Network Working Group Ed Belove (Harvard)
Request for Comments: 56 Dave Black (Harvard)
Bob Flegel (Utah)
Lamar G. Farquar (Utah)
June 1970 Third Level Protocol
In our view of the world each host has a set of four programs to allow a
user teletype to communicate with a foreign monitor. The exact
implementation of these programs is highly installation-dependent. Thus
all explanations are meant to describe functional characteristics rather
The four programs come in two male/female pairs. A user employs a send-
logger at his site to communicate with receive-logger at the appropriate
foreign site in order to establish a full duplex link between the user's
teletype and the foreign machine's monitor. This puts him in the
equivalent of a pre-logged in state at the other machine. After the
link has been established, the two loggers drop out of the picture, and
the user is left talking to a sender in his machine, whose main function
is to take input from the user's teletype and send it down the link that
was established by the loggers to the receiver in the foreign host which
passes it along to its monitor (making it look like input from a local
teletype). Replies from the foreign monitor are given by it to the
receiver, which transmits them back along the link to the sender, which
outputs them on the user's teletype. The sender and receiver in each
machine must either exist in multiple copies, one for each network user,
or there must be a single copy which can handle all of the network
users. The loggers, however, need be able to handle only one user at a
time, since their task is quickly accomplished, leaving them free to
satisfy other requests. However there should be some method of queuing
requests that can not be satisfied immediately. A less satisfactory
alternative would be to give a busy message to any user who tries to use
the logger while it is busy. (This, of course, does not preclude the
possibility of an installation having a re-entrant logger, or of having
multiple copies of the logger.)
The receive-logger should be user zero in every machine and should
always be listening to socket zero. (This same thing can be accomplished
by having the NCP intercept all messages to user zero, socket zero, and
send them to the receive-logger; but it is simpler and cleaner to have
the logger actually be user zero and have the NCP handle its messages
the same as everyone else's.)
When the send-logger is called, it pulls a pair of unused sockets (2N
and 2N+1) from a pool of free sockets and CONNECT from 2N+1 to User 0,
Socket 0 in the desired foreign host. This activates the receive-logger,
which accepts the connection if it has an available slot for the foreign
teletype. He then immediately closes down this connection to allow links
from other sources to be initiated. If, on the other hand, there is no
room for the foreign teletype (or if, for some other reason, the
receive-logger does not wish to connect) the attempted link to socket
zero is refused. This notifies the send-logger that he cannot log on the
foreign host and it then notifies the user of this fact. There is no
guarantee, however, that the close was actually sent by the foreign
logger. It could have been sent by the NCP if, for example, the pending
call queue for the socket was overloaded.
If the link to socket zero has been accepted (thus indicating that the
receive-logger can accommodate the request) after closing that link, the
receive-logger picks an available pair of sockets (2M and 2M+1) from its
pool, and connects from 2M+1 to 2N. (It found the identity of 2N when
its listen was answered by the link with 2N+1.) The send-logger has
meanwhile listened to socket 2N and now accepts the link, and CONNECTS
from 2N+1 to 2M. The receive-logger has been listening to this socket
and accepts the attempted link.
At this point, there is a full duplex connection between the two
loggers. They then activate the sender and receiver, which handle all
other communication between the user and the foreign monitor. (The
senders and receivers can be part of the loggers, or can be called by
When the user is finished and escapes back to his monitor, it is up to
the sender to close down the links. On the receiving end, it would be
highly desirable for the NCP to notify the receiver of this fact, so it
could log the user off (if he had failed to do that himself) and could
free any resources that he had been using.
A more formal outline of the proposed protocol described in the scenario
1. Stable state: receive-logger at foreign host listening to User 0,
2. Local user calls send-logger.
3. Send-logger calls CONNECT (port, 2N+1, <foreign host#,0,0>).
4. Send-logger calls LISTEN (port, <local host#, user#, 2N>).
5. Foreign logger's LISTEN is answered, and he is told local user
number, host and #2N+1.
6. Foreign logger looks for available sockets (2M and 2M+1). If they
exist and it is able to establish connection, it accepts and then
immediately closes the link.
7. Foreign logger calls CONNECT (port, 2M+1, <local host#, user#,
8. Foreign logger calls LISTEN (port, <local host#, user#, 2M>).
9. Send-logger has listened to 2N and accepts link, then calls
CONNECT (port, 2N+1, <foreign host#, user#,2M>).
10. Receive-logger, which is listening on 2M, accepts link.
11. Loggers activate appropriate handlers.
12. When the user is finished, sender closes down both links.
This basic method of establishing a full duplex connection should be
standard throughout the network. The particular way each installation
handles the implementation of the sender, receiver, and the two loggers
is of no consequence to the network and is highly machine dependent.
(Even the fact of needing a sender and receiver is machine dependent in
that some members of the network might be able to handle their functions
in other ways.) However, some conventions must be established regarding
communication between the sender and receiver, or their equivalents.
Network Standard Code
In order to facilitate use of the network, we propose the convention
that all teletype-to-foreign-monitor communication be done using 128
character USASCII. (This is the code used by the IMP's and is in the
appendix to the IMP operating manual.) It makes sense to require
machines to make only one conversion to a standard code, than to have to
make conversions to every code on the net.
In addition, since most of the network machines use ASCII as their
internal character code, it will be no trouble for them. Even those
machines that use a different code must translate to and from ASCII in
order to communicate with local teletypes. Extending this translation to
the network should cause very little trouble. We envision this
translation as taking place in the sender and receiver, but again that
is implementation dependent.
If ASCII is adopted as a standard, we would suggest that all non-ASCII
machines create a monitor to the machine's internal code. This would
make the complete character set available to those who wished to use it
(and were willing to write a simple conversion routine for the local
machine.) In this way, those users who wanted to could use any machine
on the net from their teletype, without requiring their machines to have
records of all the network codes, and yet could use the full power of
the foreign machine if they wanted.
Again, this standard applies only for teletype-to-foreign-monitor
A standard way of handling the break character has to be established for
the network and be included in the protocol. Problems with the break
character arise in several contexts. First, there are two distinct
purposes served by the break character. One is as a panic button. This
says, "I do not care what is happening, stop and get me out to monitor
level now." This command is executed immediately upon receipt, and is
most commonly used to get out of a program that one does not want to be
in (e.g., one that is in an infinite loop, etc.)
The other purpose that is served is that of an exit from a subsystem, or
on a machine with a forking structure as a method to get back to the
next higher level fork. This second purpose is not an immediate one in
that the user wants the system to finish all that he has told it to do
We assume that there does not exist in every system 1) a way of
performing each of these functions, or 2) a clear cut distinction
between the calling and operation of the two. Furthermore, there are
subtle distinctions as to how each system treats the commands.
The panic button function can easily be performed by the proposed
control command <INT>. This function must be accomplished by using a
control command, since a program can enter a state where it is accepting
no input: hence, the program cannot be aborted by sending it a message
down the teletype link. There is no reason to worry about the race
condition caused by sending this command down the control link since its
whole purpose is to force the machine to disregard everything else the
user has sent.
In our implementation of this, we would ask the user to specify to the
logger a seldom used character that he wants to be his foreign panic
button. Then, it would be a simple task for the sender to map this
character into an <INT> command, which the foreign machine must
interpret properly. This scheme would work well for most machines, but
some may lend themselves to different ways of generating the <INT>.
The other problem that presents itself is what to do if the foreign
machine's "exit" character is the same as the local machine's. The
problem is that while a user is talking to a foreign machine, he would
want to be in a transparent mode, where everything he types is sent
directly to the other machine. The way he would get himself out of this
mode is to type either his machine's "exit" character or its panic
button. Thus, if the foreign machine has the same one, there would be no
way to send it. The way out of this is the same as above--merely a
mapping of another seldom used character into the foreign machine's
"exit" character. This type of mapping can be carried as far as each
installation deems necessary. Giving the user complete control over
translation is helpful in that it allows him to user characters that his
teletype cannot generate.
Command Message Formats
Each site should establish its now conventions about when to send a
monitor command string, and in what size chunks. When performing a
routine operation, one might want to send several command lines as a
single message. If working with the monitor as usual, a reasonable break
point might be at every carriage return. When using a highly interactive
language such as QED, one might decide character-by-character
transmission was a necessity. We feel that each user should have the
choice between these three methods (and possible more). Furthermore, the
user should be able to change between each mode at will. The differences
in syntax of the send-message commands mentioned above should be noted.
For the first, a special send-message command character must be defined,
and it should not be sent along with the message. For the second, the
carriage return acts dually as the send-message command and as a command
delimiter. Therefore it must be sent with the message. Finally, the case
of character-by-character transmission with its implicit send command
should pose no significant problems.
The preceding discussion is meant to imply also that the receiver must
be able to buffer up each of the above types of transmission into a form
acceptable to its own monitor interface.
In addition, all echoing should be done in the local host, with the
foreign machine suppressing its echoes (if it can.)
We would like to thank Carl Ellison (of Utah) for his valuable
suggestions and criticisms of this work, and Jim Curry (of Utah) for his
encouragement and support of the effort.
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Jon Ribbens 7/97 ]