Telnet Communications Model and the Network Virtual Terminal (NVT)
(Page 4 of 4)
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.
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