单选题 In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.
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 (66) between sending the notification and notification arriving at the source, thousands of additional (67) may arrive. In ATM network, a major tool for preventing (68) is (69) control. When a host wants a new virtual (70) , it must describe the traffic to be offered and the service expected.