Path MTU Probe — Finding the Packet Size That Breaks Things
This determines the largest packet that can cross a network path without fragmentation, by probing with progressively larger sizes and observing where responses stop. The answer is the path MTU, and when it is smaller than the endpoints assume, the resulting failures are among the most confusing you will encounter.
Why run it
Because of a specific and very recognisable symptom: connections that establish successfully and then hang the moment real data flows. Small requests work, large responses stall. Interactive commands are fine, file transfers freeze. That pattern is an MTU problem almost every time, and it is nearly impossible to diagnose by any other means — every component reports itself healthy, because from each one's perspective it is. Tunnels and VPNs are the usual cause, since encapsulation reduces the space available for payload.
Why automate it
Path MTU is a property of the route, and the route changes. A tunnel is introduced, a provider changes encapsulation, traffic fails over to a backup path with different characteristics. Because the symptom is intermittent and application-specific, it is typically misattributed to the application for a long time before anyone suspects the network. Probing on a schedule means you have the MTU history to point at, which collapses a multi-day investigation into a glance at a chart.
What it gives you
The explanation for a class of failure that otherwise consumes days and gets blamed on the wrong system. Tracked over time, it turns an intermittent mystery into a dated, attributable change.