## 2 Peer-to-Peer Approach for Decentralized MG's Control
To address these shortcomings, the communications network should move from the conventional centralized infrastructure to a decentralized one. A decentralized communication infrastructure, removes the central controller as a single point of failure, providing a significant improvement in terms of reliability. Therefore, the hierarchical management scheme of microgrids should be implemented employing a decentralized architecture. Decentralized architectures for power systems are usually implemented through Multi-Agent Systems (MAS) technology [44; 45; 46; 47]. In this structure each DER unit is considered as an agent. An agent is a computer system capable of performing tasks autonomously and with the ability to communicate with its neighboring agents to solve problems through cooperation, coordination and negotiation. Most MAS implement hierarchical topologies where some agents (super-agents) have authority over the actions of other agents [48] to form distributed decentralized systems (see Figure 2b). However, these agents do not have the capacity to reorganize themselves [49]. In addition the individual behavior of the agents is easy to know while the whole system is unreachable [50]. The absence of such functionality might result in a lack of knowledge about the global status of the microgrid, as well as in a sub-optimal allocation and a poor management of the critical resources [49]. Therefore, a new paradigm based on peer-to-peer (P2P) communications could be a promising solution for improving the use of DERs and the network performance in microgrids [17; 51; 52; 53]. P2P systems are formed by peers. A peer is an agent [49] that can act both as server and client, characteristic that allows agents be proactive. P2P systems avoid a single point of failure and are much scalable because the available resources grow with the number of nodes joining the network. Nodes  
Figure 2: (a) Centralized; (b) Distributed and (c) Pure P2P Communication Architectures. are capable to cooperate to achieve a common goal and they have self-organization capabilities. The connectivity between nodes, as Figure 2c depicts, is carried out by creating virtual links, overlays, which are built on top of the IP network. Some advantages of these systems are their greater flexibility, their extensibility and their adaptive reconfiguration. This implies that each node communicates with each other to create self-organizing overlay structures on top of the subjacent physical networks [54]. It is worth pointing out that P2P networks have a highly distributed nature, so they offer much large scalability than possible with other networks that are based on centralized or partially distributed systems, due to the fact each peer could act as a server. Therefore, the typical bottlenecks of centralized and partially distributed systems are avoided because the number of servers increases linearly with the one of clients. P2P architectures can be classified as Structured or as Unstructured according to their logical topology and degree of decentralization. On the one hand, an unstructured P2P network has a random and unstructured mesh network topology. There is not an algorithm for organization. The information and data resources are distributed among peers. A Broadcasting lookup technique is used to locate resources and data retrieval in unstructured P2P, in such a way that each peer propagates a request to its directly connected peers. The propagation remains until the message time to live (TTL) threshold (typically four) has been reached [55]. This broadcast creates a large amount of signal traffic and uses of a lot of network bandwidth [56]. These characteristics do not result in a scalable and efficient system. On the other hand, a structured P2P network has a dedicated network and a well-defined topology where peers are responsible for information and data resources. In structured overlays, a Distributed Hash Table is used for routing in order to locate resources in the network.