Measure the test taker's understanding about the layer-7 structure of OSI.

For more information about the 7-layer structure of the OSI network-Linux general technology-Linux technology and application, see the following. Physical layer is the lowest layer of the OSI model. It is built on the basis of physical communication media and serves as an interface between the system and communication media, it is used to implement transparent bit stream transmission between data link entities. The design must ensure that when one party sends a binary "1", the other party also receives "1" rather than "0 ".

The physical layer is the only layer in OSI designed for communication media. It provides connections with communication media and describes the mechanical, electrical, functional, and procedural features of such connections, to establish, maintain, and release physical connections between data link entities. The physical layer provides correct transfer of bit information to the upstream layer.

The Physical Layer Protocol defines a series of standards for the hardware interface, such as how many volts are used to represent "1", how many volts are used to represent "0", and how long a bit lasts; whether the transmission is bidirectional or unidirectional; how the connection is terminated after the initial connection is established and completed; how the sender and receiver respond to the communication; the size and number of connectors between the devices; the purpose of each line.

The main task of the data link layer is to enhance the function of transmitting original bits in the physical layer, so that the network layer is displayed as an error-free link. It establishes, maintains, and releases data link connections between adjacent network entities, and transmits data link data units (frames, frames ). It is the first layer of bit collection and processing by package. It completes the final encapsulation before sending the package and performs the first inspection on the arrived package. Its main functions are as follows:

Data Link connection establishment and release: both parties contact each other before and after each communication to confirm the start and end of a communication. The data link layer generally provides three types of services: no response, no connection, no connection, and connection-oriented. Composition of data link data units: Add Data Link Protocol control information on the basis of data delivered by the upper layer to form data link protocol data units. Split data link connection: when the data volume is large, to improve the transmission rate and efficiency, the original data transmitted on a physical link is transferred over multiple physical links (opposite to multiplexing ). Demarcation and synchronization: identifies the start and end of data link data units from the bit streams transmitted on the physical connection, and identifies each field in the data link, in order to achieve correct receipt and control. Sequence and traffic control: it is used to ensure that the data units sent by the sender can be transmitted to the receiver in the same order, and keep the matching between the sending rate and the receiving rate. Error Detection and recovery: detects transmission, format, operation, and other errors, and recovers the errors. If the error cannot be recovered, report it to the relevant network entity.

The network layer is related to the Operation Control of the subnet. One of the key issues is to determine how the Group chooses a route from the source to the target. This layer maintains the route table and determines which route is the fastest and when to use the alternative route. A route can be a fixed static route table in the network, almost unchanged, or determined at the beginning of each session (such as through Terminal negotiation ), you can also flexibly decide routes for each group based on the current network load.

Another important feature of the network layer is transmission and traffic control. When there are too many groups in the subnet, it provides an effective traffic control service to control the packets transmitted over the network connection, to avoid information congestion or congestion.

The network layer provides two types of network services: connectionless services (datagram services) and connection-oriented services (virtual circuit services ). The network layer makes the higher layer independent from the data transmission and exchange technologies used to connect the system.

The IP protocol works at this layer and provides "connectionless" or "datagram" services.
The basic function of the transport layer is to receive data from the Session layer. When necessary, they are divided into smaller units for transmission to the network layer, and ensure that the information of each segment that arrives at the other side is correct. Moreover, these tasks must be completed efficiently.

The transport layer adds a layer of software on the basis of the network layer, so that it can shield the differences between various communication subnets and provide a service that can meet the requirements of user processes, it has a constant universal interface, allowing the user process to conveniently use network resources and communicate with each other by simply understanding the interface.

In general, the Session Layer creates a transmission connection for each request, and the transport layer creates an independent network connection for it. If the transmission connection requires high information throughput, the transport layer can also create multiple network connections for it to distribute data on these network connections to improve the throughput. On the other hand, if it is not cost-effective to create or maintain a network connection, the transport layer can reuse several transmission connections to one network connection to reduce costs. In any case, the transport layer is required to make multiplexing transparent to the Session Layer.

The transport layer is the real "end-to-end" layer from the source to the target. That is to say, a program on the source machine uses the packet header and the control packet to communicate with similar programs on the target machine. In the layers below the transport layer, the Protocol is the protocol between each machine and its directly adjacent machine, rather than the Protocol between the final source machine and the target machine, there may be multiple routers among them. Figure 1-1 illustrates this difference. Layer 1-Layer 3 are linked, and Layer 4-Layer 7 are end-to-end.

The TCP protocol works at this layer and provides reliable connection-based services. It provides reliable data transmission between two endpoints and end-to-end error recovery and traffic control.
The session layer allows users on different machines to establish session relationships, that is, formal connections. This formal connection provides high reliability for sending and receiving information. The purpose of the session layer is to effectively organize and synchronize conversations between users of the session service for cooperation and manage data exchanges between them.

One of the Session Layer services is the management dialog, which allows two-way transmission of information at the same time or only one-way transmission at any time. If it belongs to the latter, it is similar to a single-line railway. The Session Layer will record the next line. A session-related service is token management. tokens can be exchanged between the two parties. Only one Party holding the token can perform some key operations.

Another session service is synchronization ). When a large amount of information is sent continuously, in order to make the sent data more precisely structured, you can set synchronization points in the data sent to record the status of the sending process, in addition, when a session is interrupted due to an error, the session entity can resume session transmission from a synchronization point without resuming the session from the beginning.

There is no special Session Layer in the TCP/IP protocol system, but some functions of this layer are implemented in its transport layer protocol.

The presentation layer (presentation layer) provides some specific functions. Because these functions are often requested, people hope to find a common solution, instead of letting every user implement them. It is worth mentioning that the lower layers of the presentation layer only focus on reliable transmission of bit streams, while the presentation layer focuses on the syntax and semantics of the transmitted information.

The presentation layer is not fully defined and widely used. For example, the presentation layer is not defined in the TCP/IP protocol system. The presentation layer converts any data used at the application layer to provide standardized application interfaces and public communication services. For example, data format conversion, data compression/decompression, and data encryption/Decryption may be performed in the presentation layer.

The application layer contains a large number of commonly needed protocols. This layer handles security issues and resource availability. In recent years, application-layer protocols have developed rapidly. application-layer protocols that are frequently used include FTP, TELNET, HTTP, and SMTP.

The layers of the OSI model clearly define tasks that only deal with the upper and lower adjacent layers: accept the services provided by the lower layer and provide services to the upper layer. Because all network protocols are layered, like stacks, they are often collectively referred to as protocol stacks. Their working mode is generally: when sending, they receive data from the upper layer, split and package it, and then hand it to the lower layer; when receiving, they receive data packets from the lower layer, split the package, and then hand it to the upper layer. In this way, the transmission process of a package is: the application program of the sending host transfers data to the network protocol stack implementation (network communication program), and the network protocol stack implements data packaging layer by layer, finally, it is sent to the physical layer on the data link. After receiving the data, the host will split the package layer by layer and pass it up until it reaches the application layer, and the application will obtain peer-to-peer data.