OSI seven layer model and blockchain

1. The so-called network layering is to send or forward the data, pack or unpack the data, and load or unpack the control information, which are completed by different hardware and software moles. In this way, the complex problem of communication and network interconnection can be simplified. OSI (open system interconnection) divides the network into seven layers: physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer. Physical layer: the physical layer is the lowest layer of the reference model. This layer is the data transmission medium of network communication, which is composed of cables and devices connecting different nodes. The main function is: using the transmission medium to provide physical connection for the data link layer, responsible for processing data transmission and monitoring data error rate, so as to facilitate the transparent transmission of data flow. Data link layer: the data link layer is the second layer of the reference model. The main functions are: on the basis of the services provided by the physical layer, the data link connection is established between the communication entities, the data packets in the unit of "frame" are transmitted, and the error control and flow control methods are adopted to make the error physical lines become error free data links. Network layer: network layer is the third layer of the reference model. The main functions are: to create logical links for data transmission between nodes, to select the most appropriate path for packets through the communication subnet through routing algorithm, and to achieve congestion control, network interconnection and other functions. Transport layer: transport layer is the fourth layer of the reference model. The main function is to provide users with reliable end-to-end services, deal with packet errors, packet order, and other key transmission problems. The transport layer shields the details of the lower layer data communication to the upper layer, so it is the key layer in the computer communication architecture. Session layer: session layer is the fifth layer of the reference model. The main functions are: responsible for the transmission link between the two nodes, so as to ensure that the point-to-point transmission is not interrupted, and manage the data exchange and other functions. Presentation layer: presentation layer is the sixth layer of the reference model. The main function is: to deal with the expression of information exchanged in two communication systems, mainly including data format transformation, data encryption and decryption, data compression and recovery and other functions. Application layer: the application layer is the highest level of the reference model. The main function is to provide many services for application software, such as file server, database service, e-mail and other network software services.

2. From top to bottom: application layer, presentation layer, session layer, transport layer, network layer, data link layer, physical layer

physical layer: the physical layer transmits bit information through links. It mainly deals with the following problems: the choice of the size and shape of the connector, the function of each pin, the electrical signal transformation of data bits and the synchronization of bit level. Usually, a network can have several different physical layer types, even a node may have many different physical layer types, because different technologies require their own physical layer
2) data link layer: the data link layer (sometimes called the link layer) transmits block information through physical links. It is mainly responsible for the following tasks: data error checking, coordinating the use of shared media (such as in a LAN) and addressing (when multiple systems can access, such as in a LAN). In addition, different links usually have different data link layer implementations; Moreover, the same node can support several different data link layer protocols, and each type of link connected by the node has its own protocol
3) network layer: the network layer enables any pair of systems in the network to communicate with each other. A fully interconnected network means that each node is directly connected to other nodes, but this topology can not be used in the case of many nodes. A typical case is that the network layer must find a path through a series of connected nodes, and each node on the path must forward packets in the appropriate direction. The main tasks of network layer processing are: routing calculation, packet segmentation and reorganization (when different links in the network have different maximum packet size restrictions) and congestion control
transport layer: transport layer establishes a reliable communication link between two systems. It mainly deals with some errors caused by the network layer, such as packet loss and plicate packet errors, as well as packet reordering, segmentation (so that transport layer users can handle large packets) and reloading (so as to avoid inefficient segmentation and reloading in the network layer). In addition, it also helps the transport layer to rece the data transmission rate when the network congestion occurs
5) session layer: i.s.o. thinks that the session layer does not play a significant role in the Internet system. The services provided by the i s o session layer go beyond the simple full plex reliable communication flows provided by the transport layer, such as session control (realizing the special communication mode between systems) and link (bundling a group of packets so that they are either sent or not sent). No matter what this layer is, it has nothing to do with the underlying network devices such as bridges and routers
6) presentation layer: this layer is designed for consistent data representation, so that people can define their own data structure without worrying about bit / byte order or floating-point number representation. ISO has established the standard in ASN. 1 (Abstract Syntax Notation 1). Although I don't like ASN. 1 very much, because it is too complex and inefficient (in terms of space and processing), it is used by many iete standards
7) application layer: bridging and routing are attractive because people need to make use of the corresponding applications of these functions. Applications include file transfer, virtual terminal and web browsing. There are usually multiple applications running at the same time on a node.

3. 1. Physical layer: physical medium; Such as the interface type of network cable, the interface type of optical fiber, the transmission rate of various transmission media, such as twisted pair Coaxial cable and other 2 data link layer: for example, the switch (the switch works in the data link layer) mainly encapsulates and de encapsulates the MAC address (the address of the network card) of the data received from the physical layer. 3 network layer: for example, the router (the router works in the network layer); It mainly encapsulates and unpacks the IP address of the data received from the lower layer. Transport layer: it is responsible for establishing, maintaining and terminating communication between two nodes in the network, and defines some protocols and port numbers for data transmission, such as TCP and UDP. 5 session layer: establish data transmission path through transport layer (port number: transmission port and receiving port) 6 presentation layer: translation between application and network Officer: if your application needs to send data to the Internet, it needs presentation layer to translate it into "language" of Internet transmission. If the received data is interpreted, encrypted and decrypted, compressed and decompressed, the application layer is responsible for providing the interface to the software so that the program can use network services. Such as FTP, web, QQ and other applications!

4. OSI is an open reference model for communication system interconnection and a well-defined protocol specification
OSI has seven layers, which are application layer, presentation layer, session layer, transport layer, network layer, data link layer and physical layer from top to bottom
among them, the upper layer, namely layer 7, 6, 5 and 4, defines the functions of the application program, and the lower three layers, namely layer 3, 2 and 1, mainly face the end-to-end data flow through the network
(1) application layer: an application that communicates with other computers. It is the communication service of the corresponding application. Examples: Telnet, HTTP, FTP, WWW, NFS, SMTP, etc
(2) presentation layer: the main function of this layer is to define data format and encrypt. Examples: encryption, ASII, etc
(3) session layer: defines how to start, control and end a session, including the control and management of multiple two-way hours, so that the application can be notified when only a part of the continuous message is completed, so that the data seen by the presentation layer is continuous. In some cases, if the presentation layer receives all the data, the data is used to represent the presentation layer. Examples: RPC, SQL, etc
(4) transport layer: the functions of this layer include whether to select error recovery protocol or error free recovery protocol, multiplexing the input of data streams of different applications on the same host, and reordering the received packets in wrong order. Examples: TCP, UDP, SPX
(5) network layer: this layer defines the end-to-end packet transmission, defines the logical address that can identify all nodes, and also defines the way of routing implementation and learning. In order to adapt to the transmission medium whose maximum transmission unit length is less than the packet length, the network layer also defines the segmentation method of how to decompose a packet into smaller packets. Examples: IP, IPX, etc
(6) data link layer: defines how to transmit data on a single link. These protocols are related to the media under discussion. Examples: ATM, FDDI, etc
(7) physical layer: OSI's physical layer specifications are standards related to the characteristics of transmission media. These specifications usually refer to the standards formulated by other organizations. Connector, pin, pin usage, current, current, coding and optical molation are all included in various physical layer specifications. The physical layer often uses multiple specifications to define all the details. Example: rj45802.3, etc

5. TCP / IP is the pronoun of a group of protocols, it also includes many protocols, forming a TCP / IP protocol cluster. The TCP / IP protocol cluster is divided into four layers. IP is located in the second layer of the protocol cluster (corresponding to the third layer of OSI), and TCP is located in the third layer of the protocol cluster (corresponding to the fourth layer of OSI)
TCP / IP communication protocol adopts a four layer hierarchical structure, and each layer calls the network provided by its next layer to fulfill its own requirements. These four layers are:

application layer: the layer of communication between applications, such as simple e-mail transfer (SMTP), file transfer protocol (FTP), network remote access protocol (telnet), etc

transport layer: in this layer, it provides data transmission 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, which is responsible for transmitting data and determining that the data has been delivered and received<

interconnection network layer: it is responsible for providing basic packet transmission function, so that each packet can reach the destination host (but not check whether it is received correctly), such as Internet Protocol (IP)

network interface layer: the management of the actual network media, defining how to use the actual network (such as Ethernet, serial line, etc.) to transmit data
OSI is the abbreviation of open system interconnection, which means open system interconnection reference model< The OSI seven layer model is a kind of framework design method, which realizes reliable communication between different systems and networks through seven hierarchical structure models, Therefore, its most important function is to help different types of hosts achieve data transmission

physical layer: the lowest or first layer of OSI model, which includes physical networking media, such as cable connectors. The physical layer protocol generates and detects voltage to send and receive signals carrying data. Insert a network interface card into your PC, and you've built the foundation of computer networking. In other words, you provide a physical layer. Although the physical layer does not provide error correction service, it can set the data transmission rate and monitor the data error rate. Network physical problems, such as wire disconnection, will affect the physical layer
data link layer: the second layer of OSI model, which controls the communication between network layer and physical layer. Its main function is how to transfer data reliably on unreliable physical circuit. In order to ensure the transmission, the data received from the network layer is divided into specific frames that can be transmitted by the physical layer. Frame is a structure packet used to move data. It includes not only the original data, but also the network addresses of sender and receiver, as well as error correction and control information. The address determines where the frame will be sent, and the error correction and control information ensures that the frame will arrive error free
the function of the data link layer is independent of the network and its nodes and the type of physical layer used, and it does not care whether wo rd, e Xcel or I NTE r NTE are running. Some connecting devices, such as switches, work in the data link layer because they need to decode the frame and use the frame information to send the data to the correct receiver
network layer: the third layer of OSI model, whose main function 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 a in one network to node B in another network by comprehensively considering the transmission priority, network congestion, quality of service and the cost of optional routing. Because the network layer deals with routing, and the router connects all segments of the network and intelligently guides data transmission, it belongs to the network layer. In the network, "routing" is based on addressing scheme, usage mode and accessibility to guide data transmission
transport layer: the most important layer in OSI model. The transmission protocol controls the flow at the same time or sets the appropriate transmission rate based on the speed of the data that the receiver can receive. In addition, the transport layer compulsorily divides the longer packets according to the maximum size that the network can handle. For example, Ethernet cannot receive packets larger than 1500 bytes. The transport layer of the sender node divides the data into smaller pieces, and arranges a sequence number for each piece, so that when the data reaches the transport layer of the receiver node, it can be reorganized in the correct order. This process is called sorting
one of the transport layer services is TCP (transmission control protocol) in TCP / IP protocol suite, and the other is SPX (sequential packet switching) in IPX / SPX protocol suite
session layer: responsible for establishing and maintaining communication between two nodes in the network. The functions of session layer include: establishing communication link, keeping communication link unblocked, synchronizing the dialogue between two nodes, deciding whether communication is interrupted and deciding where to resend when communication is interrupted
you may often hear people call the conversation layer the "traffic police" of network communication. When you dial to your ISP (Internet service provider) to connect to the Internet, the session layer on the ISP server negotiates the connection between you and the session layer on your PC client. If your phone line accidentally falls off the wall jack, the session layer on your terminal will detect the connection interruption and initiate the connection again. The session layer sets the communication deadline by determining the priority and time of the node communication
presentation layer: the translator between the application and the network. In the presentation layer, the data will be formatted according to the scheme that the network can understand; This format also varies depending on the type of network used
represents the decryption and encryption of management data, such as the processing of system password. For example, when you query your bank account on the Internet, you use a secure connection. Your account data is encrypted before it is sent. At the other end of the network, the presentation layer will decrypt the received data. In addition, the presentation layer protocol decodes and encodes the image and file format information
application layer: it is responsible for providing interface for software so that the program can use network services. The term "application layer" does not refer to a special application running on the network. The services provided by the application layer include file transfer, file management and e-mail information processing.

6. OSI (open system interconnection) seven layer network model is called open system interconnection reference model. It is a logical definition and a specification. It logically divides the network into seven layers, The order from top to bottom is:
7 application layer
6 presentation layer
5 session layer
4 transport layer
3 network layer
2 data link layer
1 physical layer
each layer has related and corresponding physical devices, such as routers (network layer) and switches. OSI seven layer model is a kind of framework design method. The main purpose of building seven layer model is to solve the compatibility problem of heterogeneous network interconnection. Its main function is to help different types of hosts to realize data transmission. Its greatest advantage is to distinguish the three concepts of service, interface and protocol, and to realize reliable communication between different systems and networks through seven hierarchical structure models

TCP / IP is the pronoun of a group of protocols. It also includes many protocols, forming a TCP / IP protocol cluster. The TCP / IP protocol cluster is divided into four layers. IP is located in the second layer of the protocol cluster (corresponding to the third layer of OSI), and TCP is located in the third layer of the protocol cluster (corresponding to the fourth layer of OSI). TCP / IP communication protocol adopts a four layer hierarchical structure, and each layer calls the network provided by its next layer to fulfill its own requirements. These four layers are: application layer: the layer of communication between applications, such as simple e-mail transfer (SMTP), file transfer protocol (FTP), network remote access protocol (telnet), etc. Transport layer: in this layer, it provides data transmission 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 data and determining that the data has been delivered and received. Interconnection network layer: it is responsible for providing basic packet transmission function, so that each packet can reach the destination host (but not check whether it is received correctly), such as Internet Protocol (IP). Network interface layer: manage the actual network media, define how to use the actual network (such as Ethernet, serial line, etc.) to transmit data

comparison between OSI model and TCP / IP model
TCP / IP model is actually a condensed version of OSI model, It has only four layers:
1. Application layer
2. Transportation layer
3. Internet layer
4. Network interface layer
compared with OSI function:
application layer corresponds to OSI application layer, presentation layer, session layer
transportation layer corresponds to OSI transport layer
Internet layer corresponds to OSI network layer
network interface layer corresponds to OSI data link layer and physical layer

7. 1. Physical layer: it mainly defines physical equipment standards, such as interface type of network cable, interface type of optical fiber, transmission rate of various transmission media, etc. Its main function is to transmit bit stream (i.e. convert 1 and 0 to current strength for transmission, and then convert to 1 and 0 after reaching the destination, which is commonly known as digital to analog conversion and analog-to-digital conversion). This layer of data is called bits
2. Data link layer: defines how to format data for transmission and how to control access to physical media. This layer usually also provides error detection and correction to ensure reliable data transmission
3. Network layer: provides connection and path selection between two host systems in different geographical locations. With the development of Internet, the number of users accessing information from different sites in the world has increased greatly, and the network layer is the layer to manage this kind of connection
4. Transport layer: it defines some protocols and port numbers (WWW port 80, etc.) for data transmission, such as TCP (transmission control protocol, low efficiency and high reliability, which is used to transmit data with high reliability and large amount of data), UDP (user datagram protocol, which is just opposite to TCP, which is used to transmit data with low reliability and small amount of data, For example, QQ chat data is transmitted in this way. The main purpose is to segment and transmit the data received from the lower layer, and reorganize it after reaching the destination address. This layer of data is often called segment
5. Session layer: establish data transmission path through transport layer (port number: transport port and receive port). It is mainly used to initiate session or accept session request between your systems (devices need to know each other, which can be IP, MAC or host name)
6. Presentation layer: to ensure that the information sent by the application layer of one system can be read by the application layer of another system. For example, a PC program communicates with another computer, one of which uses extended binary decimal interchange code (EBCDIC) and the other uses American Standard Code for information exchange (ASCII) to represent the same characters. If necessary, the presentation layer will use a common format to achieve the conversion between multiple data formats
7. Application layer: the OSI layer closest to the user. This layer provides network services for users' applications such as e-mail, file transfer and terminal emulation.

8. What you mentioned above is the processing method of information source after encryption in the computer. The switches and routers involved in the following mainly work in the data link layer and network layer, which are defined according to their main working principle and the law of address mapping

when understanding the OSI-7 layer model, we should not refer to it indivially and independently, but determine its main functions from its main working principle and role. Take the switch as an example, if viewed from the function, it also has the underlying physical layer and data layer to obtain data and translate it.

9. From bottom to top, there are 1 to 7 layers, which are:

application layer
presentation layer
session layer
transport layer
network layer
data link layer
physical layer

the upper three layers are called high layer, Define the communication and human-machine interface between applications. What does it mean? The upper three levels are responsible for transforming what the computer can understand into what you can understand, or transforming what you can understand into what the computer can understand
the next four layers are called the bottom layer, which define how to end-to-end data transmission, physical specifications and the conversion between data and photoelectric signals
the following layer by layer to explain:

application layer, very simple, is the application. This layer is responsible for determining the communication objects and ensuring that sufficient resources are used for communication, which is of course the work of the application that wants to communicate

the presentation layer is responsible for data coding and transformation to ensure the normal work of the application layer. This layer is the place where we can transform the interface and binary, that is, the transformation between our language and machine language. Data compression, decompression, encryption and decryption all take place in this layer. This layer processes the data into different formats according to different application purposes, which shows the various file extensions we see

the session layer is responsible for establishing, maintaining and controlling sessions, distinguishing different sessions, and providing three communication modes: simplex, half plex and full plex. We usually know that NFS, RPC, x
windows all work in this layer

transport layer is responsible for dividing and combining data to realize end-to-end logical connection. The data is integrated in the upper three layers, and is segmented at this layer. The segmented data in this layer is called segment. Three way handshake (three way
handshake), connection oriented or connectionless oriented services, and flow control all occur in this layer

the network layer is responsible for managing network addresses, locating devices, and determining routes. We are familiar with the IP address and router is working in this layer. The data segments of the upper layer are divided in this layer and encapsulated as packets. There are two kinds of packets. One is called data
packets, which is the user data passed down from the upper layer; The other is called route update packets (route update
packets), which are directly sent by routers to exchange routing information with other routers

the data link layer is responsible for preparing physical transmission, CRC verification, error notification, network topology, flow control, etc. We are familiar with MAC address and switch are working in this layer. The packets passed down from the upper layer are divided and encapsulated in this layer and are called frames

the physical layer is the real physical link, which is responsible for sending and receiving data in the form of bit stream.