One of the basic roles of a plugin is to monitor local or remote hosts and verify if they are working correctly. There is a choice of generic plugins to accomplish this task.
Standard networking plugins allow hosts to be monitored using ICMP ECHO (refer to http://en.wikipedia.org/wiki/Ping). This is used to determine whether a computer is responding to IP requests. It is also used to measure the time that a machine takes to respond, and how many packages are lost during the communication. These plugins also try to connect to certain TCP/UDP ports. This is used to communicate with various network-based services to make sure that they are working properly, and respond within a defined amount of time.