
The Interface Message Processor (IMP) was the
packet switching
In telecommunications, packet switching is a method of grouping Data (computing), data into short messages in fixed format, i.e. ''network packet, packets,'' that are transmitted over a digital Telecommunications network, network. Packets consi ...
node
In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex).
Node may refer to:
In mathematics
* Vertex (graph theory), a vertex in a mathematical graph
*Vertex (geometry), a point where two or more curves, lines ...
used to interconnect participant networks to the
ARPANET
The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first computer networks to implement the TCP/IP protocol suite. Both technologies became the tec ...
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
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 automation, industrial automa ...
DDP-516 minicomputer
A minicomputer, or colloquially mini, is a type of general-purpose computer mostly developed from the mid-1960s, built significantly smaller and sold at a much lower price than mainframe computers . By 21st century-standards however, a mini is ...
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 requires 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 , 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 standard, Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster ...
(IETF).
History

The concept of an interface computer for computer networking was first proposed in 1966 by
Donald Davies
Donald Watts Davies, (7 June 1924 – 28 May 2000) was a Welsh computer scientist and Internet pioneer who was employed at the UK National Physical Laboratory (NPL).
During 1965-67 he invented modern data communications, including packet s ...
for the
NPL network
The NPL network, or NPL Data Communications Network, was a local area computer network operated by the National Physical Laboratory (United Kingdom), National Physical Laboratory (NPL) in London that pioneered the concept of packet switching.
...
in England and implemented there in 1968-9.
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.
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 (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, Bernie Cosell and Paul Wexelblat
* 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)
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
The PDP-1 (Programmed Data Processor-1) is the first computer in Digital Equipment Corporation's PDP series and was first produced in 1959. It is known for being the most important computer in the creation of hacker culture at the Massachusetts ...
, where the IMP code was written and edited, then run on the Honeywell.
There was considerable technical interchange with the British team building the NPL network
The NPL network, or NPL Data Communications Network, was a local area computer network operated by the National Physical Laboratory (United Kingdom), National Physical Laboratory (NPL) in London that pioneered the concept of packet switching.
...
and Paul Baran
Paul Baran (born Pesach Baran ; April 29, 1926 – March 26, 2011) was a Polish-American engineer who was a pioneer in the development of computer networks. He was one of the two independent inventors of packet switching, which is today the do ...
at RAND but the BBN team independently developed significant aspects of the network's internal operation, such as routing, flow control, software design, and network control.
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'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, inventor, and a pioneer in many aspects of computer science. He is best known for his work on founding the field of human–computer interaction, particularly ...
'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 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 (Terminal IMP), which connected terminals (i.e., teletypes) 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 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. 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.
]
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 Bob Kahn.[
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 assigned to a device connected to a computer network that uses the Internet Protocol for communication. IP addresses serve two main functions: network interface i ...
on the Internet
The Internet (or internet) is the Global network, 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 ...
) 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 NPL network
The NPL network, or NPL Data Communications Network, was a local area computer network operated by the National Physical Laboratory (United Kingdom), National Physical Laboratory (NPL) in London that pioneered the concept of packet switching.
...
in 1968.
Later versions of the 1822 protocol, such as 1822L, are described in and its successors.
See also
* Front-end processor
* Fuzzball router
*Internet protocol suite
The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suite are ...
References
Further reading
*
*
*
*{{cite journal, author=Hubert Zimmermann , title=OSI Reference Model—The ISO Model of Architecture for Open Systems Interconnection , journal=IEEE Transactions on Communications , date=April 1980 , volume=28 , issue=4 , pages=425–432 , url=http://www.comsoc.org/livepubs/50_journals/pdf/RightsManagement_eid=136833.pdf , doi=10.1109/TCOM.1980.1094702 , s2cid=16013989 , url-status=dead , archive-url=https://web.archive.org/web/20050309080952/http://www.comsoc.org/livepubs/50_journals/pdf/RightsManagement_eid%3D136833.pdf , archive-date=2005-03-09
External links
A Technical History of the ARPANET with photos of IMP
Oral 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 BBN Report 1822, "Specification for the Interconnection of a Host and an IMP".
Networking hardware
ARPANET
Network protocols
History of the Internet
Computer-related introductions in 1969