在传输过程中，数据报将被发送到网关 G，而网关 G 又会将它转发到目的地主机。
Along its way, the datagram will get sent to Gateway G, which will in turn forward it to its destination host.
当数据报经过网关转发时，它就将生存时间字段中的值减一。
When a datagram is forwarded by a gateway, it decreases the value in the time to live field by one.
IP组播利用树转发结构，只需在组播树的分支处复制数据报文，而且在每条链路上仅转发一次。
IP multicast utilizes a tree delivery structure, on which data packets are duplicated only at fork nodes and are forwarded only once over each link.
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