Hi, I wanted to discuss the Fast Recovery code in FreeBSD, specifically
when we come out of Fast Recovery. (This is a slightly modified code)

==> IF SACK ENABLE
if (tp->t_dupacks >= tcprexmtthresh) {
/* Check for a partial ACK */
if (tcp_sack_partialack(tp, th)) {
tp->snd_cwnd += tp->t_maxseg;
needoutput = 1;
} else {
/* Out of fast recovery */
tp->snd_cwnd = tp->snd_ssthresh;
if (tcp_seq_subtract(tp->snd_max,
th->th_ack) < tp->snd_ssthresh)
tp->snd_cwnd =
tcp_seq_subtract(tp->snd_max,
th->th_ack) + tp->t_maxseg;
tp->t_dupacks = 0;
}
}

Consider that SACK is enabled. We have more than tcprexmtthresh dupacks
so far. This is the ACK that acks the lost and fast retransmitted
packet. It is not a partial ACK.

So the out of fast recovery code.

tp->snd_cwnd = tp->snd_ssthresh; /* set cwnd to ssthresh */

if (tcp_seq_subtract(tp->snd_max, th->th_ack) < tp->snd_ssthresh)
tp->snd_cwnd = tcp_seq_subtract(tp->snd_max, th->th_ack) +
tp->t_maxseg;

So if snd_max - th_ack < snd_ssthresh, i.e. we did not send
snd_ssthresh worth of data during Fast Recovery, we set

snd_cwnd = (snd_max - th_ack) + t_maxseg

This looks like a problem to me. The RFC says just set cwnd to
ssthresh. In this code if we sent little, we would scale down the
congestion window for no reason. Shouldnt we just set it to
snd_ssthresh.

Am I reading this wrong, or is this how it is supposed to be. Comments.