Fixed version of rdt 2.0 rdt 2.1

The  rdt 2.1 sender and receiver  FSM each now  have  twice  as many  states  as before. This is because  the protocols  state must  now reflect whether  the packet   currently being  sent  or expected should have  a sequence number 0 or  note  that the action  as those  states where a 0 numbered packet is being  sent or expected are mirror images of those  where a  1-  numbered packet is being  sent  or expected the  only  difference have  to do  with the  handling  of the  sequence number .

                                                            figure rdt sender , handles garbled it ACK/ NACKs

Protocols rdt 2.1  uses both  positive  and negative acknowledgments  from the  receiver to  the sender  when an  out of  order packet is received the received the  receiver send  a positive  acknowledgment for the  packet it has received, when a corrupted packet is received the  receiver send a negative acknowledgement. We can  accomplish  the same  effect as a NAk  if instead of sending an NAK  we instead  send  an ACK  for the  last correctly received  packet. A sender that receives two ACK for the same  packet knows that the receiver did not correctly receive the packet the  following  the packet  that is being ACK   twice.