Tcp Or Udp - HTTP/3 Explained
in recent years efforts to create new transport protocols have almost entirely been halted because of the difficulties in deploying them on the Internet. Deployment of new protocols is hampered by many firewalls, NATs, routers and other middle-boxes that only allow TCP or UDP are deployed between users and the servers they need to reach. Introducing another transport protocol makes N% of the connections fail because they are being blocked by boxes that see it not being UDP or TCP and thus evil or wrong somehow. The N% failure rate is often deemed too high to be worth the effort.