Session layer, Presentation layer, Application Layer __ Network Basics

the function of the session layer

The main function of the session layer is to establish, maintain and release the user-oriented connection among two nodes, manage and control the session, and ensure the reliable transmission of the session data. We all mention connections at the session and transport levels, so what's the difference between a session connection and a transport connection?
If you say to your secretary, call Mr. Jones, you are equivalent to the conversation layer, and the Secretary is the transport layer. Your request is equivalent to requesting a session. You make a request to establish a connection, but do not have to find the phone number, dialing, etc. The Secretary proceeded to make a telephone call and begin establishing a transport connection. When the dial is successful, the other side picks up the microphone and the transmission connection is established. Then, you take the call, the session layer (connection) is established successfully.

There are three relationships between a session connection and a transport connection: one-to-one relationship, that is, a session connection corresponds to a transmission connection; a one-to-many relationship, a session connection corresponds to multiple transport connections; multi-pair relationship, multiple session connection corresponds to a transmission relationship, like a phone call, a person can talk to another person after speaking, Without having to let the telecommunications bureau know that someone else is talking. During a session, the session layer needs to decide whether to use Full-duplex or Half-duplex communication. If Full-duplex communication is used, the session layer has little to do with conversation management; if Half-duplex communication is used, the session layer coordinates the session with 2 data tokens of a computer network technology, ensuring that only one user can transmit data at a time. When the session layer establishes a session, let a user get the token first. Only the user who obtains the token has permission to send. If the receiver wants to send data, it can ask for a token. The sender decides when to give up. Once the token is obtained, the receiver is converted to the sender. When we do a lot of data transfer, such as you are downloading a 100M file, when the download to 95M, the network disconnected, then what to do. Whether you need to start again. To solve this problem, the session layer provides a synchronization service that divides the session into distinct session units by defining checkpoints (Checkpoint) in the data stream. When a network failure occurs, the data is returned from the last checkpoint. Common session-layer protocols include Structured Query Language (SQL), remote process call (RPC), x-windows System, AppleTalk session protocol, Digital Network structure Session Control Protocol (DNA SCP), etc.

the function of the presentation layer

The presentation layer is mainly responsible for the conversion of data format, compression and decompression, encryption and decryption.

In the OSI model, the layers below the presentation layer are primarily responsible for not making errors when data is transmitted across the network. However, there is no error in the transmission of data, which does not mean that the information represented by the data is not wrong. For example, you want to go to Shanghai from Hangzhou two o'clock in the afternoon, so you said to Shanghai friends, "I two o'clock in the afternoon," but your friend understood that two o ' clock arrived in Shanghai. So this sentence, although not wrong, but because of different understanding, produced a different result.

The presentation layer is specifically responsible for these questions about how computer information is represented in the network. The presentation layer is responsible for converting between different data formats to enable the exchange of information between different computer systems. The exchange of information between the two computers, in addition to coding, includes arrays, floating point numbers, records, images, sounds and other data structures, the presentation layer in an abstract way to define the data structure used in the exchange, and in the computer internal representation and network standard representation of the conversion between.

The presentation layer is also responsible for encrypting the data to protect it during the transmission of the data. The data is encrypted at the sender end and decrypted at the receiving end. Encrypt and decrypt data using the encryption key. The presentation layer is also responsible for compressing the files, compressing the file size by algorithms, and reducing transmission costs.

Application Layer

Application layer is the highest level in the network system, and the only one layer for users, application layer will provide users with common applications, and realize the functions of network services. Commonly used e-mail, internet browsing and other network services, are the application layer of the program.

The application layer is mainly in the face of user access to the network. There are mainly applications, such as: Dns,ftp,e-mail,telnet,http.

a summary of the OSI model

The upper layer of the OSI model (session layer, presentation layer, application layer) handles user interfaces, data formats, and application access.

The lower layers of the OSI model (physical layer, Data link layer, network layer, transport layer) handle the transmission of data in network media.