What was the first message sent over ARPANET in 1969?

Answer

Researchers meant to send 'LOGIN' but the system crashed after two letters.

Context

The transmission took place on 29 October 1969 at about 22:30. Charley Kline, a UCLA student programmer working under Leonard Kleinrock, tried to log in from a UCLA computer to a machine at the Stanford Research Institute (SRI) over the new ARPANET link. The crash occurred after the letters 'l' and 'o' had gone through, so the first message was two letters long rather than the intended 'login'.

The link ran through Interface Message Processors, small minicomputers built by the firm BBN that handled routing between host computers and the telephone lines. UCLA's was the first installed, with SRI's following within weeks, and the network had four nodes by the end of 1969, with the University of California, Santa Barbara and the University of Utah joining the first two.

UCLA's account says the system was restored roughly an hour later and the full login then succeeded. The story that the pair planned a poetic 'lo and behold' is a later flourish; the accounts describe it as an interrupted login attempt, and the word 'lo' read as an accidental, fitting opening line only in hindsight. Kleinrock's own research on queueing theory and packet networks underpinned the design, which is why UCLA hosted the first node.