What are the seven levels of the OSI reference model?

The seven layers of OSI are, from low to high, physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer.

1. Application layer: the interface between network services and end users.

2. Presentation layer: data representation, security and compression. (it has been merged into the application layer in the five-layer model), and the formats are JPEG, ASCll, EBCDIC, encryption format, etc.

3. Session layer: establish, manage and terminate sessions. (it has been merged into the application layer in the five-tier model), which corresponds to the host process and refers to the ongoing session between the local host and the remote host.

4. Transport layer: defines the protocol port number for transmitting data, as well as flow control and error checking.

These protocols are TCP and UDP. Once the packet leaves the network card, it will enter the network transport layer.

5. Network layer: addressing logical addresses to realize path selection between different networks.

These protocols are: ICMP, IGMP, IP(IPV4, IPV6).

6. Data link layer: Establish logical connection, address hardware, and check for errors. Combining bits into bytes and then into frames, accessing the medium with MAC address, finding errors but unable to correct them.

7. Physical layer: establish, maintain and disconnect physical connections.

TCP/IP layered model structure, the protocols between application layers realize communication interconnection between application layers by calling transport layer, network layer and physical data link layer layer by layer.