Checksum offloading

software doesn't do checksum calcalation, it hand over an empty checksum field to the hardware and hardware does that automaticlly. 


Link:http://www.wireshark.org/docs/wsug_html_chunked/ChAdvChecksums.html


Recent network hardware can perform advanced features such as IP checksum calculation, also known as checksum offloading. The network driver won't calculate the checksum itself but will simply hand over an empty (zero or garbage filled) checksum field to the hardware.