The 12th International Symposium on Wireless Personal Multimedia Communications (WPMC 2009) SUITABILITY OF DHT-BASED PEER-TO-PEER SESSION INITIATION PROTOCOL FOR WIRELESS DISTRIBUTED SERVICES Otso Kassinen Erkki Harjula Mika Ylianttila University of Oulu University of Oulu University of Oulu Oulu, Finland Oulu, Finland Oulu, Finland ABSTRACT Providing both analytical and empirical results, we analyze the suitability of DHT-based Peer-to-Peer Session Initiation Protocol (P2PSIP) overlay networking as a substrate for wireless, distributed multimedia services. Session initiation is just one application of this framework. First, we discuss the potential of P2PSIP for different applications and analyze the security and reliability of P2PSIP. Second, we present our prototype mobile P2PSIP implementation and a P2PSIP- based distributed calendar application for wireless user groups. Third, we measure average DHT hop count and retransmission count in up to 2000-node P2PSIP overlays. Fourth, we analyze numerically the scalability and robustness of P2PSIP and client-server SIP in some generic networking scenarios. Our results demonstrate the feasibility of P2PSIP as a standardized peer-to-peer networking protocol for distributed Internet applications, both fixed-line and mobile. I. INTRODUCTION Recent years have witnessed a substantial increase in the popularity of rich Internet communication services such as online games, VoIP, instant messaging, and social networking. All these services involve communications between two or more end-user terminals on top of a suitable infrastructure. Increasingly, the access devices are wireless. Communication services can be roughly divided into two categories by their model of information exchange: 1) mediated exchange, where information is posted to a shared application environment accessible by both endpoints, for example over HTTP to a Web site; and 2) direct exchange, where application-specific information is transferred in real- time between application instances running at the endpoints. When endpoints want to interact using a direct-exchange oriented application, a session needs to be established between them. A prominent standardized solution for session management is the Session Initiation Protocol (SIP), an application-layer signaling protocol for creating, modifying and terminating sessions between two or more endpoints. SIP has a client-server architecture. A SIP network consists of a set of (fixed-line) SIP servers with different kinds of functionalities; SIP user agents, i.e. clients, use the servers to initiate direct exchange. In this paper, the traditional SIP architecture is referred to as client-server SIP. Several extensions for client-server SIP exist, including instant messaging, presence, and emergency services, and there are proposals for advanced modifications such as transparent session transfer between terminals for mobile users [1]. In both fixed-line and mobile access networks, peer-to-peer (P2P) networking has long been a subject of intensive study. In consequence, the Internet Engineering Task Force (IETF) has recently formed the Peer-to-Peer SIP (P2PSIP) working group for standardizing serverless, decentralized operation of SIP [2]. Distributed hash tables (DHT) are a key technology for building the efficient P2P overlays needed. An early DHT-based approach similar to P2PSIP is presented in [3]. It is possible to use SIP on top of a decentralized P2PSIP overlay network. However, P2PSIP has a wider scope of applicability. Whereas SIP was designed for direct-exchange communications, P2PSIP also standardizes mediated exchange; P2PSIP enables the storage, discovery and access of digital resources as its core functionalities. Session signaling is just one application of the P2P framework. Wireless use of P2PSIP (wired-wireless convergence) has been considered in the design from early on. Thus, P2PSIP provides a standard platform for both direct- and mediated- exchange oriented mobile services. However, the mobile P2PSIP related research efforts in the literature have focused on the protocol’s properties for direct exchange applications, such as the VoIP call setup delays evaluated in [4]. Content sharing systems are a typical example of mediated exchange. In [5], the popular content sharing system called BitTorrent, a DHT-based P2P system, is analyzed. In addition to content sharing, mediated exchange can be used among other things for implementing P2P mailbox-type messaging services: in [6], measurements of performance in a mobile publish/subscribe P2P system are provided. DHT algorithms - a key enabler for mediated exchange in P2P systems - have been extensively studied; for example in [7], four algorithms are evaluated in terms of bandwidth usage and lookup latency as a function of the essential parameters of the algorithms. In this paper we analyze the potential of P2PSIP for providing generic functionality for mediated-exchange services, in addition to the more established work on SIP- based, mostly direct-exchange type services on top of the decentralized platform. Our results are based on qualitative and quantitative analysis, and on an evaluation of P2PSIP overlay networks run with an actual P2PSIP implementation. II. P2PSIP FEASIBILITY ANALYSIS A. Scope and Potential for Different Network Applications The usefulness of traditional SIP networks lies not just in the unified format of the session-signaling messages. Before any negotiation for session establishment can take place, the endpoints must be able to locate each other. In an IP-based network, this ultimately translates into resolving, in one way or another, the IP address of the remote party. In SIP systems, every user has a SIP URI (address-of-record) that acts as a user ID, similar to an e-mail address. The SIP servers map their users’ SIP URIs to the users’ current IP addresses. This enables initiating a session with a user whose IP address is unknown. However, perhaps an equally profound