sidered as a challenging issue [139, 159]. In this regard, the 5G exchange (5GEx) project investigates proposing a unified European 5G infrastructure services [160, 161]. It involves cross-domain orchestration platforms and unification of NFV/SDN for multi-domain orchestration. In addition, the 5GPPP in [162] has analyzed several projects and platforms, e.g., OpenWhisky and Kubernetes, for 5G cloud services that are adopting the virtualization technology. The design complexity of NFV within 5G networks is a challenging issue due to the diverse configurations of vertical industry's applications. This issue is envisaged to result in tremendous number of independent virtual networks operating on the same infrastructure which leads, in turn, to security threats. In this context, the ETSI NFV solutions (SOL) working group provides a set of specifications and open API M. Alfaqawi et al. Table 1.6 Research works to tackle NFV challenges Reference Challenge Proposed algorithm and performance [142-144] In [142], VNF orchestration algorithm based on variable orchestration neighboring search has been proposed to provide VNF as a service (VNFaaS) for any type of topologies. Furthermore, a demonstration composed of multiple OpenStack clusters showed an improvement in terms of QoS and a high level of availability to access the VNFaaS anytime and anywhere. In [143], an E2E integration system has been designed to orchestrate the heterogeneous resources of multi-vendors considering latency and availability constraints. It presented a full-automated E2E VNFaaS. In [144] a dynamic orchestration mechanism of SFC in hybrid NFV networks was proposed based on Markov renewal process (MRP) in order to reduce the computing [145-147] In [145], an algorithm to achieve an optimal reliability with management minimum cost was proposed. The simulation results presented better performance by less computational resources and bandwidth consumptions in case of high number of services. In [146], a management method for the NFV MANO was proposed by e