Technical Information

Telnet works by emulating (imitating) an idealized ASCII data terminal at each end of the communications link. A Telnet program is the terminal emulation software you use to log in to an Internet host. The host has similar Telnet software.

The TELNET Protocol is built upon three main ideas: first, the concept of a "Network Virtual Terminal"; second, the principle of negotiated options; and third, a symmetric view of terminals and processes. (RFC 1080)

1. The "Network Virtual Terminal"

   

   
   

   Communications through telnet is not done directly terminal to terminal. Instead a virtual terminal is created which provises a standardised interface for both parties to interact. Firstly a computer requests a connection. This computer becomes a "user" and than data or services are receved from the provider of that information. This is all done in realtime transmitting messages via a bi-directional character device. This allows a multitude of applications to be run simultainously. All of this is done through the virtual terminal eliminating the need to store information of commands on the local computer. (RFC 854 )

2. Negotiated options

   

   The network virtual terminal provides only a minimum set of functionality. Many hosts would like to provide additional services. For example, in some cases it would be useful to turn remote character echo off (to save transmission bandwidth). In general terms, an option is any service or function the Network Virtual Terminal does not perform by default.
   There are 255 possible option codes. However, if there's ever a 256th option, the designers planned ahead by reserving the last code for subnegotiating in an expanded code space (RFC 861). There's also an option for reporting which options are activated, allowed, or prohibited on the current connection (RFC 859). Below are some basic negotiation messages.

   Sender Sent	Receiver Responds	Implication
   WILL	DO	The sender would like to use a certain facility if the receiver can handle it. Option is now in effect
   WILL	DONT	Receiver says it cannot support the option. Option is not in effect.
   DO	WILL	The sender says it can handle traffic from the sender if the sender wishes to use a certain option. Option is now in effect.
   DO	WONT	Receiver says it cannot support the option. Option is not in effect.
   WONT	DONT	Option disabled. DONT is only valid response.
   DONT	WONT	Option disabled. WONT is only valid response.

   
   

3. symmetric view of terminals and processes.

   This symmetry is a convention in Telnet to minimise the possibility of errors in the communications. It defines rules where by requests are recieved and translated. This occurs in such a way that it can connect users as well as programs in such a way that the proceses seem to occur simultaniously. This allows Telnet to be used for many different applications such as, video games, chat, sending files, and many other realtime opperations.