File Transfer (FTP, PuTTY, WinSCP)

Computer Network & Network Security System — Application Layer, NEC licence examination syllabus (Nepal Engineering Council).

File Transfer: FTP, PuTTY, WinSCP

Three different tools, all solving "get a file from here to there over a network."

📁 FTP — File Transfer Protocol

Two connectionsA CONTROL connection (commands like login, list files) stays open the whole session, while a separate DATA connection is opened just for each actual file transfer.

Active vs Passive modeActive: the SERVER initiates the data connection back to the client. Passive: the CLIENT initiates it — passive mode plays much nicer with firewalls, which is why it's the common default today.

🖥️ PuTTY and WinSCP

PuTTYAn SSH client — lets you securely log into and run commands on a remote machine's command line.

WinSCPA graphical SFTP/SCP client — the "drag and drop files" experience for securely transferring files to/from a remote server, built on the same secure SSH foundation as PuTTY.

💡 One-liner: "Why does FTP use two separate connections?" → Separating control (commands) from data (file content) lets the control connection stay responsive and simple, regardless of how large or slow the actual file transfer is.

Why FTP uses two connections

Two connections for one file transfer looks wasteful. It follows from a genuine design need.

The control connection stays open for the whole session carrying commands and replies; the data connection is opened per transfer and closed when it finishes. Keeping them separate means a command can be sent during a transfer — most importantly an abort — because the control channel is not blocked by the file moving on the other one.

🔀 Active versus passive, and why firewalls care

ActiveThe SERVER opens the data connection back to the client. To the client's firewall this is an unsolicited inbound connection, which it blocks — the client asked for a file and is now being connected to from outside.

PassiveThe CLIENT opens both connections. Everything is outbound from the client's side, so a stateful firewall permits it as return traffic for a conversation the client started.

💡 This is the stateful-firewall reasoning from the security chapter, seen from the application side. Passive mode became the default because active mode requires the client's firewall to accept an inbound connection it has no record of requesting — exactly what a firewall exists to prevent.

Why FTP is not used for anything sensitive

FTP sends credentials and file contents in plain text. Anyone able to observe the traffic reads both — the password included.

🔐 The secure replacements

SFTPFile transfer carried inside SSH. One connection, one port, encrypted throughout. This is what PuTTY and WinSCP use.

FTPSOrdinary FTP wrapped in TLS. Keeps FTP's two-connection design, so it inherits the same firewall awkwardness.

💡 SFTP and FTPS are commonly confused and are not variants of each other. SFTP is a different protocol that happens to move files over SSH; FTPS is FTP with encryption added. SFTP's single connection is also why it passes through firewalls cleanly where FTPS does not.

Syllabus points

FTP: control & data connection, active/passive
PuTTY (SSH client)
WinSCP (SFTP/SCP transfer)

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