Implement PLPMTUD blackhole detection (RFC 4821), inspired by code

from xnu sources.  If we encounter a network where ICMP is blocked
the Needs Frag indicator may not propagate back to us.  Attempt to
downshift the mss once to a preconfigured value.

Default this feature to off for now while we do not have a full PLPMTUD
implementation in our stack.

Adds the following new sysctl's for control:
net.inet.tcp.pmtud_blackhole_detection -- turns on/off this feature
net.inet.tcp.pmtud_blackhole_mss       -- mss to try for ipv4
net.inet.tcp.v6pmtud_blackhole_mss     -- mss to try for ipv6

Adds the following new sysctl's for monitoring:
-- Number of times the code was activated to attempt a mss downshift
net.inet.tcp.pmtud_blackhole_activated
-- Number of times the blackhole mss was used in an attempt to downshift
net.inet.tcp.pmtud_blackhole_min_activated
-- Number of times that we failed to connect after we downshifted the mss
net.inet.tcp.pmtud_blackhole_failed

Phabricator:	https://reviews.freebsd.org/D506
Reviewed by:	rpaulo bz
MFC after:	2 weeks
Relnotes:	yes
Sponsored by:	Limelight Networks

1 files changed, 11 insertions, 1 deletions

diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index 07dd5c3..7db0e50 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -675,6 +675,12 @@ just_return:
 
 send:
 	SOCKBUF_LOCK_ASSERT(&so->so_snd);
+	if (len > 0) {
+		if (len >= tp->t_maxseg)
+			tp->t_flags2 |= TF2_PLPMTU_MAXSEGSNT;
+		else
+			tp->t_flags2 &= ~TF2_PLPMTU_MAXSEGSNT;
+	}
 	/*
 	 * Before ESTABLISHED, force sending of initial options
 	 * unless TCP set not to do any options.
@@ -1303,8 +1309,12 @@ send:
 	 *
 	 * NB: Don't set DF on small MTU/MSS to have a safe fallback.
 	 */
-	if (V_path_mtu_discovery && tp->t_maxopd > V_tcp_minmss)
+	if (V_path_mtu_discovery && tp->t_maxopd > V_tcp_minmss) {
 		ip->ip_off |= htons(IP_DF);
+		tp->t_flags2 |= TF2_PLPMTU_PMTUD;
+	} else {
+		tp->t_flags2 &= ~TF2_PLPMTU_PMTUD;
+	}
 
 	if (tp->t_state == TCPS_SYN_SENT)
 		TCP_PROBE5(connect__request, NULL, tp, ip, tp, th);