When SYN or SYN/ACK had to be retransmitted RFC5681 requires us to

reduce the initial CWND to one segment.  This reduction got lost
some time ago due to a change in initialization ordering.

Additionally in tcp_timer_rexmt() avoid entering fast recovery when
we're still in TCPS_SYN_SENT state.

MFC after:	2 weeks

3 files changed, 19 insertions, 6 deletions

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index 62819cf..8bf1b0c 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -345,10 +345,16 @@ cc_conn_init(struct tcpcb *tp)
 	/*
 	 * Set the initial slow-start flight size.
 	 *
-	 * RFC3390 says only do this if SYN or SYN/ACK didn't got lost.
-	 * XXX: We currently check only in syncache_socket for that.
+	 * RFC5681 Section 3.1 specifies the default conservative values.
+	 * RFC3390 specifies slightly more aggressive values.
+	 *
+	 * If a SYN or SYN/ACK was lost and retransmitted, we have to
+	 * reduce the initial CWND to one segment as congestion is likely
+	 * requiring us to be cautious.
 	 */
-	if (V_tcp_do_rfc3390)
+	if (tp->snd_cwnd == 1)
+		tp->snd_cwnd = tp->t_maxseg;		/* SYN(-ACK) lost */
+	else if (V_tcp_do_rfc3390)
 		tp->snd_cwnd = min(4 * tp->t_maxseg,
 		    max(2 * tp->t_maxseg, 4380));
 	else {
diff --git a/sys/netinet/tcp_syncache.c b/sys/netinet/tcp_syncache.c
index 55cca73..adb175b 100644
--- a/sys/netinet/tcp_syncache.c
+++ b/sys/netinet/tcp_syncache.c
@@ -852,11 +852,12 @@ syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
 	tcp_mss(tp, sc->sc_peer_mss);
 
 	/*
-	 * If the SYN,ACK was retransmitted, reset cwnd to 1 segment.
+	 * If the SYN,ACK was retransmitted, indicate that CWND to be
+	 * limited to one segment in cc_conn_init().
 	 * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
 	 */
 	if (sc->sc_rxmits > 1)
-		tp->snd_cwnd = tp->t_maxseg;
+		tp->snd_cwnd = 1;
 
 #ifdef TCP_OFFLOAD
 	/*
diff --git a/sys/netinet/tcp_timer.c b/sys/netinet/tcp_timer.c
index e01600c..979c4b3 100644
--- a/sys/netinet/tcp_timer.c
+++ b/sys/netinet/tcp_timer.c
@@ -539,7 +539,13 @@ tcp_timer_rexmt(void * xtp)
 	}
 	INP_INFO_RUNLOCK(&V_tcbinfo);
 	headlocked = 0;
-	if (tp->t_rxtshift == 1) {
+	if (tp->t_state == TCPS_SYN_SENT) {
+		/*
+		 * If the SYN was retransmitted, indicate CWND to be
+		 * limited to 1 segment in cc_conn_init().
+		 */
+		tp->snd_cwnd = 1;
+	} else if (tp->t_rxtshift == 1) {
 		/*
 		 * first retransmit; record ssthresh and cwnd so they can
 		 * be recovered if this turns out to be a "bad" retransmit.