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This article mainly explains "what is the linux OSI seven-tier model and the TCP-IP four-tier model". Interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Now let the editor take you to learn "what is the linux OSI seven-tier model and the TCP-IP four-tier model?"

1. ISO seven-layer network model

In order to enable computers from different computer manufacturers to communicate with each other in order to establish computer networks in a larger range, the International Standardization Organization (ISO) formally launched a network system structure-OSI seven-layer network model in 1981. Since then, the major computer manufacturers quickly approached it, which greatly promoted the development of computer networks. OSI seven-layer network models are: application layer, presentation layer, session layer, transport layer, network layer, data link layer, physical layer.

1.1 Application layer

The application layer is closest to the user's OSI layer, which provides network services for users' applications (such as e-mail, file transfer, network requests, etc.). It is mainly responsible for providing interfaces to the software so that the program can use network services. In terms of implementation, it uses specific port numbers and specific data formats to encapsulate tcp or udp, thus implementing the application layer protocol.

1.2 presentation layer

The presentation layer is the translator between the application and the network, which ensures that the information sent by the application layer of one service can be read by the application layer of another service. At the presentation layer, the data is formatted according to a scheme that the network can understand, and this format varies depending on the type of network used. Some typical applications are as follows:

Decryption and encryption of data, such as the processing of system passwords

Decode and encode picture and file format information

1.3 session layer

The session layer is responsible for establishing, maintaining and terminating communication between two nodes in the network. The functions of the session layer include establishing a communication connection, keeping the communication connection unblocked during the session, synchronizing the dialogue between the two nodes, determining whether the communication is interrupted and where to start to resend when the communication is interrupted.

1.4 Transport layer

The transport layer is the most important layer in the OSI seven-layer model. The transport protocol controls the flow at the same time or specifies the appropriate transmission rate based on the speed of the data that can be received by the receiver. In addition, the transport layer forcibly divides longer packets according to the maximum size that the network can handle. For example, as we all know, the MTU of Ethernet is 1500, that is, the maximum size of packets that can be sent each time. At this time, the transport layer of the sender node divides the data into smaller data slices, and assigns a sequence number to each data slice, so that when the data reaches the transport layer of the receiver node, it can be reorganized in the correct order. The most typical application of the transport layer is tcp protocol.

1.5 Network layer

The main function of the network layer is to translate the network address into the corresponding physical address and decide how to route the data from the sender to the receiver. The network layer determines the best path from node An in one network to node B in another network by comprehensively considering transmission priority, network congestion, quality of service and the cost of optional routes. Because the network layer processes and intelligently guides data transmission, the router connects each segment of the network, so the router belongs to the network layer. In a network, "routing" guides the transmission of data based on addressing schemes, usage patterns, and accessibility. The network layer is responsible for establishing the routes they use between the source and destination machines, and there is no error detection and correction mechanism in this layer itself. therefore, the network layer must rely on reliable transmission services between end-to-end.

1.6 data Link layer

The data link layer controls the communication between the network layer and the physical layer. Its main function is how to transmit data reliably on unreliable physical lines. In order to ensure transmission, the data received from the network layer is divided into specific frames that can be transmitted by the physical layer. A frame is a structural packet used to move data, which includes not only the original data, but also the physical addresses of the sender and receiver, as well as error detection and control information. The address determines where the frame will be sent, while error correction and control information ensures that the frame arrives error-free. If the receiving point detects an error in the transmitted data when transmitting the data, it will notify the sender to resend the frame.

1.7 physical layer

The physical layer is the lowest layer, which includes physical networking media, such as cable connectors. The protocol of the physical layer generates and detects the voltage in order to send and receive signals carrying data. Insert a network interface card into your desktop PC and you will build the foundation for computer networking. In other words, you provide a physical layer, and to put it bluntly, the physical layer is all the physical, visible tools used in networking.

2. TCP/IP four-layer model

TCP/IP is synonymous with a set of protocols, it also includes many protocols, forming the TCP/IP protocol suite. The TCP/IP protocol suite is divided into four layers. IP is located in the second layer of the protocol suite (corresponding to the third layer of OSI), and TCP is located in the third layer of the protocol suite (corresponding to the fourth layer of OSI).

2.1 four-tier model

TCP/IP communication protocol uses a four-layer hierarchical structure, each layer calls the network provided by its next layer to complete its own needs. The four layers are:

Application layer: the layer of communication between applications, such as simple email transfer (SMTP), File transfer Protocol (FTP), Network remote access Protocol (Telnet), etc. Responsible for network access to the application, where different processes are identified by port numbers.

Transport layer: in this layer, it provides data transfer services between nodes, such as Transmission Control Protocol (TCP), user Datagram Protocol (UDP), etc. TCP and UDP add transmission data to the packet and transmit it to the next layer. This layer is responsible for transmitting the data and determining that the data has been delivered and received. Responsible for end-to-end communication session connection and establishment. The choice of transmission protocol depends on the mode of data transmission.

Network layer: responsible for providing basic packet delivery functions so that each packet can reach the destination host (without checking whether it is correctly received), such as Internet Protocol (IP). Responsible for encapsulating data frames into IP datagrams and running necessary routing algorithms.

Data link layer: the management of the actual network media, defines how to use the actual network (such as Ethernet, Serial Line, etc.) to transmit data, and realizes the network driver of the network card interface. Responsible for converting the binary stream into data frames, and sending and receiving data frames.

2.2 what are the protocols of the four-layer model?

Application layer {http Hypertext transfer Protocol ftp File transfer Protocol telnet Telnet ssh secure Shell Protocol stmp simple Mail send pop3 receive Mail}

Transport layer {tcp Transmission Control Protocol, udp user packet Protocol}

Network layer {ip Internet Protocol icmp Network Control message Protocol igmp Network Group Management Protocol

Data link layer {arp address translation protocol, rarp reverse address translation protocol, mpls multiprotocol label switching}

Some of the protocol notes are as follows:

ARP: (address Translation Protocol) is used to obtain hardware host addresses on the same physical network.

MPLS: (multi-protocol label switching) is a promising next generation network protocol.

IP: (Internet Protocol) is responsible for addressing and routing packets between hosts and the network.

ICMP: (network Control message Protocol) A protocol used to send and report delivery errors about packets.

IGMP: (network Group Management Protocol) a protocol used by IP hosts to report host group members to the local multicast router.

TCP: (transmission Control Protocol) provides reliable communication connections for applications. It is suitable for transmitting a large amount of data at one time. And applicable to the request to get the corresponding application.

UDP: (user packet Protocol) provides connectionless communication and does not guarantee reliable delivery packets. It is suitable for transmitting a small amount of data at a time.

2.3 the most common application layer protocols

Usually we use the ip address followed by the upper port number to uniquely determine a connection: ip is used to locate the host, and port distinguishes the application (process).

For example: http-- > 80 ssh-- > 22 telnet-- > 23 ftp-- > 21

The port number used by the system is generally 1-1024, and that available to users is 1025-65536.

Note: you can use cat / etc/services to see which application layer protocols are available and which transport layer protocols they use.

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