@HopperMCS @Yoruka @waff @rher @Zergling_man AP is a free protocol spec.

Mastodon implements AP, but always in an incompatible way, thus by default federation is broken to non-Mastodon and Pleroma and GNU Social developers have had to go and implement Mastodon's API for federation to work.

It was originally StatusNet, but as always, it was GNU who did the hard work that got it to work usably.