fill_networklayer
fill_networklayer(buf + ETH_HLEN, payload_len + sizeof(struct iphdr),
fill_networklayer(buf + ETH_HLEN + sizeof(struct iphdr),
fill_networklayer(buf + ETH_HLEN, payload_len, IPPROTO_TCP);
fill_networklayer(buf + ETH_HLEN, PAYLOAD_LEN, IPPROTO_TCP);