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Date:      Thu, 21 May 2020 19:46:11 +0000 (UTC)
From:      Richard Scheffenegger <rscheff@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-12@freebsd.org
Subject:   svn commit: r361342 - in stable/12/sys/netinet: . tcp_stacks
Message-ID:  <202005211946.04LJkBme027322@repo.freebsd.org>
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Author: rscheff
Date: Thu May 21 19:46:11 2020
New Revision: 361342
URL: https://svnweb.freebsd.org/changeset/base/361342

Log:
  MFC r360477: Correctly set up the initial TCP congestion window in all cases
  
  by not including the SYN bit sequence space in cwnd related calculations.
  Snd_und is adjusted explicitly in all cases, outside the cwnd update, instead.
  
  This fixes an off-by-one conformance issue with regular TCP sessions
  not  using Appropriate Byte Counting (RFC3465), sending one more
  packet during  the initial window than expected.
  
  PR:		235256
  Reviewed by:	tuexen (mentor), rgrimes (mentor, blanket)
  Approved by:	tuexen (mentor), rgrimes (mentor, blanket)
  MFC after:	3 weeks
  Sponsored by:	NetApp, Inc.
  Differential Revision:	https://reviews.freebsd.org/D19000

Modified:
  stable/12/sys/netinet/tcp_input.c
  stable/12/sys/netinet/tcp_stacks/rack.c
Directory Properties:
  stable/12/   (props changed)

Modified: stable/12/sys/netinet/tcp_input.c
==============================================================================
--- stable/12/sys/netinet/tcp_input.c	Thu May 21 19:45:14 2020	(r361341)
+++ stable/12/sys/netinet/tcp_input.c	Thu May 21 19:46:11 2020	(r361342)
@@ -1519,7 +1519,7 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, stru
     struct tcpcb *tp, int drop_hdrlen, int tlen, uint8_t iptos)
 {
 	int thflags, acked, ourfinisacked, needoutput = 0, sack_changed;
-	int rstreason, todrop, win;
+	int rstreason, todrop, win, incforsyn = 0;
 	uint32_t tiwin;
 	uint16_t nsegs;
 	char *s;
@@ -2432,12 +2432,6 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, stru
 		if (IS_FASTOPEN(tp->t_flags) && tp->t_tfo_pending) {
 			tcp_fastopen_decrement_counter(tp->t_tfo_pending);
 			tp->t_tfo_pending = NULL;
-
-			/*
-			 * Account for the ACK of our SYN prior to
-			 * regular ACK processing below.
-			 */ 
-			tp->snd_una++;
 		}
 		if (tp->t_flags & TF_NEEDFIN) {
 			tcp_state_change(tp, TCPS_FIN_WAIT_1);
@@ -2458,6 +2452,13 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, stru
 			tcp_timer_activate(tp, TT_KEEP, TP_KEEPIDLE(tp));
 		}
 		/*
+		 * Account for the ACK of our SYN prior to
+		 * regular ACK processing below, except for
+		 * simultaneous SYN, which is handled later.
+		 */
+		if (SEQ_GT(th->th_ack, tp->snd_una) && !(tp->t_flags & TF_NEEDSYN))
+			incforsyn = 1;
+		/*
 		 * If segment contains data or ACK, will call tcp_reass()
 		 * later; if not, do so now to pass queued data to user.
 		 */
@@ -2751,6 +2752,15 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, stru
 process_ACK:
 		INP_WLOCK_ASSERT(tp->t_inpcb);
 
+		/*
+		 * Adjust for the SYN bit in sequence space,
+		 * but don't account for it in cwnd calculations.
+		 * This is for the SYN_RECEIVED, non-simultaneous
+		 * SYN case. SYN_SENT and simultaneous SYN are
+		 * treated elsewhere.
+		 */
+		if (incforsyn)
+			tp->snd_una++;
 		acked = BYTES_THIS_ACK(tp, th);
 		KASSERT(acked >= 0, ("%s: acked unexepectedly negative "
 		    "(tp->snd_una=%u, th->th_ack=%u, tp=%p, m=%p)", __func__,

Modified: stable/12/sys/netinet/tcp_stacks/rack.c
==============================================================================
--- stable/12/sys/netinet/tcp_stacks/rack.c	Thu May 21 19:45:14 2020	(r361341)
+++ stable/12/sys/netinet/tcp_stacks/rack.c	Thu May 21 19:46:11 2020	(r361342)
@@ -5580,12 +5580,6 @@ rack_do_syn_recv(struct mbuf *m, struct tcphdr *th, st
 	if (IS_FASTOPEN(tp->t_flags) && tp->t_tfo_pending) {
 		tcp_fastopen_decrement_counter(tp->t_tfo_pending);
 		tp->t_tfo_pending = NULL;
-
-		/*
-		 * Account for the ACK of our SYN prior to
-		 * regular ACK processing below.
-		 */ 
-		tp->snd_una++;
 	}
 	if (tp->t_flags & TF_NEEDFIN) {
 		tcp_state_change(tp, TCPS_FIN_WAIT_1);
@@ -5603,6 +5597,13 @@ rack_do_syn_recv(struct mbuf *m, struct tcphdr *th, st
 		if (!IS_FASTOPEN(tp->t_flags))
 			cc_conn_init(tp);
 	}
+	/*
+	 * Account for the ACK of our SYN prior to
+	 * regular ACK processing below, except for
+	 * simultaneous SYN, which is handled later.
+	 */
+	if (SEQ_GT(th->th_ack, tp->snd_una) && !(tp->t_flags & TF_NEEDSYN))
+		tp->snd_una++;
 	/*
 	 * If segment contains data or ACK, will call tcp_reass() later; if
 	 * not, do so now to pass queued data to user.

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