Selective Repeat | Sliding Window Protocol

Sliding Window Protocol-


Before you go through this article, make sure that you have gone through the previous article on Sliding Window Protocol.


The two well known implementations of sliding window protocol are-



  1. Go back N Protocol
  2. Selective Repeat Protocol


In this article, we will discuss about Selective Repeat protocol.

Learn about Go back N Protocol.


Selective Repeat Protocol-


Selective Repeat protocol or SR protocol is an implementation of a sliding window protocol.

The features and working of this protocol are explained in the following points-




In SR protocol, sender window size is always same as receiver window size.


In SR protocol,

  • Sender window size = Receiver window size
  • The size is of course greater than 1 otherwise the protocol will become Stop and Wait ARQ.
  • If n bits are available for sequence numbers, then-

Sender window size = Receiver window size = 2n/2 = 2n-1




SR protocol uses independent acknowledgements only.


In SR protocol,

  • Receiver acknowledges each frame independently.
  • As receiver receives a new frame from the sender, it sends its acknowledgement.




SR protocol does not accept the corrupted frames but does not silently discard them.


In SR protocol,

  • If receiver receives a frame that is corrupted, then it does not silently discard that frame.
  • Receiver handles the situation efficiently by sending a negative acknowledgement (NACK).
  • Negative acknowledgement allows early retransmission of the corrupted frame.
  • It also avoids waiting for the time out timer to expire at the sender side to retransmit the frame.




SR protocol accepts the out of order frames.


In SR protocol,

  • Consider receiver receives a frame whose sequence number is not what the receiver expects.
  • Then, it does not discard that frame rather accepts it and keeps it in its window.




SR protocol requires sorting at the receiver’s side.


In SR protocol,

  • Receiver window is implemented as a linked list.
  • When receiver receives a new frame, it places the new frame at the end of the linked list.
  • When the received frames are out of order, receiver performs the sorting.
  • Sorting sorts the frames in the correct order.




SR protocol requires searching at the sender’s side.


In SR protocol,

  • Receiver does not reject the out of order frames.
  • Receiver accepts the out of order frames and sort them later.
  • Thus, only the missing frame has to be sent by the sender.
  • For sending the missing frame, sender performs searching and finds the missing frame.
  • Then, sender selectively repeats that frame.
  • Thus, only the selected frame is repeated and not the entire window.
  • That is why, the protocol has been named as “Selective Repeat Protocol“.




SR protocol leads to retransmission of lost frames after expiry of time out timer.


In SR protocol,

  • Consider a frame being sent to the receiver is lost on the way.
  • Then, it is retransmitted only after time out timer expires for that frame at sender’s side.


Efficiency of SR Protocol-


Efficiency of any flow control protocol is given by-


Efficiency = Sender Window Size in Protocol / (1 + 2a)


In selective repeat protocol, if sender window size = N, then-


Efficiency of SR Protocol = N / (1 + 2a)


To gain better understanding about Selective Repeat ARQ,

Watch this Video Lecture


Next Article- Practice Problems On Selective Repeat Protocol


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Selective Repeat | Sliding Window Protocol
Article Name
Selective Repeat | Sliding Window Protocol
Selective Repeat Protocol in computer networks is a Sliding Window Protocol. Selective Repeat ARQ or SR Protocol is an implementation of sliding window protocol like Go back N Protocol.
Publisher Name
Gate Vidyalay
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