File Transfer Protocol (FTP)

FTP is divided into two connections: the command connection and the data connection.

• Command Connection: Primarily used for sending control messages.

• Data Connection: Used for sending actual file data.

FTP Command Connection (port 21)

• The client initiates a TCP connection to start a session.

• FTP commands are usually textual and consist of verbs.

• Examples include USER, PASS, CWD, LIST, RETR, STOR, and REST.

• Typical FTP responses include:

  • 331 Username OK, password required.

  • 230 Login successful.

  • 125 Data connection already open; transfer starting.

  • 425 Can't open data connection.

  • 452 Error writing file.

FTP Response Codes

• 1xx: Positive preliminary response.

• 2xx: Positive completion response.

• 3xx: Positive intermediate response.

• 4xx: Transient negative completion response.

• 5xx: Permanent negative completion response.

FTP Data Connection (port 20)

FTP client initiates the connection on port 20. It operates separately from the command connection, which allows it to transfer files.

FTP Data Connection Modes: Active vs. Passive

1. Active Mode (PORT):

   • The command format is PORT h1,h2,h3,h4,p1,p2.

   • It suggests that the client listens on IP address h1.h2.h3.h4 and port number p1*256+p2 for a data connection.

   • The FTP server then initiates the connection to this specified port.

2. Passive Mode (PASV):

   • Once the server listens and is ready for a connection, it informs the client about the IP and port details.

FTP Flow

For both modes, the flow is as follows:

• CWD src command is sent by the client.

• 200 response is a positive completion.

• In Active mode, the PORT command specifies where the server should connect. In Passive mode, the PASV command asks the server where to connect.

• LIST command is issued to get a list of files/directories.

• 150 indicates the start of a file transfer.

• Data is then transmitted.

• 226 indicates the completion of the file transfer.