The protection fiber links are then taken into ac-count by using both the shared path protection and the
dedicated path protection methods. Both methods build a protection fiber link for each working fiber link, thus the network
is absolutely reliable for any single physical link failure, which means the simultaneous failure of bidirectional
physical links between two neighbor nodes. The FTD problem with protection can be formulated by
using an ILP model or an integer programming (IP) model, depending on the adopted protection method.