Avoid a situation where we do not set the tcp persist timer after

a zero window condition.  If you send a 0-length packet, but there
is data is the socket buffer, and neither the rexmt or persist timer
is already set, then activate the persist timer.
From FreeBSD revision 284941; OK deraadt@ markus@ mikeb@ claudio@

1 files changed, 27 insertions, 1 deletions

diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index 734403d0c61..f52ea666340 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: tcp_output.c,v 1.112 2015/06/30 15:30:17 mpi Exp $	*/
+/*	$OpenBSD: tcp_output.c,v 1.113 2015/07/13 23:11:37 bluhm Exp $	*/
 /*	$NetBSD: tcp_output.c,v 1.16 1997/06/03 16:17:09 kml Exp $	*/
 
 /*
@@ -1008,6 +1008,32 @@ send:
 				tp->t_rxtshift = 0;
 			}
 		}
+
+		if (len == 0 && so->so_snd.sb_cc &&
+		    TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0 &&
+		    TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0) {
+			/*
+			 * Avoid a situation where we do not set persist timer
+			 * after a zero window condition. For example:
+			 * 1) A -> B: packet with enough data to fill the window
+			 * 2) B -> A: ACK for #1 + new data (0 window
+			 *    advertisement)
+			 * 3) A -> B: ACK for #2, 0 len packet
+			 *
+			 * In this case, A will not activate the persist timer,
+			 * because it chose to send a packet. Unless tcp_output
+			 * is called for some other reason (delayed ack timer,
+			 * another input packet from B, socket syscall), A will
+			 * not send zero window probes.
+			 *
+			 * So, if you send a 0-length packet, but there is data
+			 * in the socket buffer, and neither the rexmt or
+			 * persist timer is already set, then activate the
+			 * persist timer.
+			 */
+			tp->t_rxtshift = 0;
+			tcp_setpersist(tp);
+		}
 	} else
 		if (SEQ_GT(tp->snd_nxt + len, tp->snd_max))
 			tp->snd_max = tp->snd_nxt + len;