Editing Packet switching (section)

==History==
{{Further|History of the Internet|Protocol Wars}}
{{See also|Datagram#History}}<!--consider merging overlapping content at some point-->

=== Invention and development ===
[[File:The idea of the data packet (Baran, 1964)-en.svg|thumb|The "message block", designed by Paul Baran in 1962 and refined in 1964, is the first proposal of a [[data packet]].<ref name=":1" /><ref name=":10" />]]
[[File:Packet-switching cost performance trends, 1960-1980.svg|thumb|Packet-switching cost performance trends, 1960-1980.<ref name="Roberts 141–172">{{Citation |last=Roberts |first=L. |title=The arpanet and computer networks |date=1988 |work=A history of personal workstations |pages=141–172 |url=https://doi.org/10.1145/61975.66916 |access-date=2023-11-30 |place=New York, NY, USA |publisher=Association for Computing Machinery |doi=10.1145/61975.66916 |isbn=978-0-201-11259-7}}</ref>]]

The concept of switching small blocks of data was first explored independently by [[Paul Baran]] at the [[RAND Corporation]] during the early 1960s in the US and [[Donald Davies]] at the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] (NPL) in the UK in 1965.<ref name=":16">{{Cite news |title=The real story of how the Internet became so vulnerable |url=http://www.washingtonpost.com/sf/business/2015/05/30/net-of-insecurity-part-1/ |url-status=dead |archive-url=https://web.archive.org/web/20150530231409/http://www.washingtonpost.com/sf/business/2015/05/30/net-of-insecurity-part-1/ |archive-date=2015-05-30 |access-date=2020-02-18 |newspaper=Washington Post |language=en-US |quote=Historians credit seminal insights to Welsh scientist Donald W. Davies and American engineer Paul Baran}}</ref><ref name="Pelkeyp42">{{cite book |last1=Pelkey |first1=James L. |url=https://www.morganclaypoolpublishers.com/catalog_Orig/samples/9781450397292_sample.pdf |title=Circuits, Packets, and Protocols: Entrepreneurs and Computer Communications, 1968-1988 |last2=Russell |first2=Andrew L. |last3=Robbins |first3=Loring G. |date=2022 |publisher=Morgan & Claypool |isbn=978-1-4503-9729-2 |page=4 |quote="Paul Baran, an engineer celebrated as the co-inventor (along with Donald Davies) of the packet switching technology that is the foundation of digital networks"}}</ref><ref name="NIHF2007">{{cite web|title=Inductee Details - Paul Baran|url=https://www.invent.org/inductees/paul-baran|publisher=National Inventors Hall of Fame|access-date=6 September 2017|postscript=none}}; {{cite web|title=Inductee Details - Donald Watts Davies|url=https://www.invent.org/inductees/donald-watts-davies|publisher=National Inventors Hall of Fame|access-date=6 September 2017}}</ref><ref>{{Cite journal |last=Edmondson-Yurkanan |first=Chris |date=2007 |title=SIGCOMM's archaeological journey into networking's past |url=https://dl.acm.org/doi/10.1145/1230819.1230840 |journal=Communications of the ACM |language=en |volume=50 |issue=5 |pages=63–68 |doi=10.1145/1230819.1230840 |issn=0001-0782 |quote=The 1960 challenge was to build a network such that a significant subset of the network could survive a military attack. [Baran] told us he knew he could design a solution once he realized that, 'given redundant paths, the reliability of the net work could be greater than the reliability of the parts.' ... In his first draft dated Nov. 10, 1965, Davies forecast today’s 'killer app' for his new communication service: 'The greatest traffic could only come if the public used this means for everyday purposes such as shopping... People sending enquiries and placing orders for goods of all kinds will make up a large section of the traffic... Business use of the telephone may be reduced by the growth of the kind of service we contemplate.'|url-access=subscription }}</ref>

In the late 1950s, the [[US Air Force]] established a [[wide area network]] for the [[Semi-Automatic Ground Environment]] (SAGE) radar defense system. Recognizing vulnerabilities in this network, the Air Force sought a system that might survive a [[nuclear attack]] to enable a response, thus diminishing the attractiveness of the first strike advantage by enemies (see [[Mutual assured destruction]]).<ref name=steward>{{cite web |last= Stewart |first= Bill |title= Paul Baran Invents Packet Switching |work= Living Internet |date= 2000-01-07 |url= http://www.livinginternet.com/i/ii_rand.htm |access-date= 2008-05-08}}</ref> In the early 1960s, Baran invented the concept of ''distributed adaptive message block switching'' in support of the Air Force initiative.<ref name=":17" /><ref>{{Cite journal |last=Baran |first=Paul |date=May 27, 1960 |title=Reliable Digital Communications Using Unreliable Network Repeater Nodes |url=http://www.rand.org/content/dam/rand/pubs/papers/2008/P1995.pdf |url-status=live |journal=The RAND Corporation |page=1 |archive-url=https://ghostarchive.org/archive/20221010/http://www.rand.org/content/dam/rand/pubs/papers/2008/P1995.pdf |archive-date=2022-10-10 |access-date=July 7, 2016}}</ref> The concept was first presented to the Air Force in the summer of 1961 as briefing B-265,<ref name=steward/> later published as RAND report P-2626 in 1962,<ref name=":1">{{cite web|url=http://www.rand.org/pubs/papers/P2626/|title=RAND Paper P-2626|last= Baran|first=Paul|year=1962}}</ref> and finally in report RM 3420 in 1964.<ref name=":10">{{cite web|url=http://www.rand.org/pubs/research_memoranda/RM3420/index.html|title=On Distributed Communications|date=January 1964|last1=Baran|first1=Paul}}</ref> The reports describe a general architecture for a large-scale, distributed, survivable communications network. The proposal was composed of three key ideas: use of a [[decentralized]] network with multiple paths between any two points; dividing user messages into ''message blocks;'' and delivery of these messages by [[store and forward]] switching.<ref name=":17">{{Cite journal|last=Baran|first=Paul|date=2002|title=The beginnings of packet switching: some underlying concepts|url=http://web.cs.ucla.edu/~lixia/papers/Baran2002.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://web.cs.ucla.edu/~lixia/papers/Baran2002.pdf |archive-date=2022-10-10 |url-status=live|journal=IEEE Communications Magazine|language=en|volume=40|issue=7|pages=42–48|doi=10.1109/MCOM.2002.1018006|issn=0163-6804|quote=Essentially all the work was defined by 1961, and fleshed out and put into formal written form in 1962. The idea of hot potato routing dates from late 1960.}}</ref><ref>{{Cite web |title=Paul Baran and the Origins of the Internet |url=https://www.rand.org/about/history/baran.html |access-date=2020-02-15 |website=RAND Corporation |language=en}}</ref> Baran's network design was focused on [[digital communication]] of voice messages using switches that were low-cost electronics.<ref name="Pelkey6.1a">{{Cite book |last=Pelkey |first=James L. |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications 1968–1988 |chapter=6.1 The Communications Subnet: BBN 1969 |quote=As Kahn recalls: ... Paul Baran’s contributions ... I also think Paul was motivated almost entirely by voice considerations. If you look at what he wrote, he was talking about switches that were low-cost electronics. The idea of putting powerful computers in these locations hadn’t quite occurred to him as being cost effective. So the idea of computer switches was missing. The whole notion of protocols didn’t exist at that time. And the idea of computer-to-computer communications was really a secondary concern. |chapter-url=https://historyofcomputercommunications.info/section/6.1/the-communications-subnet-bbn-1969/}}</ref><ref name=":5a">{{Cite book |last=Waldrop |first=M. Mitchell |url=https://books.google.com/books?id=eRnBEAAAQBAJ&pg=PT285 |title=The Dream Machine |date=2018 |publisher=Stripe Press |isbn=978-1-953953-36-0 |pages=286 |language=en |quote=Baran had put more emphasis on digital voice communications than on computer communications.}}</ref><ref>{{Cite journal |last=Kleinrock |first=L. |date=1978 |title=Principles and lessons in packet communications |url=https://ieeexplore.ieee.org/document/1455412 |journal=Proceedings of the IEEE |volume=66 |issue=11 |pages=1320–1329 |doi=10.1109/PROC.1978.11143 |issn=0018-9219 |quote=Paul Baran ... focused on the routing procedures and on the survivability of distributed communication systems in a hostile  environment, but did not concentrate on the need for resource sharing in its form as we now understand it; indeed, the concept of a software switch was not present in his work.|url-access=subscription }}</ref>

[[Christopher Strachey]], who became [[University of Oxford|Oxford University's]] first Professor of Computation, filed a [[patent application]] in the United Kingdom for [[time-sharing]] in February 1959.<ref>{{Cite web |title=Computer Pioneers - Christopher Strachey |url=https://history.computer.org/pioneers/strachey.html |access-date=2020-01-23 |website=history.computer.org}}</ref><ref name=":132">{{Cite web |title=Computer - Time-sharing, Minicomputers, Multitasking |url=https://www.britannica.com/technology/computer/Time-sharing-and-minicomputers |access-date=2023-07-23 |website=Britannica |language=en}}</ref> In June that year, he gave a paper "Time Sharing in Large Fast Computers" at the [[International Federation for Information Processing#History|UNESCO Information Processing Conference]] in Paris where he passed the concept on to [[J. C. R. Licklider]].<ref name="ctsspg">{{cite book |last=Corbató |first=F. J. |url=http://www.bitsavers.org/pdf/mit/ctss/CTSS_ProgrammersGuide.pdf |title=The Compatible Time-Sharing System: A Programmer's Guide |publisher=MIT Press |year=1963 |isbn=978-0-262-03008-3 |display-authors=etal}}. "the first paper on time-shared computers by C. Strachey at the June 1959 UNESCO Information Processing conference".</ref><ref>{{harvnb|Gillies|Cailliau|2000|page=13}}</ref> Licklider (along with [[John McCarthy (computer scientist)|John McCarthy]]) was instrumental in the development of time-sharing. After conversations with Licklider about time-sharing with remote computers in 1965,<ref name="Roberts1978" /><ref name=":15">{{cite web |last1=Roberts |first1=Dr. Lawrence G. |date=May 1995 |title=The ARPANET & Computer Networks |url=http://www.packet.cc/files/arpanet-computernet.html |url-status=dead |archive-url=https://web.archive.org/web/20160324032800/http://www.packet.cc/files/arpanet-computernet.html |archive-date=24 March 2016 |access-date=13 April 2016}}</ref> Davies independently invented a similar [[data communication]] concept,<ref>{{Cite book |last=Roberts |first=Gareth Ffowc |url=https://www.google.co.uk/books/edition/For_the_Recorde/waCYEAAAQBAJ?hl=en&gbpv=1&pg=PA102&printsec=frontcover |title=For the Recorde: A History of Welsh Mathematical Greats |date=2022 |publisher=University of Wales Press |isbn=978-1-78683-917-6 |language=en |quote=Mathematicians had already developed methods of analysing traffic jams - 'queueing theory' ... - but it needed new a insight to solve the problem of how to avoid bottle-necks between computers.}}</ref> using short messages in fixed format with high data transmission rates to achieve rapid communications.<ref>{{Cite web |last=Pelkey |first=James L. |date=May 27, 1988 |title=Interview of Donald Davies |url=http://archive.computerhistory.org/resources/access/text/2017/11/102738594-05-01-acc.pdf}}</ref> He went on to develop a more advanced design for a hierarchical, high-speed [[computer network]] including [[Router (computing)|interface computers]] and [[communication protocol]]s.<ref name=":21">{{Cite web |last=Davies |first=D. W. |date=1966 |title=Proposal for a Digital Communication Network |url=https://www.dcs.gla.ac.uk/~wpc/grcs/Davies05.pdf |quote=all users of the network will provide themselves with some kind of error control ... Computer developments in the distant future might result in one type of network being able to carry speech and digital messages efficiently.}}</ref><ref>{{Citation |last1=Scantlebury |first1=R. A. |title=A Protocol for Use in the NPL Data Communications Network |date=April 1967 |publisher=Private papers |last2=Bartlett |first2=K. A.}}</ref><ref name=":5">{{cite conference |last1=Davies |first1=Donald |last2=Bartlett |first2=Keith |last3=Scantlebury |first3=Roger |last4=Wilkinson |first4=Peter |date=October 1967 |title=A Digital Communication Network for Computers Giving Rapid Response at remote Terminals |url=https://people.mpi-sws.org/~gummadi/teaching/sp07/sys_seminar/how_did_erope_blow_this_vision.pdf |conference=ACM Symposium on Operating Systems Principles |archive-url=https://ghostarchive.org/archive/20221010/https://people.mpi-sws.org/~gummadi/teaching/sp07/sys_seminar/how_did_erope_blow_this_vision.pdf |archive-date=2022-10-10 |access-date=2020-09-15 |url-status=live}}</ref> He coined the term ''packet switching'', and proposed building a commercial nationwide data network in the UK.<ref>{{Cite book |last=Yates |first=David M. |url=https://books.google.com/books?id=ToMfAQAAIAAJ&q=%22proposed+national+packet-switched+network%22 |title=Turing's Legacy: A History of Computing at the National Physical Laboratory 1945-1995 |date=1997 |publisher=National Museum of Science and Industry |isbn=978-0-901805-94-2 |page=130 |language=en}}</ref><ref>{{Citation |last= Davies |first= D. W. |author-link= Donald Davies |title= Oral History 189: D. W. Davies interviewed by Martin Campbell-Kelly at the National Physical Laboratory |publisher= Charles Babbage Institute University of Minnesota, Minneapolis |date= 17 March 1986 |url= http://conservancy.umn.edu/handle/107241 |access-date= 21 July 2014 |archive-url= https://web.archive.org/web/20140729025914/http://conservancy.umn.edu/handle/107241 |archive-date= 29 July 2014 |url-status= dead }}</ref> He gave a talk on the proposal in 1966, after which a person from the [[Ministry of Defence (United Kingdom)|Ministry of Defence]] (MoD) told him about Baran's work.<ref>{{Cite web |title=UK National Physical Laboratories, Donald Davies |url=https://www.livinginternet.com/i/ii_npl.htm |access-date=2024-06-05 |website=LivingInternet |language=en-US}}</ref> 

[[Roger Scantlebury]], a member of Davies' team, presented their work (and referenced that of Baran) at the October 1967 [[Symposium on Operating Systems Principles]] (SOSP).<ref name=":5" /><ref>{{Cite book |last1=Hafner |first1=Katie |url=http://archive.org/details/wherewizardsstay00haf_vgj |title=Where wizards stay up late: the origins of the Internet |last2=Lyon |first2=Matthew |date=1996 |publisher=Simon & Schuster |others=Internet Archive |isbn=978-0-684-81201-4 |pages=76–78 |quote=Roger Scantlebury ... from Donald Davies' team ... presented a detailed design study for a packet switched network. It was the first Roberts had heard of it. ... Roberts also learned from Scantlebury, for the first time, of the work that had been done by Paul Baran at RAND a few years earlier.}}</ref><ref name="Moschovitisp58-9">{{harvnb|Moschovitis|1999|p=[https://archive.org/details/historyofinterne0000unse/page/58/mode/2up 58-9]}} More significantly, Roger Scantlebury ... presents the design for a packet-switched network. This is the first Roberts and Taylor have heard of packet switching, a concept that appears to be a promising receipe for transmitting data through the ARPAnet.</ref><ref name="C. Hempstead, W. Worthington">{{cite book |url=https://archive.org/details/EncyclopediaOf20thCenturyTechnologyAZMalestrom/page/n623/mode/2up |title=Encyclopedia of 20th-Century Technology |publisher=Routledge |year=2005 |isbn=9781135455514 |editor1-last=Hempstead |editor1-first=C. |volume=1, A-L |pages=574 |quote=It was a seminal meeting as the NPL proposal illustrated how the communications for such a resource-sharing computer network could be realized. |editor2-last=Worthington |editor2-first=W.}}</ref><ref>{{Cite web |title=On packet switching |url=https://www.nethistory.info/Archives/packets.html |access-date=2024-01-08 |website=Net History |quote=[Scantlebury said] We referenced Baran's paper in our 1967 Gatlinburg ACM paper. You  will find it in the References. Therefore I am sure that we introduced Baran's work to Larry (and hence the BBN guys).}}</ref> At the conference, Scantlebury proposed packet switching for use in the [[ARPANET]] and persuaded [[Lawrence Roberts (scientist)|Larry Roberts]] the economics were favorable to [[message switching]].<ref name=":18">{{cite book |last1=Naughton |first1=John |url=https://books.google.com/books?id=bbonCgAAQBAJ&q=they+lacked+one+vital+ingredient |title=A Brief History of the Future: The origins of the Internet |date=2015 |publisher=Hachette |isbn=978-1474602778 |quote=they lacked one vital ingredient. Since none of them had heard of Paul Baran they had no serious idea of how to make the system work. And it took an English outfit to tell them. ... Larry Roberts paper was the first public presentation of the ARPANET concept as conceived with the aid of Wesley Clark ... Looking at it now, Roberts paper seems extraordinarily, well, vague.}}</ref><ref name=":5b">{{Cite book |last=Waldrop |first=M. Mitchell |url=https://books.google.com/books?id=eRnBEAAAQBAJ&pg=PT285 |title=The Dream Machine |date=2018 |publisher=Stripe Press |isbn=978-1-953953-36-0 |pages=285–6 |language=en |quote=Scantlebury and his companions from the NPL group were happy to sit up with Roberts all that night, sharing technical details and arguing over the finer points.}}</ref><ref name="Abbate20002" /><ref>{{cite web |title=Oral-History:Donald Davies & Derek Barber |url=http://ethw.org/Oral-History:Donald_Davies_%26_Derek_Barber |access-date=13 April 2016 |quote=the ARPA network is being implemented using existing telegraphic techniques simply because the type of network we describe does not exist. It appears that the ideas in the NPL paper at this moment are more advanced than any proposed in the USA}}</ref><ref>{{cite journal |last1=Barber |first1=Derek |date=Spring 1993 |title=The Origins of Packet Switching |url=http://www.cs.man.ac.uk/CCS/res/res05.htm#f |journal=The Bulletin of the Computer Conservation Society |issue=5 |issn=0958-7403 |access-date=6 September 2017 |quote=Roger actually convinced Larry that what he was talking about was all wrong and that the way that NPL were proposing to do it was right. I've got some notes that say that first Larry was sceptical but several of the others there sided with Roger and eventually Larry was overwhelmed by the numbers.}}</ref><ref>{{Cite journal |last=Needham |first=Roger M. |date=2002-12-01 |title=Donald Watts Davies, C.B.E. 7 June 1924 – 28 May 2000 |url=https://royalsocietypublishing.org/doi/10.1098/rsbm.2002.0006 |journal=Biographical Memoirs of Fellows of the Royal Society |volume=48 |pages=87–96 |doi=10.1098/rsbm.2002.0006 |s2cid=72835589 |quote=Larry Roberts presented a paper on early ideas for what was to become ARPAnet. This was based on a store-and-forward method for entire messages, but as a result of that meeting the NPL work helped to convince Roberts that packet switching was the way forward.|url-access=subscription }}</ref> Davies had chosen some of the same parameters for his original network design as did Baran, such as a packet size of 1024 bits. To deal with packet permutations (due to dynamically updated route preferences) and [[datagram]] losses (unavoidable when fast sources send to a slow destinations), he assumed that "all users of the network will provide themselves with some kind of error control",<ref name=":5" /> thus inventing what came to be known as the [[end-to-end principle]]. Davies proposed that a local-area network should be built at the laboratory to serve the needs of NPL and prove the feasibility of packet switching. After a [[pilot experiment]] in early 1969,<ref name=":72">{{Cite conference |last1=Rayner |first1=David |last2=Barber |first2=Derek |last3=Scantlebury |first3=Roger |last4=Wilkinson |first4=Peter |date=2001 |title=NPL, Packet Switching and the Internet |url=http://www.topquark.co.uk/conf/IAP2001.html |conference=Symposium of the Institution of Analysts & Programmers 2001 |access-date=2024-06-13 |quote=The system first went 'live' early in 1969 |website=|archive-url=https://web.archive.org/web/20030807200346/http://www.topquark.co.uk/conf/IAP2001.html |archive-date=2003-08-07 }}</ref><ref name=":22">{{Cite journal |last1=John S |first1=Quarterman |last2=Josiah C |first2=Hoskins |date=1986 |title=Notable computer networks |journal=Communications of the ACM |language=EN |volume=29 |issue=10 |pages=932–971 |doi=10.1145/6617.6618 |s2cid=25341056 |quote=The first packet-switching network was implemented at the National Physical Laboratories in the United Kingdom. It was quickly followed by the ARPANET in 1969. |doi-access=free}}</ref><ref name="Haughney Dare-Bryan" /><ref name="Hempstead2005">{{cite book |author1=C. Hempstead |url=https://archive.org/details/EncyclopediaOf20thCenturyTechnologyAZMalestrom/page/n621/mode/2up?q=packet+switching |title=Encyclopedia of 20th-Century Technology |author2=W. Worthington |date=2005 |publisher=[[Routledge]] |isbn=9781135455514 |pages=573–5}}</ref> the [[NPL Data Communications Network]] began service in 1970.<ref name=":3b">{{Cite journal |last=Campbell-Kelly |first=Martin |date=1987 |title=Data Communications at the National Physical Laboratory (1965-1975) |url=https://archive.org/details/DataCommunicationsAtTheNationalPhysicalLaboratory |journal=Annals of the History of Computing |language=en |volume=9 |issue=3/4 |pages=221–247 |doi=10.1109/MAHC.1987.10023 |s2cid=8172150}}</ref> Davies was invited to Japan to give a series of lectures on packet switching.<ref name="frs">{{Cite journal |last1=Needham |first1=R. M. |author-link=Roger Needham |year=2002 |title=Donald Watts Davies, C.B.E. 7 June 1924 – 28 May 2000 |journal=[[Biographical Memoirs of Fellows of the Royal Society]] |volume=48 |pages=87–96 |doi=10.1098/rsbm.2002.0006 |s2cid=72835589 |quote=The 1967 Gatlinburg paper was influential on the development of ARPAnet, which might otherwise have been built with less extensible technology. ... Davies was invited to Japan to lecture on packet switching.}}</ref> The NPL team carried out [[simulation]] work on datagrams and [[Network congestion|congestion]] in networks on a scale to provide data communication across the United Kingdom.<ref name="Hempstead2005" /><ref name=":82">{{Cite thesis |last=Clarke |first=Peter |title=Packet and circuit-switched data networks |date=1982 |degree=PhD |publisher=Department of Electrical Engineering, Imperial College of Science and Technology, University of London |url=https://spiral.imperial.ac.uk/bitstream/10044/1/35864/2/Clarke-PN-1982-PhD-Thesis.pdf}} "As well as the packet switched network actually built at NPL for communication between their local computing facilities, some simulation experiments have been performed on larger networks. A summary of this work is reported in [69]. The work was carried out to investigate networks of a size capable of providing data communications facilities to most of the U.K. ... Experiments were then carried out using a method of flow control devised by Davies [70] called 'isarithmic' flow control. ... The simulation work carried out at NPL has, in many respects, been more realistic than most of the ARPA network theoretical studies."</ref><ref name="Pelkey">{{cite book |last=Pelkey |first=James |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications 1968-1988 |chapter=6.3 CYCLADES Network and Louis Pouzin 1971-1972 |access-date=2020-02-03 |chapter-url=http://www.historyofcomputercommunications.info/Book/6/6.3-CYCLADESNetworkLouisPouzin1-72.html |archive-url=https://web.archive.org/web/20210617093154/https://www.historyofcomputercommunications.info/Book/6/6.3-CYCLADESNetworkLouisPouzin1-72.html |archive-date=2021-06-17 |url-status=dead}}</ref><ref name="MCK">{{cite journal |last=Campbell-Kelly |first=Martin |date=Autumn 2008 |title=Pioneer Profiles: Donald Davies |url=http://www.computerconservationsociety.org/resurrection/res44.htm |journal=Computer Resurrection |issn=0958-7403 |number=44}}</ref><ref>{{Cite conference |last=Wilkinson |first=Peter|date=2001 |title=NPL Development of Packet Switching |url=http://www.topquark.co.uk/conf/IAP2001.html |archive-url=https://web.archive.org/web/20030807200346/http://www.topquark.co.uk/conf/IAP2001.html |conference=Symposium of the Institution of Analysts & Programmers 2001 |access-date=2024-06-13 |archive-date=2003-08-07 |quote=The feasibility studies continued with an attempt to apply queuing theory to study overall network performance. This proved to be intractable so we quickly turned to simulation.}}</ref>

[[Lawrence Roberts (scientist)|Larry Roberts]] made the key decisions in the [[request for proposal]] to build the [[ARPANET]].<ref name=":4">{{Cite news |last=Hafner |first=Katie |date=2018-12-30 |title=Lawrence Roberts, Who Helped Design Internet's Precursor, Dies at 81 |url=https://www.nytimes.com/2018/12/30/obituaries/lawrence-g-roberts-dies-at-81.html |access-date=2020-02-20 |work=The New York Times |issn=0362-4331 |quote=He decided to use packet switching as the underlying technology of the Arpanet; it remains central to the function of the internet. And it was Dr. Roberts's decision to build a network that distributed control of the network across multiple computers. Distributed networking remains another foundation of today's internet.}}</ref> Roberts met Baran in February 1967, but did not discuss networks.<ref name=":5c">{{Cite book |last=Waldrop |first=M. Mitchell |url=https://books.google.com/books?id=eRnBEAAAQBAJ&pg=PT285 |title=The Dream Machine |date=2018 |publisher=Stripe Press |isbn=978-1-953953-36-0 |pages=285–6 |language=en |quote=Oops. Roberts knew Baran slightly and had in fact had lunch with him during a visit to RAND the previous February. But he certainly didn't remember any discussion of networks. How could he have missed something like that?}}</ref><ref>{{Cite web |last=O'Neill |first=Judy |date=5 March 1990 |title=An Interview with PAUL BARAN |url=https://conservancy.umn.edu/bitstream/handle/11299/107101/oh182pb.pdf?sequence=1&isAllowed=y |page=37 |quote=On Tuesday, 28 February 1967 I find a notation on my calendar for 12:00 noon Dr. L. Roberts.}}</ref> He asked [[Frank Westervelt]] to explore the questions of message size and contents for the network, and to write a position paper on the intercomputer communication protocol including “conventions for character and block transmission, error checking and re transmission, and computer and user identification."<ref name=":242">{{cite web |last=Pelkey |first=James |title=4.7 Planning the ARPANET: 1967-1968 in Chapter 4 - Networking: Vision and Packet Switching 1959 - 1968 |url=https://historyofcomputercommunications.info/section/4.7/planning-the-arpanet-1967-1968/ |url-status=dead |archive-url=https://web.archive.org/web/20221223230647/https://historyofcomputercommunications.info/section/4.7/planning-the-arpanet-1967-1968/ |archive-date=December 23, 2022 |access-date=May 9, 2023 |work=The History of Computer Communications}}</ref> Roberts revised his initial design, which was to connect the [[host computer]]s directly, to incorporate [[Wesley A. Clark|Wesley Clark's]] idea to use [[Interface Message Processor]]s (IMPs) to create a [[message switching]] network, which he presented at SOSP.<ref>{{Cite web |last=Press |first=Gil |date=January 2, 2015 |title=A Very Short History Of The Internet And The Web |url=https://www.forbes.com/sites/gilpress/2015/01/02/a-very-short-history-of-the-internet-and-the-web-2/ |url-status=live |archive-url=https://web.archive.org/web/20150109145400/https://www.forbes.com/sites/gilpress/2015/01/02/a-very-short-history-of-the-internet-and-the-web-2/ |archive-date=January 9, 2015 |access-date=2020-02-07 |website=Forbes |language=en |quote=Roberts' proposal that all host computers would connect to one another directly ... was not endorsed ... Wesley Clark ... suggested to Roberts that the network be managed by identical small computers, each attached to a host computer. Accepting the idea, Roberts named the small computers dedicated to network administration 'Interface Message Processors' (IMPs), which later evolved into today's routers.}}</ref><ref>{{Citation |title=SRI Project 5890-1; Networking (Reports on Meetings) |url=https://web.stanford.edu/dept/SUL/library/extra4/sloan/mousesite/EngelbartPapers/B1_F20_CompuMtg.html |year=1967 |access-date=2020-02-15 |archive-url=https://web.archive.org/web/20200202062940/https://web.stanford.edu/dept/SUL/library/extra4/sloan/mousesite/EngelbartPapers/B1_F20_CompuMtg.html |publisher=Stanford University |quote=W. Clark's message switching proposal (appended to Taylor's letter of April 24, 1967 to Engelbart)were reviewed. |archive-date=February 2, 2020}}</ref><ref name=":19">{{Cite book |last=Roberts |first=Lawrence |title=Multiple Computer Networks and Intercomputer Communications |date=1967 |pages=3.1–3.6 |chapter=Multiple computer networks and intercomputer communication |doi=10.1145/800001.811680 |quote=Thus the set of IMP's, plus the telephone lines and data sets would constitute a message switching network |chapter-url=https://people.mpi-sws.org/~gummadi/teaching/sp07/sys_seminar/arpanet.pdf |s2cid=17409102}}</ref><ref name=":20">{{Cite book |last1=Tanenbaum |first1=Andrew S. |url=https://csc-knu.github.io/sys-prog/books/Andrew%20S.%20Tanenbaum%20-%20Computer%20Networks.pdf |title=Computer networks |last2=Wetherall |first2=David |date=2011 |publisher=Prentice Hall |isbn=978-0-13-212695-3 |edition=5th |location=Boston Amsterdam |page=57 |quote=Roberts bought the idea and presented a some what vague paper about it at the ACM SIGOPS Symposium on Operating System Principles held in Gatlinburg, Tennessee in late 1967}}</ref> Roberts was known for making decisions quickly.<ref name=":5d">{{Cite book |last=Waldrop |first=M. Mitchell |url=https://books.google.com/books?id=eRnBEAAAQBAJ&pg=PT279 |title=The Dream Machine |date=2018 |publisher=Stripe Press |isbn=978-1-953953-36-0 |pages=279, 284–5 |language=en |quote=Roberts was already becoming known as the fastest man in the Pentagon. ... And not for nothing was Larry Roberts known as the fastest man in the Pentagon. By the time they got to the airport, the decision had been made .... Once again, the fastest man in the Pentagon made his decision without hesitation}}</ref> Immediately after SOSP, he incorporated Davies' and Baran's concepts and designs for packet switching to enable the data communications on the network.<ref name="Abbate20002">{{cite book |last1=Abbate |first1=Jane |author-link=Janet Abbate |url=https://books.google.com/books?id=E2BdY6WQo4AC&q=packet+switching&pg=PA125 |title=Inventing the Internet |date=2000 |publisher=MIT Press |isbn=978-0262261333 |pages=37–8, 58–9 |quote=The NPL group influenced a number of American computer scientists in favor of the new technique, and they adopted Davies's term "packet switching" to refer to this type of network. Roberts also adopted some specific aspects of the NPL design.}}</ref><ref name=":6">{{cite web |title=Shapiro: Computer Network Meeting of October 9–10, 1967 |url=https://web.stanford.edu/dept/SUL/library/extra4/sloan/mousesite/Archive/Post68/ARPANETMeeting1167.html |archive-url=https://web.archive.org/web/20150627133802/https://web.stanford.edu/dept/SUL/library/extra4/sloan/mousesite/Archive/Post68/ARPANETMeeting1167.html |archive-date=27 June 2015 |website=stanford.edu}}</ref><ref>{{Cite web |title=Computer Pioneers - Donald W. Davies |url=https://history.computer.org/pioneers/davies.html |access-date=2020-02-20 |website=IEEE Computer Society |quote=In 1965, Davies pioneered new concepts for computer communications in a form to which he gave the name "packet switching." ... The design of the ARPA network (ArpaNet) was entirely changed to adopt this technique. |postscript=none}}</ref><ref>[http://www.internethalloffame.org/inductees/donald-davies "Pioneer: Donald Davies"], Internet Hall of Fame "America’s Advanced Research Project Agency (ARPA), and the ARPANET received his network design enthusiastically and the NPL local network became the first two computer networks in the world using the technique."</ref>

A contemporary of Roberts' from [[MIT]], [[Leonard Kleinrock]] had researched the application of [[queueing theory]] in the field of [[message switching]] for his doctoral dissertation in 1961–62 and published it as a book in 1964.<ref>{{cite book |last1=Isaacson |first1=Walter |url=https://books.google.com/books?id=4V9koAEACAAJ&pg=PA245 |title=The Innovators: How a Group of Hackers, Geniuses, and Geeks Created the Digital Revolution |date=2014 |publisher=Simon and Schuster |isbn=9781476708690 |page=246}}</ref> Davies, in his 1966 paper on packet switching,<ref name=":21" /> applied Kleinorck's techniques to show that "there is an ample margin between the estimated performance of the [packet-switched] system and the stated requirement" in terms of a satisfactory [[Response time (telecommunications)|response time]] for a human user.<ref>{{Cite web |last=Davies |first=D. W. |date=1966 |title=Proposal for a Digital Communication Network |url=https://www.dcs.gla.ac.uk/~wpc/grcs/Davies05.pdf |page=10, 16}}</ref> This addressed a key question about the viability of computer networking.<ref name=":92">{{cite tech report|last1=Heart|first1=F.|last2=McKenzie|first2=A.|last3=McQuillian|first3=J.|last4=Walden|first4=D.|url=https://web.archive.org/web/20230527095942/https://walden-family.com/bbn/arpanet-completion-report.pdf|title=Arpanet Completion Report|publisher=Bolt, Beranek and Newman|location=Burlington, MA|date=January 4, 1978}} pp. III-40-1</ref> Larry Roberts brought Kleinrock into the ARPANET project informally in early 1967.<ref>{{Cite web |title=SRI Project 5890-1; Networking (Reports on Meetings). [1967] |url=http://sloan.stanford.edu/mousesite/EngelbartPapers/B1_F20_CompuMtg.html |access-date=2020-02-15 |website=web.stanford.edu|archive-url=https://web.archive.org/web/20110810063347/http://sloan.stanford.edu/mousesite/EngelbartPapers/B1_F20_CompuMtg.html |archive-date=2011-08-10 }}</ref> Roberts and Taylor recognized the issue of response time was important, but did not apply Kleinrock's methods to assess this and based their design on a [[store-and-forward]] system that was not intended for [[real-time computing]].<ref>{{harvnb|Hafner|Lyon|1996}}</ref> After SOSP, and after Roberts' direction to use packet switching,<ref name=":6" /> Kleinrock sought input from Baran and proposed to retain Baran and RAND as advisors.<ref name="Abbate20003">{{cite book |last1=Abbate |first1=Janet |url=https://books.google.com/books?id=9BfZxFZpElwC&pg=PA37 |title=Inventing the Internet |publisher=[[MIT Press]] |year=2000 |isbn=978-0-2625-1115-5 |location=Cambridge, MA |pages=39, 57–58 |quote=Baran proposed a "distributed adaptive message-block network" [in the early 1960s] ... Roberts recruited Baran to advise the ARPANET planning group on distributed communications and packet switching. ... Roberts awarded a contract to Leonard Kleinrock of UCLA to create theoretical models of the network and to analyze its actual performance.}}</ref><ref>{{citation |title=Summary of ARPA ad hoc meeting |date=November 3, 1967 |url=https://archive.org/details/SummaryOfArpaAdHocMeeting/page/n1/mode/2up |quote=We propose that a working group of approximately four people devote some concentrated effort in the near future in defining the IMP precisely. This group would interact with the larger group from the earlier meetings from time to time. Tentatively we think that the core of this investigatory group would be Bhushan (MIT), Kleinrock (UCLA), Shapiro (SRI) and Westervelt (University of Michigan), along with a kibitzer's group, consisting of such people as Baran (Rand), Boehm (Rand), Culler (UCSB) and Roberts (ARPA).}}</ref><ref>{{citation |author=Judy O'Neill |title=Oral history interview with Paul Baran |date=1990 |url=https://conservancy.umn.edu/handle/11299/107101 |publisher=Charles Babbage Institute |hdl=11299/107101 |quote=BARAN: On Tuesday, 31 October 1967 I see a notation 9:30 AM to 2:00 PM for ARPA's (Elmer) Shapiro, (Barry) Boehm, (Len) Kleinrock, ARPA Network. On Monday, 13 November 1967 I see the following: Larry Roberts to abt (about?) lunch (time?). Art Bushkin = 1:00 PM. Here. Larry Roberts IMP Committee. On Thursday, 16 November 1967 I see 7 PM Kleinrock, UCLA - IMP Meeting.}}</ref> The ARPANET working group assigned Kleinrock responsibility to prepare a report on software for the IMP.<ref>{{citation |title=Meeting of the ARPA Computer Network Working Group at UCLA |date=November 16, 1967 |url=https://archive.org/details/MeetingOfTheArpaComputerNetworkWorkingGroupAtUcla}}</ref> In 1968, Roberts awarded Kleinrock a contract to establish a Network Measurement Center (NMC) at [[UCLA]] to measure and model the performance of packet switching in the ARPANET.<ref name="Abbate20003" /> 

[[Bolt Beranek & Newman]] (BBN) won the contract to build the network. Designed principally by [[Bob Kahn]],<ref name="Hafner1">{{harvnb|Hafner|Lyon|1996|pp=[https://archive.org/details/wherewizardsstay00haf_vgj/page/116/mode/2up?q=kahn 116, 149]}}</ref><ref name="Pelkey6.1b">{{Cite book |last=Pelkey |first=James L. |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications 1968–1988 |chapter=6.1 The Communications Subnet: BBN 1969 |quote=Kahn, the principal architect |chapter-url=https://historyofcomputercommunications.info/section/6.1/the-communications-subnet-bbn-1969/}}</ref> it was the first wide-area packet-switched network with distributed control.<ref name=":4" /> The BBN "IMP Guys" independently developed significant aspects of the network's internal operation, including the routing algorithm, flow control, software design, and network control.<ref name=":2A3">{{cite journal |last1=Roberts |first1=Lawrence G. |date=November 1978 |title=The Evolution of Packet Switching |url=http://www.ismlab.usf.edu/dcom/Ch10_Roberts_EvolutionPacketSwitching_IEEE_1978.pdf |url-status=dead |journal=IEEE Invited Paper |archive-url=https://web.archive.org/web/20181231092936/http://www.ismlab.usf.edu/dcom/Ch10_Roberts_EvolutionPacketSwitching_IEEE_1978.pdf |archive-date=31 December 2018 |access-date=September 10, 2017 |quote=Significant aspects of the network's internal operation, such as routing, flow control, software design, and network control were developed by a BBN team consisting of Frank Heart, Robert Kahn, Severo Omstein, William Crowther, and David Walden}}</ref><ref name="F.E. Froehlich, A. Kent">{{cite book |author=F.E. Froehlich, A. Kent |url=https://books.google.com/books?id=gaRBTHdUKmgC&pg=PA344 |title=The Froehlich/Kent Encyclopedia of Telecommunications: Volume 1 - Access Charges in the U.S.A. to Basics of Digital Communications |date=1990 |publisher=CRC Press |isbn=0824729005 |page=344 |quote=Although there was considerable technical interchange between the NPL group and those who designed and implemented the ARPANET, the NPL Data Network effort appears to have had little fundamental impact on the design of ARPANET. Such major aspects of the NPL Data Network design as the standard network interface, the routing algorithm, and the software structure of the switching node were largely ignored by the ARPANET designers. There is no doubt, however, that in many less fundamental ways the NPL Data Network had and effect on the design and evolution of the ARPANET.}}</ref> The UCLA NMC and the BBN team also investigated network congestion.<ref name="Hafner1" /><ref>{{IETF RFC|334}}</ref> The Network Working Group, led by [[Steve Crocker]], a graduate student of Kleinrock's at UCLA, developed the host-to-host protocol, the [[Network Control Program (ARPANET)|Network Control Program]], which was approved by Barry Wessler for ARPA,<ref>{{IETF RFC|53}}</ref> after he ordered certain more exotic elements to be dropped.<ref>{{cite tech report|last1=Heart|first1=F.|last2=McKenzie|first2=A.|last3=McQuillian|first3=J.|last4=Walden|first4=D.|url=https://web.archive.org/web/20230527095942/https://walden-family.com/bbn/arpanet-completion-report.pdf|title=Arpanet Completion Report|publisher=Bolt, Beranek and Newman|location=Burlington, MA|date=January 4, 1978|page=III-63}}</ref> In 1970, Kleinrock extended his earlier [[Analytics|analytic]] work on message switching to packet switching in the ARPANET.<ref name=":12">{{Cite thesis |last=Clarke |first=Peter |title=Packet and circuit-switched data networks |date=1982 |degree=PhD |publisher=Department of Electrical Engineering, Imperial College of Science and Technology, University of London |url=https://spiral.imperial.ac.uk/bitstream/10044/1/35864/2/Clarke-PN-1982-PhD-Thesis.pdf}}  "Many of the theoretical studies of the performance and design of the ARPA Network were developments of earlier work by Kleinrock ... Although these works concerned message switching networks, they were the basis for a lot of the ARPA network investigations ... The intention of the work of Kleinrock [in 1961] was to analyse the performance of store and forward networks ... Kleinrock [in 1970] extended the theoretical approaches of [his 1961 work] to the early ARPA network."</ref> His work influenced the development of the ARPANET and packet-switched networks generally.<ref>{{Cite book |last=Abbate |first=Janet |url=https://archive.org/details/inventinginterne00jane/page/230/mode/2up?q=%22On+Kleinrock%27s+influence%22 |title=Inventing the Internet |date=1999 |publisher=MIT Press |others=Internet Archive |isbn=978-0-262-01172-3 |pages=230 |quote=On Kleinrock’s influence, see Frank, Kahn, and Kleinrock 1972, p. 265; Tanenbaum 1989, p. 631.}}</ref><ref name=":42">{{Cite book |last=Davies |first=Donald Watts |url=https://archive.org/details/computernetworks00davi/page/86/mode/2up?q=kleinrock+kleinrock%27s |title=Computer networks and their protocols |date=1979 |publisher=Wiley |others=Internet Archive |isbn=978-0-471-99750-4 |pages=See page refs highlighted at url}}</ref><ref>{{Cite journal |last=Kleinrock |first=L. |date=1978 |title=Principles and lessons in packet communications |url=https://ieeexplore.ieee.org/document/1455412 |journal=Proceedings of the IEEE |volume=66 |issue=11 |pages=1320–1329 |doi=10.1109/PROC.1978.11143 |issn=0018-9219|url-access=subscription }}</ref> 

The ARPANET was demonstrated at the [[International Conference on Computer Communications|International Conference on Computer Communication]] (ICCC) in Washington in October 1972.<ref name="Pelkey8.32">{{cite book |last=Pelkey |first=James |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications 1968–1988 |chapter=8.3 CYCLADES Network and Louis Pouzin 1971–1972 |chapter-url=https://historyofcomputercommunications.info/section/8.3/CYCLADES-Network-and-Louis-Pouzin-1971-1972/}}</ref><ref name="Hafnerp2222">{{harvnb|Hafner|Lyon|1996|p=[https://archive.org/details/wherewizardsstay00haf_vgj/page/222/mode/2up 222]}}</ref> However, fundamental questions about the design of packet-switched networks remained.<ref name="Pelkey8.4b">{{cite book |last=Pelkey |first=James |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications 1968–1988 |chapter=8.4 Transmission Control Protocol (TCP) 1973-1976 |quote=Arpanet had its deficiencies, however, for it was neither a true datagram network nor did it provide end-to-end error correction. |chapter-url=https://historyofcomputercommunications.info/section/8.4/transmission-control-protocol-(tcp)-1973-1976/}}</ref><ref>{{Cite book |last=Pouzin |first=Louis |chapter=An integrated approach to network protocols |date=May 1975 |title=Proceedings of the May 19-22, 1975, national computer conference and exposition on - AFIPS '75 |chapter-url=https://dl.acm.org/doi/10.1145/1499949.1500100 |publisher=Association for Computing Machinery |pages=701–707 |doi=10.1145/1499949.1500100 |isbn=978-1-4503-7919-9|s2cid=1689917 }}</ref><ref name=":2A" />

Roberts presented the idea of packet switching to communication industry professionals in the early 1970s. Before ARPANET was operating, they argued that the router buffers would quickly run out. After the ARPANET was operating, they argued packet switching would never be economic without the government subsidy. Baran had faced the same rejection and thus failed to convince the military into constructing a packet switching network in the 1960s.<ref name="Roberts 141–172"/>

The [[CYCLADES]] network was designed by [[Louis Pouzin]] in the early 1970s to study [[internetworking]].<ref name=":28">{{Cite book |last=Abbate |first=Janet |url=https://books.google.com/books?id=E2BdY6WQo4AC&pg=PA125 |title=Inventing the Internet |date=2000 |publisher=MIT Press |isbn=978-0-262-51115-5 |pages=124–127 |language=en |quote=In fact, CYCLADES, unlike ARPANET, had been explicitly designed to facilitate internetworking; it could, for instance, handle varying formats and varying levels of service |author-link=Janet Abbate}}</ref><ref name="Kim2005">{{cite book |last1=Kim |first1=Byung-Keun |url=https://books.google.com/books?id=lESrw3neDokC&pg=PA54 |title=Internationalising the Internet the Co-evolution of Influence and Technology |date=2005 |publisher=Edward Elgar |isbn=1845426754 |pages=51–55 |quote=In addition to the NPL Network and the ARPANET, CYCLADES, an academic and research experimental network, also played an important role in the development of computer networking technologies}}</ref><ref name=":7">{{Cite news |date=2013-11-30 |title=The internet's fifth man |newspaper=The Economist |url=https://www.economist.com/news/technology-quarterly/21590765-louis-pouzin-helped-create-internet-now-he-campaigning-ensure-its |access-date=2020-04-22 |issn=0013-0613 |quote=In the early 1970s Mr Pouzin created an innovative data network that linked locations in France, Italy and Britain. Its simplicity and efficiency pointed the way to a network that could connect not just dozens of machines, but millions of them. It captured the imagination of Dr Cerf and Dr Kahn, who included aspects of its design in the protocols that now power the internet.}}</ref> It was the first to implement the end-to-end principle of Davies, and make the host computers responsible for the reliable delivery of data on a packet-switched network, rather than this being a service of the network itself.<ref name="Bennett20092">{{cite web |last1=Bennett |first1=Richard |date=September 2009 |title=Designed for Change: End-to-End Arguments, Internet Innovation, and the Net Neutrality Debate |url=https://www.itif.org/files/2009-designed-for-change.pdf |access-date=11 September 2017 |publisher=Information Technology and Innovation Foundation |pages=7, 9, 11 |quote=Two significant packet networks preceded the TCP/IP Internet: ARPANET and CYCLADES. The designers of the Internet borrowed heavily from these systems, especially CYCLADES ... The first end-to-end research network was CYCLADES, designed by Louis Pouzin at IRIA in France with the support of BBN’s Dave Walden and Alex McKenzie and deployed beginning in 1972.}}</ref> His team was thus first to tackle the highly-complex problem of providing user applications with a reliable [[virtual circuit]] service while using a [[best-effort service]], an early contribution to what will be the [[Transmission Control Protocol]] (TCP).<ref>{{Cite book |last=Green |first=Lelia |url=https://www.worldcat.org/title/504280762 |title=The internet: an introduction to new media |date=2010 |publisher=Berg |isbn=978-1-84788-299-8 |series=Berg new media series |page=31 |oclc=504280762 |quote=The original ARPANET design had made data integrity part of the IMP's store-and-forward role, but Cyclades end-to-end protocol greatly simplified the packet switching operations of the network. ... The idea was to adopt several principles from Cyclades and invert the ARPANET model to minimise international differences.}}</ref>

[[Bob Metcalfe]] and others at [[Xerox PARC]] outlined the idea of [[Ethernet]] and the [[PARC Universal Packet]] (PUP) for internetworking.<ref name="Moschovitisp78-9">{{harvnb|Moschovitis|1999|p=[https://archive.org/details/historyofinterne0000unse/page/78/mode/2up 78-9]}}</ref>

In May 1974, [[Vint Cerf]] and [[Bob Kahn]] described the [[Transmission Control Program]], an internetworking [[Communication protocol|protocol]] for sharing resources using packet-switching among the nodes.<ref>{{Cite journal|last1=Cerf|first1=V.|last2=Kahn|first2=R.|date=1974|title=A Protocol for Packet Network Intercommunication|url=https://www.cs.princeton.edu/courses/archive/fall06/cos561/papers/cerf74.pdf |archive-url=https://ghostarchive.org/archive/20221010/https://www.cs.princeton.edu/courses/archive/fall06/cos561/papers/cerf74.pdf |archive-date=2022-10-10 |url-status=live|journal=IEEE Transactions on Communications|volume=22|issue=5|pages=637–648|doi=10.1109/TCOM.1974.1092259|issn=1558-0857|quote=The authors wish to thank a number of colleagues for helpful comments during early discussions of international network protocols, especially R. Metcalfe, R. Scantlebury, D. Walden, and H. Zimmerman; D. Davies and L. Pouzin who constructively commented on the fragmentation and accounting issues; and S. Crocker who commented on the creation and destruction of associations.}}</ref> The specifications of the TCP were then published in {{IETF RFC|675}} (''Specification of Internet Transmission Control Program''), written by Vint Cerf, [[Yogen Dalal]] and Carl Sunshine in December 1974.<ref>{{cite IETF |title=Specification of Internet Transmission Control Protocol |rfc=675 |last1=Cerf |first1=Vinton |last2=Dalal |first2=Yogen |last3=Sunshine |first3=Carl |date=December 1974 |publisher=[[IETF]]}}</ref> 

The [[X.25|X.25 protocol]], developed by [[Rémi Després]] and others, was built on the concept of [[virtual circuit]]s. In the mid-late 1970s and early 1980s, national and international [[public data network]]s emerged using X.25 which was developed with participation from France, the UK, Japan, USA and Canada. It was complemented with [[X.75]] to enable internetworking.<ref name=":27">{{Cite web |last=Postel |first=Jon |date=August 29, 1979 |title=Comparison of X.25 and TCP Version 4 as Cable-bus Network Protocols |url=https://www.rfc-editor.org/ien/ien130.pdf}}</ref> 

Packet switching was shown to be optimal in the [[Huffman coding]] sense in 1978.<ref>{{Cite journal |last1=Camrass |first1=R. |last2=Gallager |first2=R. |date=1978 |title=Encoding message lengths for data transmission (Corresp.) |url=https://ieeexplore.ieee.org/document/1055910 |journal=IEEE Transactions on Information Theory |language=en |volume=24 |issue=4 |pages=495–496 |doi=10.1109/TIT.1978.1055910 |issn=0018-9448|url-access=subscription }}</ref><ref>{{Cite web |title=Reflections on an Internet pioneer: Roger Camrass |url=http://stories.clare.cam.ac.uk/reflections-of-an-internet-pioneer/ |access-date=2024-07-01 |website=stories.clare.cam.ac.uk |language=en}}</ref>  

In the late 1970s, the monolithic Transmission Control Program was layered as the Transmission Control Protocol (TCP), atop the [[Internet Protocol]] (IP). Many [[Internet pioneers]] developed this into the [[Internet protocol suite]] and the associated Internet architecture and governance that emerged in the 1980s.<ref>{{Cite web |last1=Cerf |first1=Vinton G. |last2=Postel |first2=Jon |date=August 18, 1977 |title=Specification of Internetwork Transmission Program: TCP Version 3 |url=https://www.rfc-editor.org/ien/ien21.pdf |page=iii, 75-87}}</ref><ref>{{Cite web |last=Postel |first=Jon |date=September 1978 |title=Specification of Internetwork Transmission Control Protocol: TCP Version 4 |url=https://www.rfc-editor.org/ien/ien55.pdf |pages=iii, 85-97}}</ref><ref>{{Cite web |last=Cerf |first=Vinton G. |date=1 April 1980 |title=Final Report of the Stanford University TCP Project |url=https://www.rfc-editor.org/ien/ien151.txt}}</ref><ref name="Moschovitisp118">{{harvnb|Moschovitis|1999|p=[https://archive.org/details/historyofinterne0000unse/page/118/mode/2up 78-9]}}</ref><ref name="ISINames-2003">[https://www3.isi.edu/news/story/54 "ISI Names Dr. Paul Mockapetris Visiting Scholar"] {{webarchive|url=https://web.archive.org/web/20120826032920/http://www3.isi.edu/about-news_story.htm?s=54|date=2012-08-26}}, Information Sciences Institute, University of Southern California, 27 March 2003</ref><ref>[http://portal.acm.org/citation.cfm?doid=205447.205462 "Congestion avoidance and control"], Van Jacobson, ACM SIGCOMM Computer Communication Review - Special twenty-fifth anniversary issue, Highlights from 25 years of the Computer Communication Review, Volume 25 Issue 1, Jan. 1995, pp.157-187</ref>

For a period in the 1980s and early 1990s, the network engineering community was polarized over the implementation of competing protocol suites, commonly known as the [[Protocol Wars]]. It was unclear which of the Internet protocol suite and the [[OSI model]] would result in the best and most robust computer networks.<ref name="ieee201703">{{cite magazine |author=Andrew L. Russell |date=30 July 2013 |title=OSI: The Internet That Wasn't |url=https://spectrum.ieee.org/osi-the-internet-that-wasnt |magazine=[[IEEE Spectrum]] |volume=50 |issue=8}}</ref><ref>{{Cite web |last=Russell |first=Andrew L. |title=Rough Consensus and Running Code' and the Internet-OSI Standards War |url=https://www2.cs.duke.edu/courses/common/compsci092/papers/govern/consensus.pdf |url-status=live |archive-url=https://web.archive.org/web/20191117080112/https://www2.cs.duke.edu/courses/common/compsci092/papers/govern/consensus.pdf |archive-date=2019-11-17 |publisher=IEEE Annals of the History of Computing}}</ref><ref>{{Cite book |last1=Davies |first1=Howard |chapter-url=https://books.google.com/books?id=DN-t8MpZ0-wC&q=%22protocol+wars%22&pg=PA106 |title=A History of International Research Networking: The People who Made it Happen |last2=Bressan |first2=Beatrice |date=2010 |publisher=John Wiley & Sons |isbn=978-3-527-32710-2 |pages=106–110 |language=en |chapter=The Protocol Wars}}</ref>

Leonard Kleinrock carried out theoretical work at UCLA during the 1970s analyzing throughput and delay in the ARPANET.<ref>{{Cite web |last=Postel |first=J. |date=7 February 1979 |title=Internet Meeting Notes -- 25 & 26 January 1979 |url=https://www.rfc-editor.org/ien/ien76.pdf |access-date=9 February 2022 |page=5 |quote=Vint noted that UCLA's internet work is primarily theoretical research on throughput and delay analysis. This work is headed by L. Kleinrock.}}</ref><ref>{{cite tech report |url=https://walden-family.com/bbn/arpanet-completion-report.pdf |title=Arpanet Completion Report |last1=Heart |first1=F. |last2=McKenzie |first2=A. |last3=McQuillian |first3=J. |last4=Walden |first4=D. |date=January 4, 1978 |archive-url=https://web.archive.org/web/20230527095942/https://walden-family.com/bbn/arpanet-completion-report.pdf |url-status=dead |archive-date=2023-05-27 |publisher=Bolt, Beranek and Newman |location=Burlington, MA}}</ref><ref name=":43">{{Cite book |last=Davies |first=Donald Watts |url=https://archive.org/details/computernetworks00davi/page/86/mode/2up?q=kleinrock+kleinrock%27s |title=Computer networks and their protocols |date=1979 |publisher=Wiley |others=Internet Archive |isbn=978-0-471-99750-4 |pages=See page refs highlighted at url |quote=In mathematical modelling use is made of the theories of queueing processes and of flows in networks, describing the performance of the network in a set of equations. ... The analytic method has been used with success by Kleinrock and others, but only if important simplifying assumptions are made. ... It is heartening in Kleinrock's work to see the good correspondence achieved between the results of analytic methods and those of simulation.}}</ref> His theoretical work on [[hierarchical routing]] with student [[Farouk Kamoun]] became critical to the operation of the Internet.<ref name=":52">{{Cite book |last=Davies |first=Donald Watts |url=https://archive.org/details/computernetworks00davi/page/110/mode/2up?q=%22kleinrock+and+kamoun%22 |title=Computer networks and their protocols |date=1979 |publisher=Wiley |others=Internet Archive |isbn=978-0-471-99750-4 |pages=110–111 |quote=Hierarchical addressing systems for network routing have been proposed by Fultz and, in greater detail, by McQuillan. A recent very full analysis may be found in Kleinrock and Kamoun.}}</ref><ref>{{Cite book |last1=Feldmann |first1=Anja |title=Proceedings of the 2009 workshop on Re-architecting the internet |last2=Cittadini |first2=Luca |last3=Mühlbauer |first3=Wolfgang |last4=Bush |first4=Randy |last5=Maennel |first5=Olaf |date=2009 |publisher=Association for Computing Machinery |isbn=978-1-60558-749-3 |series=ReArch '09 |location=New York, NY, USA |pages=43–48 |chapter=HAIR: Hierarchical architecture for internet routing |doi=10.1145/1658978.1658990 |quote=The hierarchical approach is further motivated by theoretical results (e.g., [16]) which show that, by optimally placing separators, i.e., elements that connect levels in the hierarchy, tremendous gain can be achieved in terms of both routing table size and update message churn. ... [16] KLEINROCK, L., AND KAMOUN, F. Hierarchical routing for large networks: Performance evaluation and optimization. Computer Networks (1977). |chapter-url=https://core.ac.uk/download/pdf/326320693.pdf |s2cid=2930578}}</ref> Kleinrock published hundreds of research papers,<ref name=":26">{{Cite web |title=Leonard Kleinrock |url=https://www.internethalloffame.org/inductee/leonard-kleinrock/ |access-date=2023-03-13 |website=Internet Hall of Fame |language=en-US}}</ref><ref name=":32">{{Cite web |title=Kleinrock (Leonard) papers |url=https://oac.cdlib.org/findaid/ark:/13030/c8kd240b/entire_text/ |access-date=2023-04-04 |website=oac.cdlib.org}}</ref> which ultimately launched a new field of research on the theory and application of queuing theory to computer networks.<ref name=":12" /><ref name="Abbatep81">{{Cite book |last=Abbate |first=Janet |url=https://archive.org/details/inventinginterne00jane/page/80/mode/2up |title=Inventing the Internet |date=1999 |publisher=MIT Press |others=Internet Archive |isbn=978-0-262-01172-3 |page=81}}</ref>

Complementary [[metal–oxide–semiconductor]] ([[CMOS]]) [[VLSI]] (very-[[large-scale integration]]) technology led to the development of high-speed [[broadband]] packet switching during the 1980s{{ndash}}1990s.<ref name="Hayward">{{cite journal |last1=Hayward |first1=G. |last2=Gottlieb |first2=A. |last3=Jain |first3=S. |last4=Mahoney |first4=D. |title=CMOS VLSI Applications in Broadband Circuit Switching |journal=IEEE Journal on Selected Areas in Communications |date=October 1987 |volume=5 |issue=8 |pages=1231–1241 |doi=10.1109/JSAC.1987.1146652 |issn=1558-0008}}</ref><ref name="Hui">{{cite journal |last1=Hui |first1=J. |last2=Arthurs |first2=E. |title=A Broadband Packet Switch for Integrated Transport |journal=IEEE Journal on Selected Areas in Communications |date=October 1987 |volume=5 |issue=8 |pages=1264–1273 |doi=10.1109/JSAC.1987.1146650 |issn=1558-0008}}</ref><ref>{{cite book |last1=Gibson |first1=Jerry D. |title=The Communications Handbook |date=2018 |publisher=[[CRC Press]] |isbn=9781420041163|url=https://books.google.com/books?id=Tokk5bZxB0MC&pg=SA34-PA4}}</ref>

=== The "paternity dispute" ===

Roberts claimed in later years that, by the time of the October 1967 SOSP, he already had the concept of packet switching in mind (although not yet named and not written down in his paper published at the conference, which a number of sources describe as "vague"), and that this originated with his old colleague, Kleinrock, who had written about such concepts in his Ph.D. research in 1961-2.<ref name=":19" /><ref name=":18" /><ref name=":20" /><ref>{{Cite journal |last=Kirstein |first=Peter T. |date=2009 |title=The early history of packet switching in the UK |journal=IEEE Communications Magazine |volume=47 |issue=2 |pages=18–26 |doi=10.1109/MCOM.2009.4785372 |s2cid=34735326 |quote=It is more difficult to establish at this time, however, whether Larry intended to switch the fragments as independent packets in the ARPAnet before he heard of the NPL work; certainly he now claims that this was always his intention.}}</ref><ref>{{Cite web |last=technicshistory |date=2019-06-02 |title=ARPANET, Part 2: The Packet |url=https://technicshistory.com/2019/06/02/arpanet-part-2-the-packet/ |access-date=2024-06-21 |website=Creatures of Thought |language=en |quote=The above description of how packet-switching came to be is the most widely-accepted one. However, there is an alternative version. Roberts claimed in later years that by the time of the Gatlinburg symposium, he already had the basic concepts of packet-switching well in mind, and that they originated with his old colleague Len Kleinrock, who had written about them as early as 1962, as part of his Ph.D. research on communication nets. It requires a great deal of squinting to extract anything resembling packet-switching from Kleinrock’s work, however, and no other contemporary textual evidence that I have come across backs the Kleinrock/Roberts account.}}</ref> In 1997, along with seven other [[Internet pioneers]], Roberts and Kleinrock co-wrote "Brief History of the Internet" published by the [[Internet Society]].<ref>{{citation |author= Barry M. Leiner, Vinton G. Cerf, David D. Clark, Robert E. Kahn, Leonard Kleinrock, Daniel C. Lynch, Jon Postel, Larry G. Roberts, Stephen Wolff |title=Brief History of the Internet |publisher=Internet Society |date=1997|url=https://www.internetsociety.org/resources/doc/2017/brief-history-internet/}}</ref> In it, Kleinrock is described as having "published the first paper on packet switching theory in July 1961 and the first book on the subject in 1964". Many sources about the history of the Internet began to reflect these claims as uncontroversial facts. This became the subject of what [[Katie Hafner]] called a "paternity dispute" in The New York Times in 2001.<ref name=":8">{{citation |author=Katie Hafner |title=A Paternity Dispute Divides Net Pioneers |date=November 8, 2001 |url=https://www.nytimes.com/2001/11/08/technology/a-paternity-dispute-divides-net-pioneers.html?pagewanted=all |newspaper=New York Times |quote="The Internet is really the work of a thousand people," Mr. Baran said. "And of all the stories about what different people have done, all the pieces fit together. It's just this one little case that seems to be an aberration."}}</ref>

The disagreement about Kleinrock's contribution to packet switching dates back to a version of the above claim made on Kleinrock's profile on the UCLA Computer Science department website sometime in the 1990s. Here, he was referred to as the "Inventor of the Internet Technology".<ref>{{cite web |last1=UCLA Computer Science Dept. |title=Leonard Kleinrock, Professor (archived) |url=http://www.lk.cs.ucla.edu |website=UCLA Computer Science Dept. |access-date=28 December 2023 |archive-url=https://web.archive.org/web/20040227150208/http://www.lk.cs.ucla.edu:80/index.html |archive-date=Feb 27, 2004}}</ref> The webpage's depictions of Kleinrock's achievements provoked anger among some early Internet pioneers.<ref name=":0">{{cite book |last1=Isaacson |first1=Walter |url=https://books.google.com/books?id=4V9koAEACAAJ&pg=PA245 |title=The Innovators: How a Group of Hackers, Geniuses, and Geeks Created the Digital Revolution |date=2014 |publisher=Simon & Schuster |isbn=9781476708690 |pages=244–6 |author-link=Walter Isaacson}}</ref> The dispute over [[Scientific priority|priority]] became a public issue after Donald Davies posthumously published a paper in 2001 in which he denied that Kleinrock's work was related to packet switching. Davies also described ARPANET project manager [[Lawrence Roberts (scientist)|Larry Roberts]] as supporting Kleinrock, referring to Roberts' writings online and Kleinrock's UCLA webpage profile as "very misleading".<ref>{{citation |author=Donald W. Davies |title=An Historical Study of the Beginnings of Packet Switching |date=2001 |url=https://academic.oup.com/comjnl/article-abstract/44/3/152/415514 |journal=The Computer Journal |quote=I can find no evidence that he understood the principles of packet switching.}}</ref><ref name=":9">{{Cite journal |last=Harris |first=Trevor, University of Wales |date=2009 |editor-last=Pasadeos |editor-first=Yorgo |title=Who is the Father of the Internet? The Case for Donald Davies |url=https://www.academia.edu/378261 |url-status=dead |journal=Variety in Mass Communication Research |language=en |publisher=ATINER |pages=123–134 |isbn=978-960-6672-46-0 |archive-url=https://web.archive.org/web/20220502025941/https://www.academia.edu/378261/Who_is_the_Father_of_the_Internet_The_Case_for_Donald_Davies |archive-date=May 2, 2022 |quote=Leonard Kleinrock and Lawrence (Larry) Roberts, neither of whom were directly involved in the invention of packet switching ... Dr Willis H. Ware, Senior Computer Scientist and Research at the RAND Corporation, notes that Davies (and others) were troubled by what they regarded as in appropriate claims on the invention of packet switching}}</ref> [[Walter Isaacson]] wrote that Kleinrock's claims "led to an outcry among many of the other Internet pioneers, who publicly attacked Kleinrock and said that his brief mention of breaking messages into smaller pieces did not come close to being a proposal for packet switching".<ref name=":0" /> 

Davies' paper reignited a previous dispute over who deserves credit for getting the ARPANET online between engineers at [[Bolt, Beranek, and Newman]] (BBN) who had been involved in building and designing the ARPANET IMP on the one side, and ARPA-related researchers on the other.<ref name=":2A3" /><ref name="F.E. Froehlich, A. Kent" /> This earlier dispute is exemplified by BBN's [[Will Crowther]], who in a 1990 oral history described Paul Baran's packet switching design (which he called [[hot-potato routing]]), as "crazy" and non-sensical, despite the ARPA team having advocated for it.<ref>{{citation|author=Judy O'Neill | title=Oral history interview with William Crowther| date=12 March 1990| hdl=11299/107235|url=https://conservancy.umn.edu/handle/11299/107235|quote=...there were all sorts of crazy ideas about, and most of them didn't make any sense. There was this 'hot potato' routing which somebody was advocating, which was just crazy.}}</ref> The reignited debate caused other former BBN employees to make their concerns known, including Alex McKenzie, who followed Davies in disputing that Kleinrock's work was related to packet switching, stating "... there is nothing in the entire 1964 book that suggests, analyzes, or alludes to the idea of packetization".<ref>{{citation |author=Alex McKenzie |title=Comments on Dr. Leonard Kleinrock's claim to be "the Father of Modern Data Networking" |year=2009 |url=http://alexmckenzie.weebly.com/comments-on-kleinrocks-claims.html |access-date=April 23, 2015}}</ref>

Former [[Information Processing Techniques Office|IPTO]] director [[Robert W. Taylor|Bob Taylor]] also joined the debate, stating that "authors who have interviewed dozens of Arpanet pioneers know very well that the Kleinrock-Roberts claims are not believed".<ref>{{citation|author=Robert Taylor|title=Birthing the Internet: Letters From the Delivery Room; Disputing a Claim|date=November 22, 2001|url=https://www.nytimes.com/2001/11/22/technology/l-birthing-the-internet-letters-from-the-delivery-room-disputing-a-claim-325210.html|newspaper=New York Times|author-link=Robert Taylor (computer scientist)}}</ref> Walter Isaacson notes that "until the mid-1990s Kleinrock had credited [Baran and Davies] with coming up with the idea of packet switching".<ref name=":0" />

A subsequent version of Kleinrock's biography webpage was copyrighted in 2009 by Kleinrock.<ref>{{citation |author=Leonard Kleinrock |title=Leonard Kleinrock - UCLA Dept. of Computer Science |url=https://www.lk.cs.ucla.edu/index.html |archive-url=https://web.archive.org/web/20231205193349/https://www.lk.cs.ucla.edu/index.html |archive-date=December 5, 2023 |quote=He developed the mathematical theory of data networks, the technology underpinning the Internet, while a graduate student at MIT in the period from 1960-1962.  In that work, he also modeled the packetization of messages and solved for a key performance gain that packetization provides.}}</ref> He was called on to defend his position over subsequent decades.<ref name=":14">{{citation |title=Letters to the editor |journal=IEEE Communications |date=February 2011|doi=10.1109/MCOM.2011.5706298 |url=https://ieeexplore.ieee.org/document/5706298|url-access=subscription }}</ref> In 2023, he acknowledged that his published work in the early 1960s was about message switching and claimed he was thinking about packet switching.<ref>{{Cite AV media |url=https://www.inc.com/computerfreaks |title=Computer Freaks |date=June 22, 2023 |last=Haughney Dare-Bryan |first=Christine |type=Podcast |publisher=Inc. Magazine |series=Chapter Two: In the Air}}</ref> Primary sources and historians recognize Baran and Davies for independently inventing the concept of digital packet switching used in modern computer networking including the ARPANET and the Internet.<ref name=":16" /><ref name="Pelkeyp42" /><ref name="Abbate20002" /><ref name=":13">{{Cite book |last1=Norberg |first1=Arthur L. |title=Transforming computer technology: information processing for the Pentagon, 1962-1986 |last2=O'Neill |first2=Judy E. |date=1996 |publisher=Johns Hopkins Univ. Press |isbn=978-0-8018-5152-0 |series=Johns Hopkins studies in the history of technology New series |location=Baltimore |pages=153–196}} Prominently cites Baran and Davies as sources of inspiration, and nowhere mentions Kleinrock's work.</ref><ref>{{cite report |url=https://apps.dtic.mil/sti/pdfs/ADA115440.pdf |title=A History of the ARPANET: The First Decade |date=1 April 1981 |publisher=Bolt, Beranek & Newman Inc. |pages=13, 53 of 183 |quote=Aside from the technical problems of interconnecting computers with communications circuits, the notion of computer networks had been considered in a number of places from a theoretical point of view. Of particular note was work done by Paul Baran and others at the Rand Corporation in a study "On Distributed Communications" in the early 1960's. Also of note was work done by Donald Davies and others at the National Physical Laboratory in England in the mid-1960's. ... Another early major network development which affected development of the ARPANET was undertaken at the National Physical Laboratory in Middlesex, England, under the leadership of D. W. Davies. |archive-url=https://web.archive.org/web/20121201013642/http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA115440 |archive-date=1 December 2012 |url-status=live}}</ref>

Kleinrock has received many awards for his ground-breaking [[applied mathematical research]] on packet switching, carried out in the 1970s, which was an extension of his pioneering work in the early 1960s on the optimization of message delays in communication networks.<ref name=":12" /><ref>{{Cite web |title=Leonard Kleinrock |url=https://samueli.ucla.edu/people/leonard-kleinrock/ |access-date=2024-01-20 |website=UCLA Samueli School Of Engineering |language=en-US}}</ref> However, Kleinrock's claims that his work in the early 1960s originated the concept of packet switching and that his work was a source of the packet switching concepts used in the ARPANET have affected sources on the topic, which has created methodological challenges in the historiography of the Internet.<ref name=":8" /><ref name=":0" /><ref name=":9" /><ref name=":14" /> Historian Andrew L. Russell said "'Internet history' also suffers from a ... methodological, problem: it tends to be too close to its sources. Many Internet pioneers are alive, active, and eager to shape the histories that describe their accomplishments. Many museums and historians are equally eager to interview the pioneers and to publicize their stories".<ref>{{Cite conference |last=Russell |first=Andrew |date=2012 |title=Histories of Networking vs. the History of the Internet |url=https://arussell.org/papers/russell-SIGCIS-2012.pdf |conference=2012 SIGCIS Workshop |page=6}}</ref>