HOST/IMP Interfacing

The hardware and software interfaces between HOST and IMP is an area of particular concern for the HOST organizations. Considering the diversity of HOST computers to which a standard IMP must connect, the hardware interface was made bit serial and full-duplex. Each HOST organization implements its half of this very simple interface.

The software interface is equally simple and consists of messages passed back and forth between the IMP and HOST programs. Special error and signal messages are defined as well as messages containing normal data. Messages waiting in queues in either machine are sent at the pleasure of the machine in which they reside with no concern for the needs of the other computer.

The effect of the present software interface is the needless rebuffering of all messages in the HOST in addition to the buffering in the IMP. The messages have no particular order other than arrival times at the IMP. The Network Control Program at one HOST (e.g., UTAH) needs waiting RFNM's before all other messages. At another site (e.g., SRI), the NCP could benefit by receiving messages for the user who is next to be run.

What is needed is coding representing the specific needs of the HOST on both sides of the interface to make intelligent decisions about what to transmit next over the channel. With the present software interface, the channel in one direction once committed to a particular message is then locked up for up to 80 milliseconds! This approaches one teletype character time and needlessly limits full-duplex, character by character, interactions over the net. At the very least, the IMP/HOST protocol should be expended to permit each side to assist the other in scheduling messages over the channels.