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The TCP/IP Guide

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Table Of Contents  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

Previous Topic/Section
Telnet Connections and Client/Server Operation
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Pages in Current Topic/Section
123
4
Next Page
Telnet Protocol Commands
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Telnet Communications Model and the Network Virtual Terminal (NVT)
(Page 4 of 4)

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.

Key Concept: The Telnet NVT format is based on 7-bit US ASCII, with each byte carrying one character. The standard specifies that devices must handle all standard printable ASCII characters, as well as three mandatory control characters. Two of these are the carriage return (“CR”) and line feed (“LF”) characters; when combined, these define the logical end of a line of text. The Telnet standard also describes the interpretation of five other optional ASCII control characters.


Half-Duplex and Full-Duplex Modes

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.


Previous Topic/Section
Telnet Connections and Client/Server Operation
Previous Page
Pages in Current Topic/Section
123
4
Next Page
Telnet Protocol Commands
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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