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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP Interactive and Remote Application Protocols                 9  Telnet Protocol

Telnet Connections and Client/Server Operation 1 2 3 4 Telnet Protocol Commands

End-of-Line Representations

The Telnet NVT scheme defines the combination of the carriage return (“CR”) and line feed (“LF”) characters to represent the end of a line of ASCII text. The literal meaning of these two characters is “return to the left margin” (from the “CR”) and “go to the next line” (from the “LF”). However, NVT treats the “CR+LF” sequence as more than just two independent characters; they are taken collectively to define a logical “end of line” character. This is necessary because not all terminal types define an end of line using both “CR” and “LF”. Translation of end-of-line characters between the native and NVT formats is one of the functions that Telnet client and server software must perform to ensure compatibility between terminals and hosts.

Another artifact of the age of Telnet is that for maximum compatibility, the Network Virtual Terminal specification is designed under the assumption of half-duplex operation: only one device can transmit at a time. A device that is sending data is supposed to end its transmission with the special Telnet Go Ahead command, telling the other device that it may now transmit (the next topic describes Telnet protocol commands). This is similar to how people using walkie-talkies end each transmission with “over” to tell their partner that they may now respond.

Of course, modern networks operate in a full-duplex mode, and using half-duplex communication would be needlessly inefficient. In most cases the Telnet client and server agree to use an option (Suppress Go Ahead) that eliminates the need to send this command. However, having this as the default is a good example of how NVT acts as a “least common denominator” in Telnet, in case the simpler operating mode is needed by either device.

Telnet Connections and Client/Server Operation 1 2 3 4 Telnet Protocol Commands