1. What is the OSI seven-layer model
The OSI seven-layer model is also called " OSI reference model " . Its full name is " Open System Interconnection Reference Model " . agreement . _ So why the OSI reference model?
reason
At that time, a network architecture represented by IBM 's SNA appeared in the network industry . The focus of these architectures was the internal network connection of each company. There was no unified standard , so it was difficult to interconnect them.
In this case, ISO proposed the OSI reference model, whose biggest feature is openness . Network products from different manufacturers can realize interconnection, interoperability and portability as long as they follow this reference model . Simply put, any system that follows the OSI standard can communicate with each other as long as they are physically connected.
2. Explanation of the functions of each layer
First, let's look at the order of the seven-layer model, from bottom to top : physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer.
Next, I will explain to you the functions of each layer separately:
1. Physical layer (first layer)
Although it is at the bottom, it is the foundation of the entire open system . The physical layer provides transmission media and interconnection devices for data communication between devices, and provides a reliable environment for data transmission.
The main function of the physical layer is to use the transmission medium to provide a physical connection for the data link layer and realize the transparent transmission of the bit stream.
2. Data link layer (second layer)
The data link layer is the second layer of the OSI model and is responsible for establishing and managing links between nodes . Due to the existence of various interferences in the computer network, the physical link is very unreliable.
The main function of the data link layer is: on the basis of the bit stream provided by the physical layer, through error control and flow control methods , the physical line with errors becomes an error-free data link , that is, to provide reliable communication through the physical medium. The method of transferring data.
3. Network layer (third layer)
The network layer is the third layer of the OSI model. In the network layer, a logical address is used to identify a point, the upper layer data is encapsulated into a data packet , and the source address and the logical address of the destination end are encapsulated in the header of the packet . The network layer selects the best path according to the logical address of the data packet header, and sends the data to the destination. Common protocols are: IP, ARP, RARP, etc.
The main function of the network layer is to realize the transmission of data from the source to the destination.
4. Transport layer (fourth layer)
The transport layer is the fourth layer of the OSI model. This layer provides the functions of establishing, maintaining and tearing down transport connections , and acts as a link between the preceding and the following . The two most common protocols at this layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) .
The main function of the transport layer is to implement data communication between user processes on different hosts in the network . Provide users with reliable error and flow control to ensure the correct transmission of messages, and at the same time shield the details of lower-layer data communication from the upper layer, that is, transparently provide messages to users.
5. Session layer (fifth layer)
The session layer is the fifth layer of the OSI model and is the interface between user applications and the network.
The main functions of this layer are: organizing and coordinating the communication between two session processes, and managing the data exchange . When establishing a session, users must provide the remote address they want to connect to . And these addresses are different from MAC addresses, they are specially designed for users, and are easier for users to remember than MAC addresses.
The specific functions of the session layer are as follows:
①Session management: Allow users to establish, maintain and terminate sessions between two physical devices, and support data exchange between them.
② Session flow control: Provide session flow control and cross-session functions.
③ Addressing: Use the remote address to establish a session connection.
④Error control: Logically speaking, the session layer is mainly responsible for the establishment, maintenance and termination of data exchange, but the actual work is to receive data from the transport layer and be responsible for correcting errors.
6. Presentation layer (sixth layer)
The presentation layer is the sixth layer of the OSI model, which interprets commands and data from the application layer and gives corresponding meanings to various grammars.
The main function of the presentation layer is to deal with the presentation of user information, such as encoding, data format conversion, and encryption and decryption.
The specific functions of the presentation layer are as follows:
①Data format processing: Negotiate and establish the format of data exchange, and resolve the differences in data format representation among various applications.
② Data encoding: handle character set and digital conversion.
③ Compression and decompression: In order to reduce the amount of data transmission, this layer is also responsible for data compression and recovery.
④Encryption and decryption of data: It can improve the security of the network.
7. Application layer (seventh layer)
The application layer is the highest layer of the OSI reference model. It is the interface between computer users, various applications and the network . The network service protocols of the common application layer are: HTTP, HTTPS, FTP, POP3, SMTP, etc.
The main function of the application layer is to directly provide services to users and complete various tasks that users want to complete on the network.
Based on the work of the other 6 layers, it is responsible for completing the connection between the application program and the network operating system in the network , establishing and ending the connection between users, and completing the various network services and applications required by network users. Oversight, management and service agreements . In addition, this layer is also responsible for coordinating the work among various applications.
The specific functions of the application layer are as follows:
①User interface: The application layer is the direct interface between the user and the network, as well as the application program and the network, enabling users to interact with the network.
② Realize various services: various applications in this layer can complete and realize various services requested by users.