A method for computing a difference between a first propagation delay of a first transmission line and a second propagation delay of a second transmission line. The first and second transmission lines connect a first node and a second node of an optical network. The method comprises, at the second node: receiving first data from the first node along the first transmission line; computing a time offset between a first clock signal recovered from the first data and a second clock signal generated at the second node; receiving second data from the first node along the second transmission line; and computing the difference on the basis of the first data, the second data and the time offset.