Hello,
I've setup ipipv6 tunnel (ipv4 over ipv6). I see "Clamp TCP MSS" as an option of the tunnel. Also I see in some guidelines that I should add firewall rule for TCP MSS clamping
Should I use both of this settings or just one?　Which one is preferable?

Thank you!

Clamp in the tunnel interface is the "automated" way to do it, as long as you can calculate PDMTU (not blocking ICMP messages, aka "ping"). You can do the same with mangle, even do the calculation yourself and fix the TCP MSS to a fixed value.

Regards.