• Suppress Go Ahead
  The original telnet implementation defaulted to "half duplex" operation. This means that data traffic could only go in one direction at a time and specific action is required to indicate the end of traffic in one direction and that traffic may now start in the other direction. [This similar to the use of "roger" and "over" by amateur and CB radio operators.] The specific action is the inclusion of a GA character in the data stream.Modern links normally allow bi-directional operation and the "suppress go ahead" option is enabled.

• echo
  The echo option is enabled, usually by the server, to indicate that the server will echo every character it receives. A combination of "suppress go ahead" and "echo" is called character at a time mode meaning that each character is separately transmitted and echoed.There is an understanding known as kludge line mode which means that if either "suppress go ahead" or "echo" is enabled but not both then telnet operates in line at a time mode meaning that complete lines are assembled at each end and transmitted in one "go".

• linemode
  This option replaces and supersedes the line mode kludge.

• remote flow control
  This option controls where the special flow control effects of Ctrl-S/Ctrl-Q are implemented.Telnet control functionsThe telnet protocol includes a number of control functions. These are initiated in response to conditions detected by the client (usually certain special keys or key combinations) or server. The detected condition causes a special character to be incorporated in the data stream.

• Interrupt Process
  This is used by the client to cause the suspension or termination of the server process. Typically the user types Ctrl-C on the keyboard. An IP (244) character is included in the data stream.

• Abort Output
  This is used to suppress the transmission of remote process output. An AO (238) character is included in the data stream.

• Are You There
  This is used to trigger a visible response from the other end to confirm the operation of the link and the remote process. An AYT (246) character is incorporated in the data stream.

• Erase character
  Sent to the display to tell it to delete the immediately preceding character from the display. An EC (247) character is incorporated in the data stream.

• Erase line
  Causes the deletion of the current line of input. An EL (248) character is incorporated in the data stream.
  
• Data Mark
  Some control functions such as AO and IP require immediate action and this may cause difficulties if data is held in buffers awaiting input requests from a (possibly misbehaving) remote process. To overcome this problem a DM (242) character is sent in a TCP Urgent segment, this tells the receiver to examine the data stream for "interesting" characters such as IP, AO and AYT. This is known as the telnet synch mechanism.A DM not in a TCP Urgent segment has no effect.

The telnet command
On most Unix systems a telnet session can be initiated using the telnet command.

telnet <remote host>
but if the user just types telnet then various options and subcommands are available which can be used to study the behaviour of the session.

Here's an exmaple of a telnet session from scitsc to ccub

bash$ telnet
telnet> toggle options
Will show option processing.
telnet> open ccub
Trying 63.14.239.165 ...
Connected to laynetworks.com.
Escape character is '^]'.
SENT do SUPPRESS GO AHEAD
SENT will TERMINAL TYPE (reply)
RCVD do TERMINAL TYPE (don't reply)
RCVD will SUPPRESS GO AHEAD (don't reply)
RCVD will ECHO (reply)
SENT do ECHO (reply)
RCVD do ECHO (reply)
SENT wont ECHO (reply)
UNIX(r) System V Release 4.0 (ccub)
RCVD dont ECHO (don't reply)
login: Login timed out after 60 seconds
Connection closed by foreign host.