Historically we have two fields in tcpcb to describe sender MSS: t_maxopd,

and t_maxseg. This dualism emerged with T/TCP, but was not properly cleaned
up after T/TCP removal. After all permutations over the years the result is
that t_maxopd stores a minimum of peer offered MSS and MTU reduced by minimum
protocol header. And t_maxseg stores (t_maxopd - TCPOLEN_TSTAMP_APPA) if
timestamps are in action, or is equal to t_maxopd otherwise. That's a very
rough estimate of MSS reduced by options length. Throughout the code it
was used in places, where preciseness was not important, like cwnd or
ssthresh calculations.

With this change:

- t_maxopd goes away.
- t_maxseg now stores MSS not adjusted by options.
- new function tcp_maxseg() is provided, that calculates MSS reduced by
  options length. The functions gives a better estimate, since it takes
  into account SACK state as well.

Reviewed by:	jtl
Differential Revision:	https://reviews.freebsd.org/D3593

1 files changed, 8 insertions, 13 deletions

diff --git a/sys/netinet/tcp_timer.c b/sys/netinet/tcp_timer.c
index 9767eb709056..fb4ff11a4a72 100644
--- a/sys/netinet/tcp_timer.c
+++ b/sys/netinet/tcp_timer.c
@@ -660,7 +660,6 @@ tcp_timer_rexmt(void * xtp)
 	 */
 	if (V_tcp_pmtud_blackhole_detect && (((tp->t_state == TCPS_ESTABLISHED))
 	    || (tp->t_state == TCPS_FIN_WAIT_1))) {
-		int optlen;
 #ifdef INET6
 		int isipv6;
 #endif
@@ -684,8 +683,7 @@ tcp_timer_rexmt(void * xtp)
 			tp->t_flags2 |= TF2_PLPMTU_BLACKHOLE;
 
 			/* Keep track of previous MSS. */
-			optlen = tp->t_maxopd - tp->t_maxseg;
-			tp->t_pmtud_saved_maxopd = tp->t_maxopd;
+			tp->t_pmtud_saved_maxseg = tp->t_maxseg;
 
 			/* 
 			 * Reduce the MSS to blackhole value or to the default
@@ -694,13 +692,13 @@ tcp_timer_rexmt(void * xtp)
 #ifdef INET6
 			isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) ? 1 : 0;
 			if (isipv6 &&
-			    tp->t_maxopd > V_tcp_v6pmtud_blackhole_mss) {
+			    tp->t_maxseg > V_tcp_v6pmtud_blackhole_mss) {
 				/* Use the sysctl tuneable blackhole MSS. */
-				tp->t_maxopd = V_tcp_v6pmtud_blackhole_mss;
+				tp->t_maxseg = V_tcp_v6pmtud_blackhole_mss;
 				V_tcp_pmtud_blackhole_activated++;
 			} else if (isipv6) {
 				/* Use the default MSS. */
-				tp->t_maxopd = V_tcp_v6mssdflt;
+				tp->t_maxseg = V_tcp_v6mssdflt;
 				/*
 				 * Disable Path MTU Discovery when we switch to
 				 * minmss.
@@ -713,13 +711,13 @@ tcp_timer_rexmt(void * xtp)
 			else
 #endif
 #ifdef INET
-			if (tp->t_maxopd > V_tcp_pmtud_blackhole_mss) {
+			if (tp->t_maxseg > V_tcp_pmtud_blackhole_mss) {
 				/* Use the sysctl tuneable blackhole MSS. */
-				tp->t_maxopd = V_tcp_pmtud_blackhole_mss;
+				tp->t_maxseg = V_tcp_pmtud_blackhole_mss;
 				V_tcp_pmtud_blackhole_activated++;
 			} else {
 				/* Use the default MSS. */
-				tp->t_maxopd = V_tcp_mssdflt;
+				tp->t_maxseg = V_tcp_mssdflt;
 				/*
 				 * Disable Path MTU Discovery when we switch to
 				 * minmss.
@@ -728,7 +726,6 @@ tcp_timer_rexmt(void * xtp)
 				V_tcp_pmtud_blackhole_activated_min_mss++;
 			}
 #endif
-			tp->t_maxseg = tp->t_maxopd - optlen;
 			/*
 			 * Reset the slow-start flight size
 			 * as it may depend on the new MSS.
@@ -748,9 +745,7 @@ tcp_timer_rexmt(void * xtp)
 			    (tp->t_rxtshift > 6)) {
 				tp->t_flags2 |= TF2_PLPMTU_PMTUD;
 				tp->t_flags2 &= ~TF2_PLPMTU_BLACKHOLE;
-				optlen = tp->t_maxopd - tp->t_maxseg;
-				tp->t_maxopd = tp->t_pmtud_saved_maxopd;
-				tp->t_maxseg = tp->t_maxopd - optlen;
+				tp->t_maxseg = tp->t_pmtud_saved_maxseg;
 				V_tcp_pmtud_blackhole_failed++;
 				/*
 				 * Reset the slow-start flight size as it