# Comprehensive Comparison of VNE Solutions Based on Different Coordination Approaches
## 1 Introduction  
Network Virtualization (NV) is predicted to tip the balance of the structural technologies that constitute the current internet working ecosystem toward more intangible architectures [1; 2; 3], particularly in the context of 5G networking technologies [4; 5; 6; 7; 8; 9]. By its definition, NV enables the abstraction of network and computational resources and, depending on the applied scenario, it either implements a combination of multiple physical networks to a single virtual software-based entity or partitions one physical network into separate heterogeneous virtual ones. However, in order to materialize any of the aforementioned concepts in actual real life scales, the commercial hosting infrastructures transcend legacy limitations and transform into outsourced virtual environments, enabling the provisioning of hosting services called Infrastructure as a Service (IaaS). IaaS is classified as a cloud commodity that performs transparent and on-demand delivery of hardware and software resources horizontally to corporations and domestic users alike [10; 11]. By investing in the IaaS market, which embraces a pay as you use billing model, corporations avoid any long term commitment in the context of creating and sustaining infrastructures with internal means. IaaS also facilitates the administration of any instant provisioned computing set-up, by offering a unified and flexible management over the internet. The business architecture of the virtualization and infrastructure market was shaped accordingly. The infrastructure Provider (InP) and the Service Provider (SP) are the two roles that have been decoupled from the single traditional entity of the Internet Service Provider (ISP) [12]. Two more roles can spring from the SP, thus, establishing a quad layered NV business architecture, as shown in Figure 1. In this version of the model, all processes are performed by the InP, the virtual network provider (VNPs), the virtual network operator (VNO), and the SP. While the group of the SPs are in charge of providing services to the end-users, the VNPs focus on instantiating virtual networks responsible for satisfying requests stemmed from the interacting SPs. On the other hand, the responsibility of the VNOs is to manage the successfully deployed virtual networks. The VNP and VNO roles alleviate the burden of task orchestration from the SPs, presenting the opportunity to focus only on service provisioning [13]. However, the aforementioned radical innovations have also induced chain renovations to other established networking concepts and simultaneously created the necessary space for new ones to emerge. A virtualization mechanism that may prove to become the focal point of internet modernization is Virtual Network Embedding (VNE). The VNE study focuses on innovative solutions that incorporate optimal mapping of Virtual Networks (VNs) and their demands onto physical resources by adhering to specific optimization objectives. Here follows a summary of those distinctive objectives, researched throughout the existing literature. Limiting energy consumption--Energy consumption has always been a catalyst for an increase in the profit margins of the InPs. Moreover, the race for the fourth industrial revolution and the emergence of immersive ideas, such as functionality and resource disaggregation, encountered in environments, like 5G and beyond [12, 14, 15], have spawned large scale data-centers with a constantly increasing energy footprint. Consequently, the IaaS industry has to deal with unceasingly increasing energy expenses as well as the obligation to comply with collections of legislation that aim to limit the environmental impact by the InPs' operating cycle. In the context of VNE, the energy consumption of the SN can be managed and restricted to an accepted minimum by mapping the VNs in specific ways. Specialized VNE algorithms can grant the mapping operations energy awareness by rearranging the mapped virtual resources onto the underlying SN and, in the process, idling physical ones [16].