FTP protocol details

One 、FTP Overview of the agreement

1、 File transfer protocol (File Transfer Protocol) It is the most widely used file transfer protocol on the Internet , be used for Internet Two way transfer of control files on .

2、FTP Provide interactive access , Allows customers to specify the file type and format , And allow access to files .

3、FTP Shielding the details of each computer system , Therefore, it is suitable for transferring files between computers in Heterogeneous Networks .

Two 、FTP The features of the agreement

1、 File transfer protocol FTP Only some basic services of file transfer are provided , It USES TCP Reliable mode of transportation .

2、FTP Its main function is to reduce or eliminate the incompatibility of processing files under different operating systems .

3、FTP Using client server mode . One FTP The server process can provide services for multiple client processes at the same time ,FTP The server process of consists of two parts ： One is the main process , Responsible for accepting new requests ; In addition, there is a slave process , Responsible for handling individual requests .

3、 ... and 、FTP working process

1、FTP There are control connections and data connections ,

The standard port of control connection is 21, Used for sending FTP Command message .

The standard port for data connection is 20, For uploading 、 Download data .

2、FTP The difference between the protocol and other protocols is client access FTP The server side needs to establish two TCP Connect , One is used to transmit FTP command , One for transmitting data

3、FTP There are active mode and passive mode ,

Active mode ： The service side from 20 The port initiates the connection to the client , The working process is shown in the figure below ：

  In the first step ,FTP Client submit PORT Command and allow the server to connect its data port back and forth (1027 port ).

  In the second step , The server returns a confirmation .

In the third step ,FTP The server sends... To the client TCP Connection request , The target port is 1027, The source port is 20. Establish a connection to transmit data .

Passive mode ： The server passively waits for the customer at each port within the specified range , Passive initiated connection .

In the first step , The command port of the client is connected to the command port of the server , And send orders "PASV".

In the second step , The server returns the command "PORT 2024" Tell the client ： Which ports does the server use to monitor data connections .

In the third step , The client initializes a data connection from its own data port to the data port specified by the server .

In step four , The server returns a message to the data port of the client "ACK" Respond to .