  We derive relativistic corrections for one-way and two-way time and frequency
transfer over optical fibres neglecting no terms that exceed 1 ps in time and
$10^{-18}$ in fractional frequency, and estimate their magnitude in typical
fibre links. We also provide estimates of the uncertainties in the evaluation
of the relativistic corrections due to imperfect knowledge of parameters like
the coordinates of the fibre and stations, Earth rotation, or thermal effects
of the fibre index and length. The links between Teddington(UK) and Paris(F) as
well as Braunschweig(D) and Paris(F), that are currently under construction,
are studied as specific examples.
