Date: Thu, 14 Mar 2013 18:32:17 +0530 From: Bejoy Thomas <bejoygthomas@yahoo.com> To: freebsd-net@freebsd.org Subject: Question regarding FreeBSD Congestion control Message-ID: <EE7BE1B3-8CD1-4899-AF80-B6ECEABD33B8@yahoo.com>
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I have been looking at the FreeBSD congestion control code and had some =
questions on the SACK enabled code flow . =20
With SACK the congestion window is retained as 1 MSS when the Fast =
Retransmit is exited and Fast Recovery is entered,so cwnd growth seems =
to be much slower than slow start, as we we are re transmitting drops =
from the sack hole or new data. But in non-SACK case the congestion =
window is restored to cwnd/2 +(dupacks-snd_limited)*MSS when Fast =
retransmit is exited as per RFC3782. Could some one familiar with the =
code please throw some light on why we fall back to a cwnd of 1MSS as we =
enter fast recovery with SACK enabled.
/*Congestion signal before ack. =
*/
cc_cong_signal(tp, th, =
CC_NDUPACK);
cc_ack_received(tp, th, =
CC_DUPACK);
tcp_timer_activate(tp, TT_REXMT, =
0);
tp->t_rtttime =3D 0;
if (tp->t_flags & =
TF_SACK_PERMIT) {
TCPSTAT_INC(
=
tcps_sack_recovery_episode);
tp->sack_newdata =3D =
tp->snd_nxt;
tp->snd_cwnd =3D =
tp->t_maxseg; <<<<<<<<<
(void) tcp_output(tp);
goto drop; =
<<<<<<<<<<
}
tp->snd_nxt =3D th->th_ack;
tp->snd_cwnd =3D tp->t_maxseg;
(void) tcp_output(tp);
KASSERT(tp->snd_limited <=3D 2,
("%s: tp->snd_limited too =
big",
__func__));
tp->snd_cwnd =3D =
tp->snd_ssthresh +
tp->t_maxseg *
(tp->t_dupacks - =
tp->snd_limited); <<<<<<<<<
Bejoy Thomas=
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