server IP address :: FFFF :127.0.0.1
// Client port: 49345
// Client IP address :: FFFF :127.0.0.1
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Receiving and responding to messages sent by clients:

const net=require('net');
var server=net.createServer();// If the default configuration parameters do not need to be modified, you can omit them
server.on('connection',(serverSocket)=>{
    console.log("Received a client connection request...");
    console.log("Server port: %s",serverSocket.localPort);
    console.log("Server IP address: %s",serverSocket.localAddress);
    console.log("Client port: %s",serverSocket.remotePort);
    console.log("Client IP address: %s",serverSocket.remoteAddress);
    serverSocket.on("data",(data)=>{The message sent from the client to the server is received
        console.log("Received client message: %s",data);
      	serverSocket.write("Server receives data:"+data);// The server sends a message to the client
    })
})
server.listen('8000', () = > {console.log('Listening on port 8000... ')})// Listening on port 8000...
// Received a client connection request...
// Server port: 8000
// Server IP address :: FFFF :127.0.0.1
// Client port: 49381
// Client IP address :: FFFF :127.0.0.1
// Received client message: a
// Received client message: b
// Received client message: c
// Received client message: d
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