RFC 56 (rfc56) - Page 1 of 6

Third Level Protocol: Logger Protocol

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

                            Logger Protocol

                          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