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IPv4 Flag Day – TCP/IP – 1983 AD

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IPv4 Flag Day

Jon Postel (1943–1998)

The original ARPANET relied on Interface Message Processors (IMPs) to forward packets and a combination of complex protocols. These protocols included the ARPANET Host-to-Host Protocol (AHHP), the Network Control Protocol (NCP), and the Initial Connection Protocol (ICP). All communications between hosts had to go through IMPs, and IMPs were expensive.

The first ARPANET IMP was installed in September 1969, and the network was in regular operation by 1971. Just two years later, in 1973, work started on a complete network redesign. At the core of the new design was the Internet Protocol (IP), a connectionless protocol that moved packets between hosts. IP was designed to be fast, predictable, and extensible, but it didn’t guarantee that packets would ever be delivered. Instead, IP provided a “best-effort network” that followed the end-to-end principle—intelligent endpoints are responsible for managing their own communications. Also called the stupid network, this network had just one job: to move packets from one host to another.

Above the IP layer was the Transmission Control Protocol (TCP), which allowed two computers on the ends of the network to communicate over a reliable stream of 8-bit characters. Programs using TCP simply created a connection to a “socket” on a remote computer and then sent and received characters as necessary. The computer’s operating system then packaged the characters into packets, sent the packets, and retransmitted the packets if they got lost. TCP made it easy for programmers to develop services such as remote terminal, file transfer, and email.

For many years, ARPANET supported both NCP and TCP/IP. In 1981, Jon Postel warned the ARPANET’s users that ISI, the network’s operator, could disable NCP on the entire ARPANET simply by telling the IMPs to drop NCP traffic. In the middle of 1982, he did just that, briefly—and the sky didn’t fall. A few months later he did it again, this time for two days. Finally, Postel made the change permanent on January 1, 1983, which became known as “flag day.”

SEE ALSO Interface Message Processor (IMP) (1968), ARPANET/Internet (1969), NSFNET (1985), ISP Provides Internet Access to the Public (1989), World IPv6 Day (2011)

January 1, 1983, became known as “flag day,” when ARPANET permanently transitioned from NCP to TCP/IP.

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