Telnet Protocols(6)

　　A similar problem could occur with Multics or some IBM system as a　line at a time server. The user at a using site gets his process at　the serving site into an output loop and wants to break the process　without having to release his Telnet connection. The buffers clog　the connection, transmission is choked off, and the , Break, or　other user control signal gets stuck in the pipeline.

　　Example -- Solution:

　　The user at the using site knows he is entering a special control　signal (Break, Attn, , etc.) and follows it with a SYNCH.

　　(The local instructions at using sites for accomplishing this may　differ from site to site.)

　　User to Using Site Telnet

　　Send SYNCH.

　　Using Site Telnet to Serving Site Telnet:

　　DATA MARK in Data Stream.

　　Using Site Telnet to Using Site NCP:

　　Send an INS.

　　Serving Site NCP to Telnet Server:

　　Interrupt "INS received".

　　Serving Site Telnet:

　　Examines the input data stream (looking for special control　signals) until it sees DATA MARK then resumes normal　handling.

　　Thus, depending on the server's local implementation to　provide adequate service, a special handling of the data　stream can be invoked whenever an INS is received in order　to get to the special character. When it sees DATA MARK, it　recognizes it as a synchronization point and knowing that　the special character has been passed on, strips the DATA　MARK from the data stream and returns to normal mode.

　　If the DATA MARK arrives before the INS, the serving site　should not process the data stream further until an INS is　received.

　　This approach to handling selected special characters or signals　relieves the using Telnet processes from having to recognize the　special serving site characters, as well as from having to know how　the serving site wants to handle them. At the same time, the　procedure requires only a minimum level of user understanding of the　serving site. This seems appropriate, since the Telnet ASCII　conventions are providing a Network Virtual Terminal, not a Network　Virtual User.

　　Notice that the correct order is (1) special character or signal　(e.g. BREAK or ), then (2) SYNCH.

　　User Telnet Signals

　　The ability of the user to cause the using site Telnet to send any　combination of ASCII characters in a string, and only that　combination, is viewed as important to the user utility of the Telnet　ASCII conventions. Because of this, some user sites may find it　necessary to provide special local Telnet signals from the human user　to the using site Telnet.

　　Example:

　　A user on a line at a time system (Multics, System 360, GCOS,　etx.), which require an end of line signal before processing the　user's input, is working through the Network on a serving site　that operates a character at a time. The application is a　debugging aid that permits the user to type in "location=" to　which it will respond with n where n represents the current　contents of that location. The serving site process does not　expect to see the "location=" followed by a carriage return line　feed sequence. The user at the using site should be able to type　in the location, follow it with a signal (to the user Telnet) to　suppress the end of line convention, followed by the end of line　signal, and expect the "location=" to be transmitted immediately　without an end of line sequence being transmitted to the server.

　　Example:

　　In another case, a using site has decided that it is convenient to　accumulate four characters at a time and transmit them to the　serving site, unless an end of line signal is observed, in which　case the end of line sequence is sent preceded by whatever number　of characters have been accumulated (presumably three or less).