Network Working Group J. White
Request for Comments: 206 Computer Research Lab, UCSB
NIC: 7176 9 August 1971
A User TELNET
Description of an Initial Implementation
PREFACE
The User TELNET described in this document has been implemented
within UCSB's Online System by Mark Krilanovich and makes teletype-
compatible time-sharing systems in the Net accessible to Online
System users.
Contents
Page
I. Function................................................... 1
II. Invoking TELNET............................................ 2
III. The Virtual Teletype....................................... 5
IV. Returning to TELNET........................................ 11
V. Breaking TELNET Connections................................ 12
VI. Abnormal Network Conditions................................ 12
FIGURES
Page
Figure 1. Network Sites........................................ 4
Figure 2. TELNET Character Set................................. 6
Figure 3. Connect Error Codes.................................. 13
Figure 4. Input Error Codes.................................... 13
Figure 5. Output Error Codes................................... 14
I. Function
This document describes a program whose function is to make an Online
System terminal _appear_ to any teletype-compatible, time-sharing
system in the Network _as if it were directly connected to that
system_. By invoking this program from his terminal, an OLS user can
effectively dial up a system at another site and use it as if he were
a local user of that system. The program performs the following
major functions:
1) it resolves keyboard and display device incompatibilities
2) performs character set transformations, and
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RFC 206 A User TELNET August 1971
3) makes its presence and that of the Network transparent to the
user.
The program which performs these functions is called a _User TELNET_.
It operates in conjunction with another program called a _Server
TELNET_ executing at the site whose system is being used. A Server
TELNET exists at each site whose system is accessible from the Net.
Similarly, a User TELNET must exist at a site before users at that
site have access to remote systems.
User and Server TELNETs access the Net through their respective NCPs,
and operate under a Network-standard protocol which specifies the
hand-shaking procedure by which the two parties connect themselves to
one another, as well as the format of data sent over a pair of
Network connections established between them. A User TELNET makes
such protocol considerations transparent to its users.
The TELNET protocol is based upon the notion of a virtual teletype,
employing a 7-bit ASCII character set. The primary function of a
User TELNET, then, is to provide the means by which its users can
'hit' all the keys on that virtual teletype. If the user's keyboard
happens in fact to actually be a teletype, then the mapping procedure
is a very simple one. However, in UCSB's case, where a user's input
device is an OLS keyboard (a device very different from a teletype),
the mapping is more complex.
The primary function of a Server TELNET is to map the keys on the
virtual teletype into that set of keys _its_ local users can push on
_their_ keyboards. Again, if those keyboards are teletypes, the
mapping done by the Server TELNET is straightforward. In other
cases, the task may be very complex.
A similar set of mappings transforms ouput generated by the remote
system into a form displayable on the user's output device (in UCSB's
case, this device is normally a storage scope).
II. Invoking TELNET
This and succeeding sections describe a preliminary version of a User
TELNET (hereafter referred to simply as TELNET) currently implemented
within the Online System. This initial implementation does not
provide all of the services that a final version must provide, nor
does it provide all the conveniences that the next version will
offer. It's a first pass which will be upgraded in the near future.
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RFC 206 A User TELNET August 1971
TELNET is accessible from NET, a subsystem (like MOLSF and COL) of
OLS. A user is by default prohibited from loading NET. To have
access to NET enabled fro his user number, a user need only contact
the Computer Center. Assuming the foregoing, Net can be loaded with
the key sequence:
KEYBOARD ENTRY OLS QUERY/RESPONSE
SYST WORK AREAS UPDATED
LOAD NET LOAD NET
RETURN FILE LOADED
or by specifying the system name 'NET' at login.
Once in NET, TELNET is invoked by going to Level II Real and hitting
_LOG_. TELNET responds with a query for site number. The user
should enter in decimal the number of the site (as indicated in
Figure 1) to which he desires access, followed by _RETURN_. TELNET
will then query the user for the Server TELNET's socket_number. By
convention, this number is normally 1, but in certain cases some
other socket may be appropriate. In any case, the user should enter
the socket number in decimal and hit _RETURN_. The dialogue, then,
goes like this:
KEYBOARD ENTRY OLS QUERY/RESPONSE
II LOG FOREIGN SITE NO. = (site number)
site number RETURN FOREIGN SCK NO. = 1
1 RETURN
When the second RETURN is depressed, TELNET will attempt to contact
the designated Server TELNET and establish a duplex connection for
the user. Once the connection has been established, TELNET will
erase the tube and position the carriage to the upper left-hand
corner of the screen. From this point on, the user is effectively
connected to the remote system. TELNET enters a mode in which keys
pushed by the user are mapped into their virtual-teletype equivalents
and incoming text similarly transformed and displayed on the scope.
If the remote system to which the user is connected normally issues a
login message, that message will be the first to be displayed. In
any case, the user should proceed by logging in to the remote system
according to the conventions appropriate to that system.
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RFC 206 A User TELNET August 1971
Figure 1.
Network Sites
Institution Location Computer Site Site # Site #
Name (HEX) (DEC)
------------------------------------------------------------------------
UCLA Los Angeles Sigma-7 UCLA 01 1
UCLA Los Angeles IBM 360/91 UCL1 41 65
Stanford Research
Institute Menlo Park PDP-10 SRI0 02 2
Stanford Research
Institute Menlo Park PDP-15 SRI1 42 66
UCSB Santa Barbara IBM 360/75 UCSB 03 3
University of Salt Lake City,
Utah Utah PDP-10 UTAH 04 4
Bolt, Beranek, Cambridge,
and Newman Mass. PDP-10 BBN0 05 5
Bolt, Beranek, Cambridge,
and Newman Mass. DDP-516 BBN1 45 69
MIT Cambridge,
Mass. GE 645 MIT0 06 6
MIT Cambridge,
Mass. PDP-10 MIT1 46 70
Rand
Corporation Santa Monica IBM 1800 RAND 07 7
(IBM 360/65)
System Development
Corporation Santa Monica DDP-516 SDC 08 8
(IBM 360/67)
Harvard Cambridge,
University Mass. PDP-10 HRV0 09 9
Harvard Cambridge,
University Mass. PDP-1 HRV1 49 73
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RFC 206 A User TELNET August 1971
Lincoln Lexington,
Laboratory (MIT)Mass. IBM 360/67 LINO 0A 10
Lincoln Lexington,
Laboratory (MIT)Mass. TX2 LIN1 4A 74
Stanford
University Stanford PDP-10 STAN 0B 11
University of Urbana, Ill. PDP-11 ILl 0C 12
Illinois
Case Western
Resevrve Cleveland,
University Ohio PDP-10 CASE 0D 13
Carnegie-Mellon Pittsburgh,
University Pa. PDP-10 CARN 0E 14
Burroughs Paoli, Pa. B-6500 BURR 0F 15
(Illiac IV)
III. The Virtual Teletype
The algorithm by which TELNET maps the OLS keyboard into the virtual
teletype's character set, and by which it maps that same character
set into the set of characters which can be displayed on the user's
storage scope is defined in Figure 2. A line of that figure reads as
follows:
For line 11:
The key labeled 'LF' (meaning 'Line Feed') on the virtual
teletype, sometimes referred to as 'control-J', is struck by
pressing either _SUB_, _CASE J_, or [1] on an OLS keyboard.
TELNET sends to the remote system an 8-bit character with the
value X'0A'. Whenever 'LF' is received from the foreign system,
TELNET displays it by rolling the carriage down one line.
As indicated in the figure, _CASE_ substitutes for the CNTRL key on a
teletype. Hence, for example, 'control-C' is represented by the key
pair 'CASE C'. Note, however, that _CASE_ and 'C' are hit in
sequence, whereas on a teletype the CNTRL key is held down while 'C'
is struck. 'CASE A' - 'CASE Z' each have an equivalent on the upper
keyboard, and the position of that key on the upper keyboard
corresponds to that of its counterpart on the lower keyboard. Hence,
LS equivalent to CASE A.
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RFC 206 A User TELNET August 1971
Although TELNET provides the user with the means for transmitting
both upper- and lower-case alphabetics (hitting 'A' sends a lower-
case 'A'; holding down _SHIFT_ and hitting 'A' sends upper-case 'A'),
there is no provision in OLS for displaying lower-case characters on
the storage scope. Hence, TELNET maps lower-case alphabetics into
upper-case before displaying them.
The four virtual teltype keys 'ENQ', 'ACK', 'BEL', and 'NAK' are
displayed by TELNET in a special manner. Two lines are reserved for
these characters at the top of the display area, and whenever TELNET
receives such a character from the remote system, it displays it in
its assigned position within the two-line field. TELNET always
positions those four characters at the top of the display area,
regardless of the user's current position on the scope, then returns
to it.
A storage scope has both positive and negative attributes relative to
a teletype. Display is much quieter on a scope than on a teletype.
However, the noise made by the return of a teletype carriage is often
a valuable cue to the user, frequently signifying that the previous
line of input from the user has been accepted by the system. As a
substitute for this particular cue, TELNET displays an underline
('_') at the left edge on the next line whenever it receives a
carriage return (CR) from the remote system.
Figure 2. TELNET Character Set
To | | Also |Push | Or |Code Sent |Upon receipt
Send| (Explanation)| Known as |Either| |is (in hex)|Displayed as
----+--------------+----------+------+---------+-----------+------------
NUL |NULL | ^@ | [2] | | 00 |
| | | | | |
SOH |Start of | [3]^A | LS |[4]CASE A| 01 |
| Heading | | | | |
| | | | | |
STX |Start of Text | ^B | ATAN | CASE B | 02 |
| | | | | |
ETX |End of Text | ^C | LOG | CASE C | 03 |
| | | | | |
EOT |End of | | | | |
| Transmission| ^D | REFL | CASE D | 04 |
| | | | | |
ENQ |Enquiry | ^E | [4] | CASE E | 05 | [5]
| | | | | |
ACK |Acknowledge | ^F | UP | CASE F | 06 |
| | | | | |
BEL |Bell | ^G | DOWN | CASE G | 07 |
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RFC 206 A User TELNET August 1971
BS |Backspace | ^H | EVAL | CASE H | 08 |
| | | | | |
HT |Horizontal | | | | |
|Tab. | ^I | INV | CASE I, | 09 |
| | | | TAB | |
| | | | | |
LF |Line Feed | ^J | SUB | CASE J, | 0A | Carriage
| | | | [1] | | rolled down
| | | | | |
VT |Vertical Tab. | ^K | MAX | CASE K | 0B |
| | | | | |
FF |Form Feed | ^L | MOD | CASE L | 0C |
| | | | | |
CR |Carriage | | | | |
| Return | ^M | DEL | CASE M, | 0D |[6]Carriage
| | | | RETURN| | returned
| | | | | |
SO |Shift Out | ^N | ARG | CASE N | 0E |
| | | | | |
SI |Shift In | ^O | DIFF | CASE O | 0F |
| | | | | |
DLE |Data Link | | | | |
| Escape | ^P | SUM | CASE P | 10 |
| | | | | |
DC1 |Device | | | | |
| Control 1 | ^Q | [7] | CASE Q | 11 |
| | | | | |
DC2 |Device | | | | |
| Control 2 | ^R | [8] | CASE R | 12 |
| | | | | |
DC3 |Device | | | | |
| Control 3 | ^S | RS | CASE S | 13 |
| | | | | |
DC4 |Device | | | | |
| Control 4 | ^T | SQ | CASE T | 14 |
| | | | | |
NAK |Negative | | | | |
| Acknowledge | ^U | CONJ | CASE U | 15 |
| | | | | |
SYN |Synchronous | | | | |
| Idle | ^V | EXP | CASE V | 16 |
| | | | | |
ETB |End of | | | | |
| Transmission| | | | |
| Block | ^W | [9] | CASE W | 17 |
| | | | | |
CAN |Cancel | ^X | COS | CASE X | 18 |
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RFC 206 A User TELNET August 1971
EM |End of Medium | ^Y | SQRT | CASE Y | 19 |
| | | | | |
SUB | Substitute | ^Z | SIN | CASE Z | 1A |
| | | | | |
ESC | Escape |Alt Mode, | NEG | | 1B |
| | ^[ | | | |
| | | | | |
FS | File | ^ | PROD | | 1C |
| Separator | | | | |
| | | | | |
GS | Group | ^] | SORT | | 1D |
| Separator | | | | |
| | | | | |
RS | Record | ^^ | CONV | | 1E |
| Separator | | | | |
| | | | | |
US | Unit | ^<- | PWR | | 1F |
| Separator | | | | |
| | | | | |
SP | Space | |SPACE | | 20 |blank
| | | | | |
! | Exclamation | | ! | | 21 | !
| Point | | | | |
| | | | | |
" | Quotation |Diaeresis | " | | 22 | "
| Marks | | | | |
| | | | | |
# | Number Sign | | # | | 23 | #
| | | | | |
$ | Dollar Sign | | $ | | 24 | $
| | | | | |
% | Percent | | % | | 25 | %
| | | | | |
& | Ampersand | | & | | 26 | &
| | | | | |
' | Apostrophe |Acute | ' | | 27 | '
| | Accent, | | | |
| | Closing | | | |
| | Single | | | |
| | Quote | | | |
| | | | | |
( | Opening | | | | 28 | (
| Parenthesis| | ( | | |
| | | | | |
) | Closing | | | | 29 | (
| Parenthesis| | ( | | |
| | | | | |
* | Asterick | | * | | 2A | *
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RFC 206 A User TELNET August 1971
+ | Plus | | + | | 2B | +
| | | | | |
, | Comma |Cedilla | , | | 2C | ,
| | | | | |
- | Hyphen |Minus | - | | 2D | -
| | | | | |
. | Period |Decimal | . | | 2E | .
| | Point | | | |
| | | | | |
/ | Slant | | / | | 2F | /
| | | | | |
: | Colon | | : | | 3A | :
| | | | | |
; | Semicolon | | ; | | 3B | ;
| | | | | |
< | Less than | | < | | 3C | <
| | | | | |
= | Equals | | = | | 3D | =
| | | | | |
> | Greater than | | > | | 3E | >
| | | | | |
? | Question Mark| | ? | | 3F | ?
| | | | | |
@ | Commercial At| | @ | | 40 | @
| | | | | |
[ | Opening | | [ | | 5B | [
| Bracket | | | | |
| | | | | |
\ | Reverse Slant| |CASE /| | 5C | \
| | | | | |
] | Closing | | ] | | 5D | ]
| Bracket | | | | |
| | | | | |
^ | Circumflex | | ^ | ^ | 5E | ^
| | | | | |
_ | Underline | | <- | _ | 5F | _
| | | | | |
` | Grave Accent | Opening |CASE `| | 60 |`
| | single | | | |
| | quote | | | |
| | | | | |
{ | Opening Brace| |CASE (| | 7B | {
| | | | | |
| | Vertical Line| | | | | 7C | |
| | | | | |
} | Closing Brace| |CASE )| | 7D | }
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RFC 206 A User TELNET August 1971
~ | Tilde | Overline,| - | | 7E | ~
| | General | | | |
| | Accent | | | |
| | | | | |
DEL | Delete | Rubout | BACK | | 7F |
| | | | | |
0-9 | Numerics | | 0-9 | | 30-39 | 0-9
| | | | | |
A-Z | Upper Case | |SHIFT | | 41-5A | A-Z
| Alphabetics | |[9] | | |
| | | A-Z | | |
| | | | | |
a-z | Lower Case | | | | 61-7A | A-Z
| Alphabetics | | A-Z | | |
| | | | | |
Table Notes
[1] Down arrow
[2] Superscript 1
[3] Superscript 2
[4] Circle with Dot in the Middle
[5] Superscript 3
[6] Superscript 6
[7] Circle with Plus (+) in the Middle
[8] Circle with Slash (/) in the Middle
[9] Superscript 5
Notes for Figure 2. TELNET Character Set
Superscript 1 - All of the following keys send NULL:
SET, CLR, cent sign, Superscript 0-9
Superscript 2 - '^A' is read 'Control A'. Same for '^B', '^C', etc.
Superscript 3 - ENQ, ACK, BEL, and NAK are displayed as '',
'', '', and '', respectively, in an
area at the top of the screen reserved especially for
those characters.
Superscript 4 - ^A is sent by hitting th keys 'CASE' and 'A' in turn.
Same for ^B, ^C, etc.
Superscript 5 - Upper case 'A' is sent by holding down the SHIFT key
while 'A' is struck. Same for upper case 'B', 'C',
etc.
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RFC 206 A User TELNET August 1971
Superscript 6 - CR also causes an underline '_' to be displayed in
the left margin as a substitute for the often useful
cue provided by the noisy return of a teletype
carriage.
A storage scope is also a much faster display device than a teletype.
However, in some situations this apparent attribute can be
troublesome. In particular, when a system displays a whole series of
lines of text without requiring input from the user, the display may
be wrapped around from the bottom of the scope to the top, and begin
overlaying itself before the user has had some time to read it all.
In such cases, the following strategy is sometimes useful (and will
make sense once the next section -- Section IV -- has been read): hit
_II_ before the display begins to wrap around upon itself; the
display will stop. After the text has been read, erase the scope and
hit _LOG_ and display will resume.
If the user hits _ERASE_ while in TELNET, that key will have its
usual effect; it will have _no_effect upon the remote system.
However, because of a quirk in the current implementation of TELNET,
_ERASE_ will have the one additional effect of causing the display of
incoming text to be suspended until another key on the virtual
teletype is struck. The user is thus cautioned against erasing the
scope while waiting for a response from the remote system. However,
the user can extricate himself from the situation described by
following the _ERASE_ with NUL on the virtual teletype.
IV. Returning to TELNET
The user can at any time escape from TELNET (and hence from the
remote system to which he is attached) by depressing a level key
(e.g., I, II,...,_SYST, TYPE, LIST_) or of course, by hitting
_RESET_. Any time thereafter, the user may return to TELNET by
hitting _II LOG_ (on NET) once again, and resume operation on his
remote system _exactly where he left off_. When _'II LOG'_ is
pressed this second time, instead of querying the user for site
number as before, TELNET will notice that the user is already
connected to a remote system and simply resume where _it_ left off.
After he escapes from TELNET and before he returns to it, the user
may engage himself in any other Online System activity short of
logging out but including changing systems, while still retaining the
option of subsequently returning to TELNET and resuming activity on
the remote system to which he remains connected. The user can
exploit this property of TELNET to obtain hard copy of a selected
portion of his dialogue with the remote system. The user simply
brings his system to the point at which a listing is thereby desired,
escapes from TELNET by hitting _'TYPE SEL' 1,4 RETURN'_, thereby
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RFC 206 A User TELNET August 1971
selecting the line printer as a secondary output device, and then
returns to TELNET by pressing _'II LOG'_. All subsequent
communication with the remote system will be recorded for eventual
output on the printer. Logging off of OLS while escaped from TELNET
causes the TELNET connection to be broken automatically by the
system.
V. Breaking TELNET Connections
At any time while the user is in contact with TELNET, after the
connection to the remote system has been established, the user may
terminate that connection by hitting _'CON RETURN'_. TELNET will
respond _'SOCKETS PURGED_'. \ subsequent call to TELNET (i.e., a
subsequent instance of _'II LOG'_) will illicit the original query
for remote site number. The user is then free to connect himself to
another foreign system.
The user is cautioned that breaking Network connections as just
described is not necessarily equivalent to logging out of the remote
system. Some Server TELNETs may, when they detect the breaking of
connections, log the user off of their system; others may leave the
user logged on. The user should be aware of the conventions of the
systems he uses and, where appropriate, log himself off before
breaking connections.
The key sequence _'I DEL RETURN'_ will also terminate a TELNET
connection, but all other connections that may have been previously
established using the operators of Level I are closed as well.
VI. Abnormal Network Conditions
While the user is in TELNET and attached to a remote system, TELNET
monitors the user's Network connections. If any abnormal condition
is detected, it terminates its monitoring and issues a message of the
form:
{CONNECT}
{INPUT} ERROR - CODE = n
{OUTPUT}
where 'n' is one of the values listed in Figures 3-5. A CONNECT
error may occur during TELNET's initial attempt to establish
connections to the remote system; an INPUT error may occur when
TELNET attempts to extract incoming data from the NET; and an OUTPUT
error may occur as TELNET tries to insert outgoing data into the Net.
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RFC 206 A User TELNET August 1971
Figure 3. Connect Error Codes
4 - A previous invocation of TELNET left a local receive
socket open; the send socket is closed. Hit _'I DEL
RETURN'_.
8 - A previous invocation of TELNET aborted, and _'I DEL
RETURN'_ failed to close the Network connections.
The remote site is probably dead.
12 - All communication paths between UCSB and the
specified remote site are in use.
16 - UCSB's NCP is running at capacity or is drained or
stopped.
20 - The connection attempt was refused by the remote
site.
28 - No such remote site.
36 - Remote site was discovered to be dead.
44 - The Operator has stopped or reset the NCP.
60 - Either the specified remote site is not accepting
input from the net, or there was a failure in the
subnet.
68 - The specified remote site has broken all existing
connections to UCSB.
72 - The remote site is known to be dead.
Figure 4. Input Error Codes
8 - Connections to the server TELNET at the remote site
have been broken.
16 - Connections to the Server TELNET at the remote site
are being broken.
52 - An interrupt was sent by the remote system. Resume
by hitting _LOG_.
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RFC 206 A User TELNET August 1971
Figure 5. Output Error Codes
8 - Connections to the Server TELNET at the remote site
have been broken.
16 - Connections to the Server TELNET at the remote site
are being broken.
20 - same as 8.
36 - Remote site has died.
44 - The Operator has stopped or reset the NCP.
52 - An interrupt was sent by the remote system. Resume
by hitting _LOG_.
60 - Either the remote site has stopped accepting input
from the Net or there was a failure in the subnet.
64 - same as 60.
68 - The remote site has broken all existing connections
to UCSB.
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RFC, FYI, BCP