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The Advanced Research Projects Agency Network (ARPANET) developed by ARPA of the U.S. Department of Defense was the world's first operational packet switching network, and the progenitor of the global Internet.

Packet switching, now the dominant basis for both data and voice communication worldwide, was a new and important concept in data communications. Previously, data communications was based on the idea of circuit switching, as in the old typical telephone circuit, where a dedicated circuit is tied up for the duration of the call and communication is only possible with the single party on the other end of the circuit.

With packet switching, a system could use one communication link to communicate with more than one machine by assembling data into packets. Not only could the link be shared (much as a single mail person can be used to post letters to different destinations), but each packet could be routed independently of other packets. This was a major advance.


Background of the ARPANET

The earliest ideas of a computer network intended to allow general communication between users of various computers were formulated by J.C.R. Licklider of Bolt, Beranek and Newman (BBN) in August 1962, in a series of memos discussing his "Galactic Network" concept. These ideas contained almost everything that the Internet is today.

In October 1962, Licklider was appointed head of the Behavioral Sciences and Command and Control programs at ARPA (as it was then called), the United States Department of Defense Advanced Research Projects Agency. He then convinced Ivan Sutherland and Bob Taylor that this was a very important concept, although he left ARPA before any actual work on his vision was performed.

ARPA and Taylor continued to be interested in creating a computer communication network, in part to allow ARPA-sponsored researchers in various locations to use various computers which ARPA was providing, and in part to quickly make new software and other results widely available. Taylor had three different terminals in his office, connected to three different computers which DARPA was funding: one for the SDC Q-32 in Santa Monica, one for Project Genie at the University of California, Berkeley, and one for Multics at MIT. Taylor later recalled:

"For each of these three terminals, I had three different sets of user commands. So if I was talking online with someone at S.D.C. and I wanted to talk to someone I knew at Berkeley or M.I.T. about this, I had to get up from the S.D.C. terminal, go over and log into the other terminal and get in touch with them.
I said, oh, man, it's obvious what to do: If you have these three terminals, there ought to be one terminal that goes anywhere you want to go where you have interactive computing. That idea is the ARPAnet." [1].

Roughly contemporaneously, a number of people had (mostly independently) worked out various aspects of what later became known as "packet switching"; the people who created the ARPANET would eventually draw on all these different sources.

Origins of the ARPANET

At the end of 1966, Taylor brought Larry Roberts to ARPA from MIT Lincoln Laboratory to head a project to create the network. Roberts had some initial experience in this area: two years previously, in 1965, while at MIT Lincoln Laboratory, he had connected the TX-2 to System Development Corporation's Q-32 over a telephone line, conducting some of the earliest experiments in which two computers communicated that way. Roberts' initial concept for the network for ARPA was to hook the various time-sharing machines directly to each other, through telephone lines.

At a meeting at the University of Michigan in Ann Arbor, Michigan in early 1967, many of the participants were unenthusiastic at having the load of managing this line put directly on their computer. One of the participants, Wesley Clark, came up with the idea of using separate smaller computers to manage the communication links; the small computers would then be connected to the large time-sharing mainframe computers which were the typical machines to be connected to the ARPANET. This concept allowed most of the detailed work of running the network to be offloaded from the large main-frames; it also meant that correct operation of the network as a whole was not subject to the vagaries of individual host implementations, and that DARPA would have complete control over the network itself.

Initial planning for the ARPANET began on that basis, with a number of working groups on specific technical subjects meeting during the late spring and summer of 1967.

Roberts then proceeded to author a "plan for the ARPANET", which was presented at a symposium in Gatlinburg, Tennessee in October, 1967. Also presenting there was Roger Scantlebury, from Donald Davies' group at NPL. (Roberts had previously encountered Davies at a conference in Britain about time-sharing, in November, 1965.) He discussed Davies' packet switching ideas with Roberts, and introduced Roberts to Paul Baran's work.

The exact impact of all this is unclear, and somewhat controversial; memoirs by different people involved in the process give sharply conflicting accounts, often in conflict with their earlier recorded statements. The general view of most historians is that all four (Baran, Kleinrock, Davies and Roberts) had important contributions:

  • Davies was instrumental in passing on the knowledge of packet switching that he and Baran had developed to Lawrence Roberts Template:NamedRef
  • Roberts' ideas for the network were modified by his discussions with Scantlebury. .. According to his later description, upon returning to Washington from the Gatlinburg meeting [Roberts] was influenced by Baran's reports Template:NamedRef
  • If anyone influenced Roberts in his earliest thinking about computer networks, it was Kleinrock. ... Baran's insights into data communications intrigued [Roberts] ... The Gatlinburg paper presented by Scantlebury on behalf of the British effort was clearly an influence, too. Template:NamedRef

Creation of the ARPANET

By the summer of 1968, a complete plan had been prepared, and after approval at ARPA, a Request For Quotation (RFQ) was sent to 140 potential bidders. Most regarded the proposal as outlandish, and only 12 companies submitted bids, of which only four were regarded as in the top rank. By the end of the year, the field had been narrowed to two, and after negotiations, a final choice was made, and the contract was awarded to BBN on 7 April, 1969.

BBN's proposal followed Roberts' plan closely; it called for the network to be composed of small computers known as Interface Message Processors (more commonly known as IMPs). The IMPs at each site performed store-and-forward packet switching functions, and were connected to each other using modems connected to leased lines (initially running at 50 kbit/second). Host computers connected to the IMPs via custom bit-serial interfaces to connect to ARPANET.

BBN initally chose a ruggedized version of Honeywell's DDP-516 computer to build the first generation IMP. The 516 was originally configured with 24 kbytes of core memory (expandable) and a 16 channel Direct Multiplex Control (DMC) direct memory access control unit. Custom interfaces were used to connect, via the DMC, to each of the hosts and modems. In addition to the lamps on the front panel of the 516 there was also a special set of 24 indicator lights to show the status of the IMP communication channels. Each IMP could support up to four local hosts and could communicate with up to six remote IMPs over leased lines.

The small team at BBN (initially only seven people), helped considerably by the detail they had gone into to produce their response to the RFQ, quickly produced the first working units. The entire system, including both hardware and the world's first packet switching software, was designed and installed in nine months.

Initial ARPANET deployment


The initial ARPANET consisted of four IMPs. They were installed at:

The first ARPANET link was established on October 29, 1969, between the IMP at UCLA and the IMP at SRI. By December 5, 1969, the entire 4-node network was connected [2].

Software and protocol development

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When the ARPANET was first deployed, not much work had been done of how the computers attached to it would communicate with each other. Over the next several years, a considerable amount of work went into this.

At the lowest levels of the Network Control Program (something that would later come to be called a protocol suite) were a set of basic communication protocols that allowed hosts to exchange data. On top of them, other protocols allowed applications to perform more specific functions.

In 1983, TCP/IP protocols replaced NCP as the principal protocol of the ARPANET, and the ARPANET became just one component of the fledgling Internet.


In 1971, Ray Tomlinson of BBN sent the first network email [3]. By 1973, the File Transfer Protocol (FTP) specification had been defined and implemented, enabling file transfers over the ARPANET. At that point, 75% of the ARPANET traffic was email.

A Network Voice Protocol (NVP) specifications was also defined (RFC 741) and then implemented, but conference calls over the ARPANET never worked well, for technical reasons; packet voice would not become a workable reality for several decades.

Growth of the network

In March, 1970, the ARPANET reached the U.S. East Coast, when an IMP at BBN itself was joined up to the network. Thereafter, the network grew quickly: 9 IMPs by June of 1970, and 13 by December; 18 by September, 1971 (at which point twenty-three hosts, at universities and government research centers, were connected to the ARPANET); 29 by August, 1972, and 40 by September, 1973.

At that point two satellite links, across the Pacific and Atlantic Oceans to Hawaii and Norway respectively, had been added to the network. From Norway, a terrestrial circuit added an IMP in London to the growing network.

By June, 1974 there were 46 IMPs, and the network reached 57 in July, 1975. By 1981, the number of hosts had grown to 213, with a new host being added approximately every twenty days.

After the ARPANET had been up and running for several years, ARPA looked for another agency to hand off the network to; ARPA's primary business was funding cutting-edge research and development, not running a communications utility. Eventually, in July 1975, the network was turned over to the Defense Communications Agency, also part of the Department of Defense.

In 1984, the U.S. military portion of the ARPANet was broken off as a separate network, the MILNET.

Later hardware developments

Support for inter-IMP circuits of up to 230.4 kbit/s was added in 1970, although considerations of cost and IMP processing power meant this capability was not much used.

1971 saw the start of the use of the non-ruggedized (and therefore significant lighter) H-316 as an IMP. It could also be configured as a Terminal IMP (TIP), which added support for up to 63 ASCII serial terminals through a multi-line controller in place of one of the hosts. The 316 featured a greater degree of integration than the 516, which made it less expensive and easier to maintain. The 316 was configured with 40 Kbytes of core memory for a TIP. The size of core memory was later increased, to 32 Kbytes for the IMPs, and 56Kbytes for TIPs, in 1973.

The Honeywell based IMPs were eventually superseded by multi-processor BBN Pluribus IMPs in 1975. These in turn were later phased out in favor of machines called C/30s, which were custom built by BBN.

The original IMPs and TIPs were phased out as the ARPANET was shut down after the introduction of the NSFNet, but some IMPs remained in service as late as 1989.

The ARPANET and nuclear attacks

A common semi-myth about the ARPANET states that it was designed to be resistant to nuclear attack. The Internet Society writes about the merger of technical ideas that produced the ARPANET in A Brief History of the Internet, and states in a note:

It was from the RAND study that the false rumor started claiming that the ARPANET was somehow related to building a network resistant to nuclear war. This was never true of the ARPANET, only the unrelated (sic) RAND study on secure voice considered nuclear war. However, the later work on Internetting did emphasize robustness and survivability, including the capability to withstand losses of large portions of the underlying networks.

The myth that the ARPANET was built to withstand nuclear attacks however remains such a strong and apparently appealing idea — and of course "a good story" — that many people refuse to believe it is not true. However it is not, unless one means that these ideas influenced the ARPANET development by way of the RAND research papers. The ARPANET was designed to survive network losses, but the main reason was actually that the switching nodes and network links were not highly reliable, even without any nuclear attacks.


On 26 March, 1976 Queen Elizabeth II sent out the first royal email, from the Royal Signals and Radar Establishment.


Support and style of management by ARPA was crucial to the success of ARPANET. The ARPANET Completion Report, published jointly by BBN and ARPA, concludes by stating: is somewhat fitting to end on the note that the ARPANET program has had a strong and direct feedback into the support and strength of computer science, from which the network itself sprung. Template:NamedRef

References in film and media

See also


Further reading

  • Arthur Norberg, Judy E. O'Neill, Transforming Computer Technology: Information Processing for the Pentagon, 1962-1982 (Johns Hopkins University, 1996) pp. 153-196
  • A History of the ARPANET: The First Decade (Bolt, Beranek and Newman, 1981)
  • Katie Hafner, Where Wizards Stay Up Late: The Origins of the Internet (Simon and Schuster, 1996)
  • Janet Abbate, Inventing the Internet (MIT Press, Cambridge, 1999) pp. 36-111
  • Peter H. Salus, Casting the Net: from ARPANET to Internet and Beyond (Addison-Wesley, 1995)
  • M. Mitchell Waldrop, The Dream Machine: J. C. R. Licklider and the Revolution That Made Computing Personal (Viking, New York, 2001)

Detailed technical reference works

  • Larry Roberts and Tom Merrill, Toward a Cooperative Network of Time-Shared Computers (Fall AFIPS Conference, October 1966)
  • Larry Roberts, Multiple computer networks and intercomputer communication (ACM Symposium on Operating System Principles. October 1967)
  • D. W. Davies, K. A. Bartlett, R. A. Scantlebury, and P. T. Wilkinson. A digital communications network for computers giving rapid response at remote terminals (ACM Symposium on Operating Systems Principles. October 1967)
  • Frank Heart, Robert Kahn, Severo Ornstein, William Crowther, David Walden, The Interface Message Processor for the ARPA Computer Network (1970 Spring Joint Computer Conference, AFIPS Proc. Vol. 36, pp. 551-567, 1970)
  • Stephen Carr, Stephen Crocker, Vinton Cerf. Host-Host Communication Protocol in the ARPA Network (1970 Spring Joint Computer Conference, AFIPS Proc. Vol 36, pp. 589-598, 1970)
  • Severo Ornstein, Frank Heart, William Crowther, S. B. Russell, H. K. Rising, and A. Michel, The Terminal IMP for the ARPA Computer Network (1972 Spring Joint Computer Conference, AFIPS Proc. Vol. 40, pp. 243-254, 1972)
  • John McQuillan, William Crowther, Bernard Cosell, David Walden, and Frank Heart, Improvements in the Design and Performance of the ARPA Network (1972 Fall Joint Computer Conference, AFIPS Proc. Vol. 41, Pt. 2, pp. 741-754, 1972)
  • Feinler, E.; Postel, Jon B. ARPANET Protocol Handbook (Network Information Center, Menlo Park, 1978)
  • Lawrence Roberts, The Evolution of Packet Switching (Proceedings of the IEEE, November, 1978)

External links

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