OSI notes sorting

1、OSI Basic concepts

OSI（Open System Internet） : Open system interconnection , Is a public reference model , Generally used in theoretical research , It is also the standard for defining data generation .

2、OSI Seven layer model and its functional understanding

OSI The model divides the work of network communication into 7 layer .1 to 4 Layers are considered lower layers , Related to data movement .5 to 7 The first floor is the top floor , Contains application level data . Each layer is responsible for different functions . From low to high, it can be divided into ： The physical layer 、 Data link layer 、 The network layer 、 Transport layer 、 The session layer 、 Presentation layer and application layer .

first floor ： The physical layer

stay OSI In the seven level reference model , The physical layer is in the OSI The lowest level of the reference model , The physical layer mainly uses the mechanical characteristics of the hardware 、 Optical properties 、 Electrical characteristics, etc , Defines the standards for physical devices ,eg： Interface type of network cable 、 Transmission rate of various media, etc , It uses the transmission medium to provide physical connection for the data link layer , Realization Bit stream Transparent transmission . The main transmission medium is optical fiber 、 Twisted pair 、 Coaxial cable .

understand ： Define physical device standards , Increase the transmission data rate , Digital to analog conversion .

The second floor : Data link layer

The data link layer is OSI The second layer in the reference model , Between the physical layer and the network layer . The data link layer is divided into logical link control sublayer and media access control sublayer , stay LLC The layer can resolve the related protocols used by the upper layer , stay MAC Layer for physical addressing 、 Burning, etc . Data is processed at this layer Framing , And handle flow control , Thus, a data link is established between the communicating entities .

understand ： Defines how data is transferred , And how to let the control layer access the physical media . And provide detection and correction , To confirm the reliable transmission of data .

The third level : The network layer

The network layer is OSI The third layer in the reference model , The network layer through addressing 、 Addressing provides communication for different hosts on the network , The data in the data link layer is converted to Data packets , Then choose the path 、 Segment combination 、 flow control 、 Congestion control transmits information from one network device to another network device .

understand ： By addressing 、 Addressing and other operations to select the optimal path , Data transfer

The fourth level : Transport layer

The transport layer is OSI The fourth layer in the reference model , The function of this layer is Distinguish traffic 、 Define the transmission mode of data . The two most common protocols in the transport layer are transmission control protocol TCP（ Connection oriented reliable transport protocol ） And user datagram protocol UDP（ Non connection oriented unreliable transport protocol ）.

TCP: Connection oriented reliable transport protocol , Used to send large amounts of data . For example, I sent 1000 Pack out , Another computer needs to tell me if I have received 1000 A package . If the corresponding number of packets is not received , Then the computer will tell me that those bags are missing , Then you need to send it again . In this way, we can ensure that the other party has completely accepted these data .

UDP： Non connection oriented unreliable transport protocol , Used to send a small amount of data . For example, I sent 10 Pack out , If oneortwo are lost on the way, it will have little impact on the computer , This is applicable to live broadcast , Or games .

understand ： Distinguish traffic , Define how traffic is transmitted , Select reliable transmission or unreliable transmission through flow characteristics .

The fifth floor : The session layer

The session layer is OSI Refer to the fifth layer in the model , This layer establishes for each type of data transmitted （ management 、 maintain 、 End ） A virtual connection （ It can prevent different flows from affecting each other ）, Each kind of data can produce a special channel .

understand ： By realizing the function of automatic addressing , To establish and manage communication between applications .

The sixth floor : The presentation layer

The presentation layer is OSI The sixth layer in the reference model , This layer converts logical languages into binary languages , And translate , encryption .

understand ： On the basis that the program can automatically contract and address , In order to realize the communication between two different systems , Need to translate the language , encryption

The seventh floor : application layer

The application layer is OSI The seventh layer in the reference model , This layer can accept user data , It's a human-computer interface , User input data , Operating system for processing , The processed data is transformed through the application layer .

3、TCP Three times mobile phone system （ It's very important ）

How to ensure connection oriented ： Three times mobile phone system （ Confirm retransmission ）

Work steps

step1： The first handshake ： The client makes the first step to the server , Issue a synchronization request package to a bound port （Syn+Req The synchronous request package host request server port is open ）, Server consent , Then officially send .

step2： The second handshake ： The server requests that the port of the client is open , And the server sends Ack+Syn+Req Package to client , To confirm that the client has received Syn paragraph .

step3： The third handshake ： The client agrees , The client responds to the server with a Ack Data packet to indicate that the server's Syn paragraph .

4、TCP/IP Model

TCP/IP The model is actually OSI A simplified version of the model , It is also the standard for defining data generation , It is divided into four levels ：

1. application layer （ Corresponding OSI The application layer of the model 、 The presentation layer 、 The session layer ）

2. Host to host layer （ Corresponding OSI The transport layer of the model ）

3. The Internet layer （ Corresponding OSI The network layer of the model ）

4. Network interface layer （ Corresponding OSI The data link layer of the model 、 The physical layer ）