Network Working Group J. Davidson
Request for Comments: 563 University of Hawaii
NIC: 18775 28 August 1973
References: RFC357, RFC560
Comments on the RCTE TELNET Option
RFC560 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)
RFC357 (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."
RFC357 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 RFCshows that they are
fundamentally different.