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Difference between Stop and Wait, GoBackN and Selective Repeat

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  • Difficulty Level : Easy
  • Last Updated : 22 Mar, 2022

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 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 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: 

PropertiesStop and WaitGo Back NSelective Repeat
Sender window size1NN
Receiver Window size11N
Minimum Sequence number2N+12N
Efficiency1/(1+2*a)N/(1+2*a)N/(1+2*a)
Type of AcknowledgementIndividualCumulativeIndividual
Supported order at the Receiving endIn-order delivery onlyOut-of-order delivery as well
Number of retransmissions in case of packet drop1N1
Transmission Type Half duplexFull duplexFull duplex
Implementation difficultyLow ModerateComplex

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.

Reference –
Book – Computer Networks by Tanenbaum

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