This article briefly describes the necessary network knowledge, which is a prerequisite for understanding other network knowledge. After mastering the OSI seven-layer model, you have a clear understanding of network knowledge at a macro level. Let's cut to the topic:
Suppose a user of system A wants to transmit data to a user of system B.
A system user's data is sent to the application layer first.
After this layer attaches control information AH (header) to it, it is sent to the presentation layer.
The presentation layer performs the necessary transformation on the data and sends it to the session layer after adding the header PH.
The session layer also adds a header SH to the transport layer. The transport layer segments the long message and adds the header TH to the network layer.
The network layer converts the information into message packets, and adds the group number NH to the data link layer.
The data link layer converts the information with headers and trailers (DH and DT) into frames, which are sent to the opposite party (system B) bit by bit via the physical layer.
After the B system receives the information, it strips off the control information layer by layer according to the opposite action to the A system, and finally transmits the original data to the users of the B system.
It can be seen that only the physical layer in the two systems is real communication, while the other layers are virtual communication.