选题背景和研究意义 弹性重叠网 Overlay Network ost-1 ha it-4 t-5 host-1 D host IP Layer Network host-3 host-48 弹性重叠网 选题背景和研究意义 host-2 a b a b c c a b d a b host-1 host-5 A B C D host-1 host-3 host-4 host-5 IP Layer Network Overlay Network host-2 host-3 host-4