N  = window size
Sn = sequence number
Sb = sequence base
Sm = sequence max
ack = ack number
nack = first non acknowledged
Receiver:
Do the following forever:
  Randomly accept or reject packet

  If the packet received and the packet is error free
        Accept packet
        Send a positive ack for packet
  Else
        Refuse packet
        Send a negative ack for packet

Sender:
Sb = 0
Sm = N − 1
ack = 0
Repeat the following steps forever:
  Send packet with ack
  If positively ack is recieved:
         ack++
         Transmit a packet where Sb <= ack <= Sm.
         packets are transmitted in order
  Else
          Enqueue the nack into the queue
  //check if last packet in the window is sent
  if(ack==Sm)
    if(queue is not empty)
          // start from the first nack packet
         nack = queue.front();
         empty the queue
         ack = nack

    Sm = Sm + (ack − Sb)
    Sb = ack