Symptom detection using behavior probability density, network monitoring of multiple observation value types, and network monitoring using orthogonal profiling dimensions

Yet another network monitoring system includes data structures for maintaining information regarding historical activity of a network and emergent activity of a network. Those data structures include observable values for multiple profile dimensions, including source/destination address, application, location, and time. The data structures also include observable values for combinations of more than one of those multiple profile dimensions, including, e.g., (source address)×(application), and the like. It is expected that only a relatively sparse set of combinations of more than one of those multiple profile dimensions would have meaningful information associated therewith. The network monitoring system maintains those data structures only for those combinations of more than one of those multiple profile dimensions for which maintaining that information would be substantially meaningful.