Path MTU

What it does: finds the largest packet that reaches a target WITHOUT being fragmented — the real end-to-end MTU. It binary-searches ICMP echo sizes with the Don't-Fragment bit set.

Safe to run: yes, read-only ICMP.

Reading results: a path MTU below 1500 means a link in the path (VPN, PPPoE, GRE tunnel) is clamping it — the usual cause of 'small pages load, big transfers or some HTTPS sites hang.' If the host won't answer a DF ping at all, a firewall is dropping DF ICMP; try another target.