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

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 %s' % (destination, str(length))
            print cmd
            self._sendline(cmd)
            self._expect(cmd)
            # wait echo reply
            time.sleep(1)
        except Exception, e:
            ModuleHelper.WriteIntoDebugLogger("ping() Error: " + str(e))