These are desirable features in heterogeneous mobile networks of arbitrary size [1] in which people no longer only communicate in the traditional way, but also share other information that is relevant to the context of their activities [2]. In fact, nowadays, many phone services rely on context-aware information provided by embedded sensors, which the users share. This imposes the need to achieve fast and seamless data delivery between network devices, for which SIP offers reliable support. In the context of mobile sensing, P2PSIP protocols facilitate decentralized and fast communications between sensor-enabled terminals. Nevertheless, in order to make P2PSIP protocols feasible in mobile sensing networks, it is necessary to minimize overhead transmissions for management purposes, thereby reducing battery lifetime.

P2P services are often based on overlay networks, in which resource management is a key challenge. P2P network states are strongly transient, and routing survivability to failures is a very important design consideration [3]. In practice, churn (peers leaving or joining the network) can affect the accuracy of the routing and resource tables at the peers, because certain entries (corresponding to individual peers) may be missing or stale. This affects both the efficiency and consistency of operations, such as lookups. While incomplete distributed hash tables (DHT) degrade performance (the less complete the registration information, the more hops the lookup procedure needs), stale contacts increase latency, due to timeouts (when peers that have waited a defined time for a response decide that the requested peer no longer belongs to the overlay).

Since timeout intervals typically last no longer than a few round-trip times, latency can easily exceed the needs for common real-time operations. This is a greater problem in mobile wireless networks, where topology is inherently more unstable than in the wired Internet.Peers that participate in a P2PSIP [4] overlay not only act as conventional SIP user agents (UAs), but also, collectively, play the normal roles of a central server. In such a system, registrar servers (where registration information is placed), proxy servers (intermediary nodes) and message routing functions of the traditional SIP protocol are replaced by a distributed P2P overlay. The core functions of this overlay are storage, discovery and access to digital resources.

Currently, the P2PSIP working group of the Internet Engineering Task Force (IETF) is considering proposals for the P2PSIP peer protocol based on distributed SIP (dSIP) [4] and Kademlia [5]. dSIP is an SIP-based protocol that has been proposed as a generic framework for a distributed SIP location service. In other words, it is a protocol for Batimastat resource lookup in peer-to-peer SIP networks. It is simple and reusable with traditional SIP UAs. We describe Kademlia in Section 2.3.