In low-speed network, it is usually adequate to wait for congestion to occur and then react to it by telling the source of packets to slow down. In high-speed networks, this approach often works poorly, because in the (71) between sending the notification and notification arriving at the source thousands of additional (72) may arrive．InATMnetwork，amajortoolfor preventing (73) is (74) control. When ahostwants a new virtual (75) , it must describe the traffic to be offered and the service expected.

A．packets

B．cells

C．message

D．files