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General Description

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 than design.

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 them, etc.)

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 above follows:

  1. Stable state: receive-logger at foreign host listening to User 0, Socket 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#, 2N>).

  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.