When customers of retail Internet service start demanding a NAT
standard, then that's when the IETF might want to think about
documenting the standard that the market seems to want.
here's the only thing that a NAT standard should say:
an intermediary MUST NOT alter the source or destination field in an IP
header.
an intermediary MUST NOT route an IP packet to other than its intended
destination.
an intermediary MUST NOT alter the payload of an IP packet.