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))
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