One of the things you'll want to monitor in your network is latency. Latency is a measure of how long it takes for traffic to get from one device to another device. Latency is a combination of propagation delay (how long it takes the bits to flow from device to device over an interface) and nodal processing delay (how long it takes bits to flow from interface to interface through a device). Usually, you don't focus on one component of latency over the other. Examine the network topology shown in Figure 8-3.
In this simple network, traffic flows from router1 through the network to router5. That path will take some amount of time on a normal day. If you know what that average time (or network latency) is, you can basically monitor your network and evaluate the current performance against the expected performance. So when you see a spike in network latency, you know that something has run amok in your network.
The Junos OS supports a tool called real-time performance monitoring (RPM). RPM essentially is a set of tests, run periodically, that help you measure the latency between two devices on a network.
RPM works only if the two devices you're measuring both run the Junos OS.