Interface Message Processor

The Interface Message Processor (or IMP, as they were universally known) was the packet switch which the ARPANET was made out of; the IMPs were connected to each other via high-speed dedicated point-point telephone links, connected to the IMPs via a modem. All ARPANET hosts connected to an IMP, using the Host-to-IMP Protocol.

A message from one Host to another was sent via the source Host's IMP, then routed through some number (possibly zero) intermediate IMPs, then through the destination Host's IMP (possibly the same IMP as the first one, in which case the message never went over a telephone line), on to the destination Host.

The IMPs were Honeywell DDP-516 'ruggedized' minicomputers, with added special high-speed modem interfaces, and asynchronous bit-serial Host interfaces. Later, other variant minicomputers were also used; the Honeywell DDP-316 (the non-ruggedized version of the 516), the Pluribus, and the BBN C/30 (which had microcode which emulated the Honeywell machines).

Further reading

• F.E. Heart, R.E. Kahn, S.M. Ornstein, W.R. Crowther, and D.C. Walden, "The interface message processor for the ARPA computer network", Proceedings AFIPS, 1970 SJCC, Vol. 36, pp. 551-567.
• J.M. McQuillan, W.R. Crowther, B.P. Cosell, D.C. Walden, and F.E. Heart, "Improvements in the Design and Performance of the ARPA Network", Proceedings AFIPS, 1972 FJCC, Vol. 40, pp. 741-754.
• John M. McQuillan, David C. Walden, "The ARPA Network Design Decisions", in "Computer Networks", Vol. 1, No. 5, August 1977, pp. 243-289.

External links

• Resurrection of the 1973 IMP code - While focused on the retrieval, and running under a simulator, of the original IMP code, this site contains much other IMP-related material too (manuals, etc)
• ARPANET Technical Information