TCP checksum calculation

[bsd_newbie]

Can someone point me to the code that tries to redo tcp checksum as a result of mucking up with the ip source addrees.

[hydra]

Check out: /usr/src/sys/netinet. The checksum calculation is platform specific, the amd64 algorithm is for example located in: /usr/src/sys/amd64/amd64/in_cksum.c.