The second imperfection of the network layer is that segments may be lost. Since the receiver sends an acknowledgment segment after having received each data segment, the simplest solution to deal with losses is to use a retransmission timer.

A retransmission timer starts when the sender sends a segment. The value of this retransmission timer should be greater than the round-trip-time, for example, the delay between the transmission of a data segment and the reception of the corresponding acknowledgment. Note that TCP sends an acknowledgment for almost every segment! We’ll look at this in more detail in later lessons. When the retransmission timer expires, the sender assumes that the data segment has been lost and retransmits it.