Difference between Stop and Wait, GoBackN and Selective Repeat

Reliable data transfers is one of the primary concerns in computer networking. This service department lies in the hands of TCP. There major flow control protocols – Stop and Wait, Go Back N, and Selective Repeat.

  1. Stop and Wait
    The sender sends the packet and waits for the ACK (acknowledgement) of the packet. Once the ACK reaches the sender, it transmits the next packet in row. If the ACK is not received, it re-transmits the previous packet again.

  2. Go Back N
    The sender sends N packets which is equal to the window size. Once the entire window is sent, the sender then waits for a cumulative ACK to send more packets. On the receiver end, it receives only in-order packets and discards out-of-order packets. As in case of packet loss, the entire window would be re-transmitted.



  3. Selective Repeat
    The sender sends packet of window size N and the receiver acknowledges all packet whether they were received in order or not. In this case, the receiver maintains a buffer to contain out-of-order packets and sorts them.The sender selectively re-transmits the lost packet and moves the window forward.

Differences:

Properties Stop and Wait Go Back N Selective Repeat
Sender window size 1 N N
Receiver Window size 1 1 N
Minimum Sequence number 2 N+1 2N
Efficiency 1/(1+2*a) N/(1+2*a) N/(1+2*a)
Type of Acknowledgement Individual Cumulative Individual
Supported order at Receiving end In-order delivery only Out-of-order delivery as well
Number of retransmissions in case of packet drop 1 N 1

Where,

  • a = Ratio of Propagation delay and Transmission delay,
  • At N=1, Go Back N is effectively reduced to Stop and Wait,/li>
  • As Go Back N acknowledges the packed cumulatively, it rejects out-of-order packets,
  • As Selective Repeat supports receiving out-of-order packets (it sorts the window after receiving the packets), it uses Independent Acknowledgement to acknowledge the packets.



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