'Expire probes and "memorized" flows
Each of these should only have a limited lifetime.'

def _do_expire(self):

    t = time.time()
    for (ip, expire_at) in self.outstanding_probes.items():
        if (t > expire_at):
            self.outstanding_probes.pop(ip, None)
            if (ip in self.live_servers):
                self.log.warn('Server    %s    down', ip)
                del self.live_servers[ip]
    c = len(self.memory)
    self.memory = {k: v for (k, v) in self.memory.items() if (not v.is_expired)}
    if (len(self.memory) != c):
        self.log.debug('Expired    %i    flows', (c - len(self.memory)))
