GLU: acquiring Nurbs parameters evaluated by the tessellator

I’m studying the Nurbs tessellator of GLU, and I can collect all the vertices generated by the tessellator using the callback function.
The problem is that I don’t know the corrisponding values of U and V on which the Nurb is evaluated and from which the vertices are obtained.
Is there a way to get them?

Thanks in advance for any help.