I am an assistant professor at the University of Pennsylvania. My research interests include distributed systems, networking, social networks, and security. I am particularly interested in large-scale distributed systems that span multiple administrative domains, such as cloud computing platforms or the Internet's interdomain routing system. Recently I have been working on accountability for distributed systems, on Internet access networks and on fighting botnets. In 2010, I was awarded the Otto Hahn Medal by the Max Planck Society for my research on accountability.