#1457 mod_s2s_keepalive doesn't do anything on failure
I have mod_s2s_keepalive loaded on my server. It will periodically send a ping to all s2s links.
However, it will not take any action if the ping it sent expires, especially not try to tear down and restart the s2s connection. Thus, broken s2s links (think yax.im <--> muc.xmpp.org) tend to stay broken / unidirectional for a long time until manual admin intervention.