Difference between Stop and Wait, GoBackN and Selective Repeat

Reliable data transfers are one of the primary concerns in computer networking. This service department lies in the hands of TCP. Their 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 a row. If the ACK is not received, it re-transmits the previous packet again. 
   
2. Go Back N –
   The sender sends N packets which are 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 packets of window size N and the receiver acknowledges all packets 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 the Receiving end	–	In-order delivery only	Out-of-order delivery as well
Number of retransmissions in case of packet drop	1	N	1
Transmission Type	Half duplex	Full duplex	Full duplex
Implementation difficulty	Low	Moderate	Complex

Where, 

• a = Ratio of Propagation delay and Transmission delay,
• At N=1, Go Back N is effectively reduced to Stop and Wait,
  
• 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.

Reference –
Book – Computer Networks by Tanenbaum

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