Giovanna Melideo

Experiences and issues in the realization of e-government services

Certification of the executed service is a critical issue for an e-government infrastructure. In fact, given the legal value that is often attached to data managed and exchanged by public administrations, being able to document the actual execution of e-services is of the utmost importance. This is made more complex in cases, as often happens in the public administration sector, where e-services are based on legacy systems managed by autonomous and independent organizations. In this paper we discuss, starting from real-life e-government systems developed in Italy, the introduction, within the standard three tier architecture for e-services, of new control components, based on efficient algorithmic techniques, providing solutions for this issue.

On Nash equilibria for multicast transmissions in ad-hoc wireless networks

We study a multicast game in ad-hoc wireless networks in which a source sends the same message or service to a set of receiving stations via multi-hop communications and the overall transmission cost is divided among the receivers according to given cost sharing methods. We assume that each receiver gets a certain utility from the transmission and enjoys a benefit equal to the difference between his utility and the shared cost he is asked to pay. Assuming a selfish and rational behavior, each user is willing to receive the transmission if and only if his shared cost does not exceed his utility. Moreover, given the strategies of the other users, he wants to select a strategy of minimum shared cost. A Nash equilibrium is a solution in which no user can increase his benefit by choosing to adopt a different strategy. We consider the following reasonable cost sharing methods: egalitarian, semi-egalitarian next-hop-proportional, path-proportional, egalitarian-path-proportional and Shapley value. We prove that, while the first five cost sharing methods in general do not admit a Nash equilibrium, the Shapley value yields games always converging to a Nash equilibrium. We then turn our attention to the special case in which the receivers’ set R is part of the input (that is only the stations belonging to R have a positive utility which is set equal to infinity) and show that in such a case also the egalitarian and the egalitarian-path-proportional methods yield convergent games. In such a framework, we show that the price of anarchy is unbounded for the game yielded by the egalitarian method and provide matching upper and lower bounds for the price of anarchy of the other two convergent games with respect to two different global cost functions, that is the overall cost of the power assignment, that coincides with the sum of all the shared costs, and the maximum shared cost paid by the receivers. Finally, in all cases we show that finding the best Nash equilibrium is computationally intractable, that is NP-hard.

A Reference Architecture for the Certification of E-Services in a Digital Government Infrastructure

Certifying the execution of a service is a critical issue for an e-government infrastructure. In fact being able to document that an e-service was actually carried out, given the legal value that is often attached to data managed and exchanged by public administrations, is of the utmost importance. This is made more complex in cases, like it often happens in the public administration sector, where e-services are based on legacy systems managed by autonomous and independent organizations. In this paper we discuss the introduction, within the standard three tier architecture for e-services, of an architectural subsystem providing certification functions. This architecture features both physical and functional independence from the application level and is made up by new control components providing a highly efficient solution for certification requirements. Our solution has been successfully tested in real-world systems developed in Italy to support digital government functions.

Game Theoretical Issues in Optical Networks

In this paper we focus on the problem in optical networks in which selfish or non-cooperative users can configure their communications so as to minimize the cost paid for the service. Such a cost depends on the personal configuration and on the one of the other users. During a series of time steps, at each of which only one user can move to a better configuration, a Nash equilibrium is eventually reached, that is a situation in which no user can select an improved solution and thus is interested in further modifications. In such a setting, the network provider must determine suitable payment functions covering the network costs that induce Nash equilibria with the best possible global performances. We first present results in the classical scenario in which we are interested in optimizing the optical spectrum, that is in minimizing the total number of used wavelengths. We then outline possible settings in which the approach can be eventually applied to minimize the cost of optical routing due to specific hardware components such as ADMs or filters, that are typical examples of expensive elements whose price can be shared among different lightpaths under specific constraints

Quality of service in wireless networks

In this paper we revise some of the most relevant aspects concerning the quality of service in wireless networks, providing, along the research issues we are currently pursuing, both the state-of-the-art and our recent achievements. More specifically, first of all we focus on network survivability, that is the ability of the network of maintaining functionality as a consequence of a component failure. Then, we turn our attention on data access and network services in a distributed environment. Finally, we analyze a basic network optimization task, that is routing design in wireless ATM networks.

Mobile network creation games

We introduce a new class of network creation games, called mobile network creation games, modelling the spontaneous creation of communication networks by the distributed and uncoordinated interaction of k selfish mobile devices. Each device is owned by a player able to select a node in an underlying positions graph so as to minimize a cost function taking into account two components: the distance from her home position, and the number of players she is not connected to, with the connectivity costs being prevailing, i.e., the Nash Equilibria are stable solution states in which communication is possible among all the players. We show that the game always admits a Pure Nash equilibrium, even if the convergence after a finite number of improving movements is guaranteed only when players perform their best possible moves. More precisely, if initial positions are arbitrary, that is not necessarily coinciding with the home ones, an order of kD best moves is necessary (and sufficient) to reach an equilibrium, where D is the diameter of the positions graph. As for the Nash equilibria performances, we first prove that the price of stability is 1 (i.e. an optimal solution is also a Nash equilibrium). Moreover, we show that the lack of centralized control of mobile devices is a major issue in terms of final performance guaranteed. Namely, the price of anarchy is Θ(kD). Nevertheless, we are able to prove that if players start at their home positions, in Θ(k min {k2,D}) best moves they reach an equilibrium approximating the optimal solution by a factor of Θ(k min {k,D}).

On nash equilibria for multicast transmissions in ad-hoc wireless networks

We study a multicast game in ad-hoc wireless networks in which a source sends the same message or service to a set of receiving stations via multi-hop communications and the overall transmission cost is divided among the receivers according to given cost sharing methods Receivers enjoy a benefit equal to the difference between the utility they get from the transmission and the shared cost they are asked to pay Assuming a selfish and rational behavior, each user is willing to receive the transmission if and only if his shared cost does not exceed his utility Moreover, given the strategies of the other users, he wants to select a strategy of minimum shared cost A Nash equilibrium is a solution in which no user can increase his benefit by seceding in favor of a different strategy We consider the following reasonable cost sharing methods: the overall transmission cost is equally shared among all the receivers (egalitarian), the cost of each intermediate station is divided among its downstream receivers equally (semi-egalitarian) or proportionally to the transmission powers they require to reach their next-hop stations (proportional), and finally each downstream user at an intermediate station equally shares only his required next-hop power among all the receivers asking at least such a level of power (Shapley value) We prove that, while the first three cost sharing methods in general do not admit a Nash equilibrium, the Shapley value yields games always converging to a Nash equilibrium Moreover, we provide matching upper and lower bounds for the price of anarchy of the Shapley game with respect to two different global cost functions, that is the overall cost of the power assignment, that coincides with the sum of all the shared costs, and the maximum shared cost paid by the receivers Finally, in both cases we show that finding the best Nash equilibrium is computationally intractable, that is NP-hard.

Truthful Mechanisms for Generalized Utilitarian Problems

In this paper we investigate extensions of the well-known Vickrey-Clarke-Groves (VCG)mechanisms to problems whose objective function is not utilitarian and whose agents’ utilities are not quasi-linear. We provide a generalization of utilitarian problems,termed consistent problems, and prove that every consistent problem admits a truthful mechanism. These mechanisms,termed VCG-consistent (VCGc) mechanisms,can be seen as a natural extension of VCG mechanisms for utilitarian problems.

On the minimal information to encode timestamps in distributed computations

Timestamping protocols are used to capture the causal order or the concurrency of events in asynchronous distributed computations. In this paper we give an answer to the open problem issued by Schwarz and Mattern [Distrib. Comput. 7 (3) (1994) 149-174] about the minimum amount of information managed by protocols which represent causality in an isomorphic way. We point out that to encode each timestamp an amount of non-structured information (i.e., the number of bits) of ⌈log2((m+1)n - Σk=3n(nk)(2k-35))⌉ bits is necessary.

An Improved Approximation Algorithm for the Minimum Energy Consumption Broadcast Subgraph

In an ad-hoc wireless network each station has the capacity of modifying the area of coverage with its transmission power. Controlling the emitted transmission power allows to significantly reduce the energy consumption and so to increase the lifetime of the network. In this paper we focus on the Minimum Energy Consumption Broadcast Subgraph (MECBS) problem [1,2,6], whose objective is that of assigning a transmission power to each station in such a way that a message from a source station can be forwarded to all the other stations in the network with a minimum overall energy consumption. The MECBS problem has been proved to be inapproximable within (1–e) ln n unless NP ⊆ DTIME(n O(log log n)) [2,6], where n is the number of stations. In this work we propose a 2H n − − 1-approximation greedy algorithm which, despite its simplicity, improves upon the only previously known ratio of 10.8 ln n [1] and considerably approaches the best-known lower bound on the approximation ratio.