Understanding Network Model OSI model

Transport layer （Transport Layer）

The function of the transport layer is to provide error free transmission links for the session layer , Ensure that the information transmitted between the two devices is correct , The data unit transmitted by the transport layer is segment (segment).
The transport layer receives data from the session layer , And pass it to the network layer , If the session layer data is too large , The transport layer cuts it into smaller data units —— Segment for transmission .
The transport layer is responsible for creating end-to-end communication connections . Through this layer , The applications on the hosts of both sides of the communication communicate directly through each other's address information , It doesn't matter how many intermediate nodes there are in the network .
The transport layer can establish a separate connection for each session layer request , You can also establish a separate connection for multiple session layer requests according to the usage of the connection , This is called multiplexing , But anyway , This transport layer service is transparent to the session layer .
An important work of the transport layer is error checking and retransmission . The packet may have errors in network transmission , Disorder may also occur 、 Lost, etc , The transport layer must be able to detect and correct these errors . When a packet in a data stream is transmitted in the network, if it reaches its destination through different paths , It may cause a change in the order of arrival . The receiver's transport layer should be able to identify the order of packets , And before passing the contents of these packages to the session layer, restore them to the order in which they were sent . The receiver transport layer not only reorders the packets , Also verify that all packages have been received . If there are errors and losses , Receiver must , The other party must be requested to retransmit the lost packet .
In order to avoid the sending speed exceeding the processing capacity of the network or the receiver , The transport layer is also responsible for performing flow control （flow.
control), Reduce traffic when resources are insufficient , And increase traffic when resources are sufficient .

The session layer , Presentation layer and application layer

The session layer (Session Layer)
The session layer uses the end-to-end services provided by the transport layer , Provide session services to presentation layer or session users , Just like his name , The conversation layer establishes the conversation relationship , And keep the conversation smooth , Decide whether the communication is interrupted and where to start sending again . for example , A user logs in to a remote system and exchanges information with it , The session layer manages this process , Control which party has the right to send information , Which party must receive the information , It's actually a synchronization mechanism .
The session layer also handles error recovery . for example , If a user is sending the contents of a large file on the network , And the network suddenly broke down , When the network returns to work , Does the user have to start retransmission from the starting position of the file ？ The answer is No , Because the session layer allows users to insert checkpoints in a long information flow , Just retransmit the data discarded after the last checkpoint .
If the transmission is interrupted occasionally at the lower layer , The session layer will try to re-establish communication , For example, when a user dials to ISP（ Internet service providers ） When you request to connect to the Internet ,ISP The session layer on the server provides information to users PC The session layer on the client makes a write connection , If the user's telephone line occasionally falls off the jack on the wall , The session layer on the terminal will detect the disconnection and re initiate the connection .
The presentation layer (Presentation Layer)
Responsible for the application layer information “ Express ” Into a format , So that the interrupt device can be correctly identified , It mainly focuses on the semantics and syntax of information transmission , In the presentation layer , Data will be encoded in a consistent way , So that computers using the same presentation layer protocol can identify data with each other . for example , A picture can be expressed as JPEG Format , It can also be expressed as BMP Format , If the other program does not recognize our representation , This picture cannot be displayed correctly .
The presentation layer is also responsible for data encryption and compression . encryption （encryption) It is a certain conversion of data coding , A process that prevents unauthorized users from intercepting or reading . If someone intercepts data without authorization , What you see will be encrypted data . Compress （compression) It refers to reducing the number of bits of information on the basis of maintaining the original meaning of the data . If transmission is expensive , Compression will significantly reduce costs , And increase the amount of information sent per unit time .
application layer (Application Layer)
The application layer is OSI At the top of , It deals directly with users and Applications , Responsible for providing interfaces to software to enable programs to use network services . The network services here include file transfer 、 file management 、 E-mail message processing, etc . It must be emphasized that the application layer is not equivalent to an application . for example , Send e-mail on the network , Your request is transmitted to the network through the application layer .