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OSI reference model
OSI is the abbreviation of English Open System Interconnect, Chinese translation open system interconnection, an abstract seven-layer model, but its core idea is not abstract, very specific, its core idea:
1) The host at both ends of the communication can understand each other's language
This requires the host to send data using the same protocol format.
2) I want the communication system to be modular, and each module provides a standard interface.
Each module provides a standardized process interface for directly coupled modules. There is no hard and fast rule for each module, but the external interface must be standard so that the coupled modules can be seamlessly connected.
The benefits of doing this are:
(1) Each communication module can be independently developed due to the use of standard interfaces, increasing freedom and increasing production efficiency
(2) Increasing the repetitive utilization of code. Due to the standardized external interface of the communication module, the application can directly use the off-the-shelf communication module without re-encoding, which greatly reduces the burden on the developer and indirectly provides production efficiency.
The above benefits are for beginners, such as in the clouds. Human learning has a characteristic. When encountering an abstract problem that is difficult to understand, we always hope to find something comparable in real life. Let's talk about the standardization of the automobile industry.
The earliest American car manufacturers, such as Ford and GM, each produced in their own small workshops, the output was very low, the tire size was not constrained, and how big the tires were.
Ford improved the car assembly line and went through a number of processes.
1)Punch press frame housing 2) Car body reverse welding 3) Exfoliation 4) Assembly of girders, shockproof, engine 5) Interior decoration 6) Installation of a radiator and hydraulic system 7) Installation of tires
A small car was born. With the growing scale of the industry, the industrial division of labor is becoming more and more refined. With auto parts suppliers, such as tires, if the automakers have different tire size specifications, it will undoubtedly increase. The cost of the supplier, and the standardized tire size, the supplier only needs to produce several standard sizes, no matter which car manufacturer can use, the economic cost of scale will naturally decline, so standardization is a win-win situation, the above is the car Standardization of industry.
Computer networks follow the same standardized evolution path. As we all know, the earliest computer network was built by Americans. The name is ARPA. ARPA is the abbreviation of the English Advanced Research Projects Agency. Chinese is the US Department of Defense Advanced Research Projects Agency, ARPA. The network uses TCP/IP as the computer network interconnection protocol, and these protocols are implemented in the BSD-based operating system. The popularity of TCP/IP has attracted the attention of the industry, so TCP/IP is the framework model, the International Organization for Standardization (ISO) and The International Telegraph and Telephone Consultative Committee (CCITT) jointly proposed and developed this seven-layer OSI reference model:
1) Application layer 2) Presentation layer 3) Session layer 4) Transport layer 5) Network layer 6) Link layer 7) Physical layer
After the user data passes through the pipeline of the above seven processes, a binary stream consisting of 0 and 1 is born. According to whether the physical layer is fiber, cable or air, the binary stream is converted into an optical signal, electrical signal, and electromagnetic wave signal in a physical medium (physical layer). In the transmission, after several exchange switches (link layer) exchange, through a number of relay routers (network layer) forwarding, and finally reach the end of the data.
The above seven processes eventually produce a binary stream. In addition to the physical layer, each process adds a string of its own protocol headers before the original data. Each layer of the active host of each protocol is generated. It is natural to understand these protocols. The opposite end of the head, for example:
Link layer protocol header. This protocol header can only be understood by the source host and the terminating router. It only survives the relay router that the source host is connected to. In other words, its lifetime is limited to one link, as long as it is terminated. The router can receive it smoothly, and its mission is completed, then the router will strip it from the binary stream, and the rest of the network is the protocol header of the network layer.
The network layer protocol header, which contains the destination network address, is used to indicate the router along the way, where is the destination of this string of binary streams, the router queries its own network address table, and decides to send it to the next destination closer to the destination. Jump the router. If an outbound interface is found to be directly connected to the next hop router, then it is necessary to consider how to add the protocol header of the link layer. According to different interface types, a link layer protocol header suitable for the link is added, and then the physical layer media is transmitted by optical, electrical, and electromagnetic waves.
According to the similar steps above, a string of binary streams changes the link-specific protocol header every time a router passes along the path, but the network layer protocol header does not change, and finally, the string of binary streams reaches the endpoint.
At this point, the network layer protocol header has also completed its mission, stripping off the network layer protocol header, before stripping off, first read the transport layer protocol code of the network layer protocol header, and send the serial binary stream to the transport layer, leaving the head array the protocol header of the transport layer.
The transport layer protocol, according to the session layer protocol code in the transport layer protocol header, first strips off the transport layer protocol header, and sends the serial binary stream to the session layer, and the remaining header array is the session layer protocol header.
The session layer protocol, according to the layer protocol code in the session layer header, strips off the session layer protocol header and sends the string binary stream to the presentation layer. The remaining header array is the presentation layer protocol header.
The presentation layer protocol, according to the application layer protocol code in the presentation layer header, first strips off the presentation layer protocol header, and sends the serial binary stream to the application layer, and the remaining header array is the application layer protocol header.
The application layer protocol, after the relay of the one-hop one-hop router and the layer-by-layer protocol header of the destination host, the data sent by the source host to the destination host finally reaches the endpoint, and the data is text, picture, audio, video, it is finally explained by the application layer protocol.
