UDP, defined in RFC 768 (August 1980), is a thin envelope around IP that adds only source/destination ports and a checksum. It does not retransmit, does not order packets, does not adapt its rate. Everything else — reliability, congestion control, encryption — has to be built on top by the application. That apparent weakness is the whole point: by leaving those decisions to the application, UDP lets streaming protocols make different tradeoffs than HTTP does.
In streaming, UDP is the substrate for everything that needs low latency or selective reliability. SRT and RIST build adaptive retransmit windows on top of UDP for contribution; WebRTC's media path uses SRTP over UDP and tolerates packet loss to avoid the wait of retransmission; QUIC (RFC 9000, May 2021) implements its own ordered, reliable streams over UDP and serves HTTP/3. The 2026 trend is that more and more of the streaming stack quietly moves to UDP underneath while keeping a familiar API on top.
The operational catch with UDP is firewalls. Many corporate and ISP networks rate-limit or block arbitrary UDP, and almost all NATs handle UDP differently from TCP. A WebRTC or QUIC pipeline must include fallback to TCP-based alternatives (TURN over TCP/443, HTTP/2 fallback for QUIC) for the percentage of users whose network silently kills UDP.
