Comments on the RCTE TELNET Option

RFC 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.)
   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.

An 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.
   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."
   RFC 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.