called the P2PSIP peer protocol. Since P2P SIP is about peer-to-peer networks for real-time communication, it is expected that most peers will be coupled with SIP entities. 9.4.2 An Architecture for P2P SIP The P2P SIP architecture defined in [15] has a separate layer, independent of SIP that provides the P2P overlay functions and SIP is an application using the services of the layer. A peer in the P2P SIP system is comprised of two layers: the P2P overlay layer and the SIP layer. The P2P overlay layer handles all the P2P overlay functions, but does not interpret semantic meanings of the resources placed or looked up by the higher layers, including SIP. From the perspective of SIP, the P2P overlay layer mainly provides a loca- tion service for various SIP resources. DHT-based structured P2P networks support efficient search mechanisms when the resource names are precisely known and meet the requirements of P2P SIP well. Therefore, DHT is pro- posed to be used for the P2P overlay layer. P2P overlay networks can be comprised of peers with different amount of physical resources operating in various network environments. Thus, peers are classified into two types: super nodes (SNs) and ordinary nodes (ONs). For DHT-based P2P overlay networks, super nodes participate in building and maintaining the DHT, whereas ordinary nodes do not participate in DHT. Super nodes serve search requests by routing the search traffic in the DHT or generating replies to search requests. An ordinary node relies on a super node to discover resources from other peers. In this sense, super nodes form a distributed storage and search service network. Since only the super nodes form a DHT-based P2P overlay, there is a hier- archical structure in network: the core consisting of super nodes forming a DHT with a periphery of ordinary nodes. Each ordinary node is associated with one or more super nodes. An ordinary node uses the storage and lookup services of the overlay network core through the associated super nodes. On the other hand, SIP has it