The Interface Message Processor (IMP) was the

packet switching In telecommunications, packet switching is a method of grouping Data (computing), data into ''network packet, packets'' that are transmitted over a digital Telecommunications network, network. Packets are made of a header (computing), header and ...

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 A rugged computer or ruggedized computer is a computer specifically designed to operate reliably in harsh usage environments and conditions, such as strong vibrations, extreme temperatures and wet or dusty conditions. They are designed from incept ...

Honeywell Honeywell International Inc. is an American publicly traded, multinational conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four areas of business: aerospace, building technologies, performance ma ...

DDP-516 The Honeywell 316 was a popular 16-bit minicomputer built by Honeywell starting in 1969. It is part of the Series 16, which includes the Models 116 (1965, Discrete component, discrete), 316 (1969), 416 (1966), 516 (1966) and DDP-716 (1969). They w ...

minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold at a much lower price than mainframe and mid-size computers from IBM and its direct competitors. In a 1970 survey, ...

with special-purpose interfaces and software. In later years the IMPs were made from the non-ruggedized

Honeywell 316 The Honeywell 316 was a popular 16-bit minicomputer built by Honeywell starting in 1969. It is part of the Series 16, which includes the Models 116 (1965, discrete), 316 (1969), 416 (1966), 516 (1966) and DDP-716 (1969). They were commonly used fo ...

which could handle two-thirds of the communication traffic at approximately one-half the cost. An IMP requires the connection to a host computer via a special bit- serial interface, defined in ''

BBN Report 1822 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 ru ...

''. The IMP software and the ARPA network communications protocol running on the IMPs was discussed in , the first of a series of standardization documents published by what later became the

Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and ...

(IETF).

History

The concept of an "Interface computer" was first proposed in 1966 by

Donald Davies Donald Watts Davies, (7 June 1924 – 28 May 2000) was a Welsh computer scientist who was employed at the UK National Physical Laboratory (NPL). In 1965 he conceived of packet switching, which is today the dominant basis for data communic ...

for the

NPL network The NPL network, or NPL Data Communications Network, was a local area computer network operated by a team from the National Physical Laboratory in London that pioneered the concept of packet switching. Based on designs first conceived by Donal ...

in England. 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 The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Ad ...

(ARPA) to discuss interconnecting machines across the country. Larry Roberts, who led the ARPANET implementation, initially proposed a network of host computers. Wes Clark suggested inserting "a small computer between each host computer and the network of transmission lines", i.e. making the IMP a separate computer. The IMPs were built by the Massachusetts-based company

Bolt Beranek and Newman Raytheon BBN (originally Bolt Beranek and Newman Inc.) is an American research and development company, based next to Fresh Pond in Cambridge, Massachusetts, United States. In 1966, the Franklin Institute awarded the firm the Frank P. Brown ...

(BBN) in 1969. 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". The team working on the IMP called themselves the "IMP Guys": * Team Leader: Frank Heart * Software: Willy Crowther,

David Walden David Corydon Walden (June 7, 1942April 27, 2022) was an American computer scientist who contributed to the engineering development of the ARPANET, a precursor of the modern internet. He specifically contributed to the Interface Message Processo ...

, Bernie Cosell and Paul Wexelblat * Hardware:

Severo Ornstein Severo M. Ornstein (born 1930) is a retired computer scientist and son of American composer Leo Ornstein. In 1955 he joined MIT's Lincoln Laboratory as a programmer and designer for the SAGE air-defense system. He later joined the TX-2 group and ...

, Ben Barker * Theory and collaboration with the above on the overall system design:

Bob Kahn Robert Elliot Kahn (born December 23, 1938) is an American electrical engineer who, along with Vint Cerf, first proposed the Transmission Control Protocol (TCP) and the Internet Protocol (IP), the fundamental communication protocols at the hear ...

* Other: Hawley Rising * Added to IMP team later: Marty Thrope (hardware), Jim Geisman, Truett Thach (installation), Bill Bertell (Honeywell) BBN began programming work in February 1969 on modified Honeywell DDP-516s. The completed code was six thousand words long, and was written in the Honeywell 516 assembly language. The IMP software was produced primarily on a PDP-1, where the IMP code was written and edited, then run on the Honeywell. BBN designed the IMP simply as "a messenger" that would only "store-and-forward". BBN designed only the host-to-IMP specification, leaving host sites to build individual host-to-host interfaces. The IMP had an error-control mechanism that discarded packets with errors without acknowledging receipt; the source IMP, upon not receiving an acknowledging receipt, would subsequently re-send a duplicate packet. Based on the requirements of ARPA's request for proposal, the IMP used a 24-bit

checksum A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify dat ...

for error correction. BBN chose to make the IMP hardware calculate the checksum, because it was a faster option than using a software calculation. The IMP was initially conceived as being connected to one host computer per site, but at the insistence of researchers and students from the host sites, each IMP was ultimately designed to connect to multiple host computers. The first IMP was delivered to

Leonard Kleinrock Leonard Kleinrock (born June 13, 1934) is an American computer scientist and a long-tenured professor at UCLA's Henry Samueli School of Engineering and Applied Science. In the early 1960s, Kleinrock pioneered the application of queueing theory ...

's group at UCLA on August 30, 1969. It used an SDS Sigma 7 host computer.

Douglas Engelbart Douglas Carl Engelbart (January 30, 1925 – July 2, 2013) was an American engineer and inventor, and an early computer and Internet pioneer. He is best known for his work on founding the field of human–computer interaction, particularl ...

'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 third IMP was installed in University of California, Santa Barbara on November 1, 1969. The fourth and final IMP was installed in the University of Utah in December 1969. The first communication test between two systems (UCLA and SRI) 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. BBN developed a program to test the performance of the communication circuits. 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." A variant of the IMP existed, called the TIP, which connected terminals as well as computers to the network; it was based on the Honeywell 316, a later version of the 516. Later, some Honeywell-based IMPs were replaced with multiprocessing BBN

Pluribus The Pluribus''Pluribus'' is the ablative plural of the Latin word for "more" or "above." multiprocessor was an early multi-processor computer designed by BBN for use as a packet switch in the ARPANET. Its design later influenced the BBN Butter ...

IMPs, but ultimately BBN developed a microprogrammed clone of the Honeywell machine. IMPs were at the heart of the ARPANET until DARPA decommissioned the ARPANET in 1989. Most IMPs were either taken apart, junked or transferred to

MILNET In computer networking, MILNET (fully Military Network) was the name given to the part of the ARPANET internetwork designated for unclassified United States Department of Defense traffic.DEFENSE DATA NETWORK NEWSLETTEDDN-NEWS 26 6 May 1983 MILNE ...

. Some became artifacts in museums; Kleinrock placed IMP Number One on public view at UCLA. The last IMP on the ARPANET was the one at the University of Maryland. Interface Message Processor Front Panel

BBN Report 1822

''BBN Report 1822'' specifies the method for connecting a host computer to an IMP. This connection and protocol is generally referred to as ''1822'', the report number. The specification was written by

. The initial version of the 1822 protocol was developed in 1969. Since it predates the

OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

by a decade, 1822 does not map cleanly into the OSI layers. However, it is accurate to say that the 1822 protocol incorporates the

physical layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip. Th ...

, the

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer ...

, and the

network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of trans ...

. The interface visible to the host system passes network layer addresses directly to a physical layer device. To transmit data, the host constructs a message containing the numeric address of another host on the network (similar to an

IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...

on the

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...

) and a data field, and transmits the message across the 1822 interface to the IMP. The IMP routes the message to the destination host using protocols that were eventually adopted by Internet routers. Messages could store a total length of 8159 bits, of which the first 96 were reserved for the header ("leader"). While packets transmitted across the Internet are assumed to be unreliable, 1822 messages were guaranteed to be transmitted reliably to the addressed destination. If the message could not be delivered, the IMP sent to the originating host a message indicating that the delivery failed. In practice, however, there were (rare) conditions under which the host could miss a report of a message being lost, or under which the IMP could report a message as lost when it had in fact been received. The specification incorporated an alternating bit protocol, of the type proposed by Donald Davies' team for the

in 1968. Later versions of the 1822 protocol, such as 1822L, are described in and its successors.

References

External links

A Technical History of the ARPANET with photos of IMPOral history interview with Severo Ornstein

Charles Babbage Institute The IT History Society (ITHS) is an organization that supports the history and scholarship of information technology by encouraging, fostering, and facilitating archival and historical research. Formerly known as the Charles Babbage Foundation, ...

, University of Minnesota. Ornstein was principal hardware designer of the IMP. *Internet STD 39, also known as

, "Specification for the Interconnection of a Host and an IMP". Networking hardware ARPANET Network protocols History of the Internet

History

BBN Report 1822

See also

References

Further reading

External links