单项选择题

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 (1)between sending the notification and notification arriving at the source, thousands of additional（2）may arrive. In ATM network, a major tool for preventing （3）is （4）control. When a host wants a new virtual（5）it must describe the traffic to be offered and the service expected.

（5）是（）

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