Telnet/FTP

Telnet and FTP are two very commonly used application-layer protocols on the Internet. They have been around for over 20 years now.Telnet is an application-layer protocol and allows a user to connect to an account on another remote mechine. A client program on one machine can connect with a server program running on another machine using this protocol. Users utilizing Telnet interact with the remote machine in the same way as they would with a local machine. Telnet was one of the earliest protocols and in the early days was used primarily to allow users in one location to access accounts or machines in another location.

The default port (reserved by IANA) that the telnet client connects to on the server side is port 23. The client/server TCP connection is maintained throughout the telnet session. Since telnet can work across different platforms, telnet assumes that the hosts run a general network virtual terminal, which is a simple character device with a keyboard and printer. Data typed by a user on the keyboard are transmitted to the remote server, and the received data from the server are output to the printer.


In order to differentiate between a telnet control message and user data, each control message is preceded with a special octet, eight bits of 1s. Initial control messages during a telnet setup indicate the capabilities of the two endpoints to each other. After this is completed, the server requests an ID and password for logging in. The user types in his or her ID and password, and this is transferred to the server over the TCP connection. Once authenticated at the server, the connection is opened and data start to flow in both directions.

The File Transfer Protocol (FTP), as the name indicates, is used to download or copy files between remote machines connected via an IP network. FTP is also a client/server protocol. One of the interesting aspects of FTP is that it uses separate TCP connections to send control messages and to send data. The default port assigned by IANA for FTP on an FTP server is 21. FTP also uses TCP as the underlying transport protocol. A file is transmitted as a stream of bytes, and the closing of the TCP connection setup indicates the completion of the transmission. The control connection persists across multiple file transfers. But every file transfer requires a separate data connection. FTP is in the process of being enhanced in the IETF by a working group known as FTPext.