docs: def ping(self, destination, length=20):
          """ send ICMPv6 echo request with a given length to a unicast destination
              address
  
          Args:
              destination: the unicast destination address of ICMPv6 echo request
              length: the size of ICMPv6 echo request payload
          """
          print '%s call ping' % self.port
          print 'destination: %s' %destination
          try:
              cmd = 'ping %s -c 1 -s %s -I %s'  % (destination, str(length), WPAN_INTERFACE)
              if self._is_net:
                  ssh_stdin, ssh_stdout, ssh_stderr = self.handle.exec_command(cmd)
              else:
                  self._sendline(cmd)
                  self._expect(cmd)
              # wait echo reply
              time.sleep(1)
          except Exception, e:
              ModuleHelper.WriteIntoDebugLogger('ping() Error: ' + str(e))