Discussion of Telnet Protocol(3)

　　The convention of using "I ECHO", "YOU ECHO" seems to fill both the　requirements for dynamic echo control and for a minimum　implementation of TELNET Protocol. [3] An agreed-upon exchange to　pass echo control (i.e., two sites exchange the I ECHO/YOU ECHO　codes) results in passing the control from one site to the other.

　　Example: A serving site is exchanging control information with　the USER in an area where the serving system asks for pass word　and wants to suppress the printing of the pass word at the using　site's user terminal. (In this case, the using site has the　ability to control the print capability at the user's terminal.)

　　Using site has been echoing to the user's terminal.

　　Serving Site to Using Site (--->)

　　I ECHO

　　Using Site to Serving Site (<---)

　　YOU ECHO

　　--->Pass word:

　　<--- (User enters password at terminal)

--->　　---> (No echo sent)

　　---> YOU ECHO

　　<--- I ECHO

After the exchange, the original normal condition is re-established. If the using site did not have dynamic echo controlnstalled in its TELNET implementation, the serving site would have signaled I ECHO several times, received no response, and assumed that the using site could not comply proceeding to call for the pass word without the normal protection of inhibiting print.

TELNET control signals are of two types: one that results in transmission of signals down the network to a receiving site; the other intended for the user/process site only. The latter type will be discussed later. So far, we have discussed the former type, specifically dealing with echo control.

The use of ESC should not be considered a TELNET-wide standard, but a convention limited to the 7 bit ASCII mode of transmission. Other conventions, to be incorporated later, may include binary transmission, EBCDIC, etc. Presumably, each will have its own convention for an escape character to extend its code set.

Since it is expected that conventions other than ASCII will be implemented under TELNET, a code to indicate a DATA TYPE representing each set of conventions will be employed. The control code X'AO' has been selected to represent the ASCII convention in TELNET. Since a number of applications may wish to transmit transparently (i.e., 8 bit binary data), X'Al' is being reserved for that purpose. The TELNET control code X'A2' is reserved for an expected set of EBCDIC conventions. The DATA TYPE is expected as the first byte of data over a TELNET connection. Minimum implementations will be aided by providing a default. That is, if the first byte over a connection has the high order bit set as zero, then the transmission has begun in ASCII mode.

Each set of conventions, i.e., each DATA TYPE will be expected to have a convention for that DATA TYPE to signal that it is returning to control mode. This return may be for the purpose of making use of an existing control codes or to change data type. X'88' is used [4].

Example: At the using site, a terminal has a special device on it (e.g., plotter, laboratory instrument, control box, etc.) that is controlled by binary code in 8 bit bytes. The terminal uses a special "enter" code that routes signals to the device and cuts off printing at the terminal until a special "leave" signal is received from the driving process. The driving process in this case is at a remote serving site. It is assumed in this example that a DLE convention is used for transparent transmission, a single DLE signal representing return to control. Normal transmission has been in ASCII.

Driving Process (at Serving Site) to Using Site) ---->　　After the exchange, the original normal condition is re-established. If the using site did not have dynamic echo control　installed in its TELNET implementation, the serving site would　have signaled I ECHO several times, received no response, and　assumed that the using site could not comply proceeding to call　for the pass word without the normal protection of inhibiting　print.

　　TELNET control signals are of two types: one that results in　transmission of signals down the network to a receiving site; the　other intended for the user/process site only. The latter type will　be discussed later. So far, we have discussed the former type,　specifically dealing with echo control.