Low latency networks are the ideal media; in typical intranet all the hosts can be reached in a few milliseconds. Monitoring tools, starting from typical default ping utility (the one that sends ICMP Echo packets and waits for response), are consistent in their results if typical response is above 10-15 milliseconds. When response time drops significantly and is circa 1ms or even less, the results may begin to vary considerably.

Imagine you need a monitor to react to a derivative of several performance values. For example, you could need to only trigger alert if CPU load and free memory have both crossed certain thresholds. If those monitors are related to the same host, you can always use generic monitor type, such as Script or Program, Python script etc. and do whatever math is required. What should you do if the performance values can only be taken from different hosts? There are several solutions.