|Title||Discussion of Telnet Protocol
Network Working Group T. O'Sullivan
Request for Comments: 139 Raytheon
NIC: 6717 7 May 1971
Discussion of TELNET Protocol
The attached discussion is an extension of RFC 137, NIC #6717, and is
presented to provide useful background to designers and implementers
to help them interpret the proposed Protocol and evaluate it in
preparation for further discussion at the Atlantic City meetings.
While the views in the discussion represent those of various TELNET
committee members, they should not be interpreted as being the agreed
view of committee. They are the author's understanding of some of
the arguments and background to the PROTOCOL proposed in the TELNET
* See Footnotes to attached discussion for changes to RFC 137.
Discussion of TELNET PROTOCOL
The use of a standard, network-wide, intermediate representation of
terminal code between sites eliminates the need for using and serving
sites to keep information about the characteristics of each other's
terminals and terminal handling conventions, but only if the user,
the using site, and the serving site assume certain responsibilities.
1. The serving site must specify how the intermediate code will be
mapped by it into the terminal codes that are expected at that
2. The user must be familiar with that mapping.
3. The using site must provide some means for the user to enter
all of the intermediate codes, and as a convenience, special
control signals, as well as specify for the user how the
signals from the serving site will be presented at the user
Other schemes were considered but rejected. For example, a proposal
that the using site be responsible to transmit to and from the code
expected by the serving site was rejected since it required that the
using site keep tables of all serving site codes and provide mapping
for each case. The information would require constant maintenance as
new hosts were added to the network.
Since it is not known how the current or future sites will specify
the mapping between the network-wide standard code (7 bit ASCII in an
8 bit field) and the codes expected from their own terminals, it
seems necessary to permit the user to cause every one of the 128
ASCII codes, plus (for full user power) selected control signals
(either of a TELNET control nature, or of a special terminal nature
such as break or attention).
There was strong feeling about the importance of the user/system
interface at the using site, but equally strong feeling that this
problem is one of local implementation and should reflect the using
site installation philosophy rather than the subject to network-wide
standards. Some topics of consideration in this area are:
1. How to represent special graphics, not available at the using
site, at the user's terminal.
2. Treatment of upper/lower case problem on TTY 33 and 35.
a. Representing lower-case output.
b. Providing users with shift and shift lock signals.
3. Incorporating editing capability in TELNET.
4. Extending user options in Network mode not available to local
e.g., hold output
5. Permit users to specify how keyboard input is to be translated,
e.g., let a character from the terminal cause a specified
string to be sent by the user's TELNET.
In early discussions, there was pressure to get a simple statement of
protocol out early to permit early use of selected systems. The
counter pressure to provide a richer set of protocol in the first
release was also present. Work started in the direction of the
latter, but the complexities introduced were not necessary for early
use of the network. The proposed solution to the TELNET protocol
problem seems to provide a mechanism for a minimum implementation (to
be discussed later) while providing a basis for developing richer
sets of protocol for present and future use in terminal applications,
process-process communications, and use by other conventions to pass
data or control information.
The understanding that ASCII be used as a network-wide code has been
established for some time. Its use in TELNET provided a problem with
respect to the limitation of a maximum character set of 128. Some
systems provide for more than this number in their operation, and
therefor, as serving sites cannot map on a one for one basis.
Each such serving site could probably provide a reasonably useful
character set, including all system control signals, by mapping 128
of its codes and just not provide a network user access to the other
codes. However, any character left out might later be used in a
major application at that site as a special control signal. This
could result in denying network users the facility offered by that
application. Serving sites are, therefor, encouraged to provide a
full mapping between the ASCII code and the code used on the serving
The ASCII code for ESC (known to some as ALT MODE) has been selected
as an escape . For each serving site character not mapped on a
one for one basis, the serving site can specify an escape character
or string of escape characters (preferably a printable graphic) to
represent it. Thus, the user could enter the full set of serving
site code from any network terminal operating through the Network
Virtual Terminal (NVT) ASCII convention. The serving site, in
generating output directed at the user's terminal, would be expected
to map out such a character and transmit the appropriate ESC
character or string of ESC characters.
Example: A serving site, whose normal code is EBCDIC, has
specified that cent () has not been mapped on a one for one
basis and that to transmit the character, users must enter ESC
followed by C. At a using site, the TELNET implementers have
decided to try to print out all ESC characters using \ to indicate
ESC. On receipt of the representation for cent, the user would
see \C on his print-out.
The representation of the end of a physical line at a terminal is
implemented differently on network HOSTS. For example, some use a
return (or new line) key, the terminal hardware both returns the
carriage or printer to start of line and feeds the paper to the next
line. In other implementations, the user hits carriage return and
the hardware returns carriage while the software returns to the
terminal a line feed. The network-wide representation will be
carriage return followed by line feed. It represents the physical
formatting that is being attempted, and is to be interpreted and
appropriately translated by both using site and serving site.
Example: A Multics user is working, through the network, on some
serving site HOST. In the course of the session, the user has
numerous occasions to hit New Line on his Mod 37 TTY. Each time
the Multics system is awakened by a New Line interrupt, the line
of buffered characters is passed to TELNET where it is scanned for
special characters. If none is found, carriage return followed by
line feed is inserted where New Line was entered, and the line is
turned over to the NCP for transmission. When the TELNET finds
the carriage return line feed sequence in the data stream coming
from the serving site, the two characters are replaced with New
Line code and sent to the terminal.
The decision to have the assumed condition for echo be that the using
site will provide any echo necessary for its terminals was taken
because of the difficulties faced by some installations that cannot
turn off their echo or that have terminals that print locally as a
result of key strokes. Serving sites could take the position "let
the user turn my echo off", but this seems an unnecessary burden on
the user. In addition, some serving sites may choose not to supply
any echo service, in which case the no echo assumption will supply a
network-wide condition, while other assumptions would give a mixed
starting connection. 
The convention of using "I ECHO", "YOU ECHO" seems to fill both the
requirements for dynamic echo control and for a minimum
implementation of TELNET Protocol.  An agreed-upon exchange to
pass echo control (i.e., two sites exchange the I ECHO/YOU ECHO
codes) results in passing the control from one site to the other.
Example: A serving site is exchanging control information with
the USER in an area where the serving system asks for pass word
and wants to suppress the printing of the pass word at the using
site's user terminal. (In this case, the using site has the
ability to control the print capability at the user's terminal.)
Using site has been echoing to the user's terminal.
Serving Site to Using Site (--->)
Using Site to Serving Site (<---)
<--- (User enters password at terminal)
---> (No echo sent)
---> YOU ECHO
<--- I ECHO
After the exchange, the original normal condition is re-
established. If the using site did not have dynamic echo control
installed in its TELNET implementation, the serving site would
have signaled I ECHO several times, received no response, and
assumed that the using site could not comply proceeding to call
for the pass word without the normal protection of inhibiting
TELNET control signals are of two types: one that results in
transmission of signals down the network to a receiving site; the
other intended for the user/process site only. The latter type will
be discussed later. So far, we have discussed the former type,
specifically dealing with echo control.
The use of ESC should not be considered a TELNET-wide standard, but a
convention limited to the 7 bit ASCII mode of transmission. Other
conventions, to be incorporated later, may include binary
transmission, EBCDIC, etc. Presumably, each will have its own
convention for an escape character to extend its code set.
Since it is expected that conventions other than ASCII will be
implemented under TELNET, a code to indicate a DATA TYPE representing
each set of conventions will be employed. The control code X'AO' has
been selected to represent the ASCII convention in TELNET. Since a
number of applications may wish to transmit transparently (i.e., 8
bit binary data), X'Al' is being reserved for that purpose. The
TELNET control code X'A2' is reserved for an expected set of EBCDIC
conventions. The DATA TYPE is expected as the first byte of data
over a TELNET connection. Minimum implementations will be aided by
providing a default. That is, if the first byte over a connection
has the high order bit set as zero, then the transmission has begun
in ASCII mode.
Each set of conventions, i.e., each DATA TYPE will be expected to
have a convention for that DATA TYPE to signal that it is returning
to control mode. This return may be for the purpose of making use of
an existing control codes or to change data type. X'88' is used .
Example: At the using site, a terminal has a special device on it
(e.g., plotter, laboratory instrument, control box, etc.) that is
controlled by binary code in 8 bit bytes. The terminal uses a
special "enter" code that routes signals to the device and cuts
off printing at the terminal until a special "leave" signal is
received from the driving process. The driving process in this
case is at a remote serving site. It is assumed in this example
that a DLE convention is used for transparent transmission, a
single DLE signal representing return to control. Normal
transmission has been in ASCII.
Driving Process (at Serving Site) to Using Site) ---->
Using Site to Serving Site <----
ENTER code...8 bit binary bytes...
Using Site TELNET to Terminal |
Enter code...8 bit binary bytes...
Turn printer off, feed transparently to special device, look
for LEAVE signal
8 bit binary bytes...LEAVE signal...single DLE
8 bit binary data...LEAVE signal MESSAGE
During this sequence of exchanges - at the terminal, feed binary
data to special device until LEAVE signal is sensed, strip off
LEAVE signal, turn on printer and block data path to special
device, print MESSAGE at terminal.
There is a special control signal on some terminals that has no
corresponding bit pattern in ASCII, but is transmitted by a special
electrical signal. This control signal is ATTN on a 2741 and BREAK
on a teletype. The ASCII DATA TYPE in TELNET will use the code X'81'
to represent BREAK. (There is a corresponding control signal for use
from serving sites to using sites for reverse break, and it is
assigned the code X'82').
Some systems treat the break as an extra code available for use in
conjunction with the data stream. For example, one system uses break
as a special editing code meaning "delete the current line to this
point". In these cases, the code may simply be inserted in the data
stream with no special additional action by the user.
Other systems use BREAK or ATTN in a special interrupt fashion, to
mean stop processing the application and give me the supervisor, or
cancel the present job, etc. (Other systems use normal characters
for this purpose, such as "Control C".) In these cases, because of
differences in the ways both serving and using sites operate, it is
necessary to take a route in addition to the normal TELNET data
stream to signal that the special control signal is imbedded in the
The PDP-10 normally will, when it fills its input buffer, continue
to accept characters from a terminal examining each to see if it
is a control character, then act on it if it is or throw it away
if it is not.
Since the TELNET server at the serving site is at the mercy of the
NCP with respect to controlling the bunching, and therefor,
arrival at the TELNET of bursts of characters, TELNET
implementations might be expected to choke off flow to the buffers
until they are ready to accept characters without throwing them
Under this condition, the serving process might be outputting to the
using terminal, the input buffers fill up, and a control C get stuck
in the data stream that has been choked off.
A similar problem could occur with the Multics or some IBM system as
a server. The user at a using site gets into an output loop at the
serving site and wants to break the process without having to release
his TELNET connection. The buffers clog the connection, transmission
is choked off, and the control C break, or other user control signal
gets stuck in the pipeline.
_Example - Solution_
The user at the using site knows he is entering a special control
signal (break, ATTN, control C, etc.) and follows it with an X'80'.
(The local instructions at using sites for accomplishing this may
differ from site to site.)
Using Site TELNET to Serving Site
Insert X'80' in Data Stream
Using Site TELNET to Using Site NCP
Send an INS
Sending Site NCP to TELNET Server
Look out, here she come
Serving Site TELNET
Does its special thing until it sees X'80' then resumes
Thus, depending on the server's local implementation to provide
adequate service, a special handling of the data stream can be
invoked whenever an INS is received in order to get the special
character. When it sees X'80', it recognizes it as a SYNC character
and knowing that the special character has been passed on, strips the
X'80' from the data stream and returns to normal mode.
If the X'80' arrives before the INS, a counting scheme can keep the
activity appropriate to the serving site conditions.
This approach to handling selected special characters or signals
relieves the using TELNET processes from having to recognize the
special serving site characters, as well as from having to know how
the serving site wants to handle them. At the same time, the
procedure requires only a minimum level of user understanding of the
serving site. This seems appropriate, since the TELNET ASCII
conventions are providing a Network Virtual Terminal, not a Network
The ability of the user to cause the using site TELNET to send any
combination of ASCII characters in a string, and only that
combination, is viewed as important to the user utility of the TELNET
ASCII conventions. Because of this, some user sites may find it
necessary to provide special local TELNET control signalling from the
user to the using site.
A user on a line at a time system (Multics, System 360, GECOS,
etc.) is working through the Network on a serving site that
operates a character at a time. The application is a debugging
aid that permits the user to type in a memory location = to which
it will respond with n where n represents the current contents of
that location. The serving site process does not expect to see
the location = followed by a carriage return line feed sequence.
The user at the using site should be able to type in the location,
follow it with a signal to suppress the end of a line convention,
followed by a new line or return, and expect the location number =
to be transmitted immediately without an end of line sequence.
In another case, a using site has decided that it is convenient to
accumulate four characters at a time and transmit them to the
serving site, unless an end of line is observed, in which case the
end of line sequence is sent preceded by whatever number of
characters have been accumulated, (presumably three or less). In
the same debugging application, the address is such that the end
does not correspond with the four character buffer demarcation.
The user should have the ability to enter a code for "transmit
immediately" in place of the Carriage Return in order to preserve
neat formatting, and expect the address to be sent to the serving
TELNET controls have been discussed and those introduced to date
are probably sufficient for an early implementation of TELNET
ASCII convention. There will be a need to establish a mechanism
for the controlled assignment (on request by Network Sites), and
announcement of DATA TYPE and CONTROL codes.
It should be noted that some controls are network-wide TELNET
controls, while others are specific to the ASCII Data Type. It
should be further recognized that some local control messages do
not require a corresponding network-wide code.
While it is recognized that even a minimum implementation of
TELNET for a using site is expected to permit the user to send any
selected ASCII string (and only that string) to the serving site,
it is not necessary for a serving site to implement a full mapping
from ASCII to local code, nor is it necessary for either the using
or serving sites to implement all control codes.
_Example - Using Site_
A minimum implementation of the TELNET protocol for the using site
would permit ignoring (and stripping) any control signals from the
serving site since they would all either require agreement or
acknowledgement (e.g., DATA TYPE, ECHO CONTROL, etc.) or can be
ignored with no particularly harmful results (e.g., reverse
_Example - Serving Site_
A minimum implementation of the TELNET protocol for the serving
site could provide one for one mapping for the most important 128
serving system controls and graphic signals, and ignore all
It would be helpful if a minimally implemented receiving site, when
it recognizes an incoming control signal for which appropriate
reaction is not available, could respond with X'87' (The following
not implemented at this site) and follow it with the code just
Whenever an ASCII TELNET connection is lost, it should be assumed
that the process at the other end of the connection has been quit,
aborted, failed, etc. In this way, a minimum using site installation
can fail to implement the break and break synchronization, and have
the user rely on the using site local procedure for leaving a running
local process and returning to the supervisor to break a connection
to a remote serving site.
User recognizes that he is caught in an output loop and wishes to
stop his user process at the serving site. The serving site
requires a break, but the using site minimum implementation has
not made it available. Even if it had, the INS was not
implemented and could not be used to unblock the input pipe.
Locally, the using site convention for leaving a process and
getting to supervisory level is to hit the attention key on the
2741 terminal. The user does this and is passed to the supervisor
where he signals to release the TELNET connection. The serving
site, seeing that an ASCII TELNET connection has been lost,
assumes that the user is ended either normally or abnormally.
Serving site cancels the user's process. The user tries again by
re-establishing the connection, logging in again, re-initiating
the process, etc.
Other conventions under TELNET may make quite different assumptions
about lost connections, and some may go as far as dynamic
establishing and releasing of connections.
The proposed TELNET ASCII implementation leaves much uncovered, but
seems to permit early simple implementation with varying levels of
capability, along with the capacity to expand in several ways to meet
There is an important open question. Should a PROTOCOL such as
TELNET provide the basis for extending a system to perform functions
that go beyond the normal capacity of the local system. For example,
a local system may not provide functions such as Hold Output, Kill
Print, etc., but it could extend it for network purposes through
TELNET. If so, to what extent should such extensions be thought of
as Network-wide standards as opposed to purely local implementations.
 Please drop the (s) at the end of "character" in paragraph 3,
page 3, RFC 137, NIC #6714.
 Also make note that the starting assumption in the initial
exchange between using site and serving site will be that the using
site will (if necessary) provide echo and the serving site will not.
 Note: Please change RFC #137, NIC #6714, page 4 - Code X'85' to
 Please note on page 4 of RFC 137 that the receipt of an X'88'
should be responded with by the receiver sending a double signal,
i.e., X'88'X'88' if the new DATA TYPE can be handled.
 Cent sign
[This RFC was put into machine readable form for entry]
[into the online RFC archives by Lorrie Shiota, 1/02]