IEEE Wireless Communications • April 2009 30 1536-1284/09/$25.00 © 2009 IEEE Client (IP a :Port a ) User A Private IP network A C.1 C.2 C.3 A CCEPTED FROM O PEN C ALL INTRODUCTION Peer-to-peer (P2P) communication has become one of the major trends in wireless Internet. In a P2P system, a participating peer plays the roles of both client and server. Logically, the peers estab- lish P2P connections directly with each other without passing through network servers. An Internet-based P2P communication session con- sists of two phases: the identification phase and the communications phase. In the identification phase, the calling peer identifies the transport address of the called peer (i.e., the Internet Proto- col [IP] address and the port number). In the communications phase, the calling peer directly connects to the called peer based on the transport address retrieved in the identification phase. Existing IP-based P2P systems can be catego- rized into two types: hybrid P2P and pure P2P. Hybrid P2P systems utilize centralized servers to maintain the mapping between the identifications and the transport addresses of the participating peers. The P2P connection procedure of a hybrid P2P system consists of the following steps: Step A.1 To join the system, a peer starts the P2P application, which attaches the peer to the IP network. Step A.2 The peer registers to the centralized server by providing its identification and the transport address. Step A.3 To initiate a P2P connection, the call- ing peer queries the centralized server to retrieve the transport address of the called peer. Step A.4 Upon receipt of the transport address of the called peer, the calling peer establishes a P2P connection directly to the called peer without involving the centralized server. In this procedure, Steps A.1 and A.2 are exe- cuted to maintain the mapping between the identi- fication and the transport address. Step A.3 executes the identification phase, and the commu- nications phase starts with Step A.4. A hybrid P2P example is the conventional Session Initiation Pro- tocol (SIP) system, where the peers are SIP user agents (UAs), and the centralized server is a SIP registrar server that maps the SIP uniform resource identifier (SIP-URI) to the transport address (IP address and port number) for each registered UA. A hybrid P2P system provides effi- cient query at the cost of maintaining the address mapping in a centralized server (i.e., Steps A.1 and A.2), which may incur a scalability problem. In contrast, the identification mechanism of a pure P2P system is distributed among the partic- ipating peers. Depending on how the peers com- municate in the identification phase, a pure P2P system can be unstructured or structured. In an unstructured P2P system, a query for identifying the transport address of a participating peer is flooded through the network. Unstructured P2P is not required to maintain centralized mapping between the identifications and the transport addresses at the cost of expensive flooding query. In contrast, in a structured P2P system, the iden- tification phase is achieved by a peer querying other peers in a specific routing structure (e.g., a ring, a grid, or a tree). A structured P2P exam- ple is the peer-to-peer SIP system [1] that uti- lizes the distributed hash table for transport address registration and query. Structured P2P provides efficient query in a decentralized fash- ion at the cost of maintaining the distributed mapping tables among the participating peers. To implement a P2P system for mobile devices that have the capability to access mobile telecom- munications networks, such as the universal mobile telecommunications system (UMTS) [2, 3], we propose a hybrid P2P system called iP2P. iP2P effectively reuses the UMTS mobility man- agement mechanism and short message service CHIEN-CHUN HUANG-FU AND YI-BING LIN, NATIONAL CHIAO TUNG UNIVERSITY HERMAN CHUNG-HWA RAO, F AR EASTONE TELECOMMUNICATIONS CO., L TD. ABSTRACT Peer-to-peer communications is a major trend in wireless Internet. In a P2P system, the calling peer must identify the network address of the called peer before establishing a P2P connection. This article proposes iP2P, a hybrid P2P system for mobile devices. iP2P utilizes the short mes- sage service as the control protocol to identify the address of the called peer. Our approach provides an efficient identification mechanism without the requirement for the maintenance of a centralized registrar server in a hybrid P2P system. We also show how iP2P can integrate effectively with the existing Network Address Translation traversal mechanisms to solve the private IP address issue. I P2P: A P EER - TO -P EER S YSTEM FOR M OBILE D EVICES The authors propose iP2P, a hybrid P2P system for mobile devices. iP2P utilizes the short message service as the control protocol to identify the address of the called peer.