RFC 4891:Using IPsec to Secure IPv6-in-IPv4 Tunnel...
RFC-Ref

decapsulator


Click on the red underlined text to get to the source

... address checks as mitigating measures: o To mitigate threat (1), the decapsulator verifies that the IPv4 source address ...
... configured tunnel endpoint. The decapsulator may also implement IPv4 ingress filtering ...
... interface. o To mitigate threat (2), the decapsulator verifies whether the inner IPv6 address is a valid ...



Google
Web
RFC-Ref