This is an excerpt from A Brief History of the Future: the origins of the Internet by John Naughton.  First published by Weidenfeld & Nicolson in 1999.  Subsequent revised editions published by Phoenix. (c) John Naughton 1999.
 Chapter 10: Casting the Net

"Network. Anything reticulated or decussated at equal distances, with interstices between the intersections."
  Samuel Johnson , A Dictionary of the English Language,1755

The strange thing about the ARPANET was that it worked more or less as advertised from the word go.  BBN delivered IMPs at the rate of one a month and the network grew apace.  The University of California at Santa Barbara got the third one in November 1969 and Utah took delivery of the fourth early in December.  IMP Number 5 was delivered to Bolt Beranek and Newman itself early in 1970 and the first cross-country circuit -- a 50 kilobit per second line from Leo Kleinrock’s machine in Los Angeles to BBN’s in Boston -- was established.  This meant not only that the Net now spanned the continent, but also that BBN was able to monitor it remotely.

  By the Summer of 1970, the graduate students on the Network Working Group had worked out a provisional version of the Network Control Program (NCP) -- the protocol which enabled basic  communications between host computers  -- and IMPs 6,7,8 and 9 had been installed at (respectively) MIT, RAND, System Development Corporation and Harvard.  Towards the end of the Summer, AT&T  (whose engineers were presumably still baffled by the strange uses the researchers had discovered for telephone lines) replaced the UCLA-BBN link with a new one between BBN and RAND.  A second cross-country link connected the University of Utah and MIT. By the end of 1971 the system consisted of 15 nodes (linking 23 hosts).  In August 1972 a third cross-country line was added.  By the end of the year ARPANET had 37 nodes.  The system was beginning to spread its wings -- or, if you were of a suspicious turn of mind, its tentacles.
  Researchers were also beginning to get a realistic idea of what you could do with it.  They could log in to remote machines, for example,  and exchange files securely.  Later in 1970 they also saw the first stirrings of electronic mail.  And, as always, there were some restless kids who wanted to do some wacky things with their new toy.  In January 1971, for example, two Harvard students, Bob Metcalfe and Danny Cohen, used a PDP-10 minicomputer to simulate an aeroplane landing on the flight deck of an aircraft carrier and then displayed the images on a graphics terminal located down the Charles River in MIT.  The graphics were processed on the MIT machine and the results (in this case the view of the carrier’s flight deck) were then shipped back over the Net to the PDP-10 at Harvard, which displayed them.  It was the kind of stunt which makes non-technical people wonder what students are smoking, but in fact the experiment had a serious purpose because it showed that the Net could move significant amounts of data around (graphics files tend to be large) at a rate which approximated to what engineers call ‘real time’.  Metcalfe and Cohen wrote an RFC3 describing their achievement under the modest header “Historic Moments in Networking”.
  The emerging ARPANET was a relatively closed and homogeneous system.  Access to it was confined to a small elite working in Pentagon-funded computing laboratories.  And although it wove together a number of varied and incompatible mainframe computers (the hosts), the subnetwork of IMPs which actually ran the network was comprised of identical units controlled, updated and debugged from a single Network Control Center located in Bolt Beranek and Newman’s offices in Boston.  The subnetwork showed its military colours in that it was designed to be exceedingly efficient and reliable in its performance and behaviour.  Indeed, one of the reasons the designers chose the Honeywell 516 as the basis for the IMPs was that it was a machine capable of being toughened for military use -- which meant that it could also (in theory) be secured against the curiosity of graduate students.  (This was before the folks at ARPA or BBN realised how crucial those students would be in getting the damn thing to work.)
  The modern Internet is significantly different from the system that BBN built.  In the first place, it is comprised of an unimaginable variety of machines of all ages, makes and sizes, running a plethora of operating systems and communications software.  Secondly, it is in fact a network of networks: the components of the modern Internet are themselves wide-area networks of various sorts.  There is no Network Control Centre probing the nodes, installing software updates from afar and generally keeping an eye on things.  And yet, in the midst of all this headless, chaotic variety, there is order.  The system works.  Packets get from one end of the world to the other with astonishing speed and reliability. 
  If you have any doubts, try this.  I have on my laptop a lovely little program called PING.  Its purpose is to send out test packets to a destination anywhere on the Net in order to test how reliably they reach their destination and how long they take in transit.  Now let’s PING a node in San Francisco: it’s, the site which provides those live pictures of the Bay Area which first captured my attention.  After 22 pings, I call a halt and the program summarises the results in a table:
  Packet loss=9.09%
  Min rtt=441
  Max rtt=650
  Avg rtt=522
  The first row indicates that the site address has been translated into the underlying Internet address -- the set of four numbers which uniquely identifies the computer to be PINGed.  The second row shows that 20 of the 22 packets (i.e. 90.91 per cent) reached their destination.  The third row reveals that the minimum time for the round trip was 441 milliseconds (thousandths of a second), the maximum was 650 and the average worked out at 522.  That is to say, it took, on average, just over half a second for a packet to travel from my study in Cambridge, UK to KPIX’s machine in San Francisco and back again.
  Looking back at the ARPANET from the vantage point of the contemporary Net is a bit like comparing a baby chimpanzee with a human child .  The resemblances are striking and the evolutionary link is obvious.  It is said that something like 98 per cent of the genetic material of both species is indistinguishable.  Yet we are very different from chimps.
  So it is with the two networks.  The Internet has inherited many of its dominant characteristics from its pioneering ancestor, but it is significantly different in one respect: its ability to diversify.  ARPANET could never have expanded the way the Internet has: its design required too much control, too much standardisation for it to match the variegated diversity of the online world.  For it to metamorphose into the network which now girdles the globe, something had to change.
In a way, the ARPANET was a ‘proof of concept system’.  It took a set of ideas many people thought impracticable, and created a working network out of them.  By 1972 it was clear that the project was, in technical terms, a runaway success.  But as many inventors know, designing a better mousetrap is no guarantee that the world will beat a path to your door.  Even geniuses have to blow their own trumpets sometimes.

  Within BBN, one man -- Robert Kahn -- understood this better than most.  He was born in Brooklyn, New York, in 1934, got a Bachelor’s degree in electrical engineering from City College of New York and then went to Princeton where he picked up a Master’s and a doctorate.  In 1964 Kahn went to MIT as an assistant professor, and two years later took temporary leave of absence (as many MIT engineering faculty members did) to work at BBN.  He never made it back to MIT, because when the company decided to bid for the ARPANET contract, Kahn was persuaded to take on the job of overall system design and was hooked for life.
  Kahn was a systems engineer, not a hacker.  While others were concerned with the individual components which made up the network, he was the guy who knew that systems were more (and often less) than the sum of their parts – that the whole network would behave in ways that could not be predicted from a study of its individual components.  His professional instincts told him, for example, that the routing algorithms – the procedures which governed the way the IMPs processed packets – would be critical.  “It was my contention”, he recalled, “that we had to worry about congestion and deadlocks”.
 What do you do when the network just fills up?  Things might come to a grinding halt.  I was busy at work designing mechanisms that would prevent that from happening or, if it did happen, that would get you out of it.  And the prevailing feeling of my colleagues was it's like molecules in a room; don't worry about the fact that you won't be able to breathe because the molecules will end up in a corner.  Somehow there will be enough statistical randomness that packets will just keep flowing.  It won't be able to somehow block itself up.
  Determined to prove the optimists wrong, Kahn and a colleague named Dave Walden went out to California early in 1970 to test the fledgling Net to destruction.  “The very first thing that we did”, he remembers,
"was run deadlock tests.  And the network locked up in twelve packets.  I had devised these tests to prove that the network could deadlock.  There was no way to convince anybody else, particularly the person writing the software, that the network was going to deadlock - except by doing it."
  From the outset Kahn had been of the view that the network project should be a large scale experiment, not something cooked up in the rarefied atmosphere of an individual lab or a number of geographically-proximate institutes.  He felt that limited systems might not scale up and wanted a continent-wide network, using long-distance phone lines, from the word go.  And his view prevailed, which is why from early in the project the ARPANET spanned the continental United States.
  In the middle of 1971 Kahn turned his mind to the problem of communicating this astonishing technical achievement to the movers and shakers of the US political, military, business and telecommunications communities.  After a meeting at MIT of some of the main researchers involved on the project, it was decided that the thing to do was to organise a large-scale, high-profile, live demonstration of what the network could do.  Casting round for a suitable venue, he hit on the first International Conference on Computer Communication, scheduled to be held in the Washington Hilton in October 1972, and negotiated with the organisers an agreement that ARPA could mount a huge, live exhibition of the network in action.
   The goal of the exhibition was to be the most persuasive demo ever staged -- “to force”, in Kahn’s words, “the utility of the network to occur to the end users”.  It was to be the event that made the world take notice of packet switching, because up to that point the technology had been more or less invisible outside of the elite circle of ARPA-funded labs.  “A lot of people were sceptical in the early days”, said Kahn.
"I mean, breaking messages into packets, reassembling them at the end, relying on a mysterious set of algorithms, routing algorithms, to deliver packets.  I'm sure there were people who distrusted airplanes in the early days.  "How are you going to ensure that they are going to stay up?"  Perhaps this was the same kind of thing.
  Kahn and his team put an enormous effort into mounting the demo.  They toured the country bullying, cajoling, tempting researchers and computer equipment manufacturers into participating.  On the days before the conference opened, hackers gathered from all over the country to begin the nightmarish task of assembling and testing all the kit in the designated hall of the hotel. The atmosphere was slightly hysterical as the clock ticked away and computer equipment behaved in its customary recalcitrant way whenever a live demo approaches.  Everyone was “hacking away and hollering and screaming”  recalls Vint Cerf, one of the graduate students involved.  Kahn himself observed later that if someone had dropped a bomb on the Washington Hilton during the demo it would have wiped out the whole of the U.S. networking community in a single strike.
  Anyone who has ever relied on a computer network for a critical presentation knows what a high-risk gamble this was.  And yet it paid off: the system worked flawlessly10 and was seen by thousands of visitors.  The Hilton demo was the watershed event that made powerful and influential people in the computing, communications and defense industries suddenly realise that packet switching was not some toy dreamed up by off-the-wall hackers with no telecoms experience, but an operational, immensely powerful, tangible technology. 
So the ARPANET worked.  The question for Kahn (and indeed for the agency) was: what next?  Answer: the world.  But how?  The ARPANET model was not infinitely extensible for the simple reason that extending it would have required everyone to conform to the requirements of the US Department of Defense.  And other people had other ideas.  Indeed, even as the ARPANET was being built, other packet-switched networks had begun to take shape.  The French, for example, had begun work on their Cyclades network under the direction of a computer scientist called Louis Pouzin.  And Donald Davies’s team at the British National Physical Laboratory were pressing ahead with their own packet-switched network.

  Within the United States also, people had constructed alternative systems.  In 1969, ARPA had funded a project based at the University of Hawaii, an institution with seven campuses spread over four islands.  Linking them via land-lines was not a feasible proposition, so Norman Abramson and his colleagues Frank Kuo and Richard Binder devised a radio-based system called ALOHA.  The basic idea was to use simple radio transmitters (akin to those used by taxi-cabs) sharing a common radio frequency.  As with ARPANET, each station transmitted packets whenever it needed to.  The problem was that because the stations all shared the same frequency sometimes the packets ‘collided’ with one another (when two or more stations happened to transmit at the same time) with the result that packets often got garbled.  Abramson and his colleagues got round this by designing a simple protocol: if a transmitting station failed to receive an acknowledgement of a packet, it assumed that it had been lost in transmission, waited for a random period11 and then re-transmitted the packet.
  ARPA was interested in this idea of radio links between computers -- for obvious reasons.  There was a clear military application -- an ALOHA-type system based on radio transmitters fitted in moving vehicles like tanks could have the kind of resilience that centralised battlefield communications systems lacked.  But the limited range of the radios would still pose a problem, necessitating relay stations every few miles -- which re-introduced a level of vulnerability into the system.  This led to the idea of using satellites as the relay stations -- and in the end to the construction of SATNET, a wide-area network based on satellites.  In time,  SATNET linked sites in the US with sites in Britain, Norway, Germany and Italy.  Another attraction of these systems is that they offered far greater flexibility in the use of shared (not to mention scarce and expensive) transmission capacity.
  As these other packet-switching networks developed, Bob Kahn (who had now moved from BBN to ARPA) became increasingly preoccupied with the idea of devising a way in which they and ARPANET could all be interlinked.  This was easier said than done.  For one thing, the networks differed from one another in important respects -- the size of a packet, for example.  More importantly, they differed greatly in their reliability.  In the ARPANET, the destination IMP (as distinct from the host computer to which it was interfaced) was responsible for reassembling all the parts of a message when it arrived.  IMPs made sure that all the packets got through by means of an elaborate system of hop-by-hop acknowledgements.  They also ensured that different messages were kept in order.  The basic protocol of the ARPANET -- the Network Control Program -- was therefore built on the assumption that the network was reliable.
  This did not hold for the non-ARPA networks, where the governing assumption was, if anything, exactly the opposite.  There is always interference in radio transmissions, for example, so the ALOHA system had to be constructed on the assumption that the network was inherently unreliable, that one simply couldn’t count on a packet getting through.  If no acknowledgement was received, a transmitting host would assume it had got lost and dispatch an identical packet; and it would keep doing this until it received acknowledgement of receipt.  Also the non-ARPANET systems differed in other fundamental respects -- right down to what they regarded as the size of a standard packet and the fact that they had different rates of transmission.  Clearly linking or ‘internetting’ such a motley bunch was going to be difficult.
  To help solve this problem, Kahn turned to a young man who had been with the ARPANET project from the beginning.  His name was Vinton (‘Vint’) Cerf.
  The partnership between Kahn and Cerf is one of the epic pairings of modern technology, but to their contemporaries they must have seemed an odd couple.  Kahn is solid and genial and very middle-American -- the model for everyone’s favourite uncle -- who even as a young engineer radiated a kind of relaxed authority.  Cerf is his polar opposite -- neat, wiry, bearded, cosmopolitan, coiled like a spring.  He was born in California, the son of an aerospace executive, and went to school with Steve Crocker, the guy who composed the first RFC.  His contemporaries remember him as a wiry, intense child whose social skills were honed (not to say necessitated) by his profound deafness.  They also remember his dress style -- even as a teenager he wore a suit and tie and carried a briefcase.  “I wasn't so interested in differentiating myself from my parents”, he once told a reporter, “but I wanted to differentiate myself from the rest of my friends just to sort of stick out”.  Coming from a highly athletic family, he also stood out as a precocious bookworm who had taught himself computer programming by the end of tenth grade and calculus at the age of 13.  His obsession with computers was such that Crocker once claimed Cerf masterminded a weekend break-in to the UCLA computer centre simply so that they could use the machines.
  Cerf spent the years 1961-’65 studying mathematics at Stanford, worked for IBM for a while and then followed Crocker to UCLA where he wound up as a doctoral student in Leo Kleinrock’s lab, the first node on the ARPANET.  In this environment Cerf’s neat, dapper style set him apart from the prevailing mass of untidy hackers.  He was one of the founder members of the Network Working Group, and his name figures prominently in the RFC archive from the beginning.  He and Kahn had first worked together in early 1970 when Kahn and Dave Walden conducted the tests designed to push the ARPANET to the point of collapse in order to see where its limits lay.  “We struck up a very productive collaboration”, recalled Cerf. "He would ask for software to do something, I would program it overnight, and we would do the tests....  There were many times when we would crash the network trying to stress it, where it exhibited behavior that Bob Kahn had expected, but that others didn't think could happen. One such behavior was reassembly lock-up. Unless you were careful about how you allocated memory, you could have a bunch of partially assembled messages but no room left to reassemble them, in which case it locked up. People didn't believe it could happen statistically, but it did."
  The other reason Kahn wanted Cerf for the internetworking project was because he had been one of the students who had devised the original Network Control Protocol for the ARPANET.  Having got him on board, Kahn then set up a meeting of the various researchers involved in the different networks in the US and Europe.  Among those present at the first meeting were Donald Davies and Roger Scantlebury from the British National Physical Laboratory, Remi Despres from France, Larry Roberts and Barry Wessler from BBN, Gesualdo LeMoli from Italy, Kjell Samuelson from the Royal Swedish Institute, Peter Kirstein from University College, London, and Louis Pouzin from the French Cyclades project.
  “There were a lot of other people”, Cerf recalls, "at least thirty, all of whom had come to this conference because of a serious academic or business interest in networking.  At the conference we formed the International Network Working Group or INWG.  Stephen Crocker, …, didn’t think he had time to organize the INWG, so he proposed that I do it."
  Having become Chairman of the new group, Cerf took up an Assistant Professorship in Computer Science at Stanford and he and Kahn embarked on a quest for a method of creating seamless connections between different networks.  The two men batted the technical issues back and forth between them for some months, and then in the Spring of 1973, sitting in the lobby of a San Francisco hotel during a break in a conference he was attending, Cerf had a truly great idea.  Instead of trying to reconfigure the networks to conform to some overall specification, why not leave them as they were and simply use computers to act as ‘gateways’ between different systems?  To each network, the gateway would look like one of its standard nodes.  But in fact what the gateway would be doing was simply taking packets from one network and handing them on to the other.
  In digital terms, this was an idea as profound in its implications as the discovery of the structure of the DNA molecule in 1953, and for much the same reasons. James Watson and Francis Crick uncovered a structure which explained how genetic material reproduced itself; Cerf’s gateway concept provided a means by which an ‘internet’ could grow indefinitely because networks of almost any kind could be added willy-nilly to it.  All that was required to connect a new network to the ‘network of networks’ was a computer which could interface between the newcomer and one network which was already connected.
  In some ways, the idea of using gateways was analogous to Wesley Clark’s 1967 brainwave of using IMPs as intermediaries between host computers and the network.  But thereafter the analogy breaks down because ARPANET hosts were absolved of responsibility for ensuring the safe arrival of their messages.  All they had to do was to get the packets to the nearest IMP; from then on the sub-network of IMPs assumed responsibility for getting the packets through to their destinations.  The Network Control Protocol on which the network ran was based on this model.  But it was clear to Cerf and Kahn that this would not work for an ‘internet’.  The gateways could not be expected to take responsibility for end-to-end transmission in a variegated system.  That job had to be devolved to the hosts. 
  And that required a new protocol.
In a remarkable burst of creative collaboration, Cerf and Kahn laid the foundations for the ‘network of networks’ during the Summer and Autumn of 1973.  In traditional ARPANET fashion, Cerf used his graduate students at Stanford as sounding-boards and research assistants, and frequently flew to Washington where he and Kahn burned much midnight oil on the design.  In September they took their ideas to a meeting of the INWG at Sussex University in Brighton, UK and refined them in the light of discussions with researchers from Donald Davies’s and Louis Pouzin’s labs.  Then they returned to Washington and hammered out a draft of the scientific paper which was to make them household names in the computing business.  It was a joint production, written, Cerf recalled, with “one of us typing and the other one breathing down his neck, composing as we’d go along, almost like two hands on a pen”.  By December, the paper was finished and they tossed a coin to see who would be the lead author.  Cerf won -- which is why he has ever since been popularly known as “the father of the Internet”.

  “A Protocol for Packet Network Interconnection” by Vinton G. Cerf and Robert E. Kahn was published in a prominent engineering journal in May 1974.  It put forward two central ideas.
   One was the notion of a gateway between networks which would understand the end-to-end protocol used by the hosts that were communicating across the multiple networks.  The other was that packets would be encapsulated by the transmitting host in electronic envelopes (christened ‘datagrams’) and sent to the gateway as end-to-end packets called ‘transmission-control-protocol’ or TCP messages.  In other words, whereas the ARPANET dealt only in packets, an internet would deal with packets enclosed in virtual envelopes.
  The gateway, in the Cerf-Kahn scheme, would read only the envelopes: the contents would be read only by the receiving host.  If a sending host did not receive confirmation of receipt of a message, it would retransmit it -- and keep doing so until the message got through.  The gateways -- unlike the IMPs of the ARPANET -- would not engage in retransmission.  “We focused on end-to-end reliability”, Cerf said.  The motto was “don’t rely on anything inside those nets.  The only thing that we ask the net to do is to take this chunk of bits and get it across the network.  That’s all we ask.  Just take this datagram and do your best to deliver it”.
  The TCP idea was the electronic equivalent of the containerisation revolution which transformed the transport of international freight.  The basic idea in the freight case was agreement on a standard size of container which would fit ships’ holds, articulated trucks and rail wagons.  Almost anything could be shipped inside a container, and special cranes and handling equipment were created for transferring containers from one transport mode to another.  In this analogy, the transport modes (sea, road, rail) correspond to different computer networks; the containers correspond to the TCP envelopes; and the dockside and trackside cranes correspond to the Cerf-Kahn gateways.  And just as the crane doesn’t care what’s inside a container, the computer gateway is unconcerned about the contents of the envelope.  Its responsibility is to transfer it safely onto the next leg of its journey through Cyberspace.
In July 1975, the ARPANET was transferred by DARPA to the Pentagon’s Defense Communications Agency as a going concern.  Since the agency’s prime mission was to foster advanced research, not run a network on a day-to-day basis, it had been trying to divest itself of the Net for some time.  Having done so, it could concentrate on the next major research task in the area -- which it defined as the ‘internetting’ project.

  The first actual specification of the TCP protocol had been published as an Internet Experiment Note in December 1974.  Trial implementations of it were begun at three sites -- Stanford, BBN and University College, London -- so the first efforts at developing the Internet protocols were international from the very beginning. 
  The earliest demonstrations of TCP in action involved the linking of the ARPANET to packet radio and satellite networks.  The first live demo took place in July 1977.  A researcher drove a van on the San Francisco Bay-shore Freeway with a packet radio system running on an LSI-11 computer.  The packets were routed over the ARPANET to a satellite station, flashed over the Atlantic to Norway and thence via land-line to University College, London.  From London they travelled through SATNET across the Atlantic and back into the ARPANET, which then routed them to a computer at the University of Southern California.  “What we were simulating”, recalls Cerf, "was someone in a mobile battlefield environment going across a continental network, then across an intercontinental satellite network, and then back into a wireline network to a major computing resource in national headquarters. Since the Defense Department was paying for this, we were looking for demonstrations that would translate to militarily interesting scenarios. So the packets were traveling 94,000 miles round trip, as opposed to what would have been an 800-mile round trip directly on the ARPANET. We didn't lose a bit!"
The Cerf-Kahn TCP proposal was a great conceptual breakthrough but in itself it was not enough to enable reliable communications between wildly different networks.  In fact it took six years of intensive discussion and experimentation to develop the TCP concept into the suite of inter-related protocols which now governs the Internet.
  These discussions are chronicled in the archive of Internet Experiment Notes – the Internet equivalent of ARPANET’s ‘Request for Comment’ papers.  The record suggests that the evolution of the TCP protocol into its present form was driven by two factors – the intrinsic limitations of the original Cerf-Kahn concept, and the practical experience of researchers at the Xerox Palo Alto Research Center (PARC), the lab which Bob Taylor had set up after he left ARPA and which invented much of the computing technology we use today – from graphical user interfaces like Microsoft Windows or that of the Apple Macintosh, to Ethernet local area networking and laser printing.
  The PARC people were deep into networking for the simple reason that they couldn’t avoid it.  Having decided years earlier that computers should have graphic displays (rather than just displaying characters on a screen) they had faced the problem of how the contents of a screen could be printed.  This they solved by inventing the laser printer, a wonderful machine which could translate a pattern of dots on a screen into an equivalent pattern on paper.  But this in turn raised a new problem – how to transmit the screen pattern to the printer.  At a resolution of 600 dots per inch, for example, it takes something like 33 million bits to describe a single A4 page!  The PARC guys were then placed in the absurd situation of having a computer which could refresh a screen display in one second, a printer which could print the page in two seconds, and a cable between the two which took nearly 15 minutes to transfer the screen data for that page from one to the other.
  The Ethernet local area networking system was PARC’s solution to the transmission problem.  It was invented in 1973 by a group led by Bob Metcalfe – the Harvard graduate student who used to test the ARPANET by shipping computer-game graphics across it – and inspired by the ALOHA packet radio system.  Like the Hawaiian system, Ethernet used packets to transfer data from one machine to another, and it adapted the same approach to the problem of packet collision: each device on the network listened until the system was quiet, and then dispatched a packet.  If the network was busy, the device waited for a random number of milliseconds before trying again.  Using these principles, Metcalfe & Co designed a system that could ship data down a coaxial cable at a rate of 2.67 million bits per second – which meant that the time to transmit an A4 page from computer to printer came down from 15 minutes to about 12 seconds and local area networking was born.
  With this kind of performance, it was not surprising that PARC rapidly became the most networked lab in the U.S.  Having a fast networking technology meant that one could rethink all kinds of basic computing assumptions.  You could, for example, think about distributed processing – where your computer subcontracted some of its calculations, say, to a more powerful machine somewhere else on the network.  And in their quest to explore the proposition that “the network is the computer”, PARC researchers attached all kinds of devices to their Ethernets – from fast printers and ‘intelligent’ peripherals to slow plotters and dumb printers. 
  More significantly, as the lab’s local area networks proliferated, the researchers had to build gateways between them to ensure that networked resources would be available to everyone.  And this in turn meant that they had to address the question of protocols.  Their conclusion, reached sometime in 1977, was that a single, one-type-fits-all protocol would not work for truly heterogeneous internetworking.  Their solution was something called the PARC Universal Packet (forever afterwards known as Pup) which sprang from their need for a rich set of layered protocols, allowing different levels of service for different applications.  Thus, you could have simple but unreliable datagrams (very useful in some situations), but could also have a higher level of functionality which provided complete error control (but perhaps lower performance).
  As it happens, some people on the INWG were moving towards the same conclusion – that a monolithic TCP protocol which attempted to do everything required for internetworking was an unattainable dream.  In July 1977, Cerf invited the PARC team, led by John Shoch, to participate in the Group’s discussions.  They came with the authority of people who had not only been thinking about internetworking but actually doing it for real.  In the end, TCP went  through four separate iterations, culminating in a decision to split it into two new protocols: a new Transaction Control Protocol and an Internet Protocol (IP).
  The new TCP handled the breaking up of messages into packets, inserting them in envelopes (to form what were called ‘datagrams’), reassembling messages in the correct order at the receiving end, detecting errors and re-transmitting anything that got lost.
  IP was the protocol which described how to locate a specific computer out of millions of interconnected computers, and defined standards for transmitting messages from one computer to another. IP handled the naming, addressing, and routing of packets and shifted the responsibility of error free transmission from communication links (gateways) to host computers.
  The evolving suite (which eventually came to encompass upwards of a hundred detailed protocols) came to be known by the generic term ‘TCP/IP’.  Other parts of the terminology evolved too: the term ‘gateway’, for example, eventually came to be reserved for computers which provided bridges between different electronic mail systems, while the machines Cerf-Kahn called gateways came to be called ‘routers’.  The model for the Internet which they conceived therefore became that of an extensible set of networks linked by routers.
  Military interest kept the internetting research going through the late 1970s and into the early 1980s.  By that time there were so many military sites on the network, and they were using it so intensively for day-to-day business, that the Pentagon began to worry about the security aspects of a network in which military and scientific traffic travelled under the same protocols.  Pressure built up to split the ARPANET into two networks – one (MILNET) exclusively for military use, the other for the original civilian crowd.  But because users of both networks would still want to communicate with one another, there would need to be a gateway between the two – which meant that there suddenly was an urgent practical need to implement the new internetworking protocols. 
  In 1982 it was decided that all nodes connected to the ARPANET would switch from the old NCP to TC/IP.  Since there was some reluctance in some sites to the disruption this would cause, a certain amount of pressure had to be applied.  In the middle of 1982, NCP was ‘turned off’ for a day -- which meant that only sites which had converted to the new protocol could communicate.  “This was used”, Cerf said, “to convince people that we were serious”.  Some sites remained unconvinced so in the middle of the Autumn NCP was disabled for two days.  After that, the ARPANET community seems to have been persuaded that the inevitable really was inevitable and on January 1, 1983, NCP was consigned to the dustbin of history.  The future belonged to TCP/IP.
  Cerf recalls the years 1983-’85 as “a consolidation period”.  The great breakthrough came in 1985 when -- partly as a result of DARPA pressure -- TCP/IP was built into the version of the Unix operating system developed at the University of California at Berkeley.  It was eventually incorporated into the version of Unix adopted by workstation manufacturers like Sun – which meant that TCP/IP had finally made it to the heart of the operating system which drove most of the computers on which the Internet would eventually run.  The Net’s digital DNA had finally been slotted into place.
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