Selective repeat protocol, also called Selective Repeat ARQ (Automatic Repeat Request), is a data link layer protocol that uses a sliding window method for the reliable delivery of data frames. Here, only the erroneous or lost frames are retransmitted, while the good frames are received and buffered.

It uses two windows of equal size: a sending window that stores the frames to be sent and a receiving window that stores the frames received by the receiver. The size is half the maximum sequence number of the frame. For example, if the sequence number is from 0 to 15, the window size will be 8.

Working Principle

Selective Repeat protocol provides for sending multiple frames depending upon the availability of frames in the sending window, even if it does not receive acknowledgement for any frame in the interim. The maximum number of frames that can be sent depends upon the size of the sending window.

The receiver records the sequence number of the earliest incorrect or un-received frame. It then fills the receiving window with the subsequent frames that it has received. It sends the sequence number of the missing frame along with every acknowledgement frame.

The sender continues to send frames that are in its sending window. Once, it has sent all the frames in the window, it retransmits the frame whose sequence number is given by the acknowledgements. It then continues sending the other frames.

Sender Site Algorithm of Selective Repeat Protocol

begin
   frame s; //s denotes frame to be sent
   frame t; //t is temporary frame
   S_window = power(2,m-1); //Assign maximum window size
   SeqFirst = 0; // Sequence number of first frame in window
   SeqN = 0; // Sequence number of Nth frame window
   while (true) //check repeatedly
      do
         Wait_For_Event(); //wait for availability of packet
         if ( Event(Request_For_Transfer)) then
            //check if window is full
            if (SeqN–SeqFirst >= S_window) then
               doNothing();
            end if;
            Get_Data_From_Network_Layer();
            s = Make_Frame();
            s.seq = SeqN;
            Store_Copy_Frame(s);
            Send_Frame(s);
            Start_Timer(s);
            SeqN = SeqN + 1;
         end if;
         if ( Event(Frame_Arrival) then
            r = Receive_Acknowledgement();
            //Resend frame whose sequence number is with ACK
            if ( r.type = NAK) then
               if ( NAK_No > SeqFirst && NAK_No < SeqN ) then
                  Retransmit( s.seq(NAK_No));
                  Start_Timer(s);
               end if
                  //Remove frames from sending window with positive ACK
               else if ( r.type = ACK ) then
                  Remove_Frame(s.seq(SeqFirst));
                  Stop_Timer(s);
                  SeqFirst = SeqFirst + 1;
               end if
         end if
         // Resend frame if acknowledgement haven’t been received
         if ( Event(Time_Out)) then
            Start_Timer(s);
            Retransmit_Frame(s);
         end if
end

Receiver Site Algorithm of Selective Repeat Protocol

Begin
   frame f;
   RSeqNo = 0; // Initialise sequence number of expected frame
   NAKsent = false;
   ACK = false;
   For each slot in receive_window
   Mark(slot)=false;
   while (true) //check repeatedly
      do
         Wait_For_Event(); //wait for arrival of frame
         if ( Event(Frame_Arrival) then
            Receive_Frame_From_Physical_Layer();
            if ( Corrupted ( f.SeqNo ) AND NAKsent = false) then
               SendNAK(f.SeqNo);
               NAKsent = true;
            end if
            if ( f.SeqNo != RSeqNo AND NAKsent = false ) then
               SendNAK(f.SeqNo);
               NAKsent = true;
               if ( f.SeqNo is in receive_window ) then
                  if ( Mark(RSeqNo) = false ) then
                     Store_frame(f.SeqNo);
                     Mark(RSeqNo) = true;
                  end if
               end if
               else
               while ( Mark(RSeqNo))
                  Extract_Data(RSeqNo);
                  Deliver_Data_To_Network_Layer();
                  RSeqNo = RSeqNo + 1;
                  Send_ACK(RSeqNo);
               end while
            end if
         end if
   end while
end