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RFC 206 - A User TELNET Description of an Initial Implementation


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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

      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.

   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.

                                 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

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.

   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] <ENQ>
    |              |          |      |         |           |
ACK |Acknowledge   |    ^F    |  UP  | CASE F  |    06     | <ACK>
    |              |          |      |         |           |
BEL |Bell          |    ^G    | DOWN | CASE G  |    07     | <BELL>

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     | <NAK>
    |              |          |      |         |           |
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     |

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     | *

+   | 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     | }

~   | 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 '<ENQ>',
                   '<ACK>', '<BELL>', and '<NAK>', 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.

   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

   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.

                      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_.

                               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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