Hacker News Reader

Your local router, if correctly configured should do fragmentation. But in 95% of use cases, jumbo frames aren’t worth the hassle.

Not for IPv6. And for IPv4, I believe few routers actually handle IP fragmentation properly or at all (which is why this was removed from IPv6).

This is what Path MTU Discovery is for, in principle, or what MSS clamping at your router will do.

Shouldn‘t the NAT take care of the outgoing frames as it is operating on the tcp/udp level?

No. NAT does Network Address Translation. It just changes addresses in packets. The packets are still whatever size they are.

There's fragmentation, but that's separate from NAT and often broken.