Do you want to ask about network knowledge in the interview? Just know a little

One 、TCP And UDP

TCP（Transmission Control Protocol, Transmission control protocol ） agreement ： Responsible for segmenting the files to be transferred transmitted , Generally used to establish a session , Its basic characteristic is reliable transmission 、 flow control , The so-called three handshakes 、 Four waves are also based on TCP Agreed .

UDP（User Data Protocol, User datagram protocol ） agreement ： One data packet can complete data communication , Packets are not segmented , No need to establish a session , No need for flow control , It's an unreliable transmission , On screen broadcast 、 multicast 、 Broadcasts are based on UDP agreement .

Two 、 be based on TCP Connection oriented socket of protocol （Internet Socket）

1、TCP

The Transmission Control Protocol Transmission control protocol .

2、 Streaming socket

We often talk about sockets, that is Internet Scoket, According to the data transmission mode , Can be Internet Scoket There are several kinds of , In the use of socket() When creating a network connection , It must be told which data transmission mode it is , Simply put, it is the control protocol of data transmission .

The computer field is called ： Streaming socket （Stream Sockets）, Use... In computer code SOCK_STREAM Express .

3、SOCK_STREAM Has the following characteristics

• reliable 、 Two way communication data flow ;
• Data transmission is carried out in sequence ;
• The data is accurate in transmission , It has its own retransmission mechanism ;
• The sending and receiving of data are out of sync , There is a buffer ;
• The typical application of stream format socket is HTTP agreement , Because the browser must have complete and correct data when loading and parsing .

3、 ... and 、 be based on UDP Connectionless socket for protocol （Datagram Sockets）

1、UDP

User Datagram Protocol User datagram protocol .

2、 Datagram format socket

be based on UDP Connectionless socket for protocol , The computer field is called ： Datagram format socket （Datagram Sockets）, Used in computer code SOCK_DGRAM Express .

SOCK_DGRAM It's a high efficiency 、 high velocity 、 Unreliable communication data flow . It is based on UDP Agreed , No data verification , Only data transmission .

3、SOCK_DGRAM Has the following characteristics

• There is a limit on the size of data transmitted each time ;
• The pursuit of high efficiency 、 high velocity ;
• The transmitted data may be lost or damaged （ Small probability events ）;
• Sending and receiving data is synchronous , That is, there are data boundaries ;
• A typical application of datagram format is QQ Video and voice , Because the requirements of both are high efficiency and low latency , At the same time, the loss of data with small probability will not affect the transmission quality of the whole data .

Four 、 Blocking problem of receiving data

1、TCP

If there is no data , Blocking Socket I'll wait , Until there are several ;

Non blocking Socket return WSAEWOULDBLOCK;

If there's data , Take as much as you have .

2、UDP

If there is no data , Blocking Socket I'll wait ;

Non blocking Socket return WSAEWOULDBLOCK;

If there's data , It will wait for the whole contract to arrive , And receive the whole contract , To return to .

5、 ... and 、TCP Realize data upload

1、TCP Server side

package com.tcpip;
 
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.ServerSocket;
import java.net.Socket;
 
public class TcpServerTest {
    
    public static void main(String[] args) throws Exception {
    
        //1、 Create services 
        ServerSocket serverSocket = new ServerSocket(9000);
        //2、 Listen for client connections 
        Socket socket = serverSocket.accept();
        //3、 Get input stream 
        InputStream is = socket.getInputStream();
        //4、 File output 
        FileOutputStream fos = new FileOutputStream(new File("copy.png"));
        byte[] buffer = new byte[1024];
        int len;
        while ((len=is.read(buffer))!=-1){
    
            fos.write(buffer,0,len);
        }
        //5、 close resource 
        fos.close();
        is.close();
        socket.close();
        serverSocket.close();
    }
}

2、TCP client

package com.tcpip;
 
import java.io.File;
import java.io.FileInputStream;
import java.io.OutputStream;
import java.net.InetAddress;
import java.net.Socket;
 
public class TcpClientTest {
    
    public static void main(String[] args) throws Exception {
    
        //1、 Create a socket Connect 
        Socket socket = new Socket(InetAddress.getByName("127.0.0.1"),9000);
        //2、 Create an output stream 
        OutputStream os = socket.getOutputStream();
        //3、 Read the file 
        FileInputStream fis = new FileInputStream(new File("01.png"));
        //4、 write file 
        byte[] buffer = new byte[1024];
        int len;
        while ((len=fis.read(buffer))!=-1){
    
            os.write(buffer,0,len);
        }
        //5. close resource 
        fis.close();
        os.close();
        socket.close();
    }
}

3、 test

6、 ... and 、UDP Realize data upload

1、UDP The receiver

package com.tcpip;
 
import java.net.DatagramPacket;
import java.net.DatagramSocket;
 
public class UdpServerTest {
    
    public static void main(String[] args) throws Exception {
    
        //1、 Open ports 
        DatagramSocket socket = new DatagramSocket(9090);
        //2、 Receive packets 
        byte[] buffer = new byte[1024];
        DatagramPacket packet = new DatagramPacket(buffer,0,buffer.length);
        socket.receive(packet);
        System.out.println(new String(packet.getData(),0,packet.getData().length));
        //3、 Close the connection 
        socket.close();
    }
}

2、UDP The sender

package com.tcpip;
 
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
 
public class UdpClientTest {
    
    public static void main(String[] args) throws Exception {
    
        //1、 Build a socket
        DatagramSocket socket = new DatagramSocket();
        //2、 Build a package 
        String msg = "UDP  test ";
        InetAddress localhost = InetAddress.getByName("localhost");
        int port  = 9090;
        DatagramPacket packet = new DatagramPacket(msg.getBytes(),0,msg.getBytes().length,localhost,port);
        //3、 Send package 
        socket.send(packet);
        //4、 close resource 
        socket.close();
    }
}

3、 test

7、 ... and 、 Network layer security

1、 What are the security issues ？

1. IP Deceptive attack
2. Smurf attack （DDOS A type of attack ）
3. ICMP Redirection and unreachable attacks
4. IP Address scanning attack

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