说明：  When starting with protocols that work at the upper layers of the network models, each set of data is wrapped inside the next lower layer protocol, similar to wrapping letters inside an envelope. The application creates the data, then the transport layer wraps that data inside its format, then the network layer wraps the data, and finally the link (Ethernet) layer encapsulates the data and transmits it. Each network layer either encapsulates the data stream with additional information, or manages data handling or come part of the connection