## 1 Introduction  
In the last decade, there has been a great evolution in the sensor networking world. A vast amount of small, inexpensive, energy-efficient, and reliable sensors with wireless networking capabilities is available worldwide, increasing the number of sensor network deployments [1, 2]. The adoption of IPv6, combined with the advanced networking capabilities of modern sensor nodes, enables the integration of sensor networks into the existing IP networking infrastructure [3]. IPv6 provides a huge address space for networking purposes, while concurrently leading to the rapid development of many useful applications. Following these advancements, the vision of a World-Wide Sensor Web [4] is becoming a reality, since the trend in next generation networks is to create fully interconnected infrastructures, consisting of many heterogeneous networks. The transition to large scale wireless sensor networks (WSNs) increases the complexity in their operation and management and raises stability and scalability issues [5]. This is due to the special requirements that are imposed from the sensor nodes and the characteristics of the WSNs. Battery limitations and the need for lifetime maximization make necessary the overall power consumption minimization during operation. Energy-efficient operation has to be realized taking in account the existence of heterogeneous sensor nodes with diverse computing and storage capabilities. Furthermore, the highly volatile and dynamic nature of WSN topologies--due to continuous nodes joins and leaves or mobility in the WSN field--requires the continuous intervention of the network administrator. Self-organization mechanisms are proposed in order to reduce the complexity in the operation and management of WSNs. Considering the specific characteristics of such networks, the management of sensor infrastructures should be as autonomic as possible, _i.e._, the wireless sensors should manage themselves with minimum or no human intervention [6]. Autonomic functionalities have to be developed towards the sensor network evolution focusing on the introduction of functionality that will enable the provision of advanced services with low power consumption. Establishment of cooperation among nodes, ability to retrieve the current status of each sensor node, ubiquitous access to data and optimisation of the overall sensor network performance are examples of functionalities that facilitate the efficient operation of sensor nodes from the energy and performance perspective. By developing self-functionalities, autonomic monitoring will be realized, adaption to environmental changes will be supported and operational and management cost will be reduced. A promising method to enable autonomicity in wireless heterogeneous environments is the adoption of decentralized schemes [5, 7]. Centralized components make the system vulnerable in the sense that they are single points of failure. Furthermore the entire traffic load may be needed to be directed towards them, which is energy consuming, especially in dense networks [8, 9]. On the contrary, decentralized schemes are fully scalable and achieve homogeneous load distribution. Current research efforts show that decentralization may be achieved through the exploitation of techniques that are applied in peer-to-peer (p2p) networks since relevant protocols rely on decentralized structures, such as Distributed Hash Tables (DHTs) [10]. These techniques are based on the creation of an overlay topology and the implementation of specific mechanisms over it [4]. P2p overlays may be formulated over sensor networks to eliminate the need for proxy support, to provide efficient data lookup, to limit broadcasts and to enable flexible access to the sensed data. However, the maintenance of an overlay topology is challenging in dynamic environments where nodes join or leave the network [11, 12] and requires the exchange of a large number of messages among the participating nodes [13].