As stated in Section [5.2.2], a router MUST verify the IP checksum of any packet that is received, and MUST discard messages containing invalid checksums. The router MUST NOT provide a means to disable this checksum verification.
A router MAY use incremental IP header checksum updating when the only change to the IP header is the time to live. This will reduce the possibility of undetected corruption of the IP header by the router. See [INTERNET:6] for a discussion of incrementally updating the checksum.
A more extensive description of the IP checksum, including extensive implementation hints, can be found in [INTERNET:6] and [INTERNET:7].