An inter-domain path may then appear to be optimal at the domain
level, but may actually use up a very long intra-domain path.
the addition of cyclic structures to the domain that
would enable the Bhandari algorithm to be applied directly in
finding the domain-disjoint paths
The disjoint path pairs problem
is NP-hard in general directed networks for k 2 [42], but
is solvable in polynomial time in undirected networks [43],
directed planar networks [44], and directed acyclic networks
