Network knowledge summary

It is known that TCP/IP The agreement is divided into four layers / Five floors , The application layer , Transport layer , The network layer , Access layer ( The data link 、 The physical layer ). The physical layer does some simple level transmission , Through optical fiber 、 Twisted pair 、 Radio and other external media 0、1 Transmission of level signal .

       Data link layer ： How do you know that the ports connected to each other are sent to this machine , It's time to MAC Address as tag , Due to the low reliability of some media in the physical layer , Therefore, the data link layer is required for data verification , The data link layer will cut the bit stream with unknown length of the physical layer into various segments , Each paragraph is carried out separately CRC check , take MAC Address and other information are added to Baotou , take CRC The checksum is added at the end of the package , This is the frame format of the data link layer . Bitstream cutting can also reuse the data of the physical layer .

       The network layer ： The link layer is based on broadcasting , In order to prevent the network storm under large-scale communication , It needs to be divided into multiple broadcasts , At this time, you need a virtual address —ip Address , The role of the network layer is to allocate and process each ip Address , That's gateway .

       Transport layer ： It is known that there are multiple applications on a host that need to be networked , How to allocate a packet to each application process after receiving it ？ That is to distinguish by port number . So the network layer is the data transmission between hosts , The transport layer is the data transmission between various application processes .TCP and UDP The protocol is at the transport layer .UDP According to MTU( Maximum transmission unit -1500) The maximum calculated is no more than 1472(1500-20IP Head -8UDP Head ).

       The transmission of data stream is encapsulated from top to bottom in the five layer model , First of all TCP Header encapsulation , Then the IP Header encapsulation , In the link layer, the link layer will be added in the header MAC Encapsulation of address , Add at the tail CRC Encapsulation of checksum .