Telnet Protocol specification(5)

　　THE TELNET "SYNCH" SIGNAL

　　Most time-sharing systems provide mechanisms which allow a　terminal user to regain control of a "runaway" process; the IP and　AO functions described above are examples of these mechanisms.

　　Such systems, when used locally, have accessto all of the signals　supplied by the user, whether these are normal characters or　special "out of band" signals such as those supplied by the　teletype "BREAK" key or the IBM 2741 "ATTN" key. This is not　necessarily true when terminals are connected to the system　through the network; the network's flow control mechanisms may　cause such a signal to be buffered elsewhere, for example in the　user's Host.

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

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

　　When several Synchs are sent in rapid succession, the Urgent　notifications may be merged. It is not possible to count Urgents　since the number received will be less than or equal the number　sent. When in normal mode a DM is a no operation, when in urgent　mode it signals the end of the urgent processing (this should　correspond with the end of Urgent pointer indicated by TCP).

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

　　"Interesting" signals are defined to be: the TELNET standard　representations of IP, AO, and AYT (but not EC or EL); the local　analogs of these standard representations (if any); all other　TELNET commands; other site-defined signals which can be acted on　without delaying the scan of the data stream.

　　Since one effect of the SYNCH mechanism is the discarding of　essentially all characters (except TELNET commands) between the　sender of the Synch and its recipient, this mechanism is specified　as the standard way to clear the data path when that is desired.

　　For example, if a user at a terminal causes an AO to be　transmitted, the server which receives the AO (if it provides that　function at all) should return a Synch to the user.

　　Finally, just as the TCP Urgent notification is needed at the　TELNET level as an out-of-band signal, so other protocols which　make use of TELNET may require a TELNET command which can be　viewed as an out-of-band signal at a different level.

　　By convention the sequence [IP, Synch] is to be used as such a　signal. For example, suppose that some other protocol, which uses　TELNET, defines the character string STOP analogously to the　TELNET command AO. Imagine that a user of this protocol wishes a　server to process the STOP string, but the connection is blocked　because the server is processing other commands. The user should　instruct his system to:

　　1. Send the TELNET IP character;

　　2. Send the TELNET SYNC sequence, that is:　Send the Data Mark (DM) as the only character　in a TCP urgent mode send operation.

　　3. Send the character string STOP; and

　　4. Send the other protocol's analog of the TELNET DM, if any.

　　The user (or process acting on his behalf) must transmit the　TELNET SYNCH sequence of step 2 above to ensure that the TELNET IP　gets through to the server's TELNET interpreter.

　　The Urgent should wake up the TELNET process, the IP should　wake up the next higher level process.

　　THE NVT PRINTER AND KEYBOARD

　　The NVT printer has an unspecified carriage width and page length　and can produce representations of all 95 USASCII graphics (codes　32 through 126). Of the 33 USASCII control codes (0 through 31　and 127), and the 128 uncovered codes (128 through 255), the　following have specified meaning to the NVT printer: