There is no such thing as an "MTU behind it", there is a separate path MTU for each and every ordered address pair, more or less (and that's not even static, ob

There is no such thing as an "MTU behind it", there is a separate path MTU for each and every ordered address pair, more or less (and that's not even static, obviously). And obviously I was talking about packet filters, not about some kind of application firewall, which obviously doesn't have anything to do with filtering of packets anyhow.