I used to write processing software for stock exchange data feeds such as OPRA (https://www.opradata.com/ https://www.opradata.com/), they were UDP only at the

I used to write processing software for stock exchange data feeds such as OPRA (https://www.opradata.com/ https://www.opradata.com/), they were UDP only at the time. Of course packets could be lost so there actually was a paper based form you could fill in and re-request the missed data packet by fax (no-one ever did that, though). I think by now they implemented a TCP/IP connection to request re-transmissions these days.