svn commit: r359926 - head/sys/netinet

[Michael Tuexen]

Author: tuexen
Date: Tue Apr 14 16:35:05 2020
New Revision: 359926
URL: https://svnweb.freebsd.org/changeset/base/359926

Log:
  Improve the TCP blackhole detection. The principle is to reduce the
  MSS in two steps and try each candidate two times. However, if two
  candidates are the same (which is the case in TCP/IPv6), this candidate
  was tested four times. This patch ensures that each candidate actually
  reduced the MSS and is only tested 2 times. This reduces the time window
  of missclassifying a temporary outage as an MTU issue.
  
  Reviewed by:		jtl
  MFC after:		1 week
  Sponsored by:		Netflix, Inc.
  Differential Revision:	https://reviews.freebsd.org/D24308

Modified:
  head/sys/netinet/tcp_timer.c
  head/sys/netinet/tcp_var.h

Modified: head/sys/netinet/tcp_timer.c
==============================================================================
--- head/sys/netinet/tcp_timer.c	Tue Apr 14 16:34:13 2020	(r359925)
+++ head/sys/netinet/tcp_timer.c	Tue Apr 14 16:35:05 2020	(r359926)
@@ -723,16 +723,40 @@ tcp_timer_rexmt(void * xtp)
 	    (V_tcp_pmtud_blackhole_detect == 3 && isipv6)) &&
 	    ((tp->t_state == TCPS_ESTABLISHED) ||
 	    (tp->t_state == TCPS_FIN_WAIT_1))) {
-		/*
-		 * Idea here is that at each stage of mtu probe (usually, 1448
-		 * -> 1188 -> 524) should be given 2 chances to recover before
-		 *  further clamping down. 'tp->t_rxtshift % 2 == 0' should
-		 *  take care of that.
-		 */
+		if (tp->t_rxtshift == 1) {
+			/*
+			 * We enter blackhole detection after the first
+			 * unsuccessful timer based retransmission.
+			 * Then we reduce up to two times the MSS, each
+			 * candidate giving two tries of retransmissions.
+			 * But we give a candidate only two tries, if it
+			 * actually reduces the MSS.
+			 */
+			tp->t_blackhole_enter = 2;
+			tp->t_blackhole_exit = tp->t_blackhole_enter;
+			if (isipv6) {
+#ifdef INET6
+				if (tp->t_maxseg > V_tcp_v6pmtud_blackhole_mss)
+					tp->t_blackhole_exit += 2;
+				if (tp->t_maxseg > V_tcp_v6mssdflt &&
+				    V_tcp_v6pmtud_blackhole_mss > V_tcp_v6mssdflt)
+					tp->t_blackhole_exit += 2;
+#endif
+			} else {
+#ifdef INET
+				if (tp->t_maxseg > V_tcp_pmtud_blackhole_mss)
+					tp->t_blackhole_exit += 2;
+				if (tp->t_maxseg > V_tcp_mssdflt &&
+				    V_tcp_pmtud_blackhole_mss > V_tcp_mssdflt)
+					tp->t_blackhole_exit += 2;
+#endif
+			}
+		}
 		if (((tp->t_flags2 & (TF2_PLPMTU_PMTUD|TF2_PLPMTU_MAXSEGSNT)) ==
 		    (TF2_PLPMTU_PMTUD|TF2_PLPMTU_MAXSEGSNT)) &&
-		    (tp->t_rxtshift >= 2 && tp->t_rxtshift < 6 &&
-		    tp->t_rxtshift % 2 == 0)) {
+		    (tp->t_rxtshift >= tp->t_blackhole_enter &&
+		    tp->t_rxtshift < tp->t_blackhole_exit &&
+		    (tp->t_rxtshift - tp->t_blackhole_enter) % 2 == 0)) {
 			/*
 			 * Enter Path MTU Black-hole Detection mechanism:
 			 * - Disable Path MTU Discovery (IP "DF" bit).
@@ -752,7 +776,8 @@ tcp_timer_rexmt(void * xtp)
 			 */
 #ifdef INET6
 			if (isipv6 &&
-			    tp->t_maxseg > V_tcp_v6pmtud_blackhole_mss) {
+			    tp->t_maxseg > V_tcp_v6pmtud_blackhole_mss &&
+			    V_tcp_v6pmtud_blackhole_mss > V_tcp_v6mssdflt) {
 				/* Use the sysctl tuneable blackhole MSS. */
 				tp->t_maxseg = V_tcp_v6pmtud_blackhole_mss;
 				TCPSTAT_INC(tcps_pmtud_blackhole_activated);
@@ -771,7 +796,8 @@ tcp_timer_rexmt(void * xtp)
 			else
 #endif
 #ifdef INET
-			if (tp->t_maxseg > V_tcp_pmtud_blackhole_mss) {
+			if (tp->t_maxseg > V_tcp_pmtud_blackhole_mss &&
+			    V_tcp_pmtud_blackhole_mss > V_tcp_mssdflt) {
 				/* Use the sysctl tuneable blackhole MSS. */
 				tp->t_maxseg = V_tcp_pmtud_blackhole_mss;
 				TCPSTAT_INC(tcps_pmtud_blackhole_activated);
@@ -798,11 +824,9 @@ tcp_timer_rexmt(void * xtp)
 			 * with a lowered MTU, maybe this isn't a blackhole and
 			 * we restore the previous MSS and blackhole detection
 			 * flags.
-			 * The limit '6' is determined by giving each probe
-			 * stage (1448, 1188, 524) 2 chances to recover.
 			 */
 			if ((tp->t_flags2 & TF2_PLPMTU_BLACKHOLE) &&
-			    (tp->t_rxtshift >= 6)) {
+			    (tp->t_rxtshift >= tp->t_blackhole_exit)) {
 				tp->t_flags2 |= TF2_PLPMTU_PMTUD;
 				tp->t_flags2 &= ~TF2_PLPMTU_BLACKHOLE;
 				tp->t_maxseg = tp->t_pmtud_saved_maxseg;

Modified: head/sys/netinet/tcp_var.h
==============================================================================
--- head/sys/netinet/tcp_var.h	Tue Apr 14 16:34:13 2020	(r359925)
+++ head/sys/netinet/tcp_var.h	Tue Apr 14 16:35:05 2020	(r359926)
@@ -169,6 +169,8 @@ struct tcpcb {
 	u_int	t_starttime;		/* time connection was established */
 
 	u_int	t_pmtud_saved_maxseg;	/* pre-blackhole MSS */
+	int	t_blackhole_enter;	/* when to enter blackhole detection */
+	int	t_blackhole_exit;	/* when to exit blackhole detection */
 	u_int	t_rttmin;		/* minimum rtt allowed */
 
 	u_int	t_rttbest;		/* best rtt we've seen */