High-level services allow users and programs to interact with automated services on remote machines and with remote users. This section describes programs that include Rlogin (Remote login) and TELNET (TErminaL NETwork).
TELNET is a simple remote terminal protocol that allows a user to log on to a computer across an Internet. TELNET establishes a TCP connection, and then passes keystrokes from the user’s keyboard directly to the remote computer as if they had been typed on a keyboard attached to the remote machine.
TELNET also carries output from the remote machine back to the user’s screen. The service is called transparent because it looks as if the user’s keyboard and display attach directly to the remote machine. TELNET client software allows the user to specify a remote machine either by giving its domain name or IP address.
TELNET offers three basic services. First, it defines a network virtual terminal that provides a standard interface to remote systems. Second, TELNET includes a mechanism that allows the client and server to negotiate options. Finally, TELNET treats both ends of the connection symmetrically.
Remote Login (Rlogin)
Rlogin was designed for remote login only between UNIX hosts. This makes it a simpler protocol than TELNET because option negotiation is not required when the operating system on the client and server are known in advance. Over the past few years, Rlogin has also ported to several non-UNIX environments.
RFC 1282 specifies the Rlogin protocol. When a user wants to access an application program or utility located on a remote machine, the user performs remote login. The user sends the keystrokes to the terminal driver where the local operating system accepts the characters but does not interpret them.
The characters are sent to the TELNET client, which transforms the characters into a universal character set called Network Virtual Terminal (NVT) characters and delivers them to the local TCP/IP stack. The commands or text (in NVT form) travel through the Internet and arrive at the TCP/IP stack at the remote machine.
Here the characters are delivered to the operating system and passed to the TELNET server, which changes the characters to the corresponding characters understandable by the remote computer.