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. It was the first generation of gateways, which are known today as routers. An IMP was a ruggedized Honeywell DDP-516 minicomputer with special-purpose interfaces and software. 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. An IMP required the connection to a host computer via a special bit-serial interface, defined in BBN Report 1822. The IMP software and the ARPA network communications protocol running on the IMPs was discussed in RFC 1, the first of a series of standardization documents published by the Internet Engineering Task Force (IETF).

The idea of the IMP being a separate computer was suggested by Wes Clark to Larry Roberts who led the ARPANET implementation for the Department of Defense's Advanced Research Projects Agency (ARPA). The IMPs were built by Bolt Beranek and Newman (BBN).

The original IMP team at BBN consisted of:

• Team Leader: Frank Heart
• Software: Willy Crowther, Dave Walden, Bernie Cosell
• 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)

The first IMP was delivered to Leonard Kleinrock's group at UCLA on August 30, 1969. It used an SDS Sigma-7 host computer. Douglas Engelbart's group at the Stanford Research Institute (SRI) received the second IMP on October 1, 1969. It was attached to an SDS-940 host. The first communication test between these two systems took place on October 29, 1969 when a login to the SRI machine was attempted, but only the first two letters could be transmitted. The SRI machine crashed upon reception of the 'g' character. A few minutes later the bug was fixed and the login attempt was successfully completed.

A variant of the IMP existed, called the TIP, which connected terminals instead of computers to the network; it was based on the 316. Initially, some Honeywell-based IMPs were replaced with multiprocessing BBN Pluribus IMPs, but ultimately BBN developed a microprogrammed clone of the Honeywell processor.

IMPs were at the heart of the ARPANET until it was decommissioned in 1989. The last IMP on the ARPANET was the one at the University of Maryland.