Historically, every ICMP error message has included the Internet header and at least the first 8 data bytes of the datagram that triggered the error. This is no longer adequate, due to the use of IP-in-IP tunneling and other technologies. Therefore, the ICMP datagram SHOULD contain as much of the original datagram as possible without the length of the ICMP datagram exceeding 576 bytes. The returned IP header (and user data) MUST be identical to that which was received, except that the router is not required to undo any modifications to the IP header that are normally performed in forwarding that were performed before the error was detected (e.g., decrementing the TTL, or updating options). Note that the requirements of Section [126.96.36.199] supersede this requirement in some cases (i.e., for a Parameter Problem message, if the problem is in a modified field, the router must undo the modification). See Section [188.8.131.52]).