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 %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))