def protocol_str(protocol):
    """Convert internal API protocol to string."""
    if protocol == const.PROTOCOL_MRP:
        return 'MRP'
    if protocol == const.PROTOCOL_DMAP:
        return 'DMAP'
    if protocol == const.PROTOCOL_AIRPLAY:
        return 'AirPlay'
    return 'Unknown'
Convert internal API protocol to string.