I was trying to rsync from one computer to another. The destination was Fedora 30, the current version. The source was fedora 1, a ten year old version. And rsync complained about a protocol mismatch, and much googling, trial-and-error and brain-sweat didn't give me an answer.
So I played a trick on it. I used nfs to mount an export from the old computer, to a temporary directory on the new computer. Then I could rsync just fine, because as far as rsync was concerned, it was from Fedora 30 to Fedora 30.