Karn’s Algorithm
What will be an ideal response?
It is very important to figure out when a packet should be retransmitted for the network efficiency. TCP finds the lost packets by checking the timeout of retransmit timer. Sender starts a retransmission timer after transmitting a packet. If the timer expires before an ACK arrives, the packet will be retransmitted. But setting retransmit timer is not easy. If the timer is too long, the network performance degraded. If the timer is too short, unnecessary retransmission may happen. Retransmit Timeout value (RTO) is calculated by referring to RTT (Round Trip Time) when the time received an ACK minus the time a data was sent. TCP used the weighted average of measured RTTs as estimated RTT. Usually TCP computes timeout by doubling this estimated
RTT value. Because TCP used the average value of the RTT, it is not flexible in large scale networks where RTT fluctuates widely.
Karn’s algorithm states that the current round trip time estimator (RTT) should not be updated on receiving ACK for segments that have been retransmitted. This is required since it is
impossible to know which segments that the ACK is corresponding to. It can be the first transmission that triggers this ACK, or it can also be the different retransmission(s). In order to avoid miscalculation for RTT, we should not use ACKs for retransmitted segments to update RTT. The retransmission timeout value (RTO), on the other hand, is doubled on each failure until an acknowledgement is received for a segment that was not retransmitted.
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