Interface Message Processor

Interface Message Processor
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Name	Interface Message Processor
Caption	The first IMP at the MIT Lincoln Lab.
Developer	Bolt, Beranek and Newman
Manufacturer	Bolt, Beranek and Newman
Generation	First
Released	1969
Predecessor	None
Successor	Pluribus

Interface Message Processor. It was the first generation of packet switching routers and the foundational hardware that enabled the creation of the ARPANET, the precursor to the modern Internet. Developed by the firm Bolt, Beranek and Newman under contract from the Advanced Research Projects Agency, it served as the critical network interface between host computers and the nascent packet-switched network. Its deployment beginning in 1969 marked a pivotal shift from circuit switching to a more robust and efficient digital communications paradigm.

Overview

The device functioned as a dedicated minicomputer that managed network communications, allowing disparate and often incompatible mainframe computers at research institutions to exchange data. Each connected site, such as the University of California at Los Angeles and the Stanford Research Institute, housed one of these units, which was linked via leased telephone lines to others in the network. Its primary innovation was implementing the network control protocol and reliably routing discrete blocks of data, or packets, across a decentralized system. This design was central to the network's resilience and scalability, principles that remain core to Internet architecture today.

Development and design

The project was spearheaded by Frank Heart at Bolt, Beranek and Newman, with key contributions from engineers like Severo Ornstein and Robert Kahn. The design was based on a ruggedized version of the Honeywell 316 minicomputer, later upgraded to the Honeywell 516, chosen for its reliability. The team developed specialized software and interface hardware to handle message assembly, error checking, and routing according to designs influenced by earlier work on queueing theory and packet switching concepts from Paul Baran and Donald Davies. Rigorous testing, including a famous "bread truck" test drive around Cambridge, Massachusetts, ensured the system could withstand real-world conditions.

Role in ARPANET

The first unit was installed at University of California, Los Angeles in September 1969, directly connected to a SDS Sigma 7 host, with a second soon after at the Stanford Research Institute. The first successful host-to-host message transmission occurred between these nodes in October 1969, a landmark event supervised by researchers like Leonard Kleinrock and Charles Kline. As the network grew, additional units were deployed at nodes including the University of California, Santa Barbara, the University of Utah, MIT, and the RAND Corporation. These machines formed the backbone of the expanding network, enabling early applications like remote login via the Telnet protocol and file transfer using the File Transfer Protocol.

Technical specifications

The core hardware was a modified Honeywell 516 computer with 12 kilobytes of core memory. It utilized a specialized bit-serial interface to connect to host computers and a standard modem interface for the 50 kbit/s leased telephone lines. The software, written in assembly language, implemented store-and-forward packet switching, dynamic routing via a distributed algorithm, and the IMP-IMP protocol for reliable inter-IMP communication. Error detection and retransmission were handled by a variant of the Selective Repeat ARQ method, ensuring data integrity across sometimes noisy connections.

Legacy and historical significance

The technology directly demonstrated the feasibility and power of packet switching on a national scale, providing the operational proof-of-concept for Robert Kahn and Vinton Cerf to later develop the TCP/IP protocols. Its successors, like the Pluribus multi-processor IMP, led to more advanced gateways and routers. The architectural principles it embodied—decentralization, redundancy, and protocol layering—became the philosophical and technical bedrock of the global Internet. The original units are held in institutions like the Smithsonian Institution, and key figures from the project, including Robert Metcalfe, went on to pioneer other major technologies like Ethernet.

Category:Computer networking Category:ARPANET Category:History of the Internet