"Internet" is believed to be familiar to everyone, because we are exposed to the Internet almost every day, and it has been fully integrated into our daily lives. But do you know? The various applications of the Internet are actually hierarchical, that is, the seven-layer OSI model among many networked populations. The following will teach you a deep understanding of the iOS seven-layer model.
OSI layered thinking:
因为计算机网络中存在着众多的体系结构，例如IBM公司的SNA(系统网络体系结构，7层)和DEC公司的DNA(数字网络体系结构，3层)等。由于体系结构的差异化，使得网络产品出现了严重的兼容性问题，影响了网络的快速发展。为了解决这个问题，ISO于1984年正式颁布了OSI RM。这个模型把网络通信的工作分为7层。1至4层被认为是低层，这些层与数据移动密切相关。5至7层是高层，包含应用程序级的数据。每一层负责一项具体的工作，然后把数据传送到下一层。由低到高具体分为：物理层、数据链路层、网络层、传输层、会话层、表示层和应用层。
Deep understanding of OSI seven-layer reference model:
Physical layer (introduced from the bottom): The physical layer includes the physical networking media, which is actually wiring, optical fibers, network cards, and other things used to connect two network communication devices together. It specifies the mechanical, electrical, functional, and process characteristics between the activation, maintenance, and shutdown of communication endpoints. Although the physical layer does not provide error correction services, it can set the data transmission rate and monitor the data error rate.
The second layer of the key OSI model of OSI two to four layers of data movement : the data link layer The main function of the data link layer is to control the communication between the network layer and the physical layer. It guarantees the reliable transmission of data on unreliable physical lines. It divides the data received from the network layer into specific frames that can be transmitted by the physical layer, ensuring the reliability of transmission. Its main functions include: physical addressing, data framing, flow control, data error detection, and retransmission. It is independent of the network layer and the physical layer, and does not need to care whether the computer is running software or other operations when working.
The third layer of the OSI model: Many users at the network layer often confuse the related problems of layer 2 and layer 3. In short, if you are talking about a problem related to IP address, routing protocol or address resolution protocol (ARP), then this is The third problem. The network layer is responsible for routing data packets between subnets. It determines the best path from two nodes in a network by comprehensively considering the transmission priority, network congestion level, service quality, and the cost of optional routing. In addition, it can also achieve congestion control, Internet interconnection and other functions.
The fourth layer of the OSI model: the transport layer The transport layer is the most important layer in the OSI model. It is the first end-to-end layer that serves as a buffer when two computers communicate through the network. When the service quality of the network layer cannot meet the requirements, it will improve the service to meet the requirements of the upper layers; and when the service quality of the network layer is better, it only needs to do very little work. In addition, it has to deal with end-to-end error control and flow control issues, and ultimately provide reliable and error-free data transmission for the session.
The fifth layer of the OSI model: the session layer The session layer is responsible for establishing and maintaining communication between two nodes in the network and keeping the session synchronized. It also determines whether the communication is interrupted and where to resend when the communication is interrupted.
The sixth layer of the OSI model: the role of the presentation layer is to manage the decryption and encryption of data, such as common system password processing, when your account data is encrypted before sending, at the other end of the network, the presentation layer will receive Data decryption. In addition, the presentation layer needs to decode and encode the picture and file format information.
The seventh layer of the OSI model: the application layer. Simply put, the application layer is an interface for the operating system or network applications to access network services, including information processing such as file transfer, file management, and e-mail.
The typical equipment corresponding to each layer is as follows:
Physical layer .................. Network: repeater, hub, network cable, HUB
Data link layer ...... Network: network card, bridge, switch
Network layer ………………… Network: router, firewall, multi-layer switch
Transport layer .................. Computers: processes and ports
Session layer ………… .. Computer: Establishing a session, SESSION authentication, resumable transfer
Presentation layer ………… .. Computer: encoding method, image encoding and decoding, URL field transmission encoding
Application layer ………… .. Computer: applications such as FTP, SMTP, HTTP
Advantages of OSI layering
1) People can easily discuss and learn the specification details of the protocol.
(2) The standard interface between layers facilitates the modularization of the project.
(3) Create a better interconnection environment.
(4) Reduce the complexity, make the program easier to modify, and the speed of product development is faster.
(5) Each layer utilizes the services of the next lower layer to make it easier to remember the functions of each layer.
to sum up:
OSI七层模型有效的解决了不同网络体系互连时所遇到的兼容性问题，它的出现减轻了网络的复杂程度，一旦网络发生故障，可迅速定位故障所处层次，便于查找和纠错；通过在各层上定义标准接口，它使同属一层的不同网络设备间能实现互操作；它还保证了各层之间的相对独立；而高层协议可以放在多种低层协议上运行，提高了网络的效率；因为每次更新都只需在一个层次进行，不受整体网络的制约，所以它的出现有效刺激了网络技术革新，它是网络技术发展的源动力。
