This target allows to alter the MSS value of TCP SYN packets, to control
the maximum size for that connection (usually limiting it to your
outgoing interface's MTU minus 60).  Of course, it can only be used
in conjunction with
.BR "-p tcp" .
It is only valid in the
.BR mangle
table.
.br
This target is used to overcome criminally braindead ISPs or servers
which block ICMPv6 Packet Too Big packets or are unable to send them.
The symptoms of this problem are that everything works fine from your 
Linux firewall/router, but machines behind it can never exchange large
packets:
.PD 0
.RS 0.1i
.TP 0.3i
1)
Web browsers connect, then hang with no data received.
.TP
2)
Small mail works fine, but large emails hang.
.TP
3)
ssh works fine, but scp hangs after initial handshaking.
.RE
.PD
Workaround: activate this option and add a rule to your firewall
configuration like:
.nf
 ip6tables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \\
             -j TCPMSS --clamp-mss-to-pmtu
.fi
.TP
.BI "--set-mss " "value"
Explicitly set MSS option to specified value.
.TP
.B "--clamp-mss-to-pmtu"
Automatically clamp MSS value to (path_MTU - 60).
.TP
These options are mutually exclusive.