|Title||Echoing strategy for satellite links
Network Working Group John Davidson
Request for Comments: 357 University of Hawaii
NIC: 10599 Will Crowther
Categories: Remote Controlled Echoing, Satellite, TELNET BBN
References: RFC's 346, 355, 358, 318 John McConnell
June 26, 1972
An Echoing Strategy For Satellite Links
As mentioned in RFC 346 ("Satellite Considerations" by Jon Postel)
those interactive systems which provide echoing for full-duplex
terminals over the ARPANET become more awkward to use as transmission
delays increase. The reason, of course, is that a character's round
trip time is added to the inherent echo delay of the server with the
result that the terminal echoing appears extremely sluggish.
For a terminal separated from its server by a single satellite link,
the delay will be such that even if echoing at the server were
instantaneous, the latency between keying and printing of an input
character will be nearly half a second. If, in addition, the
character is routed thru a portion of the surface net, the delay will
be of course increase. It is estimated that echo delays of at least
one second will not be uncommon.
This document describes a strategy which will eliminate the delay
associated with simple echoing and allow the transmission delay to be
hidden in the cost of computation only. This scheme is proposed as
an optional addition to existing User TELNETs; its use requires the
explicit support of a cooperating server process.
II. Standard Echo Strategy
Echoing for locally connected full-duplex terminals is normally
provided at the server by a resident system task called the (e.g.)
Terminal Handler. The Terminal Handler echoes on a one-for-one or
simple replacement basis and buffers all typed input on behalf of the
To let the user process operate most efficiently, the Terminal
Handler should collect characters until a complete command or
parameter (or whatever) has been typed. Then, presumably, the
process can do some significant computing. Since the user process
knows the syntax of the string it expects, it can specify to the
Terminal Handler those characters which delimit completed parameters.
Such characters are called 'Wakeup Characters' since the user process
is awakened as they are echoed.
Certain commands keyed by the user will require an output response
from the process. In order that the typed commands be followed by
its response and be separated from succeeding commands, the Terminal
Handler must suspend echoing of user type-ahead. It can resume
echoing (starting for type-ahead - with the unechoed characters in
the buffer) as soon as the process has stated (implicitly or
explicitly) that it has completed the output response.
Characters which cause the Terminal Handler to suspend echoing are
called 'break characters' They are specified by the user process
based upon the syntax of the expected input. Normally break
characters are also wakeup characters. As examples:
1. A text editor may gobble up typed English sentences every time
a period or question mark is echoed. The two characters are
wakeup characters only. There is no need to suspend echoing.
2. In some systems, an ESC character is used to invoke command
recognition. The user who types
CO [ESC] ABC [ESC] XYZ
should see as output
COPY (FROM FILE) ABC (TO FILE) XYZ
The ESC is both a break and a wakeup. The printout should be
the same no matter how fast the user types.
The server must provide a means for each user process to communicate
the following to the Terminal Handler:
1. the set of wakeup characters,
2. the set of break characters,
3. which break characters should and which should not be echoed,
(Some break characters - such as ESC in example 2 - should not
4. completion of an output response,
5. whether or not to echo characters. (Not echoing is useful in
"hide your input" applications.)
As far as implementation, 1. and 2. could be communicated by allowing
the user process to specify a 128-bit (for an ASCII device) table
with 1's set for each wakeup character, and another table with 1's
set for each break character. This approach becomes fairly expensive
in terms of core memory as the number of terminals becomes large; the
system must store these bit tables itself since in most cases the
user process will not be in core while echoing is being done by the
To reduce the storage requirements, the system can make known to all
its programmers a limited number, say 4, of supported break
characters for his process from, for example:
a. alphanumeric characters,
b. punctuation characters,
c. echoable control characters (including the bell and CR, etc.),
d. non-echoable control characters (Control-C, etc.),
by specifying in a system call which break set(s) should be used.
This requires no more than 4 bits of system storage per terminal, and
a single table to identify the set(s) to which each of the 128
possible ASCII characters belongs.
For the user process to communicate (3) to the Terminal Handler
(which break characters should and which should not have echoed), the
process can specify another 4 bit field with 1's set for those break
classes whose members should be echoed. For the 4 classes above,
only 3 bits would be required since members of class (d) are defined
to be non-echoable.
To communicate the completion of an output response (4), the user
process could issue an explicit system call; or, the Terminal Handler
could assume completion when the user process requests input of the
first character following the break.
"Hide your input" (5) would be communicated by a system call which
(a) "break on every character and don't echo any break characters",
or, for example
(b) "don't echo anything and break on punctuation, or any control
character" for an alphanumeric password,
depending on the syntax of the expression to be hidden.
Several new terms need to be defined, some of which are direct
extensions of the terms used in the "standard echo strategy"
description. There is no reason to insist that the four buffers
presented all be implemented as individual constructs; they are
logically separated for clarity in the discussions which follow.
Remote Controlled Echoing (RCE)
This is the name for the echo strategy described in this document.
Echoing will be controlled by the (remote) server but performed by
the User TELNET.
This is a logical buffer used by a User TELNET to hold characters
in sequence as they are received from the terminal keyboard (after
they have been converted to NVT characters).
This is a logical buffer used by a User TELNET to hold NVT
characters which have been typed but have not yet been transmitted
to the server.
This is a logical buffer used by a User TELNET to hold the NVT
characters received from the server.
This is a logical buffer residing in the User TELNET from which
characters will be sent in sequence to the terminal printer. (The
output buffer contains NVT characters which may have to be
converted to characters employed by the actual terminal.)
The 128 possible (7-bit) ASCII characters employed by the Network
Virtual Terminal can be partitioned into several quasi-equivalence
classes (for example alphabetic, numeric, punctuation characters,
etc.). These classes can be defined so that each character is a
member of at least one class, although it may belong to more than
A server process may indicate to a User TELNET that certain of
these classes (or all, or none) are to be considered break
classes. That is, a break class is an equivalence class which is
of special significance to the server process. In terms of the
discussion of section II, the Server recognized 4 equivalence
classes any combination of which might be designated as break
class by a particular process.
The RCE implementation will have more than 4 equivalence classes
(perhaps as many as 8) to provide more flexibility in the choice
of break character sets.
Two break actions are possible:
(1) a break character encountered in the input buffer IS moved to
the print buffer at the appropriate time, or
(2) a break character encountered in the input buffer IS NOT moved
to the print buffer.
The server process will specify which break action should be
followed. (The two actions correspond to echoing or not echoing
the break character.)
(This description is written in terms of the TIP which, of course,
embodies a User TELNET.)
Remote Controlled Echoing is an attempt to remove the echo
responsibility from the Terminal Handler and push it off into the
TIP; wakeup processing is still handled at the server. The process'
interface (system calls, etc.) to the server's Terminal Handler need
not change, but the (abbreviated) Terminal Handler (actually a Server
TELNET) must find a way to relay the process' echo requirements to
the TIP. It does this with TELNET commands and control information.
System calls and echo parameters (break classes, etc.) peculiar to a
particular serving Host must be interpreted by the Server Telnet
Refer to figure 1. A character received from a full-duplex
terminal will be converted to its NVT equivalent and placed in
both the transmission AND the input buffers. The TIP's
transmission strategy determines when it will be removed from the
transmission buffer; the server's RCE control commands dictate
when it will be removed from the input buffer.
A character received from the server will be placed in the output
Of the three labeled paths DISCARD, ECHO and OUTPUT, exactly one
is enabled at all times. RCE commands dictate which one. Thus
(DISCARD:) be removed in sequence from the input buffer and
(ECHO:) be removed in sequence from the input buffer and
placed in the print buffer, of
(OUTPUT:) be removed in sequence from the output buffer and
placed in the print buffer.
From the print buffer they will be converted from NVT characters
and be immediately send to the terminal's printer.
| Terminal Keyboard |
Convert to NVT characters
To +-------------------+ |
Server <----|Transmission Buffer| |
+-------------------+ | +-----From Server
^ | |
| Input Buffer | | Output Buffer |
| | |
DISCARD | +--ECHO---+ +---+ OUTPUT
| | |
V V V
Oblivion | Print Buffer |
To Terminal Printer
Figure 1. Character Flow within the TIP
Commands: Server to Host
The following are the proposed TELNET commands sent by the server
process to the TIP. Commands (2) thru (5) should not be sent if
RCE is not being used.
(1) Use Remote Controlled Echoing. The server asks the TIP to
employ the echo strategy described in this document. The
TIP can respond either YES (I will use it) or NO. (It is
suggested that the response YES also be "Use RCE" to
eliminate race conditions.)
(2) Set Break Action. This is actually 2 commands. The server
can set the break action to echo or not echo a break
(3) Set Break Classes. This command specifies those equivalence
classes which are to be considered break classes. It will
be a two (8-bit) byte command.
Note: The envisioned implementation requires the TIP to have
a table with one entry per ASCII character. Each entry is
formatted with one bit position for each equivalence class,
and a bit is set or reset according as the given character
is or is not a member of that class. The server sends a
"Set Break Classes" command (1st byte) followed by a
formatted control word (2nd byte) to the TIP with bit
positions set for those equivalence classes which represent
break classes for the server process.
When a (virtual) character is taken from the input buffer
the TIP does a table look-up indexed by the character. If a
simple ANDing of the table entry with the terminal's control
word yields a non-zero result, a break was received.
(Receipt of a break character enables the OUTPUT path.)
(4) Move to Break (MTB). This command directs the TIP to move
characters in sequence from the input buffer to the print
buffer until a break character is encountered. The break
character will be moved or discarded depending on the
previously-specified break action. (Essentially, MTB
enables the ECHO path.)
(5) Delete to Break (DTB). This command directs the TIP to move
characters in sequence from the input buffer and discard
them until a break character is encountered. The break
character will also be discarded. This provides a
convenient mechanism for hiding a user's input. (DTB
enables the DISCARD path.)
Commands: User to Server
The USER may send (via TIP) a request to the server process
requesting that it "Use Remote Controlled Echoing" to which
the server must respond "YES" or "NO".
Very simply, the Remote Controlled Echoing strategy works as
follows: The Server TELNET will tell the TIP to (essentially)
(1) echo a message,
(2) print the process' response to that message,
(3) echo the next message
(4) print the response to that message
. . .
etc., to effect an orderly listing of inputs and responses much as
would be imposed when using a half-duplex device.
The actual interaction depends on the control commands. When a
terminal-serving process is invoked on behalf of a TIP user, the
Server TELNET will send the "Use RCE" command; the TIP will
respond "YES". Then the Server TELNET will send the "Set Break
Action" and "Set Break Classes" commands to properly reflect the
break strategy requested by the terminal-serving process. Lastly
the Server TELNET will send an MTB command. This enables the ECHO
The TIP removes characters from the input buffer and places them
in the print buffer. When it encounters a break character, it
performs the break action specified, and enables the OUTPUT path.
Characters are then moved in sequence from the output buffer to
the print buffer. When an MTB (or, DTB) is encountered, it is
discarded and the ECHO (DISCARD) path is enabled.
The Server TELNET may change the break action or break classes
after an interaction, but should normally do so prior to sending
the MTB or DTB commands. It should only send an MTB (DTB) after
all process output from the previous message has been sent.
Why Does This Work?
The RCE strategy described above produces the correct result at
the user's terminal because it is in essence the same scheme used
by the Terminal Handler which normally provides the echoing at the
serving site. Initially, the characters are echoed as they are
typed; then a break character is keyed, echoing is suspended. If
the process produces any output response, it is printed before
echoing of subsequent input.
Echoing of the next command begins, if there is type-ahead, with
the characters in the input buffer, and even if the input buffer
is emptied immediate echoing of keyed input from the terminal is
provided since the ECHO path remains enabled up to a break.
(In the following, we assume all break characters are also wakeup
characters and (carelessly) treat the two interchangeably.)
Suppose a TIP user attempts to login to a remote server with the
properly formatted message
LOG NAME PASSWORD [CR]
and that the Server TELNET has requested the use of RCE.
Presuming that the break (and wakeup) characters sets are
appropriately defined to include space and CR (and that the break
action specifies they should be echoed), the primary sequence of
RCE commands which will drive the TIP to produce the correct
printout at the user's terminal is:
(1) MTB (to print "LOG "), and since the space is a break
(2) MTB (to print "NAME "),
(3) DTB (to delete "PASSWORD [CR]" (See section VI, number
11)), and perhaps a message followed by
(4) MTB (to reenable the echo path).
We investigate in some detail how interaction at the
process/Server TELNET interface causes these commands to be send
to the TIP.
When the EXEC is invoked, it issues a system call to set its break
classes. The Server TELNET interprets the system call in terms of
the classes supported by RCE, and sends the appropriate two-byte
"Set Break Classes" (SBC) command to the TIP. A space is among
the characters of the break set.
The EXEC asks for input, so the Server TELNET send MTB ((1)
above). We presume the EXEC blocks until some input is available.
The EXEC is awakened when the first space arrives; it recognizes
the LOG command to be a call upon the LOGIN subsystem which it
The LOGIN process issues a system call to set its break classes
(this time both space and CR are included, and, as before, the
Server TELNET forwards the command as an SBC). Then it asks for
input (so the Server TELNET sends MTB ((2) above)), and blocks
until the second space arrives.
When the LOGIN process has verified the existence of a user with
name NAME, it issues a system call to suppress printing of the
next parameter (the password). In compliance, the Server TELNET
sends DTB ((3) above).
Once the password has been examined and verified, a message like
[CR][LF] LOGIN COMPLETED [CR][LF]
can be sent, followed by a request for input. The Server TELNET
thus forwards an MTB ((4) above) and the sequence is completed.
Suppose in the above example the user had typed
When the LOGIN process regained control, it would have noted that
the break was a CR instead of a space. It then could have issued
which the Server TELNET would follow (when LOGIN requests print
suppression) with DTB. When the TIP had finished its output, the
DISCARD path would be enabled and the user's terminal would have
with the cursor positioned just after the =. The TIP will hide
the characters of the password.
Suppose a user were using a text editor, TEXT, to create a source
file of English sentences. The TEXT subprocess might allow only
non-formatting control characters (e.g., "Control-C") as break
characters. The RCE strategy would allow the user to receive
immediate echoing for all his input until he typed such a control
The Remote Controlled Echoing Strategy is designed to provide echoing
for a full-duplex terminal as if it were locally connected to its
server. The effect of the long transmission delays will only be
evident as an increase in the processing performed at a break. Only
in the most interactive systems will such a delay be consistently
noticeable. For example if a user invokes a long FORTRAN
compilation, the fact that its start is delayed for half a second
will not normally be evident.
Furthermore, users who are able to type several messages ahead may
only notice a processing delay as a result of the first break-
interaction; both transmission and processing of successive messages
may occur during the printing of "echoes" and responses to previous
In the standard echoing scheme, characters are buffered at the
same server as they are keyed. But the user process does not see
them until a wakeup character has been typed. This means a TIP
using RCE could buffer characters in the transmission buffer until
a wakeup occurs and then send off the whole bunch. Unfortunately
we have chosen, for simplicity, to keep all knowledge of wakeup
characters at the serving site. This means that the TIP may
buffer beyond a wakeup (if it is not also a break) and delay the
process from doing some useful work. However, since in this case
no output is expected from the process, no noticeable delay is
visible to the user, except that the next break interaction may
take a little longer.
If the TIP chooses to buffer input before transmission, it will
transmit AT LEAST at every break character. The SERVER should be
able to instruct the TIP to transmit more often if it is
Conversational output LINKING is an example where transmission
strategy is separate from the break and wakeup strategies.
Transmission should occur on every character so that the character
can be promptly printed at each linked terminal, but no break or
wakeup need occur until a special escape character is typed (this
reawakens the EXEC, for example).
Conversational output linking also introduces another funny:
What happens when the ECHO (or DISCARD) path is enabled, but the
input buffer is empty (i.e. immediate echoing is occurring) and
something arrives in the output buffer? This "something" cannot be
a response to a previously keyed command, or the ECHO path would
be disabled. (This proof is left to the reader!) It is most likely
a system message like
[CR][LF]SYSTEM WILL GO DOWN IN 5 MINUTES[CR][LF]
or, a character arriving from another linked terminal.
Since such output does not fit neatly into our RCE scheme, the
following kludge is proposed. Unsolicited output should cause the
OUTPUT path to be enabled. The occurrence of either an MTB (DTB)
Or Empty Output Buffer will reenable the ECHO (DISCARD) path.
IV. Orthogonal Issues
Several other mechanisms may reasonably be incorporated within this
proposed addition to TELNET. The problems which need some further
1) Some means should be provided for the server to clear the
input, transmission, print and output buffers.
2) Some means should be provided for the user to immediately
clear the output buffer (i.e. suppress printing of lengthy
3) The server may want to ask about the number of characters
remaining to be printed.
4) A special tag character may be required to note where
clearing of the input buffer occurred.
5) The TIP's transmission strategy should be specifiable by
the server; perhaps a "Set Wakeup Classes" command should
6) The server should be able to cause the TIP to break on the
n-th input character regardless of whatever a break
character has been encountered.
7) Should the TIP or the server be responsible for properly
echoing a formatting control character such as a TAB?
8) The proper equivalence classes of ASCII characters have to
9) How should a CR be echoed?
10) Should one-for-one echo replacement (e.g. "$" for ESC) or
multi-character substitution (e.g. "^A" for Control-A) be
provided by the TIP?
11) The proposed DTB command directs the TIP to also discard
the delimiting break character. Should the break character
discard rather be dependent on setting the Break Action to
"don't echo" before sending the DTB?
Several of these issues will be addressed in RFC 358.
This document has presented a proposed optional addition to the User
TELNET. The next step is to perform some simulations and to
incorporate RCE into an experimental version of TELNET.
No recommendation is made that the current TELNET be discarded. For
example RCE should not be used for those half-duplex devices which
perform their own "echoing". If RCE is adopted as an alternate means
of echoing, changes to those TELNETs choosing not to implement it
should be minimal. Switching from RCE to the current echo mechanism
is an immediate result of either user or server sending a "You Echo"
(or "I'll Echo").
Much of the machinery required to implement RCE already exists at the
interface between the server process and its Terminal Handler or
Server TELNET. This means that no server process need be changed,
but that the process' means of specifying break classes, break
actions and echoing conventions must be interpreted by the Terminal
Handler and reissued to the TIP in terms of the corresponding RCE
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Erik J. Verbruggen 12/97 ]