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Application Layer TCP/IP Protocols and Concepts

A multitude of protocols exist at TCP/IP’s Application Layer, which combines the Session, Presentation, and Application Layers of the OSI model.

Telnet

Telnet provides terminal emulation over a network. “Terminal” means text-based VT100-style terminal access. Telnet servers listen on TCP port 23. Telnet was the standard way to access an interactive command shell over a network for over 20 years.

Telnet is weak because it provides no confidentiality; all data transmitted during a telnet session is plaintext, including the username and password used to authenticate to the system. Attackers who can sniff network traffic can steal authentication credentials this way.

Telnet also has limited integrity: attackers with write access to a network can alter data, or even seize control of Telnet sessions. Secure Shell (SSH) provides secure authentication, confidentiality, and integrity and is a recommended replacement for Telnet.