simple mail transfer protocol (smtp august 1982, rfc 821 (j. postal) standard for the format of arpa internet text messages august 13, 1982 (d.crocker)

i always thought it was brought in place earlier but as geoff huston writes in the mythology of ip version 6 it was

In January 1983, the Advanced Research Projects Agency Network (ARPANET) experienced a "flag day," and the Network Control Protocol, NCP, was turned off, and TCP/IP was turned on.

and i learned from david h. crocker's page: "A common error about the "T" in TCP/IP is, that it stands for transport or transfer whereas the "T" stands for transmission." (out of crocker's review of: "ruling the root" by milton l. mueller). to get the tast of the discussion it's worth to read mueller's response and crocker's follow-up.

dave crocker outlined 1992 the road to a new ip and adds to his text:

If you've wondered about the beginnings of the new version of IP (IPv6) this articles gives some background. Back when IPv6's goals were clearer, so were the technical solutions.