Go Back Client-Server Java implementation

/******  GO BACK Server  ******/
 
import java.io.*;
import java.net.*;
import java.util.LinkedList;
public class GoBackNserver {
    public static void main(String args[]) {
        try {
            ServerSocket s = new ServerSocket(7079);
            Socket con = s.accept();
            BufferedReader fromclient = new BufferedReader(new InputStreamReader(con.getInputStream()));
            DataOutputStream toclient = new DataOutputStream(con.getOutputStream());
            OutputStream output = con.getOutputStream();
            LinkedList < String > list = new LinkedList < String > ();
           
            String f = fromclient.readLine();
            int nf = Integer.parseInt(f);
            System.out.println("Number of packets = " + nf);
           
            String flow = fromclient.readLine();
            int ack = 0;
            System.out.println("Flow is "+flow);
            if(flow.contains("n") || flow.contains("N")){
            for (int i = 0; i < nf; i++) {
                   
                    if (i == 2) {
                        list.add(i, "killed");
                        ack = 2;
                        continue;
                    }
                    String n = fromclient.readLine();
               
                    int num = Integer.parseInt(n);
                    String packet = fromclient.readLine();
                    list.add(num, packet);

                    if (ack >= 2) {
                        System.out.println("Not received packet" + ack + "..... Hence Discardingpacket" + num);
                        num=num-1;
                    }else
                    {
                        System.out.println("received packet" + num + " = " + packet);
                    }
                    if (list.contains("killed")) {
                        toclient.writeBytes(ack + "\n");
                        num = ack-1;
                    }
            
                    System.out.println("sending ack" + num);
                    toclient.flush();
                    toclient.writeBytes(num + "\n");
                    toclient.flush();
                }
                System.out.println(list);
           
                    for (int r = ack; r < nf; r++) {
                        String n = fromclient.readLine();
                        int num = Integer.parseInt(n);
                        String packet = fromclient.readLine();
                        System.out.println("received packet" + num + " = " + packet);
                        list.add(num, packet);
                    }
            }else{
                int k=0;
                for (int i = 0; i < nf; i++) {
                    String n = fromclient.readLine();
                    int num = Integer.parseInt(n);
                    String packet = fromclient.readLine();
                    System.out.println("received packet" + num + " = " + packet);
                    list.add(num, packet);
                    k=i;
                }
                if(k==(nf-1)){
                    for(int j=0;j                        int num=j;
                        System.out.println("sending ack" + num);
                        toclient.flush();
                        toclient.writeBytes(num + "\n");
                        toclient.flush();
                    }
                }
            }
               
            fromclient.close();
            output.close();
            con.close();
            s.close();
        } catch (Exception e) {
            System.out.println(" " );
        }
    }
}




/************ Go Back Client *************/

import java.io.*;
import java.net.*;
import java.text.DecimalFormat;
import java.text.NumberFormat;
import java.util.LinkedList;

public class GoBackNclient {
   
    public static void main(String args[]) {
        long start = System.currentTimeMillis();

        try {
            Socket con = new Socket("localhost", 7079);
            BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
            DataOutputStream toserver = new DataOutputStream(con.getOutputStream());
            BufferedReader fromserver = new BufferedReader(new InputStreamReader(con.getInputStream()));

           
            LinkedList < String > list = new LinkedList < String > ();
          
        
            System.out.println("Enter number of packets to be transmitted");
            String n = in .readLine();
            int nf = Integer.parseInt(n);
            toserver.writeBytes(nf + "\n");
          
            for (int i = 0; i < nf; i++) {
                System.out.println("Enter packet" + i + "to be transmitted");
                String packet = in .readLine();
                list.add(packet);
            }
           
            System.out.println("\nYou have entered following data "+list+"\n");
           
            System.out.println("Please enter Y for  successful sending or N for packet loss");
            String flow = in.readLine();
            System.out.println("input read is "+flow);
            toserver.writeBytes(flow+"\n");
            int discard = 0;
            int ack = 0,w=4,wind=0,p=0,q=0;
            if(flow.contains("n") || flow.contains("N")){

            for (int i = 0; i < nf; i++) {
                p=i;
                wind = windowSize(p,q);
               
                                   
                    if (i == 2)  {
                        discard = 2;
                        System.out.println("Packet lost is"+i);
                        q=w;
                        wind = windowSize(p,q);
                        wind++;
                        q=wind;
                        continue;
                    } else {
                        String str = list.get(i);
                        if (discard == 2) str = "Discarded";
                        toserver.writeBytes(i + "\n");
                        toserver.writeBytes(str + "\n");
                    }
           
                    wind++;
                    q=wind;
                    String ak = fromserver.readLine();
               
                    ack = Integer.parseInt(ak);
                    if(ack==2)
                    System.out.println("Ack received for "+(ack-1));
                    else
                        System.out.println("Ack received for " + ack);
               
            }
                   
               
                    wind=0;q=4;
                    for (int r = ack; r < nf; r++) {
                        p=r;
                        wind = windowSize(p,q);
                        q=0;
                        System.out.println("Resending packet" + r);
                        String str = list.get(r);
                        toserver.writeBytes(r + "\n");
                        toserver.writeBytes(str + "\n");
                        wind++;
                        q=wind;
                    }
               
               
                   
                }else
                {    q=0;
                    for (int i = 0; i < nf; i++) {
                    p=i;
                    wind = windowSize(p,q);

                        System.out.println("Sending packet" + i);
                        String str = list.get(i);
                        toserver.writeBytes(i + "\n");
                        toserver.writeBytes(str + "\n");
                        wind++;
                        q=wind;
                        if(i==(nf-1)){
                            for(int j=0;j                                String ak = fromserver.readLine();
                       
                                ack = Integer.parseInt(ak);
                                ack=j;
                                System.out.println("Ack received for " + ack);
                            }
                        }
                }
                       
                       
               
               
            }
            fromserver.close();
            toserver.close();
            con.close();
        } catch (Exception e) {
            System.out.println("\n\n ERROR" + e);
        }
       
    
        long end = System.currentTimeMillis();
        NumberFormat formatter = new DecimalFormat("#0.00000");
        System.out.println("\nExecution time is " + formatter.format((end - start) / 1000d) + " seconds");

    }
   
    public static int windowSize(int p,int wind)throws InterruptedException
    {
        int i=p,w=i+4,windw=wind;
        LinkedList < Integer > list1 = new LinkedList < Integer > ();
        if(i==0 || windw==4){
            System.out.println("\nBased on the window size, packets to be sent ");
            for(int q=i;q                windw=0;
                list1.add(q);
               
            }
            System.out.println(list1);
            if(windw==0){
                Thread.currentThread();
                Thread.sleep(5000);
            }
        }
    return windw;
    }
}

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