implemented using a global naming service such as Domain Name System (DNS), or a hierarchy of P2P SIP networks. P2P SIP is used instead of DNS to resolve the domain name. As an example, individual large organizations can have a local P2P SIP network that is connected to the global P2P SIP network as shown in Figure 9.9. The local domain-specific DHT has representative server nodes that are also reachable in the global DHT, for example, key private.com maps to nodes W and V in the global DHT. Any node in the domain-specific DHT can reach the global DHT, and any node in the global DHT can reach the domain-specific DHT via the representative server nodes in the domain. This hybrid architecture allows the user to register with his provider's SIP server as well as the P2P SIP network. Call setup is sent to the SIP destination, if resolved via DNS, as well as to the P2P SIP network. global (pubp2psip.net) corporation.com example.net server based private.net FIGURE 9.9 Example of hybrid systems. C 2009 by Taylor & Francis Group, LLC P2P SIP: Network Architecture and Resource Location Strategy 9.3.3 Summary The peers in SOSIMPLE replace the functionality of SIP registrars and prox- ies, each node being responsible for those roles for some portion of the overlay. SOSIMPLE meets the requirements for a practical P2P VoIP/IM system and keeps most of the advantages of a proxy-based system. Although a number new SIP headers are defined, SOSIMPLE offers compatibility in terms of reuse of existing SIP clients as well as the ability to interface to established SIP systems. Different from SOSIMPLE, the hybrid architecture proposed in [5] is achieved by implementing the P2P algorithm using SIP messaging. It provides reliability and scalability inherent in P2P systems, in addition to interoper- ability with existing SIP infrastructure. The peer nodes form the P2P network using standard SIP messages with no change in message semantics. 9.4 P2P SIP Standardization More and more companies and institutes focus their research work on P2P