def METHOD_NAME(self, zone, local_serial):
    """
    Returns a dict, the key being the outdated secondary name, and the value being the node's current zone serial.
    """
    outdated = {}
    for server in self.servers:
        remote_serial = query_serial(zone, server)
        if not remote_serial or remote_serial < local_serial:
            outdated[self.servers[server]] = remote_serial
    return outdated