Sriram Chellappan

Deploying Wireless Sensor Networks under Limited Mobility Constraints

In this paper, we study the issue of sensor network deployment using limited mobility sensors. By limited mobility, we mean that the maximum distance that sensors are capable of moving to is limited. Given an initial deployment of limited mobility sensors in a field clustered into multiple regions, our deployment problem is to determine a movement plan for the sensors to minimize the variance in number of sensors among the regions and simultaneously minimize the sensor movements. Our methodology to solve this problem is to transfer the nonlinear variance/movement minimization problem into a linear optimization problem through appropriate weight assignments to regions. In this methodology, the regions are assigned weights corresponding to the number of sensors needed. During sensor movements across regions, larger weight regions are given higher priority compared to smaller weight regions, while simultaneously ensuring a minimum number of sensor movements. Following the above methodology, we propose a set of algorithms to our deployment problem. Our first algorithm is the optimal maximum flow-based (OMF) centralized algorithm. Here, the optimal movement plan for sensors is obtained based on determining the minimum cost maximum weighted flow to the regions in the network. We then propose the simple peak-pit-based distributed (SPP) algorithm that uses local requests and responses for sensor movements. Using extensive simulations, we demonstrate the effectiveness of our algorithms from the perspective of variance minimization, number of sensor movements, and messaging overhead under different initial deployment scenarios.

Mobility Limited Flip-Based Sensor Networks Deployment

An important phase of sensor networks operation is deployment of sensors in the field of interest. Critical goals during sensor networks deployment include coverage, connectivity, load balancing, etc. A class of work has recently appeared, where mobility in sensors is leveraged to meet deployment objectives. In this paper, we study deployment of sensor networks using mobile sensors. The distinguishing feature of our work is that the sensors in our model have limited mobilities. More specifically, the mobility in the sensors we consider is restricted to a flip, where the distance of the flip is bounded. We call such sensors as flip-based sensors. Given an initial deployment of flip-based sensors in a field, our problem is to determine a movement plan for the sensors in order to maximize the sensor network coverage and minimize the number of flips. We propose a minimum-cost maximum-flow-based solution to this problem. We prove that our solution optimizes both the coverage and the number of flips. We also study the sensitivity of coverage and the number of flips to flip distance under different initial deployment distributions of sensors. We observe that increased flip distance achieves better coverage and reduces the number of flips required per unit increase in coverage. However, such improvements are constrained by initial deployment distributions of sensors due to the limitations on sensor mobility

Peer-to-peer system-based active worm attacks: modeling and analysis

Recent active worm propagation events show that active worms can spread in an automated fashion and flood the Internet in a very short period of time. Due to the recent surge of peer-to-peer (P2P) systems with large numbers of users, P2P systems can be a potential vehicle for the active worms to achieve fast worm propagation in the Internet. In this paper, we address the issue of the impacts of active worm propagation on top of P2P systems. In particular: (1) we define a P2P system based active worm attack model and study two attack strategies (an off-line and on-line strategy) under the defined model; (2) we develop an analytical approach to analyze the propagation of active worms under the defined attack models and conduct an extensive study to the impacts of P2P system parameters, such as size, topology degree, and the structured/unstructured properties on active worm propagation. Based on numerical results, we observe that a P2P-based attack can significantly worsen attack effects (improve attack performance), and we observe that the speed of worm propagation is very sensitive to P2P system parameters. We believe that our work can provide important guidelines in design and control of P2P systems as well as overall active worm defense.

Sensor networks deployment using flip-based sensors

In this paper, we study the issue of mobility based sensor networks deployment. The distinguishing feature of our work is that the sensors in our model have limited mobilities. More specifically, the mobility in the sensors we consider is restricted to a flip, where the distance of the flip is bounded. Given an initial deployment of sensors in a field, our problem is to determine a movement plan for the sensors in order to maximize the sensor network coverage, and minimize the number of flips. We propose a minimum-cost maximum-flow based solution to this problem. We prove that our solution optimizes both the coverage and the number of flips. We also study the sensitivity of coverage and the number of flips to flip distance under different initial deployment distributions of sensors. We observe that increased flip distance achieves better coverage, and reduces the number of flips required per unit increase in coverage. However, such improvements are constrained by initial deployment distributions of sensors, due to the limitations on sensor mobility

Lifetime optimization of sensor networks under physical attacks

We address a resource constrained lifetime problem in sensor networks in an operating environment where nodes can be physically destroyed. Specifically, given a limited number of sensors and a hostile environment, our goal is to maximize the network lifetime and derive a node deployment plan. The problem of physical destruction, due to hostile environments and the small size of the sensors, is a viable threat and severely constrains the practical lifetime of sensor networks. The lifetime problem we define is representative, practical, and encompasses other versions of similar problems. We also define a representative physical attack model under which we study and solve the lifetime problem. Our solutions take into account both node constraints and the goal of energy minimization. An important observation based on this work is that network lifetime is greatly affected by the presence of physical attacks, highlighting the importance of our study. Our work has a broad and immediate impact for system designers deploying networks in hostile environments.

Associating Internet Usage with Depressive Behavior Among College Students

Depression is a serious mental health problem affecting a significant segment of American society today, and in particular college students. In a survey by the U.S. Centers for Disease Control (CDC) in 2009, 26.1% of U.S. students nationwide reported feeling so sad or hopeless almost every day for 2 or more weeks in a row that they stopped doing some usual activities. Similar statistics are also reported in mental health studies by the American College Health Association, and by independent surveys. In this article, the author report their findings from a month-long experiment conducted at Missouri University of Science and Technology on studying depressive symptoms among college students who use the Internet. This research was carried out using real campus Internet data collected continuously, unobtrusively, and while preserving privacy.

Sensor network configuration under physical attacks

Sensor networks will typically operate in hostile environments, where they are susceptible to physical attacks resulting in physical node destructions. In this paper, we study impacts of physical attacks on network configuration w.r.t. lifetime. While lifetime is constrained by limited energies and has been addressed before, they are not applicable under physical attacks. In this paper, we define a practical sensor network lifetime problem under physical attacks. We then analytically determine the minimum number and sensor deployment plan to meet lifetime requirement under physical attacks. We make several important observations, including the high sensitivity of lifetime to physical attacks.

TTL Based Packet Marking for IP Traceback

Distributed denial of service attacks continue to pose major threats to the Internet. In order to traceback attack sources (i.e., IP addresses), a well studied approach is probabilistic packet marking (PPM), where each intermediate router of a packet marks it with a certain probability, enabling a victim host to traceback the attack source. In a recent study, we showed how attackers can take advantage of probabilistic nature of packet markings in existing PPM schemes to create spoofed marks, hence compromising traceback. In this paper, we propose a new PPM scheme called TTL-based PPM (TPM) scheme, where each packet is marked with a probability inversely proportional to the distance traversed by the packet so far. Thus, packets that have to traverse longer distances are marked with higher probability, compared to those that have to traverse shorter distances. This ensures that a packet is marked with much higher probability by intermediate routers than by traditional mechanisms, hence reducing the effectiveness of spoofed packets reaching victims. Using formal analysis and simulations using real Internet topology maps, we show how our TPM scheme can effectively trace DDoS attackers even in presence of spoofing when compared to existing schemes.

Providing End-to-End Secure Communications in Wireless Sensor Networks

In many Wireless Sensor Networks (WSNs), providing end to end secure communications between sensors and the sink is important for secure network management. While there have been many works devoted to hop by hop secure communications, the issue of end to end secure communications is largely ignored. In this paper, we design an end to end secure communication protocol in randomly deployed WSNs. Specifically, our protocol is based on a methodology called differentiated key pre-distribution. The core idea is to distribute different number of keys to different sensors to enhance the resilience of certain links. This feature is leveraged during routing, where nodes route through those links with higher resilience. Using rigorous theoretical analysis, we derive an expression for the quality of end to end secure communications, and use it to determine optimum protocol parameters. Extensive performance evaluation illustrates that our solutions can provide highly secure communications between sensor nodes and the sink in randomly deployed WSNs. We also provide detailed discussion on a potential attack (i.e. biased node capturing attack) to our solutions, and propose several countermeasures to this attack.

Peer-to-peer system-based active worm attacks: Modeling, analysis and defense

Active worms continue to pose major threats to the security of todays Internet. This is due to the ability of active worms to automatically propagate themselves and compromise hosts in the Internet. Due to the recent surge of peer-to-peer (P2P) systems with large numbers of users and rich connectivity, P2P systems can be a potential vehicle for the attacker to achieve rapid worm propagation in the Internet. In this paper, we tackle this issue by modeling and analyzing active worm propagation on top of P2P systems, and designing effective defense strategies within P2P systems to suppress worm propagation. In particular: (1) we define two P2P-based active worm attack models: an offline P2P-based hit-list attack model and an online P2P-based attack model; (2) we conduct a detailed analysis on the impacts of worm propagation on top of P2P-based systems, and study the sensitivity of worm propagation to various P2P system and attack-related parameters; (3) finally, we propose defense strategies within the P2P system to combat worms. Based on extensive numerical analysis and simulation data, we demonstrate that P2P-based active worm attacks can significantly enhance worm propagation, and important P2P related parameters (system size, topology degree, host vulnerability, etc.) have significant impacts on worm spread. We also find that our proposed defense strategies can effectively combat worms by rapidly detecting and immunizing infected hosts.