One-way delay measurement requires end-to-end hosts to be synchronized in clock time on network. However, there is the relative or absolute difference between two clock times by reason of clock offset, clock skew and so on. In this paper, we present a theorem, methods and simulation results of one-way delay and clock offset estimations between end-to-end hosts. The proposed theorem is a relationship between one-way delay, one-way delay variation and round-trip time, and we show that the estimation error is mathematically smaller than a quarter of round-trip time.