A segment is not acknowledged during the retransmission timeout period and is therefore retransmitted. When the sending TCP receives an acknowledgment for this segment, it does not know if the acknowledgment is for the original segment or for the retransmitted one. The value of the new RTT is based on the departure of the segment.

However, if the original segment was lost and the acknowledgment is for the retransmitted one, the value of the current RTT must be calculated from the time the segment was retransmitted. This ambiguity was solved by Karn. Karn’s algorithm is simple.

Do not consider the round-trip time of a retransmitted segment in the calculation of RTTs. Do not update the value of RTTs until you send a segment and receive an acknowledgment without the need for retransmission. TCP does not consider the RTT of a retransmitted segment in its calculation of a new RTO.