Rare events statistics of random walks on networks: localization and other dynamical phase transitions

Rare event statistics for random walks on complex networks are investigated using the large deviations formalism. Within this formalism, rare events are realized as typical events in a suitably deformed path-ensemble, and their statistics can be studied in terms of spectral properties of a deformed Markov transition matrix. We observe two different types of phase transition in such systems: (i) rare events which are singled out for sufficiently large values of the deformation parameter may correspond to {\em localized\/} modes of the deformed transition matrix, (ii) "mode-switching transitions" may occur as the deformation parameter is varied. Details depend on the nature of the observable for which the rare event statistics is studied, as well as on the underlying graph ensemble. In the present letter we report on the statistics of the average degree of the nodes visited along a random walk trajectory in Erd\H{o}s-R\'enyi networks. Large deviations rate functions and localization properties are studied numerically. For observables of the type considered here, we also derive an analytical approximation for the Legendre transform of the large-deviations rate function, which is valid in the large connectivity limit. It is found to agree well with simulations.Comment: 5 pages, 3 figure

A framework for highly reconfigurable P2P trackers

The increasing use of Peer to Peer (P2P) applications, usually ruled by selfish behaviors, is posing new challenges to the research community. As contributions of this work we firstly devise a general framework underpinning the development of highly reconfigurable P2P trackers. Following that, a novel tracker architecture is proposed and several illustrative and enhanced tracker configurations are described. As result, the devised solution turns possible that flexible, programmable and adaptive peer selection mechanisms can be introduced at the P2P application level. The proposed solution assumes the general framework of one of the most popular P2P solutions, in this case a BitTorrent-like approach. As illustrative examples of the proposed framework capabilities, several straightforward and easy to deploy tracker configuration examples are presented, including methods for qualitative differentiation of swarm peers and advanced P2P Traffic Engineering mechanisms fostering the collaboration efforts between ISPs and P2P applications. Both the framework and the devised tracker configurations are validated resorting to simulation experiments.(undefined

Flexible coordination techniques for dynamic cloud service collaboration

The provision of individual, but also composed services is central in cloud service provisioning. We describe a framework for the coordination of cloud services, based on a tuple‐space architecture which uses an ontology to describe the services. Current techniques for service collaboration offer limited scope for flexibility. They are based on statically describing and compositing services. With the open nature of the web and cloud services, the need for a more flexible, dynamic approach to service coordination becomes evident. In order to support open communities of service providers, there should be the option for these providers to offer and withdraw their services to/from the community. For this to be realised, there needs to be a degree of self‐organisation. Our techniques for coordination and service matching aim to achieve this through matching goal‐oriented service requests with providers that advertise their offerings dynamically. Scalability of the solution is a particular concern that will be evaluated in detail

Inter-arrival times of message propagation on directed networks

One of the challenges in fighting cybercrime is to understand the dynamics of message propagation on botnets, networks of infected computers used to send viruses, unsolicited commercial emails (SPAM) or denial of service attacks. We map this problem to the propagation of multiple random walkers on directed networks and we evaluate the inter-arrival time distribution between successive walkers arriving at a target. We show that the temporal organization of this process, which models information propagation on unstructured peer to peer networks, has the same features as SPAM arriving to a single user. We study the behavior of the message inter-arrival time distribution on three different network topologies using two different rules for sending messages. In all networks the propagation is not a pure Poisson process. It shows universal features on Poissonian networks and a more complex behavior on scale free networks. Results open the possibility to indirectly learn about the process of sending messages on networks with unknown topologies, by studying inter-arrival times at any node of the network.Comment: 9 pages, 12 figure

Um sistema P2P adaptável a diferentes contextos de utilização

No mundo actual em que vivemos tudo e todos estão conectados por uma rede global denominada Internet, onde as aplicações P2P proliferam. Com a crescente evolução da Internet tem-se vindo a observar um aumento na utilização de soluções P2P, que devido à sua imprevisibilidade trazem fortes problemas para os fornecedores de serviços (e.g. ISPs). Este artigo apresentará um sistema P2P baseado no protocolo BitTorrent de fácil integração na rede Internet através da sua capacidade de adaptação a diferentes contextos de utilização. Esta integração será alcançada recorrendo a vários mecanismos e estratégias implementadas no sistema desenvolvido. A arquitectura a desenvolver será avaliada e testada com o auxilio da ferramenta de simulação NS-2, com o objectivo de testar os mecanismos propostos e analisar os resultados obtidos

Context aware programmable trackers for the next generation Internet

This work introduces and proposes the concept of context aware programmable trackers for the next generation Internet. The pro- posed solution gives ground for the development of advanced applications based on the P2P paradigm and will foster collaborative efforts among several network entities (e.g. P2P applications and ISPs). The proposed concept of context aware programmable trackers allows that several peer selection strategies might be supported by a P2P tracker entity able to improve the peer selection decisions according with pre-defined objectives and external inputs provided by specific services. The flexible, adaptive and enhanced peer selection semantics that might be achieved by the proposed solution will contribute for devising novel P2P based services and business models for the future Internet

Traffic engineering approaches in P2P environments

Nowadays, P2P applications proliferate in the Internet with distinct utilization contexts, being also an attractive model for the deployment of advanced Internet services. However, there are several undesirable effects that are caused by such applications, raising coexistence problems with Internet Service Providers (ISPs). In this context, using as case study BitTorrent like applications, this work explores a collaborative framework allowing that advanced efforts could be carried on between P2P applications and network level entities. In order to illustrate such framework, several Traffic Engineering (TE) mechanisms are devised in order to align some P2P dynamics with particular objectives pursued by network administrators. The simulation results show that both the proposed framework and corresponding illustrative mechanisms are viable and can effectively foster future research efforts within this field.(undefined

Building Cooperation in VoIP Network through a Reward Mechanism

In this paper, for solving the moral hazard problem of super nodes in VOIP network and achieving better communication quality, we establish a reward mechanism based on classical efficiency-wage models. In the reward mechanism, the function of reward is to encourage super nodes to contribute their bandwidth and cover their effort costs, whereas the function of fine is to prevent opportunistic super nodes from shirking. We consider that network quality and idle bandwidth are the essential criterions for selecting qualified super nodes. Once all super nodes can satisfy specific conditions, the required reward can be derived so as to improve the VoIP platform\u27s revenue. Moreover, we also suggest several targets both in technical and economic view that the platform provider can strive in order to boost his/her market share. In addition, the case of Skype is discussed in this study and we also examine its current pricing strategy