Function tcp_newreno_partial_ack

freebsd/netinet/tcp_input.c:3980–4011

* On a partial ack arrives, force the retransmission of the * next unacknowledged segment. Do not clear tp->t_dupacks. * By setting snd_nxt to ti_ack, this forces retransmission timer to * be started again. */

 * be started again.
 */
void
tcp_newreno_partial_ack(struct tcpcb *tp, struct tcphdr *th)
{
	tcp_seq onxt = tp->snd_nxt;
	uint32_t ocwnd = tp->snd_cwnd;
	u_int maxseg = tcp_maxseg(tp);

	INP_WLOCK_ASSERT(tp->t_inpcb);

	tcp_timer_activate(tp, TT_REXMT, 0);
	tp->t_rtttime = 0;
	tp->snd_nxt = th->th_ack;
	/*
	 * Set snd_cwnd to one segment beyond acknowledged offset.
	 * (tp->snd_una has not yet been updated when this function is called.)
	 */
	tp->snd_cwnd = maxseg + BYTES_THIS_ACK(tp, th);
	tp->t_flags |= TF_ACKNOW;
	(void) tp->t_fb->tfb_tcp_output(tp);
	tp->snd_cwnd = ocwnd;
	if (SEQ_GT(onxt, tp->snd_nxt))
		tp->snd_nxt = onxt;
	/*
	 * Partial window deflation.  Relies on fact that tp->snd_una
	 * not updated yet.
	 */
	if (tp->snd_cwnd > BYTES_THIS_ACK(tp, th))
		tp->snd_cwnd -= BYTES_THIS_ACK(tp, th);
	else
		tp->snd_cwnd = 0;
	tp->snd_cwnd += maxseg;
}

int
tcp_compute_pipe(struct tcpcb *tp)