Figure20.1两台主机之间的链路 S3 Data lin Data link Physica Physical Hop-to-hop Hop-to-hop Hop-to-hop delivery delivery delivery Link 1 LAN S2 f1 LAN f1 S1 WAN f3 S3 f2 f2 LAN LAN Link 2 ●-ink3.-。 D 20.3 20.3 Figure 20.1 两台主机之间的链路