Return the nb^th layer that is an instance of cls, matching flt
values.

def getlayer(self, cls, nb=1, _track=None, _subclass=None, **flt):
        """Return the nb^th layer that is an instance of cls, matching flt
values.
        """
        if _subclass is None:
            _subclass = self.match_subclass or None
        if _subclass:
            match = lambda cls1, cls2: issubclass(cls1, cls2)
        else:
            match = lambda cls1, cls2: cls1 == cls2
        if isinstance(cls, int):
            nb = cls + 1
            cls = None
        if isinstance(cls, str) and "." in cls:
            ccls, fld = cls.split(".", 1)
        else:
            ccls, fld = cls, None
        if cls is None or match(self.__class__, cls) \
           or ccls in [self.__class__.__name__, self._name]:
            if all(self.getfieldval(fldname) == fldvalue
                   for fldname, fldvalue in six.iteritems(flt)):
                if nb == 1:
                    if fld is None:
                        return self
                    else:
                        return self.getfieldval(fld)
                else:
                    nb -= 1
        for f in self.packetfields:
            fvalue_gen = self.getfieldval(f.name)
            if fvalue_gen is None:
                continue
            if not f.islist:
                fvalue_gen = SetGen(fvalue_gen, _iterpacket=0)
            for fvalue in fvalue_gen:
                if isinstance(fvalue, Packet):
                    track = []
                    ret = fvalue.getlayer(cls, nb=nb, _track=track,
                                          _subclass=_subclass, **flt)
                    if ret is not None:
                        return ret
                    nb = track[0]
        return self.payload.getlayer(cls, nb=nb, _track=_track,
                                     _subclass=_subclass, **flt)