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.

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 IMP routes the message to the destination host using protocols that were eventually adopted by Internet routers.

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