Question

If I know four points coordinates of two lines (each line has two points) in three dimension. How can I calculate out the the common perpendicular line of these two lines? For example, two lines are L1,two points of L1 :A(x1, y1, z1), B(x2, y2, z2) and L2: C(x3, y3, z3), D(x4, y4, z4). Thanks.