Implementing Byte Stuffing using Java

Need for Byte-Stuffing
In variable-size framing at the data link layer, we need to define a way to separate one frame from the next. Byte stuffing is employed to accomplish the task. In byte stuffing an 8-bit flag (‘F’) is added at the beginning and at the end of the frame, thereby distinguishing one frame from the next. Therefore, every time a flag sequence (‘F’) is encountered, it signifies the beginning or end of a frame. This, ingenious scheme, however would give rise to a discrepancy, if the flag pattern (‘F’) would occur within the data carried by the frame itself. Byte stuffing comes to the rescue here, by stuffing the original data with an extra 8-bit escape sequence (‘E’) before the flag pattern, whenever it occurred within the data carried by a frame. The receiver would then have to de-stuff the escape sequence, in order to obtain the original data.

A simple question that might arise at this juncture is, what if the escape sequence (‘E’) formed a part of the data to be sent! This scenario is handled in exactly the same way as described above, i.e an extra 8-bit escape sequence (‘E’) is added to the original data before the escape sequence that formed part of the data.

In character-oriented protocols, where data to be carried are 8-bit characters, byte stuffing is employed to handle the problems discussed above.

To make things simpler we will consider only three types of byte sequences in the sent data, as :
F : Flag Sequence
E : Escape Sequence
D : Any other Data Sequence

For Example :

At Sender Side
Enter the Message to be Sent : 
DDEDFFDE
The data being sent (with byte stuffed) is : FDDEEDEFEFDEEF
Seding Message....
Thanks for the Feedback Server!!

At Receiver Side
Message Received...Successfully!!!
The Stuffed Message is : FDDEEDEFEFDEEF
The Destuffed Message is : DDEDFFDE
Messaging is over.....EXITING

From the above example we can see how the original data is recovered at the receiver end.

Approach

At Sender (Client) Side

1. Data of each frame at the sender side, is first stuffed with 8-bit flag sequence (‘F’) at the beginning and end of each frame.
2. Next, the data is scanned to see if any similar flag sequence (‘F’) forms a part of it or not. If yes, then before each such flag sequence, an extra escape sequence (‘E’) is stuffed.
3. Now, if any similar escape sequence (‘E’) is found to form a part of the data to be sent, then an extra escape sequence (‘E’) is stuffed before the occurrence of each such escape sequence.
4. Finally, this stuffed data is sent by the sender.

At Receiver (Server) Side

1. The receiver skips over the first and last bytes of data received, as they are merely for signalling the beginning and end of one frame, respectively and does not carry any useful data.
2. From the next byte on wards data is scanned and if two escape sequences (‘E’) are found in succession, the first one is de-stuffed. Similarly, if an escape sequence is followed by a flag sequence (‘F’), the former is de-stuffed.
3. This strategy helps the receiver recover the actual sent data, accurately.

Implementation

At Sender (Client) Side

// Java Code for Byte_Stuffing Sender
package byte_stuffing;
import java.io.*;
import java.util.*;
import java.net.*;

public class Byte_Stuffing_Client {
    public static void main(String args[]) throws IOException
    {
        InetAddress ip = InetAddress.getLocalHost();
        int port = 45678;
        Scanner sc = new Scanner(System.in);

        // Opens a socket for connection
        Socket s = new Socket(ip, port);

        // Declaring I/O Streams
        DataInputStream dis = new DataInputStream(s.getInputStream());
        DataOutputStream dos = new DataOutputStream(s.getOutputStream());

        while (true) {
            System.out.println("Enter the Message to be Sent : ");
            String data = sc.nextLine();
            String res = new String();

            // Data in each frame is stuffed by 'F' at beginning and end
            data = 'F' + data + 'F';
            for (int i = 0; i < data.length(); i++) {

                // Stuff with 'E' if 'F' is found in the data to be sent
                if (data.charAt(i) == 'F' && i != 0 && i != (data.length() - 1))
                    res = res + 'E' + data.charAt(i);

                // Stuff with 'E' if 'E' is found in the data to be sent
                else if (data.charAt(i) == 'E')
                    res = res + 'E' + data.charAt(i);
                else
                    res = res + data.charAt(i);
            }

            System.out.println("The data being sent (with byte stuffed) is : " + res);

            // Send the data to the receiver
            dos.writeUTF(res);
            System.out.println("Seding Message....");
            if (dis.readUTF().equals("success"))
                System.out.println("Thanks for the Feedback Server!!");

            // End Messaging
            dos.writeUTF("bye");
            break;
        }

        // Close all connections
        s.close();
        dis.close();
        dos.close();
    }
}

At Receiver (Server) Side

// Java code for Byte_Stuffing Receiver
package byte_stuffing;
import java.io.*;
import java.net.*;

public class Byte_Stuffing {
    public static void main(String[] args) throws IOException
    {
        // Opens a socket for connection
        ServerSocket servsock = new ServerSocket(45678);

        // Used to block until a client connects to the server
        Socket socket = servsock.accept();

        // Declaring I/O Streams
        DataInputStream dis = new DataInputStream(socket.getInputStream());
        DataOutputStream dos = new DataOutputStream(socket.getOutputStream());

        while (true) {
            String out = new String();
            // Used to read the data sent by client
            String res = dis.readUTF();
            System.out.println("Message Received...Successfully!!!");
            System.out.println("The Stuffed Message is : " + res);

            for (int i = 1; i < res.length() - 1; i++) {

               // If data contains a 'D' or 'F' do not unstuff it
                if (res.charAt(i) == 'D' || res.charAt(i) == 'F')
                    out = out + res.charAt(i);

                // If data contains 'E' followed by 'E', de-stuff the former 'E'
                else if (res.charAt(i) == 'E' && res.charAt(i + 1) == 'E') {
                    out = out + 'E';
                    i++;
                }
            }
            System.out.println("The Destuffed Message is : " + out);
            dos.writeUTF("success");
            String ch = dis.readUTF();
            if (ch.equals("bye")) {
                System.out.println("Messaging is over.....EXITING");
                break;
            }
        }

        // Closing all connections
        socket.close();
        dis.close();
        dos.close();
    }
}

The Server (Receiver) should start running prior to the Client (Sender). This is essential because the client needs an open connection (socket) to send the data and this connection is opened by the server.

Output

At Client (Sender) Side
Enter the Message to be Sent : 
DFEDDFED
The data being sent (with byte stuffed) is : FDEFEEDDEFEEDF
Seding Message....
Thanks for the Feedback Server!!

At Server (Receiver) Side
Message Received...Successfully!!!
The Stuffed Message is : FDEFEEDDEFEEDF
The Destuffed Message is : DFEDDFED
Messaging is over.....EXITING