[tor-relays] What are the v1, v2, v3, v4 connections?

"Since startup, we have initiated 0 v1 connections, 1 v2 connections, 0 v3 connections, and 12345 v4 connections." What are these different types of connections? I can't find them in the docs, so I assume I'm not looking in the right place. (And what does it mean that the 1 v2 connection sneaked in?) Bill -- William Denton Toronto, Canada <https://www.miskatonic.org/>