Round-trip time estimates may be calculated from an acknowledgement of an original packet transmission or from an acknowledgment of a retransmission without ambiguity by using different segment sizes for the same packet during transmission and retransmission. By using different segment sizes, the retransmitted segment will be acknowledged with a different acknowledgement number. In one embodiment, a TCP sender uses segment sizes that are slightly smaller than the maximum segment size for the initial transmission (e.g. MSS-4 bytes), and uses maximally sized segments during retransmission. When one of the two transmissions is explicitly acknowledged, the TCP sender can then use the different byte sequence acknowledgement numbers to discern which transmission is being acknowledged and correctly measure the round-trip time. This also allows the TCP sender to initiate the NewReno fast recovery algorithm only upon receipt of a retransmission acknowledgment. This is especially useful, for example, during periods of rapidly increasing delays caused by deteriorating network conditions or increased congestion.