Function chd_ack_received

freebsd/netinet/cc/cc_chd.c:234–302

* All ACK signals are used for timing measurements to determine delay-based * congestion. However, window increases are only performed when * ack_type == CC_ACK. */

 * ack_type == CC_ACK.
 */
static void
chd_ack_received(struct cc_var *ccv, uint16_t ack_type)
{
	struct chd *chd_data;
	struct ertt *e_t;
	int backoff, new_measurement, qdly, rtt;

	e_t = khelp_get_osd(CCV(ccv, osd), ertt_id);
	chd_data = ccv->cc_data;
	new_measurement = e_t->flags & ERTT_NEW_MEASUREMENT;
	backoff = qdly = 0;

	chd_data->maxrtt_in_rtt = imax(e_t->rtt, chd_data->maxrtt_in_rtt);

	if (new_measurement) {
		/*
		 * There is a new per RTT measurement, so check to see if there
		 * is delay based congestion.
		 */
		rtt = V_chd_use_max ? chd_data->maxrtt_in_rtt : e_t->rtt;
		chd_data->maxrtt_in_rtt = 0;

		if (rtt && e_t->minrtt && !IN_RECOVERY(CCV(ccv, t_flags))) {
			qdly = rtt - e_t->minrtt;
			if (qdly > V_chd_qmin) {
				/*
				 * Probabilistic delay based congestion
				 * indication.
				 */
				backoff = should_backoff(qdly,
				    e_t->maxrtt - e_t->minrtt, chd_data);
			} else
				chd_data->loss_compete = 0;
		}
		/* Reset per RTT measurement flag to start a new measurement. */
		e_t->flags &= ~ERTT_NEW_MEASUREMENT;
	}

	if (backoff) {
		/*
		 * Update shadow_w before delay based backoff.
		 */
		if (chd_data->loss_compete ||
		    qdly > chd_data->prev_backoff_qdly) {
			/*
			 * Delay is higher than when we backed off previously,
			 * so it is possible that this flow is competing with
			 * loss based flows.
			 */
			chd_data->shadow_w = max(CCV(ccv, snd_cwnd),
			    chd_data->shadow_w);
		} else {
			/*
			 * Reset shadow_w, as it is probable that this flow is
			 * not competing with loss based flows at the moment.
			 */
			chd_data->shadow_w = 0;
		}