Other meanings of Interface Message Processor
Computer Networking
The Interface Message Processor (IMP) was the packet-switching node that formed the backbone of the ARPANET, the precursor to the modern Internet. Developed in the late 1960s by Bolt, Beranek and Newman (BBN), the IMP was the first generation of what would evolve into today's routers. It handled the routing, error checking, and flow control of data packets between host computers, enabling the first long-distance computer communication.
The IMP was conceived in response to a 1968 request for proposals from the Advanced Research Projects Agency (ARPA) for a communication subnetwork that would link disparate computers. BBN won the contract, and a team led by Frank Heart designed the IMP based on a Honeywell DDP-516 minicomputer, modified with a custom interface and a special real-time operating system. The IMP's software implemented a packet-switching protocol that broke data into 128-byte packets, each with a header containing source, destination, and sequence information. The design emphasized reliability, with automatic error detection and retransmission, and dynamic routing that adapted to network conditions.
The first IMP was installed at UCLA in September 1969, followed by nodes at SRI, UC Santa Barbara, and the University of Utah, forming a four-node network. Each IMP connected to up to four host computers via a 16-bit parallel interface, and to other IMPs via 50 kbps leased telephone lines. Operators monitored the network from a console that displayed traffic statistics and allowed remote diagnostics. The IMP's software was updated remotely, a novel capability at the time, and it supported the first network operations center at BBN. By 1971, the network had grown to 15 IMPs, and the technology was later adopted for the Defense Data Network.
The IMP's architecture directly influenced the development of the Transmission Control Protocol/Internet Protocol (TCP/IP) suite, which replaced the original IMP-to-IMP protocol in 1983. The term 'IMP' fell out of use as more general-purpose routers emerged, but the concept of a dedicated packet-switching node persisted. The IMP's design principles—packet switching, dynamic routing, and distributed control—are foundational to modern internetworking. The last IMP was decommissioned in 1989, but its legacy lives on in every router and switch. The IMP also inspired the development of the Interface Message Processor for other networks, such as the SATNET and PRNET, which used satellite and radio links respectively.
Few know that the IMP's development included a 'hot standby' configuration for critical nodes, where a second IMP could take over instantly. The IMP's software was written in assembly language, but a high-level language called 'IMP language' was used for some parts. The first IMP was delivered with a 12K-word memory, but later versions had 16K. The IMP's console had a 'crash' button that would dump memory to a tape for debugging. The IMP also supported a 'remote reset' feature that allowed BBN to reboot a distant IMP. The IMP's design was patented, and BBN later sold a commercial version called the 'C/30' used in the ARPANET's successor, the Defense Data Network.
The IMP was a pioneering device that laid the groundwork for the Internet's infrastructure.
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