ARPANET's First Message: Why 'LO' Reached SRI Before the System Crashed
The October 29, 1969 UCLA-to-SRI login attempt, the Interface Message Processors, and what the famous two letters do—and do not—prove.
On October 29, 1969, a UCLA team attempted to log in to a host at SRI over the newly connected ARPANET. The operators transmitted L and O; the remote system crashed before G, producing the famous first message “LO.” The connection was repaired and the login completed later.
The hosts communicated through Interface Message Processors, packet-switching nodes built by BBN. The event was a milestone in a larger program involving ARPA, universities, contractors, protocol design, and earlier packet-switching research.
“LO” was not the birth of today’s Internet in one instant. ARPANET initially used NCP; the TCP/IP transition came in 1983. Accurate history distinguishes the first host-to-host test from the later internetwork architecture.
Sources: UCLA 50th-anniversary account of the first ARPANET message, Internet Society brief history