## 4 Conclusions and Trends
Also, the practical implementation requirements and specific limitations (namely related to signaling, information management, control functionalities of routers, switches or optical cross-connectors and other related routing protocol requirements), for the multiple types of multiobjective routing methods proposed in the literature, should be investigated in real-world network application contexts. This would require further empirical R&D studies with real networks involving collaborations with network and service operators and communication software/equipment suppliers, on a case by case basis, and it is a matter that justifies future research. A problematic area where multiobjective path problem formulations are raising interest is _resilient routing_ involving multiobjective multipath problems. As already noted in [193], the development of multiobjective routing approaches and of resilient routing models, in particular, will enable a mathematically consistent analysis of trade-offs between various route and network performance metrics for different routing methods corresponding to different options in terms of protection or rerouting mechanisms in each layer. This poses a significant challenge since it will require us to address the difficult issue of decomposition of the path optimization model. In routing with path protection, this is associated with the fact that the efficient solutions of the routing/path optimization problem in the physical network level (for instance an OTN optical network structure) are pairs of lightpaths, such that each of these lightpaths may correspond to different feasible paths in the next layer of the network representation (for instance, corresponding to the use of a MPLS-TP protocol technology). Note that in this type of network architecture and modeling approach, the path optimization model would also be multiobjective in this upper layer, thus leading to a complex issue of interrelated problems. Another kind of hierarchical modeling issue, relevant to multilayer networks, involves a hierarchy of the routing/path optimization formulation, which is concerned with the application of well-known optimization methods to multi-level routing, as outlined in [3] in the context of IP-based networks. These remain challenging methodological questions that deserve future research. Still in relation to resilient routing, taking into account that many network failures occur as a result of disasters (natural phenomena or as a consequence of human intervention), in such a way that these events have a very strong impact on a certain geographical area (geographically correlated failures, see [194]), there has been an increasing interest in the development of _geodiverse routing models_. These routing methods have to take into account the geodiversity property of the paths, and this can be modeled in different manners, implying specific types of constraints on the choice of the protection path(s) (a brief review is in [195]). A strategy recurring to \(k\)-geodiverse shortest paths is described in [196], where a lower bound on the geographic distance between the paths is imposed. Also, resilient routing models with preventive techniques, namely in the case of malicious attacks, have been given attention. A possible strategy is to define specific nodes where the traffic must pass, so that a security inspection is performed to identify possible threats, thence imposing a specific type of topological constraint on the path problems (see examples in [197; 198]). We think that also in the mentioned sub-areas of resilient routing there is room for the investigation of multiobjective routing approaches leading to interesting multiobjective constrained path problems.