sub process_packet {
       my($user_data, $header, $packet) = @_;

       print "time: $header->{tv_sec}\n";
   }

  # The header information is a reference to a hash containing the following fields.
  # 
  #  *   "len" - the total length of the packet.
  #  *   "caplen" - the actual captured length of the packet
  #  *   "tv_sec" - seconds value of the packet timestamp.
  #  *   "tv_usec" - microseconds value of the packet timestamp.