Fast Retransmit Algorithm

Fast Retransmit Algorithm
If sender sends a segment: 1-5 and receiver sends ACK for 6. Senders sends 6-10 and 11-15. But the data (6-10) lost. After a certain time receiver will sends ACK for 6-10 and by this time 11-15 receive at receiver end. Receiver will send ACK for 6-10 again. This is third time ACK for 6-10. So this is waste of time. We don’t want this situation. That’s why if sender gets three ACK for same segments it use Fast Retransmit Algorithm.

Fast Retransmit:

If sender gets three duplicates ACK for the same segment, then it uses fast retransmit algorithm.

Fast Retransmit Algorithm:

Event: ACK received, with ACK field value of y

                 if (y > SendBase)

        {

                       SendBase = y

                       if (there are currently unacknowledged segments)

                             start timer

                     }

                 else

{

                         increment count of dup ACKs received for y

                         if (count of dup ACKs received for y = =3)

    {

                              resend segment with sequence number y

                             }

                         break;

}

TCP segment sent algorithm:

When data will receive from upper layer then

create TCP segment with sequence number PktNum

                 if (timer currently not running)

                       Start timer

                 Send segment

 SegmentNum = SegmentNum + length(data)

That’s why it initialize

SegmentNum=1

WindowBase-1

If timer is timeout then

                 retransmit not-yet-acknowledged segment with

                         smallest sequence number

                 start timer

if ACK received, with ACK field value of y then

                 if (y > WindowBase) {

                       WindowBase = y

                      if (there are currently not-yet-acknowledged segments)

                               start timer

                      } 


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