FTP, or File Transfer Protocol, is a standard network protocol used for transferring files between a client and a server on a computer network. It operates on the application layer of the TCP/IP protocol suite and supports both anonymous and authenticated access.
The FTP protocol governs how files are transferred and managed between the client and server. It uses a control connection (typically on TCP port 21) for issuing commands and coordinating file transfers, while separate data connections are established dynamically (using TCP port 20 for active mode or other ports for passive mode) to transfer actual file data.
The data protocol number associated with FTP depends on whether it is operating in active or passive mode. In active mode FTP, the server initiates a data connection back to the client on TCP port 20. In passive mode FTP, the client initiates both control and data connections, with the server providing an IP address and port number for the data connection.
A data connection in FTP refers to the separate TCP connection established between the client and server specifically for transferring file data. In active mode FTP, the server opens a data connection to the client’s specified IP address and port number. In passive mode FTP, the client connects to the server’s specified IP address and port number for data transfer.
FTP supports different data types for transferring files, including:
- ASCII (or text) mode: Suitable for transferring text files, where line endings are converted to match the destination system’s conventions (e.g., CR-LF for Windows, LF for Unix).
- Binary (or image) mode: Used for transferring binary files, preserving the exact byte-for-byte content of the file without modification.
- EBCDIC mode: Used for transferring files between systems using EBCDIC character encoding, which is primarily found in older IBM mainframe systems.
The choice of data type in FTP depends on the nature of the files being transferred and ensures that files are correctly interpreted and usable on the destination system.