Socket Programming

Socket Programming

A network socket is a presented as the endpoint of an inter-process communication flow across a computer network. Today, most communication systems carried out between computers is based on the Internet Protocols. So, nowadays most network sockets that we use are Internet sockets. A socket address possess the combination between the IP address and a port number, much like one of the ends of a telephone connection has the combination between a phone number and a particular extension in a telephone system. Based on this address, the internet sockets used for communication delivers incoming data packets of information to the appropriate application process ends or the threads which help to establish the better communication between the components of the system.

Sockets provide the communication flow between two computers using TCP. And the client program attempts to connect the to the server after creating a socket on its end of the communication.

When the connection is made over the system, the server creates a socket object on its end of the communication. The client and server can now communicate by the means of socket like writing to and reading from the socket.

In JAVA language java.net.Socket provides a socket and java.net.ServerSocket provides the mechanism on how the server program listens to the clients where it establishes the best communication between them.

Steps that occurs when establishing connection using sockets between two computers:

1. The server instantiates a ServerSocket object, which represents the port no denoting which specific communication to occur on.
2. The server invokes the accept() method of the ServerSocket class. This method waits for a client until it connects to the server on the given port.
3. After the server is waiting, a client instantiates a Socket object, specifying which server name and the port number to connect to.
4. The specified server and the specific port number is used to connect the client by the constructor of the socket class. If communication is successfully established then the connected client possess the Socket object capable of communicating with the server.
5. In the server side, the accept() method returns a reference to a new socket on the server that is connected to the client's socket.

There are two different types of sockets:

1. Stream Socket (connection-oriented): uses TCP; telnet, ssh, HTTP...
2. Datagram Socket (connection-less): uses UDP; audio/video streaming

• An Internet socket is characterized by a unique combination of the following:

1. Local socket address:Is resembles Local IP address and port number
2. Remote socket address: Only for established TCP sockets. As discussed in the client-server section, this is necessary since a TCP server may serve several clients concurrently. The server creates one socket for each client, and these sockets share the same local socket address.
3. Protocol: A transport protocol (e.g., TCP, UDP, raw IP, or others). TCP port 53 and UDP port 53 are consequently different, distinct sockets.Fig: Process Communicating through TCP sockets

Fig: The socket primitives [Berkeley Sockets].

Example of Socket Client:- package client.server;

import java.net.*;
import java.io.*;
public class Client{

public static void main(String args[])
throws Exception{
Socket soc=new Socket("localhost",6666);
DataInputStream dataIn=new DataInputStream(soc.getInputStream());
DataOutputStream dataOut=new DataOutputStream(soc.getOutputStream());
BufferedReader buffReader=new BufferedReader(new InputStreamReader(System.in));

String str1="",str2="";
while(!str1.equals("stop")){
str1=buffReader.readLine();
dataOut.writeUTF(str1);
dataOut.flush();
str2=dataIn.readUTF();
System.out.println("Server says to the client: "+str2);
}

dataOut.close();
soc.close();
}
}

Socket Server Example:

package client.server;
import java.net.*;
import java.io.*;

public class Server{

public static void main(String args[])throws Exception{
ServerSocket serverSoc=new ServerSocket(6666);
Socket soc=serverSoc.accept();
DataInputStream dataIn=new DataInputStream(soc.getInputStream());
DataOutputStream dataOut=new DataOutputStream(soc.getOutputStream());
BufferedReader buffReader=new BufferedReader(new InputStreamReader(System.in));

String str=" ",str2=" ";
while(!str.equals("stop")){
str=dataIn.readUTF();
System.out.println("client says: "+str);
str2=buffReader.readLine();
dataOut.writeUTF(str2);
dataOut.flush();
}
dataIn.close();
soc.close();
serverSoc.close();
}
}

Stream Sockets

It is a connection Oriented Sockets used in TCP. The stream socket may serve several clients concurrently, by creating a child process for each client which helps to establish a TCP connection between the child process and the client. But those unique dedicated sockets are needed to be created for each connection when it is essential. The each socket is identified by both the local and remote IP address and port as it needs to establish the connection.

Datagram Sockets

Datagram Sockets are connectionless sockets used in UDP. Here, no child process is created for multiple clients. Single Socket serves all the clients. The socket is identified by local port no. and IP address.

Reference

1. Tanenbaum, WetherRall, Andrew S. , David J. ,Computer Networks. 5th Ed. Boston: Pearson Education, Inc
2. http://www.javatpoint.com/socket-programming
3. http://www.tutorialspoint.com/java/java_networking.htm
4. W. Stallings, “Data and Computer Communication”, Macmillan Press, 1989.
5. Kurose Ross, “Computer Networking: A top-down approach”, 2nd Edition, Pearson Education
6. Larry L. Peterson, Bruce S. Davie, “Computer Networks: A Systems Approach”, 3rd Edition, Morgan Kaufmann Publishers