Function cubic_ssthresh_update

freebsd/netinet/cc/cc_cubic.c:452–484

* Update the ssthresh in the event of congestion. */

 * Update the ssthresh in the event of congestion.
 */
static void
cubic_ssthresh_update(struct cc_var *ccv, uint32_t maxseg)
{
	struct cubic *cubic_data;
	uint32_t ssthresh;
	uint32_t cwnd;

	cubic_data = ccv->cc_data;
	cwnd = CCV(ccv, snd_cwnd);

	/* Fast convergence heuristic. */
	if (cwnd < cubic_data->max_cwnd) {
		cwnd = ((uint64_t)cwnd * CUBIC_FC_FACTOR) >> CUBIC_SHIFT;
	}
	cubic_data->prev_max_cwnd = cubic_data->max_cwnd;
	cubic_data->max_cwnd = cwnd;

	/*
	 * On the first congestion event, set ssthresh to cwnd * 0.5
	 * and reduce max_cwnd to cwnd * beta. This aligns the cubic concave
	 * region appropriately. On subsequent congestion events, set
	 * ssthresh to cwnd * beta.
	 */
	if ((cubic_data->flags & CUBICFLAG_CONG_EVENT) == 0) {
		ssthresh = cwnd >> 1;
		cubic_data->max_cwnd = ((uint64_t)cwnd *
		    CUBIC_BETA) >> CUBIC_SHIFT;
	} else {
		ssthresh = ((uint64_t)cwnd *
		    CUBIC_BETA) >> CUBIC_SHIFT;
	}
	CCV(ccv, snd_ssthresh) = max(ssthresh, 2 * maxseg);
}

DECLARE_CC_MODULE(cubic, &cubic_cc_algo);
MODULE_VERSION(cubic, 1);