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RFC 97
NETWORK WORKING GROUP NIC 5740
Request for Comments #97 John T. Melvin
Richard W. Watson
SRI-ARC
15 February 1971
A FIRST CUT AT A PROPOSED TELNET PROTOCOL
1 Introduction
This paper describes a first cut at a proposed Telnet protocol.
_Telnet_ is a process which runs at a _user's_ _site_ and allows him
to utilize a typewriter-like terminal to gain interactive service
from a remote _server_ _site over the ARPA Network. This paper was
motivated by our need to set specifications for a protocol which
would allow online access to the Network Information Center (NIC).
The Online System running at the Network Information Center we will
refer to as NLS(NIC). On thinking about the problem of setting
specifications for access to the NIC, we have tried to generalize our
ideas so that they would apply to other systems with characteristics
similar to ours. We realize that there are other terminal hardware-
software disciplines which might find it difficult to conform to all
the requirements stated here and, therefore, the final Telnet
protocol will differ from the one stated in this NWG/RFC. One
conclusion that we may all have to come to is that connection with
the network may force us toward a more standard way of handling
terminals and their character streams in our monitors and terminal
control hardware. In the meantime, we hope that this paper and
others on the same subject that may be in process, coupled with a
survey of hardware-software requirements at each site by a NWG
subgroup, can result in an initial standard network Telnet protocol
being agreed upon quickly, as it is important to get users onto the
network as soon as possible so that interactive network usage can
indicate further directions for network protocol evolution. Next we
outline some design problems, then propose some conventions to solve
these problems for access to systems such as the NLS(NIC) and
indicate some problems needing further study. The proposed
conventions for access to the NLS(NIC) are summarized in Appendix A.
2 Some Design Problems
2A. Basic Assumption
The function of the Telnet process is to make a terminal at a
user site appear over the network as logically equivalent to a
terminal "directly" connected to the server site. There are a number
of implications of this basic function.
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RFC 97 Proposed Telnet Protocol February 1971
i) The user should be able to cause generation of all codes which
a server system terminal can generate. With respect to the
Network Information Center and some other sites it would seem a
reasonable requirement to have keying conventions so that the user
can generate all 128 ASCII character codes as input to the
network. Other sites with different character codes may require a
Telnet process to provide those codes to the network.
ii) The user should be able to escape back to his local system or
escape from the server process to the server system.
iii) The Telnets of line-at-a-time systems should be able to work
with character-at-a-time systems and line-at-a-time systems and
Telnets of character-at-a-time systems should be able to work
with line-at-a-time and character-at-a-time systems.
2B Echo Control
We use the term echo control rather than the terms half duplex
or full duplex because the Telnet connection is in reality full
duplex with respect to network transmissions. Three terminal cases
need to be considered.
Case 1 - Character-at-a-time serving site echoed
Case 2 - Character-at-a-time user site echoed
Case 3 - Line-at-a-time user site echoed
Some serving sites may be able to operate with all three cases and
some convention is required to set the mode. Strictly speaking, what
characters are echoed for what keys struck is of no concern to the
serving site, although one would like to try to minimize differences
in typescript as it appears to the user.
2C Format Control Characters
The format control characters of horizontal tab (HT), vertical
tab (VT), form feed (FF), line feed (LF), and carriage return (CR),
need to be handled in a consistent way for Cases 2 and 3 above. With
Case 1 above, the situation is simplified.
2D Network Message Boundaries
The NCP to NCP protocol was specified with the goal of having
the network message boundaries being invisible to the user processes.
It would be good if this goal could be maintained, but it may be
difficult with some line-at-a-time systems.
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RFC 97 Proposed Telnet Protocol February 1971
2E An Implementation Convention
ConVentions to solve the above problems are most simply
established if we assume that the character stream received from a
Telnet process by the server site is entered into that point in the
server monitor where character input from "direct1y" connected
terminals is entered and output from the server process is entered
into the monitor point where normal character output is entered. The
server NCP receives its input at the point where normal monitor
character output is obtained. In other words, the server process
would obtain its input from the server monitor character buffers and
send its output to these buffers rather than obtainIng input directly
from NCP buffers or outputting to NCP buffers.
The Telnet process, on the other hand, would obtain and send
character streams directly from or to its local NCP.
Other situations exist where the user processes at both ends
communicate directly with the NCP. Therefore, we would recommend
that both modes of connection (user process-monitor-NCP, or user
process-NCP) be available for communication between the NCP and a
user process. These modes would be set under program control by the
user process. The initial network convention during the login
procedure and until changed by the server process would be to obtain
characters from and send characters to the monitor. The server NCP
communicates with the monitor also. The scheme is illustrated in
Figure 1.
The motivation for such flexibility may be clearer from the
discussion below.
3 Proposed Telnet Conventions
3A The server site is to assume initially that echoing is performed
by a user site process until explict1y commanded otherwise. If the
user site can send character-at-a-time, then after connection and
login have been established, tne user could switch Lo server-site-
echo by command to the server site and then command (invisible to the
server site) his local Telnet to change its echo mode also.
3B The server process is to assume it will receive the same
character set which terminals "directly" connected to it can
generate. (We recommend at least 128 character ASCII.) The user's
Telnet may have to recognize two-character sequences to enable
generation of both upper and lower-case codes and the control codes.
We recommend that the user be able to set either upper or lower case
as the default case for single case terminals and be able to specify
a case shift character. The user should also be able to specify a
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RFC 97 Proposed Telnet Protocol February 1971
character to indicate that the next character struck is to be
converted to the appropriate control character code. This latter
convention enables control codes directly generated at the terminal
to be recognized by the user's system thus enabling escape to the
user system. Creating a convention allowing all control codes to
enter the network and allowing output of the network to feed into the
server monitor before entering the server process, gives a simple
mechanism for generating an escape to many existing systems.
(The problem is more complicated than this for some systems and we
discuss it further below.)
3C We recommend that network standards be established for the
meaning of local echoes of HT, VT, and FF or a convention to be
established for sending the meaning of these characters to the server
process. The NLS(NIC), for example, needs to keep track of the
position of the print head and in the absence of such conventions
will convert these character codes to spaces and line feeds. This
means that the appearance of the page on output may differ from the
appearance on input. It would be helpful to the user if his page on
output could be formatted as it appeared on input.
3D LF characters would be handled as if they were generated by
hitting the line feed key on a terminal "directly" connected to the
server system.
3E The carriage return (CR) character can be the source of
considerable difficulty. For example, on input, different systems
and the same system at different times, can echo and transmit
different codes to the terminal and the user process. Some monitor
systems echo nothing, just a CR, or a CRLF. Some systems transmit a
CR, CRLF, or end of line code (EOL) to the user process. The user
process may control the echo or add to it. Given the combinations
which can exist at each end of the network connection and with
respect to each other, confusion can exist unless we assume the
definition of 2A and the implementation convention of 2E. These
assumptions imply that when a CR is struck, a CR gets sent over the
network. If the user monitor system or terminal control hardware
converts a CR to a CRLF or EOL, then the Telnet program must convert
it back to a CR. When the CR reaches the server monitor it will
handle it properly for the server process.
When echoing is handled by the server system, the proper code or
codes will be echoed. The user Telnet on receiving a CRLF can pad it
with the proper nulls to handle carriage movement timing for a
particular terminal.
When echoing is handled by the user system it would be ideal if
the user's Telnet or system used the same echo convention as the
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RFC 97 Proposed Telnet Protocol February 1971
server system would. This means that either the Telnet must have a
table of echo conventions for the various systems to which it can
connect, or that it can obtain this information from the server
system or process, or vice versa.
For an initial Telnet protocol this is probably not necessary.
The user system can default and echo a CRLF on each CR received.
This default should be satisfactory for all the situations we are
familiar with and for the NIC.
3F For communication from character- and line-at-a-time systems,
the Telnet process may need to recognize a character (user
assignable) which we call end of stream (EOS). This character is to
have the function defined in the following discussion. The important
point is to distinguish end-of-stream as a network function and end-
of-line as a user or server system function. Consider line-at-a-time
systems first. We have not had much experience with line-at-a-time
systems, so what follows will need further study and clarification.
As we understand it, line-at-a-time systems recognize a character
such as CR or a break signal as the code to wake up the user process
and cause transmission to it of the line of text. From the point of
view of NLS(NIC) it is important that the user be able to enter lines
of text each terminated by a CR where appropriate and at other times
to be able to enter text not terminated with a CR. (A statement for
NLS(NIC) is a string of text of "arbitrary" length and need not have
CRs in it; on output the line is folded for the user at his (user
definable) page boundary.)
As an example of what is required, consider the case where the
user's system recognizes CR as end-of-line. In this case the Telnet
would be awakened when a CR is received. We would recommend that in
this case the CR code be literally entered into the Telnet output
buffer. If a CR is preceded by an EOS character, then the CR should
not be placed in the Telnet output buffer. Transmission through the
network can take place either when an EOS is received or
automatically when the Telnet output buffer fills. Transmission to
character-at-a-time systems from line-at-a-time systems could require
the awkward striking of three keys to get one character through the
network.
Now consider transmission for a character-at-a-time system to a
server line-at-a-time system. A similar problem to the one to be
described also exists between line-at-a-time systems. Given the
definition of an EOS character different from CR, a line can be
buffered up until the EOS is received and then sent without the EOS.
How is the serving system to know that a line has been sent? One way
would be for the serving NCP to recognize message boundaries. This
convention would violate a design goal. Another way would be for the
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RFC 97 Proposed Telnet Protocol February 1971
user Telnet to request its NCI to send an INS command. The sending
of INS type of control commands might introduce race conditions in
the network and should be investigated before their use with a Telnet
process is established. Since some of the line-at-a-time systems we
know have special hardware that recognizes the end-of-line signal, we
need some way to be compatible with this hardware using software
control signals. We leave this problem for further NWG subgroup
study.
3G We now come back to the problem of interrupting or escaping in
the remote server system. In systems which do not lock out the input
keyboard when output is going on, the mechanisms and conventions
outlined above would seem adequate unless a special break signal is
the escape signal. This latter case requires more study. In systems
which allow no input while output is occurring, one may have to live
with the consequences of such a terminal discipline and be prepared
to wait until output stops before an escape code can be sent. If the
keyboard is locked and an escape break signal can be sent to the
user's system, it can prevent output from going to the terminal, but
must be prepared to continue receiving it from the server site until
the user can inform his Telnet process to send an interrupt or escape
signal to the server site. Again this is a problem for further
study.
The Online System of the Network Information Center operates on
a character-at-a-time monitor system and the conventions established
in this paper are adequate for access to it. These conventions are
summarized in Appendix A.
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RFC 97 Proposed Telnet Protocol February 1971
APPENDIX A
NETWORK CONNECTION PROTOCOL TO SRI-NETWORK INFORMATION CENTER
1 Initial Connection Protocol
Connection establishment to NIC is identical to that presented in
Section II of NWG/RFC 80 NIC (5608,); it is reproduced here:
Telnet contacts NIC
NIC <- user site
RTS <us> <l> <p>
NIC logger is socket 1
user site <- NIC
STR <l> <us> CLS <l> <us>
if accepted
CLS <l> <us>
if rejected
assuming NIC accepts
user site <- NIC
STR <ss+l> <us>
RTS <ss> <us+l> <q>
NIC receives text thru local socket ss from remote
socket us+l via link q
assuming user site accepts
NIC <- user site
STR <us> <ss+l>
RTS <us+l> <ss> <r>
NIC sends text to remote socket us thru local socket
ss+l via link r
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RFC 97 Proposed Telnet Protocol February 1971
.
.
.
user site <- NIC
ALL <q> <space>
.
.
.
NIC <- user site
ALL <r> <space>
2 Connection Breaking Protocol
A CLS trade is made between the NCPs for each of the two
connections as per Document #1 NIC (5143,).
We may decide to put a time-out into the NIC connections such
that no interaction for some (as yet unspecified) "reasonable"
length of time will result in a CLS-out of the connections being
initiated by NIC.
3 Third Level Protocol
The first 8 bits received by NIC thru socket ss should be the
message data type designating that an 8-bit ASCII stream follows, as
per NWG/RFC #63, NIC (4963,).
I.e., the first 8 bits are 00000001
The first 8 bits received by Telnet thru socket us will also
indicate a message data type of l. Each network message should have
an integral multiple of 8 bits. If a network standard is established
different from the suggestion of NWG/RFC #63, NIC (4963,), then we
would change this protocol to conform.
NIC will have NCP-generated interrupts disabled, i.e.,
INR will be ignored
INS will not be sent to the remote host
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RFC 97 Proposed Telnet Protocol February 1971
4 NLS(NIC) Character Conventions of Interest to Telnet
Echoing can either be under control of NIS(NIC) or under control
of the user site. When we refer to echoing below, we mean under
control of NLS(NIC). When echoing is handled by the user site we
would expect the user to set the NLS(NIC) output conventions to
conform to the echoing conventions at his site. NLS(NIC) assumes
echoing is handled by the user site unless explicitly commanded
otherwise.
Format affecting control characters
horizontal tab
spaces to next (user definable) stop on both echoing
and output.
if during literal input, enters file as ASCII '11.
form feed
carriage return and (user definable) appropriate number
of line feeds on echo and output.
If during literal input, enters file as ASCII '14
vertical tab
carriage return and (user definable) appropriate number
of line feeds on echo and output
if during literal input, enters file as ASCII '13
carriage return
carriage return followed by line feed on echo and
output
if during literal input, enters file as EOL (see below)
line feed
line feed on echo and output
enters file as ASCII '12 on literal input
EOL (end of line)
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RFC 97 Proposed Telnet Protocol February 1971
presently ASCII code '37
carriage return followed by line feed on echo and
output
if during literal input, enters file as ASCII '37
If the user's system automatically appends a LF to a CR
before sending it to Telnet or converts CR to some EOL code
not ASCII '37, we would expect Telnet to send NLS(NIC) just
a CR or ASCII '37. If we receive CRLF, then on output we
will send CRLFLF.
5 NLS(NIC) Interrupt Attention Convention
A (user definable) ASCII code in the text input stream is used to
abort the executing process and return control to the main NLS(NIC)
command processor.
This code is presently DEL (ASCII '177).
Escape to the NIC monitor: No escape is required as all
operations needed for use of the NIC can be performed within
NLS(NIC).
Character Set: We strongly recommend that the Telnet process
be able to generate by some set of keying conventions all 128
ASCII codes. Use of NLS(NIC) will probably feel most comfortable
from a device with upper and lower case graphics, although we can
provide service to single case devices. We can provide a useful
service if the full ASCII set cannot be sent, but would like to
minimize the special cases we have to handle. Sites which cannot
provide the full ASCII set should contact us.
+----+ |
| | Server |
| | Program |
| | |
+----+ |
^ | |
| v |
+----+ Terminal |
| | control |
| | software | SERVER
| | and | SITE
+----+ possibly |
^ | hardware |
| v |
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RFC 97 Proposed Telnet Protocol February 1971
+----+ |
| | |
| | NCP |
| | |
+----+ |
^ | |
| v |
. .
. . Figure 1 -
. .
. . Telnet Connection
^ |
| v
+----+ |
| | |
| | NCP |
| | |
+----+ |
^ | |
| v |
+----+ |
| | |
| | Telnet |
| | |
+----+ |
^ | | USER
| v | SITE
+----+ Terminal |
| | control |
| | hardware- |
| | software |
+----+ |
^ | |
| v |
+----+ |
| | User |
\ | terminal |
\--+ |
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Tony Hansen 08/08 ]
Melvin & Watson [Page 11]