The Switch to TCP/IP — 'Flag Day'

TCP/IP is the pair of protocols that lets separately built networks act as one Internet: IP carries individual packets across networks, and TCP turns them into a reliable, ordered stream between programs. It was not invented on a single day. Vint Cerf and Bob Kahn worked out the design in 1973 and published it in May 1974 in IEEE Transactions on Communications; the single protocol of that paper was split into TCP and IP in 1978; the two were standardised as RFC 791 and RFC 793 in September 1981; and on 1 January 1983 the ARPANET switched its hosts over from its old protocol, NCP, to TCP/IP. That last date — the "flag day" — is when TCP/IP became the network's working protocol rather than an experiment.

1972–74: Kahn, Cerf and the gateway

The Internet Society's history credits Bob Kahn with the idea of "open-architecture networking", introduced shortly after he arrived at DARPA in 1972. The problem was that DARPA was now funding more than one kind of packet network — packet radio among them — while the ARPANET's own host protocol, NCP, relied on the ARPANET itself for end-to-end reliability and could not address machines beyond the destination IMP. If packets were lost, NCP simply stalled. In the spring of 1973 Kahn asked Vint Cerf, then at Stanford, to work with him on the detailed design. A first written version circulated as INWG#39 at Sussex University that September.

The published paper, "A Protocol for Packet Network Intercommunication", appeared in the May 1974 issue (vol. COM-22, no. 5, pp. 637–648). Its abstract describes a protocol that handles differing packet sizes, transmission failures, sequencing, flow control, end-to-end error checking and the setting up and tearing down of process-to-process connections across different packet-switching networks. Two ideas from it are still recognisable. Networks are joined by a device the authors called a GATEWAY — today's router. And reliability is not the networks' job: each host runs "a transmission control program (TCP)" that handles sending and accepting messages for the processes it serves.

December 1974: RFC 675

In December 1974 Cerf, Yogen Dalal and Carl Sunshine issued RFC 675, Specification of Internet Transmission Control Program. Its acknowledgements credit Ray Tomlinson of BBN — known for network email — with the three-way handshake and initial-sequence-number selection that TCP still uses to open a connection. DARPA then let three implementation contracts, to Stanford (Cerf), BBN (Tomlinson) and University College London (Peter Kirstein), and the Internet Society notes that the three independent implementations could interoperate.

1978: one protocol becomes two

The 1974 design was a single program doing everything. Experience with uses that did not need every byte delivered in order — the Internet Society history mentions packet voice in the 1970s — argued for separating the layers. In 1978 Cerf, Jon Postel and Danny Cohen proposed, in the words of the IEEE Communications Society's account, "splitting TCP into two components: a host-to-host transport-level protocol (TCP) and an internetworking network-level protocol (IP)". IP would only address and forward individual packets; TCP would handle ordering and reliability on top. A third protocol, UDP, was added to give programs direct access to IP's basic datagram service.

1981: RFC 791 and RFC 793

In September 1981 the Information Sciences Institute at USC published the two specifications that are still the reference point for the protocols' names. RFC 791, edited by Jon Postel, opens: "This document specifies the DoD Standard Internet Protocol." It defines the 32-bit address — four octets — that IPv4 uses to this day. RFC 793 describes the DoD Standard Transmission Control Protocol and says it draws heavily on nine earlier editions of the ARPA TCP specification. Both were prepared for DARPA's Information Processing Techniques Office. The Department of Defense had already adopted TCP/IP as a defence standard in 1980.

1 January 1983: the flag day

With the standards in place, Postel's RFC 801 of November 1981 laid out the cutover. Its goal was blunt: "The goal is to make a complete switch over from the NCP to IP/TCP by 1 January 1983." The plan ran in stages.

The Internet Society calls the result a "flag-day" style transition, requiring all hosts to convert simultaneously, and notes that it had been planned within the community over several years and went surprisingly smoothly. On 1 January 1983 NCP was withdrawn from service. The switch also let the network be divided: the military operational traffic moved to a separate MILNET, while ARPANET carried on for research.

What came after

Two further pieces arrived within the same year. In November 1983 Paul Mockapetris published the Domain Name System, replacing the single host table with distributed name servers. And the Internet Society history singles out the inclusion of TCP/IP in the Berkeley Unix (BSD) releases as a critical step in spreading the protocols through the research community — every university running Unix on its machines could now speak the protocol.

The 1981 texts aged slowly. RFC 793 remained the TCP specification for more than forty years until RFC 9293, published in August 2022, obsoleted it and folded in the RFCs that had amended it. For how TCP and IP divide the work, see TCP/IP in the glossary; the surrounding events are on the Internet and Web timeline, and one of the protocol's designers on Vint Cerf.