Time out

The  protocols  name Go Back  N is  derived  from the  sender behaviour in the  presence  of lost or  overly  delayed packets. A timer will again  be used  to recover  from  lost data  or acknowledgment packets  acknowledgment packets  if a timeout occurs the sender  resends  all packets that have been previously  sent but  that have  not yet been acknowledged. Our sender in figure sues only a single timer. Which can be thought  of as timer  for the oldest  transmitted but not  yet acknowledged  packet. If an ACK is received but there are still additional transmitted but not yet acknowledgment  packets  teh  timer is restarted. If  there  are no  outstanding  unacknowledged  packets the timer is stopped.

                                                       figure Extended FSM description of GBN Sender

                                                            figure Extends FSM description of GBN receiver

The receiver action in GBN are also  simple. If a packet  with  sequence number n is received  correctly and is in order  the receiver sends an ACK for packet n  and  delivers the data portion of the packet to the upper layer. In  all other  cases the receiver discards the packet and resends an ACK  for the  most  recently received in order packet. Not  that since  packets are delivered one at a time to the upper layer if packet k has been  received an delivered, then  all packets with a sequence number lower than k have also  been delivered. Thus  the use  of cumulative  acknowledgments  is a natural  choice  for GBN.

In our GBN  protocols  the receiver discards out of order packets. Suppose that  packet  n is expected but  packet n +1  arrives. Because data  must be  delivered in order the receiver could  buffer packet n+1 and then  deliver this  packet to the upper layer after it  had later received and delivered packet. However  if packet  n is  lost  both n and packet n+1  will eventually  be retransmitted as  a result  of the GBN retransmission  rule at  the sender. Thus  the receiver  can simply  discard packet n+1.

Let  takes an example  to explain  the function of the GBN protocols  for the  case of a window  size of 4 packet. The  sender can sends  packets 0 to 3 but then  must wait for  one or more  of these  packets  to be acknowledged before  processing. As each  successive ACK for eg ACK  is received  the window  slides forward and the sender can transmit one new packet pkt. On the receiver side packet is lost  and thus packets 2,3, and 4 are found  t be out of order and are discarded  by it. After the timeout  of packet 2 the sender  can send the same packet which is lost. When  the lost packet is received by the receiver and receiver sends the acknowledgment back to the sender then the other  discarded packets pkt 2,3, and 4 will be delivered to the receiver .