I believe the default protocol for AH is UDP. This is a connectionless protocol and has the capabilty to transfer more data in less time.
However, if your connection is suffering then it is likely data will be lost and neither the host nor your client will know about it. Because of this I think AH does a quick connection check when you first log in and decides if your connection is suitable for UDP. If packets have been dropped then the TCP protocol is forced. TCP is connection oriented and will request retransmissions when packets are dropped.
Some firewalls and routers do not pass UDP by default or at all.