Experimental assessment of drag reduction by traveling waves in a turbulent pipe flow

We experimentally assess the capabilities of an active, open-loop technique for drag reduction in turbulent wall flows recently introduced by Quadrio et al. [J. Fluid Mech., v.627, 161, (2009)]. The technique consists in generating streamwise-modulated waves of spanwise velocity at the wall, that travel in the streamwise direction. A proof-of-principle experiment has been devised to measure the reduction of turbulent friction in a pipe flow, in which the wall is subdivided into thin slabs that rotate independently in the azimuthal direction. Different speeds of nearby slabs provide, although in a discrete setting, the desired streamwise variation of transverse velocity. Our experiment confirms the available DNS results, and in particular demonstrates the possibility of achieving large reductions of friction in the turbulent regime. Reductions up to 33% are obtained for slowly forward-traveling waves; backward-traveling waves invariably yield drag reduction, whereas a substantial drop of drag reduction occurs for waves traveling forward with a phase speed comparable to the convection speed of near-wall turbulent structures. A Fourier analysis is employed to show that the first harmonics introduced by the discrete spatial waveform that approximates the sinusoidal wave are responsible for significant effects that are indeed observed in the experimental measurements. Practical issues related to the physical implementation of this control scheme and its energetic efficiency are briefly discussed.Comment: Article accepted by Phys. Fluids. After it is published, it will be found at http://pof.aip.or

Blocking DDoS attacks at the network level

Denial of service (DDoS) is a persistent and continuously growing problem. These attacks are based on methods that flood the victim with messages that it did not request, effectively exhausting its computational or bandwidth resources. The variety of attack approaches is overwhelming and the current defense mechanisms are not completely effective. In today’s internet, a multitude of DDoS attacks occur everyday, some even degrading the availability of critical or governmental services. In this dissertation, we propose a new network level DDoS mitigation protocol that iterates on previous attempts and uses proven mechanisms such as cryptographic challenges and packet-tagging. Our analysis of the previous attempts to solve this problem led to a ground-up design of the protocol with adaptability in mind, trying to minimize deployment and adoption barriers. With this work we concluded that with software changes only on the communication endpoints, it is possible to mitigate the most used DDoS attacks with results up to 25 times more favourable than standard resource rate limiting (RRL) methods

Command & Control: Understanding, Denying and Detecting - A review of malware C2 techniques, detection and defences

In this survey, we first briefly review the current state of cyber attacks, highlighting significant recent changes in how and why such attacks are performed. We then investigate the mechanics of malware command and control (C2) establishment: we provide a comprehensive review of the techniques used by attackers to set up such a channel and to hide its presence from the attacked parties and the security tools they use. We then switch to the defensive side of the problem, and review approaches that have been proposed for the detection and disruption of C2 channels. We also map such techniques to widely-adopted security controls, emphasizing gaps or limitations (and success stories) in current best practices.Comment: Work commissioned by CPNI, available at c2report.org. 38 pages. Listing abstract compressed from version appearing in repor