|Title||NETCRT - a character display protocol
Network Working Group R. Braden
Request for Comments: 205 UCLA/CCN
NIC: 7172 6 August 1971
NETCRT - A CHARACTER DISPLAY PROTOCOL
At the May NWG, meeting, CCN circulated dittoed copies of a proposed
character-display protocol NETCRT. Since that time, NETCRT has been
revised significantly; the current version is now being published as
an RFC, as promised last May.
NETCRT was developed because a particular site (RAND) requested
Network access to URSA, CCN's display-based crje system. The primary
use of URSA at UCLA is conversational remote job entry from a display
terminal: entering and editing program text, submitting programs for
batch execution, and examining job output; URSA is not a general-
purpose time-sharing system.
URSA's text editor is designed for a fast updating character display
and cannot be used in any reasonable way from a typewriter-like
console. Therefore, a simple TELNET protocol is not adequate for
using the crje function of URSA. Furthermore, we have assumed that
other ARPA sites will have their own text editors, well matched to
their own terminals and systems. Therefore, CCN has implemented
NETRJS (see RFC #189), to provide remote job submission and retrieval
services, before implementing NETCRT.
There are a number of other functions in URSA besides crje; some of
these would probably be useful to remote users. URSA contains a
comprehensive STATus service, whose constantly-updating displays are
"windows" into the operation of the machine and the operating system,
allowing a user to watch the progress of his jobs through the system.
URSA also includes on-line data set (file) utilities, convenient for
a user with files stored at CCN. To obtain access to these
facilities, a few sites which use CCN heavily may want to implement
NETCRT. The schedule for implementation of NETCRT at CCN to allow
Network access to URSA will depend upon the existence of a user site
that wants the service and that will write a suitable NETCRT user
process. Interested sites are urged to contact the CCN Technical
Liaison, Bob Braden.
Even though the implementation schedule for NETCRT is nebulous, we
are publishing the specs now for several reasons. First, we would
like comments and criticisms. Furthermore, NETCRT contains some
features which may be useful in the protocol(s) now being developed
for full graphical displays.
NETCRT PROTOCOL - VERSION 3
The UCLA Campus Computing Network (CCN) node intends to provide
Network access to its conversational remote job entry system URSA.
The URSA system is display-oriented, supporting only character
displays with local buffers (originally IBM 2260 displays, now CCI
301 TV display consoles). This document defines a third-level
protocol called NETCRT which allows a Network user in a remote Host
to look like a CCI console to URSA. NETCRT is defined in terms of a
virtual character display ("VCD") terminal, simulated by a process in
the user host.
URSA, like many on-line console systems, attempts to provide a good
man/machine interaction by keeping tight control over the state of
the terminal. On the other hand, the Network Working Group has
deliberately built some "squishiness" into the standard Network
protocols. We believe this squishiness is a conceptual mistake when
dealing with remote man/machine interaction, and we would support
protocol revisions to allow control over the effective communication
compliance between processes in different hosts. However, this
NETCRT protocol attempts to cope with the present squishiness, which
is apparently built into a number of host's NCPs. In fact, we have
arranged things so a host can improve response time and reduce
Network traffic with NETCRT by using the message buffering inherent
in his NCP.
B. THE VIRTUAL CHARACTER DISPLAY
A VCD consists of the following virtual hardware (see Figure 1):
1. A rectangular _display screen_ capable of displaying N lines of M
2. A _local buffer_ of M x N characters used to refresh the display.
3. A _cursor register_ which addresses the characters in the buffer
(and hence on the screen). This register controls the writing of
text into the local buffer from either the keyboard or the server,
and the reading of the local buffer by the server.
4. A _keyboard_ containing text keys and control keys. Each text key
enters a character into local buffer at the current cursor address
and steps the cursor register by 1.
5. A _communication interface_ through which the server CPU can send
a stream of _command_ segments to the VCD and receive a stream of
_response_ segments from the VCD. The command segments include
control commands to the VCD and text to be written into the local
buffer. Response segments contain status indicators and text read
from the buffer. In addition, both VCD and server may send break
The current address in the cursor register, an integer between 0 and
M x N-1, is displayed as a blitch, underscore, or other visual
indication at the corresponding point on the screen; this indication
is called the _cursor_. Position 0 is the upper left corner of the
The screen is addressed in line ("row") order, and read and write
operations by the server overflow automatically from one line to the
next. The cursor register is not assumed to operate modulo M x Nxsy.
It is possible for a server command to set the cursor register to M x
N, one position beyond the last screen position; however, the server
should never set the register to an address beyond M x N, and it
should not leave the cursor at M x N when the keyboard is unlocked.
The application program or conversational system using the VCD may
format each display screen in a variety of ways, and may use a number
of styles of interaction. One consequence is that the application
program might have to look anywhere on the screen (i.e., in the local
buffer) to find the input information it requires. We may consider
three alternative mechanisms for transmitting information from the
VCD to the serving CPU:
Mechanism 1 Whenever the user presses a "Transmit" control key,
the entire M x N characters in the buffer are
transmitted to the server CPU.
Mechanism 2 When the user presses "Transmit", the string of text
between a "start" control character and the cursor is
transmitted to the server.
Mechanism 3 The server must send a read command segment to the VCD
before the "Transmit" key will have an effect. The
read command segment determines which parts of the
buffer are to be transmitted to the server.
Mechanism 1 may be faulted as too costly in transmission time and
channel capacity, while Mechanism 2 is too restrictive. The scheme
which we propose here is based on Mechanism 3, which subsumes the
The VCD is assumed to include the following control keys:
Erase Clears the display buffer to all blanks and resets the
cursor to position 0 (the upper left corner of the
Transmit Locks the keyboard and places the VCD under control of
the server CPU. Typically, the server will read
specified areas of the screen and perhaps write out
new data before unlocking the keyboard again.
Break Has the same effect as _Transmit_, and in addition
sends an interrupt message to the server CPU. The
_Break_ key always sends the interrupt, regardless of
the state of the VCD.
Reset May be used to unlock the VCD keyboard in case the
server CPU fails to respond immediately and the user
wishes to enter new or different information.
These may be called pure control keys, since they do not correspond
to any text characters. The following control key does store a
character into the display buffer:
Newline Enter a Newline (NL) character into the display buffer
and reset the cursor to the beginning of the next
line. If this character is encountered during a read
or write operation, it is executed (i.e., the cursor
is moved to the beginning of the next line) and the NL
is counted as _one_ character.
Finally, there are assumed to be keys for manually positioning the
cursor to any address on the screen. Cursor positioning keys may
include: cursor right, cursor up, cursor left (BS), cursor down (LF),
and cursor return (CR). A tab (HT) mechanism could also be defined,
although none is included here.
C. VCD STATES
The VCD has two internal states, _Local_ and _Control_ (see Figure
Local State: The keyboard is unlocked and all keys are active. The
VCD does not accept or recognize any commands from
server except (reverse) Break.
Control State: The keyboard is locked, and only the _Break_ and
_Reset_ keys are active. The VCD accepts and executes
command segments from the server, and returns response
segments as the result of read commands.
The VCD changes from Local to Control state if either:
(1) The user presses the _Transmit_ key; or
(2) the user presses the _Break_ key; or
(3) the server sends a reverse Break request.
_Transmit's_ only effect is to enter Control State; _Break_ enters
Control State and also sends a break request (INS and X'80') to the
The VCD returns to Local State when either:
1. The user presses the _Reset_ key; or
2. the VCD encounters a LOCAL command from the server and is not in
the process of synchronizing a reverse break (see section E
We should note that CCI and IBM 2260 character display consoles
actually have only one control key ("Interrupt" on CCI, "Enter" on
2260) to perform the functions of both _Break_ and _Transmit_; this
one key in fact has the function of the _Break_ key of the VCD. We
have included both _Break_ and _Transmit_ keys in the VCD for
generality, but the URSA-NETCRT interface will be programmed to allow
a Network user of URSA to either (1) employ the _Break_ key
exclusively, or (2) use either _Break_ or _Transmit_ as appropriate.
This will be achieved by URSA simply by ignoring those break requests
(INS messages) which occur while there are outstanding read commands.
D. VCD COMMANDS
The server sends the VCD a string of command segments. These are of
varying length, consisting of an op code and none or more parameters.
The commands recognized by the VCD are as follows:
1. Display & Keyboard Control Commands:
Command Parameter(s) Function
------- ------------ -----------------------------------
ERASE none Erase display and reset cursor to 0.
i.e, clear the local buffer.
BLANK none Disable display refresh (i.e., blank
the screen but do not clear the local
UNBLANK none Enable display refresh.
LOCAL none Put VCD in _local_ state. The result
is to suspend command interpretation
and unlock the keyboard.
SYNC none Used to synchronize reverse Break
from server. SYNC (X'80) is placed
in stream by server at same time that
it sends an INS. VCD enters Control
State, synchronizes INS with BREAK
command (see next section), and
continues command interpretation.
2. Cursor Control Commands:
CURSOR 16 bit integer P Set cursor register to P, where
0 <= P <= M x N.
FIND X'0001' followed Move the cursor to point to an
by one character occurrence of the character c.
c Specifically, search backwards
toward lower addresses) from the
current cursor position and take
the first occurrences of c (i.e.,
the one with the largest address).
If no occurrence is found, leave
cursor at position 0.
SAVE none Save a copy of the current cursor
address in local register S.
RESTORE none Replace cursor register contents by
WRITE n,text 16 bit integer Write n bytes of text into display
n, followed by buffer starting at current cursor
n text bytes. position and advancing cursor by 1
for each byte (except NL character
advances to beginning of next line).
Here [sigma] + n <= M x N.
READ n 16 bit integer Read n bytes starting at the cursor
n. [sigma] and advancing cursor by one
for each byte (except NL advances
cursor to beginning of next line).
NL counts as one character. Send the
text to the server as a response
segment. Must have n + [sigma]
<= M x N.
SREAD none Read S - [sigma] bytes starting from
the current cursor position [sigma]
up to (but not including) the cursor
address stored in register S. The
cursor is left in position S as a
result. Send the text to the server
as a response segment.
AWRITE n,text 16 bit integer Same as WRITE n, except characters
n, followed by are not stored in buffer if they
n text bytes. have a lower cursor address than
the value in S.
Here are some applications of these commands in URSA:
1. One elementary URSA terminal operation reads the screen from
position x up to (but not including) the current cursor position.
This could be done with the sequence of VCD command segments:
2. Another common operation in URSA is to remember the cursor, update
specific information on the screen, and replace the cursor. This
can be done by the following 8 + n byte sequence of command
WRITE n, text
3. In URSA, the area in which a user is to type his response is
usually delimited on the left by a "Start Symbol" (graphic '').
This is a historical remnant of the IBM 2260, which has only two
hardware read operators: read the full screen, and read from the
Start Manual Input Symbol ("SMI") to the cursor. The SMI read
operation can be simulated easily on the VCD as follows:
4. The _Break_ (or _Transmit_) key on the VCD may serve the function
of the INTerrupt key on a CCI console (or ENTer on an IBM 2260).
URSA will often attempt to minimize Network traffic by sending a
sequence of commands (one message if allocation allows) like the
CURSOR m |
WRITE n, text - URSA writes a request
+- +-+ |_ User types response
| _BREAK_ | |
- -User Presses | _TRANSMIT_ |key - - -+
+- -+ -+
CURSOR p - URSA reads response
At other times, URSA might send the sequence:
and wait for the INS from the user pressing _Break_ (or the
response segment triggered by the zero-length read if he presses
_Transmit_); then URSA will send the appropriate read command
F. NETWORK MESSAGE FORMATS
The VCD connects the server through ICP to a standard socket,
establishing thereby a pair of connections between the VCD and the
server. Command segments (server-to-VCD) and response segments
(VCD-to-server) are sent over these connections, without regard to
physical message boundaries, using byte size 8. The VCD is defined
to operate in a segment-at-a-time mode (rather than character-at-a-
time), with local echo. Therefore, the server never echoes under
In many cases URSA will send a sequence of command segments (as in
the examples of the preceding section) at once; if there is
sufficient allcocation they will be sent in the same message.
Response time may be improved, therefore, if the user site is able to
buffer ahead on command segments. This buffering does raise break
synchronization problems, which are solved in the following manner
for reverse (server-to-user) break:
The server sends an INS on the control link and also a SYNC
command (X'80) on the data link to the VCD. On receiving either,
the VCD enters Control State and then achieves synchronization
between the INS and BREAK; if the INS arrives first, the VCD
executes normally all commands buffered in his host, _except_ it
ignores LOCAL commands, until the SYNC appears. Having achieved
synchronization, the VCD continues normal command interpretation
(without ignoring ensuring LOCAL commands).
By this means the server can regain control of the VCD to write new
information at any time. For example, when URSA is used under
NETCRT, most WRITE or AWRITE sequences will be preceded by a BREAK
from the server, since URSA will not know the current state of the
VCD. Even if URSA left the VCD in Control State, the user might have
manually returned his VCD to Local State by pressing _Reset_.
After receiving an INS, the VCD executes rather than ignores buffered
commands so that pending writes will not be lost in case that
processing at the user side has been held up temporarily. The read
commands executed after the server sent an INS might be irrelevant to
a server, which can ignore the corresponding response segments. In
order to do so, the server simply keeps matching counts of read
commands sent and corresponding response segments received.
Command segments will use the following formats:
Form 1 (No parameters) q:OPCODE(8)
where q = X'80' means SYNC
X'91' " LOCAL
X'92' " ERASE
X'93' " BLANK
X'94' " UNBLANK
X'95' " SAVE
X'96' " RESTORE
X'97' " SREAD
Form 2 (16 bit integer) q:OPCODE(8) + n:INTEGER(16)
where q = X'9E' means READ n
q = X'9C' " CURSOR n
In both cases, 0 <= n <= M x N
Form 3 (count and text) q:OPCODE(8) + n:LENGTH(16) + (TEXT(8) = n)
where q = X'9D' means WRITE
q = X'9A' means AWRITE
q = X'9F' and n=1 means FIND
A response segment, caused by a READ or SREAD command, has the
RESPONSE <-----X'A1' + CURSOR(16) + n:LENGTH(16) + (TEXT(8) = n)
where n > 0 is the number of characters actually read. CURSOR(16) is
an integer giving the final cursor position after the corresponding
read command. Note that the command READ 0 is permissible and may be
used by the server to find the current cursor position, or to find
out when the user presses _Transmit_.
E. SCREEN SIZE
For simplicity and consistency with URSA, we have chosen to treat the
cursor as a single integer. This in turn means that VCD and server
must agree upon the number of columns M; it is also desirable for the
server to know N.
The agreement on M and N takes place through a one-sided negotiation.
The server is assumed to know what M and N values he can handle and
these are published for user sites. When the VCD is first connected
to the server, the VCD must send an Open response segment with the
values M and N:
Open segment <---- X'B1' + M(8) + N(8) + X'0000'
If the VCD fails to send this segment or the server does not like the
values, the server closes the connections and the user is considered
 Graphic representation of start symbol: shaded triangle on its
| D I S P L A Y |
| LOCAL | Address
| BUFFER |<------------+
^ ^ | |
+-----------+ text | | | |
/ |_______| | | |
| KEYBOARD | |WRITE |READ |
+-------------+ |AWRITE |SREAD |
| | |
control | v |
| VCD | | CURSOR ADDRESS|
| CONTROL |<--->| REGISTER |
^ | ^
| | |
| | |
| | |
| | |
| | v
| | +---------------+
| | | S |
| | | REGISTER |
| | +---------------+
| COMM |
| INTERFACE |
FIGURE 1. VIRTUAL CHARACTER DISPLAY
No Commands Executed
+------>| LOCAL |------+
| +--->| State | |
| | +--------------------+ |
| | | | |
| | | | |
| | |Break | |
| | | | | INS received
LOCAL | | | key | |
| | | [send INS | |
Command | | | and X'80'] | |
Executed| | | |Transmit|
| | Reset | | |
| | | | key |
| | key | | |
| | v v |
| | +--------------------+ |
| +---| Control |<------+
| | State |
| | Keyboard locked,
| | Execute Commands
After INS is
command is ignored
until SYNC (X'80')
FIGURE 2. VCD STATES
[This RFC was put into machine readable form for entry]
[into the online RFC archives by Lorrie Shiota, 2/02]