MFC r272720, 273061, 273062, 273063, 273064

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.

Note, this is turned off by default.

1 files changed, 16 insertions, 1 deletions

diff --git a/share/man/man4/tcp.4 b/share/man/man4/tcp.4
index 1a44542..dd218bc 100644
--- a/share/man/man4/tcp.4
+++ b/share/man/man4/tcp.4
@@ -38,7 +38,7 @@
 .\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd November 8, 2013
+.Dd October 13, 2014
 .Dt TCP 4
 .Os
 .Sh NAME
@@ -522,6 +522,21 @@ avoid packet drops.
 Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a
 specific connection. This is needed to help with connection establishment
 when a broken firewall is in the network path.
+.It Va pmtud_blackhole_detection
+Turn on automatic path MTU blackhole detection. In case of retransmits we will
+lower the MSS to check if it's MTU problem. If current MSS is greater than
+configured value to try, it will be set to it, otherwise, MSS will be set to
+default values (net.inet.tcp.mssdflt and net.inet.tcp.v6mssdflt).
+.It Va pmtud_blackhole_mss
+MSS to try for IPv4 if PMTU blackhole detection is turned on.
+.It Va v6pmtud_blackhole_mss
+MSS to try for IPv6 if PMTU blackhole detection is turned on.
+.It Va pmtud_blackhole_activated
+Number of times the code was activated to attempt a MSS downshift.
+.It Va pmtud_blackhole_min_activated
+Number of times the blackhole MSS was used in an attempt to downshift.
+.It Va pmtud_blackhole_failed
+Number of times that we failed to connect after we downshifted the MSS.
 .El
 .Sh ERRORS
 A socket operation may fail with one of the following errors returned: