UDP

TCP/IP achieves poor round-trip latency and cannot approach the raw capabilities of next-generation high-speed networks like Myrinet and ATM. TCP was designed for wide-area networks and is not optimized for local-area or high-bandwidth, high-latency connections.

Middleboxes block non-TCP protocols (UDP, DCCP, SCTP) in corporate networks and censoring countries, forcing applications back to TCP despite TCP being unsuitable for many use cases.

TCP's reliability guarantees, connection overhead (3-way handshake), and flow control make it unsuitable for applications prioritizing speed over reliability, such as video calls, online games, and live streams, forcing developers to use UDP instead and lose reliability benefits.