FTP File Types, Transmission Modes, Connections, and STOR vs RETR

FTP (File Transfer Protocol) can transfer any file as “bytes,” but FTP also provides transfer types that determine how the bytes are interpreted (and sometimes transformed) between the client and server. Commonly, clients choose between ASCII (text with newline/format conversion) and Image/Binary (raw bytes, no conversion). This matches how FTP “data representation” works: text can be converted between character representations, while binary is stored/forwarded byte-for-byte. 3

In addition, FTP’s data transfer is governed by three transmission modes: Stream, Block, and Compressed. These modes define how FTP structures the data and how it signals end-of-file/record conditions on the data connection. 2

Key terms:

• FTP transmission mode
• FTP transfer type
• Control connection
• Data connection
• STOR
• RETR

What kinds of file types can FTP transfer?

FTP implementations can transfer many kinds of files (text, images, archives, executables, etc.), but at the protocol level the important distinction is transfer type:

• ASCII transfer type (TYPE A): designed for text, where FTP may convert text and line endings between systems. FTP notes that ASCII is inappropriate for non-ASCII/binary data because of the representation conversions involved. 2
• Image/Binary transfer type (TYPE I / “binary mode”): sends bytes directly and is intended for “image” (raw binary) data such as images or other non-text formats. 3

Practically, that means:

• Text files (e.g., .txt, .html) → typically use ASCII for correct newline/character handling.
• Non-text/binary files (e.g., images .png, archives .zip, executables .exe) → typically use binary/image to avoid corruption from conversions.

Mermaid overview (conceptual):

The three FTP transmission modes (MODE)

FTP defines three transmission modes for data on the data connection: 2

1. Stream mode (MODE S): data is sent as a continuous stream of bytes. End-of-file/record handling differs by file structure; for stream mode, EOF may be inferred from connection behavior, while record-structured transfers use control codes.
2. Block mode (MODE B): data is sent in blocks (each with header/descriptor information), supporting features like restart and reliable parsing of boundaries.
3. Compressed mode (MODE C): similar to block mode but includes compression (e.g., run-length encoding style), with framing/descriptor support so the receiver can parse the compressed stream.

These modes are distinct from active vs passive networking behavior; they are about how the bytes are framed and how EOF/EOR are represented to the receiver. 2

Describe the functions of the two FTP connections

FTP uses two separate TCP connections between client and server: 2

1. Control connection

   • Purpose: carry FTP commands (e.g., login, listing, file transfer requests) and return status replies (numeric response codes). 2
   • Typical flow: the client issues commands like “start transfer” and the server responds with statuses such as “ready/starting transfer/complete.”

2. Data connection

   • Purpose: carry the actual file bytes (and directory listing contents during LIST-type operations). 2
   • Typical flow: when a transfer begins, FTP opens a separate data connection for that operation; after data transfer, it closes (while the control connection usually remains).

Mermaid sequence (conceptual transfer):