"Active probes are important because one can gain crucial insight into network conditions for a specific end-to-end path at a specific time, which may not be possible if one monitoring occurs at only a single point. Furthermore, active measurements generally do not require special participation by intermediate nodes, making them easy to deploy and execute. While active probe tools provide important data about specific end-to-end conditions, there are a number of drawbacks to their use. First, the act of placing a probe into the network causes a perturbation (dubbed the “Heisenberg effect” by analogy to the uncertainty principle in physics) that may lead to a change in the network's operating conditions. Because of this problem, common practice is to use active measurement tools to sample t