Network Seven layer protocol description

Since the 70 's, some of the major foreign computer manufacturers have launched their own network architecture, but they belong to the special. To enable computers from different computer manufacturers to communicate with each other in order to build a computer network in a larger context, it is necessary to establish an international network architecture standard.

ISO in 1981 formally recommended a network system structure----Seven-tier reference model, called the Open Systems Interconnection model (the opening system Interconnection,osi). Because of the establishment of the standard model, all kinds of computer networks move closer to it, which greatly promotes the development of network communication.

The OSI model has 7 layers of structure, and each layer can have several child layers. Below I briefly introduce these 7 layers and their functions.

The 7 layers of OSI are from top to bottom, respectively:

7 Application Layer

6 Presentation Layer

5 Session Layer

4 Transmission Layer

3 Network Layer

2 Data Link Layer

1 Physical Layer

At the top level, the 7, 6, 5, 4 layers define the application's functionality,

The following 3 layers, both 3, 2, and 1 are mainly oriented toward end-to-end data flow over the network.

Here is a description of the 7-tier features:

(1) Application layer: An application that communicates with other computers, which is the communication service of the corresponding application. For example, a word processing program without communication can not execute the code of communication, and programmers engaged in word processing do not care about the 7th layer of OSI. However, if you add an option to transfer files, the word Processor programmer needs to implement the 7th layer of OSI. Example: Telnet,http,ftp,www,nfs,smtp and so on.

(2) Presentation layer: The main function of this layer is to define data format and encryption. For example, FTP allows you to choose to transmit in binary or asii format. If you select Binary, the sender and receiver do not change the contents of the file. If you select the Asii format, the sender converts the text from the sender's character set to the standard Asii and sends the data. Converts the standard asii to the character set of the receiving computer at the receiving party. Example: Encryption, Asii, and so on.

(3) Session Layer: He defines how to start, control, and end a session. Includes control and management of multiple two-way hours so that the application can be notified when only part of a continuous message is completed, so that the data that the presentation layer sees is contiguous, and in some cases the presentation layer is represented by the data if all the data is received. Example: Rpc,sql and so on.

(4) Transport layer: the function of this layer includes whether to select the error recovery protocol or error-Free recovery protocol, and to reuse the input of different application data streams on the same host, and also to reorder the packets that are not received in order. Example: Tcp,udp,spx.

(5) Network layer: This layer defines the End-to-end packet transport, defines the logical address that identifies all nodes, and defines the way in which the route is implemented and how it is learned. To accommodate a transmission medium with a maximum transmission unit length less than the packet length, the network layer also defines a segmented method for breaking a packet into smaller packets. Example: Ip,ipx and so on.

(6) Data Link layer: He defines how data is transmitted on a single link. These protocols are related to the song-type media discussed. Example: Atm,fddi and so on.

(7) Physical layer: The physical layer specification of the OSI is a standard for the characteristics of the transmission medium, which is usually referenced by the standards established by other organizations. The connection head, the needle, the needle use, the current, the current, the coding and the light modulation and so on all belong to each kind of physical layer standard content. The physical layer uses multiple specifications to complete the definition of all the details. Example: rj45,802.3 and so on.

Advantages of OSI tiering:

(1) People can easily discuss and study the specification details of the agreement.

(2) The standard interface between layers facilitates the modularization of the project.

(3) Create a better interconnect environment.

(4) Reduce the complexity, make the program easier to modify, product development faster.

(5) Each layer uses the adjacent lower level service, it is easier to remember the function of the layer.

Most computer networks use a hierarchical structure, a computer network is divided into several levels, at a high-level system is only the use of lower levels of the system provided by the interface and functions, do not need to understand the low-level implementation of the function of the algorithm and protocol; the lower level is only using parameters transmitted from the high level system, This is the independence of the hierarchy. Because of this irrelevance, each module in the hierarchy can be replaced with a new module, as long as the new modules have the same functionality and interface as the old ones, even if they use different algorithms and protocols.

In order to transmit the information and data correctly between the computer and the terminal in the network, there must be a convention or rule in the sequence of data transmission, the format and content, etc.

There are three main components of a network protocol:

1, Semantics:

is to explain the meaning of the protocol elements, the semantics of different types of protocol elements are different. For example, what kind of control information needs to be issued, what kind of action to perform, and the response received.

2. Grammar:

A number of protocol elements and data are combined to express a complete content of the format to be followed, that is, the data structure of information to make a provision. such as the structure and format of user data and control information.

3. Time series:

A detailed description of the order in which the events are implemented. For example, in the communication between the two sides, the sending point sends out a data message, if the target point is received correctly, the answer source point received correctly; If you receive the wrong information, ask the source point to resend.

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