UDP has always been used in at least VoIP and games. Basically any application where data is time-sensitive, and you'd rather not get a packet at all than have

UDP has always been used in at least VoIP and games. Basically any application where data is time-sensitive, and you'd rather not get a packet at all than have it retransmitted a bunch of times before it comes through. Also DNS. But then there's QUIC, which is a better TCP built on top of UDP. It's new, but there's already some adoption. > good luck getting end-to-end transit This isn't a TCP/UDP problem, you need to go down a level. This is because of NATs, which exist because the IPv4 address space can't possibly fit all those networked devices people use these days. IPv6 solves that. Why is its adoption so slow despite widespread support? I have no idea.