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Network Working Group                                        J. DavidsonRequest for Comments: 563                           University of HawaiiNIC:  18775                                               28 August 1973References:RFC 357,RFC 560                   Comments on the RCTE TELNET OptionRFC 560 describes a Remote Controlled Transmission and Echoing TELNET   option.  Its authors provide a framework wherein a serving host may   control two aspects of TELNET communication over the (simplex) user-   to-server path.      Commands are introduced which govern         1. when (and which) characters shall be echoed by the user, and         2. when (and which) characters shall be transmitted by the            user.      Motivation for the option was based on two considerations:         1. the latency between striking and printing of a character            which is to be echoed by a remote server is disconcerting to            the human typist, and         2. character-at-a-time transmission introduces processing            inefficiencies (for IMPS, for servers, for users) and            decreases effective channel thruputs over the net.   The author feels that the RCTE description is in error (or at least   unclear [1]) in its treatment of when characters are to be   transmitted.  However, discussion of the subject in the RCTE   specification is incomplete, so it is difficult to point to a   statement which is "wrong."  Rather, the present objections are based   on inferences drawn from the sample TENEX interaction   Perhaps there is some misunderstanding of the original issues to   which RCTE now addresses itself.   Original Motivation for Remote Controlled Echoing (RCE)RFC 357 (An Echoing Strategy for Satellite Links)  introduced a need   for RCE for users who are separated from a service host by a   satellite link.  The motivation was to lessen human frustration and   confusion;  no consideration was given to resulting processing   inefficiencies or channel thruputs.   (In the remainder of this RFC,  we consider character transmission   apart from echoing considerations.)Davidson                                                        [Page 1]

RFC 563            Comments on the RCTE TELNET Option     28 August 1973   It was recognized that the human's best interests could be served if   user-to-server transmission were performed on a character-by-   character basis,  (the implicit assumption being that this insured   the most rapid server response possible).  This scheme allowed for   the classic overlap of (network) I/O and computation,  and was thus   efficient as far as the (human) user was concerned.   Concessions were made in the transmission strategy when it was   accepted that the serving process could not in fact do any   significant processing until a completed command was available.   Ideally then, users should be able to buffer characters until they   have a completed command and then fire off the entire command in a   single "packet,"  with the resultant savings in channel usage and a   greater per-packet data efficiency.  The characters which delimited   commands were called wakeup characters, in 357,  for their effect on   the serving process.  RCTE calls them transmission characters for the   effect they have at the User TELNET.   The key here is that it is quite possible for a human,  separated by   a satellite link from his remote host,  to type several completed   commands - and to therefore initiate several packet transmissions-   all the while awaiting the server's response to his first command.   Again we see the overlap of I/O and computation,  and again we   achieve maximum efficiency from the human's viewpoint.   The problem,  however,  is that wakeup (transmission) character sets   change.  And there will always be a finite amount of time [the one-   way transmission time] during which the set definitions will differ   between server and user.  This says that during such times the user   will be sending off packets which do not contain completed commands,   (or contain more than a single completed command),  or he will be   buffering characters beyond the end of a completed command.  (A   fourth alternative is that he may actually still be doing the right   thing by chance).  Buffering beyond the end of a command is the only   case which lessens processing efficiency for the human,  however.Dissatisfaction With RCTE   Here is the author's complaint:  RCTE [at least the sample   interaction which allowed transmission (by default) only at break   characters] would have the TELNET user wait until he knows exactly   the wakeup (transmission) character set being used by the server !   Ideal channel utilization might be achieved,  since no "unnecessary"   packets are sent (and, strangely, no extra characters are allowed in   the current packet) but the overlap of I/O and computation has been   eliminated,  and the human has an extra round-trip time added to the   server's processing time.  This is wrong.Davidson                                                        [Page 2]

RFC 563            Comments on the RCTE TELNET Option     28 August 1973An Alternative Implementation   Unless a round-trip time penalty is to be paid by the human at every   break interaction,  the user TELNET must transmit characters based on   the transmission character set in effect at the moment the characters   are typed.  And unless the step-by-step interaction developed in the   RCTE TENEX example was not a true representation of the relative   temporal occurances of events,  RCTE did not do this.      The sample TENEX interaction showed the user typing   (T:) LOGIN ARPA <cr>      while the break set included <space>  and <cr>.  The only      transmission characters in effect were the break characters - by      default.  The RCTE example showed that the LOGIN <space> phrase      was,  properly,  a completed command;  it was transmitted.  But      while the alternative transmission strategy of the current RFC      would "recognize" the ARPA <cr> phrase as a second completed      command,  and thus initiate a second transmission,  RCTE withholds      judgment until the server respecifies the transmission classes.      Response for the user suffers.      One might also ask what transmission strategy was to be undertaken      when two users were,  say,  linked thru a TENEX.  Transmission      should obviously be at every character.  RCTE would send the first      single character packet and then wait to be sure that a single      character did in fact delimit the next command also.  It would      wait a long time it would seem,  since no break interaction would      occur until the end of the line (<cr>).  The user would be echoing      like a champ,  but no characters would be transmitted for the      linked party's inspection.      If we adopt the convention that transmission decisions should be      based on the transmission set [and by default,  the break set]  in      effect at the time the character is typed,  then the sample      interaction might  in fact look like this:   P:  TENEX  1.31.18,   TENEX EXEC  1.50.2  <cr> <lf>@   T:  LOGIN <space>   P:  LOGIN <space> } >>>>>> NOTE: Typing and printing occurs simul-   U:  LOGIN <space>                taneously up to the <space> at                                    which point the human "types-ahead."   T:               ARPA <cr>   U:  ARPA <cr>             <<key: the user transmits a second packet.Davidson                                                        [Page 3]

RFC 563            Comments on the RCTE TELNET Option     28 August 1973   S:  <space> <IAC> <SB> <RCTE> <0>   P:  <space> AR   S: <cr> <lf> (PASSWORD): <IAC> <SB> <RCTE> <7>              [the server sends while text is printing]   P:            PA <cr> <lf> (PASSWORD):   T: WASHINGTON <space>   U:  WASHINGTON <space>   T:                    100   S:  <space> <IAC> <SB> <RCTE> <3>   P:  <space> 100   T:                             0          [Again printing is                                              simultaneous to typing]   P:                 0   T:                              <cr>   P:                  <cr>   U:  1000 <cr>   S:  <cr> <lf> JOB ...   The interaction will not necessarily be the same each time.  It   depends on the typing speed of the user and response time of the   server.  For this example,  both channel utilization and performance   for the human are perfect,  since the transmission set [even though   it was only the default break set]  did not change.Unsolicited Output   The question of unsolicited output arise again.  The treatment in 560   was simplified over that of 357 only because of the RCTE transmission   strategy.  No output could possibly be returning for a command which   hasn't been sent yet (!),  so the message must be "SYSTEM GOING   DOWN."Davidson                                                        [Page 4]

RFC 563            Comments on the RCTE TELNET Option     28 August 1973RFC 357 outlines when unsolicited output can be recognized and when   it should be printed,  in line with the alternate transmission scheme   proposed.  The requirement that such system alerts be terminated by   RCTE commands is of course the proper way to handle such interrupts;   this clarification of the unsatisfactory solution in 357 is   appreciated.TIP Buffering   RCTE as defined cannot allow a user to transmit when his buffer is   full, else he might send a break character. [presumably the buffer   fills because we are waiting for break (transmission)  redefinition].   The response to the command delimited by the break character could   return before the characters, of the command were "echoed."  RCTE   would thus demand that it be printed first,  and the listing would be   out of order.   The alternative transmission strategy eliminates this problem since   transmission of a full buffer is no worse than guessing incorrectly   that the last character in the buffer is a transmission character.A further suggestion   All server-to-user echoing could be eliminated if control bytes were   sent to indicate which break sets should be echoed and which   shouldn't.Endnotes   [1] for example:  statement 2E2F does not properly distinguish   between the "occurrence" of a break character and the "occurrence" of   a Transmission character.  The present RFC shows that they are   fundamentally different.Davidson                                                        [Page 5]