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Classes of Decentralized Peer-to-Peer Networks Essay

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Decentralized Peer-to-Peer (P2P) overlay networks are distributed systems in nature, without any hierarchical organization or centralized control. They are typically divided in two main classes: structured and unstructured [39].
Structured P2P overlay network have tightly controlled topologies and content is placed at specified locations to efficiently solve queries. Some well-known examples are Content Addressable Network (CAN) [44], Chord [15] and Pastry [45]. Such overlays use a Distributed Hash Table (DHT) as substrate, where data objects (or values) are placed deterministically at the peers whose identifiers correspond to the data object’s unique key. In DHT-based systems, node identifiers are uniform-randomly assigned to the peers
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In order to route a query, each node checks its routing table and forwards it to a node that is numerically closest to the key. Since the focus of this paper is not on structured P2P networks, for the reasons highlighted in the next Subsection 2.2, we refer to [39, 40] for more information about them.
An unstructured P2P system is composed of peers joining the network with some loose rules and without any prior knowledge of a specific topology to preserve [39]. The resulting topology may have certain properties, though the placement of objects at the peers is not based on any specific topology-related property [41]. Thus, unstructured overlays provide complete flexibility on where resources can be published by the peers; search methods are not based on hash functions and can naturally support non-exact matches.
Search techniques for unstructured P2P overlays are typically categorized in blind and informed [41, 43].
Blind search schemes employ flooding or random-based techniques to relay queries to peers in the network. Peers keep no information about the P2P network or the probable locations of objects for routing queries [41]. Flooding is the typical mechanism used to send queries across the overlay with a limited scope (e.g. Gnutella [23]). It consists in a Breadth First Search (BFS) of the overlay network: when a peer receives a query, it returns the results if present,
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