An implementation of the Go-Back-N protocol using Java. The protocol is used to transfer a
text file from one host to another across an unreliable network. the nEmulator is used to emulate
network errors, such as packet loss and duplicate packets.

1. SSH into school servers using three different terminal windows to obtain three different hosts. Example:
   Terminal Window 1: ubuntu1604-006.student.cs.uwaterloo.ca -> host1
   Terminal Window 2: ubuntu1604-004.student.cs.uwaterloo.ca -> host2
   Terminal Window 3: ubuntu1604-008.student.cs.uwaterloo.ca -> host3

2. Run:

   make clean

3. Run:

   make

4. Obtain 4 different port numbers using the following terminal command 4 times:

   python -c 'import socket; s=socket.socket(); s.bind(("", 0)); print(s.getsockname()[1]); s.close()'

   let
   9991 -> first port number
   9992 -> second port number
   9993 -> third port number
   9994 -> fourth port number

5. For the following commands, replace hosts and port numbers with their corresponding values.

6. On host1 (terminal window 1), Run:

   ./nEmulator-linux386 9991 host2 9994 9993 host3 9992 1 0.2 0

   Refer to the assignment pdf to change how the behavior of nEmulator-linux386

7. On host2 (terminal window 2), Run the following with the specified output file name

   java Receiver host1 9993 9994

8. On host3 (terminal window 3), Run the following with the specified input file name that you would like to send.

   java Sender host1 9991 9992
   -test.txt is a sample file (~47 500byte packets)

9. Wait for host2 and host3 to finish transferring.

Example:
./nEmulator-linux386 60615 ubuntu1604-004.student.cs.uwaterloo.ca 52477 37587 ubuntu1604-008.student.cs.uwaterloo.ca 59555 1 0.2 0

//receiver
java Receiver ubuntu1604-006.student.cs.uwaterloo.ca 37587 52477 output.txt

//Sender
java Sender ubuntu1604-006.student.cs.uwaterloo.ca 60615 59555 test.txt

host1-> ubuntu1604-006.student.cs.uwaterloo.ca
host2-> ubuntu1604-004.student.cs.uwaterloo.ca
host3-> ubuntu1604-008.student.cs.uwaterloo.ca
9991 -> 60615
9992 -> 59555
9993 -> 37587
9994 -> 52477