Return a Null header object reconstructed from raw bytes, or an
        Exception if we can't resurrect the packet.

def from_bytes(self, raw):
        '''Return a Null header object reconstructed from raw bytes, or an
        Exception if we can't resurrect the packet.'''
        if len(raw) < 4:
            raise NotEnoughDataError("Not enough bytes ({}) to reconstruct a Null object".format(len(raw)))
        fields = struct.unpack('=I', raw[:4])
        self._af = fields[0]
        return raw[4:]