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

Telnet Interrupt Handling Using Out-Of-Band Signaling: The Telnet Synch Function 1 2 3 4 Berkeley Remote ("r") Commands

Enabling Options Using Negotiation

The first stage in Telnet option negotiation is for the client and server to decide whether or not they want to enable a particular option. One of the aspects of Telnet’s symmetry of operation is that either device may choose to initiate the use of an option. The initiating device may either specify that it wants to start using an option, or that it wants the other device to start using it. The responding device may either agree or disagree in either case; an option can only be enabled if both devices agree to its use.

This negotiation is performed using four Telnet protocol commands, as follows:

• WILL: Sent by the initiator to indicate that it wants to start using a particular option. There are two possible replies by the responding device:
  • DO: Sent to indicate agreement that the initiator should use the option; it is then considered enabled.
    
    
  • DONT: Sent to specify that the initiator must not use the option.
  
  
  
• DO: Sent by the initiator to request that the other device start using an option. That device may respond in two ways:
  • WILL: Sent to specify that the responding device will agree to use the option; the option is enabled.
    
    
  • WONT: Sent to tell the initiator that the responder will not use the option requested.

The symmetry of Telnet, combined with the fact that both DO and WILL can be used either to initiate a negotiation or respond to one, make Telnet’s option negotiation potentially complicated. Since either device can initiate negotiation of an option at any time, acknowledgment loops could potentially arise if both devices were to try to enable an option simultaneously, or get into a situation where each kept responding to the other’s replies.

For this reason, the Telnet standard specifies restrictions on when the WILL and DO commands are used. One is that a device may only send a negotiation command to request a change in the status of an option; it cannot send DO or WILL just to confirm or reinforce the current state of the option. Another is that a device receiving a request to start using an option it is already using should not acknowledge it using DO or WILL.

Since an option may only be activated if both devices agree to use it, either may disable the use of an option at any time by sending either of these commands:

• WONT: Sent by a device to indicate that it is going to stop using an option. The other device must respond with DONT as a confirmation.
  
  
• DONT: Sent by a device to indicate that it wants the other device to stop using an option. The other device must respond with WONT.

Telnet Interrupt Handling Using Out-Of-Band Signaling: The Telnet Synch Function 1 2 3 4 Berkeley Remote ("r") Commands