Dependable performance measurement is a common requirement for all on-line services. The ongoing tendency to outsource not only infrastructure, but also software parts to several suppliers, creates a new challenge to providers. Traditional solutions cannot easily monitor service performance as perceived by end users, since providers do not have control over all hardware/software involved. Our research focuses on evaluating how data passively collected from network devices can be used to verify end-to-end service performance. Our goal is to compare this approach with current methods used to monitor distributed services. Given the constant mutation of applications on the Internet, as well as the fast increase of the number of users, we are looking for flexible and scalable solutions.