An Empirical Study of the Global Behavior of Structured Overlay Networks as Complex Systems

Sammanfattning: Distributed applications built on top of Structured Overlay Networks (SONs) operate based on certain self-' behaviors of the underlying Peer-to-Peer network. Among those, self-organization and self-healing are the two most prominent and assumed properties. The operating environment of distributed systems continues to be more inhospitable with the advance and demand of new technologies; for example in case of mobile and ad hoc networks Churn (node turnover) can be extremely high due to node mobility, frequent disconnects/reconnects and configuration changes. Also, in such dynamic environments, the system may face high Churn (node turnover) and Network partition in a frequent manner. The situation becomes worse if the self-healing behavior of underlying SON is not complete and well defined. This implies the following non-trivial questions: Can the maintenance mechanism of a SON heal the damage to the structure due to harshness of the operating environment and reverse it back? What are the pre-conditions; in other words, what properties the healing mechanism should possess in order to achieve reversibility against stressful environments? Existing literature lacks such assessment and verification study of the self-healing property of a SON.In this thesis, we investigate both the behavior and design of a system that operate in inhospitable environments. This work is relevant to systems with both peaks of high stress (e.g. partitions, churn, network dynamicity etc.) and continuous high stress. We evaluate existing overlay maintenance strategies, namely Correction-on-Change, Correction-on-Use, Periodic Stabilization, and Ring Merge. We define the reversibility property of a system as its ability to repair itself to its original state. We propose a new strategy, called Knowledge Base, to improve conditions for reversibility against inhospitable environments. By means of simulations, we demonstrate reversibility for overlay networks with high levels of partition and churn. We make general conclusions about the ability of the maintenance strategies to achieve reversibility.Identification of Phase Transitions in a SON can provide useful information about the properties of each state of the system. Also, this enables to find the critical points in the operating space and parameters influencing them. The applications running on top of the SON can potentially utilize this knowledge to adapt its operation accordingly in different system states. In this thesis, a representative ring-based SON, namely Beernet is chosen and extended to achieve reversibility. The resulting overlay, Beernet++ exhibits reversible phase transitions under churn. We analyze the critical points observed during such transitions. We present the behavior of Beernet++ for high level of churn and network partitioning, along with their interaction.