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Active measurement tools
|“||are based on the concept of sending probe packets into the network and measuring their behavior as they flow through it. The probe packets are typically emitted from a general-purpose end-host such as a personal computer. Probe packets are sent toward a destination host by providing a target IP address (or domain name) to the measurement tool. The injection of probe packets into the network provides an indication of the routing behavior, propagation delay, queuing delay, and loss that would be experienced by normal data packets. When (and if) the probes arrive at a destination, either their arrival is logged or response packets are returned to the sender. When a response packet is returned, its arrival back at the original sender is logged, constituting the conclusion of one measurement.||”|
"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 the network at sufficiently low rates so as not to significantly perturb the network — avoiding, for example, significant additions to congestion. However, the resultant measurement data are limited in their ability to capture events at time scales finer than the sampling rate and are constrained by the necessarily small number of source and receiver locations.
"A second drawback is that any one system used to conduct active measurements is limited by routing protocols and Internet topology to measuring only a portion of the Internet. Finally, active measurement tools are limited in their ability to assess aspects of volume (for example, the total amount of traffic flowing along a given path). Some of these limitations of active probes can be addressed by passive measurement tools."
- ↑ The Internet Under Crisis Conditions: Learning from September 11, at 63.
- ↑ Id. at 63-64.