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Historical development[ edit ] SETI home was established in While P2P systems had previously been used in many application domains, [3] 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 , [18] [19] 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 software , P2P 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.

Data validation and Malware The prevalence of malware varies between different peer-to-peer protocols. Files infected with the RIAA virus were unusable afterwards and contained malicious code. Modern hashing , chunk verification and different encryption methods have made most networks resistant to almost any type of attack, even when major parts of the respective network have been replaced by faked or nonfunctional hosts.

Wireless mesh network and Distributed computing The decentralized nature of P2P networks increases robustness because it removes the single point of failure that can be inherent in a client-server based system. If one peer on the network fails to function properly, the whole network is not compromised or damaged. In contrast, in a typical client—server architecture, clients share only their demands with the system, but not their resources. In this case, as more clients join the system, fewer resources are available to serve each client, and if the central server fails, the entire network is taken down.

Distributed storage and search[ edit ] Search results for the query " software libre ", using YaCy a free distributed search engine that runs on a peer-to-peer network instead making requests to centralized index servers like Google , Yahoo , and other corporate search engines There are both advantages and disadvantages in P2P networks related to the topic of data backup , recovery, and availability.

In a centralized network, the system administrators are the only forces controlling the availability of files being shared. If the administrators decide to no longer distribute a file, they simply have to remove it from their servers, and it will no longer be available to users.

Along with leaving the users powerless in deciding what is distributed throughout the community, this makes the entire system vulnerable to threats and requests from the government and other large forces. Although server-client networks are able to monitor and manage content availability, they can have more stability in the availability of the content they choose to host.

A client should not have trouble accessing obscure content that is being shared on a stable centralized network. P2P networks, however, are more unreliable in sharing unpopular files because sharing files in a P2P network requires that at least one node in the network has the requested data, and that node must be able to connect to the node requesting the data.

This requirement is occasionally hard to meet because users may delete or stop sharing data at any point. Unpopular files will eventually disappear and become unavailable as more people stop sharing them. Popular files, however, will be highly and easily distributed.

Popular files on a P2P network actually have more stability and availability than files on central networks. In a centralized network, a simple loss of connection between the server and clients is enough to cause a failure, but in P2P networks, the connections between every node must be lost in order to cause a data sharing failure. In a centralized system, the administrators are responsible for all data recovery and backups, while in P2P systems, each node requires its own backup system.

You can help by converting this article to prose, if appropriate. Editing help is available. September Content delivery[ edit ] In P2P networks, clients both provide and use resources. This means that unlike client-server systems, the content-serving capacity of peer-to-peer networks can actually increase as more users begin to access the content especially with protocols such as Bittorrent that require users to share, refer a performance measurement study [39]. This property is one of the major advantages of using P2P networks because it makes the setup and running costs very small for the original content distributor.

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