TCPMSS
       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 40).  Of course, it can only be used in conjunction with -p tcp.
       This target is used to overcome criminally braindead ISPs or servers which block ICMP Fragmentation Needed packets.  The
       symptoms  of  this  problem  are  that everything works fine from your Linux firewall/router, but machines behind it can
       never exchange large packets:
        1) Web browsers connect, then hang with no data received.
        2) Small mail works fine, but large emails hang.
        3) ssh works fine, but scp hangs after initial handshaking.
       Workaround: activate this option and add a rule to your firewall configuration like:
        iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
                    -j TCPMSS --clamp-mss-to-pmtu