Interface Message Processor

The Interface Message Processor (IMP) was the packet switching node used to interconnect participant networks to the ARPANET from the late 1960s to 1989.

[4] In later years the IMPs were made from the non-ruggedized Honeywell 316 which could handle two-thirds of the communication traffic at approximately one-half the cost.

[5] An IMP requires the connection to a host computer via a special bit-serial interface, defined in BBN Report 1822.

[7][8][9][10] 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.

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".

[11] BBN designed only the host-to-IMP specification, leaving host sites to build individual host-to-host interfaces.

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.

The specification incorporated an alternating bit protocol,[23] of the type proposed by Donald Davies' team for the NPL network in 1968.