## 4 Conclusions and Trends
Another area where challenging issues concerning the development of multiobjective routing optimization models deserve attention is _heterogeneous networks_ that is networks where an end-to-end connection can (or needs to) use different technological platforms so that a route/path has to traverse several network structures/routing domains, each of them with distinct technical features. Another issue with impact on multiobjective routing models is the consideration in various network environments of power consumption as a relevant objective function, not only from an economic perspective but foremost, because of the very significant environmental impacts of electrical energy consumption by telecommunication networks and all types of ICT (Information and Communication Technologies) structures in general (for instance, critically relevant _vis-a-vis_ cloud computing). This trend has led to the development of _'energy-aware routing methods'_. An illustrative example is in [199] where a multiobjective routing method, using a heuristic solution for WDM networks, was developed. This study shows that the minimization of power consumption and path blocking probability are conflicting objectives thence fully justifying the consideration of multiobjective routing approaches. This is a research trend where new path problem formulations that incorporate adequate energy consumption objectives/constraints for a given network environment should be developed. Concerning new modelling aspects related to the _new technological platforms_, with implications in the form of the formulated MOPPs, this will require, as far as routing models are concerned, the specification of adequate criteria/objectives and constraints, involving technical and often economical aspects, and it is an important preliminary issue. Moreover, adequate resolution methods will have to be selected or developed for new multiobjective problem formulations associated with such models. Examples of new areas of development in this context are multiobjective routing models for anycast flows (traffic patterns of one-to-one of many flows) in _cloud computing_ (see [200]) and certain unicast routing models, such that it is imposed that paths include certain intermediate nodes previously specified. For instance, single objective formulations of this type of problem, in the context of a specific resilient routing method with path protection, were addressed in [197]. These models involve the obtainment of shortest node-disjoint path pairs constrained to visit specific nodes. Next, we mention some specific modern network technological platforms where it is expected that new routing models possibly involving multiobjective formulations may be developed. Concerning transport networks, _Elastic Optical Networks_ aim at a more efficient use of the spectrum, by allocating the bandwidth in a flexible way, considering different modulation formats, tailored for each specific application. The problem of routing in EONs is not tackled per se, but rather taking into consideration the allocation of spectrum (Routing and Spectrum Assignment--RSA), sometimes jointly with modulation selection (Routing, Modulation (Level) and Spectrum Assignment--RMSA). There are many approaches for both problems in EONs, with different pros and cons in terms of spectrum fragmentation, modulation formats, quality of transmission, traffic grooming, network survivability, energy efficiency and cost [201]. In [202], a generalization of the Dijkstra algorithm is proposed to tackle the problems of RSA and RMSA where constraints related to the continuity and contiguity of the frequency slot units are taken into account. The problem of RMSA in OTNs is also tackled in [203,204] using different modeling algorithmic approaches. The _sixth generation (6G)_ of mobile networks also poses new challenges in terms of routing. The need to satisfy the increasingly demanding key performance indicators while optimizing the available network resources is an important aspect to consider.