r IP computer network is shown in Figure 9.1. The overlay network resides at the edge of IP network and is ignorant of the underlying IP network. The nodes of the overlay network use only the IP addresses from the underlying network for discovery and routing on the application layer. The term "peer-to-peer" (P2P) refers to a class of systems and applications that employ distributed resources to perform a function in a decentralized manner. A P2P network relies primarily on the computing power and band- width of the participants in the network rather than concentrating it in a number of servers. Because it exists on top of a physical layer network, a P2P network is often called a "P2P overlay" or "P2P overlay network." A typical client-server network is shown in Figure 9.2a, while a P2P network is illustrated by Figure 9.2b. In comparison with the client-server network, P2P networks are inherently scalable and reliable because there is no single point of failure. Peer-to-peer systems operate without centralized organization or con- trol and, in the purest form, have no concept of servers. All participants are peers and communicate in a distributed, potentially untrusted environment to achieve a certain objective [6,7]. FIGURE 9.1 Example of an overlay network. C 2009 by Taylor & Francis Group, LLC P2P SIP: Network Architecture and Resource Location Strategy = client = server = peer Client-server network Peer-to-peer network FIGURE 9.2 Client-server VS. peer-to-peer network. The core operation in most peer-to-peer systems is efficient location of data items. The resource location strategy of a P2P network can be clas- sified into four types: centralized unstructured directory model, pure decen- tralized unstructured flooded requests model, hybrid unstructured model, and decentralized structured distributed hash table (DHT) Model [8]. Currently, DHT-based structured P2P systems are the main technology used in P2P SIP researches, and the most popular algorithms include Chord, Pastry, and Bamboo, which can be easily i