Network model -- OSI model and tcp/ip model

OSI Model and TCP/IP Model The comparison is as follows ：

One 、OSI Seven layer model

Functions of each layer ：
application layer ：
The application layer is located in OSI The seventh layer of the model , The function is to provide... For computer users Application interface , It also provides users with various Network services . This layer protocol defines Application process Between Interaction rules , Through different application layer protocols for different applications Web applications provide services . For example, web enabled HTTP agreement , The e-mail system uses SMTP agreement .
The presentation layer ：
The role of the presentation layer is It is used for the encoding and conversion of application layer data , Provide services for the application layer up , Accept services from the session layer down . The services provided by this layer mainly include data compression 、 Data encryption as well as Data description , And make various data formats inside the computer convert into standard forms used in communication .
The session layer ： The conversation layer is responsible for establish 、 Management and termination Communication sessions between presentation layer entities . Communication in this layer consists of service requests and responses between applications in different devices .
Transport layer ： The main task of the transport layer is to Communication between two host processes Provide services , Handle Packet error , Packet order , And some other key transmission problems .
The network layer ： This layer establishes the connection between two nodes through addressing , Packets sent to the transport layer at the source end , Choose the right routing and switching nodes , It is correctly transmitted to the transport layer of the destination according to the address .IP Agreement is Intetent. It can be understood in this way , The network layer specifies the transmission route of data packets , The transport layer specifies the transmission mode of data packets .
Data link layer ： The data link layer is usually also called the link layer . Data transmission between two hosts , It's always transmitted over a link , This requires the use of special link layer protocols . When transferring data between two adjacent nodes , Data link layer take The network layer Handed over IP Datagram assembly into frame , The link between two adjacent nodes transmits frames .
The data of each frame can be divided into ： Headlines head And data data Two parts ;

• head Indicate the data sender 、 The recipient 、 data type 、 Such as MAC Address
• data It stores the data of the interaction between computers

Through the control information, we can know the start and end bit position of a frame , Besides , It can also enable the receiving end to detect the received frame Is there any error , If mistakes are found , The data link layer can simply discard this frame , To avoid continuing to occupy network resources
The physical layer ： As OSI The lowest layer in the reference model , The function of physical layer is to realize the transparent transmission of computing and bit stream between nodes . The main task of this layer is to determine some characteristics of the interface with the transmission media ,（ mechanical 、 electrical 、 Functional and process features ）, This layer is mainly related to hardware , It has little to do with software .

Two 、TCP/IP agreement

TCP( Transmission control protocol ): A connection oriented 、 reliable 、 Transport layer communication protocol based on byte stream .
IP( Internet protocol ): Protocols for packet switched data networks .

Divide
TCP/IP The protocol family is divided into five layer system or four layer system , The architecture of five layer protocol Just for the introduction network
Designed according to the principle of network , The practical application is still TCP/IP Four tier architecture , Including application layer 、 Transport layer 、 The network layer （ The Internet layer ）、 Network interface layer

Five tier system

application layer ： TCP/IP The model will OSI Session layer in reference model 、 The presentation layer 、 And the functions of the application layer are combined into one application layer to realize , Provide services for different applications through different application layer protocols . Such as FTP、Telnet、DNS、SMTP etc. .
Transport layer ： This layer corresponds to OSI The transport layer of the reference model , It provides the communication function from the source end to the end-to-end host for the upper layer entity .
The transport layer defines two main protocols ： Transmission control protocol （TCP） And user datagram protocol （UDP）, The connection oriented TCP The protocol ensures the reliability of data transmission , Facing connectionless UDP The protocol can make the data package simple 、 Fast transmission .
The network layer ： Responsible for providing communication services for hosts in packet networks , And pass Choose the right route Transfer data to target host , When sending data , The network layer encapsulates the message segments or user data generated by the transport layer into groups or packets for transmission
Data link layer ： The data link layer is used when two adjacent nodes transmit data , The network layer is handed over IP Datagram assembly into frame , The frame is transmitted over a link between two adjacent nodes .
The physical layer ： Ensure that data can be transmitted on various physical media , Provide a reliable environment for data .

3、 ... and 、 OSI Reference model and TCP/IP The difference between reference models

The same thing ：
● OSI Reference model and TCP/IP The reference models are hierarchical
● Both can provide connection oriented and connectionless communication service mechanisms

Difference ：
● OSI The seven layer model used ; TCP/IP It's a four or five story structure
● TCP/IP The reference model does not subdivide the network interface layer , Just some conceptual description ; OSI The reference model makes a clear distinction between services and protocols
● OSI The reference model is divided into seven layers , But it's more difficult to achieve .TCP/IP The reference model can be used as a simplified hierarchical structure
● TCP/IP The reason why the protocol removes the presentation layer and the session layer is that the session layer 、 The presentation layer 、 The application layer is implemented inside the application , The final output is an application data package , And it is almost impossible to realize abstract code sharing between applications , That's why OSI The layering of the application dimensions in mind is not possible .

Four 、TCP/IP Five layer protocol communication mode