RFC Request for Comments
Technical specification documents from the IETF — the definition of the internet protocols.
RFC 791 (IP), RFC 793 (TCP), RFC 2616 (HTTP/1.1), RFC 8446 (TLS 1.3). Numbered chronologically. "RFC 2119" defines keywords (MUST, SHOULD, MAY). Also a culture: "write an RFC" = formal technical design before implementation.