Thanks Mr. Brandon for the response/feedback. Rgrds 17.06.2024 22:08 tarihinde Brandon Martin yazdı:

Is something out of your control also breaking PMTU discovery (e.g. blocking all ICMP traffic)?  With working PMTU discovery, this issue should resolve itself in the ingress direction.

You should also set the MTU properly on your PPPoE tunnel interface.  If the setup inherits properly from the parent interface's MTU, this should again be automatic, but I've observed a lot of PPPoE stacks don't necessarily do that (not that I've used PPPoE in a hot minute).  That will get mean PMTU discovery between things on your LAN and the Internet happens right at your Internet edge in the egress direction and therefore doesn't incur Internet latencies.  If the "clamp MSS to PMTU" is working, this probably means you have it right since that's usually where the rule gets its PMTU (not actual end-to-end PMTUD), but I don't know Microtik for sure.

Clamping the MSS to the known PTMU will fix the opposite direction in cases where PMTUD is broken.  That's basically why it exists.