Feasible Roaming Path
A feasible roaming path is defined as a directed path in the mobility graph, which starts at (X0, 0), and all the vertices in the path are NR vertices.
Given that the roaming interval of u is equal to τ, its actual roaming path must be a feasible roaming path in length τ, starting at (X0, 0) and terminating at the user location at time τ.