A set of rules that controls network traffic—on a highway, the rules might specify that vehicles drive on the right and pass on the left, and every car and truck must display a license plate to identify it; in a tele- phone network, the rules define the form and use of unique numbers (called “telephone numbers”) to identify each node and establish con- nections between them. To assure that a network operates properly, every node and every channel must follow the rules for that particular network.