Overlay networks a scalable alternative for p2p - Internet Computing, IEEE

P eer-to-peer systems are distributed systems that operate without centralized organization or control. To find a particular piece of data within the network, P2P systems explicitly or implicitly provide a lookup mechanism, or locator function, that matches a given string, or key, to one or more network nodes responsible for the value associated with that key. P2P nodes interoperate by using the same software or the same set of network-based APIs. Current Internet P2P applications typically provide locator functions using time-to-live (TTL) controlled-flooding mechanisms. With this approach, the querying node wraps the query in a single message and sends it to all known neighbors. The neighbors then check to see whether they can reply to the query by matching it to keys in their internal database. If they find a match, they reply; otherwise, they forward the query to their own neighbors and increase the message’s hop count. If the hop count passes the TTL limit, forwarding stops. The TTL value thus defines a boundary or “horizon” for the query that controls its propagation. However, flooding-based systems don’t scale well because of the bandwidth and processing requirements they place on the network, and they provide no guarantees as to lookup times or content accessibility. Overlay networks can address these issues. Overlay networks have a network semantics layer above the basic transport protocol level that organizes the network topology according to the nodes’ content, implementing a distributed hash table abstraction that provides load balancing, query forwarding, and bounded lookup times. Overlay networks are evolving into a critical component for self-organizing systems (see, for example, the multigroup effort at www.projectiris.net). Here we outline the differences between flooding-style and overlay networks, and offer specific examples of how researchers are applying the latter to problems requiring high-speed, selforganizing network topologies.