Peer-to-Peer (P2P) Architecture

Peer-to-Peer Architecture distinguishes itself by its distribution of power and function. Rather than concentrating its power in the server, Peer-to-Peer models rely on the power and bandwidth of participants. They form ad hoc connections between nodes for sharing all kinds of information and files. Peer-to-Peer discards hierarchical notions of clients and servers (clients at the top, servers on the bottom) and replaces it with equal peer nodes that function simultaneously as clients and servers. This also discards the idea of a central server, which exists in Client-Server Architecture.

There are several classifications of Peer-to-Peer networks. These include pure/hybrid and structured/unstructured Peer-to-Peer networks. Pure P2P networks merge the role of clients and servers as equals and do not provide a central server for managing the network or a central router that forwards requests to other networks.

Hybrid P2P models, on the other hand, do contain a central server that stores peer information and responds to request for information stored on that server. In this configuration, peers host available resources since there no central server provides this function.

Peers also make central servers aware of what resources they want to share and make those resources available to peers that request them. Also, route terminals function as used addresses and are indexed to find an absolute address.

The structure of P2P networks is determined by the nature of the overlay network, which consists of all participating peers as equal nodes. Nodes in an overlay network are connected through virtual or logical links that create a path to the underlying network.

Essentially, overlay networks are network built on top of other networks. Peer-to-Peer networks are considered overlay networks because they are usually built on top of the Internet. Structured P2P networks use a global protocol so that searches can be routed TO any peers/nodes BY any peers/nodes on the network.

To retrieve rare files, more structured overlay links are required. The most common structured P2P network is the distributed hash table (DHT). DHTs are decentralized distributed systems that store names and values. Any participating node in the network can lookup and retrieve values. Maintenance of the DHT mapping is distributed among the nodes. The ownership of each file is assigned to a peer, but the addition or deletion of peers or files doesn’t cause major disruptions. This makes them very scalable.

Unstructured P2P networks establish links more arbitrarily. To join, a peer only has to copy the links of an existing node and then add its own links as it develops. To find a desired file, however, the request must be flooded throughout the network. This doesn’t always necessarily return the desired results if the file being requested is rare. There is no correlation between peer and content. Also, flooding increases network traffic, slowing down responses and file sharing.

The primary advantage of P2P networks is that all clients contribute their resources. These resources include computing power, bandwidth, and storage space. In traditional Client-Server models there are a fixed number of servers, so the addition of clients slows down network processing. In Peer-to-Peer models, as nodes are added, system resources increase (contributed by the added nodes) to accommodate demand.

Comparisons

Client-Server Architecture provides certain advantages over these other network models. For example, Client-Server models offer easier maintenance, security, and administration. For example, encapsulation makes it possible for servers to be repaired, upgraded, or replaces without clients being affected.

Encapsulation is the process by which an object can hide its data and methods without revealing them to users. Also, because all data is stored on servers, the data is more secure. Servers control access and ensure that only screened clients can access and manipulate data. Again, since data is centralized on servers, updates occur on the server and are then transmitted to clients as they request services.

In P2P models, updates must be applied and copied to peers in the network, which requires a lot labor and is prone to errors. However, Client-Server paradigms often suffer from network traffic congestion. This is not a problem for P2P, since network resources are in direct proportion to the number of peers in the network. Also, Client-Server paradigms lack the robustness of P2P networks. Robustness refers to a network’s ability to bounce back or continue functioning if one of the components fails. If a server fails in Client-Server models, the request cannot be completed. In P2P, a node can fail or abandon the request. Other nodes still have access to resources needed to complete the download.

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