TCP checksum offloading for raw sockets

Hi Guys, How to use TCP checksum offloading for raw sockets. I can see rx/tx checksumming as enabled using ethool. But packets send out from machine have IP/UDP checksum as zero only. While making packet I am making checksum value in IP/UDP header as zero. No new value updated in checksum.

ethtool -k eth0 Offload parameters for eth0: rx-checksumming: on tx-checksumming: on scatter-gather: on tcp-segmentation-offload: on udp-fragmentation-offload: off generic-segmentation-offload: on generic-receive-offload: off large-receive-offload: off

-- Cheers Mukesh