Abstract. We study distributed content replication networks formed voluntarily by selfish autonomous users, seeking access to information objects that originate from distant servers. Each user caters to minimization of its individual access cost by replicating locally (up to constrained storage capacity) a subset of objects, and accessing the rest from the nearest possible location. We show existence of stable networks by proving existence of pure strategy Nash equilibria for a game-theoretic formulation of this situation. Social (overall) cost of stable networks is measured by the average or by the maximum access cost experienced by any user. We study socially most and least expensive stable networks by means of tight bounds on the ratios of the Price of Anarchy and Stability respectively. Although in the worst case the ratios may coincide, we identify cases where they differ significantly. We comment on simulations exhibiting occurence of cost-efficient stable networks on average.

We introduce optimal algorithms for the problems of data placement (DP) and page placement (PP) in networks with a constant number of clients each of which has limited storage availability and issues requests for data objects. The objective for both problems is to efficiently utilize each client's storage (deciding where to place replicas of objects) so that the total incurred access and installation cost over all clients is minimized. In the PP problem an extra constraint on the maximum number of clients served by a single client must be satisfied. Our algorithms solve both problems optimally when all objects have uniform lengths. When objects lengths are non-uniform we also find the optimal solution, albeit a small, asymptotically tight violation of each client's storage size by εlmax where lmax is the maximum length of the objects and ε some arbitrarily small positive constant. We make no assumption on the underlying topology of the network (metric, ultrametric etc.), thus obtaining the first non-trivial results for non-metric data placement problems. ⋆ This work has been supported in part by the project BIONETS (BIOlogically inspired NETwork and Services) (FP6-027748)

Abstract.We consider the problem of updating a directed minimum cost spanning tree (DMST), when edges are deleted from or inserted to a weighted directed graph. This problem apart from being a classic for directed graphs, is to the best of our knowledge a wide open aspect for the field of dynamic graph algorithms. Our contributions include results on the hardness of updates, a dynamic algorithm for updating a DMST, and detailed experimental analysis of the proposed algorithm exhibiting a speedup factor of at least 2 in comparison with the static practice.