tcp_checksum
u32 tcp_checksum:1;
csum_info->transmit.tcp_checksum = 1;
csum_info->transmit.tcp_checksum = 1;
tcph->check = tcp_checksum(tcph, payload_len);
tcph->check = tcp_checksum(tcph, payload_len);
tcphdr->check = tcp_checksum(tcphdr, PAYLOAD_LEN + extlen);
tcph->check = tcp_checksum(tcph, PAYLOAD_LEN);