Alberto Montresor

Gossip-based aggregation in large dynamic networks

As computer networks increase in size, become more heterogeneous and span greater geographic distances, applications must be designed to cope with the very large scale, poor reliability, and often, with the extreme dynamism of the underlying network. Aggregation is a key functional building block for such applications: it refers to a set of functions that provide components of a distributed system access to global information including network size, average load, average uptime, location and description of hotspots, and so on. Local access to global information is often very useful, if not indispensable for building applications that are robust and adaptive. For example, in an industrial control application, some aggregate value reaching a threshold may trigger the execution of certain actions; a distributed storage system will want to know the total available free space; load-balancing protocols may benefit from knowing the target average load so as to minimize the load they transfer. We propose a gossip-based protocol for computing aggregate values over network components in a fully decentralized fashion. The class of aggregate functions we can compute is very broad and includes many useful special cases such as counting, averages, sums, products, and extremal values. The protocol is suitable for extremely large and highly dynamic systems due to its proactive structure---all nodes receive the aggregate value continuously, thus being able to track any changes in the system. The protocol is also extremely lightweight, making it suitable for many distributed applications including peer-to-peer and grid computing systems. We demonstrate the efficiency and robustness of our gossip-based protocol both theoretically and experimentally under a variety of scenarios including node and communication failures.

PeerSim: A scalable P2P simulator

The key features of peer-to-peer (P2P) systems are scalability and dynamism. The evaluation of a P2P protocol in realistic environments is very expensive and difficult to reproduce, so simulation is crucial in P2P research.

Anthill: a framework for the development of agent-based peer-to-peer systems

Recent peer-to-peer (P2P) systems are characterized by decentralized control, large scale and extreme dynamism of their operating environment. As such, they can be seen as instances of complex adaptive systems (CAS) typically found in biological and social sciences. We describe Anthill, a framework to support the design, implementation and evaluation of P2P applications based on ideas such as multi-agent and evolutionary programming borrowed from CAS. An Anthill system consists of a dynamic network of peer nodes; societies of adaptive agents travel through this network, interacting with nodes and cooperating with other agents in order to solve complex problems. Anthill can be used to construct different classes of P2P services that exhibit resilience, adaptation and self-organization properties. We also describe preliminary experiences with Anthill in implementing a file sharing application.

Design patterns from biology for distributed computing

Recent developments in information technology have brought about important changes in distributed computing. New environments such as massively large-scale, wide-area computer networks and mobile ad hoc networks have emerged. Common characteristics of these environments include extreme dynamicity, unreliability, and large scale. Traditional approaches to designing distributed applications in these environments based on central control, small scale, or strong reliability assumptions are not suitable for exploiting their enormous potential. Based on the observation that living organisms can effectively organize large numbers of unreliable and dynamically-changing components (cells, molecules, individuals, etc.) into robust and adaptive structures, it has long been a research challenge to characterize the key ideas and mechanisms that make biological systems work and to apply them to distributed systems engineering. In this article we propose a conceptual framework that captures several basic biological processes in the form of a family of design patterns. Examples include plain diffusion, replication, chemotaxis, and stigmergy. We show through examples how to implement important functions for distributed computing based on these patterns. Using a common evaluation methodology, we show that our bio-inspired solutions have performance comparable to traditional, state-of-the-art solutions while they inherit desirable properties of biological systems including adaptivity and robustness.

T-Man: Gossip-based fast overlay topology construction

Large-scale overlay networks have become crucial ingredients of fully-decentralized applications and peer-to-peer systems. Depending on the task at hand, overlay networks are organized into different topologies, such as rings, trees, semantic and geographic proximity networks. We argue that the central role overlay networks play in decentralized application development requires a more systematic study and effort towards understanding the possibilities and limits of overlay network construction in its generality. Our contribution in this paper is a gossip protocol called T-Man that can build a wide range of overlay networks from scratch, relying only on minimal assumptions. The protocol is fast, robust, and very simple. It is also highly configurable as the desired topology itself is a parameter in the form of a ranking method that orders nodes according to preference for a base node to select them as neighbors. The paper presents extensive empirical analysis of the protocol along with theoretical analysis of certain aspects of its behavior. We also describe a practical application of T-Man for building Chord distributed hash table overlays efficiently from scratch.

Messor: load-balancing through a swarm of autonomous agents

Peer-to-peer (P2P) systems are characterized by decentralized control, large-scale and extreme dynamism of their environment. Developing applications that can cope with these characteristics requires a paradigm shift that puts adaptation, resilience and self-organization as primary concerns. Complex adaptive systems (CAS), commonly used to explain the behavior of many biological and social systems, could be an appropriate response to these requirements. In order to pursue these ideas, this paper presents Messor, a decentralized load-balancing algorithm based on techniques such as multi-agent systems drawn from CAS. A novel P2P grid computing system has been designed using the Messor algorithm, allowing arbitrary users to initiate computational tasks.

A robust protocol for building superpeer overlay topologies

The concept of superpeer has been introduced to improve the performance of popular file-sharing applications. A superpeer is a node in a P2P network that operates as a server for a set of clients, and as an equal w.r.t. other superpeers. By exploiting heterogeneity, the superpeer paradigm allows P2P networks to run more efficiently, without compromising their decentralized nature. This paper describes SG-1, a novel generic mechanism for the construction and the maintenance of overlay topologies based on superpeers. SG-1 is based on the well-known gossip paradigm, with nodes exchanging information with randomly selected peers and re-arranging the topology according to the requirements of the particular P2P application. The resulting protocol is extremely efficient and robust, capable to deal with a continuous flow of nodes joining and leaving the system, as well as to repair a network where up to 100% of the existing super-peers have been removed.

Robust aggregation protocols for large-scale overlay networks

Aggregation refers to a set of functions that provide global information about a distributed system. These junctions operate on numeric values distributed over the system and can be used to count network size, determine extremal values and compute averages, products or sums. Aggregation allows important basic functionality to be achieved in fully distributed and peer-to-peer networks. For example, in a monitoring application, some aggregate reaching a specific value may trigger the execution of certain operations; distributed storage systems may need to know the total free space available; load-balancing protocols may benefit from knowing the target average load so as to minimize the transfered load. Building on the simple but efficient idea of antientropy aggregation (a scheme based on the antientropy epidemic communication model), in this paper we introduce practically applicable robust and adaptive protocols for proactive aggregation, including the calculation of average, product and extremal values. We show how the averaging protocol can be applied to compute further aggregates like sum, variance and the network size. We present theoretical and empirical evidence supporting the robustness of the averaging protocol under different scenarios.

Self-star Properties in Complex Information Systems

The Self-Star Vision.- The Self-Star Vision.- Self-organization.- Evolving Fractal Gene Regulatory Networks for Graceful Degradation of Software.- Evolutionary Computing and Autonomic Computing: Shared Problems, Shared Solutions?.- Self-? Topology Control in Wireless Multihop Ad Hoc Communication Networks.- Emergent Consensus in Decentralised Systems Using Collaborative Reinforcement Learning.- The Biologically Inspired Distributed File System: An Emergent Thinker Instantiation.- Evolutionary Games: An Algorithmic View.- Self-awareness.- Model Based Diagnosis and Contexts in Self Adaptive Software.- On the Use of Online Analytic Performance Models, in Self-Managing and Self-Organizing Computer Systems.- Prediction-Based Software Availability Enhancement.- Making Self-Adaptation an Engineering Reality.- An Online Control Framework for Designing Self-Optimizing Computing Systems: Application to Power Management.- Self-Management of Systems Through Automatic Restart.- Fundamentals of Dynamic Decentralized Optimization in Autonomic Computing Systems.- Self-awareness vs. Self-organization.- The Conflict Between Self-* Capabilities and Predictability.- Self-Aware Software - Will It Become a Reality?.- Supporting Self-*.- A Case for Design Methodology Research in Self-* Distributed Systems.- Enabling Autonomic Grid Applications: Requirements, Models and Infrastructure.- Pandora: An Efficient Platform for the Construction of Autonomic Applications.- Spatial Computing: The TOTA Approach.- Towards Self-Managing QoS-Enabled Peer-to-Peer Systems.- Peer-to-Peer Algorithms.- Cooperative Content Distribution: Scalability Through Self-Organization.- Design and Analysis of a Bio-inspired Search Algorithm for Peer to Peer Networks.- Multifaceted Simultaneous Load Balancing in DHT-Based P2P Systems: A New Game with Old Balls and Bins.- Robust Locality-Aware Lookup Networks.- Power-Aware Distributed Protocol for a Connectivity Problem in Wireless Sensor Networks.- Self-Management of Virtual Paths in Dynamic Networks.- Sociologically Inspired Approaches for Self-*: Examples and Prospects.

Chord on demand

Structured peer-to-peer overlay networks are now an established paradigm for implementing a wide range of distributed services. While the problem of maintaining these networks in the presence of churn and other failures is the subject of intensive research, the problem of building them from scratch has not been addressed (apart from individual nodes joining an already functioning overlay). In this paper we address the problem of jump-starting a popular structured overlay, Chord, from scratch. This problem is of crucial importance in scenarios where one is assigned a limited time interval in a distributed environment such as Planet-Lab, or a grid, and the overlay infrastructure needs to be set up from the ground up as quickly and efficiently as possible, or when a temporary overlay has to be generated to solve a specific task on demand. We introduce T-Chord, that can build a Chord network efficiently starting from a random unstructured overlay. After jump-starting, the structured overlay can be handed over to the Chord protocol for further maintenance. We demonstrate through extensive simulation experiments that the proposed protocol can create a perfect Chord topology in a logarithmic number of steps. Furthermore, using a simple extension of the protocol, we can optimize the network from the point of view of message latency.