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| 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. |
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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 | Description | |
| data | All terminal input/output data. | ||
| End subNeg | 240 | F0 | 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 | 248 | 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 |
OSI Model || TCP || UDP || RARP || IP || TELNET ||SNMP || SMTP || FTP || ICMP || IGMP || ARP
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