TCP: send full initial window when timestamps are in use

The fastpath in tcp_output tries to send out
full segments, and avoid sending partial segments by
comparing against the static t_maxseg variable.
That value does not consider tcp options like timestamps,
while the initial window calculation is using
the correct dynamic tcp_maxseg() function.

Due to this interaction, the last, full size segment
is considered too short and not sent out immediately.

Reviewed by:	tuexen
MFC after:	2 weeks
Sponsored by:	NetApp, Inc.
Differential Revision:	https://reviews.freebsd.org/D26478

3 files changed, 22 insertions, 7 deletions

diff --git a/sys/netinet/tcp.h b/sys/netinet/tcp.h
index 0a5226836f0f..faf142959375 100644
--- a/sys/netinet/tcp.h
+++ b/sys/netinet/tcp.h
@@ -80,6 +80,8 @@ struct tcphdr {
 	u_short	th_urp;			/* urgent pointer */
 };
 
+#define	PADTCPOLEN(len)		((((len) / 4) + !!((len) % 4)) * 4)
+
 #define	TCPOPT_EOL		0
 #define	   TCPOLEN_EOL			1
 #define	TCPOPT_PAD		0		/* padding after EOL */
diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index 2485651df6e7..a7611ceb66f7 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -591,6 +591,20 @@ after_sack_rexmit:
 		if (len >= tp->t_maxseg)
 			goto send;
 		/*
+		 * As the TCP header options are now
+		 * considered when setting up the initial
+		 * window, we would not send the last segment
+		 * if we skip considering the option length here.
+		 * Note: this may not work when tcp headers change
+		 * very dynamically in the future.
+		 */
+		if ((((tp->t_flags & TF_SIGNATURE) ?
+			PADTCPOLEN(TCPOLEN_SIGNATURE) : 0) +
+		    ((tp->t_flags & TF_RCVD_TSTMP) ?
+			PADTCPOLEN(TCPOLEN_TIMESTAMP) : 0) +
+		    len) >= tp->t_maxseg)
+			goto send;
+		/*
 		 * NOTE! on localhost connections an 'ack' from the remote
 		 * end may occur synchronously with the output and cause
 		 * us to flush a buffer queued with moretocome.  XXX
diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index fcd8e475e066..a99898b8081e 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -3013,7 +3013,6 @@ tcp_maxseg(const struct tcpcb *tp)
 	 * but this is harmless, since result of tcp_maxseg() is used
 	 * only in cwnd and ssthresh estimations.
 	 */
-#define	PAD(len)	((((len) / 4) + !!((len) % 4)) * 4)
 	if (TCPS_HAVEESTABLISHED(tp->t_state)) {
 		if (tp->t_flags & TF_RCVD_TSTMP)
 			optlen = TCPOLEN_TSTAMP_APPA;
@@ -3021,26 +3020,26 @@ tcp_maxseg(const struct tcpcb *tp)
 			optlen = 0;
 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 		if (tp->t_flags & TF_SIGNATURE)
-			optlen += PAD(TCPOLEN_SIGNATURE);
+			optlen += PADTCPOLEN(TCPOLEN_SIGNATURE);
 #endif
 		if ((tp->t_flags & TF_SACK_PERMIT) && tp->rcv_numsacks > 0) {
 			optlen += TCPOLEN_SACKHDR;
 			optlen += tp->rcv_numsacks * TCPOLEN_SACK;
-			optlen = PAD(optlen);
+			optlen = PADTCPOLEN(optlen);
 		}
 	} else {
 		if (tp->t_flags & TF_REQ_TSTMP)
 			optlen = TCPOLEN_TSTAMP_APPA;
 		else
-			optlen = PAD(TCPOLEN_MAXSEG);
+			optlen = PADTCPOLEN(TCPOLEN_MAXSEG);
 		if (tp->t_flags & TF_REQ_SCALE)
-			optlen += PAD(TCPOLEN_WINDOW);
+			optlen += PADTCPOLEN(TCPOLEN_WINDOW);
 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 		if (tp->t_flags & TF_SIGNATURE)
-			optlen += PAD(TCPOLEN_SIGNATURE);
+			optlen += PADTCPOLEN(TCPOLEN_SIGNATURE);
 #endif
 		if (tp->t_flags & TF_SACK_PERMIT)
-			optlen += PAD(TCPOLEN_SACK_PERMITTED);
+			optlen += PADTCPOLEN(TCPOLEN_SACK_PERMITTED);
 	}
 #undef PAD
 	optlen = min(optlen, TCP_MAXOLEN);