Passive measurement tools
|“|| are based on logging different aspects of traffic observed at specific vantage points in the network. The data that can be collected by passive means may have a variety of forms, from access logs to packet traces to detailed activity counters on routers. These data can be collected either at end-hosts or at nodes within the network. Passively collected data can be displayed in real time (as is often done by network operators) or placed in a repository for offline analysis.
"Passive measurement data can provide great insight into the activities on a link or at a node. However, they have some significant drawbacks. Such data are almost always considered proprietary and are rarely made available for general analysis. Passive collection of network data can result in extremely large data sets, which greatly complicate archiving and analysis. Passive measurement tools are also prone to various types of errors that require careful attention."