6in4, sometimes referred to as SIT,[a] is an IPv6 transition mechanism for migrating from Internet Protocol version 4 (IPv4) to IPv6.
It is a tunneling protocol that encapsulates IPv6 packets on specially configured IPv4 links according to the specifications of RFC 4213.
[citation needed] Some NAT devices automatically permit transparent operation of 6in4.
The heartbeat protocol signals the other side of the tunnel with its current endpoint location.
This problem can partially be solved by implementing network ingress filtering (not near the exit point but close to the true source) or with IPsec.