As noted in RFC 2766 [RFC2766], NAT-PT and end-to-end security do not work together. When an IPv6-only node (X) initiates communication to IPv4-only node Y, the packets from X have certain IPv6 source and destination addresses which are both used in IPSec (AH or ESP) and TCP/UDP/ICMP checksum computations. Since NAT-PT translates the IPv6 address of X into an IPv4 address that has no relationship to X’s IPv6 address, there is no way for recipient Y to determine X’s IPv6 address and in that way verify the checksums.