In all the measurements we assume that there is no administrative access to the network devices affecting the performance. If such access was possible, then it would be better to simply read the statistics gathered in the devices. Many times it is necessary to have some knowledge of the topology of the different networks handling the traffic of interest. Because of this, the tools need to do topology discovery. It is not always possible to discover all links or nodes because there exists some topologies that produce some links to be unobservable to the available discovery tools.
Measurements can be active or pasive.
In active measurements the measuring station generates traffic in order to create the conditions and the events necessary for the measurements. In the passive measurements the station simply observs its own traffic or other stations traffic and infers performance characteristics and problems from the gathered data. Both methods have specific applications. In this method, the station doing the measurements generates traffic but in small volumes, so that network performance at all places is almost not affected by the measurements. Datagram loss probability and round trip delay are measured, and information about all the nodes and links traversed by the traffic generated is indirectly calculated.