UDP and application-layer protocols

The specification of UDP, RFC 768, is dated 28 August 1980 and signed by Jon Postel. It is three pages long. It defines four header fields of 16 bits each and says, in effect, that everything else is somebody else's problem.

That restraint is the point. Forty years later the most ambitious transport protocol of the decade, QUIC, standardised in May 2021 and carrying HTTP/3, chose to run on top of those eight bytes rather than replace them.