Friday, October 1, 2010

TELNET: Terminal emulation protocol of TCP/IP

Posted by OurTech Team | Friday, October 1, 2010 | Category: |

TELNET is the terminal emulation protocol in TCP/IP environment. TELNET uses the TCP as the transport protocol to establish connection between server and client. After connecting, TELNET server and client enter a phase of option negotiation that determines the options that each side can support for the connection. Each connected system can negotiate new options or renegotiate old options at any time. In general, each end of the TELNET connection attempts to implement all options that maximize performance for the systems involved.

When a TELNET connection is first established, each end is assumed to originate and terminate at a "Network Virtual Terminal", or NVT. An NVT is an imaginary device which provides a standard, network-wide, intermediate representation of a canonical terminal. This eliminates the need for "server" and "user" hosts to keep information about the characteristics of each other's terminals and terminal handling conventions.
The principle of negotiated options takes cognizance of the fact that many hosts will wish to provide additional services over and above those available within an NVT, and many users will have sophisticated terminals and would like to have elegant, rather than minimal, services.

Option requests are likely to flurry back and forth when a TELNET connection is first established, as each party attempts to get the best possible service from the other party. Beyond that, however, options can be used to dynamically modify the characteristics of the connection to suit changing local conditions.
Modern Telnet is a versatile terminal emulation due to the many options that have evolved over the past twenty years. Options give TELNET the ability to transfer binary data, support byte macros, emulate graphics terminals, and convey information to support centralized terminal management.
 Protocol Structure - TELNET (Terminal emulation protocol of TCP/IP)TELNET commands are ASCII text. The following are the TELNET commands:

Commands Code No.
Dec Hex
data     All terminal input/output data.
End subNeg 240 FO End of option subnegotiation command.
No Operation 241 F1 No operation command.
Data Mark 242 F2 End of urgent data stream.
Break 243 F3 Operator pressed the Break key or the Attention key.
Int process 244 F4 Interrupt current process.
Abort output 245 F5 Cancel output from current process.
You there? 246 F6 Request acknowledgment.
Erase char 247 F7 Request that operator erase the previous character.
Erase line 248 F8 Request that operator erase the previous line.
Go ahead! 249 F9 End of input for half-duplex connections.
SubNegotiate 250 FA Begin option subnegotiation.
Will Use 251 FB Agreement to use the specified option.
Won't Use 252 FC Reject the proposed option.
Start use 253 FD Request to start using specified option.
Stop Use 254 FE Demand to stop using specified option.
IAC 255 FF Interpret as command. 

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