Operation
A P2P network generally consists of a large number of nodes each connected not to all other nodes, but a small subset of the nodes. If a node wants to find a resource on the network, which may be on a node it does not know about, it could simply broadcast its search query to its immediate neighbours. If the neighbours do not have the resource, it then asks its neighbours to forward the query to their neighbours in turn. This is repeated until the resource is found or all the nodes have been contacted, or perhaps a network-imposed hop limit is reached.
Query flooding is simple to implement and is practical for small networks with few requests. It contacts all reachable nodes in the network and so can precisely determine whether a resource can be found in the network (Freenet, for example, only returns a probabilistic result).
On the other hand, every request may cause every node to be contacted. Each node might generate a small number of queries; however, each such query floods the network. Thus, a larger network would generate far more traffic per node than a smaller one, making it inherently unscalable. Additionally, because a node can flood the network simply by issuing a request for a nonexistent resource, it could be possible to launch a denial-of-service attack on the network.
Read more about this topic: Query Flooding
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