FTP is the abbreviation of English File Transfer Protocol, which means file transfer protocol. Like HTTP, it is a widely used protocol on the Internet to transfer files between two computers. Compared with HTTP, FTP protocol is much more complicated. The complicated reason is that FTP protocol needs two TCP connections, one is command link, which is used to transfer commands between FTP client and server; The other is a data link, which is used to upload or download data.

FTP protocol has two working modes: port mode and PASV mode, which means active and passive in Chinese.

The process of port (active) connection is that the client sends a connection request to the FTP port of the server (2 1 by default), and the server accepts the connection and establishes a command link. When data needs to be transmitted, the client uses the port command on the command chain to tell the server: "I have opened the XXXX port, you can connect me." Therefore, the server sends a connection request from port 20 to the XXXX port of the client, and establishes a data link to transmit data.

PASV (passive) connection process is: the client sends a connection request to the FTP port of the server (2 1 by default), and the server accepts the connection and establishes a command link. When data needs to be transmitted, the server uses the PASV command on the command chain to tell the client: "I opened the XXXX port, so you can connect me". Therefore, the client sends a connection request to the XXXX port of the server and establishes a data link to transmit data.

As can be seen from the above, the connection mode of command link is the same in the two modes, but the establishment mode of data link is completely different. This is the complexity of FTP.

Second: What is HTTP?

When we want to browse a website, we only need to enter the address of the website, such as www.microsoft.com, in the address bar of the browser, but what appears in the address bar of the browser is: this website, the domain name is not from nothing, but is assigned from com.cn, and com.cn is assigned from cn. Guess where it is. Where did cn come from? I'm telling you, this is from ". This is the "root domain". The root domain is the highest level of the domain name, and the "."layer is managed by INIC (Internet Network Information Center). Domain names all over the world are like this, explained layer by layer. Our computer finally got the IP address of this website by asking DNS servers in charge of different domains. Usually we don't lose "yahoo.com.cn." So we can omit it. " . There are many backbone DNS servers in the world, the most important of which is the 13 routing server. If the routing server can't run normally, then the INTENET will be paralyzed. The names of these 13 servers are "A" to "M" respectively, of which 10 servers are located in the United States and 1 servers are located in the United Kingdom, Sweden and Japan.