Problem Definition: Write a program in Java to implement Go Back N algorithm.
The Program sends the frames from the Client to the Server with checking for missing frames via sending an acknowledgement.

P.S. Enter the inputs in the Client program after the connection is established with the Server.

/*Server Program*/

import java.net.*;
import java.io.*;
import java.util.*;
public class Server
{
public static void main(String args[]) throws Exception
{
ServerSocket server=new ServerSocket(6262);
System.out.println(“Server established.”);
Socket client=server.accept();
ObjectOutputStream oos=new ObjectOutputStream(client.getOutputStream());
ObjectInputStream ois=new ObjectInputStream(client.getInputStream());
System.out.println(“Client is now connected.”);
int x=(Integer)ois.readObject();
int k=(Integer)ois.readObject();
int j=0;
int i=(Integer)ois.readObject();
boolean flag=true;
Random r=new Random(6);
int mod=r.nextInt(6);
while(mod==1||mod==0)
mod=r.nextInt(6);
while(true)
{
int c=k;
for(int h=0;h<=x;h++)
{
System.out.print(“|”+c+”|”);
c=(c+1)%x;
}
System.out.println();
System.out.println();
if(k==j)
{
System.out.println(“Frame “+k+” recieved”+”\n”+”Data:”+j);
j++;
System.out.println();
}

else
System.out.println(“Frames recieved not in correct order”+”\n”+” Expected farme:” + j +”\n”+ ” Recieved frame no :”+ k);
System.out.println();
if(j%mod==0 && flag)
{
System.out.println(“Error found. Acknowledgement not sent. “);
flag=!flag;
j–;
}
else if(k==j-1)
{
oos.writeObject(k);
System.out.println(“Acknowledgement sent”);
}
System.out.println();
if(j%mod==0)
flag=!flag;
k=(Integer)ois.readObject();
if(k==-1)
break;
i=(Integer)ois.readObject();
}
System.out.println(“Client finished sending data. Exiting”);
oos.writeObject(-1);
}
}

/*Client Program*/

import java.util.*;
import java.net.*;
import java.io.*;
public class Client
{
public static void main(String args[]) throws Exception
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.print(“Enter the value of m : “);
int m=Integer.parseInt(br.readLine());
int x=(int)((Math.pow(2,m))-1);
System.out.print(“Enter no. of frames to be sent:”);
int count=Integer.parseInt(br.readLine());
int data[]=new int[count];
int h=0;
for(int i=0;i<count;i++)
{
System.out.print(“Enter data for frame no ” +h+ ” => “);
data[i]=Integer.parseInt(br.readLine());
h=(h+1)%x;
}
Socket client=new Socket(“localhost”,6262);
ObjectInputStream ois=new ObjectInputStream(client.getInputStream());
ObjectOutputStream oos=new ObjectOutputStream(client.getOutputStream());
System.out.println(“Connected with server.”);
boolean flag=false;
GoBackNListener listener=new GoBackNListener(ois,x);
listener=new GoBackNListener(ois,x);
listener.t.start();
int strt=0;
h=0;
oos.writeObject(x);
do
{
int c=h;
for(int i=h;i<count;i++)
{
System.out.print(“|”+c+”|”);
c=(c+1)%x;
}
System.out.println();
System.out.println();
h=strt;
for(int i=strt;i<x;i++)
{
System.out.println(“Sending frame:”+h);
h=(h+1)%x;
System.out.println();
oos.writeObject(i);
oos.writeObject(data[i]);
Thread.sleep(100);
}
listener.t.join(3500);
if(listener.reply!=x-1)
{
System.out.println(“No reply from server in 3.5 seconds. Resending data from frame no ” + (listener.reply+1));
System.out.println();
strt=listener.reply+1;
flag=false;
}
else
{
System.out.println(“All elements sent successfully. Exiting”);
flag=true;
}
}while(!flag);
oos.writeObject(-1);
}
}

class GoBackNListener implements Runnable
{
Thread t;
ObjectInputStream ois;
int reply,x;
GoBackNListener(ObjectInputStream o,int i)
{
t=new Thread(this);
ois=o;
reply=-2;
x=i;
}
@Override
public void run() {
try
{
int temp=0;
while(reply!=-1)
{
reply=(Integer)ois.readObject();
if(reply!=-1 && reply!=temp+1)
reply=temp;
if(reply!=-1)
{
temp=reply;
System.out.println(“Acknowledgement of frame no ” + (reply%x) + ” recieved.”);
System.out.println();
}
}
reply=temp;
}
catch(Exception e)
{
System.out.println(“Exception => ” + e);
}
}
}

/*Client Output

Enter the value of m : 7
Enter no. of frames to be sent:5
Enter data for frame no 0 => 1
Enter data for frame no 1 => 2
Enter data for frame no 2 => 3
Enter data for frame no 3 => 4
Enter data for frame no 4 => 5
Connected with server.
|0||1||2||3||4|

Sending frame:0

Acknowledgement of frame no 0 recieved.

Sending frame:1

Sending frame:2

Sending frame:3

Sending frame:4

Sending frame:5
*/

/*Server Output

Server established.
Client is now connected.
|0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||24||25||26||27||28||29||30||31||32||33||34||35||36||37||38||39||40||41||42||43||44||45||46||47||48||49||50||51||52||53||54||55||56||57||58||59||60||61||62||63||64||65||66||67||68||69||70||71||72||73||74||75||76||77||78||79||80||81||82||83||84||85||86||87||88||89||90||91||92||93||94||95||96||97||98||99||100||101||102||103||104||105||106||107||108||109||110||111||112||113||114||115||116||117||118||119||120||121||122||123||124||125||126||0|

Frame 0 recieved
Data:0
Acknowledgement sent

|1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||24||25||26||27||28||29||30||31||32||33||34||35||36||37||38||39||40||41||42||43||44||45||46||47||48||49||50||51||52||53||54||55||56||57||58||59||60||61||62||63||64||65||66||67||68||69||70||71||72||73||74||75||76||77||78||79||80||81||82||83||84||85||86||87||88||89||90||91||92||93||94||95||96||97||98||99||100||101||102||103||104||105||106||107||108||109||110||111||112||113||114||115||116||117||118||119||120||121||122||123||124||125||126||0||1|

Frame 1 recieved
Data:1
Error found. Acknowledgement not sent.

|2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||24||25||26||27||28||29||30||31||32||33||34||35||36||37||38||39||40||41||42||43||44||45||46||47||48||49||50||51||52||53||54||55||56||57||58||59||60||61||62||63||64||65||66||67||68||69||70||71||72||73||74||75||76||77||78||79||80||81||82||83||84||85||86||87||88||89||90||91||92||93||94||95||96||97||98||99||100||101||102||103||104||105||106||107||108||109||110||111||112||113||114||115||116||117||118||119||120||121||122||123||124||125||126||0||1||2|

Frames recieved not in correct order
Expected farme:1
Recieved frame no :2
|3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||24||25||26||27||28||29||30||31||32||33||34||35||36||37||38||39||40||41||42||43||44||45||46||47||48||49||50||51||52||53||54||55||56||57||58||59||60||61||62||63||64||65||66||67||68||69||70||71||72||73||74||75||76||77||78||79||80||81||82||83||84||85||86||87||88||89||90||91||92||93||94||95||96||97||98||99||100||101||102||103||104||105||106||107||108||109||110||111||112||113||114||115||116||117||118||119||120||121||122||123||124||125||126||0||1||2||3|

Frames recieved not in correct order
Expected farme:1
Recieved frame no :3
|4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||24||25||26||27||28||29||30||31||32||33||34||35||36||37||38||39||40||41||42||43||44||45||46||47||48||49||50||51||52||53||54||55||56||57||58||59||60||61||62||63||64||65||66||67||68||69||70||71||72||73||74||75||76||77||78||79||80||81||82||83||84||85||86||87||88||89||90||91||92||93||94||95||96||97||98||99||100||101||102||103||104||105||106||107||108||109||110||111||112||113||114||115||116||117||118||119||120||121||122||123||124||125||126||0||1||2||3||4|

Frames recieved not in correct order
Expected farme:1
Recieved frame no :4
*/