Total Number of Packets Sent and Received in Contiki

I'm trying to get the total number of packets transmitted and packets received in RPL. I'm using upd-client.c and udp-server.c and I added printf lines in both files as the following:

PROCESS(udp_client_process, "UDP client process");
AUTOSTART_PROCESSES(&udp_client_process);
/*---------------------------------------------------------------------------*/
static void
tcpip_handler(void)
{
  char *str;

  if(uip_newdata()) {
    str = uip_appdata;
    str[uip_datalen()] = '\0';
    printf("Data recv");
    //printf("DATA recv '%s'\n", str);
  }
}
/*---------------------------------------------------------------------------*/
static void
send_packet(void *ptr)
{
  static int seq_id;
  char buf[MAX_PAYLOAD_LEN];

  seq_id++;
  printf("Data send");
  PRINTF("DATA send to %d 'Hello %d'\n",
         server_ipaddr.u8[sizeof(server_ipaddr.u8) - 1], seq_id);
  sprintf(buf, "Hello %d from the client", seq_id);
  uip_udp_packet_sendto(client_conn, buf, strlen(buf),
                        &server_ipaddr, UIP_HTONS(UDP_SERVER_PORT));
}

PROCESS(udp_server_process, "UDP server process");
AUTOSTART_PROCESSES(&udp_server_process);
/*---------------------------------------------------------------------------*/
static void
tcpip_handler(void)
{
  char *appdata;

  if(uip_newdata()) {
    appdata = (char *)uip_appdata;
    appdata[uip_datalen()] = 0;
    printf("Data recv");
   // PRINTF("DATA recv '%s' from ", appdata);
    PRINTF("%d",
           UIP_IP_BUF->srcipaddr.u8[sizeof(UIP_IP_BUF->srcipaddr.u8) - 1]);
    PRINTF("\n");
#if SERVER_REPLY
    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);
#endif
  }
}

Then in the simulation script editor, I counted the printed messages:

while (true) {
if (msg) {
    if(msg.startsWith("Data send")) {
        num_messages_tx += 1;
    }
    if(msg.startsWith("Data recv")) {
        num_messages_rx += 1;
    }
}

I'm not sure what I'm doing wrong but it seems that I always have equal numbers of transmitted and received packets.

I appreciate your help.. Hanin

in the project-conf.h define below macros and the contiki link-stats.c count them for you.

#define LOG_CONF_LEVEL_MAC LOG_LEVEL_INFO
#define LINK_STATS_CONF_PACKET_COUNTERS 1