def __pack_message(operation, data):
    """Takes message data and adds a message header based on the operation.

    Returns the resultant message string.
    """
    request_id = _randint()
    message = struct.pack("<i", 16 + len(data))
    message += struct.pack("<i", request_id)
    message += _ZERO_32  # responseTo
    message += struct.pack("<i", operation)
    return (request_id, message + data)