The protocol uses several commands that control the method and various details of the interaction between the client and the server. These commands can be either mandatory commands or extensions. During the session initialization the client and the server negotiates the connection parameters with these commands. Sub-negotiation is a process during the protocol which is for exchanging extra parameters of a command (e.g.: sending the window size). The commands of the protocol are:
| Request/Response | Description |
|---|---|
| SE | End of sub-negotiation parameters. |
| NOP | No operation. |
| DM | Data mark - Indicates the position of Sync event within the data stream. |
| BRK | Break - Indicates that a break or attention key was hit. |
| IP | Suspend, interrupt or abort the process. |
| AO | Abort output - Run a command without sending the output back to the client. |
| AYT | Are you there - Request a visible evidence that the AYT command has been received. |
| EC | Erase character - Delete the character last received from the stream. |
| EL | Erase line - Erase a line without a CRLF. |
| GA | Go Ahead - Instruct the other machine to start the transmission. |
| SB | Sub-negotiation starts here. |
| WILL | Will (option code) - Indicates the desire to begin performing the indicated option, or confirms that it is being performed. |
| WONT | Will not (option code) - Indicates the refusal to perform, or continue performing, the indicated option. |
| DO | Do (option code) - Indicates the request that the other party perform, or confirmation that the other party is expected to perform, the indicated option. |
| DONT | Do not (option code) - Indicates the request that the other party stop performing the indicated option, or confirmation that its performing is no longer expected. |
| IAC | Interpret as command. |
Table 4.25. Telnet protocol commands
Published on June 04, 2020
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