What is OSI
OSI (Open System Interconnect), open system interconnection;
The purpose of OSI is to allow different networks to use the same architecture to connect to each other . Before the advent of OSI, the computer networks of different companies had different structures.
OSI model
Each layer of OSI will encapsulate the data once, and each layer of the corresponding data receiving end will decapsulate the received data, and finally get the original data. In the application, presentation, and session, these three layers call the transmitted content as Data, in the transport layer as "Segment", in the network layer as "Packet" (data packet), in the data link layer as "Frame" (frame), "bit" at the physical layer. OSI is peer-to-peer communication, for example, the sender transport layer can only communicate with the receiver's transport layer. In the process of encapsulating the original data, each layer adds the Head (head) information of the corresponding level to the original data. The processed data consists of Head + Data, and then this data is handed over to the next layer for processing. The process of decapsulation is reversed, and each layer will process the data according to the Head information corresponding to its own layer in the data, and finally restore the original data. The data processing process of each layer is mainly processed according to the protocol, so it is important to learn the corresponding protocol of each layer accordingly.
The role and protocol of each layer of OSI
TCP/IP model
Derived from OSI, the application layer, presentation layer and session layer in OSI are combined into the application layer. The principle has not changed significantly.