File transfer protocol --ftp

One .FTP Model

FTP client On the client side, a user interface, This is what users use FTP System interface for file operation . stay user interface Next , Users can use user command File operations .user command and Linux command Very much like . It will be introduced later .

Client PI Full name Client Protocol Interpreter, The function is to carry out user command and FTP command The transformation of （ The difference between the two can be seen as source code and mechanical code 、ASCII Form domain name and binary form IP The difference in address ）（ User user command I'll give it to you client PI, Turn into FTP command; and client PI Will get FTP command Turn it into a form that users can understand and give it to user interface）, And will FTP command Transferred to the Server PI and Client DTP. One Client only one PI.        

Server PI The role of is to accept Client PI Of FTP command, control Server DTP. One Server only one PI.

DTP Its full name is Data Transfer Process. It is a process established for data transmission ,Client and Server Each end needs to establish a , For data transfer of a file . Once the transfer is complete, it will be PI To release. . So it can be seen that DTP stay Server and Client Is the corresponding occurrence , And more than one pair .

 

FTP The model is characterized by , It establishes two connection channels .Control Connection Each pair of client PI and server PI Only one , be used for FTP command The transmission of . because FTP command need FTP The client side and the server side cooperate to complete , So build Control Connection It makes sense .Control connection Due to the use ASCII code , So in line with Telnet Protocol.                                                  Data Connection It's every pair client DTP and server DTP Connection channel between , Used for data transmission between the two .server The establishment of a data connection be called active,client The establishment is called passive.

FTP server Is a process on the server , Its use 21 Number port Listen to the client control connection Connection request .control connection After the foundation of ,server Accept client Of commands, return reply.

FTP client It's direct and user Dealing with , It is responsible for opening control connection. Be careful control connection Is jointly established by both parties , But only by FTP client open . however data connection Both sides can open , It depends on the actual situation . FTP client Coding method of （ Transmitted commands ） and telnet equally , Use 8 Bit ASCII.

Two .User command&FTP command

It should be noted that control connection Transmitted in FTP command（ASCII In the form of ）, What the user actually uses is User command. Both need client PI To transform .

 

  3、 ... and . Example ：downloading in Active Mode

This is a FTP An example of file transfer . So it's called active mode Because data connection By server End open（ Application for establishment ） Of .

We can see from the top control connection and data connection The difference between

1.control connection Identify a pair of server and client, For both PI Between FTP Exchange of commands ; and data connection It identifies the data transmission process of a file , For both DTP Data transmission between .

2.control connection Only by client End open. After establishment , You need a client quit Command will be released . and data connection It is dynamically established , For one file transfer only , It will be released after the transmission is completed .

3.control connection Use 21 Port no. ; however data connection But not necessarily , The port it uses requires server and client adopt control connection Negotiate .