Troubleshooting MTU in encapsulated networks
A method for finding mysterious stalls and fragmented traffic.
This note documents a practical approach used in Steppe Infrastructure Notes. The goal is a setup that can be explained, reproduced, and checked from outside the machine.
Compare small and large requests
A handshake may succeed while larger responses stall. Test progressively larger payloads instead of treating ping as a binary connectivity check. Capture the interface on both sides when possible.
ping -M do -s 1400 192.0.2.1Account for every header
The usable payload is reduced by each encapsulation layer. Draw the path and calculate the overhead rather than copying a common MTU from another provider. VLANs and additional tunnels change the answer.
ip link show
tracepath 192.0.2.1Use probing and validation
Enable MTU probing where appropriate, then validate with real TLS and HTTP transfers. A lower fixed MTU can be a useful diagnostic, but it should not become an unexplained permanent workaround.
sysctl net.ipv4.tcp_mtu_probing