The concept of an interface computer for computer networking was first proposed in 1966 by
Donald Davies for the
NPL network in England and implemented there in 1968–9. The same idea was independently developed in early 1967 at a meeting of principal investigators for the Department of Defense's
Advanced Research Projects Agency (ARPA) to discuss interconnecting machines across the country.
Larry Roberts, who led the ARPANET implementation, initially proposed a network of
host computers.
Wes Clark suggested inserting "a small computer between each host computer and the network of transmission lines", i.e. making the IMP a separate computer. The IMPs were built by the Massachusetts-based company
Bolt Beranek and Newman (BBN) in 1969. BBN was contracted to build four IMPs, the first being due at UCLA by Labor Day; the remaining three were to be delivered in one-month intervals thereafter, completing the entire network in a total of twelve months. When Massachusetts Senator Edward Kennedy learned of BBN's accomplishment in signing this million-dollar agreement, he sent a telegram congratulating the company for being contracted to build the "Interfaith Message Processor". • Software:
Willy Crowther,
David Walden, Bernie Cosell and Paul Wexelblat • Hardware:
Severo Ornstein, Ben Barker • Theory and collaboration with the above on the overall system design:
Bob Kahn • Other: Hawley Rising • Added to IMP team later: Marty Thrope (hardware), Jim Geisman, Truett Thach (installation), Bill Bertell (Honeywell) BBN began programming work in February 1969 on modified Honeywell DDP-516s. The completed code was six thousand
words long, and was written in the Honeywell 516 assembly language. The IMP software was produced primarily on a
PDP-1, where the IMP code was written and edited, then run on the Honeywell. There was considerable technical interchange with the British team building the
NPL network and
Paul Baran at
RAND but the BBN team independently developed significant aspects of the network's internal operation, such as routing, flow control, software design, and network control. BBN designed the IMP simply as "a messenger" that would only "store-and-forward". A few minutes later, the bug was fixed and the login attempt was successfully completed. BBN developed a program to test the performance of the communication circuits. According to a report filed by Heart, a preliminary test in late 1969 based on a 27-hour period of activity on the UCSB-SRI line found "approximately one packet per 20,000 in error;" subsequent tests "uncovered a 100% variation in this number - apparently due to many unusually long periods of time (on the order of hours) with no detected errors." A variant of the IMP existed, called the TIP (Terminal IMP), which connected terminals (i.e.,
teletypes) as well as computers to the network; it was based on the Honeywell 316, a later version of the 516. Later, some Honeywell-based IMPs were replaced with multiprocessing
BBN Pluribus IMPs, but ultimately BBN developed a
microprogrammed clone of the Honeywell machine. IMPs were at the heart of the ARPANET until DARPA decommissioned the ARPANET in 1989. Most IMPs were either taken apart, junked or transferred to
MILNET. Some became artifacts in museums; Kleinrock placed IMP Number One on public view at UCLA. The last IMP on the ARPANET was the one at the University of Maryland. ==BBN Report 1822==