From 28257b5cccce8b2e835710a878f3705e77563ce5 Mon Sep 17 00:00:00 2001
From: Matthew Dillon <dillon@FreeBSD.org>
Date: Thu, 10 Oct 2002 19:21:50 +0000
Subject: [PATCH] Update various comments mainly related to retransmit/FIN that
 I documented while working on a previous bug.

Fix a PERSIST bug.  Properly account for a FIN sent during a PERSIST.

MFC after:	7 days
---
 sys/netinet/tcp_output.c | 42 ++++++++++++++++++++++++++++++++++------
 1 file changed, 36 insertions(+), 6 deletions(-)

diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index 845aff8befa4..944e7eeff17c 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -215,6 +215,18 @@ tcp_output(struct tcpcb *tp)
 		}
 	}
 
+	/*
+	 * If snd_nxt == snd_max and we have transmitted a FIN, the 
+	 * offset will be > 0 even if so_snd.sb_cc is 0, resulting in
+	 * a negative length.  This can also occur when tcp opens up
+	 * its congestion window while receiving additional duplicate
+	 * acks after fast-retransmit because TCP will reset snd_nxt
+	 * to snd_max after the fast-retransmit.
+	 *
+	 * In the normal retransmit-FIN-only case, however, snd_nxt will
+	 * be set to snd_una, the offset will be 0, and the length may
+	 * wind up 0.
+	 */
 	len = (long)ulmin(so->so_snd.sb_cc, win) - off;
 
 	if ((taop = tcp_gettaocache(&tp->t_inpcb->inp_inc)) == NULL) {
@@ -252,7 +264,7 @@ tcp_output(struct tcpcb *tp)
 		/*
 		 * If FIN has been sent but not acked,
 		 * but we haven't been called to retransmit,
-		 * len will be -1.  Otherwise, window shrank
+		 * len will be < 0.  Otherwise, window shrank
 		 * after we sent into it.  If window shrank to 0,
 		 * cancel pending retransmit, pull snd_nxt back
 		 * to (closed) window, and set the persist timer
@@ -268,6 +280,12 @@ tcp_output(struct tcpcb *tp)
 				tcp_setpersist(tp);
 		}
 	}
+
+	/*
+	 * len will be >= 0 after this point.  Truncate to the maximum
+	 * segment length and ensure that FIN is removed if the length
+	 * no longer contains the last data byte.
+	 */
 	if (len > tp->t_maxseg) {
 		len = tp->t_maxseg;
 		sendalot = 1;
@@ -336,7 +354,8 @@ tcp_output(struct tcpcb *tp)
 	}
 
 	/*
-	 * Send if we owe peer an ACK.
+	 * Send if we owe the peer an ACK, RST, SYN, or urgent data.  ACKNOW
+	 * is also a catch-all for the retransmit timer timeout case.
 	 */
 	if (tp->t_flags & TF_ACKNOW)
 		goto send;
@@ -347,8 +366,7 @@ tcp_output(struct tcpcb *tp)
 		goto send;
 	/*
 	 * If our state indicates that FIN should be sent
-	 * and we have not yet done so, or we're retransmitting the FIN,
-	 * then we need to send.
+	 * and we have not yet done so, then we need to send.
 	 */
 	if (flags & TH_FIN &&
 	    ((tp->t_flags & TF_SENTFIN) == 0 || tp->snd_nxt == tp->snd_una))
@@ -794,7 +812,7 @@ tcp_output(struct tcpcb *tp)
 
 		/*
 		 * Set retransmit timer if not currently set,
-		 * and not doing an ack or a keep-alive probe.
+		 * and not doing a pure ack or a keep-alive probe.
 		 * Initial value for retransmit timer is smoothed
 		 * round-trip time + 2 * round-trip time variance.
 		 * Initialize shift counter which is used for backoff
@@ -809,9 +827,21 @@ tcp_output(struct tcpcb *tp)
 			callout_reset(tp->tt_rexmt, tp->t_rxtcur,
 				      tcp_timer_rexmt, tp);
 		}
-	} else
+	} else {
+		/*
+		 * Persist case, update snd_max but since we are in
+		 * persist mode (no window) we do not update snd_nxt.
+		 */
+		int xlen = len;
+		if (flags & TH_SYN)
+			++xlen;
+		if (flags & TH_FIN) {
+			++xlen;
+			tp->t_flags |= TF_SENTFIN;
+		}
 		if (SEQ_GT(tp->snd_nxt + len, tp->snd_max))
 			tp->snd_max = tp->snd_nxt + len;
+	}
 
 #ifdef TCPDEBUG
 	/*