Due to frequent asymmetries of forward and backward paths, in many cases the measurement of one-way metrics is required.
Active measurements rely on sending of test traffic through the network.
The approach has the disadvantage of generating additional load on network links and routers, which can significantly affect the measurement results. Active measurements have additional disadvantages, as test traffic may bother intermediate providers especially if test traffic is not recognizable as such, and as it may lead to additional costs in case of usage-based charging. In contrast, passive measurements are based on already existing traffic in the network. While passive methods may lead to additional traffic as a result of the measurement, these methods avoid the negative impact of test traffic. At the same time, passive measurements lack the advantages of test traffic generated by active methods, as the test traffic leads to controllable experiments, where pre-defined traffic patterns suitable for the measurement goals are sent over the path of interest.