build_udp_header
build_udp_header(buf + el3_len + ol3_len + ol4_len + il3_len,
build_udp_header(buf + el3_len + ol3_len,
off += build_udp_header(tbuf + off, payload_len);
cur += build_udp_header(cur, UDP_SRC_PORT, VN_PORT, pktlen);
build_udp_header(buf, sport, dport, payload_len);