On Counteracting Byzantine Attacks in Network Coded Peer-to-Peer Networks

Random linear network coding can be used in peer-to-peer networks to increase the efficiency of content distribution and distributed storage. However, these systems are particularly susceptible to Byzantine attacks. We quantify the impact of Byzantine attacks on the coded system by evaluating the probability that a receiver node fails to correctly recover a file. We show that even for a small probability of attack, the system fails with overwhelming probability. We then propose a novel signature scheme that allows packet-level Byzantine detection. This scheme allows one-hop containment of the contamination, and saves bandwidth by allowing nodes to detect and drop the contaminated packets. We compare the net cost of our signature scheme with various other Byzantine schemes, and show that when the probability of Byzantine attacks is high, our scheme is the most bandwidth efficient.Comment: 26 pages, 9 figures, Submitted to IEEE Journal on Selected Areas in Communications (JSAC) "Mission Critical Networking

Creep curve measurement to support wear and adhesion modelling, using a continuously variable creep twin disc machine

Predictive modelling of wear and adhesion at rolling-sliding contacts such as a railway rail and wheel depends on understanding the relationship between slip and shear force at the contact surface, i.e. the creep verses force curve. This paper describes a new approach to creep curve measurement using a twin disc machine running with a continuous programmed variation of creep, enabling an entire creep curve to be defined in a single experiment. The work focuses on very low levels of creep, ranging from zero to 1%, and shows clear correlation between the creep curve gradient and the full slip friction coefficient for dry and lubricated contacts. Comparison of data generated using the new approach with that generated using multiple tests each at a single creep level shows good agreement. Comparison is also made between the twin disc data and results for full size three dimensional rail-wheel contacts to examine how two and three dimensional contact adhesion data are related. The data generated has application in wear and rolling contact fatigue modelling, but the original motivation for the research was generation of creep curves to support prediction of low adhesion conditions at the rail-wheel interface based upon monitored running conditions prior to brake application. The range of contact conditions investigated includes those experienced in service and during driver training, with the correlation found between creep curve gradient (measurable prior to braking) and full slip friction coefficient (not measurable until brakes are applied) representing a key finding

Towards an engineering model for curve squeal

Curve squeal is a strong tonal noise that may arise when a railway vehicle negotiates a curve. The wheel/rail contact model is the central part of prediction models, describing the frictional instability occurring in the contact during squeal. A previously developed time-domain squeal model considers the wheel and rail dynamics, and the wheel/rail contact is solved using Kalker’s nonlinear transient CONTACT algorithm with Coulomb friction. In this paper, contact models with different degree of simplification are compared to CONTACT within the previously developed squeal model in order to determine a suitable contact algorithm for an engineering curve squeal model. Kalker’s steady-state FASTSIM is evaluated, and, without further modification, shows unsatisfying results. An alternative transient single-point contact algorithm named SPOINT is formulated with the friction model derived from CONTACT. Comparing with the original model results, the SPOINT implementation results are promising and similar to results from CONTACT

Modelling of railway curve squeal including effects of wheel rotation

Railway vehicles negotiating tight curves may emit an intense high-pitch noise. The underlying mechanisms of this squeal noise are still a subject of research. Simulation models are complex since they have to consider the non-linear, transient and high-frequency interaction between wheel and rail. Often simplified models are used for wheel and rail to reduce computational effort, which involves the risk of oversimplifications. This paper focuses on the importance to include a rotating wheel instead of a stationary wheel in the simulation models. Two formulations for a rotating wheel are implemented in a previously published wheel/rail interaction model: a realistic model based on an Eulerian modal coordinate approach and a simplified model based on a rotating load and moving Green's functions. The simulation results for different friction coefficients and values of lateral creepage are compared with results obtained for the stationary wheel. Both approaches for the rotating wheel give almost identical results for the rolling speed considered. Furthermore, it can be concluded that a model of a stationary flexible wheel is sufficient to simulate curve squeal

"Does god play dice with corrugations?": environmental effects on growth

Corrugation growth has perplexed many researchers for several decades with remaining challenges including its reliable prediction in the field. In the present paper, the effect of environmental variations on corrugation growth is investigated using field measurements and mechanics-based modelling. Statistically significant relationships between average daily rainfall, humidity and the growth of rail corrugation were investigated using meterological and railway site field monitoring of a metropolitan network test site with a recurring rail corrugation of about 95mm wavelength. Corrugation growth rate (G) was determined by systematically measuring the longitudinal rail profile with a Corrugation Analysis Trolley (CAT) on the 240m radius, narrow gauge, concrete-sleepered curve. Both roughness generated and weld initiated profile growths were investigated. The weather data was obtained from records held by the Bureau of Meteorology. The modelling is developed to provide insight and mechanics based analysis of the field corrugation growth under changes in environmental conditions and vehicle speed variability. Results show a strong correlation between variations in rainfall and corrugation growth that is consistent with changes in steady state wear. Changes in contact patch geometry and the damage mechanism as the corrugation amplitude grows are shown to have substantial but slower effects on growth. The results are used to explain observed changes in corrugation growth rate under variable speed control at the same site

Interfacial separation between elastic solids with randomly rough surfaces: comparison between theory and numerical techniques

We study the distribution of interfacial separations P(u) at the contact region between two elastic solids with randomly rough surfaces. An analytical expression is derived for P(u) using Persson's theory of contact mechanics, and is compared to numerical solutions obtained using (a) a half-space method based on the Boussinesq equation, (b) a Green's function molecular dynamics technique and (c) smart-block classical molecular dynamics. Overall, we find good agreement between all the different approaches.Comment: 25 pages, 12 figure

Semantic Coding: Partial Transmission

Shannon wrote in 1948: rdquoThe semantic aspects of communication are irrelevant to the engineering problemrdquo. He demonstrated indeed that the information generated by a source depends only on its statistics and not on the meaning of the source output. The authors derived the fundamental limits for semantic compaction, transmission and compression systems recently. These systems have the property that the codewords are semantic however, i.e. close to the source sequences. In the present article we determine the minimum distortion for semantic partial transmission systems. In these systems only a quantized version of each source source symbol is transmitted to the receiver. It should be noted that our achievability proof is based on weak instead of strong typicality. This is unusual for Gelfand-Pinsker [1980] related setups as e.g. semantic coding and embedding

Secret key watermarking with changing keys

We consider a digital watermarking application where multiple parties can embed additional information using their watermark embedder. These parties are not supposed to influence each other and each watermark detector needs to be able to decode the information embedded by any of the embedder systems. One approach would be to use a single secret key and to assign part of the payload to identify the particular embedder. However, it is generally accepted that for security reasons, each embedder should better have its own secret key. A major drawback of this last approach is related to the detector implementation complexity, which increases linearly with the number of embedders. It is shown that this drawback can be overcome by changing the key in the watermarking system dependant on features of the incoming signal