Open the file location -->/home/user/Desktop/contiki-ng/os/net/ipv6
simple-udp.c file

First, register a socket, both at the sender and receiver.

#include "net/ipv6/simple-udp.h"
simple_udp_register(struct simple_udp_connection *c,
  uint16_t local_port,
  uip_ipaddr_t *remote_addr,
  uint16_t remote_port,
  simple_udp_callback receive_callback)

  For each incoming datagram, the function above will be called with parameters that provide source/destination addresses and ports, and the payload.

c->receive_callback(c,
  &(UIP_IP_BUF->srcipaddr),
  UIP_HTONS(UIP_UDP_BUF->srcport),
  &(UIP_IP_BUF->destipaddr),
  UIP_HTONS(UIP_UDP_BUF->destport),
  databuffer, uip_datalen());