Function ng_l2tp_seq_recv_nr

freebsd/netgraph/ng_l2tp.c:1310–1414

* Handle receipt of an acknowledgement value (Nr) from peer. */

 * Handle receipt of an acknowledgement value (Nr) from peer.
 */
static void
ng_l2tp_seq_recv_nr(priv_p priv, u_int16_t nr)
{
	struct l2tp_seq *const seq = &priv->seq;
	struct mbuf	*xwin[L2TP_MAX_XWIN];	/* partial local copy */
	int		nack;
	int		i, j;
	uint16_t	ns;

	mtx_lock(&seq->mtx);

	/* Verify peer's ACK is in range */
	if ((nack = L2TP_SEQ_DIFF(nr, seq->rack)) <= 0) {
		mtx_unlock(&seq->mtx);
		return;				/* duplicate ack */
	}
	if (L2TP_SEQ_DIFF(nr, seq->ns) > 0) {
		mtx_unlock(&seq->mtx);
		priv->stats.recvBadAcks++;	/* ack for packet not sent */
		return;
	}
	KASSERT(nack <= L2TP_MAX_XWIN,
	    ("%s: nack=%d > %d", __func__, nack, L2TP_MAX_XWIN));

	/* Update receive ack stats */
	seq->rack = nr;
	seq->rexmits = 0;

	/* Free acknowledged packets and shift up packets in the xmit queue */
	for (i = 0; i < nack; i++)
		m_freem(seq->xwin[i]);
	memmove(seq->xwin, seq->xwin + nack,
	    (L2TP_MAX_XWIN - nack) * sizeof(*seq->xwin));
	memset(seq->xwin + (L2TP_MAX_XWIN - nack), 0,
	    nack * sizeof(*seq->xwin));

	/*
	 * Do slow-start/congestion avoidance windowing algorithm described
	 * in RFC 2661, Appendix A. Here we handle a multiple ACK as if each
	 * ACK had arrived separately.
	 */
	if (seq->cwnd < seq->wmax) {
		/* Handle slow start phase */
		if (seq->cwnd < seq->ssth) {
			seq->cwnd += nack;
			nack = 0;
			if (seq->cwnd > seq->ssth) {	/* into cg.av. phase */
				nack = seq->cwnd - seq->ssth;
				seq->cwnd = seq->ssth;
			}
		}

		/* Handle congestion avoidance phase */
		if (seq->cwnd >= seq->ssth) {
			seq->acks += nack;
			while (seq->acks >= seq->cwnd) {
				seq->acks -= seq->cwnd;
				if (seq->cwnd < seq->wmax)