协议栈：
Application:Handles Porn
TCP:Handles Retransmission(转发)
IP：Handles Routing(路由)
Link:Handles A Single Hop
协议栈中各层协议的关系不是递归的，而是像一条流水线一样。
The role of the IP layer is to figure out how to "route" packets to their final destination. To make this possible, every interface on the network needs an "IP address".
link-level protocol & network protocol:
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The link-level protocol describes how to get from one node to another if there're connected directory;
The "network protocol" tells us how to get from one point in the network to any other, going through other links if nessary.
