TCP · Transmission Control Protocol
The ubiquitous reliable IP transport; it runs on any network without lossless tuning but costs latency, which is why hot GPU data paths use RDMA and reserve TCP for reach and simplicity.
Current numbers
Port 502Classic Modbus TCP is unauthenticated, and FrostyGoop used it; Claroty finds over 80% of data-center OT on BACnet/Modbus-class protocols