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1. OSI seven-layer model

Physical layer:

The analog signal is converted into a digital signal by a hardware device, so there is a 0 + + 1 data stream, called a bit stream.

Data Link layer:

You can send a bitstream, but if there is no format, it will be a mess, so there are "frames". A kind of "frame" data block is used for transmission. in order to ensure the accuracy of data communication and realize effective error control, error detection and other functions (network card, switch) are added.

Network layer:

The first two layers are all about the ability to send data and whether the data is correct, but if you can connect two computers, multiple computers and only one of them can communicate, you need to route. Selective hair, then each computer must have its own identity, so the emergence of the IP protocol (router).

Transport layer:

The process of bit stream transmission can not be smooth all the time, and it is normal to have occasional interruptions. If you artificially set up a unit, divide it into segments of information and derive messages from it, combined with the upper layers, we can have the right data for the target to occur to a computer. There are two important protocols in the transport layer: TCP and UDP. TCP is inefficient, but the sending packet verifies whether it is complete, and UDP is efficient, regardless of whether others receive it or not.

Session layer:

The computer receives the sent data, but there are so many processes, which process needs to use this data, then send it to that process. For example: if port 80 is to be used, so data is communicated within the system, the data from the receiving port is sent to the demand port.

Presentation layer:

Now the required data is received correctly, but because the data may be based on security during transmission, or because of algorithmic compression, and because of different network types. Then there must be a bridge of communication to sort out and restore what should have been said, similar to the process of dismantling express delivery.

Application layer:

Provide a variety of service interfaces, users only need to operate the application layer to get the service content, so that encapsulation can make it available to more people. The main protocols included: FTP (File transfer Protocol), Telnet (Telnet Protocol), DNS (Domain name Resolution Protocol), SMTP (Mail transfer Protocol), POP3 Protocol (Post Office Protocol), HTTP Protocol (Hyper Text Transfer Protocol)

2. TCP/IP model

The TCP/IP model is actually a condensed version of the OSI model, which has only four levels:

Application layer: application layer presentation layer corresponding to OSI session layer

Transport layer: the transport layer corresponding to OSI

Internet layer: the network layer of OSI

Network interface layer: data link layer and physical layer of OSI

III. TCP/IP protocol

IP layer transmission unit is IP packet, which belongs to point-to-point transmission; TCP layer transmission unit is TCP segment, which belongs to end-to-end transmission.

Original: https://blog.csdn.net/cmyh200/article/details/82768804

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