Donald Davies(1924–2000)

Donald Watts Davies was born at 5 Treherbert Street in Treorchy in the Rhondda Valley on the seventh of June 1924, the son of a colliery clerk who died a few months after his birth. His mother took the boy and his twin sister to Portsmouth, where she had family, and he was raised and schooled there. He won an exhibition to Imperial College, London in 1942, took a first-class degree in physics in 1943 at the accelerated wartime tempo, and was at once seconded to the wartime nuclear programme, the Tube Alloys project, where he worked through the rest of the war on the calculation of neutron diffusion in uranium assemblies. He returned to Imperial after the war and took a second first, in mathematics, in 1947.

In the autumn of 1947 he joined the Mathematics Division of the National Physical Laboratory at Teddington, the great central laboratory of British metrology and applied mathematics, and was put straight onto the team Alan Turing was leading to build the Pilot ACE computer, the realisation of Turing's 1945 design for an electronic stored-program machine. The Pilot ACE ran its first program in May 1950; Davies stayed at NPL for the next forty-seven years, and from 1966 was the head of the Computer Science Division of the laboratory.

Through 1965 and into 1966 he set out, in a series of internal NPL papers and lectures, the principle he called packet switching. The conventional way of sending messages between computers in 1965 was to set up a dedicated end-to-end circuit, as a telephone call sets up a line, and send the message down it; the line sat idle most of the time, because computer traffic comes in bursts, and the system scaled badly. Davies's idea was to break every message into short fixed-length blocks, give each block a header containing the address of its destination, hand the blocks to the network as independent packets, and let each node in the network forward each packet onward toward its destination by the best route then available, the packets to be reassembled at the receiving end. The principle made bursty traffic share the same lines efficiently, made the network resilient to the failure of any one link, and scaled.

He presented the design publicly at the Association for Computing Machinery's first Symposium on Operating System Principles at Gatlinburg, Tennessee in October 1967, the same conference at which Lawrence Roberts of the United States Defense Advanced Research Projects Agency presented the early plans for what would become the ARPANET. The Davies design and the independent earlier work of Paul Baran at the RAND Corporation in California were taken together as the architectural basis of ARPANET; Roberts adopted the NPL packet size and much of the NPL terminology, including the word packet itself. At Teddington Davies built the NPL Local Network, which ran from January 1970 as one of the first packet-switched networks in the world.

From the local network at NPL the packet-switching principle spread, through ARPANET in 1969 and 1970, through the European Informatics Network in 1976, through the great public packet networks of the 1970s, and through the internet protocols of the 1980s, into the universal communications fabric on which every byte of internet traffic now moves. Davies turned in his later years to cryptography and computer security, did important work on financial network security and on the cryptanalysis of the Lorenz machine of the Second World War, and was awarded the CBE in 1983 and elected a Fellow of the Royal Society in 1987. He took the John von Neumann Medal in 1986 and was inducted into the Internet Hall of Fame on its inauguration in 2012. He died at Esher on the twenty-eighth of May 2000. The Davies name in modern computer science carries the weight of the principle worked out at Teddington in 1965 from which the architecture of the connected world descends.