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Network Working Group                                          B. CosellRequest for Comment: 435                                         BBN-NETNIC: 13675                                                     D. WaldenCategory: TELNET, Protocols, Echoing                             BBN-NETReferences: 318, 357                                      5 January 1973TELNET Issues   This RFC discusses a number of TELNET related issues which have been   bothering us [1].  The basic, central issue we started from was that   of echoing.  We worked downward from our difficulties to discover the   basic principles at the root of our unhappiness, and from there   worked back upwards to design a scheme which we believe to be better.   In this note we will discuss both the alternate scheme and its   underlying principles.   As something of a non sequitur, before discussing echoing we feel it   expedient to dismiss one possible stumbling block, outright.  HIDE   YOUR INPUT may or may not be a good idea, this question not   concerning us at the moment.  Whatever the case, the issue of hiding   input is certainly separable from that of echoing.  We, therefore,   strongly recommend that a STOP HIDING YOUR INPUT command be   sanctioned to replace the multiplexing of this function on the NO   ECHO command.  Once this has been done, the pair of commands HIDE   YOUR INPUT and STOP HIDING YOUR INPUT can be kept or discarded   together, and we can discuss the issue of echoing independently of   them.Echoing   The basic observation that we made regarding echoing was that servers   seem to be optimized to best handle terminals which either do their   own echoing or do not, but not both.  Therefore, the present TELNET   echoing conventions, which prohibit the server from initiating a   change in echo mode, seemed overly confining.  The servers are   burdened with users who are in the 'wrong' mode, in which they might   not otherwise have to be, and users, both human and machine, are   burdened with remembering the proper echoing mode, and explicitly   setting it up, for all the different servers.  It is our   understanding that this prohibition was imposed on the servers to   prevent loops from developing because of races which can arise when   the server and user both try to set up an echo mode simultaneously.   We will describe a method wherein both parties can initiate a change   of echo mode and show that the method does not loop.Cosell & Walden                                                 [Page 1]

RFC 435                      TELNET Issues                5 January 1973   This alternate specification relies on three primary assumptions.   First as above, the server, as well as the user, should be able to   suggest the echo mode.  Second, all terminals must be able to provide   their own echoes, either internally or by means of the local Host.   Third, all servers must be able to operate in a mode which assumes   that a remote terminal is providing its own echoes.  Both of these   last two result from the quest for a universal, minimal basis upon   which to build.  It is fairly easy for a Host which normally supplies   echoes to disable the appropriate code, but it will difficult for a   Host which does not do echoing to integrate such routines into its   system similarly, it is easier for a local Host to supply echoes to a   terminal which cannot provides its own, but it borders on the   impossible to undo echoing in a terminal which has automatic echoing   built into it.   Our proposed specification would use the present ECHO and NO ECHO   commands as follows: ECHO, when sent by the server to the user, would   mean 'I'll echo to you' ECHO, when sent by the user to the server,   would mean 'You echo to me'.  NO ECHO, when sent by the server to the   user, would mean 'I'll not echo to you'; NO ECHO, when sent by the   user to the server, would mean 'Don't you echo to me'.  These are, of   course, nearly the same meanings that the commands currently have,   although most current implementations seem to invert the server-to-   user meanings.   In our specification, whenever a connection is opened both server and   user assume that the user is echoing locally.  If the user would, in   fact, prefer the server to echo, the user could send off an ECHO   command.  Similarly, if the server prefers to do the echoing (for   instance, because the server system is optimized for very interactive   echoing), the server could send off an ECHO command.  Neither is   required to do anything, it is only a matter of preference.  Upon   receipt of either command by either party, if that is an admissible   mode of operation the recipient should begin operating in that mode,   and if such operation reflects a change in mode, it should respond   with the same command to confirm that (and when) the changeover took   place.  If the received command request an inadmissible mode of   operation, then the command's inverse should be sent as a refusal   (this must be NO ECHO, since neither party can refuse a change into   NO ECHO).  To state these rules more formally:      1) Both server and user assume that a connection is initially in         NO ECHO mode.      2) Neither party can refuse a request to change into NO ECHO mode.      3) Either party may send an unsolicited command only to request a         change in mode.Cosell & Walden                                                 [Page 2]

RFC 435                      TELNET Issues                5 January 1973      4) A party only changes its echo mode when it receives an         admissible request.      5) When a command is received, the party replies with its echo         mode, unless it did not have to change mode to honor the         request.   Several properties of this scheme are worthy of note:      1) NO ECHO is retained as the nominal connection mode.  A         connection will work in ECHO mode only when both parties agree         to operate that way.      2) The procedure cannot loop.  Regardless of which party (or both)         initiates a change, or in what time order, there are at most         three commands sent between the parties [2].      3) Servers are free to specify their preferred mode of operation.         Thus, human, or machine, users do not have to learn the proper         mode for each server.Three Principles   Let us mention the general principles we alluded to at the beginning   of this note.  The principles are: default implementation, negotiated   options and symmetry.  The principle of default implementation merely   states that for all options, defaults are declare which must be   implemented.  It is this principle which leads us to seek out the   'minimum' for each option (to keep the required burden on everybody   as small as possible), and prevents loops in protocol.  The principle   of negotiated options merely states that options must be agreed upon   by all (both) parties concerned.  It is this principle which dictated   the positive/negative acknowledgement scheme.  The principle of   symmetry merely states that neither party should have to 'know'   whether it is the server or the user.  Our scheme, as described thus   far, is not totally symmetrical we will consider this matter in a   later section.   The ECHOING scheme we have described, together with the principles   stated above, form the heart of our comments on the TELNET protocol.   The remainder of this note consists of further ways in which the   protocol can be expanded on the whole, these suggestions are all   really only applications and development of the principles we have   already put forward.  However, the fecundity of these expansions, and   the 'good feel' they have, make us yet more convinced of the '   rightness' of our original proposals.Cosell & Walden                                                 [Page 3]

RFC 435                      TELNET Issues                5 January 1973   Thus far, we have made a simple, concrete suggestion that we believe   should be immediately sanctioned.  Looking beyond that proposal,   however, has suggestion a large number of further, more ambitious   changes.  The remainder of this RFC describes ideas which we don't   feel have the immediacy of the proposal above, but should,   nonetheless, be kept in mind if the network community decides to   embark on revamping the protocol.Synchronization   One complaint we have heard about the present convention for   establishing an echoing mode is about the lack of a provision to   synchronize a change of echoing mode with the user-to-server data   stream our scheme, too, is guilty on this count.  John Davidson of   the University of Hawaii has documented, inRFC 357, a more elaborate   echoing scheme which doesn't have this problem.  We, however, feel   that it is possible to eliminate most of the trouble involved with   normal changing of echo mode at a more modest cost than that required   by the highly interactive scheme described by Davidson.  We can do   this by borrowing a small piece of that scheme.  The rule we would   incorporate is that whenever a party initiates a request for a change   in echo mode, it then buffers, without transmitting or processing,   all data in the user-to-server data stream until it receives an   acknowledgement, positive or negative, at which time it deals with   the buffered data in the newly negotiated mode.  Since with both our   proposed and the current schemes such a request is guaranteed to be   acknowledgement, the buffering time is bounded.   An important aspect of this technique of eliminating the   synchronization problem is that it need not ever become part of the   official protocol.  Since its operation is entirely internal to the   server or user, each may independently weigh the value of elegance   against the cost of the required code and buffer space.Other options   Abhay Bushan has suggested to us that whether the user and server   operate line-at-a-time or character-at-a-time mode (seeRFC 318)   should also be a negotiated option.  Further, he suggested that   whether the terminal follows the TELNET end-of-line convention or not   should also be negotiated.  Thus, when a connection is opened, in   addition to being set to NO ECHO mode, the terminal would also be set   to LINE-AT-A-TIME and EOL modes.  We could augment the command space   with the new commands LINE, NO LINE (=CHARACTER), EOL and NO EOL   (=separate CR and LF).Cosell & Walden                                                 [Page 4]

RFC 435                      TELNET Issues                5 January 1973   Once started in this direction, we found several further   applications.  HIDE YOUR INPUT could be made an option, as could   Davidson's echoing scheme, and even the character set to be used!   Consider that an APL subsystem might well want to suggest to its user   that EBCDIC be used for the connection.   In mentionaing that the character set could be negotiated, it was   implicit that 7-bit USASCII was the default.  The possibility of   having the default be straight binary suggests itself.  If we   augmented the protocol with a QUOTE character, the byte after which   were to be always interpreted as data, then codes 128-255 could be   retained as the 'TELNET command space' independently of the data mode   in use by merely prefixing all data bytes in this region with a   QUOTE.  If BINARY were a permissible data mode, then it is easy to   visualize many higher level protocols, e.g., perhaps, File Transfer   and Graphics, being built on top of, and into, the TELNET protocol.   What we would have accomplished is to promote TELNET from being a   constrained, terminal-oriented protocol to its being a flexible,   general protocol for any type of byte oriented communication.  With   such a backbone, many of the higher level protocols could be designed   and implemented more quickly and less painfully -- conditions which   would undoubtedly hasten their universal acceptance and availability   [3].   Looking toward a better world of the future, we have come up with a   more compact and flexible command scheme.  We'll describe it after   the next section.Symmetry   Some of the TENEX group (in particular, Thomas, Burchfiel and   Tomlinson) have pointed out to us that although we have made the   rules for the protocol symmetrical, we have not made the meanings of   the commands symmetrical.  For example, the interpretations of the   ECHO command -- 'I'll echo to you' and 'You echo to me' -- implicitly   assume that both the server and user know who is which.  This is a   problem not only for server-server connections where it is not clear   which is the user, but also for user-user connections, e.g., in   linking Teletypes together, where it is not clear which is the   server.   Responding to this, we came to understand that there are only five   reasonable modes of operation for the echoing on a connection pair   [4]:Cosell & Walden                                                 [Page 5]

RFC 435                      TELNET Issues                5 January 1973                         <------------------<   A          Process 1                        Process 2                         >------------------>                         neither end echoes                         <------------------<   B          Process 1  <--+                  Process 2                            ^                         >--^--------------->                        one end echoes for itself                         <------------------<   C          Process 1  <--------------+     Process 2                                        ^                         >--------------^--->                        one end echoes for the other                         <--------------V---<   D          Process 1  <--+           V       Process 2                            ^           +--->                         >--^--------------->                        both ends echo for themselves                         <-----V------------<   E          Process 1  <--+  V               Process 2                            ^  +------------>                         >--^--------------->                        one end echoes for both ends   The TENEX group suggested to us that four commands are sufficient to   deal with completely symmetric echoing.  We have actually already   mentioned the four commands -- the two possible meanings for each of   ECHO and NO ECHO.  Explicitly, the commands would be I'LL ECHO TO   YOU, YOU ECHO TO ME, DON'T ECHO TO ME and I'LL NOT ECHO TO YOU.   Echoing is now the negotiation of two options, and the initial,   default modes are DON'T ECHO TO ME and I'LL NOT ECHO TO YOU.   In the case where the server or user knows which he is, the   modification to the scheme is minimal since the commands never had   ambiguous meanings in these cases.  When an 'end' truly doesn't know,   then things are a little more complicated -- for example, consider   both ends in I'LL ECHO TO YOU mode, but even then the problems are   not insurmountable.Cosell & Walden                                                 [Page 6]

RFC 435                      TELNET Issues                5 January 1973   Once the principle of symmetry is adopted, it is no longer possible   to use a function in two different ways.  On pages 5 and 6 ofRFC318, Postel gives a description of INS and SYNC which indicates that   they are used to simulate a 'break' user-to-server, but flush the   output buffers server-to-user.  Since we do believe in symmetry, we   suggest that the INS/DATA-MARK be treated the same in both directions   and that a new CLEAR YOUR BUFFER option be added.Command Format   Extending full symmetry through the other options we have suggested,   we can now describes the compacted command format referred to   earlier.   Rather than having four commands for each option (I WILL, I WON'T,   YOU DO, YOU DON'T), there would be four 'prefixes' -- WILL, WON'T,   DO, DON'T -- which would be used before the single command devoted to   each option, WON'T and DON'T being the default modes.  To give an   example, assume the codes for WILL and WON'T are 140 and 141, and the   codes for ECHO REMOTE and HIDE INPUT are 132 and 133.  Then several   of the possible command combinations would be:                   140 133 -- DO HIDE INPUT                   140 132 -- DO ECHO REMOTE                   141 132 -- WON'T ECHO REMOTE                   141 133 -- WON'T HIDE INPUT   These are some of the commands that we believe should exist:   I WILL (140)   I WILL NOT (141)   YOU DO (142)   YOU DO NOT (143)   QUOTE (144)   SYNC (163)   SYNC REPLY (164)   ECHO REMOTE (132)   SEND A CHARACTER-AT-A-TIME (146)   SEND INDEPENDENT CR and LF (147)   SEND IN EBCDIC (162)   HIDE INPUT (133)   USE DAVIDSON'S ECHOING STRATEGY (145)   An important virtue of this command structure, and of our entire   viewpoint, is that Hosts need no longer even be aware of what all the   options are.  If we call the mode of operation in which every   alternative is in its default state the 'NVT', then a site, ofCosell & Walden                                                 [Page 7]

RFC 435                      TELNET Issues                5 January 1973   course, must handle an NVT, but beyond that if it merely responds no   to any command it does not understand, then it can totally ignore   options it chooses not to implement.  Thus, options would truly be   optional (for a change), not only to the user who may choose not to   invoke them, but also to the systems builders who may now choose not   to offer them!   We hereby volunteer to rigorously specify a version of TELNET which   embodies the principles we have described and to do so at any level   of complexity deemed sufficient by the network community.Cosell & Walden                                                 [Page 8]

RFC 435                      TELNET Issues                5 January 1973Appendix: A Sample Implementation   The basis scheme we described represents most of what we have been   thinking about the further extensions are just that, extensions.  We   fear, however, that some who are spiritually in league with us might   be frightened off by the magnitude of all the changes we suggest.  To   combat this, we here provide an example of how simply and straight-   forwardly the basis scheme could be implemented for the TIP [5].   For each user terminal the TIP would keep three state bits: whether   the terminal echoes for itself (NO ECHO always) or not (ECHO mode   possible), whether the (human) user prefers to operate in ECHO or NO   ECHO mode, and whether the connection to this terminal is in ECHO or   NO ECHO mode.  We call these three bits P(hysical), D(esired) and   A(ctual).   When a terminal dials up the TIP, the P-bit is set appropriately, the   D-bit is set equal to it, and the A-bit is set to NO ECHO.  The P-   and A-bits may be manually reset by direct commands if the user so   desires for instance, a user in Hawaii on a 'full-duplex' terminal   might know that whatever the preference of a mainland server, because   of satellite delay his terminal had better operate in NO ECHO mode --   he would direct the TIP to change his D-bit from ECHO to NO ECHO.   When a connection is opened from the TIP terminal to a server, the   TIP would send the server an ECHO command if the MIN (with NO ECHO   less than ECHO) of the P- and D-bits is different from the A-bit.  If   a NO ECHO or ECHO arrives from the server, the TIP will set the A-bit   to the MIN of the received request, the P-bit and D-bit.  If this   changes the state of the A-bit, it will send off the appropriate   acknowledgement if it does not, then the TIP will send off the   appropriate refusal if not changing meant that it had to deny the   request (i.e., the MIN of the P- and D- bits was less than the   received A- request).  If while a connection is open, the TIP   terminal user changes either the P- or D-bit, the TIP will repeat the   above tests and send off an ECHO or NO ECHO, if necessary.  When the   connection is closed, the TIP would reset the A-bit to NO ECHO.   While the TIP's implementation would not involve ECHO or NO ECHO   commands being sent to the server except when the connection is   opened or the user explicitly changes his echoing mode, we would   suppose that bigger Hosts might send these commands quite frequently.   For instance, if a JOSS subsystem were running, the server might put   the user in NO ECHO mode, but while DDT was running, the server might   put the user in ECHO mode.Cosell & Walden                                                 [Page 9]

RFC 435                      TELNET Issues                5 January 1973   [1] We have assumed that TELNET is defined as suggested by Jon Postel   inRFC 318.   [2] Notice that a faulty implementation could achieve the effect of a   loop by repeatedly sending a command which has previously been   refused.  We consider this a property of the implementation, not of   the scheme in general, a command which has be rejected should not be   repeated until something changes -- for instance, not until after a   different program has been started up.   [3] Will Crowther, with an eye towards building higher protocols upon   TELNET, has suggested that a SYNC command (not to be confused with   the existing SYNCH), and a SYNC REPLY be added to TELNET.  For   example, a server might want to wait until the output buffer of a   user's terminal were empty before doing something like closing the   connection or passing the connection to another server.  Although we   see no current use for the command pair, they seem to be a handy   enough building block that we recommend that they be included.   [4] It is perhaps appropriate to mention that most of the connections   in the network are TELNET connections, which are full duplex.   Wouldn't it be reasonable to make all Host/Host protocol connections   full duplex, rather than simplex? If, for some reason, one truly   needs a simplex connection, the reverse direction can always just be   ignored.   [5] Readers unfamiliar with the TIP may read the TIP Users Guide --   NIC 10916.        [This RFC was put into machine readable form for entry]    [into the online RFC archives by Helene Morin, Via Genie, 12/99]Cosell & Walden                                                [Page 10]