s own hierarchical structure. But in general, the hierarchy of peers in the P2P overlay layer is independent of the hierarchical structure of SIP. A UA with a P2P overlay layer is referred to as a P2P UA, and similarly, a proxy or a registrar with a P2P overlay layer is referred to as a P2P proxy or P2P registrar respectively. Super nodes are good candidates for P2P proxies, but not all super nodes have to become a P2P proxy. There are four types of entities in the P2P overlay network. The bootstrap server and login server in a P2P overlay network facilitate the formation of the P2P overlay network. When other means become available for their functions, they can be omitted and thus are optional. Once a node knows one or multiple C 2009 by Taylor & Francis Group, LLC P2P SIP: Network Architecture and Resource Location Strategy nodes participating in a P2P overlay network, it can start the process to join the network. The details of the entities and the related operations are similar to those described in Section 9.3.1. 9.4.3 A Hierarchical P2P-SIP Architecture In the hierarchical P2P SIP architecture outlined in [16], P2P overlays are used by SIP as the underlying peer location protocol. Heterogeneous overlays are inter-connected via a decentralized manner. Each overlay will elect one or more powerful peers to be P2P proxies, which logically construct an upper level overlay and forward messages among heterogeneous overlays. There are also several stateful peers in an overlay to inherit the controllable feature of the stateful SIP proxies. The proposed hierarchical P2P-SIP structure is illustrated in Figure 9.10. Various P2P overlays (such as Pastry, CAN) are respectively used as the underlying route discovery protocol. Thus the communication overhead will be significantly reduced, since relaying nodes need not maintain dialog states. To connect peers from heterogeneous overlays, an upper level overlay, the global P2P overlay, is formed, which interconnects the local overlays. That is, each overlay elects one or mo