Abstract: In this paper, we introduce Pacemaker, a scalable and lightweight protocol to measure reliably the availability of peers. To the best of our knowledge, Pacemaker is the only protocol resilient to the presence of selfish peers, i.e. peers lying about their availability and minimizing their contribution to the system. Pacemaker relies on a novel {\em pulse-based} architecture, where a small set of trusted peers regularly flood the network with pulses containing cryptographic values. Collecting these pulses enables peers to later prove their presence in the system at any time, using cryptographic signatures. This new architecture overcomes many limitations of ping-based systems, and can be easily deployed on ad-hoc networks and social-based topologies. Simulation results show that our protocol provides accurate availability measurements even in the presence of selfish peers. Furthermore, our results are verified by experiments in Planetlab, which also illustrate the deployability of Pacemaker in real networks.