In network security, Denial of Service (DoS) attacks target network systems with the aim of making them unreachable. Last generation threats are particularly dangerous because they can be carried out with very low resource consumption by the attacker. In this paper we propose SlowDrop, an attack characterized by a legitimate-like behavior and able to target different protocols and server systems. The proposed attack is the first slow DoS threat targeting Microsoft IIS, until now unexploited from other similar attacks. We properly describe the attack, analyzing its ability to target arbitrary systems on different scenarios, by including both wired and wireless connections, and comparing the proposed attack to similar threats. The obtained results show that by executing targeted attacks, SlowDrop is successful both against conventional servers and Microsoft IIS, which is closed source and required us the execution of so called “network level reverse engineering” activities. Due to its ability to successfully target different servers on different scenarios, the attack should be considered an important achievement in the slow DoS field.

Network coding is a class of routing algorithms offering increased throughput and improved robustness to random failures. With traditional routing, intermediate nodes in the network may only forward unmodified packets. With network coding, instead, intermediate nodes are allowed to forward linear combinations of received packets. Original data can be reconstructed after collecting sufficiently many linear combinations. Current file sharing systems offer either low overhead and high bandwidth with no privacy, or acceptable privacy at very low speed. Thanks to network coding, a general-purpose P2P network can obtain a privacy/performance tradeoff that may be considered reasonable in most real-world scenarios. In this paper we present an integrity strategy for network coding-based P2P anonymous systems, specifically designed to preserve the anonymity of peers. Our approach is significantly easier to implement than current solutions when anonymity is required. We implement the cryptographic algorithms on which our method is based and provide performance figures. We also define verification strategies which use batching for improved performances together with an efficiency analysis.

We introduce the notion of StemCerts, a digital certificate scheme that allows the user to modify some fields of a digital certificate while keeping it valid. The owner can modify a StemCert in a limited and controlled fashion without interacting with the certification authority which issued it. By modifying her identity, the user can achieve "pseudonymous anonymity" - but the CA can still associate a certificate to its owner - and/or handle temporary or permanent address changes. Modifying the expiry date allows the user to transform her certificate into a set of "one time" certificates, thus alleviating the need for revocation lists. We developed two proof-of-concept implementations for this new scheme. The first one was based on Chameleon hash functions, while the second one was based on the use of two chained, standard X.509 v3 certificates. We also present experimental data collected from the prototype implementations that show how the second prototype can easily be adopted in real environments, possibly exploiting smartcard technology.

We define a DHT system whose aim is to combine the routing efficiency typical of randomized networks, i.e. average path length O(logn/ log log n), with the ease of coding and the start-up efficiency of an optimized uniform system. Our proposed system is non-uniform, even though it is not explicitly randomized. In particular, the difference in terms of finger tables between two adjacent peers in a large system is very moderate, and predictable based on an approximation of the peers' keys. These characteristics allow us to adopt a (multi-hop) Neighbor of Neighbor routing algorithm, which asymptotically achieves O(logn/ log logn) average path length, and that in practice may even slightly outperform randomized networks containing more than 100,000 peers.

Abstract The Chord protocol is the best known example of implementation of logarithmic complexity routing for structured peer‐to‐peer networks. Its routing algorithm, however, does not provide an optimal trade‐off between resources exploited (the size of the ‘finger table’) and performance (the average or worst‐case number of hops to reach destination). Cordasco et al. showed that a finger table based on Fibonacci distances provides lower number of hops with fewer table entries. In this paper we generalize this result, showing how to construct an improved finger table when the objective is to reduce the number of hops, possibly at the expense of an increased size of the finger table. Our results can also be exploited to guarantee low routing time in case a fraction of nodes fails. Copyright © 2007 John Wiley & Sons, Ltd.

The chord protocol is the best known example of implementation of logarithmic complexity routing for structured peer-to-peer networks. Its routing algorithm, however, does not provide an optimal trade-off between resources exploited (the size of the "finger table") and performance (the average or worst-case number of hops to reach destination). Cordasco et al. showed that a finger table based on Fibonacci distances provides lower number of hops with fewer table entries. In this paper, we generalize this result, showing how to construct an improved finger table when the objective is to reduce the number of hops, possibly at the expense of an increased size of the finger table. Our results can also be exploited to guarantee low routing time in case a fraction of nodes is assumed to fail.

A cluster of PCs can be seen as a collection of networked low cost disks; such a collection can be operated by proper software so as to provide the abstraction of a single, larger block device. By adding suitable data redundancy, such a disk collection as a whole could act as single, highly fault tolerant, distributed RAID device, providing capacity and reliability along with the convenient price/performance typical of commodity clusters. We report about the design and performance of DRAID, a distributed RAID prototype running on a Gigabit Ethernet cluster of PCs. DRAID offers storage services under a single I/O space (SIOS) block device abstraction. The SIOS feature implies that the storage space is accessible by each of the stations in the cluster, rather than throughout one or few end-points, with a potentially higher aggregate I/O bandwidth and better suitability to parallel I/O.

The class of stochastic well-formed colored nets (SWN's) was defined as a syntactic restriction of stochastic high-level nets. The interest of the introduction of restrictions in the model definition is the possibility of exploiting the symbolic reachability graph (SRG) to reduce the complexity of Markovian performance evaluation with respect to classical Petri net techniques. It turns out that SWN's allow the representation of any color function in a structured form, so that any unconstrained high-level net can be transformed into a well-formed net. Moreover, most constructs useful for the modeling of distributed computer systems and architectures directly match the "well-formed" restriction, without any need of transformation. A nontrivial example of the usefulness of the technique in the performance modeling and evaluation of multiprocessor architectures is included.>

The class of Petri nets obtained by eliminating timing from generalized stochastic Petri net (GSPN) models while preserving the qualitative behavior is identified. Structural results for those nets are derived, obtaining the first structural analysis of Petri nets with priority and inhibitor arcs. A revision of the GSPN definition based on the structural properties of the models is presented. It is shown that for a (wide) class of nets, the definition of firing probabilities of conflicting immediate transitions does not require the information on reachable markings. Identification of the class of models for which the net-level specification is possible is also based on the structural analysis results. The procedure for the model specification is illustrated by means of an example. It is also shown that a net-level specification of the model associated with efficient structural analysis techniques can have a substantial impact on model analysis.>

A colored generalized stochastic Petri net (CGSPN) model was used to study the correctness and performance of the Lamport concurrent algorithm to solve the mutual exclusion problem on machines lacking an atomic test and set instruction. In particular, a parametric formal proof of liveness is developed based on the structure and initial state of the model. The performance evaluation is based on a Markovian analysis that exploits the symmetries of the model to reduce the cost of the numerical solution. Both kinds of analysis are supported by efficient algorithms. The potential of the GSPN modeling technique is illustrated on an academic but nontrivial example of an application from distributed systems.>

Ergodicity and throughput bound characterization are addressed for a subclass of timed and stochastic Petri nets, interleaving qualitative and quantitative theories. The nets considered represent an extension of the well-known subclass of marked graphs, defined as having a unique consistent firing count vector, independently of the stochastic interpretation of the net model. In particular, persistent and mono-T-semiflow net subclasses are considered. Upper and lower throughput bounds are computed using linear programming problems defined on the incidence matrix of the underlying net. The bounds proposed depend on the initial marking and the mean values of the delays but not on the probability distributions (thus including both the deterministic and the stochastic cases). From a different perspective, the considered subclasses of synchronized queuing networks; thus, the proposed bounds can be applied to these networks.>

Petri nets in which random delays are associated with atomic transitions are defined in a comprehensive framework that contains most of the models already proposed in the literature. To include generally distributed firing times into the model one must specify the way in which the next transition to fire is chosen, and how the model keeps track of its past history; this set of specifications is called an execution policy. A discussion is presented of the impact that different execution policies have on semantics of the mode, as well as the characteristics of the stochastic process associated with each of these policies. When the execution policy is completely specified by the transition with the minimum delay (race policy) and the firing distributions are of the phase type, an algorithm is provided that automatically converts the stochastic process into a continuous time homogeneous Markov chain. An execution policy based on the choice of the next transition to fire independently of the associated delay (preselection policy) is introduced, and its semantics is discussed together with possible implementation strategies.>

Recursive relations are derived for the exact computation of the steady-state probability distribution of some queuing models with passive resources that can be used to analyze the performance of multiple-bus multiprocessor system architectures. The most general case that can be shown to admit a product-form solution is described, and a recursive solution is obtained taking into account, considering different processor access rates, different memory selection probabilities, and a first-come-first-served bus scheduling policy. Several simpler cases allowing easier model solutions are also considered. Numerical evaluations for large computing systems with nonuniform memory references show the usefulness of the results.>

A class of Petri nets in which transitions can fire either immediately or after an exponentially distributed random time since their enabling (GSPN), is used to define performance models for Flexible Manufacturing Systems. An example is discussed in which the analysis of the GSPN model yields results that are more reliable than those obtained with a currently used method based on queueing networks. Detailed GSPN models are shown that allow accurate analyses of the effects that different scheduling policies and pallet mixes have on system performance.

A high-level Petri net model of the software architecture of an experimental MIMD multiprocessor system for Artificial Intelligence applications is derived by direct translation of the code corresponding to the assumed workload. Hardware architectural constraints are then easily added, and formal reduction rules are used to simplify the model, which is then further approximated to obtain a performance model of the system based on generalized stochastic Petri nets. From the latter model it is possible to estimate the optimal multiprogramming level of each processor so as to achieve the maximum performance in terms of overall throughput (number of tasks completed per unit time).