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Historical development[ edit ] SETI home was established in While P2P systems had previously been used in many application domains,  the concept was popularized by file sharing systems such as the music-sharing application Napster originally released in The peer-to-peer movement allowed millions of Internet users to connect "directly, forming groups and collaborating to become user-created search engines, virtual supercomputers, and filesystems.
The early Internet was more open than present day, where two machines connected to the Internet could send packets to each other without firewalls and other security measures.
It was developed in as a system that enforces a decentralized model of control. The basic model is a client-server model from the user or client perspective that offers a self-organizing approach to newsgroup servers.
However, news servers communicate with one another as peers to propagate Usenet news articles over the entire group of network servers. The same consideration applies to SMTP email in the sense that the core email-relaying network of mail transfer agents has a peer-to-peer character, while the periphery of e-mail clients and their direct connections is strictly a client-server relationship.
This model of network arrangement differs from the client—server model where communication is usually to and from a central server. A typical example of a file transfer that uses the client-server model is the File Transfer Protocol FTP service in which the client and server programs are distinct: Routing and resource discovery[ edit ] Peer-to-peer networks generally implement some form of virtual overlay network on top of the physical network topology, where the nodes in the overlay form a subset of the nodes in the physical network.
Overlays are used for indexing and peer discovery, and make the P2P system independent from the physical network topology.
Based on how the nodes are linked to each other within the overlay network, and how resources are indexed and located, we can classify networks as unstructured or structured or as a hybrid between the two. In particular, when a peer wants to find a desired piece of data in the network, the search query must be flooded through the network to find as many peers as possible that share the data.
Furthermore, since there is no correlation between a peer and the content managed by it, there is no guarantee that flooding will find a peer that has the desired data.
Popular content is likely to be available at several peers and any peer searching for it is likely to find the same thing. But if a peer is looking for rare data shared by only a few other peers, then it is highly unlikely that search will be successful.
The most common type of structured P2P networks implement a distributed hash table DHT  in which a variant of consistent hashing is used to assign ownership of each file to a particular peer. This makes them less robust in networks with a high rate of churn i.
Spotify was an example of a hybrid model [until ]. Currently, hybrid models have better performance than either pure unstructured networks or pure structured networks because certain functions, such as searching, do require a centralized functionality but benefit from the decentralized aggregation of nodes provided by unstructured networks.
Like any other form of softwareP2P applications can contain vulnerabilities. What makes this particularly dangerous for P2P software, however, is that peer-to-peer applications act as servers as well as clients, meaning that they can be more vulnerable to remote exploits.
Examples of common routing attacks include "incorrect lookup routing" whereby malicious nodes deliberately forward requests incorrectly or return false results, "incorrect routing updates" where malicious nodes corrupt the routing tables of neighboring nodes by sending them false information, and "incorrect routing network partition" where when new nodes are joining they bootstrap via a malicious node, which places the new node in a partition of the network that is populated by other malicious nodes.Thesis proposal architecture example, - Research paper hooks.
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