Donald Watts Davies, CBE FRS (7 June 1924 – 28 May 2000) was a Welsh computer scientist and Internet pioneer who was employed at the UK National Physical Laboratory (NPL).

His father, a clerk at a coalmine, died a few months later, and his mother took Donald and his twin sister back to her home town of Portsmouth, where he went to school.

[2] He received a BSc degree in physics (1943) at Imperial College London, and then joined the war effort working as an assistant to Klaus Fuchs[1] on the nuclear weapons Tube Alloys project at Birmingham University.

[3] He then returned to Imperial taking a first class degree in mathematics (1947); he was also awarded the Lubbock memorial Prize as the outstanding mathematician of his year.

[7][3] He applied the principle of time-sharing to the data communications line as well as the computer to invent the concept of what he called packet switching.

[3][8] Davies forecast today's "killer app" for his new communication service:[9] 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.Davies proposed dividing computer messages into very "short messages in fixed format" that are routed independently across a network, with differing routes allowed for related packets, which are reassembled at the destination.

[6][13] Davies applied queueing theory 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 for a human user.

[15][16] His work on packet switching, presented by Scantlebury, initially caught the attention of the developers of the ARPANET, a U.S. Department of Defense (DoD) network, at the Symposium on Operating Systems Principles in October 1967.

[18] 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",[19] thus inventing what came to be known as the end-to-end principle.

[20][21] Larry Roberts, of the Advanced Research Projects Agency (ARPA) of the United States Department of Defense (DoD), applied Davies' concepts of packet switching for the ARPANET, which went on to become a predecessor to the Internet.

Davies' original ideas influenced other research around the world,[29][34] including Louis Pouzin's CYCLADES project in France.

[37][38][39][40] Davies and Scantlebury were acknowledged by Cerf and Bob Kahn in their seminal 1974 paper on internetworking, A Protocol for Packet Network Intercommunication.

[44] Internetworking experiments at NPL under Davies included connecting with the European Informatics Network (EIN) by translating between two different host protocols and connecting with the Post Office Experimental Packet Switched Service (EPSS) using a common host protocol in both networks.

This philosophy goes against the widely held view that packet networks should deliver a data stream exactly equivalent to the received data stream.For a long period of time, the network engineering community was polarized over the implementation of competing protocol suites, a debate commonly called the Protocol Wars.

[4][48] He retired from NPL in 1984, becoming a leading consultant on data security to the banking industry and publishing a book on the topic that year.

[57] However, Kleinrock's later claim to have developed the theoretical basis of packet switching networks is disputed by other Internet pioneers,[58][59][60][61] including by Robert Taylor,[62] Baran[63] and Davies.

[64][41] Donald Davies and Paul Baran are recognized by historians and the U.S. National Inventors Hall of Fame for independently inventing the concept of digital packet switching used in modern computer networking including the Internet.

[77][78] NPL sponsors a gallery, opened in 2009, about the development of packet switching and "Technology of the Internet" at The National Museum of Computing.