Discussion of Telnet Protocol(4)

　　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) ---->

　　X'88'X'A1'

　　Using Site to Serving Site <----

　　X'88'X'88'

　　----------->

　　ENTER code...8 bit binary bytes...

　　Using Site TELNET to Terminal |

　　|

　　V

　　Enter code...8 bit binary bytes...

　　Terminal

　　Turn printer off, feed transparently to special device, look　for LEAVE signal

　　------------>

　　8 bit binary bytes...LEAVE signal...single DLE

　　X'A0'

　　<-----------

X'88'X'88

------------>　　X'88'X'88

　　------------>

　　Message

　　|

　　|

　　V

　　8 bit binary data...LEAVE signal MESSAGE

　　_Terminal_

　　During this sequence of exchanges - at the terminal, feed binary　data to special device until LEAVE signal is sensed, strip off　LEAVE signal, turn on printer and block data path to special　device, print MESSAGE at terminal.

　　There is a special control signal on some terminals that has no　corresponding bit pattern in ASCII, but is transmitted by a special　electrical signal. This control signal is ATTN on a 2741 and BREAK　on a teletype. The ASCII DATA TYPE in TELNET will use the code X'81'　to represent BREAK. (There is a corresponding control signal for use　from serving sites to using sites for reverse break, and it is　assigned the code X'82').

　　Some systems treat the break as an extra code available for use in　conjunction with the data stream. For example, one system uses break　as a special editing code meaning "delete the current line to this　point". In these cases, the code may simply be inserted in the data　stream with no special additional action by the user.

　　Other systems use BREAK or ATTN in a special interrupt fashion, to　mean stop processing the application and give me the supervisor, or　cancel the present job, etc. (Other systems use normal characters　for this purpose, such as "Control C".) In these cases, because of　differences in the ways both serving and using sites operate, it is　necessary to take a route in addition to the normal TELNET data　stream to signal that the special control signal is imbedded in the　data stream.

　　_Examples-Problem_

　　The PDP-10 normally will, when it fills its input buffer, continue　to accept characters from a terminal examining each to see if it　is a control character, then act on it if it is or throw it away　if it is not.