BIND9 递归查询超时机制

static inline void
fctx_setretryinterval(fetchctx_t *fctx, unsigned int rtt) {
	unsigned int seconds;
	unsigned int us;

	/*
	 * We retry every .8 seconds the first two times through the address
	 * list, and then we do exponential back-off.
	 */
	if (fctx->restarts < 3)
		us = 800000;
	else
		us = (800000 << (fctx->restarts - 2));

	/*
	 * Double the round-trip time.
	 */
	rtt *= 2;

	/*
	 * Always wait for at least the doubled round-trip time.
	 */
	if (us < rtt)
		us = rtt;

	/*
	 * But don't ever wait for more than 10 seconds.
	 */
	if (us > 10000000)
		us = 10000000;

	seconds = us / 1000000;
	us -= seconds * 1000000;
	isc_interval_set(&fctx->interval, seconds, us * 1000);
}