I am able to send packets from node to sink/root mote, but not vice verse

I tried but it giving fatal error and simulator getting close.

1.

printf("DATA sending reply\n");

uip_ipaddr_copy(&server_conn->ripaddr, &UIP_IP_BUF->srcipaddr);

uip_udp_packet_send(server_conn, "Reply", sizeof("Reply"));

uip_create_unspecified(&server_conn->ripaddr);

2.

uip_udp_packet_sendto(server_conn, buf, sizeof(buf), &nodes[total_nodes-1].ip, UIP_HTONS(12345));

I tried by two ways, but for both cases not working

should i enable some configuration in project-conf/makefile ?

Thanks