Sliding Window Protocol (Go-back-N algorithm)

#Client.py

import socket
import select

def client_program():
    n = 4
    win_start = 0
    win_end = win_start + n - 1
    host = socket.gethostname()
    port = 12366
    sender = []
    flag = 0
    client_socket = socket.socket()
    client_socket.connect((host, port))
    print('Window Size is ', n)
    print('******** Enter "bye" to close connection ***************')
    message = input("Hit any key to start sending frames -> ")
    
    while message.lower().strip() != 'bye':
        print("Sending frames...")
        if flag == 0:
            for i in range(n):
                sender.append(win_start + i)
            for i in sender:
                print("Frame -> ", i)
        else:
            print("Frame -> ", win_start)
        msg = str(win_start)
        client_socket.send(msg.encode())
        
        ready = select.select([client_socket], [], [], 1)
        if ready[0]:
            data = client_socket.recv(1024).decode()
            ack = int(data)
            if ack not in sender:
                win_start = ack
                win_end = win_start + n - 1
                flag = 0
                for i in range(n):
                    sender.pop()
            else:
                win_start = ack
                flag = 1
        else:
            print("Timeout occurred, resending frames...")
            continue
        
        print("************************************")
        print('Received ACK server: ' + data)
        message = input("Hit any key to start sending frames -> ")
    
    client_socket.close()

if __name__ == '__main__':
    client_program()

#---------------------------------------------------------------------------#
#Server.py

import socket
import random

def server_program():
    # get the hostname
    host = socket.gethostname()
    port = 12366  # initiate port no above 1024
    exp = 0
    n = 4
    new = 1
    win_start = 0
    win_end   = win_start + n - 1
    receiver = []
    server_socket = socket.socket()  # get instance
    # look closely. The bind() function takes tuple as argument
    server_socket.bind((host, port))  # bind host address and port together

    # configure how many client the server can listen simultaneously
    server_socket.listen(2)
    conn, address = server_socket.accept()  # accept new connection
    print("Connection from: ", str(address))
    
    while True:

        try:
            # receive data stream. it won't accept data packet greater than 1024 bytes
            data = conn.recv(1024).decode()
            if not data:
                # if data is not received break
                break
        
        except:
            print("Packet Loss\n")
            continue
        
        rec = int(data)
        lim = rec + n - 1
        count = 0
        flag = 0
        ack = rec
        
        # Simulating packet loss by randomly dropping packets
        if random.random() <= 0.3:
            print("Packet Loss: Frame ->", rec)
            continue
        
        randy = random.randint(1, 4)
        if new == 1 : # you received a new frame of a new window
            while(count != randy):
                temp = random.randint(rec, lim)
                
                if temp not in receiver:
                    print("Received Frame -> ", temp)
                    count+=1
                    flag = 1 # At least one new frame added in the receiver buffer
                    receiver.append(temp)
        else :
            print("Received Frame -> ", rec) # you received a new frame of an old window
            receiver.append(rec)
            flag = 1
        if(flag == 1):
            for i in range(rec,lim+1):
                if i not in receiver:
                    ack = i
                    break
                ack = i+1
        
        print( "Sending ACK    -> ", ack) # next expected frame
        print('***************************************************')
        data = str(ack)
        conn.send(data.encode())  # send data to the client

        if ack > win_end :
            win_start = ack
            win_end   = win_start + n - 1
            new = 1 # now receive a new frame of a new window
        else :
            new = 0 # now received a new frame of an old window

    conn.close()  # close the connection


if __name__ == '__main__':
    server_program()