Telnet Protocol Specification(5)

Interrupt Process (IP)

Many systems provide a function which suspends, interrupts,aborts, or terminates the operation of a user process. This function is frequently used when a user believes his process isin an unending loop, or when an unwanted process has beeninadvertently activated. IP is the standard representation forinvoking this function. It should be noted by implementersthat IP may be required by other protocols which use TELNET,and therefore should be implemented if these other protocolsare to be supported.

Abort Output (AO)

Many systems provide a function which allows a process, whichis generating output, to run to completion (or to reach thesame stopping point it would reach if running to completion)but without sending the output to the user's terminal.
Further, this function typically clears any output alreadyproduced but not yet actually printed (or displayed) on theuser's terminal. AO is the standard representation forinvoking this function. For example, some subsystem mightnormally accept a user's command, send a long text string tothe user's terminal in response, and finally signal readinessto accept the next command by sending a "prompt" character(preceded by <CR><LF>) to the user's terminal. If the AO werereceived during the transmission of the text string, areasonable implementation would be to suppress the remainder ofthe text string, but transmit the prompt character and thepreceding <CR><LF>. (This is possibly in distinction to theaction which might be taken if an IP were received; the IPmight cause suppression of the text string and an exit from thesubsystem.)

It should be noted, by server systems which provide thisfunction, that there may be buffers external to the system (inRFC854 May 1983the network and the user's local host) which should be cleared;the appropriate way to do this is to transmit the "Synch"signal (described below) to the user system.

Are You There (AYT)

Many systems provide a function which provides the user withsome visible (e.g., printable) evidence that the system isstill up and running. This function may be invoked by the user
when the system is unexpectedly "silent" for a long time,because of the unanticipated (by the user) length of acomputation, an unusually heavy system load, etc. AYT is thestandard representation for invoking this function.

Erase Character (EC)

Many systems provide a function which deletes the lastpreceding undeleted character or "print position"* from thestream of data being supplied by the user. This function istypically used to edit keyboard input when typing mistakes aremade. EC is the standard representation for invoking thisfunction.

*NOTE: A "print position" may contain several characterswhich are the result of overstrikes, or of sequences such as<char1> BS <char2>...

Erase Line (EL)

Many systems provide a function which deletes all the data inthe current "line" of input. This function is typically usedto edit keyboard input. EL is the standard representation for
invoking this function.

THE TELNET "SYNCH" SIGNAL

Most time-sharing systems provide mechanisms which allow aterminal user to regain control of a "runaway" process; the IP andAO functions described above are examples of thesemechanisms.
Such systems, when used locally, have access to all of the signalssupplied by the user, whether these are normal characters orspecial "out of band" signals such as those supplied by theteletype "BREAK" key or the IBM 2741 "ATTN" key. This is notnecessarily true when terminals are connected to the systemthrough the network; the network's flow control echanisms maycause such a signal to be buffered elsewhere, for example in theuser's host.

RFC854 May 1983

To counter this problem, the TELNET "Synch" mechanism isintroduced. A Synch signal consists of a TCP Urgent notification,coupled with the TELNET command DATA MARK. The Urgentnotification, which is not subject to the flow control pertainingto the TELNET connection, is used to invoke special handling ofthe data stream by the process which receives it. In this mode,the data stream is immediately scanned for "interesting" signalsas defined below, discarding intervening data. The TELNET commandDATA MARK (DM) is thesynchronizing mark in the data stream whichindicates that any special signal has already occurred and therecipient can return to normal processing of the data stream.

The Synch is sent via the TCP send operation with the Urgentflag set and the DM as the last (or only) data octet.

When several Synchs are sent in rapid succession, the Urgentnotifications may be merged. It is not possible to count Urgentssince the number received will be less than or equal the numbersent. When in normal mode, a DM is a no operation; when in urgentmode, it signals the end of the urgent processing.

If TCP indicates the end of Urgent data before the DM is found,TELNET should continue the special handling of the data streamuntil the DM is found.

If TCP indicates more Urgent data after the DM is found, it canonly be because of a subsequent Synch. TELNET should continuethe special handling of the data stream until another DM isfound.