Leonard Kleinrock

Leonard Kleinrock (born June 13, 1934) is an American computer scientist and Internet pioneer. He is Distinguished Professor Emeritus of Computer Science at UCLA's Henry Samueli School of Engineering and Applied Science. Kleinrock made several important contributions to the field of computer science, in particular to the mathematical foundations of data communication in computer networking. He has received numerous prestigious awards.

In the early 1960s, Kleinrock pioneered the application of queueing theory to model delays in message switching networks in his Ph.D. thesis, published as a book in 1964. In the late 1960s and 1970s, he played an influential role in the development of the ARPANET. In the 1970s, he applied queueing theory to model and measure the performance of packet switching networks and published several of the standard works on the subject. He supervised graduate students who worked on the communication protocols for the ARPANET including students whose later work on internetworking and the Internet protocol suite led to the networking technology employed in the Internet. His theoretical work on hierarchical routing in the late 1970s with student Farouk Kamoun remains critical to the operation of the Internet today.

Leonard Kleinrock was born in New York City on June 13, 1934, to a Jewish family, and graduated from the noted Bronx High School of Science in 1951. He received a Bachelor of Electrical Engineering degree in 1957 from the City College of New York, and a master's degree and a doctorate (Ph.D.) in electrical engineering and computer science from the Massachusetts Institute of Technology (MIT) in 1959 and 1963 respectively. He then joined the faculty at the University of California at Los Angeles (UCLA), where he remains to the present day; during 1991–1995 he served as the chairman of the Computer Science Department there.

Kleinrock's best-known and most-significant work is on queueing theory, a major topic of applied mathematics that has applications in many fields. His thesis proposal in 1961, Information Flow in Large Communication Nets, led to a doctoral thesis at MIT in 1962, Message Delay in Communication Nets with Storage, later published as book in 1964, Communication Nets: Stochastic Message Flow and Delay. In this work, he researched the configuration and operation of communication networks, considering design parameters such as "channel capacity, effect of priority discipline, choice of routing procedure, and design of topological structure". He analyzed delays in Plan 55-A, a message switching system operated by Western Union for processing telegrams. His thesis went on to apply probability theory to model queueing delays in a generalized communication network.

Donald Davies, in his 1966 paper on packet switching, applied Kleinrock'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 for a human user. This addressed a key question about the viability of computer networking.

A contemporary from MIT, Larry Roberts, brought Leonard Kleinrock into the ARPANET project informally in early 1967. Roberts 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 retransmission, and computer and user identification." Later that year, Roberts learned about packet switching from a paper written by Davies, presented at the October 1967 Symposium on Operating Systems Principles, and incorporated the concept into the proposal for the ARPANET.

Kleinrock was awarded a contract in 1968 to establish a Network Measurement Center (NMC) to measure and model the performance of the network. His mathematical work studied and influenced the development of the early ARPANET. In addition, Kleinrock managed the software team at UCLA — including Steve Crocker, Jon Postel, and Vint Cerf — who developed the host-host protocol for the ARPANET, the Network Control Program (NCP).

The first message on the ARPANET was sent by a UCLA undergraduate student, Charley Kline, who was supervised by Kleinrock. At 10:30 p.m, on October 29, 1969, from Boelter Hall 3420, the school's main engineering building, Kline transmitted from the university's SDS Sigma 7 host computer to the Stanford Research Institute's SDS 940 host computer. The message text was the word "login"; the "l" and the "o" letters were transmitted, but the system then crashed. Hence, the literal first message over the ARPANET was "lo". About an hour later, having recovered from the crash, the SDS Sigma 7 computer effected a full "login". The first permanent ARPANET link was established on November 21, 1969, between the Interface Message Processor (IMP) at UCLA and the IMP at the Stanford Research Institute. By December 5, 1969, the initial four-node network was established.