def s2i_frameid(x):
    """ Get pnio frame ID from a representation name

    Performs a reverse look-up in PNIO_FRAME_IDS dictionary

    :param x: a value of PNIO_FRAME_IDS dict
    :returns: integer
    """
    try:
        return {
            "RT_CLASS_3": 0x0100,
            "RT_CLASS_1": 0x8000,
            "RT_CLASS_UDP": 0xC000,
            "FragmentationFrameID": 0xFF80,
        }[x]
    except KeyError:
        pass
    try:
        return next(key for key, value in six.iteritems(PNIO_FRAME_IDS)
                    if value == x)
    except StopIteration:
        pass
    return x