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1
2Network Working Group J. Postel
3Request for Comments: 854 J. Reynolds
4 ISI
5Obsoletes: NIC 18639 May 1983
6
7 TELNET PROTOCOL SPECIFICATION
8
9
10This RFC specifies a standard for the ARPA Internet community. Hosts on
11the ARPA Internet are expected to adopt and implement this standard.
12
13INTRODUCTION
14
15 The purpose of the TELNET Protocol is to provide a fairly general,
16 bi-directional, eight-bit byte oriented communications facility. Its
17 primary goal is to allow a standard method of interfacing terminal
18 devices and terminal-oriented processes to each other. It is
19 envisioned that the protocol may also be used for terminal-terminal
20 communication ("linking") and process-process communication
21 (distributed computation).
22
23GENERAL CONSIDERATIONS
24
25 A TELNET connection is a Transmission Control Protocol (TCP)
26 connection used to transmit data with interspersed TELNET control
27 information.
28
29 The TELNET Protocol is built upon three main ideas: first, the
30 concept of a "Network Virtual Terminal"; second, the principle of
31 negotiated options; and third, a symmetric view of terminals and
32 processes.
33
34 1. When a TELNET connection is first established, each end is
35 assumed to originate and terminate at a "Network Virtual Terminal",
36 or NVT. An NVT is an imaginary device which provides a standard,
37 network-wide, intermediate representation of a canonical terminal.
38 This eliminates the need for "server" and "user" hosts to keep
39 information about the characteristics of each other's terminals and
40 terminal handling conventions. All hosts, both user and server, map
41 their local device characteristics and conventions so as to appear to
42 be dealing with an NVT over the network, and each can assume a
43 similar mapping by the other party. The NVT is intended to strike a
44 balance between being overly restricted (not providing hosts a rich
45 enough vocabulary for mapping into their local character sets), and
46 being overly inclusive (penalizing users with modest terminals).
47
48 NOTE: The "user" host is the host to which the physical terminal
49 is normally attached, and the "server" host is the host which is
50 normally providing some service. As an alternate point of view,
51
52
53
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55Postel & Reynolds [Page 1]
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59RFC 854 May 1983
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61
62 applicable even in terminal-to-terminal or process-to-process
63 communications, the "user" host is the host which initiated the
64 communication.
65
66 2. The principle of negotiated options takes cognizance of the fact
67 that many hosts will wish to provide additional services over and
68 above those available within an NVT, and many users will have
69 sophisticated terminals and would like to have elegant, rather than
70 minimal, services. Independent of, but structured within the TELNET
71 Protocol are various "options" that will be sanctioned and may be
72 used with the "DO, DON'T, WILL, WON'T" structure (discussed below) to
73 allow a user and server to agree to use a more elaborate (or perhaps
74 just different) set of conventions for their TELNET connection. Such
75 options could include changing the character set, the echo mode, etc.
76
77 The basic strategy for setting up the use of options is to have
78 either party (or both) initiate a request that some option take
79 effect. The other party may then either accept or reject the
80 request. If the request is accepted the option immediately takes
81 effect; if it is rejected the associated aspect of the connection
82 remains as specified for an NVT. Clearly, a party may always refuse
83 a request to enable, and must never refuse a request to disable some
84 option since all parties must be prepared to support the NVT.
85
86 The syntax of option negotiation has been set up so that if both
87 parties request an option simultaneously, each will see the other's
88 request as the positive acknowledgment of its own.
89
90 3. The symmetry of the negotiation syntax can potentially lead to
91 nonterminating acknowledgment loops -- each party seeing the incoming
92 commands not as acknowledgments but as new requests which must be
93 acknowledged. To prevent such loops, the following rules prevail:
94
95 a. Parties may only request a change in option status; i.e., a
96 party may not send out a "request" merely to announce what mode it
97 is in.
98
99 b. If a party receives what appears to be a request to enter some
100 mode it is already in, the request should not be acknowledged.
101 This non-response is essential to prevent endless loops in the
102 negotiation. It is required that a response be sent to requests
103 for a change of mode -- even if the mode is not changed.
104
105 c. Whenever one party sends an option command to a second party,
106 whether as a request or an acknowledgment, and use of the option
107 will have any effect on the processing of the data being sent from
108 the first party to the second, then the command must be inserted
109 in the data stream at the point where it is desired that it take
110
111
112Postel & Reynolds [Page 2]
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116RFC 854 May 1983
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118
119 effect. (It should be noted that some time will elapse between
120 the transmission of a request and the receipt of an
121 acknowledgment, which may be negative. Thus, a host may wish to
122 buffer data, after requesting an option, until it learns whether
123 the request is accepted or rejected, in order to hide the
124 "uncertainty period" from the user.)
125
126 Option requests are likely to flurry back and forth when a TELNET
127 connection is first established, as each party attempts to get the
128 best possible service from the other party. Beyond that, however,
129 options can be used to dynamically modify the characteristics of the
130 connection to suit changing local conditions. For example, the NVT,
131 as will be explained later, uses a transmission discipline well
132 suited to the many "line at a time" applications such as BASIC, but
133 poorly suited to the many "character at a time" applications such as
134 NLS. A server might elect to devote the extra processor overhead
135 required for a "character at a time" discipline when it was suitable
136 for the local process and would negotiate an appropriate option.
137 However, rather than then being permanently burdened with the extra
138 processing overhead, it could switch (i.e., negotiate) back to NVT
139 when the detailed control was no longer necessary.
140
141 It is possible for requests initiated by processes to stimulate a
142 nonterminating request loop if the process responds to a rejection by
143 merely re-requesting the option. To prevent such loops from
144 occurring, rejected requests should not be repeated until something
145 changes. Operationally, this can mean the process is running a
146 different program, or the user has given another command, or whatever
147 makes sense in the context of the given process and the given option.
148 A good rule of thumb is that a re-request should only occur as a
149 result of subsequent information from the other end of the connection
150 or when demanded by local human intervention.
151
152 Option designers should not feel constrained by the somewhat limited
153 syntax available for option negotiation. The intent of the simple
154 syntax is to make it easy to have options -- since it is
155 correspondingly easy to profess ignorance about them. If some
156 particular option requires a richer negotiation structure than
157 possible within "DO, DON'T, WILL, WON'T", the proper tack is to use
158 "DO, DON'T, WILL, WON'T" to establish that both parties understand
159 the option, and once this is accomplished a more exotic syntax can be
160 used freely. For example, a party might send a request to alter
161 (establish) line length. If it is accepted, then a different syntax
162 can be used for actually negotiating the line length -- such a
163 "sub-negotiation" might include fields for minimum allowable, maximum
164 allowable and desired line lengths. The important concept is that
165
166
167
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169Postel & Reynolds [Page 3]
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173RFC 854 May 1983
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175
176 such expanded negotiations should never begin until some prior
177 (standard) negotiation has established that both parties are capable
178 of parsing the expanded syntax.
179
180 In summary, WILL XXX is sent, by either party, to indicate that
181 party's desire (offer) to begin performing option XXX, DO XXX and
182 DON'T XXX being its positive and negative acknowledgments; similarly,
183 DO XXX is sent to indicate a desire (request) that the other party
184 (i.e., the recipient of the DO) begin performing option XXX, WILL XXX
185 and WON'T XXX being the positive and negative acknowledgments. Since
186 the NVT is what is left when no options are enabled, the DON'T and
187 WON'T responses are guaranteed to leave the connection in a state
188 which both ends can handle. Thus, all hosts may implement their
189 TELNET processes to be totally unaware of options that are not
190 supported, simply returning a rejection to (i.e., refusing) any
191 option request that cannot be understood.
192
193 As much as possible, the TELNET protocol has been made server-user
194 symmetrical so that it easily and naturally covers the user-user
195 (linking) and server-server (cooperating processes) cases. It is
196 hoped, but not absolutely required, that options will further this
197 intent. In any case, it is explicitly acknowledged that symmetry is
198 an operating principle rather than an ironclad rule.
199
200 A companion document, "TELNET Option Specifications," should be
201 consulted for information about the procedure for establishing new
202 options.
203
204THE NETWORK VIRTUAL TERMINAL
205
206 The Network Virtual Terminal (NVT) is a bi-directional character
207 device. The NVT has a printer and a keyboard. The printer responds
208 to incoming data and the keyboard produces outgoing data which is
209 sent over the TELNET connection and, if "echoes" are desired, to the
210 NVT's printer as well. "Echoes" will not be expected to traverse the
211 network (although options exist to enable a "remote" echoing mode of
212 operation, no host is required to implement this option). The code
213 set is seven-bit USASCII in an eight-bit field, except as modified
214 herein. Any code conversion and timing considerations are local
215 problems and do not affect the NVT.
216
217 TRANSMISSION OF DATA
218
219 Although a TELNET connection through the network is intrinsically
220 full duplex, the NVT is to be viewed as a half-duplex device
221 operating in a line-buffered mode. That is, unless and until
222
223
224
225
226Postel & Reynolds [Page 4]
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228
229
230RFC 854 May 1983
231
232
233 options are negotiated to the contrary, the following default
234 conditions pertain to the transmission of data over the TELNET
235 connection:
236
237 1) Insofar as the availability of local buffer space permits,
238 data should be accumulated in the host where it is generated
239 until a complete line of data is ready for transmission, or
240 until some locally-defined explicit signal to transmit occurs.
241 This signal could be generated either by a process or by a
242 human user.
243
244 The motivation for this rule is the high cost, to some hosts,
245 of processing network input interrupts, coupled with the
246 default NVT specification that "echoes" do not traverse the
247 network. Thus, it is reasonable to buffer some amount of data
248 at its source. Many systems take some processing action at the
249 end of each input line (even line printers or card punches
250 frequently tend to work this way), so the transmission should
251 be triggered at the end of a line. On the other hand, a user
252 or process may sometimes find it necessary or desirable to
253 provide data which does not terminate at the end of a line;
254 therefore implementers are cautioned to provide methods of
255 locally signaling that all buffered data should be transmitted
256 immediately.
257
258 2) When a process has completed sending data to an NVT printer
259 and has no queued input from the NVT keyboard for further
260 processing (i.e., when a process at one end of a TELNET
261 connection cannot proceed without input from the other end),
262 the process must transmit the TELNET Go Ahead (GA) command.
263
264 This rule is not intended to require that the TELNET GA command
265 be sent from a terminal at the end of each line, since server
266 hosts do not normally require a special signal (in addition to
267 end-of-line or other locally-defined characters) in order to
268 commence processing. Rather, the TELNET GA is designed to help
269 a user's local host operate a physically half duplex terminal
270 which has a "lockable" keyboard such as the IBM 2741. A
271 description of this type of terminal may help to explain the
272 proper use of the GA command.
273
274 The terminal-computer connection is always under control of
275 either the user or the computer. Neither can unilaterally
276 seize control from the other; rather the controlling end must
277 relinguish its control explicitly. At the terminal end, the
278 hardware is constructed so as to relinquish control each time
279 that a "line" is terminated (i.e., when the "New Line" key is
280 typed by the user). When this occurs, the attached (local)
281
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283Postel & Reynolds [Page 5]
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287RFC 854 May 1983
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289
290 computer processes the input data, decides if output should be
291 generated, and if not returns control to the terminal. If
292 output should be generated, control is retained by the computer
293 until all output has been transmitted.
294
295 The difficulties of using this type of terminal through the
296 network should be obvious. The "local" computer is no longer
297 able to decide whether to retain control after seeing an
298 end-of-line signal or not; this decision can only be made by
299 the "remote" computer which is processing the data. Therefore,
300 the TELNET GA command provides a mechanism whereby the "remote"
301 (server) computer can signal the "local" (user) computer that
302 it is time to pass control to the user of the terminal. It
303 should be transmitted at those times, and only at those times,
304 when the user should be given control of the terminal. Note
305 that premature transmission of the GA command may result in the
306 blocking of output, since the user is likely to assume that the
307 transmitting system has paused, and therefore he will fail to
308 turn the line around manually.
309
310 The foregoing, of course, does not apply to the user-to-server
311 direction of communication. In this direction, GAs may be sent at
312 any time, but need not ever be sent. Also, if the TELNET
313 connection is being used for process-to-process communication, GAs
314 need not be sent in either direction. Finally, for
315 terminal-to-terminal communication, GAs may be required in
316 neither, one, or both directions. If a host plans to support
317 terminal-to-terminal communication it is suggested that the host
318 provide the user with a means of manually signaling that it is
319 time for a GA to be sent over the TELNET connection; this,
320 however, is not a requirement on the implementer of a TELNET
321 process.
322
323 Note that the symmetry of the TELNET model requires that there is
324 an NVT at each end of the TELNET connection, at least
325 conceptually.
326
327 STANDARD REPRESENTATION OF CONTROL FUNCTIONS
328
329 As stated in the Introduction to this document, the primary goal
330 of the TELNET protocol is the provision of a standard interfacing
331 of terminal devices and terminal-oriented processes through the
332 network. Early experiences with this type of interconnection have
333 shown that certain functions are implemented by most servers, but
334 that the methods of invoking these functions differ widely. For a
335 human user who interacts with several server systems, these
336 differences are highly frustrating. TELNET, therefore, defines a
337 standard representation for five of these functions, as described
338
339
340Postel & Reynolds [Page 6]
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344RFC 854 May 1983
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346
347 below. These standard representations have standard, but not
348 required, meanings (with the exception that the Interrupt Process
349 (IP) function may be required by other protocols which use
350 TELNET); that is, a system which does not provide the function to
351 local users need not provide it to network users and may treat the
352 standard representation for the function as a No-operation. On
353 the other hand, a system which does provide the function to a
354 local user is obliged to provide the same function to a network
355 user who transmits the standard representation for the function.
356
357 Interrupt Process (IP)
358
359 Many systems provide a function which suspends, interrupts,
360 aborts, or terminates the operation of a user process. This
361 function is frequently used when a user believes his process is
362 in an unending loop, or when an unwanted process has been
363 inadvertently activated. IP is the standard representation for
364 invoking this function. It should be noted by implementers
365 that IP may be required by other protocols which use TELNET,
366 and therefore should be implemented if these other protocols
367 are to be supported.
368
369 Abort Output (AO)
370
371 Many systems provide a function which allows a process, which
372 is generating output, to run to completion (or to reach the
373 same stopping point it would reach if running to completion)
374 but without sending the output to the user's terminal.
375 Further, this function typically clears any output already
376 produced but not yet actually printed (or displayed) on the
377 user's terminal. AO is the standard representation for
378 invoking this function. For example, some subsystem might
379 normally accept a user's command, send a long text string to
380 the user's terminal in response, and finally signal readiness
381 to accept the next command by sending a "prompt" character
382 (preceded by <CR><LF>) to the user's terminal. If the AO were
383 received during the transmission of the text string, a
384 reasonable implementation would be to suppress the remainder of
385 the text string, but transmit the prompt character and the
386 preceding <CR><LF>. (This is possibly in distinction to the
387 action which might be taken if an IP were received; the IP
388 might cause suppression of the text string and an exit from the
389 subsystem.)
390
391 It should be noted, by server systems which provide this
392 function, that there may be buffers external to the system (in
393
394
395
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397Postel & Reynolds [Page 7]
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401RFC 854 May 1983
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403
404 the network and the user's local host) which should be cleared;
405 the appropriate way to do this is to transmit the "Synch"
406 signal (described below) to the user system.
407
408 Are You There (AYT)
409
410 Many systems provide a function which provides the user with
411 some visible (e.g., printable) evidence that the system is
412 still up and running. This function may be invoked by the user
413 when the system is unexpectedly "silent" for a long time,
414 because of the unanticipated (by the user) length of a
415 computation, an unusually heavy system load, etc. AYT is the
416 standard representation for invoking this function.
417
418 Erase Character (EC)
419
420 Many systems provide a function which deletes the last
421 preceding undeleted character or "print position"* from the
422 stream of data being supplied by the user. This function is
423 typically used to edit keyboard input when typing mistakes are
424 made. EC is the standard representation for invoking this
425 function.
426
427 *NOTE: A "print position" may contain several characters
428 which are the result of overstrikes, or of sequences such as
429 <char1> BS <char2>...
430
431 Erase Line (EL)
432
433 Many systems provide a function which deletes all the data in
434 the current "line" of input. This function is typically used
435 to edit keyboard input. EL is the standard representation for
436 invoking this function.
437
438 THE TELNET "SYNCH" SIGNAL
439
440 Most time-sharing systems provide mechanisms which allow a
441 terminal user to regain control of a "runaway" process; the IP and
442 AO functions described above are examples of these mechanisms.
443 Such systems, when used locally, have access to all of the signals
444 supplied by the user, whether these are normal characters or
445 special "out of band" signals such as those supplied by the
446 teletype "BREAK" key or the IBM 2741 "ATTN" key. This is not
447 necessarily true when terminals are connected to the system
448 through the network; the network's flow control mechanisms may
449 cause such a signal to be buffered elsewhere, for example in the
450 user's host.
451
452
453
454Postel & Reynolds [Page 8]
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456
457
458RFC 854 May 1983
459
460
461 To counter this problem, the TELNET "Synch" mechanism is
462 introduced. A Synch signal consists of a TCP Urgent notification,
463 coupled with the TELNET command DATA MARK. The Urgent
464 notification, which is not subject to the flow control pertaining
465 to the TELNET connection, is used to invoke special handling of
466 the data stream by the process which receives it. In this mode,
467 the data stream is immediately scanned for "interesting" signals
468 as defined below, discarding intervening data. The TELNET command
469 DATA MARK (DM) is the synchronizing mark in the data stream which
470 indicates that any special signal has already occurred and the
471 recipient can return to normal processing of the data stream.
472
473 The Synch is sent via the TCP send operation with the Urgent
474 flag set and the DM as the last (or only) data octet.
475
476 When several Synchs are sent in rapid succession, the Urgent
477 notifications may be merged. It is not possible to count Urgents
478 since the number received will be less than or equal the number
479 sent. When in normal mode, a DM is a no operation; when in urgent
480 mode, it signals the end of the urgent processing.
481
482 If TCP indicates the end of Urgent data before the DM is found,
483 TELNET should continue the special handling of the data stream
484 until the DM is found.
485
486 If TCP indicates more Urgent data after the DM is found, it can
487 only be because of a subsequent Synch. TELNET should continue
488 the special handling of the data stream until another DM is
489 found.
490
491 "Interesting" signals are defined to be: the TELNET standard
492 representations of IP, AO, and AYT (but not EC or EL); the local
493 analogs of these standard representations (if any); all other
494 TELNET commands; other site-defined signals which can be acted on
495 without delaying the scan of the data stream.
496
497 Since one effect of the SYNCH mechanism is the discarding of
498 essentially all characters (except TELNET commands) between the
499 sender of the Synch and its recipient, this mechanism is specified
500 as the standard way to clear the data path when that is desired.
501 For example, if a user at a terminal causes an AO to be
502 transmitted, the server which receives the AO (if it provides that
503 function at all) should return a Synch to the user.
504
505 Finally, just as the TCP Urgent notification is needed at the
506 TELNET level as an out-of-band signal, so other protocols which
507 make use of TELNET may require a TELNET command which can be
508 viewed as an out-of-band signal at a different level.
509
510
511Postel & Reynolds [Page 9]
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515RFC 854 May 1983
516
517
518 By convention the sequence [IP, Synch] is to be used as such a
519 signal. For example, suppose that some other protocol, which uses
520 TELNET, defines the character string STOP analogously to the
521 TELNET command AO. Imagine that a user of this protocol wishes a
522 server to process the STOP string, but the connection is blocked
523 because the server is processing other commands. The user should
524 instruct his system to:
525
526 1. Send the TELNET IP character;
527
528 2. Send the TELNET SYNC sequence, that is:
529
530 Send the Data Mark (DM) as the only character
531 in a TCP urgent mode send operation.
532
533 3. Send the character string STOP; and
534
535 4. Send the other protocol's analog of the TELNET DM, if any.
536
537 The user (or process acting on his behalf) must transmit the
538 TELNET SYNCH sequence of step 2 above to ensure that the TELNET IP
539 gets through to the server's TELNET interpreter.
540
541 The Urgent should wake up the TELNET process; the IP should
542 wake up the next higher level process.
543
544 THE NVT PRINTER AND KEYBOARD
545
546 The NVT printer has an unspecified carriage width and page length
547 and can produce representations of all 95 USASCII graphics (codes
548 32 through 126). Of the 33 USASCII control codes (0 through 31
549 and 127), and the 128 uncovered codes (128 through 255), the
550 following have specified meaning to the NVT printer:
551
552 NAME CODE MEANING
553
554 NULL (NUL) 0 No Operation
555 Line Feed (LF) 10 Moves the printer to the
556 next print line, keeping the
557 same horizontal position.
558 Carriage Return (CR) 13 Moves the printer to the left
559 margin of the current line.
560
561
562
563
564
565
566
567
568Postel & Reynolds [Page 10]
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570
571
572RFC 854 May 1983
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574
575 In addition, the following codes shall have defined, but not
576 required, effects on the NVT printer. Neither end of a TELNET
577 connection may assume that the other party will take, or will
578 have taken, any particular action upon receipt or transmission
579 of these:
580
581 BELL (BEL) 7 Produces an audible or
582 visible signal (which does
583 NOT move the print head).
584 Back Space (BS) 8 Moves the print head one
585 character position towards
586 the left margin.
587 Horizontal Tab (HT) 9 Moves the printer to the
588 next horizontal tab stop.
589 It remains unspecified how
590 either party determines or
591 establishes where such tab
592 stops are located.
593 Vertical Tab (VT) 11 Moves the printer to the
594 next vertical tab stop. It
595 remains unspecified how
596 either party determines or
597 establishes where such tab
598 stops are located.
599 Form Feed (FF) 12 Moves the printer to the top
600 of the next page, keeping
601 the same horizontal position.
602
603 All remaining codes do not cause the NVT printer to take any
604 action.
605
606 The sequence "CR LF", as defined, will cause the NVT to be
607 positioned at the left margin of the next print line (as would,
608 for example, the sequence "LF CR"). However, many systems and
609 terminals do not treat CR and LF independently, and will have to
610 go to some effort to simulate their effect. (For example, some
611 terminals do not have a CR independent of the LF, but on such
612 terminals it may be possible to simulate a CR by backspacing.)
613 Therefore, the sequence "CR LF" must be treated as a single "new
614 line" character and used whenever their combined action is
615 intended; the sequence "CR NUL" must be used where a carriage
616 return alone is actually desired; and the CR character must be
617 avoided in other contexts. This rule gives assurance to systems
618 which must decide whether to perform a "new line" function or a
619 multiple-backspace that the TELNET stream contains a character
620 following a CR that will allow a rational decision.
621
622 Note that "CR LF" or "CR NUL" is required in both directions
623
624
625Postel & Reynolds [Page 11]
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627
628
629RFC 854 May 1983
630
631
632 (in the default ASCII mode), to preserve the symmetry of the
633 NVT model. Even though it may be known in some situations
634 (e.g., with remote echo and suppress go ahead options in
635 effect) that characters are not being sent to an actual
636 printer, nonetheless, for the sake of consistency, the protocol
637 requires that a NUL be inserted following a CR not followed by
638 a LF in the data stream. The converse of this is that a NUL
639 received in the data stream after a CR (in the absence of
640 options negotiations which explicitly specify otherwise) should
641 be stripped out prior to applying the NVT to local character
642 set mapping.
643
644 The NVT keyboard has keys, or key combinations, or key sequences,
645 for generating all 128 USASCII codes. Note that although many
646 have no effect on the NVT printer, the NVT keyboard is capable of
647 generating them.
648
649 In addition to these codes, the NVT keyboard shall be capable of
650 generating the following additional codes which, except as noted,
651 have defined, but not reguired, meanings. The actual code
652 assignments for these "characters" are in the TELNET Command
653 section, because they are viewed as being, in some sense, generic
654 and should be available even when the data stream is interpreted
655 as being some other character set.
656
657 Synch
658
659 This key allows the user to clear his data path to the other
660 party. The activation of this key causes a DM (see command
661 section) to be sent in the data stream and a TCP Urgent
662 notification is associated with it. The pair DM-Urgent is to
663 have required meaning as defined previously.
664
665 Break (BRK)
666
667 This code is provided because it is a signal outside the
668 USASCII set which is currently given local meaning within many
669 systems. It is intended to indicate that the Break Key or the
670 Attention Key was hit. Note, however, that this is intended to
671 provide a 129th code for systems which require it, not as a
672 synonym for the IP standard representation.
673
674 Interrupt Process (IP)
675
676 Suspend, interrupt, abort or terminate the process to which the
677 NVT is connected. Also, part of the out-of-band signal for
678 other protocols which use TELNET.
679
680
681
682Postel & Reynolds [Page 12]
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684
685
686RFC 854 May 1983
687
688
689 Abort Output (AO)
690
691 Allow the current process to (appear to) run to completion, but
692 do not send its output to the user. Also, send a Synch to the
693 user.
694
695 Are You There (AYT)
696
697 Send back to the NVT some visible (i.e., printable) evidence
698 that the AYT was received.
699
700 Erase Character (EC)
701
702 The recipient should delete the last preceding undeleted
703 character or "print position" from the data stream.
704
705 Erase Line (EL)
706
707 The recipient should delete characters from the data stream
708 back to, but not including, the last "CR LF" sequence sent over
709 the TELNET connection.
710
711 The spirit of these "extra" keys, and also the printer format
712 effectors, is that they should represent a natural extension of
713 the mapping that already must be done from "NVT" into "local".
714 Just as the NVT data byte 68 (104 octal) should be mapped into
715 whatever the local code for "uppercase D" is, so the EC character
716 should be mapped into whatever the local "Erase Character"
717 function is. Further, just as the mapping for 124 (174 octal) is
718 somewhat arbitrary in an environment that has no "vertical bar"
719 character, the EL character may have a somewhat arbitrary mapping
720 (or none at all) if there is no local "Erase Line" facility.
721 Similarly for format effectors: if the terminal actually does
722 have a "Vertical Tab", then the mapping for VT is obvious, and
723 only when the terminal does not have a vertical tab should the
724 effect of VT be unpredictable.
725
726TELNET COMMAND STRUCTURE
727
728 All TELNET commands consist of at least a two byte sequence: the
729 "Interpret as Command" (IAC) escape character followed by the code
730 for the command. The commands dealing with option negotiation are
731 three byte sequences, the third byte being the code for the option
732 referenced. This format was chosen so that as more comprehensive use
733 of the "data space" is made -- by negotiations from the basic NVT, of
734 course -- collisions of data bytes with reserved command values will
735 be minimized, all such collisions requiring the inconvenience, and
736
737
738
739Postel & Reynolds [Page 13]
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741
742
743RFC 854 May 1983
744
745
746 inefficiency, of "escaping" the data bytes into the stream. With the
747 current set-up, only the IAC need be doubled to be sent as data, and
748 the other 255 codes may be passed transparently.
749
750 The following are the defined TELNET commands. Note that these codes
751 and code sequences have the indicated meaning only when immediately
752 preceded by an IAC.
753
754 NAME CODE MEANING
755
756 SE 240 End of subnegotiation parameters.
757 NOP 241 No operation.
758 Data Mark 242 The data stream portion of a Synch.
759 This should always be accompanied
760 by a TCP Urgent notification.
761 Break 243 NVT character BRK.
762 Interrupt Process 244 The function IP.
763 Abort output 245 The function AO.
764 Are You There 246 The function AYT.
765 Erase character 247 The function EC.
766 Erase Line 248 The function EL.
767 Go ahead 249 The GA signal.
768 SB 250 Indicates that what follows is
769 subnegotiation of the indicated
770 option.
771 WILL (option code) 251 Indicates the desire to begin
772 performing, or confirmation that
773 you are now performing, the
774 indicated option.
775 WON'T (option code) 252 Indicates the refusal to perform,
776 or continue performing, the
777 indicated option.
778 DO (option code) 253 Indicates the request that the
779 other party perform, or
780 confirmation that you are expecting
781 the other party to perform, the
782 indicated option.
783 DON'T (option code) 254 Indicates the demand that the
784 other party stop performing,
785 or confirmation that you are no
786 longer expecting the other party
787 to perform, the indicated option.
788 IAC 255 Data Byte 255.
789
790
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794
795
796Postel & Reynolds [Page 14]
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798
799
800RFC 854 May 1983
801
802
803CONNECTION ESTABLISHMENT
804
805 The TELNET TCP connection is established between the user's port U
806 and the server's port L. The server listens on its well known port L
807 for such connections. Since a TCP connection is full duplex and
808 identified by the pair of ports, the server can engage in many
809 simultaneous connections involving its port L and different user
810 ports U.
811
812 Port Assignment
813
814 When used for remote user access to service hosts (i.e., remote
815 terminal access) this protocol is assigned server port 23
816 (27 octal). That is L=23.
817
818
819
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853Postel & Reynolds [Page 15]
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