Telnet Protocol
The Telnet Protocol (TELNET) provides a standard method for terminal devices and terminal-oriented processes to interface.
TELNET is commonly used by terminal emulation programs that allow you to log into a remote host. However, TELNET can also be used for terminal-to-terminal communication and interprocess communication. TELNET is also used by other protocols (for example, FTP) for establishing a protocol control channel.
TCP/IP implements TELNET in the tn, telnet, or tn3270 user commands. The telnetd daemon does not provide an API to TELNET.
TCP/IP supports the following TELNET options which are negotiated between the client and server:
| Item | Description |
|---|---|
| BINARY TRANSMISSION (Used in tn3270 sessions) | Transmits characters as binary data. |
| SUPPRESS GO_AHEAD (The operating system suppresses GO-AHEAD options.) | Indicates that when in effect on a connection between a sender of data and the receiver of the data, the sender need not transmit a GO_AHEAD option. If the GO_AHEAD option is not desired, the parties in the connection will probably suppress it in both directions. This action must take place in both directions independently. |
| TIMING MARK (Recognized, but has a negative response) | Makes sure that previously transmitted data has been completely processed. |
| EXTENDED OPTIONS LIST | Extends the TELNET option list for another 256 options. Without this option, the TELNET option allows only 256 options. |
| ECHO (User-changeable command) | Transmits echo data characters already received back to the original sender. |
| TERM TYPE | Enables the server to determine the type of terminal connected to a user TELNET program. |
| SAK (Secure Attention Key) | Establishes the environment necessary for secure communication between you and the system. |
| NAWS (Negotiate About Window Size) | Enables client and server to negotiate dynamically for the window size. This is used by applications that support changing the window size. |
Note: TELNET must allow transmission of eight
bit characters when not in binary mode in order to implement ISO 8859
Latin code page.