File Transfer Protocol (FTP) is a standard network protocol used for transferring computer files between a client and server on a computer network. It operates using two separate TCP connections: a control connection on port 21 for commands and a data connection on port 20 (in active mode) for the actual file transfer. By default, FTP transmits all data, including usernames, passwords, and file content, in cleartext, which means it is not encrypted and can be intercepted.

A. SNMP (Simple Network Management Protocol) primarily uses UDP ports 161 and 162 for managing and monitoring network devices, not TCP ports 20/21.

B. SSH (Secure Shell) uses TCP port 22 and is designed for security, encrypting all data transmitted to prevent eavesdropping.

D. Telnet uses TCP port 23 for remote command-line access. While it does transfer data in cleartext, it does not use ports 20/21.

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1. Internet Assigned Numbers Authority (IANA). (2023). Service Name and Transport Protocol Port Number Registry.

Reference: The official registry lists ftp-data on TCP port 20 and ftp (control) on TCP port 21. It also lists ssh on TCP port 22

telnet on TCP port 23

and snmp on UDP port 161. This document is the authoritative source for well-known port assignments.

2. Postel

J.

& Reynolds

J. (1985). RFC 959: File Transfer Protocol. Internet Engineering Task Force (IETF).

Reference: Section "Data Connection Management" specifies the use of the server's control port minus one (21-1=20) as the default data port. The "Security Considerations" section explicitly states that the protocol does not provide for encryption.

3. Kurose

J. F.

& Ross

K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson.

Reference: Chapter 2

Section 2.5 "Application-Layer Protocols: FTP". The text describes FTP's use of two parallel TCP connections: a control connection on port 21 and a data connection on port 20. It explicitly notes

"Both the user name and password

as well as commands

are sent in cleartext..." which highlights its lack of security.