A peer-to-peer system can be defined as an overlay network built by a set of nodes on top of a physical network infrastructure and its operating protocols, such as the Internet. In a peer-to-peer network, each node maintains a limited number of connections with other nodes, called peers, and the graph of peer connections constitutes the overlay's topology. One of the most fundamental properties of existing large-scale peer-to-peer systems is a very high heterogeneity and dynamism of peers participating in the system. Studies show that the distributions of peer characteristics, such as peer session duration, available bandwidth, and storage space, are highly skewed and often heavy-tailed, with small fractions of peers possessing disproportionally large fractions of the total system resources. Such heterogeneity introduces both challenges and opportunities when designing peer-to-peer systems. The use of low-performance or low-stability nodes for maintaining system data or services can easily lead to a poor performance of the entire system, while the placement of critical data and services on the most reliable, high-capacity nodes may improve the overall system stability and performance. Current state-of-the-art peer-to-peer systems exploit their heterogeneity by introducing two-level hierarchies of peers. High capability peers, so called super-peers, form an independent sub-topology within the existing peer-to-peer network and handle the core system functionality, such as indexing peer data and handling search queries, or relaying traffic on behalf of firewalled peers. Ordinary peers connect directly to super-peers and act as their clients. However, many existing systems lack an efficient, decentralised super-peer election algorithm. In many systems, super-peers are selected manually, through an out-of-band mechanism, or are elected using simple local heuristics, which are likely to generate suboptimal super-peer sets. Sophisticated super-peer election algorithms exist, but they are usually highly specific to particular systems and are not easily portable to other application areas. This thesis presents a novel class of peer-to-peer topologies, called gradient topologies, which generalise the concept of super-peer networks. In gradient topologies, the position of each peer is determined by a continuous utility function, and the highest utility peers are clustered in a logical centre of the topology, while peers with lower utility are located at gradually increasing distance from the centre. The utility metric captures application-specific peer requirements and reflects peers' ability to contribute resources and services to the system. The gradient structure of the topology has two fundamental properties. Firstly, all peers in the system with utility above a given threshold are located close to each other in terms of overlay hops and form a connected sub-topology. Such high-utility peers can be exploited by higher level applications in a similar fashion to super-peers in traditional two-level hierarchies. Secondly, the information captured in the topology enables a search heuristic, called gradient search, that enables efficient discovery of such high utility peers. The gradient topologies have been evaluated using a custom-built simulator and compared with state-of-the-art super-peer systems. The evaluation shows that the election techniques based on gradient topologies allow more flexible super-peer criteria specification compared with the other systems. Moreover, the super-peer sets elected using gradient topologies are closer to the theoretical optimum, compared with the other systems, and have a higher average utility and stability. The experiments also show that the maintenance cost of gradient topologies, in terms of generated messages and established connections, is similar to that in the state-of-the-art systems.

Most Distributed Hash Tables (DHTs) simply consider in-terconnecting homogeneous nodes on the same overlay. How-ever, realistically nodes on a network are heterogeneous in terms of their capabilities. Because of this, traditional DHTs have been shown to exhibit poor performance in a real-world environment. Additionally, we believe that it is this approach that contributes to a limited exploitation of peer-to-peer technologies. Previous work on super-peers in DHTs was proposed to address these performance issues, however the strategy used is often based on locally clustering peers around individual super-peers. This method of super-peering, however, compromises fundamental features such as load-balancing, resilience and routing efficiency, which traditional DHTs originally promised to offer. We propose a Stealth DHT which addresses the deficien-cies of previous super-peer approaches by using the DHT algorithm itself to select the most appropriate super-peer for each message sent by peers. Through simulations and measurements, we show the fitness for purpose of our pro-posal.

This paper presents an efficient approach for improving file availability in super-peer-based peer-to-peer (P2P) file-sharing systems. In the super-peer-based P2P file-sharing system, peers are organized into multiple groups. In each group, there is a special peer called super-peer to serve the regular peers within the same group.With this property, the proposed approach utilizes the super-peer to tolerate the departure (failure) of a regular peer in order to protect shared files. Unlike traditional replication-based approaches, the proposed approach keeps track of the file queries in the super-peer to support fault tolerance. The cost of tracking the file queries is much smaller than the cost of replicating the file contents in advance. Furthermore, the proposed approach uses a logical connection technique to consider the departure (failure) of the super-peer. Finally, simulation experiments are performed to quantify the performance and overhead of the proposed approach.

ROSA is an e-learning system which enables the management of Learning Objects (LOs). However, since ROSA is a centralized system, it does not provide the integration of LOs created in other institutions. This paper presents the P2P distributed system developed for ROSA, named ROSA- P2P, which provides the physical environment to carry out the integration of these LOs. This environment includes, among other functionalities, particular strategies for: aggregation of peers and grouping of super-peers; connection/disconnection of peers; definition and election of super-peers; balancing and redistribution of peers; routing indices for peers and super-peers communication; and some issues concerning fault tolerance. Keywords: Peer-to-peer technology, Web architectures, Distributed database.

Abstract—Technological developments of the last few years have favoured the creation of distributed networking infrastructures (usually referred to as Community Networks) where the resources are made available to the members of a community of people. With emerging large-scale community infrastructures the opportunities for new commercial services and for innovative business models are becoming feasible. Since one of the most resource-demanding services today is access to user-generated content through the web, suitable content delivery services are needed in the context of Community Networks in orders to make an effective usage of shared resources. In this paper we describe two architectures we have designed to provide optimized delivery of multimedia web content (e.g. video but also other kinds of User Generated Content) within Community Networks. Both architectures work without any kind of cooperation from the original content providers but assume that a basic web caching service is provided within the network, either by commercial Service Providers or by the community members themselves. While the first architecture relies on a traditional centralized control entity, the second is designed according to the peer-topeer paradigm, in order to provide better scalability, robustness to failures and self-configurability. Experimental results are also presented aimed at evaluating the performance gains for endusers from a localized distribution of content in scenarios in which community members are distributed in clusters sparse at different geographic locations. I.

Abstract—Technological developments of the last few years have favoured the creation of distributed networking infrastructures (usually referred to as Community Networks) where the resources are made available to the members of a community of people. With emerging large-scale community infrastructures the opportunities for new commercial services and for innovative business models are becoming feasible. Since one of the most resource-demanding services today is access to user-generated content through the web, suitable content delivery services are needed in the context of Community Networks in orders to make an effective usage of shared resources. In this paper we describe two architectures we have designed to provide optimized delivery of multimedia web content (e.g. video but also other kinds of User Generated Content) within Community Networks. Both architectures work without any kind of cooperation from the original content providers but assume that a basic web caching service is provided within the network, either by commercial Service Providers or by the community members themselves. While the first architecture relies on a traditional centralized control entity, the second is designed according to the peer-topeer paradigm, in order to provide better scalability, robustness to failures and self-configurability. Experimental results are also presented aimed at evaluating the performance gains for endusers from a localized distribution of content in scenarios in which community members are distributed in clusters sparse at different geographic locations. I.

Abstract. Network signaling and control mechanisms are critical to coordinate such diverse defense capabilities as honeypots and honeynets, host-based defenses, and online patching systems, any one of which might issue an actionable alert and provide security-critical data. Despite considerable work in exploring the trust requirements of such defenses and in addressing the distribution speed of alerts, little work has gone into identifying how the underlying transport systems behave under adversarial scenarios. In this paper, we evaluate the reliability and performance trade-offs for a variety of control channel mechanisms that are suitable for coordinating large-scale collaborative defenses when under attack. Our results show that the performance and reliability characteristics change drastically when one evaluates the systems under attack by a sophisticated and targeted adversary. Based on our evaluation, we explore available design choices to reinforce the reliability of the control channel mechanisms. To that end, we propose ways to construct a control scheme to improve network coverage without imposing additional overhead. 1