IPv6 Authentication Header uses MD5 hash on the payload, the header, and unchanging fields of the IPv6 header and options to prevent sequence quessing attacks. But all these required information to calculate this hash are also available to any attacker that spies the connection (and impersonnated the real host's IP address), since they also are in the packet.
So what guarranties the attacker cannot steal the real host's connexion?
Also, what payload is processed in the MD5? The payload of the current packet or the payload of the previous packet?