How to calculate TCP checksum

Question

I am writing a Kernel Module that uses Netfilter hooks to modify some of the TCP header information and obviously, before sending, I want to re-calculate the checksum.
I al

Answer 1

@ugoren's answer is not precise. According to RFC1624 https://tools.ietf.org/html/rfc1624, this will sometimes produce -0 (0xFFFF), which is not allowed.

The correct way to compute checksum should be: new_tcp_check = ~(~old_tcp_check + ~old_val + new_val)